Sensitivity of power to change of a:f ratio
Sensitivity of power to change of a:f ratio
(OP)
These question just came up in another forum I frequent.
"Is there an optimum a/f ratio for horsepower?
If so, is it variable as a function of rpm, compression ratio, ignition lead, combustion chamber design, flame front properties, etc?
If there is a range for optimal horsepower, how much wider is this range for, say, 98% of optimal horsepower? In other words, if the range for optimal horsepower is 12.3 to 12.5, how much wider would the range be for 98% of optimal horsepower?"
The poster is an all round great guy. He is preparing an engine for the Engine Masters Competition which is an American Hot Rodding based competition based on dyno performance of engines.
He works on this with Gene Adams who is a professional drag race engine builder and they have some serious sponsors, so I am classifying them as engineering professionals.
This is work related for Gene Adams as it promotes his business.
I already answered "Yes, Yes and who knows in such detail" or words to that effect.
Does anyone here have data on what %age change you typically loose in power for say 0.1 or 0.2 change from optimum in A:F ratio.
I know the magnitude will be different in different engines and conditions due to all the reasons listed in the second question, but I am looking for typical or ballpark. A clue to any factors that cause more than normal or unexpected changes would be great.
The main aim I expect is to reduce dyno time for development of less productive factors so as to leave more time for more gainful activities.
The engine in question is a early Hemi Crysler, about 350 CI , 10.5:1 compression, single 4 barrel carby, flat tappet cam, all commercially available parts with fairly extensive modifications allowed. Petrol is provided and is a specific pump gas (98 octane I think).
The test is best average power and torque from 2500 to 6500 with an inertia dyno.
"Is there an optimum a/f ratio for horsepower?
If so, is it variable as a function of rpm, compression ratio, ignition lead, combustion chamber design, flame front properties, etc?
If there is a range for optimal horsepower, how much wider is this range for, say, 98% of optimal horsepower? In other words, if the range for optimal horsepower is 12.3 to 12.5, how much wider would the range be for 98% of optimal horsepower?"
The poster is an all round great guy. He is preparing an engine for the Engine Masters Competition which is an American Hot Rodding based competition based on dyno performance of engines.
He works on this with Gene Adams who is a professional drag race engine builder and they have some serious sponsors, so I am classifying them as engineering professionals.
This is work related for Gene Adams as it promotes his business.
I already answered "Yes, Yes and who knows in such detail" or words to that effect.
Does anyone here have data on what %age change you typically loose in power for say 0.1 or 0.2 change from optimum in A:F ratio.
I know the magnitude will be different in different engines and conditions due to all the reasons listed in the second question, but I am looking for typical or ballpark. A clue to any factors that cause more than normal or unexpected changes would be great.
The main aim I expect is to reduce dyno time for development of less productive factors so as to leave more time for more gainful activities.
The engine in question is a early Hemi Crysler, about 350 CI , 10.5:1 compression, single 4 barrel carby, flat tappet cam, all commercially available parts with fairly extensive modifications allowed. Petrol is provided and is a specific pump gas (98 octane I think).
The test is best average power and torque from 2500 to 6500 with an inertia dyno.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.





RE: Sensitivity of power to change of a:f ratio
RE: Sensitivity of power to change of a:f ratio
Patprimmer, I've got a "typical" graph of torque vs. equivalence ratio in front of me right now for a gasoline engine. In the area of max torque, the graph is reasonably flat (little change in torque for a change in fuel mixture). Max torque occurs at about 1.25 (AFR = 11.75) but approximately 98% of torque is shown from about 1.15 to 1.32 (12.75 to 11.15 AFR). This agrees with my personal experience tuning small high-out engines (130-140 hp/L).
It would appear that once you are close to about a 12:1 air fuel mixture, your efforts are better spent elsewhere. Of course, you'll have to come back and retune (or rejet) after tuning other parameters.
RE: Sensitivity of power to change of a:f ratio
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Sensitivity of power to change of a:f ratio
Thanks Greg.
I very often recommend people to that site, but forgot to this time.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Sensitivity of power to change of a:f ratio
RE: Sensitivity of power to change of a:f ratio
RE: Sensitivity of power to change of a:f ratio
RE: Sensitivity of power to change of a:f ratio
RE: Sensitivity of power to change of a:f ratio
One other area, when I was an automotive instructor, we taught that heat over time was how to think about destructive combustion temperatures. We used to take a torch and quickly pass it under are hands to show how a 2500 degree torch would not burn our skin because there is not enough time to heat sink it or what ever you trying to heat --- it takes time. Also there is only 180 degrees in the power cycle, but there are 270 degrees of relative cooling time to cool the piston that has to be apart of the consideration when evaluation the destructive combustion temperatures. Bottom line, every so-called normal combustion temperature is high enough to melt any aluminum piston, it’s the combination of heat in and heat extracted over time that makes it all work and when the balance is broken --- problems set in.
al1
RE: Sensitivity of power to change of a:f ratio
"all commercially available parts"
The early hemi manifolds I found on line are Weiand 2X4 and PAW universal base with exchangeable tops
http://www
I have to wonder if getting all 8 cylinders A/F ratio within .2 of each other might be a feat unto itself.
Is the goal to make the biggest peak number?
I think the engines showing the greatest variation or sensitivity for varying power with A/F are the ones with least "effective" combustion chambers regarding stuff like mixing (or maybe charge stratification) and turbulence. It makes sense to me that Best power requires giving ALL the oxygens something to burn, so if the lunch lady is serving small portions, some folks have to go back twice.