Axtell intake port comparison and flow chart

Here’s the Axtell intake port compared to a stock 28.5mm port. Drawings were made from port molds sent to me courtesy of Ken Canaga.

The favorable flow chart for this port can be viewed at:

head-flow-testing-t8640-375.html?hilit=axtell#p187375
(middle of page)

and

head-flow-testing-t8640-390.html?hilit=axtell
(middle of page)

You can see that there is not much difference between it and the stock 28.5mm port. Stay with stock valve diameters unless you re-angle the intake valves from 28 to 26.5 degrees or you can have catastrophic valve clash problems. You can see some smoothing of the short curve leading up to the valve seat. Larger re-angled 41mm valves allow a broader curve on the short side of the port – allowing for material removal and blending in to match a re-located valve seat. Some material is removed from the roof - again giving a smoother broader radius and to bring the ID of the port up to approx 30mm. I have accurate AutoCad drawings of this port on a cheap CD if anyone wants to print it out – make templates and duplicate it. The CD also has drawings of the exhaust port.

If the situation in the inlet port is a standing wave (sonic) what does the flow bench tell you ?

So the re-angling of the intake valves (ala' Dunstall) is more to prevent valve clash than improve flow? I get that there are several things working in concert here.

I think I read somewhere that Axtell, Jerry Branch and Dick Mann all got their combustion education from Al Gunter.

The re-angling is primarily to give you room for a larger intake valve seat and still have some space between the intake and exhaust seats. Too little space and you get cracks between the seats, and then usually drop a seat, trashing the head. But it does also help avoid valve interference. It is possible to run a larger intake valve without re-angling the guide, but it takes some care to avoid valve-to-valve interference. Axtell fit 1.532" (38.9 mm) intake valves to my 750 heads without re-angling, but with some cams, you might need to sink the exhaust a bit to get a safe interference margin.

To get really large valves, you need to re-angle both intake and exhaust guides. Steve does that on his Stage 3 heads, and fits 43 mm intakes and 36 mm exhausts. The factory race head I have had only the intake guide angle changed, and was fitted with 1.696" (43.1 mm) intakes and 1.370" (34.8 mm) exhausts.

Ken

Ken,
When you fitted the 43.1mm inlet valves, was that the only change you made at that time ? And if it was, did your dyno give evidence of an 'improvement in either mid-range torque or top-end horsepower? - One thing I always try to do is change only one thing at a time then test the bike after each modification. I think there is a situation where the valve size, taper on the inlet port and the diameter at the smallest point in the inlet tract might be at an optimum for the usable rev rage. What I have found from experience is that it is often possible to go to a place where there is no coming back from. i.e. once the head is over-ported, it is stuffed.
I am not familiar with what C.R.Axtell did with Nortons. However is it possible that some of his modifications might have been backwards steps that went un-noticed because the motor was already very fast and used in a bike that was adapted to the power characteristics ? As I've said before on this forum, just because you bolt all the fast bits together, it doesn't not necessarily mean you will have a fast bike.