TeeZee your posts have got me interested in reading the Pit-Lane thread.
http://www.pit-lane.biz/t3173-gp125-...vermars-part-2
Frits
I prefer not to talk about a stinger because that would imply a tube of some length. And length is not the issue here; the issue is flow resistance and diameters are far more decisive than lengths in that respect. Therefore I always apply a venturi as the flow-controlling element at the end of the reflector cone. It can be made exchangeable which allows experiments, and because the flow is controlled by the venturi, it allows for a tailpipe with a much larger diameter that is less susceptible to dents and deposit build-up. It also allows for a longer tailpipe in case you need to move a silencer out of the way.
You may be wondering about waves running up and down the tailpipe. Don't. Those waves will never be able to influence the events in the exhaust pipe because the flow through a correctly dimensioned venturi is sonic. Those waves are like birds flying against a storm: they will lose all their energy but they will never make any progress.
Your experience is correct: more flow restriction causes a higher average pressure in the exhaust pipe, hence less expansion and a higher average temperature of the gas in the pipe, and hence a higher speed of sound and a higher resonance rpm. And if you create too much flow restriction, the exhaust gases in the cylinder will not completely have left the building by the time the transfer ports open; they will enter the transfer ducts and that flow of hot gases will severely heat up the cylinder, the piston and the fresh mixture in the transfer ducts.
The required flow restriction depends solely on the amount of generated exhaust gas, and that is directly proportional to the generated horsepower. I am almost certain that somewhere on this forum I already posted a simple exhaust concept that included the venturi calculation. But I can't find it through the search option (forum search options and I never seem to get along) so here it is once more, together with a sketch of the exchangeable venturi (red in the sketch).
Brian, you are right about the flow restriction influencing the mixture strength of the MB40 model aero engine. For those of you who are less familiar with it: these model airplanes keep their fuel in a bladder. That bladder is contained in a tank, and the exhaust pipe pressure is fed to that tank. The pipe pressure tries to squeeze the bladder and the fuel is pushed to the inlet port via a regulator needle. But since the needle setting is optimized for each flight anyway, it doesn't matter if the fuel pressure is not always the same.
The cross-over connecting tube is connected to both inlet tracts between the carb and the reed valve, so the reeds will make sure that one crankcase cannot steal directly from its neighbour. What it can do, is steal some of the ram pressure that occurs at the end of each inlet phase. On the other hand, breathing will become much easier for each crankcase because the combined cross-area of both carbs is always available.
Air flow through each carb will become more uniform, so the suction signal at the carb's needle tube will be weaker. That means the mixture will become leaner unless you compensate with larger jetting. Did you work in that direction?
Jan Thiel
Temperatures at the exhaust flange were usually between 500 and 600°C if I remember well after 4,5 years!
Mid-exhaust temperatures were not taken,but I think they are higher.
The temperature sensor in the exhaust flange might have been cooled a little by escaping fresh charge!
We sometimes measured under-spark plug temperatures, they proved too high so we modified the head insert,
bringing the water nearer to the plug. This was very succesful, afterwards we never bothered with the sparkplug temperature anymore!
Some time ago engines mostly breathed air warmed by the radiator.
And do'nt forget the exhaust pipes!
Inside the fairing they heated the air quite a lot.
Putting them outside the fairing you lost revs.
The best solution was outside the fairing, but shielded from the airstream.
A friend of mine once tested his MBA 125 on a dyno. With and without fairing.
With fairing he lost 7HP!
So when I once had the opportunity of wind tunnel testing we looked at this.
There proved to be a quite big depression at the carburetors, at a wind speed of 120km/h.
This depression would of course have been even bigger had the engine been running!
The airbox resolved all these problems.
The bigger the better I heard at Aprilia!
Mostly there is not enough room at the bike however!
On the Aprila RSW the airbox was fitted on the spigot, of course with the air vents inside it.
On the RSA the carburetor was totally enclosed inside the airbox.
I would have liked to test with a ventilator giving real airspeed, but was told it to be too expensive.
Approximately 40.000 Euro!
You need a real big ventilator to simulate 240 km/h!
And of course it should have simulated airspeed going through the gears!
Then we could have mapped the ECU for all gears!
It proved to be cheaper to do this on-track and using telemetry.
When EGT goes up and up there should be some serious problem with the engine.
We never had pistons seize during our steady state tests.
Working on the dyno continuously 5 days a week!
I am 100% convinced our engine could have run for 6 hours at max power without seizing.
The problems arise when you close the throttle, or run part throttle!
The piston is mainly cooled by the transfer flow.
And at part throttle there is less transfer flow, causing detonation (auto ignition)
The entering fresh charge is ignited by the remaining, hot, burned gases!
You can see very severe damage to the piston after maybe 10 seconds at 20% throttle.
This still is an unresolved problem! I was thinking about a way to reduce engine power without closing the throttle. But how can you do this? I did not find a solution before I retired.
And nobody else was really interested.
At 100% throttle the engine was undestructible!
By making the transfer ports as wide as possible we had very good piston cooling.
As Frits has written, I would have liked to have also a dynamic testing possibility, with a flywheel.
In my opinion you should simulate a straight, starting at around 10.000 rpm, and shift through the gears
until you reach top speed. And with the airbox as used on the bike, with a ventilator that simulates the raising
air speed as it would be on track! Maybe it is interesting to know that without a ventilator the engine gave
less power with the airbox fitted.
I was told that such a testing system was too expensive.
And unnecessary as we won anyway!
I can also see a disadvantage of 'dynamic' testing.
Because the duration of the test is so short you can get away with very extreme (too extreme?) settings,
without damage.
Mic
How about much larger travel on the exhaust power valve.
Jan Thiel
This causes detonation (auto ignition)
The problem is that the burned gases do not exit the cilinder!
Retarding ignition also does not make sense.
As you have an AUTO-inition problem!
So the engine does not 'listen' to its ECU anymore!
What you would need is the same fresh gas flow, but with less HP!
Not easy to achieve!
A variable tailpipe might help.
Frits
Like Jan wrote, a shorter exhaust duration will worsen the detonation. What happens is this:
During normal operation, the blowdown time.area of the exhaust ports is sufficient to drop the cylinder pressure below the crankcase pressure before the transfer ports open, even at high rpm.
At part-throttle that cylinder pressure will drop to the same level, but now the crankcase pressure is much lower and exhaust gases will enter the transfer ducts, contaminating and heating the fresh mixture even before it enters the cylinder.
A theoretical solution would be a power valve that enlarges the normal exhaust timing instead of lowering it. But that is impractical as it would ruin the shape of the exhaust duct and it would cause cooling problems in the cylinder's exhaust area.
A variable tailpipe area, like Jan says, can be a more practical approach. I designed a simple solution, shown in the drawing below, but then two-stroke development at Aprilia was terminated because of Dorna's ban on two-strokes.
Jan Thiel
This could very well have been the solution.
Frits
As Jan pointed out, once you have auto-ignition, the engine does not listen to its ECU any more. So you would have to start skipping sparks well before the onset of detonation.
In a foul-stroke your proposed system does work, but a two-stroke would react far from linear. For example, if you skip 1 in 4 sparks, you will loose much more than 25% of engine power because that one missing spark will cause the gasdynamics processes to collapse. The main problem would be to realize a smooth transition from intermittent to full ignition.
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Originally Posted by Katman
. GPR50 from the looks. Dave Ds old Techo mags at a guess too.
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