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High CHT's

bluebrear

Member
I've been reading about excessive oil temps, of which, happily, I have none. Oil temp is, at highest 169 degrees, and at cruise steady 143 degrees. But, after 5 hours in my RV9A, we are still experiencing CHT's, with OAT's 16-26 F, @450 at TO for #1, being the highest. At cruise 2100 RPM, we're 370 for #1, the others are lower, but not by alot. We have a rebuilt O-320 B1A with chrome cylinders, conical mount. The prop is a metal Sensenich recommended by Van's (79") pitch. Two IA's have looked at the baffling and didn't find any obvious causes for the high temps. We have RTV'd waste areas and lowered a small amount of the intake dams on both sides of the intake per recommendation of the IA. Seemingly no change. I removed two small fiberglass rectangles from the exit of the lower cowl thinking that that would increase airflow. Still no change. VAN's tech said they have not experienced temps that high. It's built per the plans, no mods. I'm not seeing squawks from any of the 1100 or so 9 builders. Does anyone have a clue here? We're at wits end. EIS with GRT HXr.
Thanks to all.
 
A lot of questions. Not so many answers.

You do not mention quite a few things that may help in anlysis:
  1. what kind of fuel flow gets that 450 Deg?
  2. What mixture settings during climb/during cruise?
  3. Are these temps while running Full Rich, ROP (how far), LOP (how far)?
  4. What is your climb rate when seeing the high temp (Vy, Vx, cruise climb)?
  5. What is your airspeed during this high CHT time?
  6. What is the RPM, MP during these high temp readings?
  7. Where/How are your CHT probes installed?
  8. When the high temps occur what do you do to bring them down?
  9. You state your cruise CHT temp on #1 is 370 with the others not far off that. How far off are the others? The GRT should give you all cylinder readings. What are they during this high temp rate?
  10. If they are close, how close?
  11. Do they move in unison with each other when they do move?
  12. I assume the reported temperature is while running somewhere ROP but not full rich. Have you attempted to run with various differing mixture settings to see what that does to the temps?
Ok, enough with the questions. Well, maybe one other. That may also lead to a 'WHY' answer. Have you checked the induction tubes, fittings, rubber, etc? It is possible there could be an induction leak somewhere? Just one possible answer.

Maybe there are others with questions, and answers, but these are the ones I have now.
 
A couple ideas

1) Check timing. Advanced timing can cause this. Do you have electronic ignitions or mags?

2) Get rid of the #1 and #2 cylinder dams. They can be added back later after you solve other issues and determine the right height by experimenting with different widths of aluminum tape.

3) What is your climb profile like? Where in the climb are you seeing these temps? You're unlikely to be able to climb unlimited at 85 kts without the CHTs going high. Vy to safe altitude (1000' AGL or so) then lower the nose for cooling.
 
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Hi Steve,
I'm the builder/owner/pilot. My test pilot friend has flown the hours.
I can answer most of your questions.
1. Fuel flow has not been calibrated yet.
2. Mixture is full rich at climb out/cruise is leaned somewhat.
3.Temp at Full Rich TO was 455 for #1, I'm not sure of exact changes ROP/LOP
4.Climb rate at Vx was 1800 fpm
5. Airspeed at high temp was 100 kts
6. Probes are in Lycoming wells, bayonet type.
7.To lower temp, lower ROC
8.At high temp: 1/455, 2/454, 3/442, 4/409 Cruise: 1/368, 2/345, 3/333, 4/331
9. Unison; Yes
10. High temp report at full rich.
Any clues?
 
Lower the nose

My experience has been that the RV-9's will cool much better with a climb rate around 120 KTS. Even with a fixed pitch prop climb rates are usually around 900-1000 FPM. Climbing at VX for any length of time will cause high temps.

Vic
 
Get the fuel flow calibrated. Looks like flow may be too low at full power/full rich.
Small issue: I think your oil temps in cruise are on the low side, you may need a plate over the cooler for winter use.
Just to double check, since I see you have chromed cylinders:
Were the correct (not standard) piston rings installed?
 
What carb? What plane does that engine/carb combo come from? Most likely jetted too lean
 
Bill,
The engine is completely rebuilt Lycoming O-320 B1A S/N L901. Carb was never used (new condition) Marvel-Schebler MA4SPA.
 
Research how to do a test to see if its running too lean.... for example fly 75% power at 8500' and then start tp lean - you should see a rise in EGTs of say at least 100-200 degrees before they start falling off on the lean side of peak. If you see little or no rise, or it starts falling off immediately, its probably too lean. I think you can do a 75% run up on the ground too, but it might not be as accurate (watch CHTs obviously on ground run).

If you do suspect it is jetted too lean; rather than experiment yourself, you can send the carb to Marvel Shebler where they can bench test / flow test to see if its getting adequate flow for your setup and they can modify the jetting accordingly..
 
There are many different MA4SPA Carbs. The model number of it is on the tag. Most of them have different jets in them. I don't know where the B1A was used in the certified world but "most" of the carbs are too lean for our tightly cowled RV's. You'll find that a lot of discussion on here about drilling out the main jet slightly to get the needed 12-14 GPH needed at full throttle take off.
 
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My experience has been that the RV-9's will cool much better with a climb rate around 120 KTS. Even with a fixed pitch prop climb rates are usually around 900-1000 FPM. Climbing at VX for any length of time will cause high temps.

Vic
Without reading any of the other posts I am going to agree with Vic on this one. More than anything this can help greatly. With a fixed pitch prop you need to take what you can get and live with it. That means for climb you should consider what many might call a cruise climb. In my case I keep my climb speed around 115-117 mph unless some specific circumstance calls for something else. This keeps a good airflow across the cylinders. I would say start here. See what your temps do in climb at a speed somewhere in the 115-120 mph range.
 
There are many different MA4SPA Carbs. The model number of it is on the tag. Most of them have different jets in them. I don't know where the B1A was used in the certified world but "most" of the carbs are too lean for our tightly cowled RV's. You'll find that a lot of discussion on here about drilling out the main jet slightly to get the needed 12-14 GPH needed at full throttle take off.

Everyone I know with a carbed RV has had to bore out the jet to reduce CHT, primarily at take-off.
 
Hi Steve,
I'm the builder/owner/pilot. My test pilot friend has flown the hours.
I can answer most of your questions.
1. Fuel flow has not been calibrated yet.
You need to know what this is. I would prioritize doing this as soon as possible. It will be key for you in the proper operation of your engine.
2. Mixture is full rich at climb out/cruise is leaned somewhat.
One other question I did not ask is what elevation you are flying out of. My home field is at 1350 ft MSL. At that altitude I cannot run full rich without issues that I choose not to deal with. If at sea level then full rich may be necessary. Anything higher than sea level lean it out and your engine will run better, and thank you for it.
3.Temp at Full Rich TO was 455 for #1, I'm not sure of exact changes ROP/LOP
Again similar answer as #2 above. I do not know of any instances except for when I have been actually at sea level or during starting that I run full rich.
4.Climb rate at Vx was 1800 fpm
5. Airspeed at high temp was 100 kts
100 kts is acceptable but perhaps you need more airflow over the cylinders.
6. Probes are in Lycoming wells, bayonet type.
7.To lower temp, lower ROC
8.At high temp: 1/455, 2/454, 3/442, 4/409 Cruise: 1/368, 2/345, 3/333, 4/331
Yes, I also would be concerned with these climb temps. Even if mine were to make it there (mine have never been that high)I would not allow them to remain there. Since they are all in the same relative range then I would say you need to get air flowing across them better. As for cruise temps those are respectable numbers for cruise for running Rich. You can get them cooler running LOP but with a new engine and plane I would not concern yourself about that now.
9. Unison; Yes
10. High temp report at full rich.
Any clues?
As I have mentioned above, I don't think running full rich is all that good for the engine. Especially at altitude. Try leaning out more in climb and keep your air flow up with airspeed on climb out. I don't recall if you stated this was a new engine. If so then running hard and hot on break in is not necessarily a bad thing. Although, I would admit running in the 450 deg range is a little hotter than I would want myself. Is this a new engine?
 
Hi Steve,
I'm the builder/owner/pilot. My test pilot friend has flown the hours.

4.Climb rate at Vx was 1800 fpm

5. Airspeed at high temp was 100 kts

8.At high temp: 1/455, 2/454, 3/442, 4/409 Cruise: 1/368, 2/345, 3/333, 4/331

My advice is to strike your test pilot smartly with a stick. Tell him to quit doing extended climbs at Vx. Vx is for clearing trees.

Then take the air dam off #1, and slap a piece of aluminum tape across the oil cooler.

Be happy.
 
I agree with Dan. Lower the nose and climb at a higher airspeed.

You haven't calibrated fuel flow yet, but what are your EGT's reading at takeoff. As much as people will say eyndont matter, they can be an indication of running too lean at high power. My RV-6A with a 150HP O-320 was running 1450-1500 EGT's on takeoff and the CHT's quickly climbed above 400 and I even saw 450, even at a reasonably high airspeed. I drilled out the jet and verified I was hitting the full rich stop (I wasn't at first). Got the EGT's down to the low to mid 1300's and the CHT's can be kept under 400 in climb if I pull the power back a little. One more drill size to get them in the 1200's and I think I'll be good to go. The absolute EGT numbers are not always a clear indication, but they can help determine if you are running too lean, which will easily cause high CHT's. I'm ready to be flamed, as I have been in the past, but the numbers don't lie, at least in my experience. You are reading all 4 CHT's, so I assume you are reading all 4 EGT's as well. What were they att full power takeoff?
 
Count me in for agreeing with Dan regarding climb speeds.

After reaching 1,000' AGL, I typically cruise climb at 500 FPM which gets the speed up and you heading towards your destination at a good clip.
 
I agree with Dan. Lower the nose and climb at a higher airspeed.
Ummmm, I am sure glad Dan agreed with my statements and gave his advice as he did. I guess I should have waited until after he said anything to post the same advice, then I would have been in agreement with him too! Although I guess the thing I did not suggest is striking the test pilot with a stick. So, I agree with Dan. Strike that guy with a stick! :rolleyes:
 
The guy is already a stick. We'll have to deal with him some other way. I can only say thank you to all that offered their help to solve this troubling and perhaps damaging problem. We now have abundant experienced information to apply for a fix.
 
EGT's at high temp were: 1498, 1487,1449 and 1377. At cruise: 1411,1409, 1385 and 1249.
Accordingly, this seems to indicate a carb jet too lean situation. We'll work on it. Thanks
 
connectors in the circuit?

Just throwing this out - CHT thermocouple wires transmit very small voltages, so its best to minimize the use of non-thermocouple connectors and non-thermocouple wiring between the probes and the guage. I did have a similar problem with CHTs reading around 30 degrees too hot, and rewiring the thermocouple wires in a 'direct to' fashion fixed it.
 
The only EGT indication that matters is degrees from peak. Absolute values ("EGT's at high temp were: 1498, 1487,1449 and 1377") are useless for tuning, and only approximate for quickly setting inflight mixture.

Jetting very rich (or very lean) does reduce CHT, but the engine is no longer making rated power. I won't say it's a wrong approach, but it does burn cubic dollars for less whoopee. I'd suggest jetting (or swapping restrictors, or pushing buttons on the SDS control head) so all cylinders fall into the 100 to 200 ROP window, and then working out the necessary cooling to enable the use of all the expensive Lycoming power.

IO-540K angle valve, on the FAA dyno at Hughes. Best power is 100 ROP. Anywhere from 50 ROP to roughly 200 ROP keeps power within 2% of maximum. At 300 ROP power is down about 5.5%.

 
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Nice plot. I was confused by it at first until I realized richer is to the left and leaner is to the right along the x-axis.
What am I missing with respect to the purple line at 11:1 F/A and the associated calculation of being 215 degrees ROP? Anything special about that point?

thanks
erich
 
The guy is already a stick. We'll have to deal with him some other way. I can only say thank you to all that offered their help to solve this troubling and perhaps damaging problem. We now have abundant experienced information to apply for a fix.

Just thinking from a different angle, have you checked the routing of your thermocouple leads ? If they are anywhere near higher current flow wires i.e. Alternator, strobe wiring, lights, etc, it's possible your readings could be off and reading high by a significant amount.
 
What am I missing with respect to the purple line at 11:1 F/A and the associated calculation of being 215 degrees ROP? Anything special about that point?

Nope, just an artifact of some previous post. Too lazy to scan a new image;)
 
Vern, Speaking with Marvel-Shleber, I found out that my carb has one of the richest jets installed. It starts out at .101" dia. Do you have any idea how much increase in the bore would be to attain?
 
11:1 F/A

Big difference between 11:1 F/A and 11:1 A/F! :D
Takes a while to interpret this chart correctly the first time. As always, another great chart from Dan. I would like to see his reading list over the years.


Nice plot. I was confused by it at first until I realized richer is to the left and leaner is to the right along the x-axis.
What am I missing with respect to the purple line at 11:1 F/A and the associated calculation of being 215 degrees ROP? Anything special about that point?

thanks
erich
 
You might consider repositioning the entry air ramp on the top cowl. I had very similar CHT problems until I moved mine forward. This really improved entry air volume and greatly reduced temps, especially #1. Just search this forum for more detail. Worked great for mr, FP 0-360, RV7. Easy fix!
Good luck, Jim
 
Vern, Speaking with Marvel-Shleber, I found out that my carb has one of the richest jets installed. It starts out at .101" dia. Do you have any idea how much increase in the bore would be to attain?

Are you absolutely sure you're hitting the rich stop on the mixture?
 
Drill

When I did mine I drilled it in steps and ended up at a #39 drill. 0.995. That took mine from a rich to peak increase of about 50 degrees to 175 degrees and helped the cooling a lot in the climb. I would think that if yours is in fact .101 now that the problem would be more in the baffling and timing.

Even after drilling out, if I climb at VX very long i will shoot past 400. I try to climb at nothing less than 100 knots unless it's necessary.


Mine does have 9:1 pistons in it also.

Someone mentioned the ramp on the inside of the cowl and I truly believe that this is an area that would really help the cooling. DanH and I have talked about it several times and he's posted about it also. I will change mine at condition inspection time just to see what difference it makes.
 
Jim
Did you cut the dam out, shorten it and then replace a shorter section, thus moving that dam forward? If so, about how much forward?
Thanks
 
Would like some opinions about ambient temps affecting the CHT's. We're flying in Maine winter with these high CHT temps and concerned about increasing OAT's as the WX improves.
 
I would say mine are pretty similar to Kyle's: ambient + ~280F will be my average across all 4 CHTs, though I'm usually running only 65% power when I'm economy cruising. I'm assuming they'd be higher if I try to cruise faster. In addition, if my oil gets really warm because it's a hot day, I think my CHT's will rise a bit beyond what the above formula above suggests. I'm still fighting some cooling issues I think, though I'm gradually finding baffeling improvements to help.
 
Yep, I just used my Dremel to cut the ramp out, smoothed things up a bit, shortened it about an inch and a half as I recal, then glassed it back in. They say you should be able to make a fist and shove in through between the ramp and the cylinder. I'm there now. It was easy.
Jim
 
Here's a reason

In an RV-9A I looked at yesterday the CHT probe was installed in the PRIMER port of the number 3 cylinder! That's a first.

Vic
 
Jim,
How much did the temps drop, if I may ask? Just to know if I should cut the dams back. By the way, I saw a picture of a builder who had foamed the open sides of the upper cowl dams. Once smoothed, it seems like it would reduce some turbulence there.
 
Yep, I just used my Dremel to cut the ramp out, smoothed things up a bit, shortened it about an inch and a half as I recal, then glassed it back in. They say you should be able to make a fist and shove in through between the ramp and the cylinder. I'm there now. It was easy.
Jim

Jim,
How much did this lower your CHT's if I might ask. I'm about to do this myself but pondering the benefit.
Thanks
 
Jim,
How much did this lower your CHT's if I might ask. I'm about to do this myself but pondering the benefit.
Thanks
I too am interested. I can get a full fist+ in between my port inlet and #2 but can't get even a fist between my starboard inlet and #1. It looks like I could probably move my ramp about .75-1" forward before it starts sloping upward. Is that the idea? Get it angling upwards as soon as possible after the inlet?
 
I too am interested. I can get a full fist+ in between my port inlet and #2 but can't get even a fist between my starboard inlet and #1. It looks like I could probably move my ramp about .75-1" forward before it starts sloping upward. Is that the idea? Get it angling upwards as soon as possible after the inlet?

Mine is the same way Steve. DanH and I have looked at it and believe that moving it forward as far as practical would certainly help. In my case, I believe I can trim some off the aft end of the inlet itself to gain a little more room. I don't remember whether I had trimmed any of the aft end off or not when I originally built it. But as it is right now there is NO way to get a fist in there. I have a buddy with an 8A and a 360 and there is a tremendous amount of room on it and he has really cool CHT's.
 
I originally had only about an inch or maybe a bit more on my starboard side. I simply cut the ramp out, trimmed about an inch or maybe an inch and a quarter off the front end of the ramp, then epoxied it back in place, filled and smoothed things out, and that was it. Now I can just get my fist through the gap. So now my port and starboard ramps have similar gaps to their respective cylinders. The temp on #1 was my problem, and it dropped dramatically. Again, I don't claim to have any expertise in this area, but it worked for me. In any case, it was easy to do, didn't affect the paint finish on the outside of the cowl, and can certainly be undone easily. There is some talk of a small cooling drag penalty, but I could detect no performance change in speed.
Jim
 
The improvement in my case was dramatic. My #1 cylinder temp was hitting 440 on initial climb out. I now never see more than 415 at the same point in the departure. I think the tightly cowled 7 will always tend to the warm side, but I can live with what I see now. In cruise and after leaning LOP, all 4 run below 380 at 2400 rpm. By the way, I am running an aero sport power 0-360 with 8.5 Pistons, dual pmags and an Ellison TBI, all of which I love.
Good luck, hope this helps...
Jim
 
The temp on #1 was my problem, and it dropped dramatically..... There is some talk of a small cooling drag penalty, but I could detect no performance change in speed.

Interesting question. Did you notice a temperature drop for other cylinders? That would probably be the best indicator of an increase in mass flow.

 
hot engine

Bluebread

I have almost the same tempts, have fought it for 6 months, have a o320 E2D 150 hp.

Give me a call maybe finally found problem.

Gordon
386 963 5357
 
Bluebread

I have almost the same tempts, have fought it for 6 months, have a o320 E2D 150 hp.

Give me a call maybe finally found problem.

Gordon
386 963 5357
Why not post your findings here so all can benefit from your experience?
 
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