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.
Questions.
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.
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.
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.
If you take 10 pistons stamped with the same size and you start to measure them, they all will have different sizes. Imagin now you put all the pistons in the same cylinder, what you get? 10 different tolerances. Now you go through the pistons to match the prefered distance between the piston and cylinder wall.
What does the best combination look like?
Piston clearance as large or small as possible?
Is there a difference between max performance and durability?
Around 0,007 is a good value. The right one is the best one. To big and piston can tilt, to small it wears out the cylinder. If you want durability you have to go bigger in squish
Unless I’m misunderstanding what’s being suggested, accurately measuring and matching bore with skirt is standard practice for any engine. That’s to say you don’t blindly trust what the manufacturer said it is an you remeasure yourself. It’s never been a major contributor to power though.
What makes the Rotax Max more responsive to this? Maybe you talking about pin to crown distance or something else?
