Thorp Air Command - T18.net

So I had the top overhauled on my engine which was 4 new exhaust valves, hone cylinders and all new rings. Everything else was tip top shape. I should add that it is an o-290GPU converted to 290d.

Yesterday I did a break in flight with David Read. After climb out into the pattern the JPI indicated I was over 450* cht and it looked like 3&4 were both higher on cht. My oil temps were around 220* to start as well. We stayed around 4500 and powered back slightly which dropped temps to the 450 range. My front 2 cylinders were high 300* range. With 3 & 5 my egt was higher than the front 2 as well. Front 2 were similar and rear 2 were close in temps with maybe 10* or so between 3&4 with #4 higher on cht. After that first hour the oil temps dropped down to around 200*.

After the first hour of cruising around above the airport we landed and had some rich bog when I tried to do a touch and go. Did some run ups and leaning smoothed it out. I had that issue last year when the OAT temps were high along with the DA. It was mid 80's with high humidity yesterday.

I only had a single CHT on my #3 before and never noticed it that high although I flew it mostly through the winter and it was an old gauge so it could have been inaccurate.

Since the temps were consistent I think it was probably not a serious issue. The mechanic didn't silicone my baffle on the back end after re-assembly. I also noticed looking under the cowl that the pieces that wrap around the bottom of the rear cylinders was bent away from the cylinders. The rear wall of the baffle is pressed up against the cylinder fins which after reading reduces airflow around the cylinders and causes them to run hot. I'm going to adjust the spacing between the rear wall and the cylinders to see if that helps.

Any info on what else I should adjust to help with this issue. I know temps run higher during the first few hours of break in but I fel like something has changed since the fronts are lower than the rears. The A&P wasn't concerned and said to do slower climb outs and watch oil temps.

You are doing the correct thing by checking the baffling, 450 is the very top temp you want to go. For a long engine life, less then 400 is best. Leave the mixture rich and cruse climb as much as possible. You may want to do the next few flights when the OAT is cooler, until the cyl's have broken in. Keep the ground run to a minimum. Check oil consumption, and temp. You may want to also ensure the mag timing is correct, if the cyl. head temps. don't come down soon.
Best of success.

Phil
118BC

Rear/bottom baffle spacing is critical and sounds like the most probable cause of your observations. Some other points to consider: Make sure your probe wires pass nowhere near any alternator or other current carrying wires. Induced current will skew the readings. Make sure the inner-cylinder baffles are correctly installed. These are often missed during overhaul. You may want to raise the front baffles on your front two cylinders to force the air to go to the rear. Finally, check the area around the front of the case. Insure no air is sneaking by the prop hub area.

_________________
Bob Highley
N711SH
SN 835
KLAL

Thanks for the info. My oil consumption on the first flight was pretty good. 1.5 hours and around .5 qt of oil although it's hard to get an exact measurement with the clean oil.

Mag timing was just set by the a&up at 25 deg btdc. Would mag timing only effect 2 cylinders? I though about timing but then realized each mag has wires to all cylinders so I don't know how it would effect only the rear 2 cylinder temps. I'm new to airplane engines and mags so maybe I'm wrong. On taxi they all seem pretty close on temps and mag drops are equal when tested. I can always have the ap check them again though.

I kept the mixture full rich but as the day heated up it aggravates my dreaded rich bog on full throttle for take off. I'm sure the extra engine heat is causing that to worsen as well.

Tomorrow I'll take pics. So far I see that the baffles are touching the rear cylinders so I know that needs spaced out. At the bottom that wraps under the cylinder what should the spacing be? Currently mine are both bent over an inch away from the cylinders which is further than I previously recall. Most say 1/8" behind the cylinders for spacing but would making more space improve it further?

Probe wires all go through a gromet in the bottom of the firewall and everything else goes in a gromet at the top of the firewall. I think the main alt wire does pass a few inches away as it wraps around the engine although that would be close to both 2&4 and nowhere near 1&3. So I would think he problem would be on those cylinders if it was interference. Correct me if I'm wrong.

I'll check the inter cylinder baffles tomorrow when I pull the bottom cowl.

My front of the baffles is just flat. I don't have any front baffle that angles the air above the front cylinders. Maybe I need to make some in order to bring the temps up on the fronts and down on the rears. Info and pics on this would be helpful. I do have my oil cooler in front of cyl 2 with a scoop to divert air into it so I'll have to be creative to get an upward baffle in that space.

My baffle behind the firewall is solid and has good seal at the top. I checked that first thing I landed. I assume that's where you are talking about around the prop.

Thank you!

I think you are sniffing around the right tree with the airflow. I use bits of baffle seal siliconed in place to create the needed space behind the rear cylinders.

If I am thinking of the same curved pieces you are talking about, they should be reasonably snug around the cylinders. Most installations I see have something holding them tight, like a spring or a piece of safety wire. Someone did mine with threaded rod stretched between the front baffle and the rear. Typical Lycoming installations use a spring. I should have good enough access without pulling the lower cowl to get a picture if you need clarification.

Anything you can do to help smooth airflow over the front cylinders should help.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

The mag timing will affect all of the cyl's. But may add to your problem on the rear cyl's because they may get less cooling. Just one other thing to look at, make sure on impulse mags you rotate the prop past where they snap, then go back to get the correct timing set. If you can fly when it is cool outside until you get a few more hours on the engine that just adds a little more margin for you to work with. Running rich can cause some fouling issues, you may want to look at the spark plugs for soot and lead, this is just an additional thing to check so you don't have a issue later.

Phil
118BC

I figured as much. On my Cherokee there is a threaded rod and they are up against the fins on the bottom so that's why it caught my attention. I'll get those shaped back together as well.

The silicon pieces as spacer I'm curious about so if there is a way to get a pic of that it would be helpful. I was going to try to bend the baffle to create space or rivet a washer into the back baffle to space it.

I keep hearing 1/8" is plenty but I'm wondering if going to 1/4" space would provide more cooling or if it would route too much air away and not be as effective. I will be working on it today and if using 1/4" space gives more cooling I would prefer to just go straight to that verses starting with 1/8 and redoing it later for bigger gap.

Makes sense on the timing. I'll have the ap check it and adjust it. Would it be wise to have him set the timing more conservative for my break in period? Maybe 30* instead of 25*?

Now that you mention it I'm going to go back in my logs and look. I thought I saw 30 or 35* written in my logs before so maybe this o290 uses less advance than the 320 and 360 that they do more regularly in the shop. I'd be willing to sacrafice some power for the safety of lower engine temps.

I'll pull the plugs today and check them as well. I run the rem40 currently for plugs. I've never had them load up and cause any changes in mag checks like I have on my Cherokee but I think my rich condition is loading up the plugs on take off right now with how it acts. We just cleaned them all and inspected them upon assembly.

With this recent Midwest heat wave I'll be doing the rest of my flights in the early mornings before it heats up.

You may want to really look into the advance of the timing. The number reflex the degree's "Before Top Center" of the piston, the higher the number the earlier the spark occurs in the combustion cycle. The earlier the spark occurs, more heat (and possible power) is created. I would set the timing to what is recommended by Lycoming. Once the engine is broken in you should see a marked drop in CHT and oil consumption. This usually happens between 5 - 10 hours of operation. Your idea of flying during the cooler times of the day should give you a better margin to keep things cool during break in.

Phil
118BC

According to my Lycoming O290 manual, timing for a O290-D is 25, a D2/D2A is 18 and a D2B/D2C is 25. My O290-G with D2 pistons is timed at 20 (go figure, that's the way it was when I acquired it), I have 7.5:1 pistons and typically 3 & 4 have higher CHT and EGT temps.

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA

The spacers were installed on a C-177RG that I don't currently have access to but, now that I think about it, I made those spacers to protect the baffle from the engine more than to increase airflow. They weren't anything special. Just half inch square pieces of baffle seal glued to the baffle where it was making contact with the cylinder.

This is just my opinion but I don't think the amount of gap is nearly as important as getting good airflow through the fins. I have seen where guys try to use silicone sealant to do what I did with the baffle seal but they usually end up pushing sealant in between the fins which would restrict the airflow and lessen the amount of cooling air.

What kind of shape is your baffle seal in? Does it curve into the airflow? If it folds away from the airflow or blows back in flight you will lose cooling air there too.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

-D 125 hp @25 adv. comp 6.5
-D2 -D2A 135hp @ 18 adv. comp7.5
-D2B -D2C 135hp @25 adv. comp 7.0

lycoming uped the compression to get 135/140 out of the ol girl, and backed off the timing to 18. then backed off the compression and bumped the timing to maintain the hp. guess they were worried about pre ignition.

today there are a lot of guys running way more compression and way more timing.

_________________
http://www.dixiefriedfabrication.com/

Lycoming says do not exceed 500. 450 for the first 5 hours doesn't sound bad to me. you should be running the dog snot out of it for the first 5 hours of cyl. break in (75 percent or greater) then you should see temps come down.

30 degrees is more advance =more heat more chance of pre ignition. run what the book calls for no more no less.

_________________
http://www.dixiefriedfabrication.com/

I don't want to be disagreeable, but 450 is too hot in my book!
When Lycoming established 500 degrees for the FAA certification test, the engine only had to make it some limited number of hours, like 150 for certification?
We all want the 2000 TBO, plus as many more as we can get. That means cylinder temps under 400, preferably under 385.
My T-18 ran at or below 320F (O-290, chipmunk cheek cowl) running at or near full power (because I CAN!) until I added a Pmag, then it increased to 330F.
I simply can't imagine a T-18 running that hot. Something isn't right.
You mentioned the cylinder baffles are 1" away at the bottom. These must touch the cylinder fins all the way to the bottom, and are held tight with springs or safety wire, The air must flow through those fins, not past the tips.
If you open the baffle at the back of cylinders #3 & #4 without tightening the inter-cylinder baffles at the bottom, you temps will increase. RTV seal the back baffle as before. ANY air path that IS NOT THRU the cylinders is a loss of cooling. Especially the heads. Check the cooling fins for debris also.
Oil temp of 220 indicates a loss of air pressure on the "Upper Deck" so you are getting a reduction of air flow thru the oil cooler as well. A failed Vernatherm oil temperature thermostat could also be possible. You removed the winter cover on the oil cooler, right? (I made that mistake: see RANT)
Now MY RANT on vapor lock: Your indicated oil temp of 220 is taken after to oil cooler. The actual oil temp inside the engine is at least 10 degrees hotter. Your fuel temp is being driven up if you have a mechanical fuel pump. The hot oil heats the pump, the hot pump heats the fuel. You may be experiencing slight vapor lock which you are referring to as a "Rich Condition". If you are using MOGAS, switch before the next flight to 100LL AVGAS until you get control of the temperature. This is a safety issue. I've experienced in flight vapor lock from MOGAS, please don't go there. I am not opposed to running MOGAS, but right now you have a problem, so work on the safe side.
Also you may have generally hotter air temp behind and below the engine because of the high cylinder head and oil temps. This can cause vapor lock. Also, you may be missing some old exhaust pipe heat shields that prevented radiant heat from entering the fuel line or gas-collator. This condition nearly 'got' a good friend of mine.
running lean? Carb base gasket leaks? Intake tube hose clamps loose?
Bad baffle seal? Bad air seal around the propeller shaft?
A new tight engine suffers more from overheat damage than an old loose engine.
Piston ring sealing is created by high combustion pressure, and varying RPMs are good too, but I would avoid slow steep climbs, which lead to High cylinder and oil temps. Avoid break in on hot days.
High ring blow-by causes high cylinder and oil temps. Wrong piston rings? Did you put the rings in upside down? Are you losing oil out the breather? (indication of high crank case pressure / blow by). I only ask because I know I'm not immune to mistakes.
Peak combustion pressure is supposed to occur about 12 degrees after TDC, if too soon, High cylinder temps result. You could reduce advance to 20 degrees for lower cylinder temp (and higher exhaust temp) and probably see no performance loss during take off and climb, maybe a small loss of top speed. I'll refer to Bruce Finney's comment about reduced advance. You would be safer while the rings seal and the combustion surfaces accumulate the protective carbon layer that slows heat transfer. A fresh engine lacks that insulating coating on the piston crown, combustion chamber and in the exhaust port.
REM40 is a good safe spark plug. Sometimes the internal resistor fails. Check each spark plug with an Ohm meter, resistance above 5000 Ohms is a failed plug, which causes rough running, high EGT and can destroy the magneto coil. http://www.qualityaircraftaccessories.c ... -resistors
Last comment: Break in your engine on cool days. This is easy now, summer is nearly over, cooler fall weather is upon us. Good Luck is the luck you make!

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE