Lotus Elan

Ram air Weber setup leaking fuel

PostPost by: s4elan » Fri Jun 24, 2011 8:34 pm

I have a ram-air style airbox surrounding a pair of 45 DCOE Webers on my racing twincam. I've smelled fuel leaking when the throttles snap closed from 9k when braking ( and can see the residue after the race). The box is fairly tight. I believe that what is happening is the carbs are being pressurized due to their vents being inside the box. I'm pondering two solutions: 1) install a tube from the vents to the outside of the air box, thus maintaining the ram effect, or 2) Put a hole (~1" dia) in the airbox so it cannot pressurize so much ( or at all). The box has a 2 3/4" inlet that enters low in the front of box and I would put the 1" dia hole directly above that inlet. Fix #1 is based on the assumption that a pressurized box is beneficial and #2 assumes it isn't with respect to pressurization and just benefits from a cooler air charge.

Any thoughts?

Pete
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PostPost by: petersracing » Fri Jun 24, 2011 10:58 pm

Your first option means differential pressure across the jets and endless grief. Number 2 seems harmless but I bet won't solve your issue. Have you seen the standoff fuel cloud on trumpets at full noise? My fva has a three inch cloud of fuel that sits off the end of them and anything like a filter anywhere near it is going to get drenched. What is your situation ith fuel pressure management as well.
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PostPost by: bill308 » Sun Jun 26, 2011 4:25 pm

Wasn't there a turbo Esprit version that used 40 DHLA-E's and dealt with a similar issue?

IIRC, this particular version of the carb had special seals to accomodate boost. Was this a blow through system?

Pete,

Any idea how much boost you are getting inside the air box?

My recallection is that one needs a pretty high velocity to obtain pressurization benefits.

It might be worthwhile making this delta-P measurement (box pressure less ambient) on a road test. Lacking a data acquisition system, a simple clear vinyl tube manometer with a water column in the cockpit, is plenty sensitive to make this measurement, but would require observations.

(delta-P) should increase proportionally to 1/2 the velocity squared. Symbolically:

(delta-P)=1/2 ?u**2

where:

? is the air density
u is the air velocity

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PostPost by: rgh0 » Sun Jun 26, 2011 10:04 pm

Where on the carbs is the fuel leaking from? why do you beleive it is due to the RAM air effect pressuring the carbs?

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PostPost by: s4elan » Sun Jun 26, 2011 11:51 pm

Thanks to all who responded. I am including a photo to help visualize my setup.

Fuel Pressure set to 3 psi

Set up a manometer - could but presents a lot of logistic issues that I'm not ready to deal with at this time.

On the question of where the fuel leaks from - good question. There is so much residue all around the carbs and that entire side of the motor that I have no idea. My thought about it possibly being related to the ram effect is strictly supposition. I am theorizing that at full throttle the flow through the air box is such that there is not much pressure, but when off throttle completely pressure builds up. It can then pressurize the float chamber through the vent in the carb body. I may be totally off base on this.

I received a suggestion that it may be due to loose air horns. They are indeed loose because the stack up of housings does not allow the air horn clip to hold it in tightly. I have now put a little sealant on the air horn flange to carb body and made up clips to hold them in tightly. Next race is Jul 8 weekend so it will be tested. Hopefully that will cure it.

Thx,

Pete
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PostPost by: petersracing » Mon Jun 27, 2011 12:42 am

The ram tubes are way too long for the position of the closing plate on the air cleaner. At full noise there will be liquid fuel on the inside of the the plates.

Everyone who has measured the ram effect on air boxes has found the effect to be minimal on anything but an open wheeler.
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PostPost by: s4elan » Mon Jun 27, 2011 1:26 am

petersracing wrote:The ram tubes are way too long for the position of the closing plate on the air cleaner. At full noise there will be liquid fuel on the inside of the the plates.

I agree. When I reassembled it, I put in the short horns ~1.5"

Everyone who has measured the ram effect on air boxes has found the effect to be minimal on anything but an open wheeler.


Re the ram effect. I cannot state categorically if it is functioning as such but when I first installed it I used aluminized dryer style hose and it just blew out. The airbox also had an end plate blow out. Admittedly the weld wasn't the best but in my mind there must be some benefit to having that pressure, even if minimal.

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Pete
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PostPost by: rgh0 » Mon Jun 27, 2011 11:13 am

Leakage around the airhorns and venturis and condensation of the fuel standoff cloud onto the airbox and filters are the most likely cause of fuel inside the airbox..

Do you have any leakage from places on the carb body itself outside the airbox such as the starting enrichment circuit which is a common leakage point on a competition car?

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PostPost by: cabc26b » Mon Jun 27, 2011 2:36 pm

Peter and Rohan are on it -

Your fuel is beading up on the aluminum and running off. Try a session substituting a foam-filter like and ITG or pipercross and see what happpens...

Nice G4 btw ( is it an R with the IRS ??) I have a friend that runs one, he ended up venting the front clam-shell because it was bulging at speed.

G
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PostPost by: s4elan » Mon Jun 27, 2011 5:15 pm

The fuel residue inside the airbox is 95 % confined to the area within the filters on the cover plate. No evidence of running fuel - just mist.

Rohan, the fuel enrichment circuit may very well be leaking - no way to tell.There is no leakage evident anywhere until I'm running hard. How would you recommend sealing it?

cabc, not an R model. Austin A40 live axle. My front clip also bulged until I reinforced it - it lifts about 1/2" now at speed. I've seen or run with most of the G4's on the West Coast and there is a lot of variance between us. One thing for sure - they are all quick.

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PostPost by: rgh0 » Mon Jun 27, 2011 7:34 pm

I made up a gasket and sealed the air inlet on the back of the carbs as this is where the fuel comes out. The alternative is to seal down the pistons or rebuild it so the fuel does not leak past the pistons under high G loads if possible ( hard to do)

I see you are also talking to Keith at Sidedraft central there is not much worth knowing about Webers that he cant advise on.

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PostPost by: s4elan » Mon Jun 27, 2011 9:23 pm

Thanks Rohan,

You and Keith are on the same page with the cold start units. I will seal them.

btw, are you still collection dyno data? I have built another twincam and have some data from Ivey's dyno.

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PostPost by: rgh0 » Mon Jun 27, 2011 10:29 pm

yes always happy to get dyno data plus specifications for the engine

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PostPost by: twincamman » Tue Jun 28, 2011 12:24 am

well with the grand children at the cottage for the Summer this leaves more time for repair and service I have also had this leaking problem from the velocity stacks , [a steady and constant drip as in a leaky tap ] so after re jetting and re new needle and seats and increasing the 'd' spring tension on the ventures and screwing around for months with the carbs I finally junked the carbs and sprung for a new set from Dave Bean . They are not yet permanent on the car as a new throttle cable set up and brackets need fabricating [ a 4 x 8 a sheet of aluminum should suffice] . and a stripped mounting stud needs a hele coil at the front of the head I will advise on the success in the next week ..fwiw the screens are tea stainers for removing tea leaves --cost 2 bucks each at Canadian tire or your local hardware shop --.ed
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