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RV-6 O320 Fuel Flow On Takeoff

S

scottp1111

Member
Hello,

I am not sure if there is already an open thread on this anywhere on the site im sure there probably is so I apologize in advance, please feel free to move this if its in the wrong place!

I have an RV-6 with a Lycoming O320 with the 4SPA carb and the standard Van's Cowl setup. I am experiencing stupid high CHT 450+ on takeoff from sea level. I am only indicating 12.5 GPH at best on my fuel flow, and we have good reason to believe this is not enough fuel to keep the engine cool. We have checked the timing and both Bendix mags are setup for 25 degrees and all intake leaks have been resolved and we are still dealing with this problem. I have had the carb rejetted with larger jets now stepping up each time with little or no improvment. Can anyone share some target numbers with me. Currently we are running a 1290 jet...

Thanks

Scott
 
scottp1111 said:
Hello,

I am not sure if there is already an open thread on this anywhere on the site im sure there probably is so I apologize in advance, please feel free to move this if its in the wrong place!

I have an RV-6 with a Lycoming O320 with the 4SPA carb and the standard Van's Cowl setup. I am experiencing stupid high CHT 450+ on takeoff from sea level. I am only indicating 12.5 GPH at best on my fuel flow, and we have good reason to believe this is not enough fuel to keep the engine cool. We have checked the timing and both Bendix mags are setup for 25 degrees and all intake leaks have been resolved and we are still dealing with this problem. I have had the carb rejetted with larger jets now stepping up each time with little or no improvment. Can anyone share some target numbers with me. Currently we are running a 1290 jet...

Thanks

Scott
Click to expand...

....PM sent, please check!:D
 
150 or 160 hp?
FP or c/s prop?
What altitude?

Big differences in climb hp therefore fuel flow.

160 hp @ .55 lbs/hr is 14.6 gph.
105 hp (70%) @ .55 is 9.6 gph
 
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f4lcfb.jpg
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Here is a shot of my engine monitor


Pretty sure at this point with everyone's advice this engine is just running way to lean now. We had a much larger jet in the carb until we later repaired some intake leaks, at that point it became too rich in which we put a more normal jet back in it. Now it seems we are too lean again. I am shooting for about 14 GPH WOT @ SL on takeoff. Obvously this is in cruise at 1,000 feet, these pics was taken in flight for comparison purposes. The top pics are of the stock jet and the bottom pics show a small improvement with the addition of a 5% increase jet.
 
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Are you running a FP or CS prop? Depending on your configuration, a little over 12GPH may be what is called for. You won't get near 100% power with a FP prop on takeoff. I am dealing with a similar issue although my CHT's don't get that high - my goal is to get them under 400 in climb though. Are you completely confident in your baffles? A number of small improvements in mine brought down CHT's 10-20 degrees. Have you done a lean test at 8000', WOT (75% power) to see how far ROP you can go? Should be over 150 degrees I believe.

Anyways, the Lycoming charts can tell you what your full throttle fuel flow should be - mine tells me about 12.3 GPH at 2250RPM. I am at about 11.2 right now, and today I drilled my main jet from #42 to #40. That should give about a 10% increase in fuel flow if my math is right, to get me right about at that magic 12.3 number. Then I can re-evaluate. I will report back tomorrow. My full post on it from the other day is here http://www.vansairforce.com/community/showthread.php?t=144996


Chris
 
YellowJacket RV9 said:
Are you running a FP or CS prop? Depending on your configuration, a little over 12GPH may be what is called for. You won't get near 100% power with a FP prop on takeoff. I am dealing with a similar issue although my CHT's don't get that high - my goal is to get them under 400 in climb though. Are you completely confident in your baffles? A number of small improvements in mine brought down CHT's 10-20 degrees. Have you done a lean test at 8000', WOT (75% power) to see how far ROP you can go? Should be over 150 degrees I believe.

Anyways, the Lycoming charts can tell you what your full throttle fuel flow should be - mine tells me about 12.3 GPH at 2250RPM. I am at about 11.2 right now, and today I drilled my main jet from #42 to #40. That should give about a 10% increase in fuel flow if my math is right, to get me right about at that magic 12.3 number. Then I can re-evaluate. I will report back tomorrow. My full post on it from the other day is here http://www.vansairforce.com/community/showthread.php?t=144996


Chris
Click to expand...

Chris,

Im turning 2250-2300 on takeoff with my FP sensinich prop. The baffling could definitly use some more work as we both know its never totally finished. However these temps used to not be present prior to fixing some intake leaks and having to rejet.. I haven't drilled yet, I have been sending mine to D&G fuel specialties and having Russ and his team increase my flow slowly but surely and I always get it flow check and I am confident in my carb when it returns. I dont have the expertise to feel comfortable pesonally drilling. We feel that our target is 13.5 GPH on takeoff to get the cooling we are targetting.
 
CHT

Back off on your timing as low as 20 degrees and see if this helps. It won't cost you that much power. Some of the angle valve engines run 20 degrees.
then get your other problems solved and you can incrementally increase the timing back to 25 degrees. This may save you some very expensive cylinder work. Anything over 425 is way too high, absolutely no reason why it should be that high. I don't believe this is entirely fuel flow related. The other possibility is to try a different carb, the latest PN MA4SPA may help.
 
jrs14855 said:
Back off on your timing as low as 20 degrees and see if this helps. It won't cost you that much power. Some of the angle valve engines run 20 degrees.
then get your other problems solved and you can incrementally increase the timing back to 25 degrees. This may save you some very expensive cylinder work. Anything over 425 is way too high, absolutely no reason why it should be that high. I don't believe this is entirely fuel flow related. The other possibility is to try a different carb, the latest PN MA4SPA may help.
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I havent been running the plane while its in this condition. I have "test flown" for less than 5 minutes after making adjustments but I am not continually operating the aircraft at these temps. I agree these are absolutly too high, and im also looking at adding a James Cowl to help cool it down... In the middle of doing that cowl I am considering the EZ cowl flap since i will be doing a new cowl setup.

http://antisplataero.com/products/ez-cool-cowl-flap
 
No Lyc experience.... but.... is it usual for cylinders 1 & 3 to be so much hotter than 2 & 4? To my limited knowledge, that seeming like a baffling issue.
 
Are you running the baffle risers in front of #1 and #3 cylinders? If so remove them and see what happens. If they are in place and you remove them it may equalize the temps somewhat?
 
SmackSB4 said:
No Lyc experience.... but.... is it usual for cylinders 1 & 3 to be so much hotter than 2 & 4? To my limited knowledge, that seeming like a baffling issue.
Click to expand...

In my experience, with the carbureted O-320, the throttle setting greatly influences the fuel distribution between the front and rear cylinders. At WOT, the airflow is directed straight up, which is closer to the rear cylinders, and they end up richer, and thus cooler. At partial throttle settings, the mixture between front and rear changes. In fact, I can adjust my front and rear cylinder temps in a climb by slightly changing the throttle position. The front cylinders are hotter in climb, they even out in cruise.

Chris
 
There are hundreds if not thousands of o320s running standard vans cowls and baffles without temp issues. I'm sure you can figure this out without having to change cowlings. Baffles, air outlet fairings, modified baffles where the jugs have no fins, timing, plugs(maybe?), air inlet ramps, oil cooler ducting etc etc. Changing the cowling should be a last resort.
 
sblack said:
There are hundreds if not thousands of o320s running standard vans cowls and baffles without temp issues. I'm sure you can figure this out without having to change cowlings. Baffles, air outlet fairings, modified baffles where the jugs have no fins, timing, plugs(maybe?), air inlet ramps, oil cooler ducting etc etc. Changing the cowling should be a last resort.
Click to expand...

Yep your right... however visually the guy who built my cowl did less than a perfect job on my cowl. I would like to do one for more reasons than cooling. Mine looks awesome and doesn't line up perfectly. I want to do piano style hinges vs some 16 + screws. I figured the James cowl looks good and will improve cooling on top of the other mods I need to do to cool this thing
 
Is this a new engine that is not broke in yet or... how many hours on engine? You should see a good drop once the rings seat.

Baffles can't "need some work" they need to be sealed all around. Suggest having another set of eyes look over your baffles. You should be getting 13gph or slightly more. A less than ideal but effective way to get more air flowing would be to open up the cowl exit some by cutting it forward but more air flowing through the cowl will slow you down so first step is to make sure everything is sealed good so air flow is not being wasted.
 
Dbro172 said:
Is this a new engine that is not broke in yet or... how many hours on engine? You should see a good drop once the rings seat.

Baffles can't "need some work" they need to be sealed all around. Suggest having another set of eyes look over your baffles. You should be getting 13gph or slightly more. A less than ideal but effective way to get more air flowing would be to open up the cowl exit some by cutting it forward but more air flowing through the cowl will slow you down so first step is to make sure everything is sealed good so air flow is not being wasted.
Click to expand...

We opened up the bottom cowl, make small improvement. Engine is 800 hour + with 200 hour top end. We believe it's a problem with fuel flow. We changed a lot of carb jetting after sealing intake leaks and then we was way to rich. We also had the mags rebuilt. We have went through and eliminated timing and now down to low fuel flow. Pictures from 2 years ago when it was running good fuel flow was 13.8 on takeoff which indicated to us now we are way to low at 12.5
 
I have the same setup as you (150HP, FP sensi prop, same T/O RPM). With even less fuel flow than you are seeing, my temps are nowhere near 450. There has to be something else going on unless you are climbing out at 60 knots or something for a long time. I will report back tomorrow with my new fuel flow and temps.

One other thought - are both your fuel flow readings taken with the boost pump either on or off? I know my boost pump affects FF reading. I am reading slightly over 12 with pump on, but with pump off about 1GPH lower, which is the # I trust.

Chris
 
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