912 Big Bore

[Av8r3400]

I've often thought of a big bore with a turbo normalizer instead of a turbo charger. Maybe 30-32" of manifold pressure to keep rated sea level hp all the way up.

A lot less stressful to the engine.

[jiott]

I agree with AV8R3400 that it is important to not overstress the engine. I really am leery of all these big bore and turbo aftermarket mods. It is well known that the 912 80 hp engine is longer lived than the 912s 100hp with its bigger bore. Rotax deliberately used the 80 hp bore when they added the 914 turbo, rather than the 912s bore to keep engine stresses down with the added pressures of the turbo. When I was taking my flight training at Stick & Rudder in Boise in their 914 powered plane, Paul Leadabrand kept emphasizing not to use the full turbo boost any more than necessary because "it was hard on the engine".

Now I see poeple talking about aftermarket big bores (bigger than the 912uls) and aftermarket turbos delivering more boost than the 914, and then the combination of the two. All without any basic strenghtening of the engine itself! I really have my doubts about the longevity of these mods. I know this is "experimental aviation", but I believe it should be entered into with eyes wide open as to the consequences of possible short life and higher costs. Has anyone heard of these mods running trouble free for anywhere near 2000 hours? OK, I admit I am a stick-in-the-mud (probably due to my engineering background).

[n85ae]

This is making me think about my own engine (IO-240B), actually for a 20%
power boost I'd be willing to live with a 50% reduced overhaul time considering
the amount I fly this would be a good number of years before the overhaul
was actually due ...

Yeah a 150 hp IO-240B with 1000 hour TBO, I'd be fine with that

YES I would be Happy to fly with a 150 Hp engine in my plane, so before you
say this is too much power ...

Regards,
Jeff

[SkyPirate]

I would probably wait until TBO time myself and get the crank/block redone and then do the big bore or add the heads etc and do the turbo, or bite the bullet and get a new 912 ULS

[mr bill]

As I understand it, the 912 and 914 series engines have a common crankcase and crankshaft. The 914 power output is 115 hp and has a 2000 hour tbo. I feel quite comfortable putting the big bore kit on my 2000 hour tbo 912 uls. Lowell, in another thread you mentioned a service letter regarding big bore engines. Can you give us the number or the content of the letter?

[Av8r3400]

In a nutshell Rotax says that any aftermarket modification to the engine voids all warranty claims.

I'll try and find the specifics of that for you.

[HighWing]

Following is the link. I mis-spoke. It is a Service Letter rather than a Service Bulletin.

http://legacy.rotaxowner.com/si_tb_i...1-kul-1999.pdf

Interestingly, the date of the letter clearly indicated Hal is off the hook as it was posted in 1999.

[mr bill]

Thanks Lowell. My take on the letter is that it was written by Rotax lawyers. I have not heard of any problems with any of the after market kits.

[mtgsxr]

I agree with the extra stress of a big bore or a turbo kit. I think the best option for the 80hp guys would be the high compression pistons. Minimal impact to all engine components. I however worry about aftermarket piston quality.. being in the powersports business for 20 years I have seen too many failures related to aftermarket pistons. If anyone has any high time engines with the high comp pistions, I would like to hear from you.

[Av8r3400]

I would think a big bore, low compression conversion would be far less stressful to the bottom end than the violence of high compression...