Race Gas Octane/Engine Compression
07-11-2002, 04:12 PM
#1
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Race Gas Octane/Engine Compression
Out of curiosity...I have been mixing ERC-RUL 104 octane (R+M/2) gas with normal CA crap to yield gas of 96-98 octane.
If I decided not to mix and use straight race gas, I have a couple options. The RUL is formulated for normally aspirated, high compression engines. Another option is MUL/A at 101 octane and formulated for "low to medium compression". MUL/A is significantly cheaper.
I'm not sure but was under the impression that the NA 3.6L engine in a 993 has a compression ratio of about 11:1. Would this be classed as "high compression" or "medium compression"? Any risk of engine damage from using the RUL instead of MUL/A?
If I decided not to mix and use straight race gas, I have a couple options. The RUL is formulated for normally aspirated, high compression engines. Another option is MUL/A at 101 octane and formulated for "low to medium compression". MUL/A is significantly cheaper.
I'm not sure but was under the impression that the NA 3.6L engine in a 993 has a compression ratio of about 11:1. Would this be classed as "high compression" or "medium compression"? Any risk of engine damage from using the RUL instead of MUL/A?
07-12-2002, 01:56 AM
#2
RL Technical Advisor
Hi Tom:
LOL,..like people, engines like different "foods" and various parameters affect each engines' appetite for octane and fuel burn charactistics. The ratio of RON values and MON values really make quite a difference (even though the average might be identical), as does specific gravity.
Factors like:
Mechanical compression ratio
Piston dome shape & size
Plug location(s)
Ignition timing
Fuel mixture
Combustion chamber size & shape
Squish area and shape
These all affect what brand of fuel and octane numbers will make the most torque and HP in your engine. For these reasons, the best way to tell what fuel to use is by investing a half-day in some chassis dyno runs and try some different fuels. I guarantee you will be surprised by the results
Now, most stock 993 engines measure in the 10.2:1 to 10.7:1 range and thus far after 7 years of these, I've not seen any 964 or 993 engine that was truly over 11:1, as delivered from Porsche.
IMHO, you'll not risk any engine damage with these fuels as the compression ratio is rather conservative and both these fuels will fulfill the engines requirements with ease.
LOL,..like people, engines like different "foods" and various parameters affect each engines' appetite for octane and fuel burn charactistics. The ratio of RON values and MON values really make quite a difference (even though the average might be identical), as does specific gravity.
Factors like:
Mechanical compression ratio
Piston dome shape & size
Plug location(s)
Ignition timing
Fuel mixture
Combustion chamber size & shape
Squish area and shape
These all affect what brand of fuel and octane numbers will make the most torque and HP in your engine. For these reasons, the best way to tell what fuel to use is by investing a half-day in some chassis dyno runs and try some different fuels. I guarantee you will be surprised by the results
Now, most stock 993 engines measure in the 10.2:1 to 10.7:1 range and thus far after 7 years of these, I've not seen any 964 or 993 engine that was truly over 11:1, as delivered from Porsche.
IMHO, you'll not risk any engine damage with these fuels as the compression ratio is rather conservative and both these fuels will fulfill the engines requirements with ease.
07-12-2002, 02:43 PM
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Thanks Steve. At $1 a gallon more for 3 octane numbers, I'll just use more of the lower octane in the mix.
If I might pose a follow-up. You mention that the RON and MON ratio really makes a difference. Is the reason something that can be explained briefly?
ERC-MUL/A: RON=106.0 MON=96.0
ERC-RUL: RON=110.1 MON=98.0
If I might pose a follow-up. You mention that the RON and MON ratio really makes a difference. Is the reason something that can be explained briefly?
ERC-MUL/A: RON=106.0 MON=96.0
ERC-RUL: RON=110.1 MON=98.0
07-12-2002, 03:56 PM
#4
RL Technical Advisor
Hi Tom:
LOL,...Briefly yes,.easily no.
RON & MON are octane ratings that are derived by testing the fuel under different conditions.
The condensed version is that high RON fuels are tested & optimized for low and medium engine speeds and prevention of run-on. (dieseling)
High MON fuels on the other hand, are blended & tested for resistance to knocking under high RPM and loads and part throttle operation.
Using varying combinations of these two fuels of differing chemical and anti-knock characteristics results in a gasoline that is optimized for the conditions and the kind of engine at hand.
For example, a turbocharged engine or really high-compression one being operating under high RPM & peak loading, would be better served with a fuel of higher MON than RON. The exact science of blending these fuels is a result of thousands of hours in the lab and on the dyno to kind out what works and what doesn't.
For a turbo car, I'd use the very best fuels I could buy, hands down. The peak pressures and temperatures in the combustion chamber on-boost, are simply tremendous and require a fuel that won't "flash" with effective compression ratios at 14:1 or over. For your car, as long as you are not engaged in full-out racing where the engine is operated at peak RPM & loads for 20 min to 1 hour, you will be just fine with your current blending of fuels. Racing activities do require the best fuels but that can only be optimized by load-type dyno testing.
The last point I'd offer is that flame travel in the 911's combustion chamber is the biggest factor is determining what types and blends of fuels work best in these cars. Single and twin-ignition engines have different flame travel patterns and speeds so their octane appetites are different; all things being equal. This subject is extremely complex and lots more could be written about it.
LOL,...and to think that I've only just touched on this subject,...
LOL,...Briefly yes,.easily no.
RON & MON are octane ratings that are derived by testing the fuel under different conditions.
The condensed version is that high RON fuels are tested & optimized for low and medium engine speeds and prevention of run-on. (dieseling)
High MON fuels on the other hand, are blended & tested for resistance to knocking under high RPM and loads and part throttle operation.
Using varying combinations of these two fuels of differing chemical and anti-knock characteristics results in a gasoline that is optimized for the conditions and the kind of engine at hand.
For example, a turbocharged engine or really high-compression one being operating under high RPM & peak loading, would be better served with a fuel of higher MON than RON. The exact science of blending these fuels is a result of thousands of hours in the lab and on the dyno to kind out what works and what doesn't.
For a turbo car, I'd use the very best fuels I could buy, hands down. The peak pressures and temperatures in the combustion chamber on-boost, are simply tremendous and require a fuel that won't "flash" with effective compression ratios at 14:1 or over. For your car, as long as you are not engaged in full-out racing where the engine is operated at peak RPM & loads for 20 min to 1 hour, you will be just fine with your current blending of fuels. Racing activities do require the best fuels but that can only be optimized by load-type dyno testing.
The last point I'd offer is that flame travel in the 911's combustion chamber is the biggest factor is determining what types and blends of fuels work best in these cars. Single and twin-ignition engines have different flame travel patterns and speeds so their octane appetites are different; all things being equal. This subject is extremely complex and lots more could be written about it.
LOL,...and to think that I've only just touched on this subject,...
07-26-2002, 06:56 PM
#5
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My motor has a static compression ratio close to 9.0:1. I regularly run 20lbs.+ boost, with a very effecient turbo. That is 20 lbs. right up to redline 7000 rpm. That becomes a comp. ratio of approx. 20:1. The genius is in the timing, Lambda control and Sunoco GT-Plus 104 octane, unleaded. PS. no knocking and NO knock sensor.
