Best compression ratio for my 66

Reverend-Heat · Aug 18, 2004

Best compression ratio for my 66 Fastback???

I´m currently running x303 gt40 heads on my 302 with 64cc. the "nominal" compression ratio is 9.0:1

I would like to step up to get more HP. Milling down the heads to increase compression is not significant enough and also would result in reshaping the intake manifold. I´m also not changing heads. There are forged JE-pistons with a significant 18cc dome which would get me a 12.2:1 ratio. I´m using 98 octane fuel all the time which would limit early detonation. Other alterations that i feel necessary are recurving my timing to less advance and using stronger rods with floating pins.

Are there any "further" downturns of the high compression? If you feel compression is too high, why is it so and what would be a recommended comp?

please help

PS: don´t wonder thread is also in race tech forum - as i wasn´t sure where to put it

one2gamble · Aug 18, 2004

you should be fine on 98, theres a chevy 327 in my godfathers garage thats 12:1 that gets around on 91+octane booster with Iron heads.

allcarfan · Aug 18, 2004

I dont understand. You dont want to change the heads, dont want to mill the heads, but, you want to change the pistons or are the pistons already in there the ones you speak of?

Is your compression ratio right now 9.0:1? or is that what the heads make with stock pistons?

Reverend-Heat · Aug 18, 2004

one2gamble
thanks. do you know if the chevy has any other problems due to high compression - like overheating or blown gaskets?

allcarfan
currently i have the standard pistons built in, which give me 9:1. I find it in the low range for a non-blown/nitrous car. With current alumn heads (GT40)and a few other alterations in the set-up i´ve got the feeling i leave some HP unused if i don´t raise the CR.

D.Hearne · Aug 18, 2004

Why are you wasting your money running 93 octane gas with a 9.0 to 1 comp ratio ? With that motor and cam combo, it should be happy as s*^% running on 87 octane. You'd have to bump the timing up over 40 degrees total to use 93 octane, and then you'd be flirting with serious damage from doing so. With I ran a set of Canfield heads ( aluminum ) with a B303 roller ( .510 lift W/1.7 rockers) on a stock short block 5.0 using Slivolite hypereutectic pistons on 87 with NO problems. The comp ratio was 9.8 to 1 in this motor. Did the same thing with ported E7's and the B cam with a 9 to 1 ratio, and here again, no problems with detonation.

Reverend-Heat · Aug 19, 2004

D.Hearne
thanks for taking care of my purse . sorry wrong statement in my mail: i ment that i would use 98 octane if i would increase compression ratio.

In Germany we have the following pump gas classes (all unleaded):

petrol: 92 octan
super: 94
super plus: 98
and v-power (Shell): 100

don´t ask for the prices

My basic concern is not which gas to use, but if i have more advantages than disadvantage with a 12.2:1 CR - too much pressure on the rod and bearings, burning exhaust valves?, blown gaskets?...whilst not gaining much HP?

What is a good balance in gaining HP whilst not taking it over the top?

jerry S · Aug 19, 2004

I need to 'splain you sumthin bout Octane

Reverend-Heat said:
@d.hearne
thanks for taking care of my purse . sorry wrong statement in my mail: i ment that i would use 98 octane if i would increase compression ratio.

In Germany we have the following pump gas classes (all unleaded):

petrol: 92 octan
super: 94
super plus: 98
and v-power (Shell):

Right Reverend Heat, you need to remember the difference between RON, PON, and MON and posted octane in the US vs. Europe.

The US uses PON (Pump Octane Number) while in Europe you get the RON (Research Octane Number). As you can see, your 98 RON Octane in Germany is the equivalent to 94 PON octane in the US.

RON MON PON
90 83 86.6
92 85 88.5
95 87 91
96 88 92
98 90 94
100 91.5 95.8
105 95 100
110 99 104.5

D.Hearne · Aug 19, 2004

The more compression you run ( with the proper grade of fuel to allow it) the more power your motor will make. As for needing better rods, that's a 64 million dollar question. Lots of guys here used to run 12.5 to 1 with stock rods back in the 60's & 70's , and I don't recall hearing about problems with bearing wear or rod damage from the higher ratio. But better rods also aren't a bad idea. I would just shoot for a 10 to 1 ratio, which is a good compromise, if you really want to fork over the $$ for upgrades, If it were me, I'd leave the motor as is. Or just mill the heads .030 and go with a thinner gasket, according to Ford, you can mill up to .040 from the heads without manifold problems.

Best compression ratio for my 66

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