my motor is a 350 .030 over. the heads are world product 67cc heads. the cam is a rpm performer and the pistons are .125 domed I think the compression is around 10.5 - 11.1:1 but I'm not for sure. Is 93 octane good enough fuel for this engine. I can't get full advance without detonation and I keep burning head gaskets. Can anyone answer my qustion and help with my problem. I was thinking about putting on 76cc heads to lower the compression. Will this help and will lowering the compression kill HP, or will it run better because i'll be able to get full advance.

 

don't make the mistake of thinking that you need to run a certain spark advance for maximum power. It can be a different number for each engine.
What is your intial and total advance now?

 

I just calculated your compression and you're right- you're probably between 10.5 and 11, maybe higher. Too high for pump gas, using full ignition advance, especially with cast iron heads. And if you're running in detonation you will eventually pop the head gasket. Usually you can actually see where it "pushes" the metal fire ring into the water jacket, the adjacent cylinder or into open air.

Yes, lowering compression will help you make more power. You'll make more power with lower compression and full spark advance than you will with too much compression and retarded timing.

Make sure your total advance (not counting vacuum advnace) is about 32-34*. Forget about the base timing, make sure the total timing is right and let the base timing fall whereever it is. Also, it's helpful to make sure the centrifugal advance is "all in" by about 3000 RPM- not still creeping up beyond 4000.

 

How did you caculate my compression? I would like to see exactly what my compression is so I know how much to drop it. Will 76cc heads drop it below 10.1:1. Yes, they are cast iron heads. Thanks

[This message has been edited by mustangs killer (edited March 08, 2001).]

 

Guys, I really need to know what you think about this so I can decide what to do. I need to get my enigne back together soon. Thanks

 

compression is actually a small factor in the cause of detonation. There are many many things you can do to both control detonation and make more power at the same time.
The biggest factor (besides fuel makeup) in controlling detonation is to control heat. Heat is what causes the detonation in the first place.
heat in the air charge
heat in the sparkplugs
heat in the valves
heat in the piston
hot spots in the water jacket around the chambers.

There are dozens of tricks you can do to control heat & detonation. If all else fails you could run water injection that only activates when the engine is under load.
A good step would be to determine exactly why you're getting detonation. Could be too lean, or maybe too much advance. Have you double checked your timing marks on the damper?
It is normal for only a few cylinders to be doing the detonation. This is usually because of distribution problems or possibly other things like valvetrain problems. Even things like leaky valve-guides can cause detonation.
In those cases it would be a step backwards to lower compression and kill power while avoiding the real cause.

just giving you some ideas


[This message has been edited by The ODB (edited March 09, 2001).]

 

He's just got too much compression FOR HIS SETUP!!! Try some thicker (FelPro?) head gaskets (being careful about intake ports lining up properly) and some 93-94 octane gas at 32-34 degrees max advance with proper carb tuning. Chech with the gasket makers to get some info about how much you could lower your CR that way.

If a gasket change does not/cannot work, you gotta make some other change to your config/setup. My recc. change would be AL heads because they draw a lot of heat away and therefore can take more compression/advance, but STILL may need the thicker head gaskets at 11:1...

I didn't mention the octane booster because I htink it is stupid for a daily driver. Just fix the problem and make it driveable. I know I don't need another problem in my life to worry about, and octane booster in the car all the time and/or 100 octane gas is a PITA.

 

Thanks for all the help.

 

Compression plays a decent (I won't say big, nor small) effort in. As we all know according to Boyles? (maybe Charles; whose ever stated P1/T1 = P2/T2) law, pressure and temperature are directly related. As one increases so does the other (ignitionless engine styles use this to create the heat needed for combustion). Just for a little fun lets plug in some numbers!

Say we have 28" Hg at BDC (a little less than atmospheric pressure). 28 x 11 (11:1) = 308" Hg at TDC. Hummmm de humm.... so we have 28"Hg/72F = 308"Hg/T2 and I'll do it in my head and spare boring with more numbers and we get 792F!

Application of this is rather unusable under the conditions. Truely the pressure wouldn't be equal to that, nor would the temp because it would be lost to heating up other areas of the engine. However, it was just to prove a point.

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1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
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