Unless there is going to be operation above 7000 to 8000 RPM there are few
reasons for moving the opening side of the exhaust port. The stock opening
event is good up to 9,500 RPM. Also the heat load near the center of the apex
seal would not help the sealing and shorten life I would think. On the other
hand, at much lower RPMs it probably wouldn't hurt anything but torque. A
copy of the device from the exhaust ports for nonturbo engines built into the
exhaust header right outside the port, or even extending into the port would
be cheap and easy to change. Also periphery port engines are dogs at lower
RPMs. The short intake manifold length on the race car is most of that. They
are more demanding on exhaust length. They are off the cam below 7,000 and
will pull real well through 10,000. This with less inlet timing than a bridge
ported engine. A big street port would get you near stock tractability and
225 HP at 9,000 RPMs.
Lynn E. Hanover
I don't know who built that engine Lynn but the facts don't bear that out.
If you copy exactly the peripheral port configuration of the Lemans engine
you will get 240 HP at 6000 RPM with long large diameter intakes.
Power Sport is getting 210 HP at 6000 RPM with their peripheral
port configuration and long intake runners. Their dyno curve indicates
way more low RPM HP than anybody can use with a fixed pitch prop. The upper line
is the HP curve and the lower lines are the prop HP requirements
on the Power Sport dyno curve.
Paul Lamar
Exactly right, for a long intake runner. The Lamans engine has huge runners and had controlable lengths based on RPMs. Obviously they spent a bunch of time and money getting the tune right to make that kind of power and still have some width to the HP curve. Also they ran three plugs and some real good fuel. The key is the runner length. The more power you expect to recover the more precise the intake and exhaust length and diameters must be. And even then it will be for one RPM only. The SCCA peripheral port engine has two 44MM venturies to breath through. The best were under 300HP.
The best bridgeport running two 36MM venturies is about 247. The engines will not run well with overly large exhaust headers. Going down from 2" ID to 1 7/8" ID is a big improvement. Both pipes the same length exactly is very important. If you collect both pipes you can run a very short system. If you run individual pipes, they would have to be longer than you have room for.
I have not seen an engine from a named builder running without the exhaust inserts in place. They may be enlarged slightly over stock but they are in place.
I will try to get a copy of a dyno sheet of a mild bridge ported 13B running at 6,000 or lower. It has 259 at 9,000. But I doubt that anyone is looking at it at that low an RPM. Building a periphery port 13B is a lot of work to end up with only 215 HP.
If I do the ports on another one I will ask the owner to record it at 5,000, 5,500 and 6,000 just to see if my thinking is any good. How about a monster bridgeport just for the sprint to Oshkosh and back?
Lynn E. Hanover
At any constant RPM the runner length on the Lemans engine was fixed.
The three plugs did not help the power much but they did buy 3% less fuel consumption.
Much development of road racing engines is not applicable to air craft
engines as we don't really give a hoot about throttle response or what the torque is at
RPM less than peak power. Prop load as I recall is something like
the RPM squared or maybe even cubed. At take off RPM's most engines stall
the prop anyway.
Paul Lamar
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