Reducing compression ratio
Discussion
Hello everyone. First post so forgive me if I have posted in the wrong area.
I'm looking at reducing the compression ratio of my motorcycle. It's a KTM DUKE 125cc, 58mm bore, 47.2mm stroke 12.8:1 CR. I think I have a few options but I'm not sure which is best;
1) Change the piston for a deep-dished/low compression one. -I'd have to either buy a new piston or take my existing one out and measure it first in order to specify a new one.
2) Increase the thickness of the cylinder base gasket. -Simple to do, however I'm not sure if the timing chain will be able to stretch that far. Are they easy to extend?
3) Reduce the con rod length. - I dislike this, it'll require splitting the engine crankcase and the stroke will be shorter which may reduce power.
Thoughts? Thanks
I'm looking at reducing the compression ratio of my motorcycle. It's a KTM DUKE 125cc, 58mm bore, 47.2mm stroke 12.8:1 CR. I think I have a few options but I'm not sure which is best;
1) Change the piston for a deep-dished/low compression one. -I'd have to either buy a new piston or take my existing one out and measure it first in order to specify a new one.
2) Increase the thickness of the cylinder base gasket. -Simple to do, however I'm not sure if the timing chain will be able to stretch that far. Are they easy to extend?
3) Reduce the con rod length. - I dislike this, it'll require splitting the engine crankcase and the stroke will be shorter which may reduce power.
Thoughts? Thanks
It's 4 stroke. And yes I was thinking about forced induction of some sort. I know everyone would say buy a bigger bike, but I already one of those, this is just a fun little project as such.
I do have access to a milling machine, but finding a suitable grade of aluminium to produce a cylinder seems unlikely.
I do have access to a milling machine, but finding a suitable grade of aluminium to produce a cylinder seems unlikely.
I was thinking perhaps dismantling a old diesel turbocharger, and powering the compressor with an RC-car electric motor. -This does limit me to around 400W from the alternator but that should provide an additional 0.25BAR for continuous use. I'd like to put it up to 1BAR but I'd need 1.6kW of power for the compressor. From my limited knowledge a turbo won't work very well for a single cylinder with such variable exhaust gas flow.
I wouldn't advise getting a KTM DUKE 125, it's built very cheaply, the ECU has a top speed limit of 70mph. Get a Yamaha R125 or a Honda CBR 125. Lots more parts for those bikes as well!
I wouldn't advise getting a KTM DUKE 125, it's built very cheaply, the ECU has a top speed limit of 70mph. Get a Yamaha R125 or a Honda CBR 125. Lots more parts for those bikes as well!
Edited by james125 on Thursday 5th September 13:31
Edited by james125 on Thursday 5th September 13:33
I'm now looking into a small tiny turbocharger, a kp31 turbo from a Smart Fortwo. It makes 45hp on the smart which I'm probably looking at 30hp so even though it is small perhaps still on the large side.
Still looking at lowering the compression ratio. I realise I was wrong about the connecting rod, but if it is too short it'll hit the piston skirt. The piston seems to be the easiest option.
Of course I could negate any change to the engines natural compression ratio and simply bolt on the turbo, but even at some light 0.5 bar boost, that's an effective 19.2:1 compression ratio.
Still looking at lowering the compression ratio. I realise I was wrong about the connecting rod, but if it is too short it'll hit the piston skirt. The piston seems to be the easiest option.
Of course I could negate any change to the engines natural compression ratio and simply bolt on the turbo, but even at some light 0.5 bar boost, that's an effective 19.2:1 compression ratio.
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