2 cycle performance and tuning

[itjstagame]

I just picked up an old 2 cycle (because it was free :p) and I'm curious about some of the fundamental differences between 2 cycle and 4 cycle and tuning there of.

This is a 1960 125cc 1cylinder and while I figured being lighter and older it might be copareable to a newer 200 or 250cc 4 cycle the listed HP is quite low. Looking at engine specs it's compression is 6.8:1 !!

Now if this were any other engine I'd want that to be much much higher, so I'm wondering if there were reasons for having it so low, 2 cycle releated reasons.

I haven't removed the head yet (but it's only a few bolts, heh) but I suspect I can't improve too much with just smaller HG or shaved head like I would on a normal car, but IF some how I could get to 9 or 10:1 and just ran with 93 instead of 87, would I be ok? Or would it be too much force for the 2 cycle or something or may run too hot for the 2 cycle? If I try to improve the head cooling (since obviously this doesn't even have oil cooling with no oiling system) would it work? Maybe thermal glue a ton of old computer heatsinks to the existing fins to add surface area? And add a simply thermal probe and 120mm computer case fan to actively cool the heat if it hits a certain temperature and I'm going less than 20MPH or something?

Thoughts? I know I should probably just be happy to put around on my 60s conversation piece, but.... in the words of Tim Allen "MORE POWER!".

[mydlyfkryzis]

The compression can be raised, but there may be inadequate cooling.

Playing around with exhaust expansion chambers can help, playing around with the cylinder porting can help. But it is easy to turn the motor into scrap metal real easily.

The real problem is that 2 cycles, as they are "hopped up", become real peaky, with a lot of HP in a small band of RPM.

I'd get it running well stock and just enjoy the 60's, rather than create a 125cc paperweight

[BuzZz]

You're tap dancing in a minefield.

The reason the engine has such a low comp ratio is that a 2 stroke has primary and secondary compression. What is listed, and what your looking at with 6.8:1 is the secondary comp. This is measured above the piston, in the cylinder. Primary compression is below the piston, in the crankcase. It is created by the piston coming down into the crankcase, pressurizing the intake charge to force it into the cylinder when the piston moves past the intake transfer ports. So the intake charge coming into the cylinder is already under compression, add that to the 6.8:1 ratio the piston creates and it's really about 9-10:1 overall. More like about 8.5:1 in reality, but that's semantics.

You can get a very little bit more punch by shaving the head, but it probably isn't worth it. Performance will not be increased very much, you will run into pre-ignition issues and even if you did get a significant power boost, the rest of the engine is 40+ years old and never designed to handle more jam in the first place. Something else will fail, probably the rod bearings. You will also put more stress on the very critical crankshaft seals with any extra compression, and when they leak, the engine seizes.

If heat is really eating at your mind, fashion some sort of scoops to blow more air across the engine. It will be more effective than gluing a bunch of heatsinks from an electrical device all over it.

Best bet is to leave it stock. Rebuild it if you want, but forget about wringing more power out of it. Even if you did get it to make more steam, the rest of the engine/tranny won't like it, and will let you know by failing down the road.

[Axman]

Many moons ago I had a 1973 Yamaha DL250 which was a two cylinder two stroke engine. The only difference is my bike used an oil injector system. There really is no need to up any compression in the bike you should find that it is plenty powerful to get moving. My DL250 weighed 625 lbs and it still would do a wheelie quite well for a relatively heavy bike.

[crazypj]

Just to clear things up.
The real reason the compression is so low is its the UNCORRECTED compression ratio.
In a fourstroke compression is worked out from bottom dead centre to top dead centre. (swept volume )
In two stroke, COMPRESSION CANNOT START UNTIL THE EXHAUST PORT CLOSES.
Compression starts when piston is at least half way up cylinder so with your 6.8 its probably closer to 13:1, in reality a little bit lower due o internal friction

The primary compression is crankcase compression which transfers mixture from crankcase through transfer ports to cylinder/combustion chamber. and is usually less than 2~3psi. (you use 8.5~9psi to check seals/gaskets for leaks)
If you measure crankcase volume, (including area under piston) and divide it by swept volume, its really low)
Corrected compression ratio will be closer to 12.5:1 or more, even in a 1960's engine. you can work it out by calculating the swept volume from exhaust port closing and dividing by 14.7 ( gets you close to combustion chamber volume) then work out swept volume from bore and stroke, plus combustion chamber volume divide answer by ccv. Not perfect but close enough