Air volume flowrate and air mass flowrate are related by the density of air.

 

Youd be wrong.

 

Agreed. Hold a 7.5 locker in one hand and a 9'' locker in the other. Tell me it doesn't take more power to turn that. Same with the tires. Roll a 245/50/16 on a stock snowflake and a 325/50/15 on a prostar with the same initial force and see which one rolls further before stopping.

 

I hope said engine builders put less time into turning wrenches than they did into these equations, or they would take those combined years building 2 engines.

 

Your hands and arms can generate about 1/6 HP. Not a good gauge of how much power it takes to turn a drivetrain.

The power it takes to turn a rear end is something like 10-15 hp at maximum speed. That's a fraction of the power needed to run an auto trans. Same with wheels and tires, driveshafts. Those together amount to another 5-10 hp.

The biggest power draw is the transmission, and a big auto/torque converter combination that can handle big power can draw upwards of 70 hp @ max speed (i've seen the power reading from a drivetrain dyno). That's your main power loss right there.

 

The most important part. POTENTIAL. You really have to nail everything else to reach that. I have seen some stuff exceed it, but they are only for the very top of the line stuff. Max effort, no limitations race motors

 

Yes, I know it is over two years old - LOL !

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Peak_HP = Flow_CFM x .257 x Number_of_Cylinders

This is estimated potential Peak HP to expect.

You multiply .87 percent times cam's theoretical max lift , round off to nearest .050" in Flow Test, then see what CFM is at 28 inches

example : .700" Lift cam
.700 Lift times .87 percent = .609" Lift

Flow head at .600" Lift , then take CFM at 28 inches and calculate HP potential with above formula

.257 Factor = for beginning engine builders and engines near 10.0:1 Comp Ratio

.285 Factor = would be for Professional engine builders with wet sump pans, lightweight rotating assemblies, low tension great sealing rings, deep oil pans, etc.
Excellent use of inertia/wave tuning with 9.5 to 11.5:1 Comp Ratios or
11.5 to 13.0:1 CR ranges without fully utilizing inertia/wave tuning effects.

.300 to .310 Factor = Current ProStock Technology with dry sump, unlimited carburetion, Hi Comp Ratio, ultra lightweight rotating assembly, etc, max use of inertia/wave tuning, etc, 14:1 to 17:1 Comp Ratios (usually no better than .3200 efficiency or no worse than .2980 eff %)

 

This may work at the crank, at the wheel there is no formula to pin down all the variables.

 

Finally, common sense prevails in this post. You CAN NOT get crank shaft Hp from CFM alone unless your talking an all out race engine making well over 2hp/cid (over 110% VE). WAY to many variables to quantify under that. Friction power alone can throw +-30 hp into the mix. Trying to arrive at some defined HP rating based on CFM is a fools errand without a computer based software modeling program. Trying to get RWHP on a chassis dyno is pure insanity!

 

I guarantee Edlebrock, Crane Cams, & every other aftermarket manufacturer out there uses math to design a head, intake, cam, exhaust, etc...

As been stated earlier in this thread, pro engine builders will use math to determine what their goal should be. When their engine dynos a particular number they know right away if there is something wrong & if it's worth tearing it back down to find out what the issue is.

 

I don't think there's anyone out there breaking records with out of the box heads. Using the math is only good for getting a very rough estimate or just a starting point to improve upon. After that is A-B-A testing, and recording the results.

 

Apologize in advance for bumping such an old thread, but I came here via google, found some good info and I'd like to ask a follow up question.

I'm trying to reverse engineer an intake limiter, so that different engines which are different sizes get the same amount of air so they'll make approximately the same power.

I see the formulas which calculate what the theoretical max hp are, now what I'm trying to do is determine how to limit the air so the engine is only capable of hitting my set point.

 

How did this work out for you?

 

old but very interessting thread.

why does nobody measure brake specific air consumtion? this would give an indication of an engines efficiency, as opposed to bsfc that is mixture dependent.

 

Because air is free, and fuel has a cost. Which would you keep track of?

 

havent we had this discussion somwhere else a few years back?

cost doesnt matter if you are after max power, no? you could compare different engines/set ups as to what they make out of the air mass digested. after all an engine is air limited and not fuel limited.

 

The air depends on its density, which can change due to temperasture and altitude, There is little to no control over this. Too many variables.
Fuel parameters, however are far more controllable, due to its liquid state. If you control one of the two, the other's parameters will be close to what is needed.

 

airmass doesnt depend on density.

here a real world calculation (ls engine, blown, max tq, max hp):

5000 rpm, 535 bhp, 344.6 lbs/h, 11.5 afr → 5.74 lbs/min, 2'605 g/min, 29'959 g/min, 23'045 l/min*, 813 cfm, 1.51 cfm/bhp
6600 rpm, 609 bhp, 442.0 lbs/h, 11.4 afr → 7.37 lbs/min, 3'341 g/min, 38'092 g/min, 29'298 l/min*, 1034 cfm, 1.70 cfm/bhp

* convert at you favourite temp, pressure and humidity. as correction factors used on dyno are unknown, very approximate but comparable back to back. (somebody check calc please.)

im surprised there is such a large variation. what does that tell you?

 

Looks like your example engine is far more efficient at 5000rpm than at 6600rpm.

 

yes, but not in the conventionl sence of cylinder filling. unfortunately its boosted and corresponding pressure ratios are not given, so we dont know what volumetric efficiency is. however we see that the engine not only fills the cylinder less at hp peak, but also is able to make less out of the air ingested. the difference of 13% certainly reflects friction losses. maybe other factors? do thermodynamics deteriorate with rpm?

edit:
3100 rpm, 228 lbs/h x 0.205 x 11.5/229 bhp = 2.35 cfm/bhp
4000 rpm, 321 x 0.205 x 11.2 = 1.90
4500 rpm, 398 x 0.205 x 11.4 = 2.00
5000 rpm, 365 x 0.205 x 11.5 = 1.60
6800 rpm, 448 X 0.205 x 11.3 = 1.73

apparently this is not so much friction related. as the engine comes on the cam it improves dramatically. (same engine as above set up a bit differently.) looking at the jump 4500→5000 rpm im wondering if the data is correct. ok, from another run i get 1.55 for 4500 rpm.

i have no idea how dependable this data is. would be great if somebody could do this with his own data. published fuel flow is pretty rare, unfortunately.