ESE's works engine tuner

[Frits Overmars]

Looking at the Rotax drawring and my piston, I think your right. I am in the process of making another cylinder and will be using better tools so hopefully a better result.
That old cylinder made 30hp, if I can fatten up the lower part of the torque curve with better direction from the transfer ports I will be very happy.

That Rotax drawing (it's not from Rotax; I drew it in 1980) shows the original Rotax 124-cylinder of that time. Next I rotated the trailing flanks of the B-ports 15° towards the exhaust side (I would have liked to do more but then I would have struck coolant), fitted a 38 mm Mikuni and a fat pipe (official Rotax pipes then measured 100 mm at the belly; mine was 128 mm) and measured 34,5 HP/12,500 rpm at the chain. A flat tire cost us the title in the European kart championship. Life is hard.....

[breezy]

yeah went for the rotax barrel myself, had an mtx 125 in mind originally( with my gp 125 crank ,i have inside 50.6x 56) just got to have the barrel skimmed down 5mm and get a thicker bottom barrel spacer(20mm thick)..

[Frits Overmars]

yeah went for the rotax barrel myself, had an mtx 125 in mind originally( with my gp 125 crank ,i have inside 50.6x 56) just got to have the barrel skimmed down 5mm and get a thicker bottom barrel spacer(20mm thick)..

If you are going to use the Rotax cylinder in the above picture, the most important thing to watch is the coolant passages alongside the exhaust duct. Chances are that they are partly or even completely blocked by casting skin. The picture below shows an undersized coolant passage left (in the yellow circle) and a completely blocked passage right. If this is not rectified, it may kill your engine the first time you give it some stick.

[Forgi]

The exhaust nozzle I believe when used with a big T port or a 3 port with plenty of blowdown area is doing two things.............

Ok Teezee, once I get the results I'll share them!

Wobbly, I think I understand what you are saying, but lets see an example, right?
Look at this picture, this way it'll be easier to understand!

70cc 45/44 bore/stroke



As you can see the effective area and the STA of the two exhaust ports are the same, only the angles are different. It is easy to understand that up to a certain RPM you can't use a simple exhaust port to make an STA like this, without increasing the angle. (However in this case it doesn't matter.)

In theory 193 degrees could be the better because of the resonance in the exhaust. Frits has already talked about this. (190ex./130tr.)

What do you think? Although every pharameters are the same the tight nozzle couldn't work?

If it is true isn't it possible that in case of a simple port the nozzle has to be smaller than 75%? It would be logical cause if we take into account the proportions the blowdown area is smaller than the exhaust effective area in case of a T-port or having 3 ports. (of course the example above is not that case :-))

[2T Institute]

No time to dwelling on the Why?

Thought of that but doesn't explain what we have experienced

[husaberg]

Thought of that but doesn't explain what we have experienced

Good points are torque band is getting higher and wider.

Having now started to use 118mm rods in a 125 .... Good points are torque band is getting higher and wider.

[wobbly]

The problem here is that both examples you have given i would never work with.
Any 70 cc RACING engine at 10,000 rpm isnt even into the powerband, let alone pushing the limits of power with those timings.
All the work I have done is with full noise designs that are pushing the limits of what can be achieved in every area I can modify.
Trying to use my guide of making the header equal to the total Ex effective area, and making the duct nozzle = 75%, in an engine that wont make any power anyway, is
missing the point entirely.

[breezy]

If you are going to use the Rotax cylinder in the above picture, the most important thing to watch is the coolant passages alongside the exhaust duct. Chances are that they are partly or even completely blocked by casting skin. The picture below shows an undersized coolant passage left (in the yellow circle) and a completely blocked passage right. If this is not rectified, it may kill your engine the first time you give it some stick.

thank you very much for the info,ill take a look

[wobbly]

Rotax did the exact same error on the 257 single road race cylinder, many had no hole on one side and a small 3mm triangle for all of the water flow on the other side.
Dumb - about as good as the non existent QC on big end rollers, I used to buy 20 big end bearing kits, and measure every roller into +/- 1 micron diameter sets.
Some kits had 1 roller 5 microns bigger than all the rest - instant failure waiting to happen with no recourse on the wankers at the factory.

[koba]

Some kits had 1 roller 5 microns bigger than all the rest - instant failure waiting to happen with no recourse on the wankers at the factory.

Maybe that's what they were doing rather than QCing.

"Hans, pass the grease..."

[TZ350]

I am looking at trying my hand at fuel injection and would like to know how that fits with the "equivalent to a single 24mm carb" rule.



Is a 24mm venturie inlet and injectors in the engine some place equivalent to a 24mm carb??

[Frits Overmars]

Forget that 6° diffuser opening angle, TeeZee. That would be fine for continuous flow but what you have in your engine, is pulsating flow and there the main flow resistance is caused by having to accelerate the mass of mixture in the inlet tract at every turn. So keeping that tract as short as possible is much more important than having a turbulence-free diffuser angle.

[husaberg]

24mm throttle body seems fine to me EQ to a single 24mm carb i recall

[Flettner]

Much better to run a drum type throttle. You need a TPS and an in cable type liner type won't work well. Best it to have the TPS connected direct to the throttle shaft. Butterflies are not good in a twostroke inlet.

[Flettner]