Max Compression
- Thread starter Bill D
- Start date
Howard Jones
Supporter
yes
Howard Jones
Supporter
For a engine that you do not want to tear down offen and run on 91 octain Calif gas I would stay below 9.8 to 1 or so. This would be the corrected C/R after taking into account cam timing.
The reason I mention teardown is that the magazines run articles on motor builds that claim much higher C/R BUT the motors are run cool both air temp and water/oil AND the combustion chambers are brand new NASA clean.
Any deposits in the combustion chambers will cause detonation on a marginal motor with C/R and fuel octain rating issues.
On the other hand a new set of alum heads might get you a few more tenth's C/R because of their rapid rejection of hot spots.
All in all a very powerful street motor can be built on 91 octain gas. You should be able to get 1.25HP per inch at 9.5 to 1.
That is right at 400hp from a 302. You will need to use a very good head ala AFR's but it can be done with out a high compression motor. Head flow will be the key.
The reason I mention teardown is that the magazines run articles on motor builds that claim much higher C/R BUT the motors are run cool both air temp and water/oil AND the combustion chambers are brand new NASA clean.
Any deposits in the combustion chambers will cause detonation on a marginal motor with C/R and fuel octain rating issues.
On the other hand a new set of alum heads might get you a few more tenth's C/R because of their rapid rejection of hot spots.
All in all a very powerful street motor can be built on 91 octain gas. You should be able to get 1.25HP per inch at 9.5 to 1.
That is right at 400hp from a 302. You will need to use a very good head ala AFR's but it can be done with out a high compression motor. Head flow will be the key.
I'm running about 10.2:1 static compression on 91-93 octane with no problems. Mild hydraulic roller cam, cast iron 2V Cleveland heads, EFI.
Static compression is only part of the equation. Cam size and timing, ignition timing, consistant fuel/air delivery to each cylinder, combustion chamber design, etc. all contribute to detonation or lack there-of.
Static compression is only part of the equation. Cam size and timing, ignition timing, consistant fuel/air delivery to each cylinder, combustion chamber design, etc. all contribute to detonation or lack there-of.
When I did the calculations for both static and dynamic compression I came up with the following:
Using a set of Edelbrock Victor jr. aluminum heads with 60cc chambers, 210/75cc intake/exhaust ports; Crane hydraulic roller cam; 3.25 stroker crank with 5.4 rods and KB dome pistons, my static compression is 11.5:1 and my dynamic is 9.5:1.
Comments?
Bill
Using a set of Edelbrock Victor jr. aluminum heads with 60cc chambers, 210/75cc intake/exhaust ports; Crane hydraulic roller cam; 3.25 stroker crank with 5.4 rods and KB dome pistons, my static compression is 11.5:1 and my dynamic is 9.5:1.
Comments?
Bill
To further the question, what octane would be a safe bet for Bill's set up? Also, as new markets for corn are sought, I will soon have a relatively abundant supply of ethanol coming locally available as well. How might generous mixtures of ethanol and 91 octane fuel play out in a situation such as his?
Scott
Scott
Howard Jones
Supporter
You can mix 100 octain race gas with 91 octain to increase the overall octain rating. 1 gal of 100 mixed with 10 gals of 91 should get you about 92. 5 gal of 100 to 10 of 91 should get you about 95. If you are going to drive a lot of the street this is a real pain in the ass. I met a guy that could only drive one half tank distance from his house because he didn't want to carry the 100 octain to mix at fillup with him in the trunk.
Their are so many variables in this question that you will need to test to find what works best.
Aprox 10 to 1 dynamic is "about" max for 91 gas. I would just try it and see where you are. If you are marginal then fiddle with timing to see if you can eliminate knock.
Also get a idea of your air fuel ratio. You might be able to get a smog place to work with you. I would start with a target of about 11 to 1 AF ratio. Do any of you guys with EFI setups have good data on this? What do you think?
This stuff is really trial and error. IMHO a guy can really ruin a nice car with a over the top motor.
Lastly their are different makes of head gaskets that vary from one another in volume when compressed. I think you can change CR by about 1 or 2 1/10s this way.
This will require some reserch.
Oh and alcohol makes all of this worse. Less in better when it comes to alcohol.
Good luck
Their are so many variables in this question that you will need to test to find what works best.
Aprox 10 to 1 dynamic is "about" max for 91 gas. I would just try it and see where you are. If you are marginal then fiddle with timing to see if you can eliminate knock.
Also get a idea of your air fuel ratio. You might be able to get a smog place to work with you. I would start with a target of about 11 to 1 AF ratio. Do any of you guys with EFI setups have good data on this? What do you think?
This stuff is really trial and error. IMHO a guy can really ruin a nice car with a over the top motor.
Lastly their are different makes of head gaskets that vary from one another in volume when compressed. I think you can change CR by about 1 or 2 1/10s this way.
This will require some reserch.
Oh and alcohol makes all of this worse. Less in better when it comes to alcohol.
Good luck
Ian Anderson
Lifetime Supporter
Scott
I donn't want to hijack the thread but be careful eith the Ethanol.
In Zimbabwe they sold Fuel with a 85 petrol (leaded) and 15% Ethanol for years but it did cause problems.(Their ethanol came from sugar cane) (Being an local industry the mix varied considerably from nil to 50%)
When they started "blending" there were problems with street cars - they pinked / ran on, ran hot, the ethanol destroyed the rubber fuel lines, ate diaphrams in the fuel pumps, fuel evaporation problems and hence hot starting was terrible etc.
Ethanol also washes the bores once the engine stops and rapid wear was encountered.
Most people then added "Redex" or similar upper cylinder lubricants mixed in their fuel.
The biggest tell tale sign was the inside of the exhaust (tail pipe)- it changed colour with the tune of the engine from Black for over rich to Khaki for over lean - real quick check.
Good luck with the stuff but at least it is renewable.
Regards
Ian
I donn't want to hijack the thread but be careful eith the Ethanol.
In Zimbabwe they sold Fuel with a 85 petrol (leaded) and 15% Ethanol for years but it did cause problems.(Their ethanol came from sugar cane) (Being an local industry the mix varied considerably from nil to 50%)
When they started "blending" there were problems with street cars - they pinked / ran on, ran hot, the ethanol destroyed the rubber fuel lines, ate diaphrams in the fuel pumps, fuel evaporation problems and hence hot starting was terrible etc.
Ethanol also washes the bores once the engine stops and rapid wear was encountered.
Most people then added "Redex" or similar upper cylinder lubricants mixed in their fuel.
The biggest tell tale sign was the inside of the exhaust (tail pipe)- it changed colour with the tune of the engine from Black for over rich to Khaki for over lean - real quick check.
Good luck with the stuff but at least it is renewable.
Regards
Ian
It would take some research, but Brazil should have a tremendous amount of data somewhere as they have gone nationally to alcohol for some time now.
Bill
Bill
I think to go alcohol you have to approximately double your jet sizes. May be your fuel tanks would then be inadequate for a road car? The thing that scares me with alcohol is that you can't see when the thing has caught fire! I'm a bit of a wimp in that regard.
Howard Jones
Supporter
You know the more I think about this. Alcohol in the reason our fuel in Calif. is 91 instead of 92. We already have 10% alcohol in our gas. It was the replacement for MTBE when we finally got it out of our gas.
