Atkinsoning motorbike engines

[RedDevil]

Should be safe then to retard the intake valves, it only means the cutouts are there for nought, until you unretard it
You are looking for keeping it open longer during the compression stroke, but it should be well closed before the piston comes near.
Would not advance the exhaust valves though, no need to end the combustion stroke prematurely.

A metal plate behind the throttle valve... feels like venturing into Gadgetman gouge territory? Any links, any proof?
Suppose it works that would be real nice, so let's have it...?

[beatr911]

Well, let it be said that "It all depends". How far one can advance or retard cam timing depends on the engine configuration.

My experience (not just what is stated on this forum) leads me to believe that there is some room to play with cam timing on nearly every OEM engine design. Adjusting the existing cam closer toward your specific goals is entirely possible, usually with minimal cost. Remember that power output will be negatively affected at some point in the operating range. You decide what you can live with.

We are not after maximum power output here, but reduced fuel use under daily riding conditons.

Sure if one wants to spend more money, have a custom cam made, fit high compression pistons, fill and reform the ports for high velocity, etc. It's a personal choice for the goals of each person and disposable cash availability. Also, to reach the goals you'll need knowledge of how to make it all work together for best BSFC at daily riding power outputs. Most engine builders focus on maximum output which is not the same thing.

For many motorcycles, the duration of the intake cam approaches the duration an Atkinson Prius engine. I think that retarding the intake cam only, on a carbureted motorcycle engine is not likely to work due to carburetor pressure confusion. Others should try this to verify or contradict my experience to add to our collective knowledge. It costs nearly nothing and is very unlikely to harm the engine.

Retarding ONLY the intake cam reduces overlap with the exhaust, allows reversion back to the intake at low speeds, reduces dynamic (engine running) compression, allows more spark advance, increases intake valve distance to piston at TDC. Retarding both keeps overlap constant, allows reversion back to the intake at low speeds, reduces dynamic (engine running) compression, reduces exhaust cycle blow down, allows more spark advance, decreases exhaust valve distance to piston at TDC, increases intake valve distance to piston at TDC.

Advancing ONLY the intake cam increases overlap, increases dynamic compression, may require ignition retard, decreases intake valve distance to piston at TDC. Advancing both keeps overlap constant, increases dynamic compression, may require ignition retard, increases exhaust blow down, decreases intake valve distance to piston at TDC, reduces exhaust valve distance to piston at TDC. This may or may not be a big deal on your engine especially if it is operated slower which is what advancing cams optimizes.

I think I got that right.

There are negatives to each but there are also positives. Only testing will determine which solution is best for your engine configuration. Think it through, adjust your cams, turn the engine over by hand to verify you do not have piston/valve contact. Try it out!

[renault_megane_dci]

I apologise if I have been unpleasant, pedantic or whatever.

I am still reluctant to advance the inlet cam since it would require deeper valve cut outs in the piston(s) or live with a smaller than OEM valve to piston clearance (spells failure to me).

[beatr911]

No, I'm sorry. I didn't mean to come off harshly, just trying to make a clear point. I just want you (and others) to consider changing the valve timing in the situations where there is little, if any to be lost, some possibly gained and much to learn.

If you've already checked clearance and know there is not enough then the only way is how you describe. If you stated there wasn't enough clearance then I am in error.

A couple posts back I suggested (maybe not too clearly) to advance two teeth on the cam(s) turn the engine by hand to check for contact, then if OK, retard one tooth for a result of one tooth advance. This will ensure adequate valve/piston clearance. Naturally this is most useful on an engine that is already together and running well.

Experiment on! I look forward to reading what you and others will discover.

PS I've advanced the cam on my single cam truck 9 degrees with a very nice low speed torque improvement. One tooth on your bike will be more than 9 degrees I'm sure, but may very well be what you are after.

[renault_megane_dci]

I happen to own a SOHC so playing with the cam timing is always gonna be a compromise between inlet and exhaust (could not change the exhaust to inlet angle).

You mentionned your SOHC truck, I assume it is a parallel valve engine and a 2 valve and if it is not a flat head, then there is no chance the valves could ever hit the piston except maybe at full lift.
(being an austin mini enthusiast, I know than you can fit the big bore engine head to the small bore engine whereas the exhaust valve overhangs the block !)

[cRiPpLe_rOoStEr]

With a SOHC, would probably have to get a custom-profiled cam. Not a cheap game...

[serialk11r]

How about have Delta trim the exhaust cam a bit "evenly", and then have a little more overlap trimmed off the intake side? Reduced flow potential but low rpm characteristics probably improve.

[renault_megane_dci]

Actually, the target was to atkinson time a GPZ 500 but the carbs are a big issue.

So either I found an injected bike to mess with or I buy a GPZ and then an injection kit for a GPZ.

Not likely to happen for quite some time.

Also, I am worried about the antagonism between short stroke(high revving) and atkinsonned engine.
So my reference plan is to get an EL 250 (a kind of cruiser) and a GPZ or the like engine to build a squarer if not long stroke engine and then atkinson it.
Again, not likely to happen for a while since EL 250 at the right price are not very common around.

[P-hack]

I would prefer higher peak efficiency over a broad powerband.

Also, the prius and insight do not have turbo or super chargers, they are built for efficiency.

[renault_megane_dci]

Supercharging an Atkinson engine would make a Miller engine.

Broad powerband engines are a consequence of their high power to capacity ratio. They are also easier to drive «spirited»