Engine Timing Discussion

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Postby JPXman » Sun May 14, 2006 10:20 am

hey everyone,

I'm new to working on the guts of an engine so i want to ask a question about timing to the group. if your timing is too far retarded will the engine spool up to maximum possible RPM?

I mean, if you slowly advance the timing from this point will the engine RPM increase? to a point i guess and then begin to fall off and run roughly?

when we took apart my "bad" engine, we measured the timing before we took it apart. it was at 0.108" BTDC as measured by a light and the manual recommends 0.084" to 0.096". after reassembling my "bad" engine after putting in new seals and a new set of points, we set the timing at the mid-range of the factory recommended setting at 0.090". on the test stand it spooled up to 3500 rpm with a new prop (our mistake was not characterizing it first with the old prop). i figured this was because the new prop is a finer pitch (40/17) than the old prop (34/20). we put the engine back on the wing, and put the same new prop on the other engine and it spooled up to 3800 RPM!!!! that's a big difference on a big prop like that... with the 34/20 it will get up to about 4250 RPM.

So, we are going to take the engine apart again and advance the timing to 0.108" and see what we see. any insight?

Tyler
PS: the engine runs great, just lags the other engine by 300 RPM but it was enough for me to fly around for half an hour and see that the engine is running fine again and didn't quit on me :)
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Postby Guest » Wed May 24, 2006 8:11 am

Could timing make a 100-150 rpm difference ?
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Postby Guest » Wed May 24, 2006 1:06 pm

Abso-freaking-lutely!!!
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Postby JPXman » Wed May 24, 2006 9:32 pm

I guess i should have followed up this topic - it turned out that the same engine would turn identical propellers at different RPMs!!! they are both Culver props advertised as 40"/17". We did not take the other engine apart as I went back to the 34/20s and the engines. I have also bought some new points to install on the next engine we time so i'll repost here once i get that done.
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