When figuring out a combination, 2.2 hp per CFM is a pretty standard rule of thumb on a standard 2 valve head for a well thought out combination. How many of you have exceeded this and by how much?
Engine combination would be nice.
Len C
2.2 hp/cfm
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Warp Speed
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Re: 2.2 hp/cfm
I find it helpful as a guideline in stating a design. If I want x hp I'm going to need to move y cfm so now I know port and valve sized and to pump y cfm I need z displacement/rpm relationship and I'm off and running. So its helpful there.Warp Speed wrote:Does hp/cfm really mean anything?!?
It also tells you something about efficiency I suppose.
I still work at 10" and the number I used as a baseline is 2cfm/hp and it generally works out pretty close....which is like 3.4cfm/hp at 28" or 0.29hp/cfm so the 2.2 hp/cfm number in the OP is a little confine to me???? Or was that about carb cfm or something like that?
Mark
Mechanical Engineer
Mechanical Engineer
Re: 2.2 hp/cfm
Provided that you are dealing with the same fuel, this number is related to the thermal efficiency (and the air/fuel ratio), as those are the only ways it could be meaningfully different.
Re: 2.2 hp/cfm
I've got a big Chevy that does 1031 hp with 419 cfm on the intake ports....... 2.4 hp/cfm
Our Pontiac Engine Master's engine was at 739hp with 389 cfm ................. 1.9 hp/cfm
The Pontiac is MUCH more developed than the Chevy, the difference? RPM. We only run the Pontiac to 6500 for the contest, the Chevy runs to 8000. The Chevy is using the head harder because it runs more rpm. The Pontiac head was designed for low-rpm use but it still moves a ton of air, the physical size of the port is less than 2/3 the size of the Chevy port. (that and the Chevy is 137 cubic inches bigger than the Pontiac too)
All this tells me is that hp/cfm is meaningless. So is hp/cid really unless you are in a class competing hp/cid and there is an rpm limit.
Our Pontiac Engine Master's engine was at 739hp with 389 cfm ................. 1.9 hp/cfm
The Pontiac is MUCH more developed than the Chevy, the difference? RPM. We only run the Pontiac to 6500 for the contest, the Chevy runs to 8000. The Chevy is using the head harder because it runs more rpm. The Pontiac head was designed for low-rpm use but it still moves a ton of air, the physical size of the port is less than 2/3 the size of the Chevy port. (that and the Chevy is 137 cubic inches bigger than the Pontiac too)
All this tells me is that hp/cfm is meaningless. So is hp/cid really unless you are in a class competing hp/cid and there is an rpm limit.
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groberts101
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Re: 2.2 hp/cfm
I get where you're coming from but can't say I completely agree to blanket statements on the subject. For instance. What if the above numbers and output where changed so that the Chev actually made slightly more power with a slightly smaller port? Then, which engine is better tuned for the desired rpm range?900HP wrote:I've got a big Chevy that does 1031 hp with 419 cfm on the intake ports....... 2.4 hp/cfm
Our Pontiac Engine Master's engine was at 739hp with 389 cfm ................. 1.9 hp/cfm
The Pontiac is MUCH more developed than the Chevy, the difference? RPM. We only run the Pontiac to 6500 for the contest, the Chevy runs to 8000. The Chevy is using the head harder because it runs more rpm. The Pontiac head was designed for low-rpm use but it still moves a ton of air, the physical size of the port is less than 2/3 the size of the Chevy port. (that and the Chevy is 137 cubic inches bigger than the Pontiac too)
All this tells me is that hp/cfm is meaningless. So is hp/cid really unless you are in a class competing hp/cid and there is an rpm limit.
This stuff is like comparing boxers. To accurately rate them you must break it down to pound for pound. Rpm.. airflow per cc.. etc.
Re: 2.2 hp/cfm
MK E so your saying at 28" test pressure a 250 cfm head would make 850 hp I need your engine specs!!!!!
Len C
Len C
Re: 2.2 hp/cfm
900 are you sure that you couldn[t have made the same numbers on the pontiac with less head flow ? I like my pontiacs too.
Len C
Len C
Re: 2.2 hp/cfm
My point was that if you go by the 2.2 rule, the Pontiac should make 856 hp, which it would if we turned it up enough. This doesn't mean the head is sized incorrectly, it just means that we aren't using the whole potential of the head because of the rpm we are turning. The big block exceeds that number because we are pulling so hard on the head that it flows more air. CFM at 28" is almost meaningless in a running engine, CFM at 28" does allow you to develop, improve, and compare ports as long as the testing is held to a standard.groberts101 wrote:I get where you're coming from but can't say I completely agree to blanket statements on the subject. For instance. What if the above numbers and output where changed so that the Chev actually made slightly more power with a slightly smaller port? Then, which engine is better tuned for the desired rpm range?900HP wrote:I've got a big Chevy that does 1031 hp with 419 cfm on the intake ports....... 2.4 hp/cfm
Our Pontiac Engine Master's engine was at 739hp with 389 cfm ................. 1.9 hp/cfm
The Pontiac is MUCH more developed than the Chevy, the difference? RPM. We only run the Pontiac to 6500 for the contest, the Chevy runs to 8000. The Chevy is using the head harder because it runs more rpm. The Pontiac head was designed for low-rpm use but it still moves a ton of air, the physical size of the port is less than 2/3 the size of the Chevy port. (that and the Chevy is 137 cubic inches bigger than the Pontiac too)
All this tells me is that hp/cfm is meaningless. So is hp/cid really unless you are in a class competing hp/cid and there is an rpm limit.
This stuff is like comparing boxers. To accurately rate them you must break it down to pound for pound. Rpm.. airflow per cc.. etc.
Re: 2.2 hp/cfm
Maybe, maybe not. We needed enough port so that it didn't stall at .800"+ lift and we needed that lift. The port on the Pontiac is very small, only 2.6 in2 at the pinch but it flows a ton of air because it likes to be fast. This has nothing to do with liking Pontiacs, just about managing airflow. The head did what we designed it to do. If the port was much smaller we would've had too high of velocity through the port and it would've been a disaster. This particular port just moves a lot of air for the size port it is, that's all.LCaverly wrote:900 are you sure that you couldn[t have made the same numbers on the pontiac with less head flow ? I like my pontiacs too.
Len C
edit: (during development we had this same port over 430 cfm but pulled it back because it would've been too much head for what we were trying to accomplish)
Re: 2.2 hp/cfm
remember guys this is a rule of thumb not something cast in stone. Nuther rule of thumb thru carb 1.4 CFM per Hp
Len C
Len C
Re: 2.2 hp/cfm
If you average 900's numbers you get 2.15, or call it 2.2, and in the engineering world if a rule of thumb is good to 10% it's pretty good. 2.2 +/- 10% is 2.2 +/- .22 or basically 2 to 2.4, which again covers 900's numbers. So as a rule of thumb to find a ballpark it seems ok, just don't use it to measure how far apart the bases are 
(and only use it on 2 valve pushrod engines).
(and only use it on 2 valve pushrod engines).Carl Ijames, chemist not engine builder
carl ddott ijames aatt verizon ddott net
carl ddott ijames aatt verizon ddott net
Re: 2.2 hp/cfm
... 1.4 "nominal carb rating" CFM, right?
The "nominal carb rating" is simply how much flow goes through the carb with an arbitrary pressure diference across it. That arbitrary pressure difference may or may not have anything to do with what a real engine would draw through it. It so happens that the "arbitrary pressure difference" is somewhere in the ballpark of what is normally seen ... but it doesn't have to be.
Rules of thumb are fine for "goof-proofing", or "plausibility checks", and the like, but they shouldn't be blindly relied upon.
Things like "brake mean effective pressure", and "brake specific fuel consumption", have defined meanings.
Suppose we have an engine that uses 240 g/kWh BSFC (sorry, I only think metric) which is a decent number for a good spark-ignition 4-stroke gasoline engine.
Multiply by the air/fuel ratio ... assume stoichiometric ... so that's a brake specific air consumption of 3528 g/kWh = 3.528 kg/kWh.
At standard temperature (25 C) and pressure (101.3 kPa), air has a density of 1.2 kg/m3.
So therefore this engine is drawing 2.94 m3/kWh at standard temperature and pressure.
Reworded, if the engine makes 1 kW and runs for an hour, it will use 2.94 m3 of air. If it makes 0.746 kW (1 hp!) it will use 2.19 m3 per hour = 0.0365 m3/min.
1 cubic metre has 35.3 cubic feet so this is 1.3 cubic feet of air per minute.
Suggests that the "rule of thumb" for carb sizing is in the ballpark, and the original premise of the thread is not right ... or I've made a mathematical error (and if that's the case, please correct it).
Note that a fairly large number of assumptions had to be made: first that the BSFC is in a good range, secondly that the air/fuel ratio is stoichiometric, thirdly we are neglecting effects such as overscavenging (but this should be small compared to the air actually being used - otherwise there is something very wrong with the engine), fourth the barometric pressure, fifth the ambient temperature. This is why one shouldn't put too much faith in that number ...
The "nominal carb rating" is simply how much flow goes through the carb with an arbitrary pressure diference across it. That arbitrary pressure difference may or may not have anything to do with what a real engine would draw through it. It so happens that the "arbitrary pressure difference" is somewhere in the ballpark of what is normally seen ... but it doesn't have to be.
Rules of thumb are fine for "goof-proofing", or "plausibility checks", and the like, but they shouldn't be blindly relied upon.
Things like "brake mean effective pressure", and "brake specific fuel consumption", have defined meanings.
Suppose we have an engine that uses 240 g/kWh BSFC (sorry, I only think metric) which is a decent number for a good spark-ignition 4-stroke gasoline engine.
Multiply by the air/fuel ratio ... assume stoichiometric ... so that's a brake specific air consumption of 3528 g/kWh = 3.528 kg/kWh.
At standard temperature (25 C) and pressure (101.3 kPa), air has a density of 1.2 kg/m3.
So therefore this engine is drawing 2.94 m3/kWh at standard temperature and pressure.
Reworded, if the engine makes 1 kW and runs for an hour, it will use 2.94 m3 of air. If it makes 0.746 kW (1 hp!) it will use 2.19 m3 per hour = 0.0365 m3/min.
1 cubic metre has 35.3 cubic feet so this is 1.3 cubic feet of air per minute.
Suggests that the "rule of thumb" for carb sizing is in the ballpark, and the original premise of the thread is not right ... or I've made a mathematical error (and if that's the case, please correct it).
Note that a fairly large number of assumptions had to be made: first that the BSFC is in a good range, secondly that the air/fuel ratio is stoichiometric, thirdly we are neglecting effects such as overscavenging (but this should be small compared to the air actually being used - otherwise there is something very wrong with the engine), fourth the barometric pressure, fifth the ambient temperature. This is why one shouldn't put too much faith in that number ...