Lower CHT on Cyl 1 = less performce

Bob Axsom · #1 06-06-2009, 08:55 PM

Since new five years ago my Cylinder #1 has been considerably lower than the others (usually under 300 F) and it has been mentioned here that the temperature needs to be brought up to around 350 F for "best" performance. I'm flying in a race at Courtland in one week (6-13-09) and my competition is getting faster and faster so I'm getting desparate. I added three platenuts to the right side cooling air inlet ramp and I made a small deflector to mount there. I will get a test flown as soon as possible. Using my standard test method the speed will have to be greater than 184.4 kts TAS to convince me it makes any difference. We will see. I will report my results but there isn't time for the range of experiments with the deflector shapes and sizes before the race. The one I have now is in front of the large cooling fins at the "top" (outboard end) of the cylinder only. The inboard end is 0.7" above the sck ramp position and the outboard end is 0.4". I just threw in on and measured it tonight. I will work with it a little before I install for flight.

Bob Axsom

RV3 Pilot · #2 06-06-2009, 11:44 PM

(more efficient fule bur

Quote:

Originally Posted by Bob Axsom

Since new five years ago my Cylinder #1 has been considerably lower than the others (usually under 300 F) and it has been mentioned here that the temperature needs to be brought up to around 350 F for "best" performance. I'm flying in a race at Courtland in one week (6-13-09) and my competition is getting faster and faster so I'm getting desparate. I added three platenuts to the right side cooling air inlet ramp and I made a small deflector to mount there. I will get a test flown as soon as possible. Using my standard test method the speed will have to be greater than 184.4 kts TAS to convince me it makes any difference. We will see. I will report my results but there isn't time for the range of experiments with the deflector shapes and sizes before the race. The one I have now is in front of the large cooling fins at the "top" (outboard end) of the cylinder only. The inboard end is 0.7" above the sck ramp position and the outboard end is 0.4". I just threw in on and measured it tonight. I will work with it a little before I install for flight.

Bob Axsom

Who told you that? I could make an argument that the coolest running cylinder is probably the most efficient, (more efficient fuel burn) thus the other 3 should be looked at. Lots of variables obviously but.............
Tom

Bob Axsom · #3 06-06-2009, 11:51 PM

It is a very old heat is energy truth. Anyway I will find out how practical the application is in the O-360-A1A RV-6A world.

Bob Axsom

Ron Lee · #4 06-07-2009, 07:38 AM

In mine #1 is coolest and #4 hottest. Doing what you plan may raise #1 and drive more cooling air to the other cylinders. Can't see where that would be bad.

AlexPeterson · #5 06-07-2009, 08:15 AM

Bob, many of us admire your constant work on performance! Thanks!

Regarding the question of "best" performance being on warmer cylinders, it isn't true as a generalization (as was mentioned in the earlier post). Cooler cylinders will suck in a larger charge of air, hence more power per stroke, everything else being equal.

However! It completely depends on why the cylinder is cooler than the others. I have found that mixture has a much larger effect on cht's than most believe is possible. So, your #1 might have a mixture that is too lean or too rich, which would indeed be robbing you of power in that cylinder relative to the others. But, if that is the case, simply cooling the cylinder more would not change it. Have you collected and graphed the egt vs mixture vs fuel flows for power settings at or below 75%? I seem to recall that you have FI, so if there is any imbalance you can select different injector nozzles to compensate. I apologize if you have already done this...

There is a little more information at:
http://www.n66ap.alexap.com/Mixture_article.htm

osxuser · #6 06-07-2009, 11:05 AM

Desperation...

A couple of reasons people think the CHT's should be at or above 350 for race trim.

1. most efficient mixture (as seen above)
2. Taper in the cylinder bore still from not being up to operating temperature.

One thing we've also noticed racing, is oil temp has a big factor in how much power we get... we have to run Redline synthetic to get to good operating temps without burning the oil.

Bob Axsom · #7 06-07-2009, 04:04 PM

I got the deflector cleaned up, installed and sealed this morning then I went to a rose society meeting with my wife and now I'm back home. As soon as we finish dinner I'm going back to the airport. I don't know haw much time I will have but I hate to compromise the test validity. I have to safety wire two hinge pins and close an access cover which are simple but then I have to remove the wheel fairings to install the subfairings before the plane is race ready. At that point it will be in its fastest known configuration except for the addition of the deflector. I do not expect any gain and I do not have the instrumentation for some of the excelent suggestions here but I will fly the test (weather permitting) record the numbers and report the results. I have 4 cylinder digital EGT and CHT but not fuel flow. Mixture is a pretty gross operation that I need to look at more but in many races now I see no change in speed with subtle changes in mixture. My engine is an Airmotive Carbureted Lycoming O-360-A1A. Comments by Larry Vetterman and Mark Frederick on this temperature target and the tapered cylinder function are driving my thinking a lot. Soon I will know something ...

Got to go eat now.

Bob Axsom

Bob Axsom · #8 06-07-2009, 08:37 PM

It was very gusty with speeds at 20 second intervals on the three tracks varying as much as 5 kts. The SL-60 couldn't get a fix at first and I had to recycle the power and reinitialize it in flight. The sun was getting low but I figured I could get the three tracks in and get back to Drake Field before dark in my unlit racing configuration RV-6A. And so I did but the results were disappointing with a NTPS computed TAS of 178.6 kts at 6,000 ft density altitude. The CHTs were 1=312, 2=332, 3=347, 4=329 F. EGTs were 1=1286, 2=1330, 3=1348, 4=1332. I played with the mixture but in the normal operating range down to the 1100s F EGT on CYl #4 but there was no obvious direct speed response. It was only after I was setting here typing that I realized I never advanced to prop from 2450 rpm during the flight so it was a wasted effort. Will try again when time and weather permits. If there is no gain in speed I'll pull the deflector for the race in Courtland Alabama on June 13 and work with it again later.

Bob Axsom

Bob Axsom · #9 06-08-2009, 06:58 AM

Bad photo but it gives a view of the installed initial deflector.

Bob Axsom

Webb · #10 06-08-2009, 08:11 AM

Since you don't have a fuel flow meter but do have an EGT, I have a suggestion...Act like you have one. You don't have to have a fuel flow meter to determine if you have a lean cylinder. That gives you the fuel flow rate when it happens. It is going to happen anyway. You just won't know the number but it doesn't matter. The cylinders don't know what that number is.

Take it up to around 10 thousand where you won't hurt your engine. Run WOT and 2,400 rpm. Start leaning it out and record the temps. Go from a ROP normal mixture and keep leaning it out by an 1/8 turn, let stabilize for 30 seconds and record the EGT temp and go past it to the LOP side. If they all peak at the same time, they are balanced. If balanced, then I would attribute the higher temp to air flow over the cylinder. If #1 leans first, its the fuel flow to it.

There is something else you will learn about this if you don't already know. There is a mixture setting which gives you a slight rise in speed (~2 knots). Mine is around 100 ROP at 8,500 feet and about 90 ROP at 10,500 feet.