compression range for weedie/chainsaw engines
 

hello,

wondering how much compression to expect from these engines, i have a ryobi.

also what is the usual range to expect from chainsaws

thanks

RE: compression range for weedie/chainsaw engines
 

The Ryobi 31 is weaker than most chain saw engines.

Enjoy,

Jim

RE: compression range for weedie/chainsaw engines
 

Simplest thing to do is put a compression gauge on it.

Make sure the cylinder is wet. Not dry.

On some models--you can mill a few thousanths (.001--.008) off the bottom of the cylinder head and raise the compression. There is a point where you go to far and then the piston hits the top of the cylinder. KABOOM!!

But, nothing comes free. When you drop the cylinder--you increase the compression and increase the intake duration (provided it's a piston ported design). Increasing the intake is good--ya get more fuel in.

But, at the same time--you are decreasing the exhaust duration. This is major BAD. Especially on a 2-stroke engine. Getting the spent exhaust gas out the port is crucial. Lowering your exhuaust duration is going to kill any performance gains from increased compression and intake duration.

So--then you find yourself grinding on the exhuast port to take at least as much off the top as you took off the bottom of the cylinder. And thats just to get it back to where it was to start with. If you want to increase the exhaust duration beyond what it was to start with--probably a good thing--then you'll have to take off even more from the top of the port.

How you go about grinding off the exhaust port and raising it up without ruining the chrome plating has always been a mystery to me. Some guys will say that a special rotary (carbide) bit in an air powered die grinder will do the job. Others say that disturbing the chrome plating in any way will lead to it peeling off in short order.[:o]

I've never tried it with a gas engine. I want to--but I'm scared that I'll ruin a cylinder.

Best engine to experiment with--probably a Ryobi 31cc or a Homelite 25. They are cheap and readily available. For $60 you can buy a NIB weed eater.

RE: compression range for weedie/chainsaw engines
 

I'm afraid I don't know the compression of the specific engine you're asking about but I think (on the basis of my 35cc engine) it would be somewhere in the range of 5:1 to 7:1 trapped (from exhaust port closing).

To avoid the problem of chrome peeling off during porting, you have to be careful about which way the carbide bit is spinning. It should always be in the direction of the teeth on the bit, and you should use a very high RPM with very light hand pressure on the die grinder. Or you can use a fine emery bit to achieve the same effect if your die grinder cannot be reversed to match the carbide bit.

RE: compression range for weedie/chainsaw engines
 

About 100 lbs is average. Anything under 70 lbs. is pooped and hard to start. Have ground a lot of ex. ports with carbide grinders then finish with emery strip to chamfer the edges. Never had one lose the plating. Properly plated, it won't peel. Most are Nicasil.

RE: compression range for weedie/chainsaw engines
 

Hi,

I have a 25cc Homelite, a 30cc Homelite, and a Kioritz 23.6cc that are heavily ported. No problems in many, many engine runs and flights.

AV8TOR

RE: compression range for weedie/chainsaw engines
 

Unfotunatly, in the 4 strokes it is WAY low. They are designed to run in the summer, with partially clogged fins, on cheap 87 octane gas and not ping and knock to death. We might have great cooling, 93 octane gas and the ability to run much higher CR, but there isn't much market for hopping up these engines and no high CR pistons out there. The 4 strokes also have either a geartrain or belt that makes machining down the cyl base an unreasonable option.

RE: compression range for weedie/chainsaw engines
 

Subarubrat:
Had two of those. Tough little truck.:D

RE: compression range for weedie/chainsaw engines
 

Check out mine at www.subarubrat.com we are doing some great stuff with them nowdays. You can buy 3 and 4 inch lift kits now. I fabbed a full 12 in with subframe and Tcase... and lots more.