Baffled by baffle above weber trumpets

[Charlie Farley]

Ive just watched a video clip i downloaded from RacingFlix.com, features one of the 40's that ran at Spa.During the shots of the engine, i noticed a metal? tray positioned above the trumpets of the bank of webers.
What was this intended to do? Stop fuel spitting onto the engine cover above? Or were there performance related reasons behind its installation?
The video is the one from Top Gear, where Clarkson 'realised a dream', shame he couldnt see past a daft problem like his alarm on the Ford GT he bought!

 

[Bill Bayard]

Charlie,

Sounds like a "turkey pan"(cold air box). By drawing only outside air into the trumpets, you get a major boost in horsepower.

I watched a guy aim a house fan at an 8 stack EFI manifold(Cobra/302W on a chassis dyno). Voila! 20 more horsepower on the next dyno run! /ubbthreads/images/graemlins/cool.gif

At some point, I will have a turkey pan built for my car. I've had a scale engineer's drawing of my manifold for over a year now, just haven't gotten around to it yet. A local machinist, who does work for NASA, quoted $500 for a 6 piece, CNC cut, screwed together pan. I told him that I wasn't planning to drive to outer space, but the price was still $500. /ubbthreads/images/graemlins/smirk.gif


Bill

 

I seem to recall the "baffle" had a number of functions... It stopped unburnt fuel from spraying onto the rear window, it retarded flames in the event of spitting, and, perhaps most importantly, it prevented meachanics from dropping little bits an pieces down the trumpets.

 

[Chris Kouba]

Also deflected flames from any backfire action to not torch the window and any of the aforementioned spit-up gas.

CK

 

[Charlie Farley]

Many thanks for your replies.
Bill, im not sure we are talking about the same thing though. I cant imagine this costing that sort of money. This photo showed something more like a flat tray youd carry drinks on, positioned maybe three inches above the top of the trumpets.
Something we would all be able to fabricate. But thanks anyway.

 

[Ian Anderson]

I think this is what you are referring to
The "anti spit" cover on top of the carbirettor trumpets


Picture found on this thread
http://www.gt40s.com/ubbthreads/showflat.php/Cat/0/Number/60358/an/0/page/0#60358

Ian

 

[Charlie Farley]

Ian
Thanks.
Thats the ticket.
Ill have to check out your Club, see if there is a meet near me.

 

[Mike Dozier]

The "turkey pan" ( so named because it look's like the
aluminum pan many Americans cook their turkeys in on Thanksgiving) is the boxy item in the above photo underneath
the carbs. Very popular with the Cobra crowd.

MikeD

 

[Charlie Farley]

Thanks Mike,
Now i understand Bill Bayard's comment, i missed that pan completely. yes, i can see how that would help with shielding from exhaust manifolds.Coupled with the top tray, it would help alot with cold airflow.Something to consider for my future build.

 

[Jim Cowden]

In regards to the plate above the webbers.

It could have originated for several reasons.
Having made a few air boxes to me it seems that they might be trying to tune the intake system for top end power.

As the pressure wave from the induction system ( reversion) hits the plate it will send it back into the manifold runner earlyer.
Hence assisting scavinging & cylinder filling. ( getting the relationship between the waves & the intake valve opening periods)

Without the plate the wave passes outside of the trumpet giving the manifold more length basicly.
The higher the RPM the shorter the manifold needs to be.

The type of manifolds they run are pretty hard to shorten , putting the plate on would simulate the same effect.

To conferm this if it was taken off & made more mid range power what you have done is made the intake runners longer in the midrange only.
The reason new engine with variable intake runners is more common these days.

I experianced the same effect on MGB intake manifolds.
I made 13 of them for a customer .Because the eng only revs to 5.500 the manifold had to be farly long & not a lot of room for an air box.

The air box was only about 60mm high & it had trumpets in there as well.
On the dyno it made another 3 hp to the wheels at 5.500rpm but lost 6 in the mid.
Because the waves hit the inside of the lid & went back to early.( giving the effect of shorting the manifold) .
If I took the lid of it was ok.
I did fix it by putting 60mm holes over the trumpets in the lid & covered these with K&N foam.

This allowed the waves to pass in & out on lower rpm & at higher the manifold was the correct lenght.

Sorry if this is a lot of rambling but I think that maybe thats why the plate is there,If not that is the effect it will give.
Jim

 

[Ricky]

I am looking at fitting one of these reversion plates to my car primarily to stop fuel spitting onto the rear clam shell window during high throttle opening / track days which has been the primary use to date.

Reading Jims post, there appears that there could be both positive and negative performance benefits from fitment as well.

So the question is what would be an effective distance between the top of the ram tubes (trumpets) and the underside of the reversion plate? I can get a maximum of 25mm before the very back of the plate hits the clam shell window.

If fitted at say 20mm what sort of effect would I possibly end up with? (bearing in mind this is on a 347ci motor with 50mm individual TB's)

 

[Neal]

There was an urban legend that these were fitted when a mechanic dropped a nut while working on the carbs during a pitstop. Much time was wasted not knowing if the nut went into the motor. Old Shel' fitted the covers from this point on.

 

[Bill Musarra]

I thought I had a pic of the 66 Road Car with the cover on. Here is the one I found with it off. You can see the support brackets. We had just finished working on the Webers and getting ready to fire it up.

Bill

 

[Larry]

Here's a picture of my set up with a plexiglas shield.

 

[Ricky]

Great photos guy's.

Larry, What is the distance between your ram tubes and the underside of the plexi ?

Have you noticed any difference in the way the car makes power? (albeit probably small if any)

 

[Larry]

The distance between the ram tubes and the plexiglass is right at 1/2 inch and the engine makes great power.

 

[Ricky]

Many thanks. I will go with what I am planning.

 

[JimmyMac]

The answers seems simple enough,

The top plate improves cross flow of air to the venturies and provides consistant volume of air from the vents and also prevents inclusion of debris.

The cold box or turkey pan fits up to tight the rear clip deck preventing exhaust manifold convected hot air and drive train dust getting to the venturies.
This also prevents raw fuel spitting down and landing on hot exhaust headers and starting fires.

Conduction is also the reason for fitting resin gaskets between the Weber flanges and the cold box, and similarly to the inlet manifold.

Hence the expression "Cold Box"

If they don't fit tightly to the clip then they could be called real turkey pans.

 

[Mike Drew]

Another reason for this plate is to even out the flow of air into the invidual carburetor throats. When Gr4 racing Panteras were first allowed to switch from Holleys to Webers in 1973, they had problems with blown engines. That is because the back carburetor throats were more exposed to the airflow. Air actually flows up from underneath and then out of opening in the decklid on a Pantera, and the vacuum was causing the rear cylinders to suddenly go way rich, or lean, or something (I can't remember now), and BOOM! Fitting the plate atop the carburetors fixed that problem.

I can only imagine what airflow inside the GT40 bonnet looks like at 180+ mph; it is probably like a tornado in there, as air is drawn in from either side. Fitting a plate atop the carburetors presumably would prevent an unwanted 'ram' air effect into just some of the carburetors, by forcing all air to first smooth out and be drawn in from the sides, front and rear only.

Just a guess....

 

[David Morton]

"This also prevents raw fuel spitting down and landing on hot exhaust headers and starting fires".
Absolutely James.
IMHO the 'cold box' will also concentrate any fire extinguisher in the 'plenum chamber' area though I would imagine any larger fuel leaks (apart from spitting webers) would get past down through the box.

Ever thought how long it would take you to react,act, and get out?

Dave