compression ratio & pump gas...
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“... My question is simple what is the highest compression ratio i can run on 87 octaine?....”
It’s a simple question but the answer depends on a lot of variables you do not provide.
Cylinder head design and material make a huge difference....as a rule of thumb you can generally ¾ to a full point higher static compression with aluminium heads.
Cam design also has a lot to do with it as with the proper design it can effectively bleed of cylinder pressure allowing a reasonable dynamic compression ratio in spite of having a high static compression ratio.
How the engine is used is also a major factor, an engine that is subjected to heavy loads will be more susceptible to pre-ignition and detonation.
The altitude the engine operates at is also a factor, an engine operating at 4500 feet can usually get by with about ½ point more compression than one at sea level.
On a production iron head engine I would normally found in a G Body, I would say keep the compression down around 8:1 MAYBE 8.5:1. At 9:1 you are really looking at premium fuel.
One thing to keep in mind is that is for an ACTUAL compression ratio. Normally you will find that the cylinder head combustion chamber volume is a bit larger than what is listed in the book (this is especially true of production heads) resulting in a CR that is actually lower than advertised.
It’s a simple question but the answer depends on a lot of variables you do not provide.
Cylinder head design and material make a huge difference....as a rule of thumb you can generally ¾ to a full point higher static compression with aluminium heads.
Cam design also has a lot to do with it as with the proper design it can effectively bleed of cylinder pressure allowing a reasonable dynamic compression ratio in spite of having a high static compression ratio.
How the engine is used is also a major factor, an engine that is subjected to heavy loads will be more susceptible to pre-ignition and detonation.
The altitude the engine operates at is also a factor, an engine operating at 4500 feet can usually get by with about ½ point more compression than one at sea level.
On a production iron head engine I would normally found in a G Body, I would say keep the compression down around 8:1 MAYBE 8.5:1. At 9:1 you are really looking at premium fuel.
One thing to keep in mind is that is for an ACTUAL compression ratio. Normally you will find that the cylinder head combustion chamber volume is a bit larger than what is listed in the book (this is especially true of production heads) resulting in a CR that is actually lower than advertised.
Just curious...what does a compression test tell you about the CR? Is the CR identifiable with a compression tester?
This is a very common question, and yet it really doesnt mean anything.
A 9.5:1 compression iron headed v8 can be made to knock on pump gas simply by swapping out the cam.
Dynamic Compression Ratio is what will reasonably determine whether or not your engine will run on pump gas. I say reasonable, because of the variables MikeP is talking about. 8.0:1 to 8.3:1 is a "safe" DCR from what ive read to run 91+ octane. I dont know if this has been determined for 87 octane, but I would hazard a guess its probably a full point lower.
Now there will be people claiming they have gone as high as 8.9 or maybe even 9+ and run 93-94 octane safely. Sure, thats possible because of variables like ambient temperature, humidity, altitude, combustion chamber design, cooling efficiency (reverse coolant flow heads), QUENCH! etc etc.... All of these variables can soften the blow a bit on the boundaries youre pushing.
You can take a crap shoot and guess what SCR will allow you to run on pump gas, but its really just a crap shoot. Dont assume, do the calculations and take the guess work out of it. Or just build a super low compression engine with a tame cam and dont worry about it.
A 9.5:1 compression iron headed v8 can be made to knock on pump gas simply by swapping out the cam.
Dynamic Compression Ratio is what will reasonably determine whether or not your engine will run on pump gas. I say reasonable, because of the variables MikeP is talking about. 8.0:1 to 8.3:1 is a "safe" DCR from what ive read to run 91+ octane. I dont know if this has been determined for 87 octane, but I would hazard a guess its probably a full point lower.
Now there will be people claiming they have gone as high as 8.9 or maybe even 9+ and run 93-94 octane safely. Sure, thats possible because of variables like ambient temperature, humidity, altitude, combustion chamber design, cooling efficiency (reverse coolant flow heads), QUENCH! etc etc.... All of these variables can soften the blow a bit on the boundaries youre pushing.
You can take a crap shoot and guess what SCR will allow you to run on pump gas, but its really just a crap shoot. Dont assume, do the calculations and take the guess work out of it. Or just build a super low compression engine with a tame cam and dont worry about it.
quench and piston volume/shape are both huge influences. a good quench area (about .032in IIRC) and D-dish pistons work well. i have 9.7:1 (static compression) and she runs fine on 87. (IIRC my dynamic was 8.5) and my measured cylinder compression is about 180psi
here's a good calculator.
http://www.kb-silvolite.com/calc.php?action=comp
here's a good calculator.
http://www.kb-silvolite.com/calc.php?action=comp
megaladon6 said:
Quench is very important, most stock SBCs have a .65 quench which is way too big. Though with quench below .40, you have to be careful about "piston wobble" smacking the head with most factory built engines.
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