912 Big Bore

[Av8r3400]

That's surprising, Paul. On a smooth day I can see 125 mph (gps verified) from my plane…

[SkyPirate]

Im getting easily ASI indicated 115 with my 80 hp ground speed concurs on those rare no wind days I run at 5300

my field elevation is 1220 and im usually running around 3k to 4 k alt

[av8rps]

Paul
To start with I'm at 5130 field elevation. After install I increased 350 RPM where the prop was set. I have a 70" Warp Drive, taper tip and was told to leave it at the pitch it was set. After flying we found out I didn't need to change it and haven't since. To note some things from a couple of weeks ago. DA of 5100 that day, I get 5600 on take off, get 1200 FPM climb and 6100 WOT at 7500 msl straight out with little wind, GPS showing 120 MPH ground speed, pegging my 120 MPH airspeed indicator. EGTs, CHTs and oil temps very acceptable. I still need to clean up some things (snip, snip.....)

You need to add some more pitch to your prop in order to utilize the extra torque (and hp) your engine is now developing. You should NOT RUN OVER 5800 rpm, as that is Rotaxes hard number for MAX ALLOWED RPM for their 912 series engines. Max continuous rpm is 5500, so in my opinion a really good prop pitch will only allow you to get between 5600 and 5700 rpm with wide open throttle straight and level. ( I set mine for 5650 on my 912 and really like it there).

I will bet if you add some more pitch you will be much happier with your airplane and engine.

[KitfoxJ]

Thank you Paul.
Yeah, still experimenting and deciding on I what kind of flying I'm mostly going to do.
Jay

[Paul Z]

That's surprising, Paul. On a smooth day I can see 125 mph (gps verified) from my plane…

I referring to indicated airspeed on my advanced flight, not true ground speed. This year for the annual I pulled the propeller to check for corrosion, and I definately was glad I did, because surface corrosion had started. I had to repitch the prop and it's not quite right. We will see how it goes after that. When I changed from TriGear to Tail Dragger, I did picked up some cruise speed.

[Av8r3400]

You need to add some more pitch to your prop in order to utilize the extra torque (and hp) your engine is now developing. You should NOT RUN OVER 5800 rpm, as that is Rotaxes hard number for MAX ALLOWED RPM for their 912 series engines. Max continuous rpm is 5500, so in my opinion a really good prop pitch will only allow you to get between 5600 and 5700 rpm with wide open throttle straight and level. ( I set mine for 5650 on my 912 and really like it there).

I will bet if you add some more pitch you will be much happier with your airplane and engine.

Actually, Paul S, with the Zipper kits their revised and improved piston design is such that higher rpms are workable. Hal told me he has run his at 6000+ for hours at end on long cross countries without issue. The stock valve springs start to float at around 6500.

[Av8r3400]

I referring to indicated airspeed on my advanced flight, not true ground speed. ...

So was I, Paul Z. In calm air I can pretty easily redline my ASI (125 mph).

When I said verified by GPS, I meant I have checked the accuracy of my ASI with the 4-direction GPS method.

[WWhunter]

I had been told the same thing by either Hal or Jay, that is was okay to run these new cylinder/pistons at 6K rpm. I ran mine at that RPM for a short spurt when I installed them and was trying to break in the new rings. I had a hard time getting the temps up and ran it up that high. I posted on here that I had done so and was quickly told it wasn't a good idea so hadn't done it since, but then I haven't been able to fly since last fall.

I did have to increase the pitch of my prop at least a degree. With the new Zipper kit, the engine would easily over rev. I have a 3-blade Whirl wind (70") but am going to try theWhirl Wind 75" STOL prop this summer. I ordered it before Christmas but never had a chance to mount it and try it out yet. I think I read a post on here or the BCP site that the gear box might need upgrading with the use of this prop.

[av8rps]

Actually, Paul S, with the Zipper kits their revised and improved piston design is such that higher rpms are workable. Hal told me he has run his at 6000+ for hours at end on long cross countries without issue. The stock valve springs start to float at around 6500.

Hmmmm..? I've always been told the issue with over revving an engine is that you can "stretch" the connecting rods which ultimately results in crankshaft failure and/or connecting rods blowing out of the side of the engine block. But I suppose if the Zipper pistons are significantly lighter than stock pistons, inertia could be the same (as stock pistons) at the higher rpm? But I don't remember reading that the zipper pistons are that much lighter.

I'm no engine expert, so I could be way off on my theory here. Just relating what I'd been told back in the days of drag racing a 900 Kawasaki motorcycle (which ultimately exploded after big boring it and running rpm 1,000 rpm higher than stock). Now don't get me wrong, I really like what I am seeing with these big bore zipper kits for the 912, and will probably buy one myself one day. But I'm going to stick with the Rotax recommendation of no more than 5800.

Paul S

[mr bill]

Big day today! Engine ran for the first time. No prop, so limited rpm to just over 2000. Runs a little rough on one mag, but smooth on both. It has the BullyHawk soft start module. Gets one bang (backfire?) on start and smooth after that. Any ideas? This is the big bore 912 ULS. No leaks of oil or coolant after less than 2 minutes. I am happy as a clam!