How to Effectively Lower Compression In Motorbikes

[drabdullahorth]

As stated above, I need all the reccomendations I can get on lowering it as much as possible, as I need to do so for a project.

A few ideas pop up in my mind:

1. Stacking Head Gaskets. I don't know whether will it work great or not, looking forward for some advice.

2. Retarding ignition timing. Dunno how to do that in a CDI controlled bike.

3. Converting pistons to low compression. I can maybe shave a bit off the top?

[helper_learner]

Stacking Head Gaskets. I don't know whether will it work great or not, looking forward for some advice.

You can do it by changing the timing and space of exhaust valve and intake.

Retarding ignition timing. Dunno how to do that in a CDI controlled bike.

Best is to use adjustable cdi.

[drabdullahorth]

"You can do it by changing the timing and space of exhaust valve and intake."

Any guides on how to?

[VCheng]

As stated above, I need all the reccomendations I can get on lowering it as much as possible, as I need to do so for a project.

Four stroke or two stroke? How is the valvetrain being driven? Those factors will determine the optimal solution, as will the machining facilities and budget available

Changing ignition timing does nothing to change the compression ratio, which is determined by mechanical factors, as the ratio between the volumes at BDC and TDC.

[drabdullahorth]

4 stroke.
Chain drive, if that is what you mean.
My project is aimed to be as reliable, beautiful, and cheap as possible.
When you retard ignition timing, the fuel-air mix flagrates (i.e. burns) earlier, at a lower compression. This prevents knocking/detonation.

Of course, the mechanical compression doesn't change, my bad. I stated the question a bit confusingly.

[drabdullahorth]

Nobody has answered yet the trick of stacking head gaskets. Why?

[VCheng]

What is the goal here? Preventing premature detonation, or something else? Please note that retarding the timing delays the spark relative to the piston stroke cycle, so the burn is initiated later, not earlier, and, as you note, it does not change the compression itself.

Stacking head gaskets on a four stroke will need adjustment of the timing chain, and adjusting the valve timing accordingly since all the factory marks will be off.

[drabdullahorth]

Preventing detonation. Preventing any damage to the engine.

I actually want to change the ignition timing to prevent knocking. That was my original question.

For stacking head gaskets, how to adjust the valvetrain to match it perfectly?

[VCheng]

Is the engine new or old? If it has excessive carbon build up, that will form a detonation nidus independent of timing or even fuel quality. Changing compression is not easy to do properly after manufacture.

[drabdullahorth]

I'll take care of that. Assume that the engine is in pristine condition. I know how to overhaul and service bike engines. And I am ready to go the extra mile with regards to doing the job. I just need to learn how.

[VCheng]

If the engine is in "pristine" condition, then why is it having detonation problems? Going the proverbial extra mile is useful only if it is in the right direction.

[drabdullahorth]

Oh well, you are forcing me to confess, aren't you. I plan to turbo. Really hard.

[VCheng]

Okay then. Now you are talking a very different ballgame. What will be required will depend on the degree of boost, and preventing detonation will be only a minor issue compared to others that need to be solved first. I can only wish you good luck since it seems clear you need far more knowledge than is suggested by the posts above to undertake such a project by yourself.

[drabdullahorth]

Yep, I am researching a lot. This is only a small problem, which I thought to solve in the meanwhile. Expect to see a 25hp+ Rickshaw Engine Ybr in under a year.

You still didn't guide me properly on how to reduce the damn compression!

[VCheng]

Simply going to a larger naturally aspirated engine capacity will solve the issue in a more economical and reliable manner in that horsepower range.

[drabdullahorth]

125cc frames have a limit to how big the engines can be. Yamaha YBR 250 is a different and stronger frame altogether. Also, prices abhorrently skyrocket when you go for bigger engines. Furthermore, parts availability for these 40+yr old engines is pretty poor. Horsepower increases, by forced induction, if done right, can give phenomenal results. One guy, using blow-through carb turbocharging on a cheap XR200 clone, managed to get it to 67whp! I'll post his comment down below.

[VCheng]

One guy, using blow-through carb turbocharging on a cheap XR200 clone, managed to get it to 67whp! I'll post his comment down below.

At what cost? What is the cheaper and more reliable way of achieving the same or better results? I could probably get 50 hp from a 200 cc engine, but the real question is why? As a learning project, sure, but as an engineering decision, specially for mass or even limited production? Nope.

[drabdullahorth]

Not intended for production at all. This will be intended to be a personal project, my own. Where did you get that idea from?

By the way, we steered off topic here. I still need to learn on ways to reduce compression on these engines.

[helper_learner]

You can reduce the piston length by reducing the rod length to reduce the compression. Best way.

[drabdullahorth]

By cutting the rod, you mean? Thst will be horrible for the rod tensile strength, which (If you have observed from the little back-and-forth above) is not suitable for my application.