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High CHTs

paul330

Well Known Member
My Aerosport IO540 D5A4 is running beautifully - except for the CHTs.

All is well apart from the rear two cylinders. In the climb (at 120kts as opposed to best climb 100kts), they get up to around 430F. In the cruise, all is well until I lean. At peak EGT, they creep up to around 415F. To get below 400F, I need to enrich to about 15gph. The other 4 cylinders stay below 400F. Nothing gets above 435F.

So, all are within limits but I understand that below 400F is desirable. The baffling all looks good and well sealed. At the rear, I have the standard VANS set-up of air to the oil cooler, heating duct and two mag blast tubes. I have cut off the entire front dams.

How can I get the rear cylinders to run cooler?

(The engine has about 50 hours and appears to have run in with oil consumption very low - no oil added since the last change some 12 hours ago)
 
Do you have dams in front of the front cylinders ? That forces air back.

Look at clearance at the back of the baffles to see if you can get more air down the back of the rear cylinders. Also, check the inter cylinder baffles - the factory ones that come with the engine.

You could check the exit size of the hole of the cowling at the bottom or possibly put louvres on the lower cowl.

Also check again the timing of the ignition. If you are on electronic on one side, consider car plugs on that side.

All of these contributed to reducing our temps on our 7 - a different installation, but the principles are similar.
 
Here is what worked for me

1) Completely removed the air dams in front of the #1 and #2 cylinders.

2) Sealed every hole I could find around all of the baffles, including the inter-cylinder baffles. Used RTV red.

3) Adjusted the gap seal between the baffles and the cowl top so that there was very little leakage there. I used RTV red in strategic places on the baffles and a few places on the cowl.

4) Finally added an Anti-Splat-Aero cowl flap to the passengers side of the lower cowl unit positioned on the vertical surface.
See this thread:
http://www.vansairforce.com/community/showthread.php?t=141406&highlight=cowl+flap

Now I can climb through 8000 ft on a hot day at 1200 ft/min and stay below 385 deg. F.
 
Something is wrong

For a stock cowl, your CHTs are abnormally high. Some questions:
- Did you cut the stock bottom cowl louvre slots?
- What oil temps are you seeing?
- Are you running LightSpeed electronic ignitions? If so, what model?
- What GAMI spread do you have?
- Besides the two MAG blast tubes what other air are you robbing from the top of the engine?

As the earlier post state, there are things you can do to fine tune CHTs, but it sounds to me there is a root issue at play that needs to be first addressed.

Carl
 
All above I agree with except try to balance some of the temps by adding back the front dams. They are there to move more cool air to the rear cylinders. After you add them back and the front cylinders get too hot you can cut a little at a time until you balance the temps. One item that really helped mine was to pull off the original stock black baffle material that Vans supplied and I replaced it with the new silicone stuff from aircraft spruce. This sealed much better against the cowl for me and probably lowered my temps by 15 to 20 degrees. Hottest I see is 405 in 100 degree climb outs.
 
Timing is good with standard Slicks.

Everything is sealed up with red RTV but I'll go round again and check.

Air loss at back is 2 mag blast tubes, oil cooler and heating air - all stock as per plans.

I have had conflicting views on the front dams - they are currently totally cut off.

Can't comment on the GAMI spread as I am still having trouble with the EGT crimps and have yet to get all working at the same time :mad: - work in progress.....

Next thing is to try and increase the airflow behind the cylinders - probably try the "washer" approach first. I'll report back, but it's going to be a couple of weeks.

Thanks for the input.
 
Hot CHT

Paul;
I feel your pain!! I spent years getting my CHTs below 400F, what I have now is mostly cut off air dams as my 1-2 CHTs are not much cooler than the rear 2, I put a deflector over 3-4 to move more air over them to 5-6, and I even put a deflector behind 6 to force the air down into the rear. In the end, a good friend and mentor suggested I use more flaps on takeoff- Make that where you should focus next steps on. My temps go above 400, around 420F on takeoff but well within the lycoming 435F "normal" for takeoff range. Cruise however is at 55% 50F or so LOP around 370F the hottest and 340's coolest (Summer in SoCal). Since I spend 90% of my time cruising that is where my efforts went and I am happy with a 30F-40F difference between cylinders.
You have to get the injectors tuned to .5 or better, If I ran ROP my CHT's would be significantly higher too, get it tuned and those temps will go down.
Keep us updated on your progress!
Pascal
 
There are a variety or tricks to implement to cool one cylinder over another.

I'm sure all your EGTs aren't flat lined, so know why the temp for each cylinder might help for use to make better recommendations.
 
I've done some more work....

I resealed all around the baffles to seal joint. Also, peering at the back of the engine I noticed some swarf and other material that shouldn't have been there. I got some of it out but I need to get the bottom cowl off and loosen the baffles to clear the rest. I will also then try to get the washers in place.

Coolest cylinders are the middle, front are about 10F hotter, rear about 40F. I'm not worried about the difference. I tried going LOP for the first time and the temps dropped nicely so the hottest cylinder (6) was about 375F - I can live with that until the next time the bottom cowl comes off. I believe that removing the potentially fin-blocking material and using the washers will get me where I need to be.

Incidentally, at 10,000' using 19/2300 (50%), I was getting 150kts TAS @9.5gph :D
 
Keep looking

Paul,
I very much doubt that your CHT issue is baffle related.
I know you said the timing was "good" where is is set?
What is your take off fuel flow?
It needs to be 26GPH no less!
Certainly good advice on all the baffle and clearing the fins from casting flash
but your temps are ridiculously high for a stock installation.

As a comparison, I added a set of louvers on the bottom of the cowl, (2x
6 inch sections) and rarely, even on 100F days see temperature above 380 in climb and around 300 to 320 in cruise. In winter I have to tape those louvers off to keep CHTs from dropping below 300F.
I am north of you in the SF Bay area.
 
, +1 on what Ernst says. I see about the same temps in cruise as Ernst, in climb, as long as I keep the airspeed above 120kts, my temps stay below 400.
How many hours do you have on the engine? My temps did not come down until I had 20 or so hours. What is your timing? What is your FF rate? 24 to 26 is about normal.
 
Engine has 45 hours on it.

Timing is whatever it said in the manual - 23 or 25 BTDC with Slick mags. Can't remember off the top of my head but it was checked spot on 15 hours ago.

Mixture could be too lean from what you have said. My field has a density altitude of around 4-5000' so whilst it may be good for this, it could be too lean compared to sea level. I'll check the FF on my next TO but I believe it is around 23gph. I can see why this would affect the climb CHTs but if you lean to a fuel flow in the cruise, how does the max FF affect anything?

Thanks for the advice.
 
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Paul,

Please note that quite a number of fuel control units in recent years are coming out of the factory at min fuel flow or worse. I suspect the old timers who built them knew they had to achieve MAX flow and set them up right. They will be retired and replaced by who knows what cheap labour. Just a hunch. But we have seen many in the last few years.

With a DA of 5000' approximately I would expect the flow to be in the 21.5GPH range. But that is a guess off the top of my head, but a pretty good one. ;)

The cross check is simple, at full rich your EGT values assuming probes are about 2" from the flange, should be around 1280-1320dF. This is also dependent on the timing being right.

You should climb using the target EGT method, have a poke around the APS website, we have a download you can do which explains it.

I can climb in Australian summers at 120KIAS and not go past 380-385. Appropriately LOP and that is a variable, but at 10,000' that is 10-20dF LOP, and if you can get down to 9.3GPH it sounds like your GAMI spread is quite OK. Normal flow at that would be 10-10.3 GPH. Typical CHT in the cruise LOP would be 330-345 at this time of year (Summer) and 310-330 in winter.
 
My current rv10 will climb full throttle, 90kts all day and never hit 400. Bone stock cowl, baffles, louvers. Gear slot extended maybe 2 1/2 inches. They should all do this. If one can't, then something is being missed.
 
Lowering CHTs

Have you considered going lean of peak? Have you listened to or read this leaning article about the RED BOX? I fly my O-540 lean of peak which is way richer than any engine stumble. I use 6 temp probs so I can see what's happening. My plugs are always clean. Excess air can cool the temperatures.

http://www.eaavideo.org/video.aspx?v=2534345030001
 
Have you considered going lean of peak? Have you listened to or read this leaning article about the RED BOX? I fly my O-540 lean of peak which is way richer than any engine stumble. I use 6 temp probs so I can see what's happening. My plugs are always clean. Excess air can cool the temperatures.

http://www.eaavideo.org/video.aspx?v=2534345030001

Just a point of order. It is not excess air, for every intake at the same conditions there is the same amount of air, and there is no way physically possible for any "excess air" to do the cooling you get with LOP ops.

It is simple. Lower peak pressure in the cylinder caused by a greater ThetaPP angle. Nothing else.

As for the problems the thread starter is having, going LOP will lower his temperatures for sure, but he still is off the ball by a bit.
 
Thanks again for all the input.

TO fuel flow @ 5000DA is 22.5 so that sounds about right. Climb CHTs at 120kts are fine - just occasionally creeping over 400F on the rear cylinders but well below 435F.

Now I have finally got all my EGT probes working, I have found that LOP gives me considerably lower CHTs - around 370F on the rear cylinders @ 6000' and around 350C @ 10k. Bearing in mind it is summer here and the temperatures at 6000' are still around 25C, I can live with that.

So, I'm pretty happy now but will try the old "washer between cylinder and baffle" trick next time I have the lower cowl off.
 
Paul, on the weekend climbing through 4000-5000 I saw flow higher than that, I should have taken a photo.

I would try to get to a sea level port and do a takeoff there and see what flow you get passing 300'-600' when the numbers have settled down.

Should be around 25.5 GPH.
 
Might be a bit tricky around here - at least until I do a trip to Cape Town ;)

In any case, surely that only affects the climb CHTs? If you lean to a fuel flow in the cruise then surely it makes no difference what the TO fuel flow is......
 
Might be a bit tricky around here - at least until I do a trip to Cape Town ;)

In any case, surely that only affects the climb CHTs? If you lean to a fuel flow in the cruise then surely it makes no difference what the TO fuel flow is......

True. Which is why I think your cooling is not optimal because even LOP your temps are higher than most others.

But if you have insufficient flow on takeoff, you have climb limitations you otherwise should not have, and where you live that can be a limit you do not want.
 
Idle mixture is easy to adjust. Presumably, there is a screw or something on the injection unit (Precision) which can adjust the max flow......? Or is this something that should not be messed with?
 
Idle mixture is easy to adjust. Presumably, there is a screw or something on the injection unit (Precision) which can adjust the max flow......? Or is this something that should not be messed with?

Paul, if your FCU is not flowing properly, you need to remove it and send it to a facility with a proper test bench, and insist they set it to the upper limit of spec.

My partner in the Advanced Pilot Seminars Andrew Denyer does these often at his engine facility in Adelaide. If you can't get it done in RSA, Fedexit to Riverina Airmotive in Adelaide.

Your will not be the first.

And no, there is no simple screw you can tweak.
 
Just to keep this thread going as I haven't totally solved the issue......

The ol' "washer between the baffle and cylinder" trick worked beautifully on the No5 and took 20F off the temperature. It won't work for the No6 because it is 180 out and doesn't have the narrow fins at the back.

I believe that the issue is the big 4" hole for the oil cooler just above the cylinder taking too much air. Unfortunately, I can't block off any of it as the oil temperature is where I want it. I don't think it is a baffling issue as I can't find any obvious faults with it and the other cylinders are fine.

I have just come back from an air race in Botswana (see www.airrace.co.bw - it's awesome :D). At 21gph, max MAP (about 26" at 4000' DA) and 2600RPM, the CHTs were around 370F except No 6 which hovered around 400F. Oil temperature was 200-205. I feel I could have leaned a bit more for best power and an extra couple of knots but didn't want to because of the No6 temp. I made my handicap speed (173.5kts TAS) but racing is all about those extra knots (Oh, but staying on track and finding the turn points helps.....)

My plan is to fit a 17 row oil cooler instead of the stock 13 row and use a 3" duct. My hope is that the lower airflow through a bigger cooler will reduce the CHT whilst maintaining the oil temp. Before embarking on that expense, I intend to check a couple of other things:

- accuracy of probe - don't want to be chasing ghosts
- injection jet - am I getting the correct fuel into the cylinder? Doubt this is the problem because otherwise wouldn't I expect to see No6 EGT peak first?

The engine now has 100 hours on it so I don't believe that a "tight" cylinder can any longer be a possibility. Since the cylinder stays below 435F in climb and 400F in cruise, I am not overly worried but I really would like to get 30F off it so it more closely matches the others.

Question for the engine gurus: can I lean to peak at full power so I can then go to +100F? Doesn't sound like a good idea so I was using the engine power charts to estimate the required fuel flow.
 
My plan is to fit a 17 row oil cooler instead of the stock 13 row and use a 3" duct. My hope is that the lower airflow through a bigger cooler will reduce the CHT whilst maintaining the oil temp.

Don't think that plan will work. You'll cut flow area almost in half, but you won't double cooler area.

As for CHT reduction, I'm pretty sure you won't see it go down. However, one good test is worth 1000 opinions, including mine. Make some simple conical adapters (sheet metal, a pop rivet or two, and duct tape) so you can run a length of 3" duct for one flight. Yes, oil temp will be warmer than usual, but you should be able to determine if CHT changes in relation to the other cylinders.

- injection jet - am I getting the correct fuel into the cylinder? Doubt this is the problem because otherwise wouldn't I expect to see No6 EGT peak first?

Correct. If it was restricted that cylinder would peak first when pulling rich to lean.

can I lean to peak at full power so I can then go to +100F?

Answer is "maybe". A lot depends on CHT and intake air temperature. An FAA-standard detonation sweep is done with one cylinder at redline CHT, the remainder within 50F of redline, oil at max temp, and intake air at 103F. At full rated power under these conditions, the hot cylinder will probably suffer detonation at only a bit leaner than best power mixture...still well rich of peak EGT.

However, when racing at 4000 ft (26" and maybe 2650), with lower CHT and intake air temp, the risk is reduced. The EAB wildcard is ignition timing, fixed (usually magnetos) or variable advance. More advance increases detonation risk.

A bit of mild detonation won't hurt anything, so sneak up on it. Set desired power while rich, let everything stabilize, then lean a little bit at a time while watching CHT carefully. It will rise as you lean, but if at some point the CHT digits start flipping like a gas pump, richen up or pull off some MP; you just found the detonation limit.

Alternate plan; We know best power mixture will be about 125 rich of peak. Settle in at the expected flight conditions, set 24/2400 or similar, find peak, go to 125 ~150 ROP, and note the actual EGT. Now put power where you want it and readjust mixture if necessary to achieve the same target EGT. The EGT which equates to 125~150 ROP at 70 or 75% will be the same EGT at a higher power setting.
 
Thanks for the input Dan.

If I simply temporarily cover a portion of the duct inlet with AL tape, would that validate the experiment? Why do you think that restricting the oil cooler flow won't help the No6 CHT?

Finding +120F at 75% sounds like a good plan. Since the engine ran really sweetly at 2600RPM, I think I will run 2650 next year. If I can get the CHT down by 30F, then with a little leaner mixture I could possibly get 2-3kts extra. Over a near 2 hour race, that can make around 1:30m difference or 3:00m over the 2 days......
 
If I simply temporarily cover a portion of the duct inlet with AL tape, would that validate the experiment? Why do you think that restricting the oil cooler flow won't help the No6 CHT?

The corrugated interior wall suggests a thick boundary layer, so true flow area is probably less than the nominal diameter, and it would get proportionally worse as the nominal diameter is reduced. A simple intake restriction wouldn't incorporate the effect of the duct walls, which is why I suggested inserting a length of actual 3" scat hose.

As to CHT, well, been there, done that. It started when measurement found a 16F to 17F difference between OAT and the face of the oil cooler, just down a short 4" duct from the #3 baffle wall. A little bit is compression heating, but most is heat picked up from the upper cylinder fins.

One way to lower oil temperature was to duct the oil cooler air supply all the way from the front of the plenum, just behind the cowl intakes, delivering lower temperature air to the oil cooler. It would also isolate cylinder #3, which would no longer have any of its cooling air supposedly "stolen" by the oil cooler duct inlet. So I built this insulated duct system.

Duct%20Layup.jpg


Finished%20Duct.jpg


Results: As expected, dropping delivered air temperature improved oil temperatures, even though I'm sure it reduced mass flow slightly (long flat ducts, not an optimum shape).

However, I noted no change to #3 CHT when I installed the above duct system, nor when I later removed it, returning to a flared intake hole in the rear baffle wall. A little thought tells why. The key factor driving air through the cylinder fins is simple pressure delta between the upper and lower cowl volumes. An oil cooler intake near the cylinder may flow a lot of mass, but if it does not reduce pressure in the vicinity of that cylinder, the cylinder's cooling performance will not change. Setting aside frictional duct losses, pressure should be pretty much the same in the oil cooler duct as it is in the upper cowl volume. It has to be. The upper/lower cowl pressure delta is what drives air through the cooler face, just like the cylinder fins. The pressure drop comes after the cooler.

Admittedly, it's an observation and a theory. Confirmation or denial would be to install two piccolo tubes, one over each rear cylinder. One would be near the oil cooler inlet of course. Compare in-flight pressures. If the piccolo near the oil cooler duct inlet is lower by some significant amount, then (and only then) is the duct stealing from the cylinder.

Finding +120F at 75% sounds like a good plan. Since the engine ran really sweetly at 2600RPM, I think I will run 2650 next year. If I can get the CHT down by 30F, then with a little leaner mixture I could possibly get 2-3kts extra. Over a near 2 hour race, that can make around 1:30m difference or 3:00m over the 2 days......

The mixture vs power curve is fairly flat in the ROP region, but there is a little power to gain with careful knob management. Here's an example, a 540K on the FAA dyno at Hughes, 26" and 2700, pretty close to what you mentioned as your race conditions:

SFT%202700-26%2011%20AF%20EGT.jpg


Max power is about 245, at 125 ROP (1590 EGT vs 1465). Detonation onset is about 60 ROP, with full development around 50~45 ROP. The difference between heaven and **** is a spread of about 75F.

Remember, this data was taken when real hot, and the K is considered to be a worse case example. You might have a bit more leeway, or less, depending on compression and ignition advance.

First determine if you can run at best power mixture with the CHTs you have now. If so, work on CHT reduction. When you get it down by 30F or so, install a variable cowl exit to reduce mass flow at high airspeed. The restricted exit area will drive the CHTs back up to where you started, but you'll get the 2 knots.
 
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How is your exit opening on the bottom cowl? (Forgive me if I missed it.)

It should be even withe the firewall, even a little forward. This will help draw air out of the bottom cowl.
 
How is your exit opening on the bottom cowl?
It should be even withe the firewall, even a little forward. This will help draw air out of the bottom cowl.

Cutting the exit further forward is unlikely to lower local pressure just aft of the exit opening. Trimming forward of the firewall requires a slant cut, which will increase exit area. Increasing exit area practically always lowers CHT, the mechanism being decreased pressure in the lower cowl volume, thus increased deltaP across the cylinder fins. Just keep making it larger, and it will run cooler and cooler, until the exit is so large that lower cowl pressure is near zero. Unfortunately, it will also run slower, which doesn't seem to be what Paul had in mind.

http://www.vansairforce.com/community/showpost.php?p=1169174&postcount=26
 
...Just keep making it larger, and it will run cooler and cooler, until the exit is so large that lower cowl pressure is near zero. Unfortunately, it will also run slower, which doesn't seem to be what Paul had in mind.

http://www.vansairforce.com/community/showpost.php?p=1169174&postcount=26

Yes Dan, but... if the cut only slants forward an inch, what is the speed loss? 2 knots, maybe 3? Well worth the tradeoff, if all else fails. Also, IL looks better than adding louvers or a lip and simpler than adding a cowl flap.
 
If this is still a problem, you may want to consider getting your injectors balanced. In my stock Lyc IO-540, I had a similar issue where #5 & #6 CHTs ran hotter than the rest, and would easily hit 400 on climb out.

I took my plane to Airflow Performance and they did an injector balancing for me. We found that #1 & #2 were too rich, and #5 & #6 were too lean (which accounted for the higher CHTs). We installed new restrictors that balanced things out. All of my high CHT issues went away.

While I know it'd be difficult for you to take your plane to them, they can do this by shipping you restrictors and you do the experimenting until you find the right combination.
 
Not easy to see, but .. .

Just to keep this thread going as I haven't totally solved the issue.......

Paul, A careful meticulois 10-friend said he had his baffles all sealed, absolutely. Having had my under cylinder baffles off and measured, I looked at his. Huge gaps. You may have already checked these, but if not look carefully from many angles (all) . Mine (and his) did not fit well. Hope this helps . . .
 
Thanks to everyone for the input - especially Dan for his informative tutorial.

I'm not exactly cooking the cylinders at the moment so there is no rush to get this done. As Dan surmises, I don't want to do anything that costs speed as I want to race the aircraft again next year. What I want is to get the no 6 down about 30F to match the others so that I can experiment with a lower fuel flow and see if I can get a bit more power without getting the CHTs too high.

One thing I can't agree with is that the oil duct doesn't "steal" air. The argument that the pressure equalizes I don't think holds water. Since the air is a moving compressible fluid, the pressure is going to be different all over the baffles, so air going through the duct can for sure cause a lower pressure (and therefore less cooling) over the no 6 cylinder.

I'm going to try just blocking some of the duct with AL tape and see if it makes any difference. I'll also have a look at re-working some of the sealing material at the front of the engine where there may possibly be some leaks around the intakes - I'm pretty sure everything aft of that is fine. Next time I have the bottom cowl off, I'll also have a check on the lower baffle seals to make sure all is in order. However, as the cooling is good for all the other cylinders I doubt that I have a sealing problem.

As discussed earlier, I don't think that the no 6 has a weak injector or it would be peaking EGT before the others.

I'll report back - it may be a while.......
 
One thing I can't agree with is that the oil duct doesn't "steal" air. The argument that the pressure equalizes I don't think holds water. Since the air is a moving compressible fluid, the pressure is going to be different all over the baffles, so air going through the duct can for sure cause a lower pressure (and therefore less cooling) over the no 6 cylinder.

And you could be right, so measure and let's see.

The piccolo below is 12", but 6" would probably work ok, just model airplane aluminum tubing. Or you might try a few aquarium air stones, since you're trying to measure a point source. I just ordered a six-pack of the small ones, as I have some similar measurements on my own to-do list.

http://www.drsfostersmith.com/produ...MIpcmW0dKN1QIViSSBCh08zAm9EAQYBSABEgJonfD_BwE

Inexpensive Chinese manometers seem to work fine.

http://www.ebay.com/itm/LCD-Digital...606731?hash=item4402ca940b:g:hv0AAOSwlMFZKUh9

Piccolo%20and%20Probe%20Drawings.jpg
 
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And you could be right, so measure and let's see.

The piccolo below is 12", but 6" would probably work ok, just model airplane aluminum tubing. Or you might try a few aquarium air stones, since you're trying to measure a point source. I just ordered a six-pack of the small ones, as I have some similar measurements on my own to-do list.

http://www.drsfostersmith.com/produ...MIpcmW0dKN1QIViSSBCh08zAm9EAQYBSABEgJonfD_BwE

Inexpensive Chinese manometers seem to work fine.

http://www.ebay.com/itm/LCD-Digital...606731?hash=item4402ca940b:g:hv0AAOSwlMFZKUh9

[/URL]
Dan, Confused - what are exactly are you proposing to measure? Static pressure is clear, but where and why, to illuminate what? Pressure at each individual cylinder?

BTW-Paul - at these velocities and pressures the compressibility of air is not a factor. It is acting like an incompressible fluid.
 
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Yes Dan, but... if the cut only slants forward an inch, what is the speed loss? 2 knots, maybe 3? Well worth the tradeoff, if all else fails. Also, IL looks better than adding louvers or a lip and simpler than adding a cowl flap.

Slight shift from RV-10's but same issue --

That would be my question too, but for a 4 cylinder RV. Is is a lot or only a knot or two?

Trying to get cooler CHTs in climb in the AZ heat.
 
Dan, Confused - what are exactly are you proposing to measure? Static pressure is clear, but where and why, to illuminate what? Pressure at each individual cylinder?

Spot pressures in various locations...making a 3D map really. The subject of interest is how the oil cooler duct location might affect the adjacent cylinder's cooling. Since all flow is driven by pressure delta, we could survey pressures at the locations of interest to gain some insight. For example, in Paul's case one might zip-tie a bubble rock to the aft side of the #6 exhaust pushrod tube, then fly the changes in duct opening that he proposed. Since it is now winter in Oz, perhaps a quick turn around the patch with the cooler duct entirely blocked would be possible. It's a parallel valve, not as dependent on the cooler as an angle valve motor with squirts.

I'd also locate one well inside the duct, perhaps at the cooler face, then another on the exhaust side of the cooler. Perhaps another over #4. The bubble rocks come in packages of six, so place 'em all as desired, and run a bundle of tubing back to the cockpit with the ends marked. A fella with a test plan might learn a lot in a few flights with a helper to swap tubes on the manometer and record data on a clipboard.

That would be my question too, but for a 4 cylinder RV. Is is a lot or only a knot or two? Trying to get cooler CHTs in climb in the AZ heat.

Gil, you gotta do what you gotta do. Maintaining acceptable temperatures is the first priority.

How many knots? You'll hear opinions, but the number of folks who flew before and after NTPS triangles when cutting a cowl outlet is very low, and even then, results would vary based on (a) how much they cut, and (b) baffle leakage.

Ballpark? The variable exit on mine costs 2 to 4 knots when open, depending on conditions. There should be quite a few RV-6 owners with an Anti-Splat cowl flap; ask them for input. And what is the speed penalty for an open cowl outlet on a good 'ole C-182?
 
That would be my question too, but for a 4 cylinder RV. Is is a lot or only a knot or two?

How many knots? You'll hear opinions, but the number of folks who flew before and after NTPS triangles when cutting a cowl outlet is very low, and even then, results would vary based on (a) how much they cut, and (b) baffle leakage.

Ballpark? The variable exit on mine costs 2 to 4 knots when open, depending on conditions. There should be quite a few RV-6 owners with an Anti-Splat cowl flap; ask them for input. And what is the speed penalty for an open cowl outlet on a good 'ole C-182?

Hi Gil-
Just for a reference point, I just installed an AntiSplat cowl flap a couple days ago. It increases the exit air by about 11.2" square inches and in 65% power cruise it seems to have about 2-3 knots of cruise penalty. That said, I generally wouldn't need it at cruise (just climb) and, additionally, since the flap hangs down in the slipstream my un-expert opinion would surmise that just cutting back the cowl exit would have less of an impact on speed than does a cowl flap. Again, though, I wouldn't really know for sure.
Hope this helps.
 
Hi Gil-
Just for a reference point, I just installed an AntiSplat cowl flap a couple days ago. It increases the exit air by about 11.2" square inches and in 65% power cruise it seems to have about 2-3 knots of cruise penalty. That said, I generally wouldn't need it at cruise (just climb) and, additionally, since the flap hangs down in the slipstream my un-expert opinion would surmise that just cutting back the cowl exit would have less of an impact on speed than does a cowl flap. Again, though, I wouldn't really know for sure.
Hope this helps.

Thanks, the cowl flap or louvers are the backup position if needed...:)

Did you get a good reduction in CHTs during climb?
 
Thanks, the cowl flap or louvers are the backup position if needed...:)

Did you get a good reduction in CHTs during climb?
Yes, but I didn't do repeated runs to accurately quantify it.

With that qualification, sustained climb on an 85F+ day full or near-full throttle would normally have me pushing 400 or more. With the cowl flap open I could climb 5000 without surpasing 385 or so. Likewise, oil temp in climb stays down about 15-20F less than it would in climb without the flap. However, once at cruise and with the flap closed, the oil continues to climb in the direction to what I was used to seeing. Still looking for in-cruise solutions to higher-than-desired oil temps on hot days.

Again, though... the above are just off-the-cuff observations that I would not put too much faith in until I can do some more careful testing.

Just another observation, Gil... I would not put excessive stock in the "your baffles must not be well-sealed" advice. That could certainly be true, but my sustained searching on this issue over 5+ years flying is that every engine is different and what works for one doesn't always work for another. In addition, your home base is probably close to the most extreme in the US, so there's no way you're going to see temps as low as a lot of others report. Finally, there also seems to be a divide between the advice you might get from someone like Mahlon Russell and what you might get from someone like Mike Busch on CHT limits. Both know a lot more than I ever will, and I have great respect for and try to follow the advice of both, but I think Mahlon's might be a little bit more universally practical on this particular issue.

Good luck though. Thanks for all your contributions to this forum.
 
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Just another observation, Gil... I would not put excessive stock in the "your baffles must not be well-sealed" advice. That could certainly be true, but my sustained searching on this issue over 5+ years flying is that every engine is different and what works for one doesn't always work for another.

Sorry, but the Lycoming cooling charts are not wrong...and whatever cooling issues a builder may suffer are not based on luck of the draw.
 
Sorry, but the Lycoming cooling charts are not wrong...and whatever cooling issues a builder may suffer are not based on luck of the draw.

Dan, could you describe the "cylinder fin flashing problem" that does exist at various levels on many cylinders as a "luck of the draw" issue?

A variation in airflow between the fins right next to the CHT sender location could cause at least an indication of a CHT variance. The other possible variation is flashing on the cylinder side that has minimal depth fins - my old picture from my Tiger -

Cylinder-fin-flash_%28Small%29.jpg
 
Sorry, but the Lycoming cooling charts are not wrong...and whatever cooling issues a builder may suffer are not based on luck of the draw.

My only point, Dan, was that there's a lot more than baffling that may impact engine temps. Sufficient fuel flow, proper mixture distribution (especially for carbed engines that are prone to wacky distributions), inlet air vs. exit air, proper oil temperatures, etc. I'm not at all arguing that good baffling is not an important, maybe the most important factor, just that there are other things that can also play into it as well.
 
Sorry, but the Lycoming cooling charts are not wrong...and whatever cooling issues a builder may suffer are not based on luck of the draw.

Yes, and to add to that, the baffle system execution is one of the biggest variables in the entire RV line. People always say theirs are "sealed", but invariably they are not.

And as long as Van continues to ignore the "zero fin depth issue" with their poor baffle design, we will continue to fight that issue as well.

Address those two fundamental issues, and the "luck of the draw" aspects (casting flash, for example) will become noise level.
 
Yes, and to add to that, the baffle system execution is one of the biggest variables in the entire RV line. People always say theirs are "sealed", but invariably they are not.

And as long as Van continues to ignore the "zero fin depth issue" with their poor baffle design, we will continue to fight that issue as well.

Address those two fundamental issues, and the "luck of the draw" aspects (casting flash, for example) will become noise level.

Speaking of which, is there a thread on the High CHT subject that has pics of someone installing or the installed washers for the washer trick for the rear cylinders? For some reason I'm having trouble visualizing how this is implemented.
 
Dan, could you describe the "cylinder fin flashing problem" that does exist at various levels on many cylinders as a "luck of the draw" issue?

Yes...in terms of what we get off the UPS truck. How it gets installed and flown is builder choice.

That flashing provides an interesting illustration. The average aluminum baffle achieves little more than rough proximity to the ends of the fins, very often not touching them except in a few places, as well as leaving leak paths out the side edges of the wrap. To make things worse, Lycoming inserts rubber combs in the bottom fins, which block the passages even more than flash, and push the wrap away from the fin tips.

Ok, partially block the passages between fins with flash or rubber combs, and where does the air go? Was the problem the flash, or the loose-fitting wraps allowing air to bypass the pinched paths at the flash? If the wraps were really sealed to the ends of the fins, the air would just squeeze through the narrow places...a local increase in velocity, increasing turbulence, an arguable plus.

First fit attempt, my own baffle wrap, cyl#4. Bend at the big arrow is non-existent as the wraps come out of the box, and you can see the gap prior to the comb.

P1100003.JPG


Speaking of which, is there a thread on the High CHT subject that has pics of someone installing or the installed washers for the washer trick for the rear cylinders?

Left front and right rear cylinders. Read here, as well as the links: http://www.vansairforce.com/community/showthread.php?t=93949
 
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Dan, I was actually asking a more literal question-- How is the washer installed between the cylinder and bafflle in the no fin area?

On the mounting screw. Here the screw point is just above my left index finger.

Of course, the washer spaces the tin away from the head at the black rectangle as well as the no-fin depth area in the red circle. Airflow at the black rectangle is just a leak. That's why I referred you to how to make a bypass duct instead.

Intake%20Fin%20Depth.jpg


Fins.jpg
 
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