Can hp be calculated from cfm?
Can hp be calculated from cfm? If the optimum fuel to air ratio is 14.7 to 1. and we know that the ls1's burn .456 pounds of fuel per hp per hour. If you calculate the hp that you want, determin the fuel required to get that hp, then the amount of air that it will take to go with that amount of fuel, figure in 70%-100% VE and we should come up with the amount of cfm/rpm an ls1 needs to spin to get a sertin hp.
I try'd the math on a stock motor and I cant get it to come out.
I try'd the math on a stock motor and I cant get it to come out.
That math will never add up. Stochiometric fuel burn will burn down your motor at high power levels. Stock motors are usually set to somewhere in the mid 11's A/F ratio, and best power N/A is usually closer to 13/1. Best fuel economy is usually closer to 16/1. As you can see, the variables add up quickly. The only way to determine HP/CFM is on a specific combo where the A/F ratio and BSFC are already known. Anything else is nothing but a guess.
Gross horsepower does directly correlate to airflow and fuel flow. However, it is only a correlation, not a determination. That's because there are many variables related to engine design such as pumping losses, internal friction, valve and ignition timing events, optimal mixture, RPM, cylinder balance, etc.
So air flow potential versus horsepower potential is something worth thinking about, but it's only one facet of the diamond.
Steve
So air flow potential versus horsepower potential is something worth thinking about, but it's only one facet of the diamond.
Steve
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Originally Posted by Steve Bryant
Gross horsepower does directly correlate to airflow and fuel flow. However, it is only a correlation, not a determination. That's because there are many variables related to engine design such as pumping losses, internal friction, valve and ignition timing events, optimal mixture, RPM, cylinder balance, etc.
So air flow potential versus horsepower potential is something worth thinking about, but it's only one facet of the diamond.
Steve
So air flow potential versus horsepower potential is something worth thinking about, but it's only one facet of the diamond.
Steve
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From: Illinois, RT 66 dragway area
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There are a million variables, but I have heard the formula 2.2 X CFM used in the past as a very very general guide.
I have a old set of heads that flowed 377 and the motor made ~775 HP.
So the 2.056 is much closer.
I also have a set of heads that flow 530 and it's a 1200+ HP engine. So the 2.2 is closer. No matter what, everything is a guess, just use it as a starting place.
.
There are a million variables, but I have heard the formula 2.2 X CFM used in the past as a very very general guide.
I have a old set of heads that flowed 377 and the motor made ~775 HP.
So the 2.056 is much closer.
I also have a set of heads that flow 530 and it's a 1200+ HP engine. So the 2.2 is closer. No matter what, everything is a guess, just use it as a starting place.
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It can be done, but you're going at it wrong.
400hp (for example) at .456 lbm(fuel)/hp-hr, and 12.8 lbm(air)/lbm(fuel), and .078 lbm(air)/cubic foot (approximately, unknown temperature and pressure, you can look it up), 60 minutes/hour, gives 499 cfm. You don't even need RPM, or VE.
But, you really DON'T know that it is .456 BSFC. BSFC is the result from measuring power and fuel flow, typically. But you can guesstimate it by assuming an approximate value.
Remember, this would be airflow through the throttle body at steady state at ANY RPM. BSFC also changes at different RPMs, and different spark timings, and different A/F. I just picked a random air density off of google, too, so don't use .078 as gospel. This also does not assume any certain depression across the throttle (although, you can substitute a lower air density to get a higher flow rate spec).
You also cannot just divide by 8 to get a single cylinder's port flow, since ports are usually measured at a certain depression, and steady state, whereas any single cylinder only sucks for 1/3 of the time, and is not steadily sucking.
400hp (for example) at .456 lbm(fuel)/hp-hr, and 12.8 lbm(air)/lbm(fuel), and .078 lbm(air)/cubic foot (approximately, unknown temperature and pressure, you can look it up), 60 minutes/hour, gives 499 cfm. You don't even need RPM, or VE.
But, you really DON'T know that it is .456 BSFC. BSFC is the result from measuring power and fuel flow, typically. But you can guesstimate it by assuming an approximate value.
Remember, this would be airflow through the throttle body at steady state at ANY RPM. BSFC also changes at different RPMs, and different spark timings, and different A/F. I just picked a random air density off of google, too, so don't use .078 as gospel. This also does not assume any certain depression across the throttle (although, you can substitute a lower air density to get a higher flow rate spec).
You also cannot just divide by 8 to get a single cylinder's port flow, since ports are usually measured at a certain depression, and steady state, whereas any single cylinder only sucks for 1/3 of the time, and is not steadily sucking.
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depends on compression
depends on camshaft
depends on efficiency of combo
also depends on flow thru head/intake port ( not head alone )
numbers ive seen 2.0, 2.2 and 2.4 depending on how hot the combo was.
2.4 was usually a serious solid roller 14:1 or 15:1, dual carbs on sheet metal tuned runners
vacuum pump, balanced and light assembly and of course big flow numbers.
You know how a radiused entry can improve flow thru an intake port?
Or a pipe simulating header tube can improve flow thru exhaust?
In some cases, a intake designed to work with a head port can slightly improve flow thru it as a unit...
Normally drops a little.
Really hot combo's on these bbs' can achieve 2.2, but 2.0 is more likley...
I'm sure that in the most exotic config 2.6 is in the realm of possiblity, but ive not heard of much higher.
Whats F1 doing?
depends on camshaft
depends on efficiency of combo
also depends on flow thru head/intake port ( not head alone )
numbers ive seen 2.0, 2.2 and 2.4 depending on how hot the combo was.
2.4 was usually a serious solid roller 14:1 or 15:1, dual carbs on sheet metal tuned runners
vacuum pump, balanced and light assembly and of course big flow numbers.
You know how a radiused entry can improve flow thru an intake port?
Or a pipe simulating header tube can improve flow thru exhaust?
In some cases, a intake designed to work with a head port can slightly improve flow thru it as a unit...
Normally drops a little.
Really hot combo's on these bbs' can achieve 2.2, but 2.0 is more likley...
I'm sure that in the most exotic config 2.6 is in the realm of possiblity, but ive not heard of much higher.
Whats F1 doing?
Thanks for the info. I often see "tech" articles in magazines than have no real application. I've read a million cam tech articles and I still wouldn't trust myself
calculating the valve events for a motor. All I read is thiery this or thiery that. How do I go about get'n some hard numbers to calculate this sort of thing.
calculating the valve events for a motor. All I read is thiery this or thiery that. How do I go about get'n some hard numbers to calculate this sort of thing.
