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Old 05-24-2005, 10:31 PM
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Highest Compression For The Street

I was wandering whats the highest compression one can run on the street without resulting to using racingfuel/gasoline mix of some kind to eliminate detenation? This combination would use cast iron not aluminum cylinder heads.

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Old 05-24-2005, 10:42 PM
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There's a multitude of factors in this as I under stand it. But the genaric answer is 9.5:1, I believe, in stock cast iron cylinder heads. IF you step up to aluminium heads, I believe 11.5:1is the generic answer.

But timing, chamber specifics, pistons, etc... play a large roll in this.
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Old 05-24-2005, 10:56 PM
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and especially cam profiles. the more overlap and duration and more compression you can run. 10.5 is just about max but 11:1 could be possible for some. If you are asking what the max is i'd suggest staying about 10 or 10.5:1.
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Old 05-24-2005, 11:00 PM
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If you are using production components, then do not go higher then around 10:1.

I myself run arond 13:1, but I also make my own stuff.
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Old 05-24-2005, 11:47 PM
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compresssion

I stay below 11 to 1. Some say they can get up to 11.2.
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Old 05-25-2005, 07:14 AM
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I know a guy who runs 11.5:1 on 94 octane pump gas, but he does notice a little detenation when going up a steep hill. I personally run 10.1:1 on 91 octane pump gas with production iron heads with no problems at all.
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Old 05-25-2005, 07:21 AM
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Years ago Car Craft claimed that the practical limit (with some exceptions) for pump gas (93 octane) was 180 psi (on a compression test).
Anybody that knows their cranking pressure want to verify this?
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Old 05-25-2005, 08:50 AM
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compression ratios

To be on the safe side for gas availability, I would recommend not going over 9.5:1 with steel heads and 11:1 with aluminum.
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Old 05-25-2005, 11:44 AM
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i agree with sleepr and max. my street stuff is around 9.5. lonestar an brackteer are talking about big cams. so they are right too, but its gonna loose street manners.
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Old 05-25-2005, 11:51 AM
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I don't know.. My car has 10.1:1 on 91 octane wich is available at all gas stations,and I have no probs with it. My cam is a decent size for street being .49 lift on both intake and exhaust with a 236,230 duration. Runs nice, loud but nice.I forgot, steel heads to.
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Old 05-25-2005, 12:46 PM
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comp ratios

Quote:
Originally Posted by jimbobneutral
I don't know.. My car has 10.1:1 on 91 octane wich is available at all gas stations,and I have no probs with it. My cam is a decent size for street being .49 lift on both intake and exhaust with a 236,230 duration. Runs nice, loud but nice.I forgot, steel heads to.
Im not saying that 9.5:1 is an absolute ceiling, but rather about the upper ends of safe limits with most engines. Most can get away with running 10:1, or even 11:1, with steel heads on 91 or 93octane, however, there are a lot of places in the country where 91 and 93 octane gas just isnt available. Thats why I suggest a compression ratio of 9.5:1, which generally will peform quite well with 89 octane gas. Another factor is that going over 9.5:1 compression usually requires more than flat top pistons, unless you have small chamber heads, and domes add to the cost of the engine, as well as the cost of the higher octane gas. What little there is to be gained in a street engine going 1/2 a ratio higher, to me just isnt worth the added expense, along with maybe having to travel out of your way to get the higher octane fuel. BTW, I have a 351w with twisted wedge heads running a 230/235 degree, .510/.515 lift cam, and I run 9.5:1 compression, on 87 octane, and my engine was dynoed at 398 HP at 5500 RPM. I cant complain.
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Old 05-25-2005, 04:38 PM
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Man! That depends on a bunch of things! But in the case of an old V8, the cam profile is the biggest player.

You should be calculating the dynamic compression, which also uses the cam specs.

I have a 11:0 sbc with a 292H comp cam in a light car and a 5spd with a 3.90 gear and have not problems with 93 octane and 38 degrees of timing.

My 70 1/2 c10 truck with a 454 (8.75:1), with a mild rv cam, 3.08 gear, and automatic will detonate if I run my timing up too high (over 38).

In general, a sbc and handle more compression ratio than a bbc. And, a lot of little import engines can burn 87 octane fuel with 10:0 cr. So you should be more motor specific.

So in a sbc with a mild cam (under 210 degrees duration at 0.050) 9:0 should be about max.
a cam between 211 to 219, 9.5:1 cr
a cam between 220 and 230, 10:1 cr
a cam between 230 and 240, 10.5:1 cr
a cam between 241 and 250 11:1 cr

This also depends on car weight and gear. More weight and less gear (more load) would need less compression.
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Old 05-25-2005, 05:45 PM
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Quote:
Originally Posted by Rhansen
Years ago Car Craft claimed that the practical limit (with some exceptions) for pump gas (93 octane) was 180 psi (on a compression test).
Anybody that knows their cranking pressure want to verify this?
I am at 220 psi with iron heads and 91 octane.
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Old 05-25-2005, 06:39 PM
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Quote:
Originally Posted by lluciano77
I am at 220 psi with iron heads and 91 octane.
Assuming the timing is not excessively retarded that pretty much blows that theory.
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Old 05-25-2005, 07:26 PM
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Quote:
Originally Posted by Rhansen
Assuming the timing is not excessively retarded that pretty much blows that theory.
Build your next motor with a ton of compression and pump gas and maybe you'll be the 2nd in a million to get away with it. Not doubting lluciano77 but we see detonation on iron headed motors at 10:1 a lot, and that is with 93. 11:1 with aluminum heads such as AFR 195's is not easy either with pump 93. Boarderline on the edge.
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