Head CFM vs. HP per c.i.?

[thumper 823]

There are some combos here that I've seen have 300 CFM put out 115 hp while others that top out at 270 CFM and lay down 170hp.

Like others say it is the whole picture, Dyno included.

Way too many variables.

I do understand it's the whole package but I doubt 270CFM will get you 170HP.... Unless, it's a very happy dyno..

sorry  150 hp!

http://harleytechtalk.com/htt/index.php?topic=91480.0

N-gin you get the concept. A 110 CI making 150hp is 1.36 HP/CI and the heads flow about 282cfm so we are at .531 HP/CFM
Once we have examples of what is possible we can look at what is going on with similar combos that do not produce as well. The happy dyno and flow bench BS is always brought up but really is just a diversion from the subject . If you gather large data samples (like we have here on HTT) you can always disregard the very top and very bottom numbers and still have plenty of usable data to draw from when looking for trends. You also can only draw so much from programs and info gathered from autos or any other types of engines because of the unique design of the HD V-twin engine. Your not going to get a formula that spits out the HP of an engine just by plugging in the cfm number of the head from a flow bench. What you can get is an idea of what should be possible from a given CI V-twin engine of the HD design with a head that flows X cfm and when the engine comes up short you know something is wrong and you need to find out what. This is the very basis for questioning the performance of the M-8's. They have flow but as a group the power is not on par with other HD engines when you compare similar stages of builds using the hp/cfm numbers.

I have to give that 5  STAR paragraph.
There are a lot of problems depending just on a flow bench.
For every different cam ,and every different cam event, and piston position  in the bore,   a plethora of numbers arises that none has time to chase.

[Don D]

N-gin
There are some fundamental reasons why there are some excellent producers and some not so great. Airflow potential on the intake side is yet again just one data point out of the context of the whole build combination. Many, but not all, of the builds that drop like a rock after 5k are being screwed up by problems on the exhaust side which then has bad effects during overlap. The head can and is sometimes related to the issue but the pipe is usually the culprit along with a cam that doesn't cooperate and help and sometimes flat out gets in the way. When these big builds fall flat at WOT full rpm under load you will also see the MAP pressure that started at 100kpa (sea level) start to taper back when they enter the problem area.
Comes back to the method used to design the build again.
Standard logic..
Good pipe + good cam + good heads + good throttle body + good tune= good combination
There are a half dozen or so reasons why that method sucks and many times ends up with disappointing results.

A better logical approach..
A good combination of parts properly installed, rings broken in (ring seal, low blowby), and tuned right = good combination

Heads are part of that combination and play their part but remember the barn door wide open in the front all the time doesn't do squat for airflow if the back door is closed or the back door is wide open and the front door closed.

On another topic, side note, I am impressed with the results we are seeing with the M8 motor and see high aggregate torque available with simple stage 2 changes. That makes riders happy. Now they need to get the sumping fixed, a bigger issue then how to get these to run.

[1FSTRK]

Don as always many good points, maybe too many. The thread on how to build a performance HD engine could get a little long and wander a bit. To eat this elephant one bite at a time we can use things like the intake flow or any other part of the mix to set very general guide lines. For a 103 with a TR590 cam we know that 185cfm would be lacking and 350cfm would likely be coming through to big a valve and port, so looking at any combo we have to look at the pieces. On the other end if we see a know engine combo that is not working up to par and everything else is spot on can we not look to the heads from an unknown vendor that flow 30% to much air for the power being made? would we be wrong to question the valve size, port size, velocity, and so on of the head, remember we are dealing with a tried and true combo.

[Don D]

If separating out and examining only intake (not my suggestion but for discussion purposes) you could also say that 150 FPS is too slow and 600 is too high port speed. If you know the cross-sectional area of a port you can calculate the port velocity based on the cylinder diameter and the piston speed at any given RPM. This matters too and the soggy reversion laden example is one precipitate of improper head sizing, too big of a barn door. The result is again falling flat at high rpm. Cams can only crutch this. Better to get it right first.

[1FSTRK]

Let us approach this from a different angle. Here is the OP's question.

Is there a standard of how much HP you should be able to achieve given the amount of CFM your heads flow? Meaning, if your head flows 300CFM you can achieve (x) hp ci.

Now doesn't every engine sim program ask for head flow to be input?
The question isn't "can head flow alone predict the hp of an engine"
The part in red is covering all the other variables that you will have to get right to "achieve" the power potential of the flow and velocity is one of them just like compression, cam timing, and all the others.