Reducing compression ratio

[james125]

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

[stevieturbo]

Does seem like a very strange question.

If forced induction is the reason, it isnt something typically associated with 2 stroke engines.

[james125]

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.

[Retroman]

You would be best to work with the piston itself on this one.

With some pistons you can machine / skim off some metal from the top to lower the compression ratio or if not possible custom / aftermarket ones would be needed.

For the work needed (i.e the above, rejetting / swapping carb for more fuel, custom intake and exhaust manifold ect) i personally wouldn't go for it. It'll cost a lot compared to the power gain, make the engine more fragile and will be a general pain in the rear

[stevieturbo]

james125 said:

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.

Have you actually sourced a turbocharger small enough ?

[Nick1point9]

james125 said:

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.

It won't reduce the stroke, this is dictated by crank throw.

[Evoluzione]

stevieturbo said:

Have you actually sourced a turbocharger small enough ?

He says forced induction, might not be turbo.
How would a turbo go on a single cylinder anyhow? Like a one legged hamster on a wheel I would have thought....;)

A shot of Nos might be better.....

Maybe he's using an electric turbo.

Edited by Evoluzione on Wednesday 4th September 21:23

[Evoluzione]

james125 said:

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

No. 1 is the way forward, the others won't work very well.
Probably better to use high octane fuel, some clever tuning and low boost.

[P924]

How much do you want to lower the CR?

I'm guessing you don't yet know, until you know how much boost your going to run.

Out of interest which KTM is it? really want a 125 Duke

[james125]

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!

Edited by james125 on Thursday 5th September 13:31

Edited by james125 on Thursday 5th September 13:33

[Huff]

400w is just over 1/2HP to drive mechanically. Bit more with electrical losses.

0.2bar boost - supposing everything works brilliantly - means roughly pressureratio * current power output, max, less the power driving the compressor and all pumping losses.

Assuming its currently 13bhp - you'll make, what, 2HP gain overall at very best?

[james125]

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.

[HustleRussell]

Shirley it'd be easiest to fit a decompression plate or multi layer head gasket? How is the valve train driven- pushrods?

[Mr2Mike]

james125 said:

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.

The easiest way to lower compression is a thicker head gasket. Obviously this has negatives such as reduction/loss of squish band if your combustion chamber design has one, but it's cheap, simple and easily reversible. You still need to consider potential cam timing issues; if the cam chain is long enough you may be able to slot the mounting holes on the cam sprocket, assuming it's bolted to the cam like most small OHC engines.

However I suspect even the tiny Smart turbo is going to be significantly oversized for a 125cc engine. Also how do you intend to handle the fuelling requirements of a turbocharged engine? Doe the fuel injection system use MAP for engine load, and if so is the sensor usable at pressures above 1 bar (i.e. atmospheric)?

Edited by Mr2Mike on Thursday 9th January 14:09

[witko999]

james125 said:

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.

I can see how you've worked this out but it's not correct.

[McVities]

hows about a Garret GT06 - supposedly best suited for engines from 100cc-500cc

[PaulKemp]

Thicker head gasket is the easy option you'll need to do some maths but on a car 0.3mm equals about 0.5 ratio you really need to know your target ratio and boost pressure
If your dropping compression a lot then new piston with deeper bowl

[DocArbathnot]

How about a thicker base gasket?

[Kozy]

PaulKemp said:

Thicker head gasket is the easy option you'll need to do some maths but on a car 0.3mm equals about 0.5 ratio you really need to know your target ratio and boost pressure
If your dropping compression a lot then new piston with deeper bowl

Not the best solution as it opens out the squish clearance, which can actually promote detonation and have the opposite effect to that which is desired.

OP, forget 19:1 'effective compression ratio'.

Here's a simulated PV diagram for your standard engine, vs one boosted to 3psi:



Sure, peak pressure has gone up but not drastically so.

Here's how it would look at 11:1



But more interestingly, here's what it would look like if left the compression alone and simply reduced the ignition advance by 5 degrees.



Just some food for thought.

[davepoth]

I think you might be barking up the wrong tree with a turbo. Peugeot actually bolted a supercharger to a 125cc scooter, which might be something to look at.