SonexBuilders.net

...has anyone removed the cylinder shims to go up to 8:1 compression ratio?

I played around a little once changing compression ratios. Biggest thing I noticed was the push rod length and shim issues for the rockers. I probably shouldn't have bothered as it made no noticeable power decrease or increase that I could tell.

I would think the little bit of extra push rod length could be taken up by the rocker arm adjusters, no? Are you saying that you actually had to trim them? I'm in flight testing after re-engining my Sonex #325, and I'm not really able to pass my climb test at 1100 pounds yet. But so far, I'm only getting around 2800 rpm climb. When I find another couple hundred rpm, maybe that'll be the answer. But that's my rationale for wishing I had gone with 8:1 from the start. Thanks.

jwd3ca wrote:I would think the little bit of extra push rod length could be taken up by the rocker arm adjusters, no? Are you saying that you actually had to trim them?

If your engine was set up for 7:1 compression and you change to 8:1, you will definitely have to make new pushrods. There will not be enough adjustment in the rocker arms to absorb the difference. Same if you try to go the other way. Going from 8:1 to 7:1 will result in pushrods that are way too short. It's not that difficult to make new pushrods, but it's definitely necessary when changing compression ratios.

Slight thread hijack - if you need shorter pushrods is there any way to pull one of the ends off? Or do you buy new?

I have shortened my pushrods with this method. I chuck the ball end in the drill press with the bottom end in a hole drilled in a piece of wood. Depending on how much you want to remove, use a hacksaw blade and cut the top end of the tube through to the hardened nipple. You won't scratch the nipple, it's too hard. Then put the push rod on a solid surface, fit a socket over the ball end and drive it down to the new seat. You can grind the hacksaw blade to any thickness you want to remove. Takes about an hour to do four rods.

Thanks, Joe and others. Guess it comes down to 'is it worth the effort?' One opinion offered so far thinks not. Any others?

This is completely my opinion but for what it's worth, I did the exact same thing a long time ago. I removed a cylinder shim to change my compression from 7.0 to 1 up to 8.0 to 1.

After doing that I noted little to no performance change in the output of the engine. I may have gained 1-2 hp from the job but it certainly wasn't worth it.

If you have a good running engine I wouldn't mess with it.

I agree with Mike. The power increase from the compression ratio increase isn't worth the effort.

OK, that nails it. Thanks for the opinions, gents. Just going to have to find those RPMs via aeroinjector tweaking I guess. Saw 3050 static yesterday, so that's encouraging. Maybe some aerodynamics? Never installed wheel pants (but do have leg fairings) because I was previously flying from turf, now from paved runways. My larger tires/tail wheel prolly don't help, either...

I have another question, but I will open a new thread...