10-1 too high compression? - Team Camaro Tech
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post #1 of 7 (permalink) Old Apr 3rd, 02, 07:10 PM Thread Starter
Join Date: Feb 2001
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My friend just built his pontiac 400, it turned out he had 72cc heads and combined with the forged pistons he used, he now has 9.98-1 compression! For iron heads i figure this is too much with the california 91 octane gas. Everyone i have ever talked too said 9.5-1 is too high with iron heads, so i guess to compensate the high compression i will back off the timing, it is supposed to be about 10-12 initial and 36-38 at 3000 rpm, i was thinking of taking it to 8 initial and 34 total, also should i use the vacuum advance, i figured i shouldn't since he has to drive up some big hills to get to his house?
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post #2 of 7 (permalink) Old Apr 3rd, 02, 07:37 PM
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Location: Regina, Saskatchewan, Canada, Earth
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Is everything put together yet? If not run with thicker head gaskets to lower the compression. Also it will depend on the cam specs as some cam grinds are more forgiving and will let you run a bit more of a compression ratio. I personally think he will be okay with his setup as they say 9.5 as a safety (there is usually a bit of a margin). I'm not up on California gas, so maybe one shouldn't rely too much on a margin though.
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post #3 of 7 (permalink) Old Apr 3rd, 02, 07:49 PM
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pontiacs are more prone to detonation than a lot of engines, but like sneakypete said it depends on the cam specs too. A big cam with proper ignition timing will work ok, but if he is using a rv grind he will have problems. Vehicle weight and rear gearing also play an important role here.
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post #4 of 7 (permalink) Old Apr 4th, 02, 02:03 AM
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I am running 10.5:1 on my BBC 396 (402) on 92 octane with no pinging.

I have the stock 396/375 hp iron heads (L78).

I agree that timing and cam specs play a role. He may be ok with his setup.


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post #5 of 7 (permalink) Old Apr 4th, 02, 09:34 AM Thread Starter
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he is using a dual pattern cam, a comp dual energy 274, i think it has somewhere around a 480 lift. No offense but it wasn't supposed to be a debate about pinging i am quite sure it will, i was wondering about using the vacuum advance and setting the timing? it is a 68 firebird with the stock torque converter and 3.08 gears
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post #6 of 7 (permalink) Old Apr 4th, 02, 02:46 PM
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Location: Tucson, AZ, USA
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You can get away with this compression but it is dependent on some critical thigs such as what is the quench area of the chamber like??
Hopefully they are not fully open chambers..
Also, what is the quench height including the head gasket ???
Here are some typical tricks to help reduce the detonation....

Jet a bit richer..but not much...mostly in the secondary...

The power-valve/spring should be set properly...not to loose since you need it to come in sooner in mid transition since this is where the pinging starts in the RPM band..

Run a cooler plug...go colder, BUT keep an eye on the plug..back off when the plug is glazing since that is too cold a plug...

Run a lower thermostat...Like a 160 ...

Use coolers for the trans..make sure to use a big radiator...and good fans and shrouds..since heat is your enemy..

Also set the MAX advance on the distributor right before it pings...it will be somewhere around 31 to 34 degrees in your case..this is a function of stroke..the greater the stroke then the greater the piston speed for a given RPM...and it is piston speed that increases the chamber turbulance..so for the 400 figure MAX advance at 3000 to 3200 RPM which is set with the springs..
You want the advance to kicj-in about 900 RPM..
Don't do the typical thing of retarding the base timing...It's not the base timing that is the problem...It's thats by changing base timing is a poor way of reducing OVER-ALL MAX timing..instead leave base timing alone and adjust the advance tab in the ditributor...

Also...vacuum advance is ONLY used at idle and cruise mode when connected to manifold vacuum port...this helps economy when cruising...This presents a problem at idle to primary circuit transistion..since the vacuum will decrease and the mechanical advance will increase..you get roughly a cancelation since the curves are moving in opposite directions at that transition..which can make a dog for a motor...I would use the carbs ported vacuum this way you only get vacuum in cruise mode but none at idle...So..this means you will have to make your base timing higher than the 10 or 12 your using now..figure something about 16 to 20 would be better..since your not using the vacuum advance to do it for you....

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post #7 of 7 (permalink) Old Apr 4th, 02, 07:17 PM Thread Starter
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Thanks, sounds great
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