Thorp Air Command - T18.net

I misspoke before when I said 30*. I meant the other way. I was on the ipad and passively responding. I'm only 1 year into the ownership thing but used to build engines for team kawasaki and did all the engine building and tuning on my import drag car. Liquid cooled engines are a whole different beast though so I'm behind the curve.

25* is what the lycoming manual says so I'll just leave it there.

What do yours run in climb and in cruise?

Also note that I have the 2 piece top and bottom cowl style no the more common chipmonk cheek cowl. I'm not sure if that increases volume in the plenum or not. I don't have much space between the cowl and the top of my cylinders.

I typed it wrong. it was supposed to be 20* when I commented.

I have been pushing it as hard as possible with keeping the temps reasonable. The first hour we did was at 2500-2600 rpm at 4000 feet msl. It is rated to 2600 rpm with short duration of 2800 so based on the performance charts if I was below 5000 I would be above 75% if I kept it above 2400.

I reduced the quote but will respond to each questions. Your response and questions are helpful!

So after the initial 1.5 hours I did some work to the baffles.
Removed and replaced the side asbestos seals with silicone seals like my rear baffle seal.
Increased the distance to roughly 1/4" behind the rear cylinders.
Checked inter cylinder baffles: correctly installed and up against the cylinder fins.
Adjusted rear baffles so they curve up against the rear cylinders to the bottom (almost touching) (as pictured)
Sealed off any spots in the baffling that would allow air to seep out of the upper plenum
Created 2 angled air deflectors to push air above the front cylinders
Oil cooler is mounted up front with a scoop on it. I blew through it with air compressor to make sure no debris. I can see through it without any blockages. There is no thermostat on the oil cooler.
Gravity fed without fuel pump
Oil temp sensor is in the oil screen housing so I'm not sure if that is before or after the cooler. It appears my cooler return line goes straight back into the rear of the engine in a different area than my oil screen.
Running 100LL in the plane so mogas issue shouldn't be an issue. Still doesn't stop it from having vapor lock although no hard starting when it's hot which is the most common time for vapor lock to be evident from what I know.
I have 2 exhaust stacks with no cross over pipe. There is only 1 carb heat muff on it which was not removed or messed with. It is intact without any issues upon inspection.
Fuel line is centerline 6" away from each exhaust stack as is my gascolator.
Carb seal was new last winter and carb wasn't removed for top engine work. All bolts and safety wire were checked. A leak around the carb seal I think would most likely cause leak condition on all 4 cylinders.
Checked all intake gasket bolts and hose clamps on intake tubes were all tight. Intake gaskets/exhaust gaskets were all replaced. No evidence of leaking by tracts of fuel around intake or carbon on exhausts.
I'm not sure what this "propeller seal" is everyone talks about. Is it the baffle seal around the front of the engine behind the starter gear? If so the baffling is all silicone seal and in good condition. I always check it's properly positioned before flight.
Engine was done by a shop, not myself. All measurements were as needed per the engine manual stated by the AP. My #3 is a steel bore and #4 is a chrome bore so they have different rings. Fronts are both steel bore. I'll have to take his word they were properly installed.
Oil consumption was minimal on first 1.5 and today after .8. Used less than 1qt on initial flight. it was set to 6 and I was a little under 6 after the flight but not halfway between 4 and 6 markings which means it was over 5 qts left.
Only a tiny spot on the concrete under the breather after the flights. About the size of a dime. Way less than what drips out of my piper's breather.
Plugs were all cleaned and checked prior to assembly. I was the one that checked them and I replaced a single plug that was at 6k ohms with all the rest under 3k ohms resistance. I'm pulling the plugs tomorrow to check color and re-check resistors just in case. I think it would be rare for 2 plugs on opposite cylinders die immediately after inspection but I'm checking anyways. Mag drops when checking are equal and not quite 100 rpm drop when checked.


So, after today's flight. I went up earlier so temps were better but still upper 70's when I took off and mid 80's when returning with high humidity (real feel was in the 90's). The "bog" was better today and I didn't have much issue with touch and goes. After 6 touch and goes in a tight pattern I finally saw the rear cylinder temps rising above 450. I hit 460 on #4 which reduced after lowering the nose. I was running around 420-430 on the rear cylinders again today in cruise at 2500 between 2 airports. Fronts are low 300's. Oil temps were under 200* for the most part with 210* when those rear cylinders heated up after the last touch and go with a little too steep of a climb.

In straight and level cruise at 2500 if I push full throttle when the engine temps are cooled down they start to raise but I don't get the stumble I had before. So this could be vaporization of the fuel or just running rich due to hot engine bay temps when my cylinders are heated up. EGTS and chts do rise up together when pushing full throttle. Fronts come up more slowly. Cylinders 3&4 are within 1 bar of each other on my JPI in both egt and cht. When I cycle through them to see the actual temp they appear about 10-15* max difference between 3&4 and 1&2. Rear cylinders seem about 75-100* hotter compared to front cylinders.

I'm going to pull the plugs and see if they are dark or light to see if it's a lean condition or a rich condition with the stumble. Vaporzation (vapor lock) would have my plugs bone white and heat making it rich would make them dark or loaded.

I was hoping better temps after all my changes to the cowl but no luck. They are better but not a huge improvement like I wanted. One of the things the AP and I thought could be an issue is scaveging of air out of the lower cowl to allow higher flow of air through the cylinders. It appears my seal on the top plenum is good. I think the air is backing up at the rear cylinders and not moving down and out the back as much as it needs to get more change over of air. With my tight bottom cowl around the exhaust stacks and the slit type openings on the sides he thinks the air is getting trapped and not venting fast enough. He suggested a vent hole in the bottom by the exhaust similar to the piper or cessna engine vent to allow the air to flow through the engine more. This makes sense with the back of up air over the rear cylinders but I don't want to just start hacking on my lower cowl.

I honestly think cooling was an issue before even though I never saw it because I didn't have a JPI and I don't think the old single probe cht gauge was accurate. The AP that tore my engine down told me (before we even replaced stuff) that it "looked like a hot running engine". I told him otherwise since the gauge told me it was normal. Now that I'm getting high temps on the rear cylinders I'm thinking he was right about the hot running and I was just in the dark from it previously. I also operated this only october-february in Indiana weather so I've never flown it in hot and humid temps. The return flight in late sept was the hottest and it was low 80's on an old engine.

The engine also seems to produce a noticeable difference in power now. My take off rolls are a lot shorter at my home base and it wants to climb like crazy. If I picked up power from the top overhaul (all cylinders barely in the 60psi range and 1 mid 50's before overhaul) then that would also increase my engine temps as well.

Anyways, I'm beside myself and racking my brain on this so any help is welcomed. The engine is smooth and powerful so I like to think that there isn't anything serious wrong with the engine causing the temps but I don't know what to think anymore. I really don't want to cut up my cowl to increase airflow and then after I get a number of hours the temps come down anyways due to break in. I'm thinking about getting it up in cool air next weekend and just doing straight and level cruise for a number of hours and see if the temps stabilize after I get 10 hours of easy flying on it.

Are there any issues with valve adjustment that could cause cylinders to run hot? AP said after 5 hours he will check the tappet clearance again. I can't think of a scenario in my engine building experience that improper valve adjustment would cause egt and cht to both be high together. And to both be within 10* of each other means that is a pretty accurate mistake on 2 cylinders.

I can only think of two reasons for temps to be that different between serviceable cylinders... poor airflow around the cylinders and mixture. Airflow has heen covered pretty well and, given the problem is with the rear cylinders, that is my leading suspect.

As for mixture, the intake pipe connections in the sump could be loose and/or leaking, there could be leaks at the hoses that join the pipe connections to the intake tubes (especially if the hoses are old) and of course the intake tube flange gasket. Normally I wouldn't suspect 2 cylinders to be bad for mixture but stranger things have happened and these are possibles that are not likely to be found just by "looking".

I do not have information specific to the O-290-G but, according to the Type Certificate for the O-290 series aircraft engine, the CHT redline is 550* for the O-290 and 525* for all other versions using a spark plug gasket thermocouple. If using a well type thermocouple reduce by 25*. I threw this out there to point out the difference between thermocouple types and what it might do for your readings.

_________________
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

Post pictures of your cowl, front inlets, oil cooler, baffles & air exits.
I would hesitate to cut the bottom cowl open for an air exit, the T-18 side exits are just about the best that can be had.
The great radial engine fighters of late WWII used side air exits.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

I had trouble with the upload feature on this website so here is my google album with pics of the new modifications of the cowl and pics of what my cowl is. I have some under the belly showing where the AP says I should have a cutout between exhaust stacks to let the lower cowl vent the hot air down away from the cylinders. It makes sense but crazy that others don't have this issue. Although most have those side removable larger rear vent type cowls and I don't see a lot like mine with the small slits.

https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing


Today I went up when it was 70* in the morning. I put a solid 2 hours on it of straight and level cruise. Temps did start to climb when I took off. After getting to 3500 I increased power past the 2450-2500 I was previously using. I had it up to 2600-2700 for a few minutes at a time. At 2500 I was seeing low 400's on my #4, 410-420. #3 was 20-30* under that and the front 2 were 350-375 with #2 being higher by 10-15*. It only hit 440's once when I was up to 2700 rpm. I left it higher rpm to work the engine in.

Also, straight and level if I go full throttle the engine becomes rough. While watching temps I slowly leaned out the mixture and I picked up 200 rpm with leaning. So that indicates that my full throttle roughness must be a rich condition.

I took a video of 2500 rpm straight and level to capture my temps. They were as follows: (cht/egt)
1: 341 1221
2: 353 1221
3: 379 1284
4: 411 1294

Because of the oil cooler scoop on the left bank those cylinders were slightly higher. #4 cylinder is my one and only chrome cylinder. Oil temps were 190-220 depending if I was climbing, or cruise level (2500 rpm). I had a slight amount of oil from my breather after the flight but used less than .5 quart in a 2 hour flight.

I checked the torque on the intake gasket flanges and they were fine. I also checked how tight the hose clamps on the intake tubes are and they were ok as well. After landing I fueled up then had to start it to taxi to the hangar. Even hot it started right up without any issues. My idle is slightly lower than it was from the first break in flight. Hovering around 550 instead of 650 but I assume this is from the break in on the engine and it being quite warm. It was over 80* when I landed.

Jeff J, I use bayonet probe cht. So even though lycoming says 525 and 550 on these 0290 with gasket probe I would be probably still 500* limit with my bayonet probe readings.

I would NOT cut the cowling !!! Your engine is breaking in and it appears that your temps are coming down. .5 Qt's oil burn in 2 hours is not abnormal. Run it one Qt below the high mark for flights under two hours and see what happens. Continue to tighten up the baffling, watch the temp's and fly it.

Phil
118BC

You didn't show the inside top of the top cowl. The inlet opening should have a flow diffuser from the upper lip back up to the top of the cowl. A ramp at the top. This divergent duct causes a laminar expanding flow that converts inlet velocity into pressure. You need the pressure to push the turbulent air down thru the cooling fins.
Do you have a baffle seal from the lower cowl inlet lip over the top of the front cylinder shelf baffle? It would seal air to assure it goes thru the cylinders and does not bypass right down under the engine.
It looks like you have a good air dam around the front of the engine case, the propeller spinner can act like a centrifugal fan and pull top deck air forward, pumping it outward to the inlets..effectively recirculating the same air. It looks like you are OK here.
I'm thinking your side air exits are too small. I would see if they could be stretched out a little by putting a shim plate under the feet of the space bars. Or cut the tubes and spread them with a rod inside or something. Just get those openings to bow out a bit more for a test. Maybe I don't understand how that part is made, but the strap next to the fuselage looks like it should be bent inboard as it goes forward to help form an exit nozzle, and maybe it already is.
The air outlet should be bigger than the inlet because the departing air velocity is lower than the inlet air velocity.
Your lower baffles around the cylinders and heads look good right now, but the dynamic pressure could push them away...I don't see any springs/wire/all thread etc to hold them in place when the air flows thru-.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

I am running the same style cowl. Just for comparison, the left side gill on mine has an opening about 3 1/8 inches as measured in the middle of the opening. The right measured 2 3/8.

_________________
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

Not to add fuel to the fire so to speak, but are the aft CHT probes new? Are they correct? Have you shortened the wires? Are the probes the same distance from the heads as the front?
Have you used a laser temp gun in the cylinder and compared that to the reading?
I agree something seems too evenly amiss.
Also, I would not suggest touch and goes until after break in.
Cubes

This may or may not be an issue. I know EI claims in their installation manual that shortening the wires won't make a difference. I don't know about other brands.

_________________
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

Did you buy you EI temp gage as a complete system with probes? If you bought it without thermocouples, you might have a mismatched system.
http://thermocoupleinfo.com/thermocouple-types.htm
This link shows the temperature current relationship of different thermocouple types. It's possible to have combinations that read high or read low.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

He said he has JPI. I don't know which model he has but the instructions for the 760 claim wire length makes no difference there either. That is a good point on the different thermocouples.

_________________
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

Type J and Type K are the most commonly used, but the instrument vendors are not very helpful, I presume it's because they want you to buy the thermocouples from them.
The same with oil pressure and temp transducers and tank level senders. It takes a bit of research when replacing bad probes/senders.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE