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CHT and timing

Greg Arehart

Greg Arehart

Well Known Member
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
 
Hmmmmm 27 or 23

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

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 :eek:

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! :)
 
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
 
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.
 
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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.
 
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!
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PD,

I bet you did not notice any performance change either?

Your cylinders and everything else will last longer too! :)
 
gereed75 said:
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.
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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/Manuals/CdiManual_PlasmaII_ II+_III.pdf Page 33
 
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 said:
....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....:D
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For the mag, the marks are already there. The problem is, even if you make a pointer that matches the case split line,and have it within 1/8' from the flywheel with a mark on it no wider than .053 (1/2 degree) you must still be able to see it from the cockpit looking down the center of the case at 5 and 1/2 feet away from the marks. And through the canopy in a tilt up.

Allan, when you check your timing with a strobe, how close is your timing from when you set it static?

A timing system might be your next project.
 
Of course, you can time the P-Mag EI with the engine off, by just blowing in the little tube - very convenient :)
 
gereed75 said:
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
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I've done it, recently. Can't say that it was a lot of fun. With a bright timing light, working in the shade (nice to have a ginormous hangar making a shadow across from mine) it was actually easier to avoid parallax error than working on a car because I could get the light right down to the flywheel, eyeing down the case centerline. Fortunately the blast from that big, spinning double scimitar of death was blowing me backwards, but it was still unsettling. Would be even less fun on a pusher installation. I did take the trouble to highlight the timing marks in the flywheel with a fine point black sharpie pen. This was checking timing on a dual Light Speed installation.
 
Ironflight said:
Of course, you can time the P-Mag EI with the engine off, by just blowing in the little tube - very convenient :)
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So..... no timing marks to line up, just blow in the little tube?

If you line up timing marks (engine off), how do you compensate for gear lash? Here is the problem. If it is a problem.
 
gasman said:
So..... no timing marks to line up, just blow in the little tube?

If you line up timing marks (engine off), how do you compensate for gear lash? Here is the problem. If it is a problem.
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Line up the timing marks while moving in the correct direction (don't oveshoot), then blow in the tube. You'll be as accurate as the hysteresis in the gear train will allow. And mostly, you won't be near a spinning prop!:eek:
 
Here are a few photos of the (crude) timing device I made from some scrap for static timing of the mags.

A few pieces of aluminum riveted or screwed together (sorry, iphone photo is a bit fuzzy).



In place on the engine:



A simple cleco attaches the device to the back baffle support. The horizontal angle piece is set off by a spacer so that it is just to the pilot's side of the case split:



And another one to the front support with a similar spacer:



The front of the device has a piece of 0.032 Al that is perfectly aligned with the case split (which is the reference datum for how the timing marks are aligned on the starter ring gear):



I transferred the timing marks to the front of the starter ring gear with a black magic marker, and it is easy to look at the alignment by looking down the piece of aluminum.

Greg
 
This is my timing setup. The baffle support is pushed up out of the way and the straight edge is held tightly against the raised edge of the crankcase (ECI engine). I now have dual Lightspeeds but used the same setup previously for timing the single mag.

I have used the strobe light for both the mag and a the EI but its not a lot of fun.
When timing the mag with the strobe I found that the strobe sensor would not pickup the spark pulses through the shielded HT cable. I put a short non-shielded patch cable between the spark plug and the HT cable and put the strobe sensor around the patch cable.

Fin
9A
2012 dues paid

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Performance loss

RV10inOz said:
I bet you did not notice any performance change either?
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I know that when I was experimenting with 3 degrees retarded timing I was seeing approx 100 RPM loss on full power run-up. I'm running a carb, fixed pitch prop, and magnetos. When anyone is saying you won't notice any power loss most likely has a C/S prop, which mask the reduced power.
 
Hang on, from 28 to 25(correct) is what I was referring to.

Less than that you might.

The point was a bit of advance, was no gain.

DB
 
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