Engine question

[Driftwood]

I'm new to nitro buggies but I am now hooked on them! So much faster in my garden than my old Tamiya Dirt Thrasher! Too fast actually! Anyway, being new to nitro power I damaged the pull starter during my attempts to start the damn thing. Hopefully I haven't damaged the engine too, as it may have flooded. Have now worked out how to start it, but during the repair I had to remove the exhaust system. Whilst it was off I had a look down the exhaust port and noticed that the bottom edge of the hole in the liner is higher than the bottom edge of the port by a good few mm and that the piston crown at b.d.c. is lower than the bottom edge of the hole in the liner by about the same amount. Is this normal? Experience of 2 strokes in 'bikes says not and that it could restrict power, but rc motors could be completely different especially as they don't use reed vaves. Or an ignition system.
The motor in question is a Force 21 fitted to a G.V. Rex-X. Are they any good? Any recommended modifications?
More questions to follow, no doubt...

[dave g]

all depends on the port timing,if you lower the exhaust port without checking the timing of the crankvalve timing and the inlet and boost port transfer ports you could end up with throw away piston and liner.

most of the engines you see will have the lip of the liner showing thru the exhaust port,it doesnt mean the exhaust port timing isnt open right,it just means the crankcasing is open more then the exhaust port.

[Driftwood]

If it was a 4 stroke, the "lip" might be there to stop burnt exhaust gas being drawn back into the cylinder. But, being a 2 stroke you could almost run the engine on what comes out of the exhaust as there is a lot of unburnt fuel going staight through the engine and hence the need for an exhaust expansion chamber to reflect it back in. The bottom of the port wont affect the timing, only the size of the hole and therefore the amount of gas flowing through. Has anyone out there matched the bottom of the port with the piston crown at b.d.c.? Noticed any difference in engine performance?

[mark christopher]

Originally Posted by Driftwood

If it was a 4 stroke, the "lip" might be there to stop burnt exhaust gas being drawn back into the cylinder. But, being a 2 stroke you could almost run the engine on what comes out of the exhaust as there is a lot of unburnt fuel going staight through the engine and hence the need for an exhaust expansion chamber to reflect it back in. The bottom of the port wont affect the timing, only the size of the hole and therefore the amount of gas flowing through. Has anyone out there matched the bottom of the port with the piston crown at b.d.c.? Noticed any difference in engine performance?

?EH?

they dont use lips to stop gases going back, they have exhaust valves for that.

id like to see you make a 2 stroke run on its fumes, the fuel is burnt the oil is what comes out of the exhaust.

the top of the exhaust port does effect timing (exhaust port timing) it also effect the scavenging effect of 2 stroke

the expansion chamber/pipe is to scavenge the cyliner to draw more fuel/air in and act as a valve using air pulses.

[BRS]

Usualy the manufacturers make an engine case to be used by a variety of models. Same case different internals, its normal to see the liner not lining up perfectly with the cases, especialy on the exhaust side.

If you want to have a play with it get your dremmel out but dont go crying to anyone if you fuck it up

If you want more power/revs and there is room to raise the exhaust port height thats the way to go. But you will also need to look at opening up the inlet side of the crank.

Dont blame me if it ends up slower

[dave g]

actually a small amount of unburnt fuel does pass thru into the manifold,the return pulse wave from the cones in the exhaust chamber help to push that back into the combustion chamber where it meets the fresh fuel being drawn in from the rotary valve,if we didnt have the pulse wave you would see some unburnt fuel dripping from the exhaust stinger and poor fuel economy as you would have to richen the engine up somewhat.

[Driftwood]

Thanks for the responses to my query. I think BRS's comment,

Usualy the manufacturers make an engine case to be used by a variety of models. Same case different internals, its normal to see the liner not lining up perfectly with the cases, especialy on the exhaust side

is the most likely reason as to why the liner doesn't match the port. Maybe grinding the liner away could unleash hidden power but I don't think I'll bother 'cos of this prophetic warning

If you want to have a play with it get your dremmel out but dont go crying to anyone if you fuck it up

By the way, lowering the port's bottom edge won't affect the port timing. It will still open and close at the same time and therefore it's duration isn't affected. It will just be bigger. And, because the transfer ports are open at the same time as the exhaust port, lots of unburnt fuel passes straight through the engine into the exhaust. Good job someone invented the expansion exhaust system otherwise 2 strokes would lack a lot of thier power and be even more thirsty for fuel!