[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

less friction have pistons from powder aluminium :) left is Al-Si cast piston, right is forged from nano Al powder
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quote="williamsmotowerx"]In my opinion cast pistons have less friction than the forged[/quote]

Forged pistons always have a tendency to deform at the 'bottom' which is really the top.
You get worse flow, and a lower compression ratio.
With a cast piston you don't have these problems.
As the 'bottom' can be reinforced by ribs.

I have lost a lot of postings because I thought this topic had been closed......

The Tansini later called Jamathi had about 9HP, aircooled
The Dutch TT winner of 1968 had about 12HP I think, watercooled
These are estimates, because at the time we did not have a dyno...

The RSA connecting rod was only lengthened to improve inlet flow Howard.
At Bultaco I once tested a 95mm connecting rod, instead of the usual 85mm.
With the 50cc engine.
The result was less power.

The inlet disc diameter was 126 on the RSW and RSA.
A bigger diameter was tried, but proved to give no more power.
The biggest disadvantage of the RSA was the inclined carburator, causing a lot of trouble.
The carburator was made by Dell'Orto, I did not like it at all!
Japanese carburators usually don't give any problems when inclined.
The KEIHIN used on the 125 reedvalve Derbi engine 'made' by Bartol never gave any trouble.
It's only problem was very low HP, about 47,5

Nombre de messages : 15 Localisation : usa Date d'inscription : 24/12/2012

Jan Thiel a écrit:: The RSA connecting rod was only lengthened to improve inlet flow Howard.
At Bultaco I once tested a 95mm connecting rod, instead of the usual 85mm.
With the 50cc engine.
The result was less power.

I built a cr125 for motocross, going off memory 100mm stock rod, I currently have a 110mm rod in motor.

Bottom end power is lacking, I'm probably going to go back to 100mm rod and try that.

I thought main reasoning for 120mm rod was less friction. You're saying 120mm rod allowed a less obstructive intake port? That's reason for long rod?

Nombre de messages : 12 Localisation : Hellas Date d'inscription : 02/03/2014

Could you tell us a little more on how tailpipe length affects power?
Cheers

If you put a 'restrictor' just at the end of the pipe the length of the tailpipe will become a lot less critical.
You can find a design of such a restrictor in earlier posts on this site.
I think the first one to use this system was Helmut Fath.

Nombre de messages : 3 Localisation : Sweden Date d'inscription : 10/02/2015

Jan Thiel a écrit:: If you put a 'restrictor' just at the end of the pipe the length of the tailpipe will become a lot less critical.
You can find a design of such a restrictor in earlier posts on this site.
I think the first one to use this system was Helmut Fath.

What information would you normally take into account during testing when determining a good restrictor size / diameter?

I strive to run with as large restrictor diameters as possible to increase the reliability of the engine. Most often, a small peak power loss as a result of increasing the restrictor diameter can be compensated with a slightly shorter overall tuned length of the pipe. The downside is that it usually costs some low end power.

Do you know if the RSA where ever driven with different restrictor sizes on the track than what was delivered with the standard exhaust?

// Karl-Magnus

Om the 250 a diameter of 23,25 was used, on the 125 23.0
Once determined they were never changed.
And they were welded to the pipe
For the rest I fully agree with you!

Nombre de messages : 3 Localisation : Sweden Date d'inscription : 10/02/2015

Jan Thiel a écrit:: Om the 250 a diameter of 23,25 was used, on the 125 23.0
Once determined they were never changed.
And they were welded to the pipe
For the rest I fully agree with you!

Thanks for your reply Jan! Much appreciated!
(I will do my best to avoid asking already asked questions)

Where there any HP sacrificed for reliability's sake? (My guess would be Yes).
If so, would you dare to guess what could have been produced on the dyno in terms of HP if the reliability aspects was breached?

// Karl-Magnus

Never on the dyno Karl-Magnus
We always started with a too rich carburation.
Then jetted down until maximum power was reached.
We could then go still weaker, losing power, without problems.
On-track was a different story.
Carburation had to be richer because of detonation problems at part-throttle. (auto-ignition)
And because of the air-box giving higher air pressure at speed.
On the dyno the air-box gave less power....

Nombre de messages : 3 Localisation : Sweden Date d'inscription : 10/02/2015

Jan Thiel a écrit:: Never on the dyno Karl-Magnus
We always started with a too rich carburation.
Then jetted down until maximum power was reached.
We could then go still weaker, losing power, without problems.
On-track was a different story.
Carburation had to be richer because of detonation problems at part-throttle. (auto-ignition)
And because of the air-box giving higher air pressure at speed.
On the dyno the air-box gave less power....

Many thanks Jan for taking your time answering these questions!
Have a nice weekend!

// Karl-Magnus

Dear Jan!

Do you know how did the geometry of pistons used in the Aprilia look like? How much was its taper (under and above the ring, and how long), did it have ovality and if the answer is yes then how much was it?

Thanks in advance!

Nombre de messages : 98 Localisation : Italy Date d'inscription : 30/10/2012

hello

a question to all.
can someone give me a tip what can cause this problems with detonations and the crack in the center of the piston. let away the damage under the piston ring.

a few information the the engine.

94 ccm polini 52x44 CVT engine, rotary valve, 30 VHSH dellorto carburetor 95 octane unleaded fuel, driven on a smal kart track constant on 13000 rpm. exhaust temp max 350 °C about 45 mm away from the piston, NGK 10 iridium.

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it can be the ignition or the carburetor setting what i think. ignition is a non digital one set to 28° btdc like said from the factory. i have testet many carburetor settings put i dont know.

thanks Manuel

A tip , this engine is sealed ?

bad design of the cylinder head? squish, compression ratio due to octane nr.

Nombre de messages : 98 Localisation : Italy Date d'inscription : 30/10/2012

hi

laranjateam
what do you mean with sealed? the cylinder to the crankcase is sealed with a gasket, head to cylinder with O ring. and the crankcase with silicone sealing.

alcatelko
the cylinder head design is nearly like the RSA. squish is 0,9 and the compression is 15:1 on 95 octane like i said.

i think the numbers should work, but i don't know.

thanks manuel

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

Manuel Rainer a écrit:: the compression is 15:1 on 95 octane like i said.

I'm no tuner, but that seems very high for only 95

Nombre de messages : 98 Localisation : Italy Date d'inscription : 30/10/2012

JanBros a écrit:

Manuel Rainer a écrit:: the compression is 15:1 on 95 octane like i said.

I'm no tuner, but that seems very high for only 95

the RSA has 16:1 with 95 octane as a think

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

Manuel Rainer a écrit:: the RSA has 16:1 with 95 octane as a think

so with 95 octane you do not mean euro 95 Embarassed

I use 15:1 ratio at 50ccm engine and normaly I use RON 100 fuel...if is used RON 95 fuel main jet must be bigger (120 @ RON 100 fuel and 128 @ RON 95 fuel)

hi
i am not a tuner too but first for 50cc a squish at 0.9 it's big remember what Jan said 10% of the stroke or just a bit more , whatever the fuel you use try to use always the fuel which provide the higher octan ratio, also it seems you carb jetting was too lean.Maybe also not enough advance to you ignition timing.
Pierre

Nombre de messages : 98 Localisation : Italy Date d'inscription : 30/10/2012

hi

JanBros
i use normal italian 95 octane unleaded super

alcatelko
thanks for the info. i drive a 175 main jet on this 94ccm. that's also my decision to use 100 octane.

pierre95
this engine has 44 stroke. 0,9 is on the safe side. but if i want to reduce the squish, i have to modify the head geometry, otherwise i have too much compression.
i have to check the ignition in the high revs. on 13000 rpm 14°-12° i think.

thanks manuel

» [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)
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