Full Auto port shape comparison

Here are the port shape comparisons of the Full Auto to Harley XR 750 and stock small Norton ports. Pay attention to the port floors and the broad curvature leading up to the valve - this is what makes the FA & XR 750 heads work so well. These plans are helping to create some special custom manifolds and modified "cobra head" ports for a FA head in a high output race engine.

The FA intake port drawing overlaid with the Harley XR 750 port and stock small port Nort.


The FA intake port mold


The FA exhaust port drawing overlaid with the Harley XR 750 port and stock small port Nort.
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The FA exhaust port mold (not as smooth as the FA intake port).


Compare the custom re-angled FA intake manifold to the misaligned stock manifold in the image at top.

What's the inward bump all about ?

That where the valve pokes into the port.

Could we see the view from above?

I think you are over generous with the curve before the valve on the std inlet port, both my rh10s are almost non existent there

Horton

Here's the top view of the FA intake port. You can see that the width is more or less consistent as supplied. You can see the little bump/dent to strengthen the valve spring pocket area.


Splatt

My drawing and port mold is of an early 750cc 28.5mm port. I don't have an R10 port mold or drawing so I can't compare the floor radius leading up to the valve (someone send me a mold please). I always start with the 750cc 28.5mm ports for my own heads and move the bolt holes if I need it to fit an 850 (I have Ken Canagas photos of the bolt hole conversion process on the "Narley port" CD.

Here's the top view of the HD XR750 "cobra head" port shape and the Nort 750cc 28.5mm port for comparison. The XR750 port is full race and much wider leading up to and around the guide (also with bigger valve) but a narrower scaled down width and shape would work well with the FA head. I have some stage 1, 2 & 3 widths and shapes drawn up.

Here's the Harley ex port compared to the stock Norton port. If I could get an FA head with a little more height on the ex floor then the roof could relieved toward the "cobra head" shape.



See ex port floors below. The stock floor needs to be raised. The blue line is the suggested raised FA ex port floor but it needs the cobra head relief to make room for it. The original material around the guide dips into the airflow.


FA ex port top view

What effect does exhaust pipe diameter have on performance of the Commando engine ? I always use skinny exhaust pipes because I think the standing wave is more easily established. However doesn't this affect the desirable shape of the exhaust port ? My pipes are fitted to the exhaust stub with no change in shape or ID from the port outlet through the stub and right down the pipe to the collector. I note that almost every other 2 into one depicted on this forum shows larger ID pipes were used for the headers.

Its OK for the pipes to be bigger than the ex port for a racer. Even 1-3/8" street pipes drop below the bottom of the FA "D" shaped ex port. Photo below is an earlier (pre FA) welded up D port. The raised floor increases the flow because it is a more gradual curve and the bottom dam helps to prevent reversion.



The FA head is the best (and only) aftermarket head available because of its raised intake floor with its higher velocity and flow. To raise the intake floor of a factory head would require some fancy welding or a cast in place and attached shoe as Comstock used to do (there is enough room to get inside and weld up the ex floor). That's a lot of work and so its easier (but expensive) to start with the FA head. Every tuner has his own preferences with bigger valves and slight alterations etc but for a stock bike the FA head is hard to beat as it is supplied. And for racing the FA head already has the raised intake and exhaust floor to start with.

jseng1 said:

..... Every tuner has his own preferences with bigger valves and slight alterations etc but for a stock bike the FA head is hard to beat as it is supplied. And for racing the FA head already has the raised intake and exhaust floor to start with.

Click to expand...

Excellent work and great info, thanks for putting it up here Jim. Cj

acotrel said:

What effect does exhaust pipe diameter have on performance of the Commando engine ? I always use skinny exhaust pipes because I think the standing wave is more easily established. However doesn't this affect the desirable shape of the exhaust port ? My pipes are fitted to the exhaust stub with no change in shape or ID from the port outlet through the stub and right down the pipe to the collector. I note that almost every other 2 into one depicted on this forum shows larger ID pipes were used for the headers.

Click to expand...

:shock: :shock: :shock:
Holy crap batman for once I think I agree with you, now just don't start prattling on about how 26 degrees head angle, twin actuated rear suspension followers and deeply valanced mudguards and all the rest of the waffle push you wide on tight circuits causing you to have a mild head injury