Thorp Air Command - T18.net

I was recently looking inside a Lycoming sump. If I recall correctly, the intake tubes were attached to the center plenum with the back 2 cylinder's tubes above front 2 cylinder's tubes.
This was a rear horizontal carb sump, as used on a Grumman Tiger. If all the Lycoming 4 cylinder sumps are so configured, then I could understand the front cylinders getting more fuel.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

I think the most common sump has the carb mount on the bottom. Internally the branches are all but equal in length however externally the tubes to the front cylinders are longer. In this configuration I think it is the carb butterfly that causes the different mixtures between front and rear cyls. This is a part throttle problem as the butterfly deflects the main air flow which favours the front cyls. Full throttle the effect is much less.

Tony
#867
ZK-VMS

Correct. I think mine is slightly effected by full throttle as well. I removed my nozzle and it is at the machine shop to be drilled with the atomization holes like the new nozzles they sell for 160$. That should keep from having only 1 stream of fuel spraying directly to the front of the carb causing an issue.

Here is my most recent JPI data if anyone wants to look in ez trends. -900 ft DA, temp 40's F*, went to 4000ft. Performed left and right mag stress tests as described by savy aviation. Front cylinders are in the mid 1200's with full throttle and full rich. Rears are low 1400's. CHTs similar across the board for both fronts and both rears with about 20-30* difference from front to rear. So it's not just high engine temps keeping egt up. It has to be a mixture distribution issue because the spread decreases significantly with lower rpm and fronts drop considerably more than rears when I do full throttle (and enrichment circuit opens).

Well I guess I can't upload JPI data. Anyone want the data to look at just email me: jeffbinder@gmail.com

Have you tried running at cruise power with carb heat?
I was able, while running above 4000 feet, to nearly equalize the exhaust temps by a combination of part throttle and carb heat, then lean to the exhaust temp I wanted. The fuel tends to stay atomized in the air better so you get less 'wet flow' on the walls of the intake plenum and runners, and the carb butterfly angle also has some effect. Also, the mixture becomes richer with carb heat, as the same amount of fuel is drawn in, but the warm air is less dense. So you lean as required. The power reduction wasn't too bad, and speed was still pretty good but I can't remember what is was.
If you drill atomization holes in the jet, doesn't that admit air into the jet to emulsify the fuel for better atomization? if air is drawn into the jet, then that means less fuel passes thru and you get a leaner mixture?
Just asking...

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

Unfortunately my carb heat isn't the greatest design. It's an all or nothing system. I have a heat muff with 1 scat tube to the intake pipe. There isn't any forced air through the heat muff to force air into my intake. My flapper valve closes off the ram air so it draws air from the engine bay. So when it's only partial closed I don't draw warm air due to the ram air. So my drop is too much to fly it full carb heat.

As for the holes, it shouldn't change fuel ratio because it is a set amount of fuel coming through the nozzle. The air hits the fuel sooner through the holes and just causes it to be dispersed around more.

You should be able to lean out the rpm drop with carb heat on since the drop comes from having a richer mixture. Mine doesn’t get any ram air with carb heat on either. Just draws air through a muff on the tailpipe. Moving the valve in the carb heat box to a partial setting would still alter the airflow and possibly draw some warm air in the process since the ram air access is being restricted.

_________________
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

Ok, I'll give it a shot and see what I come up with. In cruise I'm only 60-80 difference which is actually not bad for a carbed engine. My main concern is the high RPM lean rear cylinders since that is most of the time a takeoff type situation. I don't like running cylinders on the lean side (still ROP but sitting at max power 100-120* ROP, high 1300 to low 1400F*) verses my front cylinders in the mid to low 1200's. I'd like my rears to be in the mid-low 1200's on take off for safety and longevity.

I guess I can test the carb heat at altitude with higher RPM setting. Even with leaning and carb heat the higher intake air temps reduces my detonation margin so that makes me a little nervous to test it on take off. I should have the same spread results at altitude with high rpm so I'll give that a go.

With the nozzle drilled to a #37 already and most only using a #39 I feel like my engine should be rich enough. It appears based on front cylinders that it is so I don't want to drill it out even further than it is just to get my rear cylinders in a safer range.

Also, I'm trying to figure out at what altitude and percentage power I'm using. The charts I used for c152 and c172 manuals seem easier to follow than the one for my 290. I dont have a manifold pressure gauge so the advanced alt chart for my 290 makes me question my power output. I don't know what MP line to use since I can't see it on my plane. I'm also not sure if I should base it on the 2800 rpm redline that is a short duration redline or the 2600 continuous redline when I set my cruise power based on altitude.

I wouldn’t want to risk it low either. If you climb high enough it is hard to hurt a normally aspirated engine because it cannot make it’s rated power. One of the prop manufacturers asked for my max throttle performance information at 8500 ft density altitude for that very reason.

_________________
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

So after having a friend drill the nozzle atomization holes I installed it and tested today. On full throttle take off all the temps are in 1200's and equal. At WOT 3500 feet the rears are equal to the front and below 1300. When I start pulling throttle back the egt spread goes lean again on the rear cylinders like before. If I push full throttle again the temps stabilize. Not a problem having a spread in cruise as the engine isn't producing power or a risk for detonation. So the project worked on keeping me safe on take off! I also had a shorter take off roll than any other time in the plane so I think I picked up some power by having proper fueling. The rear cylinders were lean of max power on take off apparently.