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-   -   CYL #6 CHT Dilemma (https://vansairforce.net/community/showthread.php?t=199804)

rvanstory 10-04-2021 06:20 PM

CYL #6 CHT Dilemma
 
I like running LOP and have been playing with injector restrictors to get my GAMI spread as close as possible. My #6 cylinder is always my hottest, regardless of what I've tried. I do NOT want to start cutting for cowl flaps or other mods.

Right now, it seems I have a choice between the "lesser of two evils". With the stock restrictor in #6 (.0280) I have "ok" CHT's in climb with only #6 going above 400 to about 405 in climbout. I'm ok with that. But in cruise, #6 will run around 385-390. A little higher than I want since other CHT's are at or below 360.

When I put a 1/2 size restrictor in the #6 nozzle, my cruise CHT on #6 goes down to around 365 (perfect!). BUT, my climb out CHT gets as high as 421!

So, question is, am I better with cooler #6 in climb? Or better with a cooler #6 in cruise?

BobTurner 10-04-2021 07:09 PM

You seem to be trying to use the fuel flow apertures to adjust the CHT. Not the right approach.
1. Do the so-called Gami test: slowly lean, and note the fuel flow when each cylinder reaches peak EGT. Do this for your favorite cruise condition.
2. Note the first to peak (going rich to lean) is the leanest cylinder. Increase the diameter of the injector aperture for that cylinder. Last to peak is the richest. Decrease its aperture size. etc. etc., until they all reach peak EGT at nearly the same time, as you slowly lean.
3. Check that you are still getting minimum fuel flow (24 gal/hr IIRC) at full throttle seal level takeoff.
Now attack any CHT issues, by adjusting cooling air/baffling. Many insert a washer between the aft cylinder and the baffling to get more air in there, although that's usually for #5. Use RTV to seal up all escape paths for the cooling air. Double check that all baffling is in proper place.

rvanstory 10-05-2021 06:23 AM

Quote:

Originally Posted by BobTurner (Post 1559817)
You seem to be trying to use the fuel flow apertures to adjust the CHT. Not the right approach.

I'm not trying to use fuel flow to adjust CHT's, I'm changing the restrictors to adjust GAMI spread. I did exactly as you described and found the #2 CYL was the one that was too lean and outside my desired spread of .5 GPH or less. I increased it's restrictor and got the desired GAMI spread of .3.

However, I also find my #6 is the leanest. #6 is the last to peak (richest cylinder). By the time I get #6 EGT's to 50 degrees LOP, my leanest cylinders are getting up to 90 degrees LOP, a little leaner than I'd like.

So, I decided to put a restrictor in #6 so it would peak a little sooner (not last). But a 1/2 step restrictor (.0275) AFFECTED climb out temps. I was surprised it did, but it seems logical (less fuel, less cooling).

That leads to my question. Better to live with 20 degree hotter CHT is cruise or climb? It seems that 1/2 step restrictor is affecting CHT, even if I didn't plan on that happening.

N427EF 10-05-2021 06:37 AM

Quote:

have been playing with injector restrictors
Use a more scientific approach rather than "playing " with restrictor sizes.
Closing your GAMI spread requires an approach where you analyze fuel flow for all cylinders.
Also, Bob is right in suggesting the added washer between the rear baffles and cylinder, although getting the fuel flow issues solved first would be my suggestion as well.

Carl Froehlich 10-05-2021 08:27 AM

Get the GAMI spread right (not hard to get to 0.0 - 0.1 GPH). After that, look for what is causing your out of line high CHT on #6. Some thoughts:
- As was posted, do the washer spacer on the #6 baffle fastener screw.
- If you are dragging cabin heat air off the rear baffle, try adding a restrictor on the air to heat muff flange. I put round piece of aluminum over the flange with a 3/4 hole in the center. The restrictor is held on place with some aluminum tape, and the SCAT hose goes over it. There is still way too much cabin heat so not a problem. I note the cabin heat flow is reduced but now hotter. On the coldest day I just had the rear heat cracked open and the front heat off - still plenty warm.
- Do the other normal dives into baffling to see what else can be done.

Carl

BillL 10-05-2021 10:24 AM

What's all this talk about screws and washer spacing - - that is a bandaid for the shallow fins - - it only applies to aft #5 and front of #2. #6 has full fin depth on the aft side.

Randy, I have done many tricks too assist my #4 CHT being 20F higher than the rest. A couple of things might help you. 1. clean the flashing under the spark plug where the air flows through the head. Maybe 10F benefit, no serious mods to do there. Get a round file, but be warned they are hard to find. and 2. Inspect the baffle opening under that head There are two gaps. Measure them - I'll bet they are 1/4" smaller than the other heads. Pay particular attending to the gap around the CHT probes. Don't bother with the barrel gap it has no influence on the CHT. Now, this all assumes you have swapped probes to be sure it is a real head/cooling issue and not a probe variation.

lr172 10-05-2021 12:35 PM

Quote:

Originally Posted by rvanstory (Post 1559800)
I like running LOP and have been playing with injector restrictors to get my GAMI spread as close as possible. My #6 cylinder is always my hottest, regardless of what I've tried. I do NOT want to start cutting for cowl flaps or other mods.

Right now, it seems I have a choice between the "lesser of two evils". With the stock restrictor in #6 (.0280) I have "ok" CHT's in climb with only #6 going above 400 to about 405 in climbout. I'm ok with that. But in cruise, #6 will run around 385-390. A little higher than I want since other CHT's are at or below 360.

When I put a 1/2 size restrictor in the #6 nozzle, my cruise CHT on #6 goes down to around 365 (perfect!). BUT, my climb out CHT gets as high as 421!

So, question is, am I better with cooler #6 in climb? Or better with a cooler #6 in cruise?

390 at LOP cruise seems pretty high, unless only 10-20* LOP, and might indicate an issue other than mixture, eventhough a fatter mixture makes it better. My GAMI spread is about .3 (not possible to get any better in my configuration) and at 8K' in LOP cruise, I see between 330 and 355 on #6 (also my hottest) in the summer, depending upon ambients. I generally cruise at 50-80* LOP


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