Why a slide throttle? The intake systems is an aerodynamic device. Anything you put in the air stream like a butterfly valve or venturi is going to cut the amount of air getting in the engine. Less air less HP. When the slide throttle is wide open, which is 99% of the time over 8500 feet, the p-port slide throttle intake system is perfectly streamlined.
An efficient motor is an easy breathing motor.
A p-port slide throttle intake system is easier to build as there are no flanges bolted to the side of the block. Just the two p-port tubes. Side port manifolds are much more complicated as there are as many as six tubes to deal with. The transition from oval on the block to round tubes is also difficult to make and weld. Of course a p-port engine will easily generate 250 HP while a side port engine is lucky to see 200 HP. It is also lighter weight than any cast aluminum manifold could be.
The slide throttle shown on this web site is the rectangular home made version. No plasma or laser cutter required. Nothing you can not do with a modern saber saw, Sawsall or hole cutter and a file. You will need some tig welding for the overall p-port manifold and slide throttle. I am sure a competent fabricator can make the whole thing for you if you ask.
The front and rear plates are 1/8th inch thick aluminum so the two inch runner tubes can be directly tig welded to them. We have replaced the old solid Teflon with two pieces of .015 inch thick etched on one side and bonded Teflon sheet and the slide. This material is available from McMaster Carr. It must be bonded to the covers because when the engine is at idle and the slide is nearly closed the pressure in the manifold is very low and the thin Teflon will be sucked into the cavities formed by the large holes in the slide. This will form a bulge in the Teflon sheet. The aluminum must be treated with a kit available in boat stores or anodized as epoxy will not stick to untreated aluminum.
The slide is now 1/4 inch thick aluminum. We sell a low cost 2 rotor kit which will save you a lot of time as the tubes are tig welded to the slide covers.
Why Teflon? Teflon has a very low coefficient of friction. When you close the throttle the prop drives the engine and the engine pumps all of the air out of the intake system up to the slide throttle. This can result in 15 psi behind the slide on both holes. The area of the holes is about 3 square inches each. That means there is a force on the slide of about 90 pounds total pressing the Teflon against the slide housing. In effect this would jam the throttle closed if it were metal on metal instead of Teflon on metal. The coefficient of friction of Teflon is .06 so the force on the slide and the throttle cable is limited to only about six pounds.
Made with virgin Teflon® polytetrafluoroethylene (PTFE) resins, this material surpasses most plastics when it comes to electrical insulation properties, low friction chemical resistance, and performance in extreme temperatures.
The glass filled stuff is a no no and that may have happened with Mark Steitle and why he noticed higher throttle cable forces. We actually tried that and I was rather disappointed in the friction performance.
Recently we discovered how easy it is to coat metal with Teflon. You buy Teflon paint and paint the metal. Then you bake it in an oven about 400 F or so for a couple of hours. Something like this is the way they make Teflon coated frying pans. You could coat the housing and may reduce the coefficient of friction even lower. I have not tried this yet so you are on the bleeding edge if you try it.
Some metal suppliers like Industrial Metals Supply in San Fernando in Southern California will shear the metal blanks for as little as 50 cent a cut. You will have to cut the large holes and drill the small holes.
http://www.imsmetals.com/contact/index.htm
Aircraft Spruce sells the throttle cables.
The Teflon itself serves as a gasket. That keeps raw fuel from leaking out onto the hot exhaust manifold. Not a good thing. The raw fuel will leak into the engine once it rises above the lower level of the runner tubes.
Here is a step by step how to assemble the slide throttle thanks to Craig Smith
Paul Lamar 11/24/2007