Disassembled my 20B yesterday that I purchased from JDM two years ago. Was so nice inside that I wish I hadn't even taken it apart. No rotor ridge, oil rings look brand new. Splitting the shaft was a non event. No tool or even a hammer. Just slid it right off. I loosened the front nut about two months ago. Maybe it being loose and the temp changes made it break loose. Were 90 one day and 30 the next for a month or so. Will start doing my porting Sunday.

 

Bob Mears

 

Hear is a response to a porting question I just answered on the Nopistons web site. It may or may not be of interest.

 

Lynn E. Hanover

 

Well OK, I think its great that we can accomplish what is "in effect" a cam regrind with just a die grinder or similar tool.

First, read and memorize, all of the tech articles on Paul Yaws web site www.yawpower.com to help establish what it is you are after, and what effect each change you make in the ports, will make on performance.

Play with the Bernoulli web site and note that when a tube increases in diameter that the flow velocity decreases.

Determine what closing point you want to establish and make your own template up keyed on the dowel holes, so that you can transfer that markup to all of the irons.

use Prussian blue to mark the irons, or Dye chem, or just a Magic marker, to make a background to mark up the port location.

after you have the port marked off the way you want it, lay down several layers of duct tape just outboard of the porting lines, so you will not hose the irons when the grinder hops out of the hole. And it will.

I keep a shallow angle while porting. Here you want to change port timing with as little increase in bowl volume as is possible.

I would build one of Mr. Hanover's flow benches, and become a near genius in just a few hours, but it is not mandatory for success.

Remember that the engine with the widest power band, will outperform anything with a short peaky power band. Old saws such as "HP sells engines while torque wins races" may have been encountered. The stock Trans has wide spaced gears for easy driving, and building an engine that falls off of the tune between gears, will be slower than stock and a misery to drive. Not what you were looking for is my guess.

For two reasons, I would not move the outer line at all. That soon un supports the leading end of the side seal and leads to poor sealing, as the square tip of the seal gets rounded off. Also, should you want to Bridgeport later on, the bridge will be more narrow than is good practice.

So the upper line (the closing line) is determined by what you want from the engine. Here is what I do. This may not be universal, so others may want to jump in with their thoughts. I do the big degree wheel thing and scribe a light line along the rotor, to indicate the closing line I want.

I then remove the rotor and alter that scribed line up a bit at the outer end, and down a bit at the inner end.
That way the trailing end of the side seal will not meet the edge of the closing line all at the same time, but come back aboard the iron from the inner end to the outer end, with no drama, no bounce or chatter, and no wear at all. It is not uncommon to reuse the side seals at year end, so long as end play is below .002".

The closing line is polished to a chrome like finish to reduce side seal wear, and improve flow.


I start the side seals off at zero clearance. So long as the side seal and corner seals pop back up from spring pressure when depressed, that is all it needs to run and get a near perfect seal.

I could get a zero hot leak down on my piston engines, and very low ring drag.

For street use, you may want to run some measurable amount of clearance, but the more you start with the sooner you will need a rebuild.

The inner edge of the port is determined by the track of the outer oil scraper lip, path. In some irons there is metal available to be reduced. Just chamfer the edge of the bowl along here. Put in a tiny radius and polish it. There is nearly nothing to be gained by increasing bowl volume, as it is too large in stock trim already.

Where rules permit it, some builders fill in some bowl volume. The velocity in the runners is lost when flow reaches the increased volume of the bowl. (Bernoulli) So I just smooth and polish the outer walls where the flow tends to want to go anyway, and smooth but do not polish the inner walls where you are attempting to trip the boundary layer (a bit) and add energy to the flow in an effort to hold some of the flow against the inner part of the turn into the bowl. (Google vortex generators)

Only gross discontinuities are removed from the runners. Here, the highest possible velocity is what you are after, and increasing the diameter, slows velocity. So just a cleanup here.

Match the intake runner to the manifold. If there is a mismatch, at all make the runner bigger to avoid a sharp ledge facing the flow. An exact match is ideal. I still use a stock gasket and once the gasket fits the manifold I glue it on. Never needs to come off. If I need a new iron, I match it to that gasket, and all is well.

A broad power band is what you want. An engine with 240 HP from 6,500 to 9,600 with beat the snot out of an engine with 260 HP from 8,900 to 9,200 RPM and little else. If he cannot cover his gear spreads between shifts, your Mother can drive around him.


Lynn E. Hanover