High compression on pump gas

[Anonymous]

I've noticed lately that whenever these 8 cylinder car magazines (Hot Rod or Craf Craft) refer to high compression (at least 10:1) they always refer to using pump gas - usually 91 or 92 octane gasoline.

How is it they can achieve a 10:1 compression ratio and still use 91 octane gas? What am I missing? Why does this apply to our sixes?


-Chris

 

[Anonymous]

I have read in a performance book that u can go as high as 17:1 on pump gas with a good water injection system. as long as u dont get detonation or can surpress the detonation, you can use quite high compression ratios.

 

[CobraSix]

I think geezer is running 10.5:1 on pump gas.

Remember, detonation is a combination of factors, CR being only one of them. TIming is a nother big factor. That's why a lot of newer cars running higher compressions, because the ECU can retard the timing as RPMS go higher, thus helping to avoid detonation.

Slade

 

[sefus]

im running 10.5 on 92ish octane in my mopar. i read a great article in popular hotrodding about how to supress it had to do with heat treating your pistons, polishing around the valves, venting hot air out not raming cold air in and a bunch of other stuff. i always hate how companies and whoever always say 10:1 is too much for the street and over 500 lift is too much for the street. I'll show them street cars with over 700 lift and 14:1 compression. its all in what you can handle(put up with).

 

[Lazy JW]

Modern engines using EFI have a distinct advantage in using oxygen sensor feedback to monitor air/fuel ratios, plus they use that little knock sensor gizmo that can detect detonation at the very onset before it is even close to being audible to the human ear. This allows them to run higher static compression ratios on pump gas by retarding the timing during operating conditions that are likely to cause knocking. Knowledgeable hotrodders can run higher ratios by using a whole bag full of tricks, like having plenty of overlap on their cams, special heat resistant coating of engine parts, etc. but you had best know what works or it will be expensive Air-cooled Volkswagens have a similar problem as they are VERY sensitive to overheating and detonation will destroy them quicker than sooner. VW recommended 93 octane leaded fuel on 7.5:1 compression stock, the folks that seem to have the best success run them at 6.6:1 now. If you just arbitrarily cram a bunch of compression in your Ford six without doing anything else, you will probably have trouble. Unless you live above 10,000 feet elevation
Joe

 

[Anonymous]

Knowledgeable hotrodders can run higher ratios by using a whole bag full of tricks, like having plenty of overlap on their cams,...

This one of the items I see more an more in the magazines. IIRC, this is one of the major factors in allowing higher compression ratios in the hot rod build ups. I could see how this would allow more exhaust (heat) to be released into the atmosphere rather than being transfered to the block. So, how much of an over lap are we talking about for a 200-6?

As far as timing, how would you retard the engine running in the higher rpm range that is not computer controlled? Limit the amount of vacuum advance being applied?

I think I read somewhere that most of these big block build ups were tuning a total of 35 or so degrees of advance. So, if that's the case, maybe a majority of the solution is the camshaft. After all, they are not computer controlled with sophisticated EFI systems. Hmmm....


-Chris

 

[CobraSix]

35* is not a lot of advance. You can run higher CR, but you'll not be able to do what a lot of people do here and advance initial timing to 18* or so.

Remember, vacuum advance only supposed to help under light loads. You could, possible use a dual vac advance unit, and only hook up the vacuum retard side of it. Maybe that will help. You can also use something like the multi function ignition controller by MSD. It allows you to plot your own ignition curve based on RPM.

Also, another thing to remember, the more overlap your cam has, the harder time it has idling at a decent speed.

Slade

 

[wsa111]

LET ME ADD MY 2 CENTS. THE COMPRESSION RATIO IS A CRITICAL BALANCE WITH THE INTAKE VALVE CLOSING. YOU CAN CRANK IN MORE CR IF YOU HAVE A CAMSHAFT WITH A LATER INTAKE VALVE CLOSING. IF YOU STAY BELOW 200 LBS CRANKING COMPRESSION YOU SHOULD BE OK. THIS IS A FINE LINE.
THE SHORTER THE CAM THE LESS CR YOU CAN GET AWAY WITH.
THE IDEAL IGNITION ADVANCE IS 12-14 DEGREES BEFORE TDC WITH 12 DEGREES IN THE DISTRIBUTOR X 2 = 24 + 12 OR 14 FOR A TOTAL OF 36 -38 TOTAL DEGREES ADVANCE. THE VACUUM ADVANCE WILL HAVE TO BE TAILORED FOR YOUR OWN SPECS. THIS CAN ONLY BE DONE WITH A 68 OR LATER DISTRIBUTOR WHICH HAS MECHANICAL ADVANCE PLUS VACUUM ADVANCE. HOPE THIS HELPS. IF YOU ARE NOT SURE YOU NEED TO GET A DISTRIBUTOR CUSTOM TAILORED BY SOMEONE LISTED IN HEMMINGS-DISTRIBUTOR DYNAMICS OR HAVE SOMEONE WITH A SUN DISTRIBUTOR MACHINE TO CUSTOM TAILOR THE PROPER ADVANCE CURVE. V/R WILLIAM THIS IS JUST THE TIP OF THE ICEBURG

 

[MustangSix]

Cam selection is critical to keeping the cylinder pressure manageable. Static CR is only one piece of the equation.

Also, when you read those magazine articles you will also probably notcie that they usually use aluminum heads on high CR engines. An alloy head can tolerate as much as a point more CR than iron, all else being equal. But for example, I rarely see iron headed 302's tolerate more than 9.5:1 on pump gas unless they have EFI, knock sensors, and computer controlled ignitions.

 

[Anonymous]

You can run higher CR, but you'll not be able to do what a lot of people do here and advance initial timing to 18* or so...

Right now I cannot achieve an initial advance of 12* not to mention 18! I'm sure this is mainly in part because of my current rebuild. Terrible CR. I think a few members on this forum helped on figuring my compression ratio - 7.8:1 give or take a few. However, now that the car is running, with about 10* intial advance, I thought it might be best to go ahead and get the head milled. My thinking is that I could only benefit from a higher ratio, say 8.5 or 9:1.

By-the-way, anyone know what the factory specifications are from a late '70's 200 camshaft?

Oh, one more thing. One could set the initial advance with the distributor in place, correct? I would think that it doesn't matter because you can always adjust the distributor after the fact. Just asking - thought I read somewhere that removing the distributor is best. Don't remember why but it was pointed out in the article.


-Chris

 

[Lazy JW]

Actually, the distributor NEEDS to be in place to set the initial timing. Think of it as the starting point for the advance curve, the initial timing is where the engine idles, and as the speed increases the advance mechanism moves the timing further ahead to compensate for the varying needs of the engine. This can be done with vacuum, or centrifugal weights, or a combination of both. I am a bit puzzled as to why you are unable to set your initial timing any further ahead than 8 degrees?
Joe

 

[Anonymous]

Well, actually, Joe, I can set the initial to 10 degrees but anything above that and the engine doesn't run very smooth. I thought it might be my carburetor but that's not it (rebuilt twice). I was going to replace it during the rebuild but opted against it...no money at the time.


-Chris

 

[Anonymous]

Cranking pressure is not a reliable indicator of required octane.
A simple comparison:
Motor #1 has small stock cam, 8-1 CR = 160 psi gauge
Motor #2 has very late intake closing, 11-1 CR = 160 psi.
As the rpm goes up, the combustion pressure for #1 rises and peaks sooner but much lower than #2. #1 can use 91 octane, #2 shouldn't.
Overlap does not affect octane requirement, except that high overlap will allow (and require) more initial spark due to exhaust gas pollution at and near idle. Doesn't change cranking pressure or DCR.

Here are some factors that cause ping and knock, and require either high octane gas, less spark advance, adjustment or replacement of some components, etc. All affect octane tolerance:
» “Looseâ€￾ piston to head clearance @ TDC (.060â€￾ or more)
» “Tallâ€￾ 1st gear ratio (Powerglide, Powerflite, Super Turbine 300, Fordomatic)
» High (low number) axle ratio
» High vehicle weight
» 195° thermostat
» High compression with mild cam
» Large bore size
» Spark plug position near exhaust valve
» Combustion chamber shape
» Lean mixture
» Hot spark plug
» Tight converter with low stall speed
» High initial ignition advance
» Fast advance curve
» Weak spring in vacuum advance canister
» Any cooling problems (rust or corrosion in the water jacket, radiator clogged, fan too small, rad not sealed to opening, no shroud, etc.)
» Low octane
» Amount of water in the head
» Effective coolant flow
» Thermostat response time
» Water pump speed
» Exhaust valve seat width

 

[Anonymous]

Good post.

>>Coated piston heads
>>Coated cylinder walls
>>etc.

I understand the idea is to lower the chances of heat dispensation into the cylinder head as well as the block. Or at least the metal surfaces that see the most heat buildup. This way you can still maintain a higher DCR and still avoid predetenation, right?


-Chris

 

[rickwrench]

Detonation (ping, pre-ignition, knock, etc.) is, of course, caused by the fuel mixture igniting from a source other than the ol' sparkplug zap.
It's all about hot spots and how NOT to have them.
If you want to run 10, 11, 12 /1 ratios in any engine, there had better not be ANY hot spots in the combustion chamber.
Polishing the piston face and chamber surfaces is a good start, as polished surfaces reflect heat better than a rough, as cast surface. Ceramic coating the chamber and piston tops is better yet as these coatings reflect even more heat back into the combustion chamber.
There is/was a good article in Carcraft this last month about sparkplug electrodes. They went into good detail (for Carcraft), but the most important thing to watch for is that the end of the negative electrode (the bent piece) is nice and smooth on the end, with no stamping flash (which will act like a little tiny glow plug). A simple preventative measure, easily taken care of with a points file, is to round of any sharp edges on the negative electrode. You would be surprised at the difference this makes in ANY engine.
Carbon build-up also causes hot spots, like a little charcoal briquet glowing in the combustion chamber. And so we have the three-fold benefit of water injection. Water injection cools the intake charge, and slows the burn rate. Both important factors in preventing pre-ignition. Maybe even better in the long run is the fact that water injection keeps the combution chamber spotlessly clean, no carbon build up at all.
Cam timing is important, as the right cam can bleed off cylinder pressure at lower rpms. But you'll lose some low end torque, too.
In the previous posts on this thread, nobody has metioned QUENCH DISTANCE! (edit-sorry panic, just now saw your post)
This is one of the most important factors when building an engine to run high compression, and no pre-ignition. A 10/1 engine with a quench distance of .035" - .040" and dished pistons will be MUCH less susceptible to pre-ignition than a 9/1 engine with flat-top pistons sitting .050" down in the hole - and an .040" head gasket. The late heads have a very nice, large, quench surface, too. When the piston reaches TDC, the A/F mixture is forced violently out of the "quench" area and into the combustion chamber at very high speed. This creates high turbulence in the chamber for great combustion, and that quick, high speed flow also cools hot spots on the combustion chamber sufaces.
10/1 on premium pump gas should be absolutely no problem, IF you build the engine right.
Rick(wrench)

My corvair's AIRcooled quench-milled head 140 engine is 10/1, runs on 91 octane CA "premium" with no ping and set to full factory advance.

 

[MustangSix]

Here are some factors that cause ping and knock, and require either high octane gas, less spark advance, adjustment or replacement of some components, etc. All affect octane tolerance:
» “Looseâ€￾ piston to head clearance @ TDC (.060â€￾ or more)
» “Tallâ€￾ 1st gear ratio (Powerglide, Powerflite, Super Turbine 300, Fordomatic)
» High (low number) axle ratio
» High vehicle weight
» 195° thermostat
» High compression with mild cam
» Large bore size
» Spark plug position near exhaust valve
» Combustion chamber shape
» Lean mixture
» Hot spark plug
» Tight converter with low stall speed
» High initial ignition advance
» Fast advance curve
» Weak spring in vacuum advance canister
» Any cooling problems (rust or corrosion in the water jacket, radiator clogged, fan too small, rad not sealed to opening, no shroud, etc.)
» Low octane
» Amount of water in the head
» Effective coolant flow
» Thermostat response time
» Water pump speed
» Exhaust valve seat width

I wonder if it might be possible to develop some algorithm that might predict knock sensityivity based on this and other data, such as static CR, cam spec, head composition, etc. Or is there a program that might be used to predict a "knock point"? The rule of thumb of "octane/10" is a little too vague sometimes.

 

[addo]

Wouldn't there be so many dynamically varying parameters it's all but impossible to make use of? Temperature gradient in the engine materials, charge density, driving style all spring to mind. I can't see industry favouring it (R&D) because the knock sensor is a fairly suitable compromise for most applications.

OK, I'm biased because with Oz fuel, knock is a given.

Adam.

 

[wsa111]

i have read the many posts on this subject. the above posts are very professional. many have experience with mopar,chevy or what have you.
the bottom line is what works on the ford six. the best advise is from someone who has done this before on this perticular engine.
to give an example this engine i built is a 67 200,block decked .025 head milled .015 with a crane 194 degree camshaft on the intake at .050. this engine will not knock on 89 octane fuel with 12 degrees initial + 26 degrees mechanical for a total of 38 degrees.
i am in the process of building an other 200 with a later head milled .070 with flat top pistons. compression will be about 9.8, but will use a camshaft in the 264-268 area. my cranking compression with the small cam and the original head is 175 psi guage. i'll let you know the results with this later combination. thanks william
attention,mustang geezer & az coupe pm me or post your results on your engines-thanks

 

[CobraSix]

I agree...some more practical examples will be beneficial. I'm really suprised geezer hasn't weighed in yet.

When I get my engine together, I'll post results. I'll be running 9.5:1, with the COmp Cam 260 (forget the .5 lift duration). I'm hoping to get by with 89 octane, but we'll see.

Slade

 

[xctasy]

Three important things:-

1. The heat stove under the carb rasies the fuel octane needed by 8 points on a Mini engine. How much morw with our log head sixes. If you have a Clifford port divider, there is less heat soak, with a set of headers, even less.

2. The second point is what David Vizard has siad for years. I haven't done the sums on the peak flow of 50/50 water alchol or just straight water at 45 psi with two 15 thou jets, but that will allow you to go to 12:1 compression with no knock if the set-up can add water evenly at low manifold vaccums (excluding when cranking on startup!). The use of an adjustable pressure switch or MAP sensor is the trick. I'd look at the 2-bbl carb setup which mounts to the log like the Mustang Geezer. The idea of single point dispensing of water worries me, as the cylindrs to detonate first are no 5 and 6, and they would be starved with a stock 1.75 or 1.5" log head unmodified. The H2o must mix into the atomised fuel, not under it. I guess you could make a set of brass port injectors, and run it like an EFI system, but I'm a bit a head of myself here.

3. Water injection never reduces pressure or heat at the exhast... it goes up. The piston may not be able to cope with 12:1 comp, even if the car doesn't knock. There is an emperical formula for the bulk thickness of a forged or non-forged piston, and if you over step the mark, it'll get killed dead. This goes for the block, head gaskets, ring lands, everything that seals is subject to more pressure, and there is a simple list of things you must know before you overtax the engine.

There is a chart used on the Pinto OHC and Mini A-seires engines from yonks ago, and this traces octane demand from the effective compression the cam provides. As Panic says, the key is what the effective compression is. Cranking pressure is an indicator only. The Indicated Brake Effective Mean Pressure curve is the key instigator of pressure. The hard edges and the four items Panic mentioned last week on the power developed in high compression engines is pivotal in the point at which detonation sets in.

As we discussed it, some engines love to go stratospheric on compression, others hate it. Untill now, I've believed that there is a proportional gain, but there isn't at all.

I favour the algorithim approach jack describes. The key ingredients were shown by Whittey in a post long ago, and by Panic.

I can get the stuff from my books and chart it. Then come up with a first cut. The basic cannon fodder is here on these links.

Read and feed!

Panic's gem on why high compression doesn't give proportional increases in power http://victorylibrary.com/mopar/otto-c.htm

Then the henpeck of it http://fordsix.com/forum/viewtopic.php?t=12838

Effective compression calcs. Hey Panic, spot the error first starter for ten... http://fordsix.com/forum/viewtopic.php?t=5518

Whittey found a better source of the calcs..http://cochise.uia.net/pkelley2/DynamicCR.html