Basics of Boosting Engine Power

[Steve Best]

i took off 2 mm and had to put in a gasket because i thought i could hear knocking . a friend just said it was the massive compression but it works well . mine was lathed so i know it was 2 mm for sure . 1.5 it optimum stock

Hi Timmy, this was an old thread but Jag had it nailed pretty good back then and is a pretty good reference on where to start even now. The balance on new engines seems to be better, so not as much a concern.

Now the bad news. The stock head is weak and warps as is. Taking 2mm off it will weaken it even further. You need a new head. That is why the recommendation is to only sand it enough to get it flat. Usually that is about 0.1mm or so.

The best way to raise compression on the stock head is to set the squish distance (between piston and head) to a recommended 0.5mm to 1.0mm range. First you have to measure it with 1.5mm soft solder (rosin core electronic solder works well). Put it on top of the piston parallel with the pin and squish it over top dead center. Then measure it. Then replace the headgasket with twisted teflon plumbers tape and shim the base gaskets to get the gap you need. This also raises the cylinder for a bit more rpm.

Raising the cylinder often kills a bit of low end power, but this is usually made up fro by the raise in compression. Compression usually mostly affects very low rpm torque. Compression also increases head, piston and cylinder heating, detonation and stripping the chrome from the cylinder walls, so go cautiously here.

One thing I would suggest is NOT port matching the intake to the cylinder. This mismatch helps low end power a lot. Do all your mods one at a time and you will see the benefit or loss of each mod.

Widening the intake and the exhaust port AT THE CYLINDER WINDOW will improve torque and rpm. do it at the top of the exhaust and bottom of the intake. Here is a terrible picture of what you are trying to achieve on the exhaust port:

roughly this shape:

Stock width (untouched) at the bottom, the width increase only at the top and only extending about 6mm into the port. Don't widen the entire port all the way to the pipe. You will kill power. A bit of radius on the top will save your ring life.

Extended tip plugs worked worse in my testing. No power improvement to iridium either, although they may last longer and combat fouling from poor jetting. Jet correctly and you won't have the problem.

Combustion chamber shape, compression, pipe and intake design will all contribute to better low rpm power.

The stock NT carb seems to wok pretty good and make as much power as any other 14mm carb. No need to go any larger until you are making double the stock hp.

The biggest thing to building power is measurable testing. Have a hill and a speedo or stopwatch so you can quantify everything you do. Keep notes. Do mods one at a time ONLY. These motors are so simple that cylinders, heads, carbs and exhausts can be swapped in 15 minutes.