Topic: how to port 500/550 four cylinder head?

[scottly]

I found this article quite interesting as a reference read some time ago when I was considering engine mods on a BMW airhead project I have underway. It, along with recommendations and other research from machinists have formed my position. You may find it useful as well.

What article??

[scottly]

Cal jumped to the conclusion that mil meant millimeter

I didn't jump to any conclusion, I mixed up what I read in 2 separate parts of the original thread. It was a mistake on my part, satisfied?

[Scott S]

A noticeable difference over my other stock 550. Especially paired with the CB650 cam.

Did you happen to ride it after porting, and before the 650 cam? I'm just curious as to your impression. No doubt some decent porting and the cam would wake these engines up (a point I made early on).

 No. The porting and cam were done at the same time.

[camelman]

Cal demands real numbers.

And to an earlier point made, porting doesnt increase air volume, it increase air flow rate. No amount of porting alone will increase in measurable volume changes (aside from decimal place volumetric calculations) into the combustion chamber. That is essentially controlled by the cam/inlet valve, as you certainly know. So, without a substantial change in volume, even the negligible compression change due to a 10mil of head milling, theres no way he would see sufficient performance gains (as claimed).

I struggle to understand your sniping over it all.

I'd challenge anyone to send a bone stock motor to the dyno, get a true baseline, then send your head to Mike R and have him perform his best porting magic using stock valves, and compare the difference. On a 50BHP motor, I would be absolutely astonished if you saw a 5BHP gain. Which will result in a 3HP at the wheel. You think 3HP at the wheel is enough difference to experience under throttle? I don't. You'd achieve similar or better results with a tuned exhaust and dyno tuned motor than from porting. No 2 ways around it.

Volumetric flow rate = (volume of air per unit of time)

If flow rate increases, then the volume of air per unit of time will increase. If the same camshaft is kept in the engine, then the period of time for the air/fuel mixture to enter the cylinders is kept the same, but now there is a higher flow rate filling the cylinder. The result is an increased fresh mixture volume inside the cylinder throughout the RPM and throttle range, which results in higher cylinder pressures and more power. All of that with only porting.

BTW, decimal place volumetric calculations are real changes. Just because the change occurs on the backside of a decimal point it doesn't mean it is an insignificant change. If a volumetric efficiency change increases by 0.05, then that can represent well more than 5% of engine power because these engines operate below a volumetric efficiency of 1.00. In fact, these engines operate more around a volumetric efficiency of 0.85 at full throttle.

I think 3HP at the wheel is noticeable.

[seanbarney41]

I have lost interest in this thread....maybe if the op comes back with some more educated questions, I will jump back in.  Otherwise, I got butt dyno calibrations to do, on junky old low tech motorcycles with "ruined" cylinder heads. 

[camelman]

Cal, so much of what you're saying is wrong that I don't care to correct it. I stand by my claims, but not your incorrect and conflated analysis.

[scottly]

Cal, go back and read the article you linked. Does it say other mods need to be made for porting to increase power? No, it does not. Speaking of numbers, that article stated that for every 1 CFM in increased flow, that cylinder would have an increase of .255 HP. Looking at the graph from the old Branch article shows an increase of about 9 CFM at a lift of .300", with the stock carb, which would add up to 9.18 HP.

[Trad]

Cal, go back and read the article you linked. Does it say other mods need to be made for porting to increase power? No, it does not. Speaking of numbers, that article stated that for every 1 CFM in increased flow, that cylinder would have an increase of .255 HP. Looking at the graph from the old Branch article shows an increase of about 9 CFM at a lift of .300", with the stock carb, which would add up to 9.18 HP.

I just re-read the article. You are correct my friend! Porting alone will yield noticeable gains! 9.18 HP is pretty impressive. It will obviously be further increased with additional performance mods. (hotter cam, higher compression pistons, oversized valves). After reading that again I'm fairly confident that a stock engine with a correctly ported head would yell noticeable gains on a SOHC4. I want someone to actually Dyno this.