This is how I measured my 200Ts squish clearance

[MattG]

It's Shed tuning, Spud Not commercially viable granted, but fun for those that enjoy that sort of thing.

 

But in fairness if you can run the piston up close to the squish band it will reward you with a little bit of extra oomph but more importantly an all round nicer engine. The turbulence you can get from tight squish is underrated IMHO, a more dependable / reliable combustion process due to the extra gas movement, so even if the mixture isn't spot on (it never is, through the whole rev range) - combustion will be better. So hopefully a more crisp engine response. Just makes the saw nicer to use. Innit....

 

Starting it might need a bit more care though, especially a top handle saw like this one - you listening MattG?

 

bmp01

Ha ha! You wanna try drop starting my 064 on a cold day, cos it aint got a decomp......so the 200 with raised comp will be a piece of p!$$ !

 

But yeah, getting squish right, also means more of the fuel/air charge will be confined to the actual combustion, and yes, thats the crucial detail.....not just a marginal compression ratio increase.

 

Matt

[MattG]

A couple of piccies as promised:

Firstly here's the setup rig to machine the squish band.....started on P80, then worked down to P240. Secondly, after a few minutes grinding, managed to rid the squish band of those stupid manufacturing artefacts, and hey presto, now have a proper squish.

[bmp01]

Ok, let me explain it again. .... don't use the hammer to flatten those lumps. Get-it?

 

Well, it looks shiny. Interested to see how it goes now.

 

Regarding the starting thing, it is worth labouring the point as it's a safety thing.

Its not the force needed to start the thing - I'm sure you will cope with that.

It IS about how much the saw reacts to the the pull cord force. In this respect a lighter saw and especially a top handle one is worse than a heavy one, we all know that. But adding in the higher compression on a small saw really makes things surprisingly jumpy.

 

bmp01

[spudulike]

The elasto start mechanism helps a lot once the compression gets up to the 200psi mark!

[MattG]

I know what you mean on the safety thing. Even a small saw can kick back nasty on a misfire at start time. FWIW I always cold/hot start anything with the chain break on, I've got a pretty face and I sure don't want it scratching!!

 

Well, the next job, is that given that I've lowered the cyl by 0.5mm by the gasket delete....the next thing is to raise the exhaust port roof by the corresponding value - I'm not after higher revs - just perfecting the specification of things as much as I can. And hopefully maximising the torque output.

[bmp01]

Well, the next job, is that given that I've lowered the cyl by 0.5mm by the gasket delete....the next thing is to raise the exhaust port roof by the corresponding value - I'm not after higher revs - just perfecting the specification of things as much as I can. And hopefully maximising the torque output.

 

That's a whole big can of worms you're about to open. ...

By the same logic you'd need to change inlet and transfers too, except with the inlet that would be adding material. And in raising the exhaust you are effectively undoing the compression ratio increase.

I think if you are contemplating that level of change, rather than blindly following the manufacturers spec you would be better to aim for a known tuning spec, which will be driven by opening and closing timing (angle in crankshaft degrees). Folded into that you need to look at port and passageway areas.......

 

My going in point (with out research) would be to leave timing as it falls with the lowered cylinder, 0.5 mm isn't going to equate to very many degrees. Go wider on the exhaust port would be the next consideration. Exhaust is pretty torturous on these though.

Also, consider modifying piston with chamfer to change timing - gives option of a 'u' turn as you can subsequently put std piston in.

 

bmp01

Edited May 5, 2017 by bmp01
additional info added

[MattG]

I think I'll be good

 

I'm not trick enough to dare to touch the transfers, and the gasket delete will have increase inlet duration already.

 

10-4 on what you said re. the CR, however at operating revs. the actual cranking pressure will be different.

 

I've done this stuff before like 25 years old on MX bikes, widening ports (in my humble opinion) is more contentious, since you can run into ring bulge issues. Experienced this myself, when I made my Yammy DT175's exh too wide. F**king blistering performance, but destroyed piston and rings every 400 miles!!!

[spudulike]

Something tells me that you will get 190-200psi on a healthy MS200 and will gain torque and speed in the cut:thumbup:

[bmp01]

I think I'll be good

 

I'm not trick enough to dare to touch the transfers, and the gasket delete will have increase inlet duration already.

 

10-4 on what you said re. the CR, however at operating revs. the actual cranking pressure will be different.

 

I've done this stuff before like 25 years old on MX bikes, widening ports (in my humble opinion) is more contentious, since you can run into ring bulge issues. Experienced this myself, when I made my Yammy DT175's exh too wide. F**king blistering performance, but destroyed piston and rings every 400 miles!!!

 

I think it will be good too. Update when it's done please.

 

 

DT's, that brings back some memories. I had a secondhand125 air cooled that came with an aftermarket exhaust. On / off power band, noise, blue smoke, piston slap - come to mind. Happy days.

 

There a rule about exhaust port width vs cylinder diameter. I wonder whIch side of the line your DT was

 

bmp01

[MattG]

I think it will be good too. Update when it's done please.

 

 

DT's, that brings back some memories. I had a secondhand125 air cooled that came with an aftermarket exhaust. On / off power band, noise, blue smoke, piston slap - come to mind. Happy days.

 

There a rule about exhaust port width vs cylinder diameter. I wonder whIch side of the line your DT was

 

bmp01

Yup, I'll certainly keep you updated.... I've just very carefully taken off ~0.5mm of the exh roof. Much trickier than my old Yammies (mine was air cooled too - DT175MX - cracking bike), and now the jug's all slapped back down with a slip of sealant (instead of the stock gasket). I'm gonna give it a final press/vac test, though I can't see why the base wont seal pretty nice as is, but just to be sure.

 

I probably won't touch it for a little while now, plenty more things to do round here alas. But I'll get a youtube of her running when it's all good.