High CHTs

paul330 · #1 10-24-2016, 10:57 AM

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)

mike newall · #2 10-24-2016, 11:20 AM

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.

Auburntsts · #3 10-24-2016, 11:21 AM

Assuming your baffles are in good shape, this baffle thread might help: http://www.vansairforce.com/communit...ad.php?t=37835

Besides checking timing, as Mike suggested, have you tried running LOP in cruise instead of ROP to get your CHTs down?

dwilson · #4 10-24-2016, 11:23 AM

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/communit...ight=cowl+flap

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

Carl Froehlich · #5 10-24-2016, 12:22 PM

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

Strasnuts · #6 10-24-2016, 11:32 PM

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.

paul330 · #7 10-25-2016, 03:17 AM

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

- 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.

ReidVaitor · #8 10-26-2016, 12:46 PM

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

rleffler · #9 10-26-2016, 02:14 PM

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.

paul330 · #10 11-13-2016, 09:37 AM

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