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Baffle Question, #4 hotter, Question . . pict

BillL

Well Known Member
In 20 hours of flight #4 CHT has always been hotter. It does not seem that the GAMI test points to it being out of line A/F wise.

The #2 head was blocked and un blocked with zero difference to #4.

The oil shutter was operated full open and full closed, no change in #4 CHT.

The fin flashing under the plug was reduced and dropped the diff from ~35F to ~15-20F. All other heads are within a 2F bracket.

edit: Swapped the probes from 2 & 4 and the high temp became #2 high as it was now in #4 head.

Has anyone determined that the baffle flap wrapped over the fins in the picture below has any effect? I am thinking it is the only thing left and a bit tentative to just cut it off without and external stiffener. The trouble would be worth it if anyone has data it will have an effect, though.

Any experience with this in VAF land?

[ok I admit, 15-20F is not bad, it just has me stumped. I gots to know . . ]

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Watching with interest as my #4 CHT is always about 15F hotter as well. I always thought it must be due to a lot of air exiting through the oil cooler, but at the same time when the upper plenum pressure is stable in flight that shouldn't really matter.
 
Watching with interest as my #4 CHT is always about 15F hotter as well. I always thought it must be due to a lot of air exiting through the oil cooler, but at the same time when the upper plenum pressure is stable in flight that shouldn't really matter.

Yeah . . . me too, wonderful to have data, I did the test several times to be sure (5 minutes at a time), and looked and plotted the G3X data - nothing.
 
Yeah . . . me too, wonderful to have data, I did the test several times to be sure (5 minutes at a time), and looked and plotted the G3X data - nothing.

I took oil cooler air from behind #3, then ducted it from the cowl inlets, then changed the cooler size and went back to taking it behind #3. It was easy to see the effect on oil temps, but there was no significant change to #3's CHT.

Bill, be bold, bend that tab down 90 degrees and report!
 
That tab is definitely going to restrict airflow through those fins, probably cuts flow almost in half. As Dan said, if you gently straighten it and allow the airflow to increase, those fins that are right next to the exhaust will really transfer some heat away from the head.

My .02
 
Bill,
Nothing to do with the lower tab and it was also in conjunction with remote mounting my oil cooler but I attribute a 20+ degree drop in #4 CHT's with the curvy baffling additions.

IMG_9404-L.png


Unrelated but oil temps increased slightly with my remote mount because I lost some plenum pressure to the cooler face, more mods for another time.....
 
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Thanks Andy!

I took oil cooler air from behind #3, then ducted it from the cowl inlets, then changed the cooler size and went back to taking it behind #3. It was easy to see the effect on oil temps, but there was no significant change to #3's CHT.

Bill, be bold, bend that tab down 90 degrees and report!

That tab is definitely going to restrict airflow through those fins, probably cuts flow almost in half. As Dan said, if you gently straighten it and allow the airflow to increase, those fins that are right next to the exhaust will really transfer some heat away from the head.

My .02

I bent the tab out of the way and there was no change in #4 temp relative to the others, still 15-20F.

If, one day something changes, I will post.
 
#4 Cylinder Temp

Hi Bill,

I had the same problem (#4 ran 20 degrees F hotter than next hottest) and alway attributed it to the pick up point behind #4 for my firewall mounted oil cooler. My GAMI spread was also good at less than .5 gph--which lead me to believe it to be cooling air flow and not air/fuel flow. After ten years of tweaking the cooling airflow with minimal success I concluded that it MUST be associated with fuel/air flow--so I called Don at AirFlow Performance and explained the situation. He suggested some simple tests which I did then recommended a 1/2 step down in FI restrictor for #4--went down from a .024 to a .0235 restrictor. BINGO--now #4 runs in the middle of the pack for cht with a total cht spread averaging 8 degrees. My GAMI spread is now .2 gph.

Hope this helps.

Cheers,

db
 
On cylinder #3 we use the "washer trick" to promote airflow where the fins are shallower than we might like. Would a similar tactic be appropriate here?
 
#4 Cylinder Temp

Hey Kurt,

The reason the washer trick works on cylinder #3 is due to the cooling fin depth on the back side of the #3 cylinder. On the back side of cylinder 3 (and #1) the cooling fins taper to "0" depth at the mid cylinder casting seam. When you put a washer between the cylinder at that seam and the inside surface of the baffle wall, the washer thickness allows some cooling air to flow past this blockage and reach the lower head fins--therefore improving the cooling of that cylinder. Since all cylinders are the same it is also true on cylinder #2 except in the front. If you have the opportunity, take a look at a cylinder off an aircraft--it will be apparent what I am talking about. If you already knew this please just disregard.

Many of us have actually incorporated bypass ramps into our baffles at the rear of cylinder #3 and at the front of cylinder #2 to correct this flow issue.
However, since the back of #4 cylinder has continuous fin depth, a ramp does not provide the same benefit.

Cheers,

db
 
Hi Bill,

I had the same problem (#4 ran 20 degrees F hotter than next hottest) and alway attributed it to the pick up point behind #4 for my firewall mounted oil cooler. My GAMI spread was also good at less than .5 gph--which lead me to believe it to be cooling air flow and not air/fuel flow. After ten years of tweaking the cooling airflow with minimal success I concluded that it MUST be associated with fuel/air flow--so I called Don at AirFlow Performance and explained the situation. He suggested some simple tests which I did then recommended a 1/2 step down in FI restrictor for #4--went down from a .024 to a .0235 restrictor. BINGO--now #4 runs in the middle of the pack for cht with a total cht spread averaging 8 degrees. My GAMI spread is now .2 gph.

Hope this helps.

Cheers,

db

Dave, it is time to go do my cruise performance documentation, my skill in tweaking the mixture knob is getting better and I'll do another GAMI test to see what it yields, this seems to be the only thing left. Thanks for your experience, I'll post the final fix.
 
#4 CHT

Bill,

Just a comment--I understand that Don at AirFlow Performance will help diagnose FI issues even if you do not use one of his AirFlow Performance FI systems. Give him a call -- incredibly knowledgable and sure took care of my issue in short order!! He also contributes knowledge to this forum from time to time.

Best of luck and of course,

Cheers,

db
 
Hey Kurt,

The reason the washer trick works on cylinder #3 is due to the cooling fin depth on the back side of the #3 cylinder. On the back side of cylinder 3 (and #1) the cooling fins taper to "0" depth at the mid cylinder casting seam. When you put a washer between the cylinder at that seam and the inside surface of the baffle wall, the washer thickness allows some cooling air to flow past this blockage and reach the lower head fins--therefore improving the cooling of that cylinder. Since all cylinders are the same it is also true on cylinder #2 except in the front. If you have the opportunity, take a look at a cylinder off an aircraft--it will be apparent what I am talking about. If you already knew this please just disregard.

Many of us have actually incorporated bypass ramps into our baffles at the rear of cylinder #3 and at the front of cylinder #2 to correct this flow issue.
However, since the back of #4 cylinder has continuous fin depth, a ramp does not provide the same benefit.

Cheers,

db

I understand the logic and the fact that there's little or no fin depth on the intake side of a parallel valve cylinder head. But are we all just assuming there's no benefit to getting some additional space between the head and the baffle, or do we know for a fact from testing?

I don't want to repeat someone's tests that may have already been conducted and concluded there's no benefit. But if it's not actually been tested, then I'm willing to try it and document results.

Not behind #4.

Dan, have you conducted this test in the past? You're the testing guru 'round these parts! :)
 
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I understand the logic and the fact that there's little or no fin depth on the intake side of a parallel valve cylinder head. But are we all just assuming there's no benefit to getting some additional space between the head and the baffle, or do we know for a fact from testing?

Not tested, but since the heads are designed to work when siamesed together, the exhaust side of #3 provides the air duct to feed the lower fins of #1 and #3. It stands to reason that the exhaust side of any "lone" head has roughly double capacity, while the intake side of a lone head has essentially zero. Also, spacing baffles away from fins is counterproductive, since you want air to be in contact with the fins to transfer heat. The reason we bypass the zero fin depth on the equator of the intake side is to deliver air to the lower fins.

I would strongly suspect that adding capacity to any lone exhaust side would simply reduce overall delta p
 
I understand the logic and the fact that there's little or no fin depth on the intake side of a parallel valve cylinder head.

...or angle valve.

But are we all just assuming there's no benefit to getting some additional space between the head and the baffle, or do we know for a fact from testing?

I don't want to repeat someone's tests that may have already been conducted and concluded there's no benefit. But if it's not actually been tested, then I'm willing to try it and document results.

Dan, have you conducted this test in the past? You're the testing guru 'round these parts! :)

Nope, I have not tested additional airflow space on the exhaust side of a cylinder. Go for it, and report back!
 
Bill, I found a number of air leaks around fins on #4. In a darkened hangar, take a flashlight from below and above, while observing for light "leaks". Hard to describe in words where I found them, and I would not be able to take pictures anyway because they are out of direct sight, but there were some surprisingly large leaks near aft side of #4, near the crankshaft end of the fins. One cannot directly see these spots, and I just put blobs of RTV on my finger and did my best to plug the leaks, using the light as a guide.
 
Bill, I found a number of air leaks around fins on #4. In a darkened hangar, take a flashlight from below and above, while observing for light "leaks". Hard to describe in words where I found them, and I would not be able to take pictures anyway because they are out of direct sight, but there were some surprisingly large leaks near aft side of #4, near the crankshaft end of the fins. One cannot directly see these spots, and I just put blobs of RTV on my finger and did my best to plug the leaks, using the light as a guide.

Good thought, Alex, but I wrapped the fins with silicone impregnated fiberglass from the centerline down to the baffle opening below. Then, the top of the baffle was sealed to the wrap with an RTV bead.

Michael, My engine cooling flow is so low that when pressure testing the upper, I could fully open and close the oil cooler shutter and the pressure did not change, nor did the CHT's. My inlets (medium SJ rings) must be large relative to the engine flow restriction. Lower plenum is a fraction of an inch of water. Virtually zero. Bad for drag, me thinks.
 
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