Secret Headflow Info

[snowman]

Here is some of the "Secret" head flow info: Normally aspirated engines only.
1. 0.43 X Max AF at 10" water = HP per cylinder

2. 2000 X AF at 10" water/disp per cyl = RPM for peak HP

3.RPM of peak HP x 0.75 = RPM for peak torque

4. To convert flow at pressures differen't than 10" of water, the factor is the square root of the ratio of the pressures. E.G. the air flow at 10" vs the air flow at 25" is 0.63 times the air flow at 25"

This info is very useful for head flowing. There is another sort of basic limit on HP that is you can't get more than 2 HP per cubic inch without a blower or turbo. The max HP is the head flow or cubic inch limit whichever is less.

I have grossly simplified the whole thing to the following:

For a 4 cylinder moter the max HP is equal to the max head flow at 28" of water or 2 HP per cubic inch whichever is less.

For an eight cylinder engine it is Twice this ammount, for a 6 cylinder half way inbetween.

All this is sort of a sanity check plus is very helpful to know where you can or can't get more power A much more accurate way is to use a simulation like Engine Dyno 2000

[jluetjen]

There is another sort of basic limit on HP that is you can't get more than 2 HP per cubic inch without a blower or turbo.

2. 2000/disp per cyl = RPM for peak HP

Doe! I guess that BMW's been wrong about their 900 HP @ 19000 RPM 3 liter (186 cu in) F1 motor. They've either got a blower hidden in there or their only making 372 HP. Heck the DFV was making 408 HP (2.19 HP/cu in) back in 1967. In 1965 Honda had a 1.5 liter F1 motor that put out 230 HP which works out to 2.47 HP per cu in.

Now looking at your peak HP formula and BMW's F1 engine; 2000/18.6 inches = 107.5 RPM????

Maybe there is a reason that these formulae have been secret - or maybe they're intended only for tractor motors.

[snowman]

They must be out to get me for giving away the Secret. They had a gremlin remove the multiplier of AF at 10" of water.

Working this backwards on the BMW engine gives an AF of 177 CFM at 10" water. Pretty good.

2000 X AF at 10"/disp per cyl = RPM for peak HP

The 2 HP per cubic inch works for almost any engine you or I could ever build. In fact only the people who make engines that run at 12000 plus RPM need worry about getting a lot more than the 2 HP number.

[jluetjen]

Oh - now it's a little clearer. It sounds like they're really "rules of thumb" to consider when planning out the porting and flow of a potential motor. BTW - you might want to go back and re-edit your original message so that other readers don't get thrown by the error in the equations.

[Mr Beau]

BMEP calculations are a little better for estimating maximum power from a given displacement and engine speed, and even then, there's a lot of assumptions (though a whole lot less than the above air flow calcs). Honda gets over 2 HP / CI on the S2000 engine, and it is an emissions engine with warranty. Touring car engines (which are still pretty high tech, but with rev limit and "obtainable" technology) are more like 2.4 HP / CI.

[snowman]

Is that a non turbo Honda engine? The guide is only a guide not exact and only for normally aspirated engines. NASCAR only gets about 2 HP per cubic inch and its hard to do better even without restrictions.

Rember this is NOT what you will get, its the MOST you can probably get if you do everything else right. If you are getting more than this you already know to much and wouldn't benefit by this crude guide.

[mrrobbied]

actually the honda s2000 engine does not have forced induction. not to mention all the motorcycle engines out there producing similar sort of horsepower per capacity.

[Mr Beau]

Jack,

I seem to remember one of Vizard's early books with a formula similar to what you've listed. Is that where you got this from?

The Honda I mentioned is not turboed (nor are the 2L 300 HP touring cars). It wouldn't take much to improve upon the 2 HP / CI that the Honda currently makes. There's not a lot left on the table, but there are definitely gains to be had due to production / OEM requirements.

[snowman]

Are any of these 2 cycle engines?

[Mr Beau]

Nope, all are 4 stroke 4 cylinders. I believe the Honda even meets ULEV (or better) emission standards.

[jluetjen]

I think that the trick with the Honda S2000 engine is the variable valve timing, mapped ignition combined with the wide flow capabilities of the 4 valve head. NASCAR engines are allowed none of these things. You basically have an engine which pulls comparable to any standard 2 liter econobox engine at low rev's, but adopts cam timing like a 1970's F1 engine at high rev's, and has an induction system which can support the requirements of both configurations.

[snowman]

Variable cam timing isn't something anyone can just do to their engine and if you don't have it your stuck with much lower power per cubic inch. There is a limit as you can only get so much into a finite size cylinder without extra pressure, in fact the 2 HP per cubic inch requires headers and induction tuning to get more than atmospheric pressure into the cylinder. I kind of suspect that 120 percent fill is the absolute limit without a blower.

It all boils down to how much fresh air you can get into a cylinder at atmospheric pressure, once it is completely filled with fresh air you can assume you have somehow mixed gas at about 12:1 with the air and calculate the power that results. If the head can flow more than the cylinder can use then the limit is the cylinder size, hence you can calculate the max HP per cubic inch. I have not done this calculation myself to know the answer.

BMW's F1 engine, the one with four valves, used in the M5, M6 cars did not get 2 HP per cubic inch without the turbo. And they probably tuned the thing at the top end for max power, achieving over 110 percent volumetric effenciency. Can variable cam timing do more at the top end? How? as the cam in the fixed tuned gave up all the low end for the high end.

[snowman]

I just looked up the Honda S2000. Its a 2L engine and gets 240HP at 9000 RPM. This is just UNDER 2 HP per cubic inch.

[snowman]

Originally posted by jluetjen
[B]Doe! I guess that BMW's been wrong about their 900 HP @ 19000 RPM 3 liter (186 cu in) F1 motor. They've either got a blower hidden in there or their only making 372 HP. ]

The following is all I could find on the BMW engine. Where can I get more detailed info? Also is the HP based on forced induction of the car traveling very fast?
Engine Specification - BMW P83 V10

Technical Specification: 10 cylinders in V configuration, normally aspirated

Cylinder Angle: 90°

Displacement: 2,998 cc

Cylinders: Four valves per cylinder

Valve Drive: Pneumatic

Engine block: Aluminium

Cylinder Head: Aluminium

Crankshaft: Steel

Oil System: Dry sump lubrication

[jluetjen]

You're not going to find out much more then that for at least a few years -- that's all confidential. A number of sources are pretty good at ferreting out some conclusions. My personal favorites are RaceTech Magazine (either on-line or print) , or their competitor Racecar Engineering. Based on competitors assessments, (nobody will talk about their own HP, but they will talk about everyone elses ) lift drag estimates, accoustic readings and such stuff they can usually come up with some pretty good concensus estimates. The other thing is that in general F1 engine's don't make a whole lot more torque per liter (BMEP) then they did 15 or 20 years ago, they just make it over a wider range and at higher rev's. So it winds up that the engines which rev the highest make the most HP. Engine rev's are pretty obvious on the telemetry that SpeedChannel will show on occasion. If you know the torque plus or minus, then you have a good idea of the BMEP which you can then combine with the rev's and voila: HP estimates -- plus or minus 10 or 20 HP.

[mrrobbied]

i am still trying to figure out where you got this 2hp per CI rule. fine the honda engine is 122ci and 244hp. hmm... not much difference there but like i wrote earlier, a lot of motorcycle engines blow that figure away, for example. the yamaha yzf-r1 engine. 1litre (61CI) and 150hp. by your reasoning it should only get 122hp and that was before they went to EFi, it's predecesor got even more power, again the yzf-r6 600cc and 120hp. i am sure almost all the high end production motorcylces have similar figures and yes they are all 4 stroke engines. Motorcycle engineers would look at this thread and laugh.

[snowman]

Its just a rule of thumb, a sanity check. If you are getting more than 2hp per cubic inch in any car engine you know way to much to go by rules of thumb.