Another enigma....

Y'all might remember that I have been chasing a hot start issue on my O-360-A1A RV-6. I replaced the MA4-5 carb with an overhauled one, thought that solved the problem, but it did not.

Then I took the Slick mags and had them serviced. They were in rough shape with about 800 hours on them. After new points, point cams, bearings, seals, etc. they are like new.

I had my A&P install and time them. He has all the neat gizmos for timing, like the proper pins, the laser thingy that he attached to the starter gear at the timing mark, and the gold box that beeps when the mags are in the right spot.

We put them in and she started great. I have flown at least a half-dozen flights and the engine seems to run better, there is definitely much less RPM drop on run-up.

I have not flown long enough on one flight, and it has not been hot enough to get the engine really heat soaked, so I don't know if the hot start issue is resolved...

But, that is not my new situation. Since I reinstalled the overhauled mags my CHT has increased about 150?! It doesn't seem to matter what I do, it still reads about 150? higher than before. Where it used to read about 250? during climb-out it now reads 400?. That is either at WOT and 2700 RPM and full rich and 100 MPH, or climb cruise at 24" and 2400 RPM and 110 MPH.

The original probe was a ring type under the #4 spark plug. Today I replaced that with a new bayonet probe, which I am told is more reliable and accurate than the ring type.

There was no change in the CHT reading.

So, I have some questions:

1) If all I did was overhaul the mags, how could that affect CHT?
2) Is 400? on #4 too high?
3) What could have caused the CHT to increase by about 150? if all other parameters are the same?

I asked my A&P if the timing could be off. He said he doubted it because it starts so good. He said that next time I pull the cowl he will check it again, and he suggested I move the probe to #2 cylinder and see what the readings are.

Any thoughts, ideas, or suggestions would be appreciated.

Thanks!!

Hey Chris,
The new temp. of 400 is probably the correct one.
250 would not have been correct. Something he did while under the cowl corrected a bad reading.

Did you check the ignition timing before removing the mags for overhaul??

Chris,
Your magneto timing previously was most likely late [retarded]. Late ignition timing will lower the CHT temps because the fuel is still burning as it leaves the cylinder. Late timing will also increase your EGT for the same reason. Fuel is now burning in the exhaust. Consider your higher CHT a good thing. Now, all the heat from your fuel is being expended in the cylinder, where it can do some useful work. No, 400F is not to high.

Charlie

If at all possible, invest in a multi probe engine monitor.
The kind that shows all CHTs and all EGTs, even better if it has data logging.

The new temp. of 400 is probably the correct one.
250 would not have been correct. Something he did while under the cowl corrected a bad reading.

Click to expand...

I would agree with that assessment.

I'm guessing that the change in your timing (probably advanced) has resulted in higher Internal Cylinder Pressures due to the earlier combustion event resulting in higher CHTs.

I usually go with opinions from guys named Charlie.

I agree that your timing was (very?) late with your old mags. I bet your fuel flow is lower and power is better now.

Charlie (E, not K)

Thanks for all the replies.

I purchased the plane the last week of August and have put on 26 hours. During the pre-buy we checked the timing and found it at 27°. We reset it to 25°.

When we reinstalled the mags on 10/30 we set it again to 25°.

Power does seem to be a bit better, but fuel flow does not seem to have changed. But, in all honesty, I have not focused too much on fuel flow, other than to get it off 18 GPH at full power, full RPM, and full rich!!!

Thanks,

I would also suggest you climb at a higher speed. When not staying in the circuit I use 120 Kt. in my 7A

George in Langley BC said:

I would also suggest you climb at a higher speed. When not staying in the circuit I use 120 Kt. in my 7A

Click to expand...

Heat is directly related to power. If your CHT's were really 250 and now 400, you would have noticed a significant difference in "same RPM" power. This type of change can't be created by timing unless it was dramatic enough to cause a noticeable power difference. EGT's yes, CHT no.

I am with the camp that something else was causing the low readings. 250 is abnormally low for 75% power output. Do some searches here.

Larry

Heat is directly related to power. If your CHT's were really 250 and now 400, you would have noticed a significant difference in "same RPM" power.

Click to expand...

If you are referring to CHT?.that statement is completely wrong. It is directly related to Internal Cylinder pressure and Peak ICP is the thing that drives the CHT, an you can generate the same power with varying peak pressure and thus lower CHT.

Higher CHT from advanced timing or detonation is possible but i doubt it has anything to do with the original post question.

The issue here is check the mags are timed at 25DBTDC, and then go look at the cooling airflow, and remember airflow is not doing what you think it is doing under there.

Dan Horton DanH has numerous good posts on cooling airflow. And it often is opposite what you expect.

I suggest your single robe before was wrong.

Go buy an EMSD10 from Dynon and a probe pack, set it up right and the education it will provide will be an eye opener. You might need some education as well. Refer my footnote below.

I'd love to have a D10, but I have literally zero panel space for any new instrument, and don't have $2k to put into the D10 and probes.

The old ring probe and new bayonet probe read the same temps.

There are significant baffle areas that need to be addressed, and they are on my list of things to do. However, nothing in the baffle or cooling flow system changed during the mag replacement.

There is an extender cable, but it is inside the cabin so the connections would not have been disturbed during the mag change.

I have had the cowling off a number of times so know how to install it correctly. When I put the top cowl on I always stick my arm inside the oil door and run my hand across the top baffle to make sure it is folded forward and in good contact with the cowl.

I am glad to hear that 400F is not too hot. When I fly again next week I will take her up to my normal CC cruise altitude of 7,500' and go to cruise power, usually 18/2400, and see what temps I get there.

So, my original question still stands. What could have caused such a difference in temps if the cooling is the same and flight operation is the same? If it was a timing issue that caused such a huge change wouldn't I see some other symptoms, too?

I have searched the forum and find that the temp I see now is acceptable, in general. Therefore I am not too concerned about that temp. What I would like to understand is why it changed...

range should improve

your range should improve with the higher CHT but 400F may be slightly high. how about using 380F max on climb out?

800 hrs on mags and never looked at? my 172 said 500 hrs they must be insp.

I too would like to see an answer to the original posters question.

I just installed my two slick 4300 series mags back from their 500 hr inspection and am seeing a HUGE CHT increase. What use to be 350-375 F on climb out is now 475-500 F. I aborted a take-off today at 2000' as #3 was 450 F and climbing rapidly #1, #2 and #4 were right behind at 425 F. All EGT'S were only 1150-1250 F. By the time I could get from the hangar to the run-up area (200 yards) do a quick run-up and get on the runway CHT'S were already 350 F. I backed the timing from 25 BTDC down to 20 degrees today and ran out of time to test fly it. Maybe tomorrow.

9A
O-320 aerosport
carburetor
538 hrs TT
slick 4300 series mags
14 gal/hr on climb
fixed fitch prop
2300 rpm on climb
100 knots indicated on climb

pulling my hair on this one, any help appreciated

doug

i agree that something was not right before the mags got pulled. i have never seen a cyl run that cold in a climb, in any airplane. I would say 400 is in the right ballpark for a 100kt climb.

doug, i would double check the timing and make sure the pin was in the right hole when they were stabbed. timing above 25 can cause the chts to get that hot. I just replace a mag that was wearing out and slipped upwards and I was seeing 430 - 435 in the climb.

bob burns
RV-4 N82RB

dougknight said:

I too would like to see an answer to the original posters question.

I just installed my two slick 4300 series mags back from their 500 hr inspection and am seeing a HUGE CHT increase. What use to be 350-375 F on climb out is now 475-500 F. I aborted a take-off today at 2000' as #3 was 450 F and climbing rapidly #1, #2 and #4 were right behind at 425 F. All EGT'S were only 1150-1250 F. By the time I could get from the hangar to the run-up area (200 yards) do a quick run-up and get on the runway CHT'S were already 350 F. I backed the timing from 25 BTDC down to 20 degrees today and ran out of time to test fly it. Maybe tomorrow.

9A
O-320 aerosport
carburetor
538 hrs TT
slick 4300 series mags
14 gal/hr on climb
fixed fitch prop
2300 rpm on climb
100 knots indicated on climb

pulling my hair on this one, any help appreciated

doug

Click to expand...

My guess is that the mags are not timed to the engine correctly.
My suggestion would be that someone other than the person that timed the mags after installation, check that work and verify it is correct.

cht

When we purchased an RV6 a few years ago we chased high cylinder temps for a year installing dams in top cowling washers in barrel barfing you name it turned out to be wrong leads for temp gauge wrong k value.
Bob

Chris,
I will tell you that the 400 degree is not abnormal on a climb. I had major heating issues after completing my RV7 where my temps were in the 415 range until I got to altitude.

What I did was to install louvers in the bottom cowling to help get the airflow better but that only help a little bit.. Then I installed one Antisplat electric cowll
Flap and WOW did that make a huge difference. The temps on a normal climb at 100 mph are now down to 380. I can do pattern work and I will not see 360.
I only installed one cowl flap about 2 inches ahead of the duez fasteners on the cowling and thought I'd see what the results would be. It was amazing... For the price a quick answer to the heat issue.
If your timimg is correct. Your mag drop correct. This might be an alternative for you

Jack

Thought I would post my results. My problem was not timing nor mag related at all. Ends up my CHT gauge died. I have the van's EGT and CHT gauges with the ECI rotary switch and probes.

Went to start it for a test flight this morning and as soon as I hit the master switch the gauge went to 225 F without even starting the engine. Pushed it back into the hanger and drove to van's for a new gauge. Installed it and flew with CHT's in the mid 300's. life is good.

doug