CHT and timing

Greg Arehart · #1 10-07-2012, 09:32 PM

After my recent engine rebuild, I've had high CHTs (higher than normal) and have been trying to track down the problem. Turns out that my timing was advanced by about 1.5-2 degrees (~27 instead of 25) and shifting the timing made at least 20 degrees of CHT difference. Before, I couldn't run above about 2300 RPM without the temps going over 400, and now I can run full throttle without getting over 400.

Probably something a lot of folks already know, but it was a definite learning experience for me to find out that only a slight difference in timing could affect such a dramatic CHT difference.

Greg

scsmith · #2 10-07-2012, 10:42 PM

Usually, slightly retarded timing will cause the engine to run hotter. Are you sure your timing wasn't 23 degrees BTDC, rather than 27?

Lots of us have electronic ignition, which runs at more advanced timing, with favorable effects on everything. I have a Light Speed plasma 2, and I don't recall the max advance off hand, but my CHT is 360 in climb, and 295 in cruise.

RV10inOz · #3 10-08-2012, 01:08 AM

Quote:

Usually, slightly retarded timing will cause the engine to run hotter. Are you sure your timing wasn't 23 degrees BTDC, rather than 27?

Not so, advancing the spark timing will raise the CHT.

There is very little value in having earlier spark timing in a "typically" fixed RPM engine, the old trick of a few extra degrees will make your car go harder was a bit of a street myth, but most youngsters gave it a go.

In an aero engine with effectively a static speed (2300-2600) the changing of spark timing has little effect on HP at the prop, you get a higher Peak Pressure PP and it moves closer to TDC so while you gain a higher force on the piston, you may not really gain any extra torque due to the angle of the crank. So Theta PP is what really matters.

Now moving the PP closer to TDC does load p the pistons, bearings, con rods pins and crank etc, and it disturbs the thermal boundary layer inside the combustion chamber, which increases heat transfer through the head, and hence you see higher CHT.

Now tell me why would you want to do that

So yes a couple of degrees will do that.

10/10 to you Greg for spotting that something was not quite right and doing something about it!

Ironflight · #4 10-08-2012, 06:53 AM

Great post Greg - you beta me to something I was going to write up! On our recent trip out west (high and hot conditions) we were seeing CHT's on the Val that we just didn't like - made it hard to climb out of a high airport. I have been running the "B" curve on the P-Mags ever since we installed them (this is a more advanced curve) - just as we have with the engine on Tsam, and have had no issues. I decided when we got back to install the P-Mag jumper to take us to the "A" curve (a couple of degrees less advanced) and see what happened.

I test flew it that way for the first time this weekend, and we are back to a very happy engine! CHT's are back the way I like them (barely touch 400 in a long climb). A couple of degrees makes a huge difference!

Paul

gereed75 · #5 10-08-2012, 09:14 AM

A related question - Beyond the static set-up of the mag and/or electronic ignition timing, how does one check the timing more accurately??

Discussions of +/- 2 degrees don't mean doodoo if you can't measure it accurately.

Pictures and arrows would be nice.

aerhed · #6 10-08-2012, 09:56 AM

Not sure what you mean by "static". Mags are fixed timing, of course (no advance). Most important thing when setting a mag is good technique that eliminates gear play and yields an accurate point break.

RV10inOz · #7 10-08-2012, 10:43 PM

Quote:

I test flew it that way for the first time this weekend, and we are back to a very happy engine! CHT's are back the way I like them (barely touch 400 in a long climb). A couple of degrees makes a huge difference!

PD,

I bet you did not notice any performance change either?

Your cylinders and everything else will last longer too!

gasman · #8 10-09-2012, 12:25 AM

Quote:

Originally Posted by gereed75

A related question - Beyond the static set-up of the mag and/or electronic ignition timing, how does one check the timing more accurately??

Discussions of +/- 2 degrees don't me doodoo if you can't measure it accurately.

Pictures and arrows would be nice.

Lightspeed recommends a timing light. I can't see how you can get close enough to read the timing marks to within less than 1 degree.

http://www.lightspeedengineering.com...;20II+_III.pdf Page 33

gereed75 · #9 10-09-2012, 09:35 AM

Gasman, That is what I was talking about. Did'nt know Klaus had updated his manual with this much detail.

Agree that I am not sure you will get it any closer using this method over the "static" methods. Good luck getting accurate reliable readings hanging out in the prop blast.

Does anyone out there do this on a routine basis??

Thanks

PerfTech · #10 10-09-2012, 10:25 AM

Quote:

Originally Posted by gasman

Lightspeed recommends a timing light. I can't see how you can get close enough to read the timing marks to within less than 1 degree.

http://www.lightspeedengineering.com...%20II+_III.pdf Page 33

....You can line-up the marks you want, then transfer them to the back of the starter ring and sheet metal with chalk or a paint stick. Then you can see them clearly from behind. My timing light will illuminate them from the cockpit. Allan....