Hey guys,

Is there a "rule of thumb" so-to-speak for the potential hp a motor is capable of (proper cam/intake/fuel/ect...)based on the head flow rate? ..... ie. multiply the intake flow rate by 2 and that is the potential HP you could make if the components were carefully chosen........ (300cfm could make 600HP) I was just wondering if there was a general rule for this. I am generally referring to a pump gas engine (SBC).

How much HP could be made from xxx cfm on pump gas if you used forced induction? ie turbo's or superchargers.......


My guess right now is about 1.5 - 2 times the intake flow for a N/A engine and about 3.5 times the intake flow rate for a intercooled turbo combo. This is assuming pump gas.......

 

Blazer,

There is no "rule-of-thumb" that I am aware of. There are too many other variables, including total displacement, valve size and lift, stroke length, con rod length, exhaust flow, mechanical compression ratio, quench area, top ring depth, ring spacing and piston skirt length, combustion chamber design, etc.

There are a couple of other "rules-of thumb" that are commonly tossed around that may relate to your question. A NA gasoline engine should generally be able to produce at least 2 HP per cubic inch of displacement at peak efficiency. NASCAR crews are able to make 800HP with 358 CID without too much trouble. A boosted engine should be able to make at least 3 HP per cubic inch at leak power. NHRA crews can typically get 1,500 HP on gasoline from 500 CID.

For a daily driver/street engine, some of that power would usually be sacrificed in the interest of reliability. A very conservative "rule" is for a NA engine is 1.25 HP per CID, and 2 HP per CID for a boosted engine.

As for how this relates to a specific head flow rating, I've never seen such a "formula", but I'm sure there are a lot of other opinions out there.

 

I've never seen one, but it seems like there should be some manner of rough equivalence... after all, work (or energy) is obtained one fuel molecule at a time, and each fuel molecule requires a certain number of oxygen molecules to release the energy, and since power equals energy per time, then it's reasonable to conclude that power also equals molecules per time, which is another way of saying "flow" as in cubic feet (molecules) per minute (time)...

The burning process releases a certain percentage of the heat energy in each molecule regardless of CID, country of origin, specific intended purpose, etc. Internal combustion engines all operate within a fairly narrow range of thermal efficiency. Thus it is reasonable to assume that any engine would produce a similar amount of energy, within a range of maybe 10 or 15%, per each fuel molecule burned; and therefore a certain amount of horsepower or kilowatts or BTU/hr or whatever per given unit of flow. The thermal efficiency can be optimized by the choice of compression ratio, ignition timing, cylinder head and piston materials, and so on; but there's a theoretical efficiency that one can only approach, and not exceed.

Using any form of boost, the same thing should apply; if you double the pressure (15psi above atmospheric in the intake), you should approximately double the flow, which should approximately double the power, assuming that there aren't flow restrictions elsewhere in the system.

But again, I've never seen anyone calculate this, although it's intuitive that it should be true. Maybe someone with a math & physics background and too much time on their hands could do this for us.

 

OK, here is something interesting.

In the February 2002 issue of Car Craft, on page 54, under the "How to Port Cylinder Heads" article, they state that AirFlow Research has quantified this question with a formula, which they claim is "fairly well established." It reads thusly:

hp = cfm x 0.2575 x number of cylinders

So just plug in your numbers and it should be pretty close.

Let's take a stock L31 Vortec head as an example. Its maximum flow is supposed to be 226 cfm at .500 lift. (This is according to Destop Dyno 2000's flow file.)

Therefore we arrive at 226 x 0.2575 x 8 = 465.56 hp

This seems to jive with all of the dyno runs I've seen lately in the hot rod mags, using these heads.

And it is probably good to keep in mind that this would doubtless be under ideal atmospheric and mechanical conditions. That is to say the perfect temperature and humidity, and also the optimum valve angles, camshaft (lumpy solid lifter), compression ratio (like 10:1) and exhaust (headers).

Hah! You learn something new every day, eh?