[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked)

Hello:
My name is Jeff Neilan and I live in Somerset, Pa. USA. I've been reading this post and gathering all the info I can. Thanks to everyone.
I have a question for Howard Gifford. You said you incorporated some of the wisdom in this thread into your 440 cc motor. Could you elaborate more on what you did and what increases in power did you notice with each change or modification? Nice work on moding the rotary valve timing.
Also very nice work on hydroforming the pipes.
Thank you.

Frits you said you had those plugs for the gudgeon pin in 12mm. Do you care to sell any at all ??

Hi Jeff. Where I made gains was in the porting. I basically copied as close as one can the port arrangement of the RSA. Look carefully at the photos of the RSA cylinder and you will see that the cylinder wall dividers are quite narrow. With careful porting I widened and squared the A and B ports and the boost exhaust to match the photos. I left the main exhaust and the C port alone as they already seemed bigger than the photos. I raised the B ports to the height of the C port. Port opening timing I used is as close to matching what Frits specified. I also enlarged and blended the boost exhaust ports all the way out to the end of the exhaust flange. I measured the crancase compression and added epoxy in the case to get a primary compression ratio of 1.45-1.

Stay tuned because I have a new engine in the works with a 200 mm diameter rv plate. This will allow me to space the intake ports dead center on the connecting rod and hopefully will increase air flow. I plan to make the inlet ports rectangular 60 mm wide and only 34 mm high. With a big port the area will be greatly increased. I am still restricted to 34 mm carbs. Hopefully the engine will flow more air with a less restricted inlet tract even with the small carbs.

BTW Frits i tried your idea of a connecting tube between the carbs and it resulted in a drastic loss of power and a reduction in both air flow and fuel flow. I suspect the connecting tube allows one crankcase to rob from the other.

I do like the idea of a pressurized air box. You are right Jan that an airbox shows slightly less hp on the dyno but I am curious if you ever tested a pressurized airbox on the dyno. I was thinking of connecting an electric blower to the airbox with a sensor for measuring the inches of pressure and controlling it with some sort of waste gate. ( a spring loaded flap!). Also with your airbox were the carbs totally inside the box or did you just seal around the spigot and route the vent hoses into the air box?

wax a écrit:

Frits you said you had those plugs for the gudgeon pin in 12mm. Do you care to sell any at all ??

................. click.

Jan Thiel a écrit:

GrahamB,
Putting them outside the fairing you lost revs.
The best solution was outside the fairing, but shielded from the airstream.

At the moment my carb is outside the fairing and pointing forward & about 20° to the side. Do I lose revs? If so, can you explain why? I dont't think I understood why one loses revs with the carb outside the fairing.

It would be very difficult to say without some instrumentation, but remember the basic principle of a venturi: when air rushes past at greater speed, it has lower pressure. So if the bell mouth was pointing directly out the side of the bike, the air-stream passing around the fairing would effectively be sucking on the carb. That is not good for getting air into the engine!

If you point it directly forwards, it will be pressurized but it will also swallow rocks. And with a carb, you need to ensure the float bowl is at the same pressure as the air outside the bell-mouth, or the mixture will be wrong. At 20°... who can say? It might depend on the wind direction (like the first airbox equipped CBR600's, which were erratic in a cross wind). Fitting an airbox seems like a good solution, even if it's just a forward-facing scoop with some grease inside to catch the stones.

Or you can get accurate pressure sensors now from any wrecked fuel-injected road-bike, so you could try data-logging the pressure.

Haufen,

I was talking about exhaust pipes, not about carburators!

Howard,

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.

so a airbox has several advantages, another project for the winter. it should be also pretty good to get the loudness down quite a bit... but I guess the effect will not be that big on smaller bikes which go max. 100 km/h and only on tight karttracks.

something frontfacing, that is not sucking air from coolers/exhaust and has somekind of stone/trashstop in it should do it. volume as big as possible...thanks graham for the great explenation!!!

Thank you Jan,

Our snowmobiles travel at less speed than 240 KPH.( I wish) Maximum speed is in the 165 KMH range dropping to 100 in the corners. Would there be enough of a benefit from a forward facing scoop at these speeds to pressurize an airbox besides the obvious cold air benefit? What area would be advisable for such a scoop to feed 2-VM34mm carbs. Dyno testing shows the engine consumes around 190 cfm at full song. From the photos it lookslike the area of your scoop is around 60 mm X 70 mm
With your telemetry what kind of pressurizatiion numbers did you get at various speeds.

Thank you Howard for your response about the port shaping. Have you any idea how much gain you got from those changes?

About the drawings of the Aprila RSA 125. Looking over those drawings it seems some of the drawings are of a different engine or a different state of tune. In other words a couple of the drawings don't match other ones, and I don't see a sectional one of the boost port, (port c), in relation to the other transfer ports. Just wondering if I missed one somewhere.

Also was there any work done inside the air box to promote laminar air flow?

Other questions are what the deck height is at TDC?
Also about the head. I saw a picture of the head as a toroidal shape. How close was the plug to the piston head and what was the spuish band ratio used and was the head changed according to the race track?

jfn2 a écrit:

About the drawings of the Aprila RSA 125. Looking over those drawings it seems some of the drawings are of a different engine or a different state of tune. In other words a couple of the drawings don't match other ones, and I don't see a sectional one of the boost port, (port c), in relation to the other transfer ports. Just wondering if I missed one somewhere.

The 2D drawing from Frits was of the old APC cylinder which came around year 2000 on the RSW125/250.

The RSA125 used a much newer APF cylinder.

Howard Gifford a écrit:

Maximum speed is in the 165 KMH range dropping to 100 in the corners.

You can work out the max theoretical pressure benefit in Pa as
0.5*density*vel²
with SI units.
165km gets you 1.3kPa, so 1.3% more pressure. However you want density and the thermodynamics give you some temp along with the pressure... so multiply by about 2/3 to get the density increase, about 0.85%.

That's the same benefit as keeping the air colder by 2.3°C.

However I imagine there are carb icing issues on a snow mobile...

Frits, you and I think very much alike regarding the tail end of the pipe. I'd like to gather my thoughts before saying more.

On a different question more aimed at Jan, how do you go about choosing the deck height? That's the term I know but maybe you call it something else: where the case stops and the cylinder starts. I understand you have to clear the bearings and all, but where is the best place to design the split?

That is easy to answer Brian.

You have the crankshaft, and an inlet hole somewhere, on the side or behind.
And you need some thickness above it, about 10mm.
That's all!

Howard Gifford a écrit:

Frits i tried your idea of a connecting tube between the carbs and it resulted in a drastic loss of power and a reduction in both air flow and fuel flow. I suspect the connecting tube allows one crankcase to rob from the other.

My internet has been down for quite some time (long story), so only now can I answer.
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, breating 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?

Hi Frits. I tried richer jets but the engine didnt like them. The bsfc went up but the power went down.. This is for a rotary motor remember, with a single inlet plate that controls both inlet ports. There is a substantial time that both inlet ports are open so I suspect the suctiion from one side reaches into the open crankcase of the other. Airflow went down to 165 cfm with the crossover and it is around 190 without it.

Jan or others, how did you control the tuned pipe wall temperature (or EGT directly?) when testing on dyno? This seemed the most difficult thing to mimic when testing either the GP engines at QUB or my R/C boat engines, in the lab. The inertial dyno or computer controlled brake seem to work best because we can match the test engine's acceleration with reality. On steady-state testing, EGT would simply climb 500, 600, 700, 750 °C until way past reality and the piston would seize.

This has always been one of Jan's greatest handicaps. He has asked for an inertial test bench over and over, but Aprilias race director Witteveen, or The Great Leader as we call him, never deemed it necessary....

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.

Jan Thiel a écrit:

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.

Modern car engines reduce engine power by retarding the spark timing. Should work on a two-stroke also (probably within smaller boundaries as you don't want to burn into the exhaust port). Depending on how much (more) throttle opening you would need to avoid detonation, I think it could be possible to make the RSA engine bullet proof at part load also.

If you imagine a diagram showing throttle opening on one axis and rpm on the other axis, can you describe the area where the engine was prone to detonation?

Jan Thiel a écrit:

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.

From what I heard Malossi has a testbench on which they can simulate the tracks they race on to finetune CVT and jetting on their racing scooters.

Reading the stories about the development of the RSA engine makes your results even more impressive to me. You had no indication system, no fuel balance, no exhaust gas analysis and no possibility to dynamically test the engine on a dyno. I guess at least Honda had almost all of that equipment if not all and used it regularly, and yet their engine could not keep up with yours! Hondas biggest mistake of that time probably was not to hire you, give you enverything you want (including tons of money and testing equipment) and let you do whatever you want.

So, a huge virtual hat's off from me to you for your achievements!

Car engine are able to turn off the fuel , and compress air flow only


(injection)

Remember that retarding the ignition is used at high rpm to increase exhaust temps and effectively shorten the pipe. So it's likely to increase the heating of the front edge of the piston...

GrahamB a écrit:

Remember that retarding the ignition is used at high rpm to increase exhaust temps and effectively shorten the pipe. So it's likely to increase the heating of the front edge of the piston...

Yes I know, but part load egt is usually lower than full load egt. And I think most of us would prefer higher part load egt over part load detonation. Of course, how far one could go and how far one would need to go would have to be tested, and how much would be needed would depend on the engine.

Toopack a écrit:

Car engine are able to turn off the fuel , and compress air flow only


(injection)

Yes but 4 stroke doesn't need the fuel/oil mixture for lubrication like the 2 stroke does.