[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

Frits Overmars a écrit:

Daniel a écrit:

I agree, that the larger the crankcase volume, the smaller the pressure drop will be. But in my mind this is, because the pressure in the crankcase right before transfer opening is already lower with a large crankcase than with a small crankcase.

If the crankcase would behave like a pump, you would be right. But when the engine is running in the power band, it is no longer a pump; it is a resonator.

I know that it is more a resonator than a pump. But I thought that it is still "a bit" a pump (factor 1 in my "theory"). To be precise, I thought that it is a pump exactly until that point, where the transfers open. And so I thought, that the crankcase pressure was lower right before transfer opening with a larger crankcase, because the compression of the relatively small piston results in a higher pressure if acting on a smaller volume. Maybe I forget the influences of the previous cycle

All in all: You still think, the bigger, the better, Frits?

Daniel A. a écrit:

Frits Overmars a écrit:

If the crankcase would behave like a pump, you would be right. But when the engine is running in the power band, it is no longer a pump; it is a resonator.

I thought that it is a pump exactly until that point, where the transfers open.

That is right. But it is a very ineffective pump: its volume ratio is only between inlet closing and transfer opening.

Citation :

I thought, that the crankcase pressure was lower right before transfer opening with a larger crankcase. Maybe I forget the influences of the previous cycle

Yes, that may be the case.

Citation :

All in all: You still think, the bigger, the better, Frits?

A large crankcase volume is good but with a very large case the flow velocity inside the case will become so low that fuel and air separate. If we had direct fuel injection, I would definitely say 'the bigger the better'. In practice this means opening the crankcase to the atmosphere as soon as the pressure inside the case allows it. With a good engine running at rpm of maximum torque that would be about 15° after BDC. In theory that is also valid for an engine with a carburetter but in practice the pulse signal to the carburetter jets would be so weak that it would be difficult to achieve good carburation.

Okay, so no matter if the crankcase is "small" or "large", in the one case the pump is "very ineffective" and in the other case it is "very very ineffective". So regarding this the crankcase volume doesn't allow big differences.

But I still don't understand, why a smaller pressure drop means, that the mixture is delivered with a higher average pressure :/

Regards

Sorry to get in between your conversations but only for this simple mathematical idea

Inicial pressure - final pressure/2 = mean (average) pressure

If final pressure is near inicial pressure -> small pressure drop, mean (average) pressure value remains higher than if final pressure value is lower - > bigger pressure drop

Thank you Frits and Jan for this great topic that allowed me to confirm some ideas that were wrong in books but right for my brain

Regards

I asked this question on the kiwi biker forum but couldn't get an answer. On the rsa did you try running it with a wasted spark on the dyno and if so was there a gain to be had at all. I know on a two stroke there is gains to be had by indexing the plug to help reduce fouling of the plug. I am wondering the extra spark at bdc could help achieve the same thing

There was no wasted spark on the RSA.
What do you mean by 'indexing the plug'?

By aiming the side electrode of a spark plug towards the exhaust port on a two stroke jetski engine we used to get another 150 rpm , 1mph on a full throttle run.
It doesn't sound much but when your drag racing towards the first turn it was worth it. It was a back to back increase every time

wax a écrit:

By aiming the side electrode of a spark plug towards the exhaust port on a two stroke jetski engine we used to get another 150 rpm , 1mph on a full throttle run.
It doesn't sound much but when your drag racing towards the first turn it was worth it. It was a back to back increase every time

Really?
Why that should happen? Shouldn't the spark be at the center always?

yes it is, but the side electrode is not

And what is the root cause of the improvement?
Better plug cooling, or there is a better start of the combustion?

The root cause of the improvement well its hard to prove what causes it. Im thinking better combustion. but you cant deny the facts on a flat out run there was roughly 150 rpm and 1mph top speed gain up the start straight when your going for the hole shot. you can back to back it all the time and get the same results. Im actually quite shocked that it seems like a new thing to some people

Could you make a little sketch please for better understanding? :)

Regards,
Daniel

I don't know if this was asked earlier, I am more or less certain that there is no gain, but I would like a hint from the experts.

Does covering the header of the exhaust with some isolating material have any benefit in performance?

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romeuh80 a écrit:

I don't know if this was asked earlier, I am more or less certain that there is no gain, but I would like a hint from the experts.

Does covering the header of the exhaust with some isolating material have any benefit in performance?

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No, I do'nt think so.
But it helps to make the engine reach the same revs on the circuit as it does on the dyno.
And it could help to keep down crankcase temperature.
I would have liked to paint the exhaust with isolating paint.
Preferably on the inside!
Doing so might have made different exhaust dimensions necessary.
It would have been very interesting.
But I never had the time to do it, which I still regret!

Working at the dyno daily, the big-end was changed after about 220 tests.
Each test taking about 6 minutes.
And also the exhaust was cleaned at the inside, removing the carbon deposit.
After this it took about a week for the same maximum power to return.
And you also needed a richer carburation, because with a clean exhaust pipe the engine had more tendency to detonate.
The teams that went to the races always cleaned their exhaust pipes on the saturday evening, before the race.
I tried to convince them not to do this, but nobody believed me!
To a lesser degree the same thing happened with the dampers.
Filling them with new glass fiber always gave less power and more detonation, for about 3-4 days!

romeuh80 a écrit:

I don't know if this was asked earlier, I am more or less certain that there is no gain, but I would like a hint from the experts.

That's a pretty trick NX

A question for Jan and Frits, with all your knowledge on disc valve engines. How much of a power impovement could be made from a late Suzuki RG500 power valvfrom a curious 2-stroke nut. Thanks in advance. P.G.

sorry all, last post incomplete. That a Mk10 RG500 power valve motor, currently producing about 130 bhp.
Thank again.

Paul Gane a écrit:

A question for Jan and Frits, with all your knowledge on disc valve engines. How much of a power impovement could be made from a late Suzuki RG500 power valvfrom a curious 2-stroke nut. Thanks in advance. P.G.

That is an impossible question to anwer, Paul. I do not even know whether a Suzuki MK10 RG500 is a roadracer or a street bike, let alone what the tech specs look like.

If you plan to modify the original Suzuki parts, it would depend on your skills, which I have no idea of, and on the suitability of those parts: is there material where you need it, are they strong enough, etc.

If you have unlimited funds, you could fit carbs, disk valves, crankshafts, pistons, cylinders, heads, pipes and electronics from the Aprilia RSA and then it should be able to produce over 200 rear wheel HP. But the cooling would be grossly insufficient, the clutch and gearbox would probable not live very long, etc.

wax a écrit:

I asked this question on the kiwi biker forum but couldn't get an answer. On the rsa did you try running it with a wasted spark on the dyno and if so was there a gain to be had at all. I know on a two stroke there is gains to be had by indexing the plug to help reduce fouling of the plug. I am wondering the extra spark at bdc could help achieve the same thing

If my memory doesn't deceive me, Frits has said in the past that the 'gap' between the electrodes should face towards the inlet side -for regular plugs- in order for the fresh stream to scavenge the residuals from the area inbetween.
For racing type plugs, such as the RSA's, it should be placed looking either right or left (the front being the exhaust), for the same reason as above.

Frits, I hope it is not the other way around regarding the two plug types..

Spoiler:

Jan Thiel a écrit:

Working at the dyno daily, the big-end was changed after about 220 tests.
Each test taking about 6 minutes.
And also the exhaust was cleaned at the inside, removing the carbon deposit.
After this it took about a week for the same maximum power to return.
And you also needed a richer carburation, because with a clean exhaust pipe the engine had more tendency to detonate.
The teams that went to the races always cleaned their exhaust pipes on the saturday evening, before the race.
I tried to convince them not to do this, but nobody believed me!
To a lesser degree the same thing happened with the dampers.
Filling them with new glass fiber always gave less power and more detonation, for about 3-4 days!

Thanks for that Jan, very interesting! Do you remember how much power was down with the cleaned exhaust pipe? And did you try to build an exhaust pipe with slightly smaller internal diameters to have the same power with a clean exhaust?

Haufen a écrit:

Jan Thiel a écrit:

Working at the dyno daily, the big-end was changed after about 220 tests.
Each test taking about 6 minutes.
And also the exhaust was cleaned at the inside, removing the carbon deposit.
After this it took about a week for the same maximum power to return.
And you also needed a richer carburation, because with a clean exhaust pipe the engine had more tendency to detonate.
The teams that went to the races always cleaned their exhaust pipes on the saturday evening, before the race.
I tried to convince them not to do this, but nobody believed me!
To a lesser degree the same thing happened with the dampers.
Filling them with new glass fiber always gave less power and more detonation, for about 3-4 days!

Thanks for that Jan, very interesting! Do you remember how much power was down with the cleaned exhaust pipe? And did you try to build an exhaust pipe with slightly smaller internal diameters to have the same power with a clean exhaust?

Maybe a little carbon filling the "safe" spaces in the exhaust would help the effect? Or is nonsense

I am testing tail pipe restrictors. I don't have a Dyno to test the different sizes. And I not sure of any difference, I was with ~21 baffle end, putted one 20mm restrictor, tested and I am not sure off any difference, carburation seems to be about the same, overev and power too....With 20,5 same happens.

I Know this was discussed a bit earlier in this topic, maybe I should post this in another topic. But I would like straight words from everyone with experience, not just frits or Jan on what happens to power curve, carburation, etc when changing only this restrictor.

Would it only change the peak power upper a bit, would it give more power?

About linning the exterior of the exhaust it insulating materials, be it paint or aluminium tape, perhaps ceramic coating like is used in four stroke after market exhaust lines might be a nice thing to try/test, it works wonders in the four bangers..

wax a écrit:

On the rsa did you try running it with a wasted spark

Wasted spark on a two stroke? What would that be, firing both plugs together on a 180° twin?

GrahamB a écrit:

wax a écrit:

On the rsa did you try running it with a wasted spark

Wasted spark on a two stroke? What would that be, firing both plugs together on a 180° twin?

firing the spark plug at bdc 180 degrees apart ???
Like they do on two stroke jetski engines

Regarding crankcase volume,will making it as small as possible make it more efficient as a pump
and as large as possible more like a resonator?relatively speaking.