What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Carburetor mixture distribution

Don Jones

Well Known Member
I am early in Phase 1 testing on the RV9A with 17 hours on it so far. It has performed extremely well and has only had a few minor glitches to deal with. The engine is running strong and oil consumption stabilized in the first 4 hours. Early on I noticed I had the typical fuel distribution issues common to carbureted Lycoming engines. Wide open throttle produced very uneven EGT’s as well as causing cylinder head temps to be 50 to 60 degrees apart. (Forget lean of peak ops all together) Simply closing the throttle slightly brought everything inline. Not the best situation, so I started doing lots of research on correcting the issue. I found several articles from sources like NASA, Navion, etc. pertaining to uneven fuel distribution caused by turbulence entering the carburetor from poor air box design. Since I fabricated my own air box I became convinced this was the source of my distribution issues. I found over the years several manufacturers had to include air straightening devices in their air box to correct similar issues. Most consisted of a few well-placed blades to help straighten the air and direct it through the carb. I didn’t want to go that route, so enter 21st century materials. Cars today use a honeycomb airflow straightener in front of the airflow sensor to straighten the air and prevent inaccurate readings caused by turbulent flow. The material they used appeared a little restrictive to me for use in a 320 cubic inch engine, so after some searching I found someone who sold a replacement with 1/8” honeycomb cells made from aircraft grade aluminum and 1/4" thick. I took the material, sandwiched it between two pieces of aluminum, potted the thing together with epoxy with spacers in the bolt locations and mounted it between the air box and carburetor.

straightenersm.jpg



I now have 4 hours on it since the mod and the results so far are impressive. I gained 50 RPM in static run up. The engine is smoother and more responsive. The really amazing part is, it will now run 20-30 degrees lean of peak with fuel flow under 7 GPH . I feel more could be had if I had electronic ignition to help light the lean mixture as anything over 30 degrees LOP causes an intermittent slight miss. I have more testing to do before I call it a complete success, but at this point I am convinced it’s a win.

Disclaimer: I am not an engineer and I didn't stay at a Holiday Inn last night. Do your own research and choose wisely
We are talking about a powerplant modification. It's in early testing and I make no claims as to it's safety. If you choose to
make a modification like this you do so at your own risk.
 
Last edited:
Air Straightener

So where can we purchase this material? I've been looking for this type of solution for a long time.....


I am early in Phase 1 testing on the RV9A with 17 hours on it so far. It has performed extremely well and has only had a few minor glitches to deal with. The engine is running strong and oil consumption stabilized in the first 4 hours. Early on I noticed I had the typical fuel distribution issues common to carbureted Lycoming engines. Wide open throttle produced very uneven EGT?s as well as causing cylinder head temps to be 50 to 60 degrees apart. (Forget lean of peak ops all together) Simply closing the throttle slightly brought everything inline. Not the best situation, so I started doing lots of research on correcting the issue. I found several articles from sources like NASA, Navion, etc. pertaining to uneven fuel distribution caused by turbulence entering the carburetor from poor air box design. Since I fabricated my own air box I became convinced this was the source of my distribution issues. I found over the years several manufacturers had to include air straightening devices in their air box to correct similar issues. Most consisted of a few well-placed blades to help straighten the air and direct it through the carb. I didn?t want to go that route, so enter 21st century materials. Cars today use a honeycomb airflow straightener in front of the airflow sensor to straighten the air and prevent inaccurate readings caused by turbulent flow. The material they used appeared a little restrictive to me for use in a 320 cubic inch engine, so after some searching I found someone who sold a replacement with 1/8? honeycomb cells made from aircraft grade aluminum and 1/4" thick. I took the material, sandwiched it between two pieces of aluminum, potted the thing together with epoxy with spacers in the bolt locations and mounted it between the air box and carburetor.

straightenersm.jpg



I now have 4 hours on it since the mod and the results so far are impressive. I gained 50 RPM in static run up. The engine is smoother and more responsive. The really amazing part is, it will now run 20-30 degrees lean of peak with fuel flow under 7 GPH . I feel more could be had if I had electronic ignition to help light the lean mixture as anything over 30 degrees LOP causes an intermittent slight miss. I have more testing to do before I call it a complete success, but at this point I am convinced it?s a win.
 
Also since this is on the carburetor side of the air filter, what type of epoxy was used?
 
When the honeycomb finally fatigues and starts coming apart, what keeps it out of the engine?

Other than that -- nice.

Dave
 
down stream fuel distribution

You may have a game changer here if your results are repeatable. I'd given up on anything close to lean of peak with my carb'd 0360. I'd understood that the problem with fuel distribution in carbs was uneven fuel mixing down stream from the venturi. How does this straightener effect that? Any expert opinions?
 
This is very interesting, seems effective. We do maintenance and inspections any way so a check on the condition periodically should be no problem. It makes sense to me and if we can get the cylinder flows a little closer to each other that is good and should help some.

Bird
 
mixture distribution

A couple days ago I posted this somewhere else on the forum. Bart at Aero Sport??? in Canada has some alternative ways of dealing with this. Superior has a sump that improves mixture distribution and Bart makes some further improvements to the Superior sump. If I were to make a flow divider for the top of the air box it would be made of steel. I think the openings can be much larger than 1/8".
Another thing that allegedly works is wrapping the leanest intake pipes with insulation-cooler mixture equals richer mixture.
 
Also since this is on the carburetor side of the air filter, what type of epoxy was used?
I used JB Weld. I felt the temps were fairly low here, but I haven't measured it. The air box itself is laid up with West Systems...no signs of any issues so far.

When the honeycomb finally fatigues and starts coming apart, what keeps it out of the engine?

Other than that -- nice.

Dave
The only thing between it and the engine is the carb venturi, so if for some reason it suffered a catastrophic failure it could end up going through the engine. However, the material is tough stuff and extremely rigid. ( I read somewhere it's 2024 T3, but can't find the reference) Back when I was racing, I saw something similar in use on the old Preditor carburetors that became popular for a while among some drag racers. 500 CI at 8000 rpm has gotta be a lot more abusive than our Lyco's

Change it every condition inspection?
That's a cheap enough possibility. I only have $50 and a few hours in it.


You are exactly right, it is nothing new. I found all of the links you posted during my research. Fact is, I bet the Ellison version would be a bolt in, but for 90 bucks and the fact it's kinda crude looking, I didn't give it much thought. The honeycomb is supposed to be extremely effective at straightening the air and I felt it would work better than Ellison's.

A couple days ago I posted this somewhere else on the forum. Bart at Aero Sport??? in Canada has some alternative ways of dealing with this. Superior has a sump that improves mixture distribution and Bart makes some further improvements to the Superior sump. If I were to make a flow divider for the top of the air box it would be made of steel. I think the openings can be much larger than 1/8".
Another thing that allegedly works is wrapping the leanest intake pipes with insulation-cooler mixture equals richer mixture.

I considered trying the Superior sump route, however after reading about distribution issues from turbulent air entering the carb it became clear to me in order to make a manifold modification everything would have to be flowed together on a wet flow bench. The carburetor, air box, even the ram air effect would have an effect on manifold tuning. I could see using a Superior sump with Bart's mods AND using an airflow straightener together as being an awesome combination, possibly nearing fuel injection distribution, course by the time you do all that you could almost buy a fuel injection system;) If you want one, which I don't, but that's another story.
 
Last edited:
Another thing that allegedly works is wrapping the leanest intake pipes with insulation-cooler mixture equals richer mixture

I think you got that temp/mixture thing backwards. Think of what happens when you apply carb heat.

JimS
 
I notice they're selling this as a better flowing alternative to "restrictive" 1/16" honeycomb. 1/8" holes are still pretty small though. Any noticeable decrease in MP with this installed?
 
Nice work Don, will brew one up and test with you. :)

That would be awesome. I would love the extra feedback. I also have another prototype I made with different straightening material I am going to have a local friend try. I never tried it because I felt the honeycomb was a better choice. The material is a high temp plastic material thas has small square holes. I salvaged it from an old Bosch airflow sensor.
 
Last edited:
Great discussion

McMaster Carr has some honeycomb material, search honeycomb.

1/2 inch diameter holes, this is a very interesting thread, can't wait to hear how it plays out.
 
Screenshot

Here is a screen shot from Sunday. Now that the weather is warming up I
am having cooling issues. Working on the fix for that now:( Gotta get those
wheel pants and gear leg fairings on too.:)

screenshot1.bmp
 
Last edited:
Here is a screen shot from Sunday. Now that the weather is warming up I
am having cooling issues. Working on the fix for that now:( Gotta get those
wheel pants and gear leg fairings on too.:)

screenshot1.bmp

This change happened after the install? Seem like you're in slow cruise mode? :)
 
This change happened after the install? Seem like you're in slow cruise mode? :)

No, cooling has always been marginal. It's a Sam James cowl and plenum.
I left too much spinner gap and think it's pressurizing the lower cowl. I have the baffles tight and sealed up. It was actually worse before the mod was installed. Wheel pants will fix the speed:D
 
Last edited:
Carb heat?

Hi Don,

This is very interesting - I had similar issues until I started playing with Carb heat - I know your airbox set up is different, but it might be worth exploring. I "think" my carb heat sets up turbulence in the the airflow that helps balance the mixture - the heat might help atomize the fuel, too.

I run LOP almost all the time with carb heat fully or partially on. 150+KTAS on 6GPH at 7500. I love it!

My only concern would be if the grid breaks up and gets sucked up. That said, I love the innovation! Good luck!
 
You may have a game changer here if your results are repeatable. I'd given up on anything close to lean of peak with my carb'd 0360. I'd understood that the problem with fuel distribution in carbs was uneven fuel mixing down stream from the venturi. How does this straightener effect that? Any expert opinions?

Install dual electronic ignition and you can run LOP with a stock LOP w/o carb heat.

I typically run my O-360 this way with no issues and get between 100 and 50* LOP.

Now to try this mod and see how that improves things.
 
If the grid breaks up and gets sucked it, then the material and thickness would be significant.
Thin aluminum hex would probably break up, get sucked into the cylinder, smoosh around and fly out the exhaust without much damage...
If made of steel, it still might break up, but once inside the cylinder, could do more damage.
On the other hand, the material used in mass airflow sensors is proven to NOT break up, so there is value in that approach.
One thing for sure; on a single engine plane, the big fan must not stop until you say so!!
 
Don,

Have a micrometer handy? Can you measure the thickness of the aluminum cell wall?
 
Don,

Have a micrometer handy? Can you measure the thickness of the aluminum cell wall?

I will have to find some of the scrap or measure next time I have the thing in my hand. Just a guess, but it's around .005 or so.
 
Last edited:
Don-
Great work! Got any pics of your airbox, and the installed straightener?

Thanks,

Craig

I don't have pics right now. I am actually working on a replacement for the airbox. I am replacing it with a piece of 3" mandrel bent aluminum tubing. It should clean up the air entering the carb even more. I will post more info and pictures as my experiments progress. I just finished painting the wheel pants so I can get them on this weekend. I will also post some before and after engine monitor graphs when I get a chance. I have been working on cooling. It's still running warmer than I would like in cruise and climb. It's been like that since before the carb air straightener. The straightener evened the temps out, but hasn't lowered them, well not much anyway.
 
I have a friend in the automotive electronics industry and I sent him your link to this story.

He mentioned that the early hot-wire systems had a heckuva time with this turbulence problem and often had flow-straighteners both before, and after the hot wire sensor, to get it to read correctly, so to the automotive guys, it's been around a while.

Best,
 
So Pierre;
Are you suggesting flow straightening on both the inlet and outlet of the carb? What are you trying to do, make me think?
I'm envisioning a honeycomb or square Ellison style inlet air straightener, and one of those tea strainer bowl shaped screens that are an aftermarket part for Harley's, to break up the fuel droplets and even the fuel distribution.
https://www.denniskirk.com/H16008.s...m_medium=cse&gclid=CI-GlNiF_7UCFdE-Mgod-iAAOw
Patent Abstract:
A dished screen placed in engine intake manifold openings beneath a carburetor in the path of intake air flow with, under a one-barrel carburetor one recess dished screen being used, and with a two-barrel carburetor two recess dished screens being used. The screen units are positioned so that fuel laden air from the carburetor passes through screening of the screen units with the mesh and wire size of the screening coupled with depressed dished shape of the screen units such as to optimize vaporization atomizing of fuel in the fuel air mixture being fed to the engine. The proper shape and screening size of the dished screen units is an excellent trade-off of intake air/fuel mixture flow obstruction for more efficient fuel utilization resulting in more power and significantly improved mileage and decreased undesired emissions.

I'm just sayin'. But isn't VANs FAB (filtered air box) supposed to assure smooth inlet air flow to the carb?
Anyway, the screens are plenty strong, and would address the 'what if the inlet straightener breaks up and goes into the engine' issue. Although some unacceptable power loss may occur.
 
Last edited:
Harleys need all the help they can get, but that's another story:)

It seems that the turbulence is more of a problem than droplets breaking up, if I understand the original OP's post.

Best.
 
I have considered the straightener in the intake manifold side of the carb. Not sure if there would be any additional benefit other than possibly the better atomization of the fuel and/or minimizing the effects of throttle blade induced turbulence. From here a wet flow bench would be in order. Not sure all that development work would ever be worth it, other than for my own recreation and education. Maybe someday. Like I say, I am still working on the inlet side. The beauty of the Sam James air filter is it leaves me free to do pretty much what I want on the air box inlet to the carb. I should have the new mandrel bent pipe ready in a few weeks for testing.
 
Seems I have heard before about a insert for the sump to match up the carb. bore size to the sump intake diameter. I know my carb. bore is much smaller than the the sump bore and have wondered about the turbulence it that area. If I can't locate something I think I'll be machining a sleeve to match them up.
 
Seems I have heard before about a insert for the sump to match up the carb. bore size to the sump intake diameter. I know my carb. bore is much smaller than the the sump bore and have wondered about the turbulence it that area. If I can't locate something I think I'll be machining a sleeve to match them up.

Lycoming service bulletin SB258 is what you seek if the sump has a bore of around 2.25" at the carb flange that tapers smaller down in the sump. Causes lots of turbulence and mixture distribution issues.
 
Before and After

Here are a couple of the data captures from before and after install of the air straightener. These are the EGT graphs, the CHT graphs show a similar smoothing and evening out of the data.

Here is the before

before.jpg


Here is the after

after.jpg
 
Sam James fab?

So what is the difference between the airbox on a James cowl and VANS? Would this same strightener work with VAN'S fab? Results look really promising Don!
 
So what is the difference between the airbox on a James cowl and VANS? Would this same strightener work with VAN'S fab? Results look really promising Don!

Sam James uses a conical filter housing then ducts the air through a 3" scat tube to an airbox mounted to the carb. Van's filter does do some air straightening, however I feel it would benefit from use of an air straightener as well.
 
CHT graphs

I thought I would post what the straightener has done for CHT's as well. They are running high, but have since first flights and have been manageable. I put the wheel pants on today and CHT's are significantly lower, but more on that later. Here are a couple of graphs pre and post carb air straightener. CHT's while warmer than I would like show some impressive evening out. The peak in the middle of the after flight was a climb to a higher altitude.
Here is the before:

chtbefore.jpg


Here is the after:

chtafter.jpg
 
Airbox Picture

Don-

Is the current airbox the one pictured in your thread about getting the airplane's pink slip?

Thanks,

Craig
 
Honeycomb source

I found this thread extremely interesting and have been thinking about giving it a try my next annual when I pull everything apart. The posted results were pretty impressive.

Im running an O-360 and have found I can run LOP at some low power settings but never any higher. Right now for putting around depending on the day I'll run 2000 to 2150 rpm and can run LOP down to 4.5gph to maybe 5gph and still do 120kts to 135kts. Great for just boring holes. It would be nice to expand that LOP window some.

I ordered some honeycomb to try from www.saxonpc.com

I saved my old airbox plate that was cracking so I have a perfect template to try and make a couple different versions. The one round piece with the small honeycombs is just how I received it. FIts perfectly. Its the default one on their site for MAF. It was $12.48.

The second, larger one I ordered at 120mm and 1/2in thick. It was $9.85. Just to try a larger honeycomb and maybe compare results down the road.

With shipping it was 26.52 total! for two different pieces to try. Ive got some left over plate from the replacement airbox plate I made and I've got some proseal that should still be good from resealing the fuel sending units last year.


I wont be installing one for months but just thought I'd share a good source in case anyone else was thinking of making one.







Uploaded with ImageShack.us
 
LOP with a Carb

I'll venture to say that you would have similar LOP results when running the throttle wide open at high altitudes (without the honeycone). Let us know the results after you run with the honeycone, especially when running around the 2250 RPM area at lower altitudes. I think that is where you will see the greatest help from the honeycone air straightener.


I found this thread extremely interesting and have been thinking about giving it a try my next annual when I pull everything apart. The posted results were pretty impressive.

Im running an O-360 and have found I can run LOP at some low power settings but never any higher. Right now for putting around depending on the day I'll run 2000 to 2150 rpm and can run LOP down to 4.5gph to maybe 5gph and still do 120kts to 135kts. Great for just boring holes. It would be nice to expand that LOP window some.

I ordered some honeycomb to try from www.saxonpc.com

I saved my old airbox plate that was cracking so I have a perfect template to try and make a couple different versions. The one round piece with the small honeycombs is just how I received it. FIts perfectly. Its the default one on their site for MAF. It was $12.48.

The second, larger one I ordered at 120mm and 1/2in thick. It was $9.85. Just to try a larger honeycomb and maybe compare results down the road.

With shipping it was 26.52 total! for two different pieces to try. Ive got some left over plate from the replacement airbox plate I made and I've got some proseal that should still be good from resealing the fuel sending units last year.


I wont be installing one for months but just thought I'd share a good source in case anyone else was thinking of making one.







Uploaded with ImageShack.us
 
Thanks for the ideas. I just got a Rotec throttle body, and I understand they benefit from inlet air straightening.
My Lycoming 4 banger has much higher EGT on cylinders 1 & 2, as many do. I understand the Ellison & Rotec units can provide better mixture distribution than the MA-4, so I went with it.
With the Marvel Shebler, I noticed more even exhaust temps when using carb heat and part throttle. Perhaps the heat & turbulence improved the mixture distribution with the updraft sump? I understand that this is not the way you are going but perhaps pre-Venturi straightening is valid like post Venturi mixing is, but greater power output should be expected using your techinque, if the honeycomb has negligible pressure drop...
 
It has been a while since anyone has posted an up-date on their results... Is the original honeycomb still in-service and working well? Have any of the other attempts shown similar improvement ? Please post if you have made this modification.

Thanks !
 
I still have mine in service. It is still working well, though I feel I need to upgrade to electronic ignition to consistently run it lean of peak. The honeycomb still looks like the day I put it in.
 
Last edited:
I tried this set up on my -4 with 0-320 carb & did not find any difference at all with or without the honeycomb. Removed the honeycomb.
 
Progress please?

Hi Guys,
Thank you for those posting on this subject.
I have just bought an RV4 (O320 160hp) which I want to fit inverted systems to. Some Q's please?
1. Any further comment on carb' orientation (across or fwd/aft)?
2. Service difficulties/problems?
3. Suggestions?
4. Ellison or Rotec?
5. Mogas or Avgas?
Thanks in advance.
Neil
 
Inside the FAB

I found this thread extremely interesting and have been thinking about giving it a try my next annual when I pull everything apart. The posted results were pretty impressive.

Im running an O-360 and have found I can run LOP at some low power settings but never any higher. Right now for putting around depending on the day I'll run 2000 to 2150 rpm and can run LOP down to 4.5gph to maybe 5gph and still do 120kts to 135kts. Great for just boring holes. It would be nice to expand that LOP window some.

I ordered some honeycomb to try from www.saxonpc.com

I saved my old airbox plate that was cracking so I have a perfect template to try and make a couple different versions. The one round piece with the small honeycombs is just how I received it. FIts perfectly. Its the default one on their site for MAF. It was $12.48.

The second, larger one I ordered at 120mm and 1/2in thick. It was $9.85. Just to try a larger honeycomb and maybe compare results down the road.

With shipping it was 26.52 total! for two different pieces to try. Ive got some left over plate from the replacement airbox plate I made and I've got some proseal that should still be good from resealing the fuel sending units last year.


I wont be installing one for months but just thought I'd share a good source in case anyone else was thinking of making one.







Uploaded with ImageShack.us
Don put his air straightener between the carb and FAB but it seems inside the FAB would be an easier installation. You could mount it with a new set of bolts so as to not interfere with any of the original FAB installation. Any one try this? So far Don reports great success and an RV4 owner reports no effect. Any other updates?
 
Back
Top