Compression ratio calculation

[xctasy]

Hi Xctasy,

Your first remarks were a bit over my head, so I'm going to have to digest them for a while.

I think when you say cold cranking compression, you're referring to dynamic compression, and to the fact that a cam with more duration and overlap will reduce the dynamic compression. So I could run a 10:1 static compression ratio, but a considerably larger than stock cam (more duration and overlap), such as say one of the Clay Smith 274 cams, and the dynamic compression would not be so high as to cause knocking, even though the static compression is rather high for a street engine running on pump gas.

Now you've told me something that I haven't even heard a hint of before ... that "forged pistons reduce knock, and have about a 100 degree less combustion temperature under load." Can you explain why that is so? If so, a set of forged pistons might aid in controlling knock, and of course increase durability as you mention.

Thank you,
Bob

Forged pistons allways run cooler for any given combustion chamber or working indicated mean pressure than cast alloy pistons. For this reason, they hurt emissions on engines, and aren't CARB approved unless the engine came out with them in the first place.

Its to do with metelurgy ( grain structure and eutectics). The modern forged piston is dimensionally stable, strong, and able to operate as well as a cast piston. There are different types of cast pistons, high sand semi forged pistons that a technically alloy castings, which can work as well as forged pistons, and these are often used instead of forged pistons in some of Fords turbo engines, but generally, a forged piston will allow more compression, more boost, and more duration before detonation sets in. Ford engineers do elaborate tests and through that work, they win cost verses performance edges. Like in 1992, they tested the standout 5.0 HO EFI, and were able to do away with that years brilliant TRW Forged piston, as it was costing the Cleveland plant a motsa to build that 225 hp engine. The next year, the forged pustons were gone, and durability was just as good. The reason was Ford did the testing to see where detonation set in, and having a hotter temperature wasn't a concern. Since that time, no need for forged pistons on the GT40 engines. So the work was a cost saving for Ford. But they also make botch ups, even with the very best engineers in the world. The brilliant cast alloy Zollener shallow deck pistons and the problem Ford had with plateau honed cylinders killed a lot of 4.6 and 5.4 SOHC engines in 1997, and Ford then had to spend millions reworking there production tollerances. So a cast piston is a cheap option, which can cause problems.

The 1979-1981 carb turbo 2300 had the first low power use of TRW Forged pistons, but they didn't help the Mustang Turbo stop dropping rods or rattling its way to oblivion with just 127, 131 or 140 hp of load. Its problem was mixture distribution and running a 9.5:1 compression ratio. The forged pistons the 2300 ran for that car probably helped Ford climb out of the extent of warranty claimes made on that disasterous engine.

The working combustion temperature and pressure in a same Compression ratio combustion chamber is raised as the top dead centre parking point is moved towards a perfect O deck (where a piston is ture with the deck), and gets worse as it goes down to that awfull 123 thou depth found on all 250 US engines, and on some 240 and 300 I6's as well.

Leedging is therefore the exposed heated annular part at the top of the piston to cylinder wall. Lower the piston parks, more thats exposed, more detonation potential. Squench is a combination of Squish and Quench and is different when the piston is raised or the hesd is decked. So Ledging, squish and quench vary with different tollerances of piston to block deck height, and between the common 25 and 45 thou gaskets. The lads at Dearborn and Geelong Australia had very differEnt deck clearances on there 250 derivatives, and used composite gaskets in Australia, and steel gaskets in the US. From 1971 to June 1976, the 155 hp gross L code Aussie engine had a ballistic 9.2:1 compression ratio, 97 RON/MON leaded fuel, and was a very strong engine. Every year from 1968 to 1981, the US 250 lost power from that intial 155 gross, and ended up with 99 hp net just because it had to run pump grade 87 or the occasional splash of 83. The engine was tuned down to avoid combustion temperatures and had to use the same head and gasket as the 200. That's why the 103 to 123 ledge, and why the we are discussion knock resistance. Anything that improves the stock knock resitance is good.

I presonally would never, ever deck a cast iron Ford block unless it was a 460, but lots of people here have done it ,and people here find that is the best step to getting performance and knock resistance back. I personally have seen blocks crack at the front of 250 log head or X-flow combos over here in NZ when they are decked, and US blocks are much thinner castings than our Antipodean engines. So take a risk and go to it, but I'd say a nice Jeep 258 piston answers all your problems. If you can cope with a 70 thou overbore, then the ledging, squish and quench is optimized in one hit.

 

[62Ranchero200]

Hi Xctasy,

Your first remarks were a bit over my head, so I'm going to have to digest them for a while.

I think when you say cold cranking compression, you're referring to dynamic compression, and to the fact that a cam with more duration and overlap will reduce the dynamic compression. So I could run a 10:1 static compression ratio, but a considerably larger than stock cam (more duration and overlap), such as say one of the Clay Smith 274 cams, and the dynamic compression would not be so high as to cause knocking, even though the static compression is rather high for a street engine running on pump gas.

Now you've told me something that I haven't even heard a hint of before ... that "forged pistons reduce knock, and have about a 100 degree less combustion temperature under load." Can you explain why that is so? If so, a set of forged pistons might aid in controlling knock, and of course increase durability as you mention.

Thank you,
Bob

Forged pistons allways run cooler for any given combustion chamber or working indicated mean pressure than cast alloy pistons. For this reason, they hurt emissions on engines, and aren't CARB approved unless the engine came out with them in the first place.

Its to do with metelurgy ( grain structure and eutectics). The modern forged piston is dimensionally stable, strong, and able to operate as well as a cast piston. There are different types of cast pistons, high sand semi forged pistons that a technically alloy castings, which can work as well as forged pistons, and these are often used instead of forged pistons in some of Fords turbo engines, but generally, a forged piston will allow more compression, more boost, and more duration before detonation sets in. Ford engineers do elaborate tests and through that work, they win cost verses performance edges. Like in 1992, they tested the standout 5.0 HO EFI, and were able to do away with that years brilliant TRW Forged piston, as it was costing the Cleveland plant a motsa to build that 225 hp engine. The next year, the forged pustons were gone, and durability was just as good. The reason was Ford did the testing to see where detonation set in, and having a hotter temperature wasn't a concern. Since that time, no need for forged pistons on the GT40 engines. So the work was a cost saving for Ford. But they also make botch ups, even with the very best engineers in the world. The brilliant cast alloy Zollener shallow deck pistons and the problem Ford had with plateau honed cylinders killed a lot of 4.6 and 5.4 SOHC engines in 1997, and Ford then had to spend millions reworking there production tollerances. So a cast piston is a cheap option, which can cause problems.

The 1979-1981 carb turbo 2300 had the first low power use of TRW Forged pistons, but they didn't help the Mustang Turbo stop dropping rods or rattling its way to oblivion with just 127, 131 or 140 hp of load. Its problem was mixture distribution and running a 9.5:1 compression ratio. The forged pistons the 2300 ran for that car probably helped Ford climb out of the extent of warranty claimes made on that disasterous engine.

The working combustion temperature and pressure in a same Compression ratio combustion chamber is raised as the top dead centre parking point is moved towards a perfect O deck (where a piston is ture with the deck), and gets worse as it goes down to that awfull 123 thou depth found on all 250 US engines, and on some 240 and 300 I6's as well.

Leedging is therefore the exposed heated annular part at the top of the piston to cylinder wall. Lower the piston parks, more thats exposed, more detonation potential. Squench is a combination of Squish and Quench and is different when the piston is raised or the hesd is decked. So Ledging, squish and quench vary with different tollerances of piston to block deck height, and between the common 25 and 45 thou gaskets. The lads at Dearborn and Geelong Australia had very differEnt deck clearances on there 250 derivatives, and used composite gaskets in Australia, and steel gaskets in the US. From 1971 to June 1976, the 155 hp gross L code Aussie engine had a ballistic 9.2:1 compression ratio, 97 RON/MON leaded fuel, and was a very strong engine. Every year from 1968 to 1981, the US 250 lost power from that intial 155 gross, and ended up with 99 hp net just because it had to run pump grade 87 or the occasional splash of 83. The engine was tuned down to avoid combustion temperatures and had to use the same head and gasket as the 200. That's why the 103 to 123 ledge, and why the we are discussion knock resistance. Anything that improves the stock knock resitance is good.

I presonally would never, ever deck a cast iron Ford block unless it was a 460, but lots of people here have done it ,and people here find that is the best step to getting performance and knock resistance back. I personally have seen blocks crack at the front of 250 log head or X-flow combos over here in NZ when they are decked, and US blocks are much thinner castings than our Antipodean engines. So take a risk and go to it, but I'd say a nice Jeep 258 piston answers all your problems. If you can cope with a 70 thou overbore, then the ledging, squish and quench is optimized in one hit.

Hi All,

I was tossing and turning all last night, thinking about all this. Ran a quick calculation this morning. If I use

CI 50 CC aluminum head
255 flattop pistons with 0.085 more compression height
No further decking or milling

(let's call this the 255 scenario)

I get an 11.4 CR - way too high for a street car driven on pump gas. I'd have to mill a 10 CC dish into the 255 flattops to get the CR down to 10:1. Even then, I'd still have .088 quench height (deck height + compressed gasket thickness) - about twice the ideal quench height.

If I use a flattop 250 piston with "normal" compression height and deck the block 0.085, the result is the same. If I use the 6.5 CC dish pistons, the lower CR resulting from the dish would allow me to deck the block .065 and end up with a 10.05 CR. Then, I'd still have a .108 quench height - 2 1/2 times the idea quench height.

I think my conclusion here is that for a 250 with it's tall deck and the 50 CC CI aluminum head, ideal quench or quench anywhere close to ideal is something that only racers can have, because only racers can run the high CRs that would result from either using the 255 pistons or decking the block .100 or more. If building a 250 for the street, that must be able to run on pump gas in real world conditions (heat, traffic, bad gas), it's not going to be possible to approach anywhere near ideal quench. Even if I was independently wealthy and could afford to have any combination of custom pistons, rods and crank made, every combination that would approach ideal quench would create a CR that wasn't doable for a street engine running on pump gas.

Thank you,
Bob

 

[First Fox]

If that's the way the math plays out for you, there is still one more option: Reverse dome pistons. Ever heard of 'em? Basically a high, flat "Dome" on the quench side of the piston, opposite of where a dome is normally placed in order to achieve good quench, and a "dish" in essence on the chamber side of the piston to lower C/R to a manageable level. These would have to be custom made, but are used quite often and might be the way to go if you math is right on this and you want to stick with the 250.

I personally never did the math using one of the AL heads myself because I know I cant afford one, but it seems to me that Mike should have probably had 2 different size chambers cast when he got these made. I bet a 60ish cc chamber would allow you to run a stock type piston and still solve your quench problem while gaining all the benefits from the AL head.

Makes me wonder why he chose to use such a small combustion chamber.

 

[CZLN6]

Howdy All:

Hey FF- "Makes me wonder why he chose to use such a small combustion chamber?"

Probably because there are 10 times more 200s out there and 50 cc chambers is more ideal for them. Also He was interested in an efficient chamber shape an 50 ccs is the way it came out.

Bob- You could always go back to the log head. I know, it's not as glamorous but it is an option. Just a thought.

Bubba mentioned offset grinding the crank, which I had forgotten. If you have to turn the journals off-set grinding is quite reasonable and it gains a little in stroke/displacement and reduces deck height. Just another thought, as if you needed another thought.

As I said, there is no easy way to build an ideal 250 with a 9:1 CR. Some where there has to be a compromise.

Keep it coming.

Adios, David

 

[62Ranchero200]

Howdy All:

Hey FF- "Makes me wonder why he chose to use such a small combustion chamber?"

Probably because there are 10 times more 200s out there and 50 cc chambers is more ideal for them. Also He was interested in an efficient chamber shape an 50 ccs is the way it came out.

Bob- You could always go back to the log head. I know, it's not as glamorous but it is an option. Just a thought.

Bubba mentioned offset grinding the crank, which I had forgotten. If you have to turn the journals off-set grinding is quite reasonable and it gains a little in stroke/displacement and reduces deck height. Just another thought, as if you needed another thought.

As I said, there is no easy way to build an ideal 250 with a 9:1 CR. Some where there has to be a compromise.

Keep it coming.

Adios, David

Hi David,

There were so many thoughts going around in my head last night that I could not sleep.

I would like to focus on one question for a moment: what is the deck thickness of a virgin 250 block? I looked for this information both in the Falcon Six Handbook and online, without success.

How much can it be safely decked in a N/A, street RPM, street CR situation (say 5,000 RPM and 9.5 - 10.0 CR)? Could it be that the deck failures that Xctasy has seen are in supercharged, turbocharged, or NOS applications, and/or with high RPM, high CR race motors? Has anyone seen a 250 deck failure in a N/A, street RPM, street CR motor?

250 blocks are only going to get harder to get as time goes on, and no one is making any new ones, so I'd prefer not to lose one to deck failure.

Thank you,
Bob

 

[First Fox]

Howdy All:

Hey FF- "Makes me wonder why he chose to use such a small combustion chamber?"

Probably because there are 10 times more 200s out there and 50 cc chambers is more ideal for them. Also He was interested in an efficient chamber shape an 50 ccs is the way it came out.


Adios, David

I would agree there are more 200 out there, but it seems that a lot of guys that are trying to make power and would thus purchase this head would want the extra cubes and are using, or swapping to the 250. I am not sure I have seen many 200's being built with this head, and certainly nothing smaller, but I have seen a few 250's. This build is a perfect example of that, Bob wants the extra displacement and the head doesn't really work for him because of the chamber volume. I don't think that having another choice on the chamber size is unreasonable.


And Bob, have you considered keeping and building the 200 to make this work out a little better for you? You are surely not too far in to change your build to the stock 200 and would save all the hassle of body mods and all else. I am SURE someone here would buy your newfound 250 if you decide to go that route.

FWIW, the 250 swap seemed like too much of a hassle for me and 200's are plentiful and cheap. I am in the process of building a turbo 200 to replace my high mileage 200 in my car now, and when THAT 200 is out, I will freshen it up to replace the 170 in my falcon. No 250 swap on the horizon for this guy. 8)

 

[Gene Fiore]

Makes me wonder why he chose to use such a small combustion chamber.

I'm wondering if Mike's machinest could increase the combustion chamber size of the aluminum head to 60 ccs or so. Is that even feasible? Just thinking out loud.

 

[62Ranchero200]

I would agree there are more 200 out there, but it seems that a lot of guys that are trying to make power and would thus purchase this head would want the extra cubes and are using, or swapping to the 250. I am not sure I have seen many 200's being built with this head, and certainly nothing smaller, but I have seen a few 250's. This build is a perfect example of that, Bob wants the extra displacement and the head doesn't really work for him because of the chamber volume. I don't think that having another choice on the chamber size is unreasonable.


And Bob, have you considered keeping and building the 200 to make this work out a little better for you? You are surely not too far in to change your build to the stock 200 and would save all the hassle of body mods and all else. I am SURE someone here would buy your newfound 250 if you decide to go that route.

FWIW, the 250 swap seemed like too much of a hassle for me and 200's are plentiful and cheap. I am in the process of building a turbo 200 to replace my high mileage 200 in my car now, and when THAT 200 is out, I will freshen it up to replace the 170 in my falcon. No 250 swap on the horizon for this guy. 8)

Hi FF,

My experience with the Ford sixes is limited, but one principle from my previous V-8 projects is that larger displacement is an ideal way to build a motor with more torque, which in turn is ideal for a street car. For a race car, a smaller high CR, high RPM, big cam engine may get the desired results, but that's not very streetable. A larger, lower CR, lower RPM, smaller cam engine will probably last longer too, all other things being equal.

Have to commit to some decisions or I'll get into an infinite regression of indecision, comparing all possibilities.

Had already moved my radiator up as much as possible even before I started the 250 project. I already have a C-4 in the car, which will accept the 250 bellhousing and flexplate with minimal difficulty. The remaining mods to put the 250 in my round body Falcon Ranchero are well documented in the Falcon Six Handbook. May even decide to get a custom hood or hood scoop made rather than even attempting to lower the 250.

Finally, if myself or anyone else ever wanted to put a V-8 in the car, after the 250 swap it would already have a SBF-compatible bellhousing and a V-8 radiator.

Thank you,
Bob

 

[62Ranchero200]

Howdy All:

Hey FF- "Makes me wonder why he chose to use such a small combustion chamber?"

Probably because there are 10 times more 200s out there and 50 cc chambers is more ideal for them. Also He was interested in an efficient chamber shape an 50 ccs is the way it came out.

Bob- You could always go back to the log head. I know, it's not as glamorous but it is an option. Just a thought.

Bubba mentioned offset grinding the crank, which I had forgotten. If you have to turn the journals off-set grinding is quite reasonable and it gains a little in stroke/displacement and reduces deck height. Just another thought, as if you needed another thought.

As I said, there is no easy way to build an ideal 250 with a 9:1 CR. Some where there has to be a compromise.

Keep it coming.

Adios, David

Hi David,

Is quench so important for a street engine that reverting to the log head, with its larger 62 CC combustion chamber, would outweigh all the benefits of the aluminum head?

* Greatly improved port and combustion chamber design
* Larger intake and exhaust valves
* The ability to use a detachable intake manifold with greatly improved fuel distribution, especially to #1 and #6
* The superior heat dissipating ability of aluminum

I'm not referring to the "wow factor" of the aluminum head. I would like to weigh benefits that would figure into the final, measurable performance of a street 250.

Thank you,
Bob

 

[First Fox]

[Hi FF,

My experience with the Ford sixes is limited, but one principle from my previous V-8 projects is that larger displacement is an ideal way to build a motor with more torque, which in turn is ideal for a street car. For a race car, a smaller high CR, high RPM, big cam engine may get the desired results, but that's not very streetable. A larger, lower CR, lower RPM, smaller cam engine will probably last longer too, all other things being equal.

Have to commit to some decisions or I'll get into an infinite regression of indecision, comparing all possibilities.

Had already moved my radiator up as much as possible even before I started the 250 project. I already have a C-4 in the car, which will accept the 250 bellhousing and flexplate with minimal difficulty. The remaining mods to put the 250 in my round body Falcon Ranchero are well documented in the Falcon Six Handbook. May even decide to get a custom hood or hood scoop made rather than even attempting to lower the 250.

Finally, if myself or anyone else ever wanted to put a V-8 in the car, after the 250 swap it would already have a SBF-compatible bellhousing and a V-8 radiator.

Thank you,
Bob

Cool, I don't care to talk you out of a decision either way, none of my business. Just noticing some shortcomings of this build and bringing them to your attention.

My experience is mainly with V8's also, but basic building is the same across the board, and combustion chamber design and the quench/turbulence issue is an important issue for any engine if your goal is efficiency and power, and on this build it is seriously lacking.

As to the cubic inches vs torque statement, of course there is merit to that, BUT I would certainly say that an engine with a 50 CID advantage and lower compression ratio would ultimately make less power than one 50 inches smaller and an idealized combustion chamber, and higher compression. Compare a well built, aluminum head, 10:1, 302 to a smogger iron head, low compression, factory 351 windsor, a 50 cubic inch difference. Not only would the 302 make more power, it would likely make more torque at nearly all RPM.

Cylinder pressure and proper combustion makes torque, not necessarily cubic inches.

Anyway, good luck with the build man, whichever way you decide to go.

 

[CNC-Dude]

Cubic inch is King, and as long as you build a purpose built 250 with the correct compression with the aluminum head, it will always spank a comparably built 200 with the same compression and like components. Go for the torque and lower RPM grunt the bigger engine has to offer. Ross and many other piston companies have a quality forged dish piston that can accomodate your desired compression ratio, and can even mirror image the combustion chamber to give you great quench to boot. Having the correct quench is key to any engine build, whether its a street engine or a race engine. Go with the aluminum head and adjust your compression with a custom piston if needed.

 

[Jakes66snake]

CNC-Dude, I'm not saying your wrong by no means...... but most people I know would preferr the 302 over the 351, 429 to 460, chevy 350 to 400. But then on the other hand everyone wants to stroke the 302 to 347. Rod ratio and compression height is the only thing I can figure. That being said I'm in the planning stage of my build, but every topic I read I say "I should do that"...info overload!! I have a 200 and a 250 but looking at CI dyno page the 200 seem to clean up over the 250's on RWHP...IDK what to do. Like this thead, costom pistons plus more machine work just to get the proper height and compression for what I've seen on here, little gains. Again IDK, this post is more of a question than statment

 

[62Ranchero200]

CNC-Dude, I'm not saying your wrong by no means...... but most people I know would preferr the 302 over the 351, 429 to 460, chevy 350 to 400. But then on the other hand everyone wants to stroke the 302 to 347. Rod ratio and compression height is the only thing I can figure. That being said I'm in the planning stage of my build, but every topic I read I say "I should do that"...info overload!! I have a 200 and a 250 but looking at CI dyno page the 200 seem to clean up over the 250's on RWHP...IDK what to do. Like this thead, costom pistons plus more machine work just to get the proper height and compression for what I've seen on here, little gains. Again IDK, this post is more of a question than statment

Jakes66snake,

Couldn't agree more - this thread has definitely raised more questions than it has answered. I appreciate everyone's expert input more than I can say, but I certainly can't say that the sum total of the input is steering me in any specific direction. While everyone seems to agree that a zero deck height is close to ideal, no one has any specific combination of parts that will achieve this with reasonable CR for my build (250, 50 CC aluminum head).

Really don't think it's going to be possible to get ideal quench height on the 250 with the 50 CC aluminum head and a CR that will work for pump gas. In the end, it may be as David mentioned, that with a 250 there is no ideal solution and compromise is the key to the build. If it were a race-only build, I could design it with a high CR and run 105 octane racing or aviation fuel, so achieving ideal quench height wouldn't be a problem.

Have to wonder if anyone else has attempted this build, with a 250 and the 50 CC aluminum head, and a CR that will work for pump gas. Anyone on this forum who has done so, please respond.

Thanks,
Bob

 

[xctasy]

Here is the summary of best choices. It's hard to sumarise stuff untill everyone understands the details, and one has to sift through a tedeous historical and geneolgical blurbs to figure out where the heck the Ford 250 fits in the small six puzzle. Especially so it, like David or FordSedanDelivery, you've worked on cars all your life...sometimes explaining why you'd rather use this or that piston, or deck this or that block its most likely harder fro them to Explain than to Do, and when we question the wise masters out of respect, we risk the good sentiments we have being missed, and we end up feeling like were piddling in their pockets a little :nono: Such is the quest for knowledge, it cannot be packed and unpacked without a good deal of mental effort.

Collectively, we have actually been given all the answers, it just that we are all learning and haven't realised it yet.

As David S has said many times, with Fords, there are no absolutes. My take in it is that because of this, you end up doing something contrary to good practice always due to the parts scrammble beteen FoMoCo's engines. You have no choice but to find out what others have done, and copy a combination that works.

To recap, the stock US pistons are known to crack in service, so don't use them unless you are prepared to have to replace them. This in not US bashing, the 200/250/255/HSC product is okay, but not as perfect as it could be. I'd not use the standard US cast alloy pistons personally, but thats because Aussies make sedan racing spec cast alloy inline six cylinder pistons, and have blowing them up since 1948 at every local race track.
Don't deck the block unless you have to, but on the 250, your stuck with having to. So there is no absolute. I'd not deck a block personally
Don't over bore a thin wall block, but in some cases, with a 250, you have to. I'd not over bore a small six block above 60 thou personally unless I was sleeving the block.
stock rods in 250's are sometimes cast, not forged. Your better off with forged, but sometimes, you just have to use whats on offer. I'd not use cast rods personally

A lot of terms have been used here with respect to the pistons, heads, head gaskets, chambers and decks, and there isn't any crash course except by slowly spoon feeding yourself. When the penny drops, you'll see that you can use the CI head and cheap pistons and gaskets, spend less money and get a better result just because you under stand the problem and can then find a solution.

The problem with the 250 is much like the 351C or 400 Ford when you want to run same heads, pistons and gaskets as the 302C engines like we did in Australia or New Zealand just after the lead got taken out of our gasoline, and we suddenly had to make big inch engines live with 91 octane gas. Compression ratio skyrockets when you put the better cylinder heads on the bigger engines, and no one seams to know quite how to deal with it.

Well, there is a simple answer, you fiddle around with pistons , head gaskets, block register and head cc's until you find a combo you like. Your about to find out that there are better, cheaper parts around than what you see on the parts suppliers list for US Fords.

For example, the Australian Ford 250 log and 2V engines had a 12.5 cc piston, a 59 to 62 cc head, a 41 thou gasket, and its was just fine for fuel down to 93 octane. Its block was 9.38" tall, not 9.469" like the US version you guys use. If you added 15.5 cc 1976-1987 ACL 3.3 x-flow pistons, you could run 91 octane. They also have 22.9 and 27.9 cc chamber pistons to lower the compression even more, straight out of the ACL catalogue, and you can use 87 octane fuel. All Ford Aussie pistons have a 16 thou below the block deck register, or 1.529 to 1.531". The Aussie pistons don't crack, and can be used forperformance applications, but they are still cast alloy. They were used in AIT, Noramalair Garret, Mike Vine and Dick Johnston turbo applications from 255 to 360 hp from 1982 to 1987, and they didn't break. As long as your not using low tension rings and the spec for the 4500 rev limited 1986-1993 Aussie X flows, the aftermarket cast pistons will do fine service.


So the best answer for you is
1. to deck the block 89 thou, use the cheap, common Aussie X-flow ACL 4.1 liter 22.9 or27.9 ccc pistons which are almost the same as the Aussie 250 log head pistons except have a bigger dish, and use the bullet proof 5.88" Aussie Forged rods and leave the chambers at 50 cc and use the 13 US dollar app630 permaset monotorque gasket set. Any fuel down to 87 octane will work

Second best answer is
2. to deck the block 89 thou, use the cheap, common Aussie X-flow ACL 4.1 liter 15.5 dish pistons which are almost the same as the Aussie 250 log head pistons, and use the bullet proof 5.88" Aussie Forged rods and open up the chambers 9 cc to 59 cc and use the 13 US dollar app630 permaset monotorque gasket set. 91 to 93 octane will be best, but any alloy head should be able to handle 87 octane


The third asnwer is to use

3. over bore 70 thou for big 258 Jeep/AMC pistons like Powerbands, ream out the rods, and add a 41 thou APP630 gasket, then you can use your stock 50 cc head.

Any one of those 3 will get great results.

If you do a search, the parts cost is very low in US dollars for any of these combinations listed above.

The US 250 is the best kept secrete, you just have to use either standard 1971 to 1987 Aussie know how, or AMC parts.

Lastly, even though I suggested Aussie pistons to cope with the deck and durabity and compression problems with the 250 engine, I only use CP 4.0 DOHC Turbo spec pistons in my engines, with Melling CSL331F 3/32th Sleeves to create a 48 thou under size engine with a 90 thou thick sleeve , US made versions of the Ford Corsair/Nissan pIntara/240K KE 24 Nissan conrod, and that takes care of compression ratio, wall thickness, rod strength, piston strength with a huge 120 thou over bore to 3.800, leaving a 2300 Lima/400 Chev style 280 thou gap between the liners. These Chevy trailblazer sleeves eliminate the need to mess around with the decking the block, and the whole pacakage provides similar strength to the 5.4 rods/5.0 piston combo Does10s uses, with greatly reduced cost. And its helpfull that the rods can take 550 hp of absue, along with the pistons. Because I use a cross flow block, the 195 thou flange allows me to fiddle around with the deck so it fits the non cross flow CI head adaptor.

I think any 250 block is an asset that shoudn't be messed around with too much, and there are common parts that seam to produce a better result than factory replacment parts.

Hope this helps :mrgreen:

 

[wsa111]

Just remember if your cranking compression goes over 195 you can always put more cam in the engine to lower the CCR.
A larger cubic inch engine will make a cam seem milder compared to the same cam in say a 200 engine.
If you have the proper quench & keep the CCR around 180-185 with the distributor curved & the total advance correct you will be fine.
A leaner fuel mixture will invite detonation so you will have to monitor the A/F ratio.
I think you need more cam if your compression ratio goes over 9.5-9.8 area.


_________________
67 mustang,C-4, with mod. 80 log head, custom 500 cfm carb with annular boosters, hooker headers, dual exhaust with X pipe, flowmaster mufflers, duraspark II distributor with MSD-6AL. Engine 205" .030" over with offset ground crankshaft & 1.65 roller rockers.

 

[FalconSedanDelivery]

with the right cam , ( and Dist Curve ) 10.5-1 wont be too much on 93 , I build SBF , and SBC motors all the time that are 10.5-10.75 ( with aluminum heads ) , and a healthy cam , its all in the Total Package , not just a compression ratio deal

 

[62Ranchero200]

So the best answer for you is
1. to deck the block 89 thou, use the cheap, common Aussie X-flow ACL 4.1 liter 22.9 or27.9 ccc pistons which are almost the same as the Aussie 250 log head pistons except have a bigger dish, and use the bullet proof 5.88" Aussie Forged rods and leave the chambers at 50 cc and use the 13 US dollar app630 permaset monotorque gasket set. Any fuel down to 87 octane will work

Second best answer is
2. to deck the block 89 thou, use the cheap, common Aussie X-flow ACL 4.1 liter 15.5 dish pistons which are almost the same as the Aussie 250 log head pistons, and use the bullet proof 5.88" Aussie Forged rods and open up the chambers 9 cc to 59 cc and use the 13 US dollar app630 permaset monotorque gasket set. 91 to 93 octane will be best, but any alloy head should be able to handle 87 octane

The third asnwer is to use

3. over bore 70 thou for big 258 Jeep/AMC pistons like Powerbands, ream out the rods, and add a 41 thou APP630 gasket, then you can use your stock 50 cc head.

Any one of those 3 will get great results.

Hope this helps :mrgreen:

Hi Xctasy,

Thank you so much for all the information. Do you know if the Aussie 27.9, 22.9, and 15.5 dish pistons are available in the standard US 250 bore sizes (3.68" and the standard overbore sizes), do they accept the same size piston pin as the US 250 pistons (0.9122"), and what is the compression height? Is there a web site where I could see these Aussie pistons so that I could see all the specs plus a visual of the piston top? I guess all the specs on the Aussie web sites are in centimeters?

I'm going to have think about all of this before I make a final decision, and also waiting for Mike to answer a couple of questions, for example: can he enlarge the combustion chambers on the CI aluminum head, and if so by how much.

Thank you,
Bob

 

[CNC-Dude]

The aftermarket is your friend, there is no need to try and make parts work that were not made for this engine to start with. Achieving 9:1 or so in compression the right way is much better than getting 9:1 the wrong way, ie. with excessive deck height creating poor quench, or having to mill the blocks deck surface in excess to try and get it, etc... all those factors add to the cost and will likely exceed the costs of buying a custom and correct piston to begin with. Add that to the import duty and taxes you will have to pay to get the other pistons from OZ, and still not have the desired quench or compression without further expense of block machining and/or head mods....Getting the correct piston to begin with will put you on the right track from the beginning.
Also, you need to be focusing on your DCR, not your static compression, it is irrelevant. The DCR is what your engine is going to be seeing, and is the most important portion of the whole compression scenario. It has been found that a DCR of between 8:1-8.2:1 is the max limit for using pump gas. You can have a static compression of 10:1, and still have a DCR of 8.2:1 depending of which cam you select and how it is degreed in your engine, so again, the static compression number is not an accurate number to be trying to use as a standard to obtain. Even with a theoretical 9:1 static compression ratio, you can exceed the DCR requirements the engine needs to run pump gas by installing the wrong cam as well. Hope that helps.

 

[xctasy]

So the best answer for you is
1. to deck the block 89 thou, use the cheap, common Aussie X-flow ACL 4.1 liter 22.9 or27.9 ccc pistons which are almost the same as the Aussie 250 log head pistons except have a bigger dish, and use the bullet proof 5.88" Aussie Forged rods and leave the chambers at 50 cc and use the 13 US dollar app630 permaset monotorque gasket set. Any fuel down to 87 octane will work

Second best answer is
2. to deck the block 89 thou, use the cheap, common Aussie X-flow ACL 4.1 liter 15.5 dish pistons which are almost the same as the Aussie 250 log head pistons, and use the bullet proof 5.88" Aussie Forged rods and open up the chambers 9 cc to 59 cc and use the 13 US dollar app630 permaset monotorque gasket set. 91 to 93 octane will be best, but any alloy head should be able to handle 87 octane

The third asnwer is to use

3. over bore 70 thou for big 258 Jeep/AMC pistons like Powerbands, ream out the rods, and add a 41 thou APP630 gasket, then you can use your stock 50 cc head.

Any one of those 3 will get great results.

Hope this helps :mrgreen:

Hi Xctasy,

Thank you so much for all the information. Do you know if the Aussie 27.9, 22.9, and 15.5 dish pistons are available in the standard US 250 bore sizes (3.68" and the standard overbore sizes), do they accept the same size piston pin as the US 250 pistons (0.9122"), and what is the compression height? Is there a web site where I could see these Aussie pistons so that I could see all the specs plus a visual of the piston top? I guess all the specs on the Aussie web sites are in centimeters?

I'm going to have think about all of this before I make a final decision, and also waiting for Mike to answer a couple of questions, for example: can he enlarge the combustion chambers on the CI aluminum head, and if so by how much.

Thank you,
Bob

For example, the Australian Ford 250 log and 2V engines had a 12.5 cc piston, a 59 to 62 cc head, a 41 thou gasket, and its was just fine for fuel down to 93 octane. Its block was 9.38" tall, not 9.469" like the US version you guys use. If you added 15.5 cc 1976-1987 ACL 3.3 x-flow pistons, you could run 91 octane. They also have 22.9 and 27.9 cc chamber pistons to lower the compression even more, straight out of the ACL catalogue, and you can use 87 octane fuel. All Ford Aussie pistons have a 16 thou below the block deck register, or 1.529 to 1.531". The Aussie pistons don't crack, and can be used forperformance applications, but they are still cast alloy. They were used in AIT, Noramalair Garret, Mike Vine and Dick Johnston turbo applications from 255 to 360 hp from 1982 to 1987, and they didn't break. As long as your not using low tension rings and the spec for the 4500 rev limited 1986-1993 Aussie X flows, the aftermarket cast pistons will do fine service.

Like the Aussie JP twin row timing chain for the 200, the 200/250 ACL piston stuff fits the stock US engines that use the same parts. Wrist/piston/Gudgeon pins are the same.

Some of the Aussie pistons are found in the US Silvolite website under Australian engines as well as the ACL website. Duralite Pistons and Duratech Pistons have been made in Aussie or Tiwan or Germany's Mahale division for years and are excellent race spec cast alloy pistons.

Page numbered 46,47, 48,49 have the info. but they are 80,81,82,83 of 275 in the pdf file.
26.8 cc is listed as #6KRY4100,
21.9 is also listed as 21.8cc, and is #6KRY4101,
15.3 cc is #6MKRY4102,
15.3 CC 6MKRY4102 vs Flatop 6MKRY9113
XY, XA, XB 250 9.2:1 10.4:1
XC, XD, XE 4.1 9.0:1 10.1:1
XF ULP Non E.F.I. 10.0:1 11.0:1
XF ULP E.F.I. 9.6:1 10.6:1

Of course, even there they mucked up in the orginal copy, as the #6MKRY9113is no longer made, and they claimed the XF Falcon unleaded cross flow had C.F.I injection. It never did..XF is the model, in this instance, it is X-flow, but so are the XC, XD and XE Falcons and TE and TF Cortinas, later F100's after 1977-1986.


The deal is that ACL's pistons dont match the early log and 2v engines orginal pistons, as they were 5.5 for the 188/200/221, and 12.5 (I think) for the 250. There were also factory low compression pistons for the 200, which were just stock 250 log/2v pistons. So the cat is a bit dicey, and within one day, and sane American citizen would have the cat complier sacked and exiled to a cheese factory in NZ. It's full of mistakes, but they are intentional to ensure ACL aren't claiming an exact interchange. Years are worng for the 250 engine, the US 240 AND 300 engines are included, but there is no differentialtion between the Australian 250 engined F100 which was sold for a decade or more down here.

And please don't have a fit, its listed as a mitsubishi catalogue....go ask!

http://mitsubishilinks.com/fsm/acl_pist ... s_pp99.pdf

In some other catalogues, there are other errors, like the competition based Larry Perkins recomended ACL Race Pistons on http://www.aclperformance.com.au/tech_p_cast.htm

On that, the info on the wrist pins for the 250 and 200 X-flow, are wrong in some cases. They are all 0.9112, but the copy editor (probably an expat Kiwi) made a mistake with the ACL Race pistons. They are certainly stock US wrist pin diameter. ACL Race are only flat top and dish, 0 and 8.0 or 8.6 cc, not a first choice. Although there is a #6MKRY9412 1.169" piston which the OHC boys use to over bore there 4.0 engines out to 250 cubic inches...The Ford 188 is called a 3.1 here, but has the stock US 200 3.3 bore and piston, with the 170 engines stroke . There is also a 221, called a 3.6, which has a 2500HSC stroke crank. The 188, 200, 200 x-FLOW, 250 X-FLOW and 4.1 X-flow all use the same ACL Race pistons #6MKRY9410, #6MKRY9411, #6MKRY9412

the Aussie conrod for the 200 is 6.27" long, not the US 4.715" low deck, and the Aussie 200/3.3 from 1971 to 1987 is a 9.38" deck engine, not 7.803"

Aussie engine details are on pages numbered 10 and 11 on this catalogue http://www.acl.com.au/web/aclwebsite.ns ... 20AR98.pdf

 

[xctasy]

Oh yeah, Option 4, from a while back. I suddenly remembered this one, which is almost the same as the Rover 4600 piston balldrick uses in his supercharged XKNO1 1961 Falcon. Since the Buick V6 and Rover engines from the same GM family. Its the N code 89-93 GM 3300 piston, the little brother to the GM 3800 found in all the front drive Buicks and Oldsmobiles

See viewtopic.php?f=3&t=67792, Crossflow tips!!

Yeah I am guessing its a mistake on their part too, but new plan that wouldnt involve expensive sleeving. These pistons http://www.summitracing.com/parts/UEM-1743H-5MM/ have a compression height of 1.310 in and a bore of 3.720 only requiring a .040 overbore which is very conservative. If you used these pistons with 300 rods which have the same big end dia as the small six engines the piston would only poke out -.0057 inches before machining. Which is pretty darn close to zero decked. the piston pin hole would have to be opened up by .007 inches so very minor, and with a 62cc head 9.5:1 compression ratios are a possibility because these pistons have a 12.67 cc dish!

This piston allows the stock shallow deck GM 3800 piston with 12.67 cc fit a 6.21" Ford 300 rod if its wrist pin is bronze bushed to suit