Re: Compression Ratio for pump gas
[Re: Justins72]
#1217357
04/17/12 03:03 AM
04/17/12 03:03 AM
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Joined: Jan 2003
Posts: 70,126 Here
DirectSubjection
Tacohead. The First and Only
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Tacohead. The First and Only
Joined: Jan 2003
Posts: 70,126
Here
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Generally, aluminum heads will allow you to run a bit more compression than iron heads would for the grade of gas you want to use. Higher compression will also allow a more radical cam but you'll get a rough idle and low vacuum. If you just want to cruise around as you said without a radical cam, then aim for a compression in the 9s - adequate for many nice street cams and you won't have to worry about detonation. Perfect with EFI too. ![](/ubbthreads/images/graemlins/Twocents.gif)
Ride eternal, shiny and chrome
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Re: Compression Ratio for pump gas
[Re: Justins72]
#1217361
04/17/12 04:39 AM
04/17/12 04:39 AM
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Joined: May 2008
Posts: 5,399 Aurora, Colorado
451Mopar
master
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master
Joined: May 2008
Posts: 5,399
Aurora, Colorado
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What engine? I prefer to use closed chamber heads and flat top pistons setup with about 0.040" quench. For the street, with premium pump gas, I would try to get about 9.0:1 for mild/economy/RV type engines using a small cam around 205-210 duration @ 0.050", 9.5:1 for mild street performance using a cam around 215-225 @ 0.050" duration, 10:1 compression if using a hot street cam in the 230-245 @ 0.050" duration, 11:1 for a street/strip or mild bracket engine with a 250-260 @ 0.050" duration cam.
Depending on the engine and heads, you may need dished or "reverse dome" quench pistons (when using open chamber heads) to get the desired compression ratio and good quench. The heads can also be milled to reduce chamber volume or small amounts of material can be removed from the chamber to slightly increase the chamber volume.
The main thing to avoid are pistons that sit too far down the cylinder bore.
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Re: Compression Ratio for pump gas
[Re: Justins72]
#1217362
04/17/12 04:53 AM
04/17/12 04:53 AM
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Joined: Jul 2005
Posts: 712 California
BigDaddy440
super stock
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super stock
Joined: Jul 2005
Posts: 712
California
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You may want to focus on choosing your camshaft first so you can use the specs and calculate your actual compression numbers.
What options are you considering that will effect your compression ratio? I'm assuming pistons and cylinder head choice.
There are two basic types of compression:
1) Static Compression (this is the number that most use when BS-ing about their engine) Static compression numbers do not consider valve opening and closing events. Compression with valves shut all the time.
2) Dynamic Effective Compression does take into consideration the opening and closing of valves (controlled by the camshaft). It is important to note that DE Compression numbers are on a different scale that Static numbers. For example you can only run about 8.3 to 8.4:1 D.E Compression on pump gas before running into problems.
I built a pretty hot 347ci small block Ford a few years back. With aluminum heads I was running about 8.4:1 D.E (or a tad less) which resulted in about 210-212psi in each cylinder. I ran 91 octane California gas and it didn't ping, but I new was pushing it.
If you're talking static numbers anything in the 9s will almost certainly be fine on pump gas regardless of your cam specs. As you get into the mid to high 10s (static compression) calculating D.E compression is a good idea. Depending on your stroke, cam specs, pistons, combustion chambers, head material, and gasket thickness you might find that you have less actual compression that you thought.
I once calculated the D.E compression on my last 440 combo and I was surprised that with the cam I was using I had far less actual compression that I thought. Even though I had a Static Ratio of 10.9:1 (flat tops with milled heads) my D.E ratio was in the mid 7s if I remember correctly. I had been running 91 octane gas in that 440 and probably could have easily ran 87 or 89 without any issues.
There are D.E compression calculators available for free online. You'll need several specs including bore & stroke, head gasket thickness as well as actual head gasket bore diameter. Full cam specs, valve relief specs (or other piston head info) as well as how far your piston is in the hole at TDC are also needed to calculate true compression.
Good luck. -Dan
1969 A12 Roadrunner 1970 Plymouth Cuda 1968 Dodge Dart
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Re: Compression Ratio for pump gas
[Re: Justins72]
#1217367
04/18/12 11:50 PM
04/18/12 11:50 PM
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Joined: Nov 2011
Posts: 2,154 Its a TRAP!
DARTH V8Я
Oh No!! I just had a moron attack!
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Oh No!! I just had a moron attack!
Joined: Nov 2011
Posts: 2,154
Its a TRAP!
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I prefer tiny chambers with a dished piston, imo promotes better, less turbulant flame travel. I've built smallblocks with dome/big chamber, flat/medium chamber/ & dish/small chamber, I always get away with more compression on less octane with dish/small chamber. Cam overlap is also crutial!
When it takes more than a sweet mullet to prove you rule at the trailer park..
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