ESE's works engine tuner

[oktrg500]

Hi, I'm a 48 year old (Or, is it young?) Norwegian. I've been lurking around this thread for some time now. I'm currently in the process of building a tuned Rotax 122 engine. The bike will be used mostly for fun on road and track days. My only concern is getting as much power as possible from the engine.

The cylinder has been reworked by a known UK tuner. To go with this I've got a VHM head for use with flat top piston. And a good selection of different reeds, carbs, std and custom exhausts, different ignition CDI's, etc.

I just can't make up my mind if I should go for a 1 or 2 ring piston. It seems to me both have their pros and cons and I would appreciate very much any advice. For now, I'm thinking 1 ring is the best.

Thank you.

[Martin1981]

hello. 1 ring! there is no need for a second ring. good 1 ring pistons with the ring peg at "18 o clock" position (so you can widen the b ports) are easy available for this engine.

i dont know why but i doubt this vhm heads a little bit. i think it would be better to not use a flat piston and have a stock head machined to desired specification.

[oktrg500]

hello. 1 ring! there is no need for a second ring. good 1 ring pistons with the ring peg at "18 o clock" position (so you can widen the b ports) are easy available for this engine.

It seems I just can not let go of the thought of the rings dissipating heat from the piston. Thus - two rings should dissipate more heat than one ring. At the same time, I've got the impression the ability of the heat dissipation from the rings might be overrated.

i dont know why but i doubt this vhm heads a little bit. i think it would be better to not use a flat piston and have a stock head machined to desired specification.

I've had a std head machined down as much as possible (0,5mm), but this only brought down the squish band from the usual 1,8/1,9mm to 1,3mm. The VHM heads are said to dissipate heat much better than the stock 122 heads. Maybe I should get a VHM head for domed piston and machine it to the desired specification. Thanks.

[Frits Overmars]

Hi, I'm a 48 year old (Or, is it young?)

Young, definitely. Velkommen, ung mann.

It seems I just can not let go of the thought of the rings dissipating heat from the piston.

I can't let go of the thought that a ring rubbing against a cylinder bore generates heat .

[wobbly]

The most powerfull 125 known to man quite happily got rid of the heat generated thru a single ring.
There have been cases of huge overbored ie oversquare cylinders that had piston heat trouble and twin rings
did help in this particular scenario, but that was a special case where the real issue was lurking elsewhere and the rings just
ameliorated the symptoms.

[F5 Dave]

Ohh fancy word for the day.

Right its a sunny day so I`m off down to the garage to ameliorate my transfers.

[senso]

My two year old crm 50 cylinder retired after running for 30 minutes with no cooling and still managing not to seize, so for science I cut him in half(sorry buddy) to see and measure in fact how much wall thickness I still have to play around with in the new cylinder, the conclusion is: not much.
I talked with a machinist friend about the possibility of welding the exhaust duct to gain a couple mm's but he doesn't want to do it, he says it might crack, so I will resort to a maximum of 190º of exhaust timing at 80% piston diameter.

About the duct shape, it should be straight or is it ok to leave it like it was, sort of a deLaval nozzle(more or less expected I grinded the exhaust with the burs in steep angle and very slowly due to the known limitation of the wall thickness)?

[TZ350]

maximum of 190º of exhaust timing at 80% piston diameter.

What is the transfer timing (STA)? I know low and wide is the prefered option for transfers but to improve the exhaust timing can you raze the cylinder a little without upsetting the transfers and spigot the head into the cylinder to get the correct squish clearance. And if you are widening the exhaust a lot it may only require grinding it up just a little to get 194 - 196 duration to get a satisfactory STA for blowdown. Anyway looks like it was a very good idea to have cut the old cylinder.

[senso]

Unfortunately the transfer timings from stock are atrocious, almost 130º if I remember correctly, but you are in the right direction about raising the cylinder, its common to do that with this engines, there is not a lot of cylinders for them and sadly almost all of them are expensive copies of the original cylinder, almost all of them in cast iron too.

I will look to the STA values and report back, I don't have a goal for maximum power, just would like to have a bit more, she was fun all right, but could be even better.

[peewee]

hey guys what you think. i was sucessfully able to fill in the front side of B with low temp rod and straighten it. kind of tricky but it was my first time so it should get easier. next is try to see if i can get a outer wall over the aux and create the passage

[TZ350]

hey guys what you think.

Very clever, looks good to me.

[wobbly]

A far bigger issue with that 50cc is the HUGE idiotic size of the duct exit - looks real easy though to counterbore it and fit an insert to get
the exit area down near 90% of the effective port size.
This will pick up a major amount of power with a new pipe having the way smaller entry size, that then creates naturally steeper diffuser etc.

[F5 Dave]

S' ' funny I was just thinking that 50 could do with some low temperature filler on the waterway and peewee posts about it.

[Frits Overmars]

... how much wall thickness I still have to play around with in the new cylinder, the conclusion is: not much... I will resort to a maximum of 190º of exhaust timing.

I'd say you stopped grinding just in time. But radiusing the port's upper edge may still give you a bit more timing and quite a bit more flow.

About the duct shape, it should be straight or is it ok to leave it like it was, sort of a deLaval nozzle?

Your duct may look like a DeLaval nozzle, but to the exhaust gases it isn't. A DeLaval nozzle should offer the outflowing gases a nicely guided duct for controlled expansion. In the case of an opening exhaust duct the bulk flow occurs while only the top part of the duct is open, so the gases dash into the deep without any guidance at the bottom. Take Wobs advice.

[senso]

I think the stock exhaust diameter duct is already in the 26 to 27mm when stock, but machining a bit of the cylinder and sticking a plug is not a bad idea.

I was mistaken, the maximum I can go is 22mm from top of cylinder to top edge of the exhaust port, with the stock 91mm con-rod and 41.4mm its just shy off 186º, the transfer all sit at 32.5mm from top, about 124º timing for A and B, being the C a bit lower with around 117-118º.
That gives 31º of blow-down, but with limited area.

In that cut cylinder I was trying do give it a radiused edge but its a tight spot for my limited tool to put a radius on it, but I will try to do it, worse case scenario I will do it by hand with sand-paper.

The engine was also running with 1.1mm squish, now I dropped it to 0.8mm(the minimum allowed by the gasket, and the head is to thin and breaks when o-ringed so its no an option).

I still don't like a lot of the idea of raising the cylinder and machining the top, because I just gain timing in all the ports, so the blow-down remains the same and 1mm will leave the cylinder with almost 130º in the transfers.

I don't have an accurate port-map with me now, but the exhaust port with 80% width has about 575-600mm^2 total area, following the 90% rule from Wobbly it would need at max an exhaust duct with 27.6mm in diameter, I think that cylinder has 28mm so not far off.

I found a couple pictures of the cylinder in one piece, I think there is still a bit of exhaust area to be gain, but I'm kinda shy to square the exhaust port and then have the piston rings stuck in it, what is your opinion about the shape Frits?





Thanks for the advice.