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Putting shims on a 912ULS head

by Phil. Meredith in General Tech Discussion 12 months ago

I have a Cygnet amphibian here in Samoa. The highest octane fuel available is 91 but this is not guaranteed as a minimum octane.

I have fitted 1.6mm shims on each cylinder.

Can anyone tell me what the reduction in rated horsepower would be with this larger cylinder capacity?

I have been unable to locate a formula to figure it out.

thanks

Re: Putting shims on a 912ULS head

by Ed Midgley » 12 months ago

Ed Midgley

4 35 77

You should be good.
Those shims should lower the compression ratio to slightly less than the 80HP 912UL

If you didn’t install oversized O rings you could have some oil leaks.

Ed

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Re: Putting shims on a 912ULS head

by Rotax Wizard » 12 months ago

Rotax Wizard

2 963 4.14k

Hi Phil...you can find the formulas online. Here is an article on how to do it for an automotive engine, same process.

https://www.motortrend.com/how-to/calculate-compression-ratio/

Cheers

Thank you said by: Phil. Meredith

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Re: Putting shims on a 912ULS head

by Peter Kl. » 12 months ago

Peter Kl.

7 230 376

Interesting matter. Found this calculator online:

https://www.bgsoflex.com/crchange.html

Example: if one lowered the compression ratio of a 100hp engine with CR =10.5 to 9.0 the reduction in hp will „only“ be 4%. That to me does not sound too significant.

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