Compression ratio calculation

[62Ranchero200]

Greetings Ford Six Fans,

Below is my baseline compression math for my 250 with large log head (with the original stock setup, except composite head gasket instead of steel shim head gasket). This may not be my final configuration, but I wanted to find a baseline.

Assumptions:

3.68" (virgin) bore
3.91" stroke
6.5 CC piston dish
0.123" deck height
~.050" compressed head gasket thickness
62 CC combustion chamber



Formulas used were:

Vmax = Pi * Radius squared * (Stroke + Deck Height + Compressed Gasket Thickness) + Dish volume + Combustion chamber volume

Vmin = Dish volume + (Pi * Radius squared * [Deck Height + Compressed Gasket Thickness]) + Combustion Chamber Volume

Please give me a reality check - any corrections are welcome.

Thank you,
Bob the Builder

 

[Gene Fiore]

Very close to the numbers I get with the program I use. I got 7.87

 

[FalconSedanDelivery]

Now you can see why I milled the Deck .100

 

[First Fox]

Hi Bob. Now that you've done the math, the only reality check I see in this is that your compression ratio absolutely sucks. Flat top pistons are available for you and will raise the C/R as will an overbore. If some reasonably priced flat tops don't do it, you can shave the head and/or deck the block for an additional increase.

Something in the neighborhood of 9:1 would be much more pleasant to drive and make noticeably more power. You might be able to go a bit higher depending on what you are using for a cam.

 

[62Ranchero200]

Now you can see why I milled the Deck .100

Question for you and Dave RE: CR and detonation on the 250: would the CI aluminum cylinder head significantly reduce the tendency for detonation at any given CR? Because

1. The CI aluminum head includes much better combustion chamber design with more "squish"
2. The associated intake manifold would offer much better fuel distribution, eliminating the leanness that might happen at #1 and #6 with a log manifold, even with a 2V conversion on the log manifold.

If so, I might be able to run a bit more CR with a CI aluminum head?

Thanks
Bob

 

[62Ranchero200]

Hi Bob. Now that you've done the math, the only reality check I see in this is that your compression ratio absolutely sucks. Flat top pistons are available for you and will raise the C/R as will an overbore. If some reasonably priced flat tops don't do it, you can shave the head and/or deck the block for an additional increase.

Something in the neighborhood of 9:1 would be much more pleasant to drive and make noticeably more power. You might be able to go a bit higher depending on what you are using for a cam.

Hi FF,

You are absolutely right, but that first calc was just to establish a baseline. I'm sure I want more CR than that (exactly as you said, 9:1 or close to run on my locally available 93 Octane without octane booster), but I'm getting to the point where every decision on the build effects every other decision. :roll:

I THINK I'm going to either have a 2V conversion done on my large log head, or get a CI aluminum cylinder head! To some degree, this is going to factor into my decision of which pistons / how much decking I can run, at the very least because the combustion chambers are about 50 CC on the CI head vs. 62 CC on the large log head. Also, the fuel distribution will be much better with the CI intake manifold so #1 and #6 will not have a tendency to run lean.

I'm using the Clay Smith 264/264/110 cam so I think that will effectively lower the CR a bit.

Thanks
Bob

 

[Jakes66snake]

Normally you run atleast .5 higher on compression when going to and alum head. I don't think its for added power, more so you don't lose it... something to do with cylinder temp.
Squish band is better controled with piston height and is most important. On any given design piston close to zero deck height and either dish or dome to match head design.

 

[First Fox]

Oh man, if you can swing the aluminum head, then by all means go for it! The better cooling characteristics/heat dissipation of the aluminum head allows for higher compression ratios and the tighter chamber design will also reduce detonation tendencies. Win/win.

My 2 percent of a dollar is this: If you can afford the aluminum head, then buy it and build around IT. It is a far superior design and its only downfall is the cost, but when you figure in having an old worn out head rebuilt, new guides, new valves, seats, deck milling, log modification etc. the aluminum head is a much better value.

With that cam you could probably even pull off 9.5 or 10 to 1 with that head. It would be a matter of having a good curve in the distributor proper jetting, and running 92 octane.

This build may become interesting to watch.

 

[62Ranchero200]

Hi Bob. Now that you've done the math, the only reality check I see in this is that your compression ratio absolutely sucks. Flat top pistons are available for you and will raise the C/R as will an overbore. If some reasonably priced flat tops don't do it, you can shave the head and/or deck the block for an additional increase.

Something in the neighborhood of 9:1 would be much more pleasant to drive and make noticeably more power. You might be able to go a bit higher depending on what you are using for a cam.

Hi FF,

Now that I have verified my CR calculator, I ran the follow comparison for my 250.

All scenarios are with composite gasket, ~.050 compressed thickness

62 CC chambers = '80 large log head (E0BE) with 62 CC combustion chambers
50 CC chambers = Classic Inlines aluminum head with 50 CC combustion chambers

6.5 CC pistons = pistons with stock style, non-California 6.5 CC dish
Flattop pistons = pistons with stock compression height (not 255 pistons), but no dish

No block decking or head milling taken into account just yet

In order of increasing CR:

6.5 CC pistons, 62 CC chambers - 7.9 CR
Flattop pistons, 62 CC chambers - 8.4 CR
6.5 CC pistons, 50 CC chambers - 8.9 CR
Flattop pistons, 50 CC chambers - 9.5 CR

IMO, a 9.5 CR is slightly too high for a street driven car with 93 octane gas - I would have to constantly be aware of the potential for detonation, how much advance I was running, how hot the engine was, etc. The 8.9 CR with 6.5 CC pistons and the CI aluminum head with 50 CC chambers is looking good as a starting point - then I can decide if / how much I want to deck the block to raise the CR a bit more.

Thanks
Bob

 

[FalconSedanDelivery]

I use a simple rule of thumb , you can get away with more , IF the combo is dialed in good , BUT just move the decimal over one point and use that octane , example 9.0-1 = 90 octane , as posted before aluminum allows a bit higher , BUT not always , chamber design along with deck and squish , can allow as high as 11-1 on 92-93 , BUT it usually take Electronic management / fuel injection to be safe , If I were to use a Aluminum head ( future plans for my 250 ) I would run about 9.5-9.75 using 91-93 , and make sure the Dist is Curved properly

 

[62Ranchero200]

I use a simple rule of thumb , you can get away with more , IF the combo is dialed in good , BUT just move the decimal over one point and use that octane , example 9.0-1 = 90 octane , as posted before aluminum allows a bit higher , BUT not always , chamber design along with deck and squish , can allow as high as 11-1 on 92-93 , BUT it usually take Electronic management / fuel injection to be safe , If I were to use a Aluminum head ( future plans for my 250 ) I would run about 9.5-9.75 using 91-93 , and make sure the Dist is Curved properly

Thanks FSD,

9.3 CR with my locally available 93 octane sounds like a good rule of thumb to me. I could start with the 6.5 CC dished pistons and the CI head (~8.9 CR) and deck the block to raise the CR slightly. I don't want to push it to the absolute limit for the 93 octane though because I want to "set it and forget it". I don't want to have to worry about tweaking my timing as a permanently ongoing process, or worry that if the car gets a little hot in the 100 degree Houston summers it might experience detonation.

I'm sure the potential of electronically managed FI is impressive, but I'm not ready to tackle that yet. I'm following a modular V8 build EFI on the ranchero.us forum (my other internet home, since my six is in a '62 Ranchero) and it's mind boggling in its complexity.

Thanks
Bob

 

[FalconSedanDelivery]

Always try to get compression , by Reducing squish , in other words it would be better to deck the block than to use a flat top piston , IF the deck was still excessive , shoot for as close to Zero as you can , because the gasket is very thick .045-.050+

 

[CZLN6]

Howdy Bob and All:

Great stuff and I apoligize if I end up plowing the same ground. Yes, Aluminum dissipates heat faster/better and therefore allows more CR. The Design of the separate manifold on Mike alloy head promotes better fuel distribution. Your Clay Smith cam will reduce dynamic CR not static CR. The difference is the amount of cylinder pressure determined by when the valves open. Less cylinder pressure= less heat, allowing more static CR. For example, when you install a performance cam you will lose low end power but gain high end. Increasing the CR gain back some of the low end power at little or now expense. With your performance cam you can safely add two or three tenth to the right of the decimal. For example, on an otherwise stock engine with a 9:1 CR can safely increase to 9.2:1 or 9.3:1 with no concern.

FSD hit the real nail on the head on how to gain CR. By reducing the quench you will not only control knock, but you will increase combustion efficiency. I repeat, IF you were to Deck the block .070" and use the .085" taller V8 pistons you would be very close to an optimum quench height assuming a .045" compressed head gasket. BUT- you will need to reduce CR no first, by not milling the head any more then necessary to true it and by milling "D" shaped dishes into the piston face mimicking the shape of the combustion chambers in the head. I'll take your starting numbers and run some CRs for you.

Bottom line, with your cam a goal CR of 9.3:1, give a take a point will be safe, even with a tank of 90 octane gas now and then. That's assuming you get your distributor recurved, and that you will be sane with initial advance, especially in the hot summer month.

Be back latter.

Adios, David

 

[62Ranchero200]

Howdy Bob and All:

Great stuff and I apoligize if I end up plowing the same ground. Yes, Aluminum dissipates heat faster/better and therefore allows more CR. The Design of the separate manifold on Mike alloy head promotes better fuel distribution. Your Clay Smith cam will reduce dynamic CR not static CR. The difference is the amount of cylinder pressure determined by when the valves open. Less cylinder pressure= less heat, allowing more static CR. For example, when you install a performance cam you will lose low end power but gain high end. Increasing the CR gain back some of the low end power at little or now expense. With your performance cam you can safely add two or three tenth to the right of the decimal. For example, on an otherwise stock engine with a 9:1 CR can safely increase to 9.2:1 or 9.3:1 with no concern.

FSD hit the real nail on the head on how to gain CR. By reducing the quench you will not only control knock, but you will increase combustion efficiency. I repeat, IF you were to Deck the block .070" and use the .085" taller V8 pistons you would be very close to an optimum quench height assuming a .045" compressed head gasket. BUT- you will need to reduce CR no first, by not milling the head any more then necessary to true it and by milling "D" shaped dishes into the piston face mimicking the shape of the combustion chambers in the head. I'll take your starting numbers and run some CRs for you.

Bottom line, with your cam a goal CR of 9.3:1, give a take a point will be safe, even with a tank of 90 octane gas now and then. That's assuming you get your distributor recurved, and that you will be sane with initial advance, especially in the hot summer month.

Be back latter.

Adios, David

David,

I really appreciate your input. Do you know of a reference article (on this forum or elsewhere) that can refresh my memory on squish, quench, deck height, combustion chamber shape, etc as it relates to CR, piston selection and decking/milling?

Thanks,
Bob

 

[First Fox]

Looks like you have advice to chew on for a while Bob, but please remember this when decking the block: The reason to do that is to raise, and therefore correct the deck height for improved quench, not for raised compression. The deck height needs to be correct for a given piston and you should raise the compression ratio by the other means mentioned. This is very important for fighting off detonation as is a a good distributor curve.

As far as the compression ratio and detonation, there really is no "set it and forget" if you are looking for max power. If you run a stock engine and want maximum performance from it, you will always be on the edge of detonation, and it may ping if you get bad gas, or the ambient temps is high like you suggested. If you expect those circumstances, you would want to be conservative on the ignition timing. That process does not change with increased compression ratios. In order to achieve maximum power, you need to be on the edge of detonation. Always. When reliability and low maintenance are more important, you again keep the timing conservative. You will find that when you fix the quench problem and raise the compression ratio, you will almost always need less ignition timing and that is a good thing. When you raise the compression on a stock engine and keep the original smog era timing curve, it will likely suffer from detonation as it does not need or like all the timing. I do believe this is where plenty of people have the fear of high compression ratios/detonation issues.

To reiterate, higher compression ratios do not necessarily equate to detonation problems, you just need to have the the proper parts and have it tuned correctly. By adding the aluminum head, fixing the quench problem by zero decking (or even having the piston slightly higher than the deck) proper ignition timing and jetting, you will be able to run safely at 9.5 to 1 But the "set it and forget it" part will depend on your tuning skills.

 

[Jakes66snake]

Don't forget about cam choice.... static vrs dynamic compression. Does anyone know if there is an aftermarket ign controler that uses a knock sensor that works with DSII?

 

[CZLN6]

Howdy Back Bob and All:

"Do you know of a reference article (on this forum or elsewhere) that can refresh my memory on squish, quench, deck height, combustion chamber shape, etc as it relates to CR, piston selection and decking/milling?"

Good questions. I can't think of a source right off, but all have been discussed here at some time or other. But, to review, "Quench" is the effect of a fresh intake charge of fuel and air being squeeze out of the tight, non combustion chamber or piston dish, areas between the top (non-dish area) of the piston and the cylinder head (not the combustion chambers). the process of being squeezed out of the quench areas at high speed create turbulence in the combustion chamber of the head and piston dish. Turbulence is good for an efficient burn on combustion. Quench happens best in a head with a "wedge" shaped combustion chamber, and it happens best when the "deck clearance" is in the .030" to .040" range. Any more and the quench effect is diminished. Any less and the risk of parts occupying the same space at the same time is risky.

Deck clearance is the total of "deck height", the distance from the top of the piston to the top of the block. The other dimension included in deck clearance is the compressed thickness of the head gasket, which varies from .025" on a stock, OEM steel shim gasket to .055" for an aftermarket FelPro composite gasket.

The combustion chamber shape on our engines is typically referred to as a "wedge shape"- somewhat like a bath tub but the spark plug side is deeper. The chamber shape on our engine head is irregular and is as cast and not machined. The chambers can benefit from reshaping to lessen intake valve shrouding, polishing to remove hot spots and to reflect heat back into the chamber. It is beneficial to maintain as much deck around the chambers as possible when reshaping so as to maximize the amount of "quench-to-bore" ratio. That is the amount of quench area remaining where the flat, non-chamber part of the head and the top, not the dish, part of the piston come together. You will note the the round dish in the pistons does not match the wedge chamber of our head- less than ideal, but probably more cost efficient for the Dearborn bean counters. Many modern, high efficiency engines use a chamber that includes the chamber in the head as well as the piston dish

All of the above go into the combination of compression ratio as well as combustion efficiency. Reducing deck height and tightening deck clearance improves efficiency. Decking the block is one way to reduce deck clearance. Using a piston with a taller "pin height" is another. "Pin height" is the distance from the center of the wrist pin to the top of the piston. The other factor is the thickness of the head gasket. Thinner is better. Decking, or milling the top of the block is one way to reduce piston deck height. Finding a piston with a taller pin height is another. Both will increase the compression ratio and increase efficiency.

I'm going to quit for now. This is alot to comprehend. Don't hesitate to ask questions for clarification. I want you to know that I did run some numbers for your engine. The bad news is your basic spec come out to 7.9:1 CR. That's the place to start.

Let me know when you're ready for more.

Adios, David

 

[62Ranchero200]

Howdy Back Bob and All:

"Do you know of a reference article (on this forum or elsewhere) that can refresh my memory on squish, quench, deck height, combustion chamber shape, etc as it relates to CR, piston selection and decking/milling?"

Good questions. I can't think of a source right off, but all have been discussed here at some time or other. But, to review, "Quench" is the effect of a fresh intake charge of fuel and air being squeeze out of the tight, non combustion chamber or piston dish, areas between the top (non-dish area) of the piston and the cylinder head (not the combustion chambers). the process of being squeezed out of the quench areas at high speed create turbulence in the combustion chamber of the head and piston dish. Turbulence is good for an efficient burn on combustion. Quench happens best in a head with a "wedge" shaped combustion chamber, and it happens best when the "deck clearance" is in the .030" to .040" range. Any more and the quench effect is diminished. Any less and the risk of parts occupying the same space at the same time is risky.

Deck clearance is the total of "deck height", the distance from the top of the piston to the top of the block. The other dimension included in deck clearance is the compressed thickness of the head gasket, which varies from .025" on a stock, OEM steel shim gasket to .055" for an aftermarket FelPro composite gasket.

The combustion chamber shape on our engines is typically referred to as a "wedge shape"- somewhat like a bath tub but the spark plug side is deeper. The chamber shape on our engine head is irregular and is as cast and not machined. The chambers can benefit from reshaping to lessen intake valve shrouding, polishing to remove hot spots and to reflect heat back into the chamber. It is beneficial to maintain as much deck around the chambers as possible when reshaping so as to maximize the amount of "quench-to-bore" ratio. That is the amount of quench area remaining where the flat, non-chamber part of the head and the top, not the dish, part of the piston come together. You will note the the round dish in the pistons does not match the wedge chamber of our head- less than ideal, but probably more cost efficient for the Dearborn bean counters. Many modern, high efficiency engines use a chamber that includes the chamber in the head as well as the piston dish

All of the above go into the combination of compression ratio as well as combustion efficiency. Reducing deck height and tightening deck clearance improves efficiency. Decking the block is one way to reduce deck clearance. Using a piston with a taller "pin height" is another. "Pin height" is the distance from the center of the wrist pin to the top of the piston. The other factor is the thickness of the head gasket. Thinner is better. Decking, or milling the top of the block is one way to reduce piston deck height. Finding a piston with a taller pin height is another. Both will increase the compression ratio and increase efficiency.

I'm going to quit for now. This is alot to comprehend. Don't hesitate to ask questions for clarification. I want you to know that I did run some numbers for your engine. The bad news is your basic spec come out to 7.9:1 CR. That's the place to start.

Let me know when you're ready for more.

Adios, David

Hi David,

I really appreciate you taking the time to answer my questions and run CR calculations for me. I think I understand the basics of the quench, deck height, and CR discussion. However, lack of experience with Ford sixes (my first experience with them was when I bought the Ranchero in October of last year) and this aspect of engine building may cause me to miss some subtleties that others such as yourself may catch.

I hesitate to go the "255 piston, milling combustion-chamber shaped dishes into the pistons" approach because it seems like a lot more variables are introduced. It would be hard to predict the exact volume of the milled dishes without an exact definition of the dish shape and depth. If the exact volume of the milled dishes can't be predicted, it's going to be difficult to calculate what the CR would be with that approach. I'm also not sure how much you can mill into the 255 flattop pistons without weakening the top of the pistons too much - after all the pistons were not designed for that, right? Mike or some others may have done this before and therefore may know exactly what to do, but my local machine shop may not understand exactly what needs to be done.

Do you think I'm being paranoid?

Would feel more confident going with an approach that doesn't involve so many unknowns, like maybe 6.5 CC dished pistons and decking the block to get CR back up to at least 9:1, if that's feasible.

One other possibility ... my machinist says that he knows the guy that used to own J.E. pistons. The guy has started his own business, is local to me, and apparently can make custom pistons at reasonable prices. What if he could make pistons just like the 6.5 CC dished pistons, but with the higher compression height like the 255 pistons? Then no custom milling would be necessary, and with the dish volume known, the CR could be calculated.

Thanks
Bob

 

[CZLN6]

Howdy Back:

"Do you think I'm being paranoid?"

NO, just cautious. Let me run some numbers with that combo of parts and less decking of the block. I'll be back.

What material is the J.E. guy making pistons with?

Adios, David

 

[CZLN6]

Howdy:

Ok, assuming an overbore of .030" with a dished 6.5 cc piston, a Victor head gasket @ .045" compressed thickness, cleaning up and smoothing the chambers and milling the head only enough to true the mating surface, and decking the top of the block .070" will yield a static CR of 9.1:1.

That will leave .053" deck height. That plus the .045" thickness of the head gasket will give you a deck clearance of .098". While this is way too much for ideal quench effect, it is way better than the .148" you started with.

Your probably asking why only .070" off the top of the block? Because that amount give you a CR very close to your goal. And, because I couldn't recommend removing any more material due to structural integrity and maintaining the depth of the head bolts. At this dimension you will still need to use hardened washers on each head bolt and you must verify that the head bolts are not bottoming out in its hole.

I'd sure like to hear more about the piston maker and possibilities.

Adios, David