Weber 48ida's in action

[shelbydoug]

https://bringatrailer.com/listing/1966-mclaren-m1b-2/?utm_source=dailymail&utm_medium=email&utm_campaign=2019-11-22

For those of you who don't understand what fuel reversion is on independent runner manifolds is, watch this video of the Mclaren on the track. Notice that when the car is under accelleration there are "puffs of 'smoke'" coming out of the stacks.

Those puffs are raw atomized unburned fuel being pushed back out of the carb throats. That's caused by camshaft timing.

You will also notice that the plume is just over the top of the velocity stacks. Those are stock height stacks and are only about 2-3/4" tall.

IF you do any research on the PanteraGp4 cars, you will find that they all used Weber 48ida's on a Detomaso IR manifold. When you see the actual pictures of those cars you will see that they have doubled the height of the stacks to 5 to 6 inches high. Besides changing the torque curve of the engine a bit, it also keeps that fuel plume in the stacks.

There aren't a lot of cars that can run with 5 inch stacks.

Frankly, I seem to think that I remember that car being run with 5" tall stacks at some point back in the day? Maybe I'm confusing that with another car?

[kjspeed]

The shorty stacks on mine leave fuel mist on the underside of the hood. Quite scary when you think about it (so I try not to). I first noticed it when starting the car with the hood up one day. Each stack puffed out a mist of fuel as the engine idled. Under full acceleration it seems to all get sucked in, so I keep my foot in it most of the time! 

[BGlover67]

I saw that car on Bat and thought what a great looking racer she was.  When I think of race cars from when I was a kid, I think of cars like this. t's up to $220K with 3 hours or so left to go.  Wonder what it will top out at?

[SFM5S000]

Doug, it's interesting to see reversion at speed with the wind in the video. I've seen numerous webered CanAm cars with Webers run, I even posted a bunch of photos (on the forum V1.0) driving behind a King Cobra at Sears Point/ Infineon/ now Sonoma Int'l chasing it with my AMG at the time. I think that was SAAC 35 and never saw reversion like that.
In that video of that Mclaren more specifically that 374 chevy motor, the cam one can tell has a very radical profile. The driver seemed to be constantly stabbing at the throttle seemingly clearing the throats.
As I recall your Webered Pantera has 5" velocity stacks. What is the lobe separation in degrees on your cam?
I'm running 10* on my 65 and funny thing. The only indicator of reversion is on the underside of the porthole hood directly above #5, and just that cylinder and spark plug seem to run lean. It has me baffled and has for some time.
I thought about that shield that the 68/69 John Weir Gulf GT40's ran (looked like an upside down cookie sheet on "V" shaped stands mounted to the carbs) with the edges bent down. My assumption on that was to combat reversion and maintain that cloud. But that was under the rear window/bodywork. Wind turbulence was coming from the sides.
Anyway, that sheet would never fit under my hood.

Cheers
~Earl J

[deathsled]

I saw the puffs of smoke. I keep toiling over whether to put Webers on big red. I think this is enough to dissuade me now. I'll stick to the Cobra hi rise and Holley 650 double pumper.

[SFM5S000]

BUT, there ain't no 14" air cleaner!

[shelbydoug]

The shorty stacks on mine leave fuel mist on the underside of the hood. Quite scary when you think about it (so I try not to). I first noticed it when starting the car with the hood up one day. Each stack puffed out a mist of fuel as the engine idled. Under full acceleration it seems to all get sucked in, so I keep my foot in it most of the time! 

When I had your setup on mine I just shortened the front two stacks and auxiliary venturis to clear the hood. I had to paint the inside of the hood since the flames (seen mostly at night starting the engine) clearly left ring marks. 

The neighbors liked to watch me start the car in the dark so they could see the flames coming through the stacks. I suppose like Romans at the Colosseum watching for blood?

The point of using a "Weber cam", is to eliminate the reversion. Generally speaking it's caused by the overlap but more exactly, a "late" closing intake valve timing.
The number shown to me on total overlap was 28 degrees to eliminate the reversion. That's like a stock 4v hydraulic camshaft.
It's my opinion after running various combinations that even the "Weber cam" can't eliminate the reversion, just reduce it.

I thought this video was a great illustration at what that reversion looks like.



Obviously one can run and survive with this "issue". The Daytonna Coupes ran with short stacks and the Boss 302 factory race cars with the dual Dominator Holleys also throw up some pretty good reversion.

Considering that neither used special camshafts to eliminate it, I'm guessing that the teams came to the same conclusion that I did, you give away hp with a "Weber type cam".



110 Earl. Just a Compcams 605 lift solid lifter. Screams like an F1 car right off of idle. It matches the characteristics of the A3 heads and 180 headers so it might not work as well in other chassis that can't use them?

The Cam Techniques "weber Cam" was bought out by Comp Cams. I believe I have the original spec sheet here that John Barnes gave me and it is a 114 but it's for a Cleveland and I don't know if they grind a 289 the same way? I never ran them on my 347. On that one I'm constantly battling leaky Holley's. I think the Webers are MUCH less of a problem.



The inlet valves on the Holley's keep sticking open if the car sits 2 weeks and flood the engine. I can't get the fuel level right on them. The Webers do not have that issue.

It could be that the Gene Berg glass ball inlet valves in the Webers are the fix and that the Holley inlet valves are not really Viton any more?

[shelbydoug]

BUT, there ain't no 14" air cleaner!

No worry. You're safe as long as you don't put air cleaners on those! The fuel plumes will soak the filter elements and have this little issue of catching fire when you go to restart the engine.
That's how you would determine if you have the reversion...soaked air filter elements.

I put screens on mine to keep from sucking in unsuspecting birds and small animals. Walmart. $.99 each.

I'm thinking of putting Budweiser decals on the stacks. They do look like beer cans? The foam stays in the carbs with them though.

5" stacks. Cloud hovers around 3", stays in stacks. Only about 2" left to the roof for clearance.