Maximising rotax performance

Hi all,

My rotax barrell is nearing its end if its life, worn nikasel. It was a strong motor, but has since been left behind in the last couple of years. Its a pre cnc barrel.

I recently had the botttom end freshened up so we are good to go for a while there.

I am confident in engine building, and happy to throw a few gaskets and time at it.

I dont have access to a dyno, so i might be chasing my tail, and may end up sending it away for a dyno session but happy to play that game.
Yes i am chasing max hp.

Squish. Is 1.05 or less best? Im happy to buff my piston regularly to not go under 1.00 if needed. We get checked semi regularly.

Piston to bore. Rotax recommend .04 to .05mm with a limit of .08mm. Sounds easy enough in theory, but with 3 different barrel sizes, and pistons having a tolerance of 0.03 per piston size. Do i have to buy a barrel, then buy a piston a touch oversize. And then hone to size to get it correct? Again yes i am chasing max hp.

How in spec are new motors. I hear rumours of tuners taking out of the box motors then making 2+hp extra then stock. What are they doing?
Are they machining barrels and heads to make hp, but staying within the tolerance given from rotax for barrel and head height etc.

Matching the piston to the cylinder is the biggest gain. My tuner goes through 10 pistons before he finds the right combo. You have to have alot of postons in stock.

Buuut. You should not aim for max power. Your aim is to have the most power through the whole band. It doesnt bring you the benefit to have a peak power spot.

Just imagine, you have like 30 hp on one spot but the rest is not good compared to an engine with maybe 29 peak but more power through the band.

You just need to have a bench to find this out

Look at the picture attached. Both have the same peak but if you look at the band you see clarly the blue will be faster than the red one.


You can find also another up to 0.5 hp by fiddeling with the reed petals

Even there equity between the cylinders is now alot better finding a good one is still the holy grail. And in combination between all the factors you can have an engine which goes pretty good.

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Thanks for the reply.
Yeah my curent dyno sheet has a time between 2 rpm points which is most critical. Effectively it is a measure of power across the curve.

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What are the differences in the pistons?

I understand theres variability on cylinders (Especially prevo), but I’m curious to know what kind of differences are being observed with pistons.

Every piston is slightly different.

If you look at the sheet from gary, you will see that a piston,even one size has tolerances. Also the cylinder. You try to match them that you have the perfect match between them.