Thorp Air Command - T18.net

After testing with the jet at a #37 drill bit it appears to be slightly rich when I have a 1500DA and 75*F so I have to lean out just slightly on WOT for take off. The last couple of days we have had a -500 to -900 ft DA and only low 30's. When I took off in climb my CHTs stayed mid 300's but my rear 2 EGT both rose to high 1400's and just broke 1500. It was climbing fast and my trim wasn't aligned so while I was waiting for the slow electric trim to balance me out I just reduced throttle slightly. I think that pull back in power reduced my mixture too much (no more economizer). I pushed full throttle and the EGTs came right down. So there is a change in front/rear egt that depends on throttle position.

My fronts were still running in the high 1200's. So it seems like I have a mixture distribution issue between front and rear cylinders. The carb gasket has been changed along with the intake gaskets. The intake pipe hoses were checked and all still tight. I think it would be a large coincidence to have equal intake leaks on only the 2 rear cylinders that has stayed the same with gasket changes.

Plugs were checked and cleaned. I did the ignition stress test at altitude and everything was just as savy analysis listed as normal. I swapped plugs front to rear and the issues still remain based on cylinder and didn't follow the plugs.

In straight and level cruise when I pull back the throttle to settle in at 2500 rpms the egt's balance out within 50-80* of each other with the fronts almost dead on and the rear almost dead on. When leaning yesterday in the cool temps my #3 was leanest at 1480*.

I was reading about an airflow disturbance with 0320 that causes rear cylinders to get less fuel and it's fixed by lycoming SB-258 airflow adapter. I don't see much difference between the carb outlet, gasket and then sump inlet which is the issue for the SB on the 320. The only other thing I could think is the aerator drill holes in the sides of the main nozzle that I do not have. I think this was also another issue with an MA4 carb and not my MA3. Does anyone have the hole size and locations so I could drill mine to see if it makes any change?

So far with a #37 hole I think I'm pretty rich although the numbers on the egt say otherwise. Should I go to yet another size larger drill for the main nozzle and see? It just frustrates me that my cherokee (with 0320) I can go full throttle in any weather with full mixture an no issues lean or rich no matter 100* F or 30* F outside.

I'm building a cable for my JPI 700 in order to download and analyze the cht/egt graphs.

Most places say you want 250* F rich of peak for WOT on take off although based on egt I'm not there and with the high DA and hot days it bogs with not quite that much fuel and I have to lean it back to near max power egt temp.

Thanks for any help in smoothing out this minor issue for me. I get tired of always messing with mixture and want it to be like my piper: push the black knob and just fly the plane.

Jeff

Other carb thread if anyone needs an update:
viewtopic.php?f=19&t=8153&start=15

If anyone has an extra main nozzle I can experiment with I would gladly purchase it. I just can't justify spending 150$ on a brand new nozzle when I'll need to drill out the main jet right off the bat.

Can you post some pics of your air intake by chance ? Dan

What kind of exhaust are you running? I have some thoughts but need more info before I comment. Try header wrap around your intake runners. A small increase in intake temp can transmogrify into high EGT's.

Here is the front view from the day I bought it. It uses a round car type plenum with an automotive round filter. It isn't sealed to the cowl. There is a small gap around the bottom edge of the intake and cowl (1/4").

I have straight exhaust that is 2 into 1 on each side with no cross over. On cylinder 4 when I had high cht I used insulating wrap on the intake runner but not on #3 because it is already shielded from the exhaust by the carb heat muff.

36* outside temps when this happened Saturday. The intake temp I think would be similar on front and rear runners with the exhaust blocked on my rear cylinders.

The temps were below 1400* climb out when oat was in the 70s which is why I'm thinking lean on the rear cylinders from distribution issues in the plenum.

I thought I'd post a few pictures of my baffle set-up, after receiving a request. I've got so many pictures of engine detail that it's hard to pick to show specifics you might be interested in. I used a thin (1/16") grey silicone rubber when, as far as I know, no one was using silicone for that. I used mostly 6061-T6, and made wooden form blocks to form the flanges, annealing as necessary. Maybe I just got lucky, but I've never heard of anyone on this forum measuring CHTs that were any closer together than what I have measured on mine, and I have very good overall cooling too. The maximum difference I measured between my hottest and coldest cylinders was 12 deg. F. (in a steep climb), the max. difference in cruise was 8 deg. F. The maximum measured temperature during the test flight was 328 deg. F. To clarify, I have an O-320-D2A, the carburetor is an MA-4SPA (part #10-3678-32), and I did change the nozzle on it to the "pepper-pod" style (part #47-828). After a few hours of tests I decided to ream the nozzle I.D. @ the inlet to 0.104" from the original 0.093". This was done late in 1995, and the test flight measuring temperatures was in 1997. I don't have an engine monitor, but I used 4 bayonet-style probes, and ran the wires in through the cabin heat box to an electronic digital thermometer on the seat next to me. All pretty "old school" compared to today.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Can you post pics of the intake of the cowl and exit? Do you happen to know what ratio of difference it is?

Also, it appears that even your valve covers are in area of ram air. Are there any block off plates to force air into the cylinders instead of around the valve covers?

Maybe more airflow in and out is the key. Yours looks very open with lots of flow even if it isn't 100% forced into the fines. Maybe that keeps the air from backing up in the cowl and holding heat in.

I'll just say that I find it very hard to believe a naturally aspirated motor could produce 1500egt. Injected, yes. My mind keeps going back to the probes for the rear cylinders.
Cubes

The probes were all placed in boiling water together. The temp of the water monitored and all probes were within 3* of each other and within 5* of the temp of the water. The position of the probes in the exhaust might be a reason.

That being said what does everyone see for egt on wot for take off? I assume the peak egt varies based on many factors and can vary. I know it usually is #3 that peaks first but sometimes it has been 2 or 4. Does that mean at wot take off my egt is different than when I'm in cruise? I see upper 1400 for peak in cruise. I'm not sure what to base my take off egt temp on. Manual says never operate above 75% leaker than 150* rich of peak. Is that based on my cruise peak egt or is there a higher take off egt peak?

It's actually not at all "open", and almost 100% of the air is forced over and through the cylinder fins and out the bottom. That's one of the things that make it efficient. There are very few, if any, air "leaks". You can see the silicone material that runs along the sides at the rocker box covers. That is attached to aluminum pieces that closely follow the contour of the engine. The following picture shows that. This is basically John Thorp's original design.. it's very tight and efficient. It was taken when I had P-Mag ignition units along with the automotive style spark plug wires. I went back to mags sometime after that.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

I went with the enclosed set up copied from Dan's El Gato . Like John's the air is forced over the cylinders and then out thru the bottom . There are 2 scatt tubes on the rear , one for cabin heat and one for the oil cooler . Never had a problem with overheating .

RB

Here are a few other pictures... I don't have much showing the air intakes and outlets on the cowl.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

What do you think about a intake leak where the rear intake tubes enter the oil pan. Looks like you have checked everything else. One way to check is to spray WD 40 on the area with the cowling off and the engine running, you look for a engine stumble when you spray an area. You must be EXTREMLY careful not to get any body parts near the rotating propeller, and have someone you fully trust sitting in the cockpit. If one of the rear intake tubes were leaking at the pan it could affect more then one cylinder. Just a thought.

Phil
118BC

That is always a possibility. I can talk to the field ap and see if he would help tie off the plane and test that.

In cruise Wednesday at 5500 ft I was testing throttle positions. The position that gave me 2450 rpm was only a 60* egt spread (at number 3). Increasing the throttle continued to widen that gap up to 170* when I was at 2600 rpm. That's as far as I pushed it. Below 2450 the egt spread showed 70-90 depending the position. So it appears it does change based on throttle plate angle.

More open throttle plate would increase manifold pressure, correct? If that is the case would an intake leak then become less evident past the throttle plate due to a "lack" of vacuum pulling air in through that leak?

My thoughts were also turbulence created by the throttle blade is helping atomize the mixture. I have read about ppl making airfoils to smooth air on the carb inlet side. A few on vans did this and have equal cylinders now.

I'll ask about performing the leak test. I use propane when I test motorcycles because it will cause a nice rise in rpm around the leak.

Based on my front egt I'm plenty rich. I just need to get those rear egt to match it and all will be great.