A different Weber question

StanJ · 03-08-08, 11:11 PM

Admittedly, this question isn't precisely GT40 related...but I've noticed over the years that many of you here have a lot of practical experience using Weber carbs, and I hope that I can benefit from some of that experience. I recently undertook a project to design and develop a plenum-style intake manifold and related parts for an inline four cylinder engine used in a CORR Pro-Lite off-road race truck. The class requires the use of a pair of Weber 48 DCOE side-draft carbs. With the current IR-style intake manifold and using 42mm (max allowed) venturis in the carbs, the engine produces a peak of 320 BHP at 7100 rpm. My customer routinely operates the engine in excess of 8500 rpm however, and lately has been getting spanked by the factory-backed trucks which have better heads, camshafts, and exhaust systems. When you determine the RPM at which the current intake flow path length (17.4") "tunes", a big part of the problem becomes apparent. We've improved the cylinder head quite a bit and the new manifold design (along with revised valve timing/ header dimensions to compliment) should bring the power up to around 395 at 8100 with no reduction in "bottom end" (in this case 6000 rpm) torque. Now to my question...

The old design utilizes rubber-encased (for vibration isolation?) carb mounting pads approx .875" thick. Generally, how necessary are these? Granted, an inline four cylinder tends to be a bit more "buzzy" vibration-wise than a typical V-8, but is it a given that they need these isolators? In conjunction with the new manifold design, that .875" puts the air cleaner just far enough "out" that a tube in the truck's frame will have to be moved. My customer is willing to break out the hot-wrench if he must, but I'd rather not make him do that if he doesn't really need to.

Russ Noble · 03-08-08, 11:59 PM

Stan,

The flexible mount is to stop fuel frothing. The same can be achieved by the common practice to 'o' ring the manifold face and clamp the carbs up with special spring type washers which retains the flexibility leaving maybe only a .060" gap between the carb and the manifold face. It's years since I had anything to do with these things but that worked well back then. Others may have more up to date info.

Cheers

Jac Mac · 03-09-08, 12:41 AM

Stan, I would look at on an individual basis in this case- set up a video camera & film the carbs while racing ( or observe them on a dyno pull) to see if the motor goes thru a vibration period that affects the carbs. You may be able eliminate them completely, but with that runner length I imagine the carbs are some distance out from the cylinder head face, or have you incorporated some 'length' into the plenum ( sorry-perhaps I shouldnt ask that

)
Russ suggestion of removing the isolaters and machining the manifold face for o-rings has some merit for shifting the carbs inboard a little.

StanJ · 03-09-08, 08:21 PM

Russ and Jac,

I think I might have unintentionally confused you regarding the runner lengths. In an IR intake system, the cylinder “sees” the combined length of the cylinder head intake port, intake manifold runner, carb venturi, trumpet, along with any/all gaskets, spacers or isolators as the functional resonating length. In my client’s original system (show here sans cylinder head) that total was 17.4 inches, which was way wrong on several counts for the intended application…the refraction wave arriving back at the cylinder during the overlap period only from 5300 through about 6400 RPM, and then not again until 8600 RPM, which is not only past the limits my client would prefer to stay within, but during the span between is actually a substantial detriment to power production.

In the new design shown below, the plenum chamber (rather than the end of the carb trumpet) provides a pressure boundary to initiate the refraction wave movement back toward the cylinder, causing the cylinder to “see” an effective length of 8.9 inches. This system effectively “tunes” twice during the engine’s operating range, with the effect peaking the first time (4th order resonance) at 6500 RPM and then again (3rd order) at 7900. Hopefully, my fellow engineers/racers here will forgive me the weight of the substantial bracing; these trucks often are subjected to vertical stress akin to driving off the roof of a house during competition. I’ll be machining the manifold from billet 6061 T-6 in two pieces as shown, then welding the plenum “end”/carb mounting plate on before taking a final light cut to true any warpage.

Also Russ, here is a pic of the isolators and the O-ring style gaskets that the original system used. From your description, I’m fairly sure that the gaskets (which appear to be Redline pieces) are what you are referring to. If you’ve found these by themselves to be effective at reducing fuel frothing in the past, then I feel a bit better about omitting the larger (and much heavier) isolators from the new system.

Jac Mac · 03-09-08, 08:43 PM

I thought that might be what your up to Stan, come up with a similar system for all the IDA lovers on their GT40's & they might be able to keep up with the 4bbl guys

Can understand the need for the bracing, we had to fit support struts to DCOE manifolds fitted to pushrod 4cyl Fords when fitted to rally cars etc for same reason. The Twin Cams were not as susceptable as the intake manifold runners were part of the cyl head.

Jac Mac

MikeP · 03-09-08, 09:04 PM

We used to use misab plates between carb and manifold with spring washers on the outside of carb flanges on racing Imp engines (4 cylinder 10,000rpm) with no problems. They are not very thick when installed and with the spring washers amd cut carb vibration and fuel frothing but are solid enough to support the carbs without needing bracketry. Readily available from most motorsport outlets or Ebay.
Cheers
Mike

StanJ · 03-10-08, 07:57 PM

Quote:

Originally Posted by jac mac

I thought that might be what your up to Stan, come up with a similar system for all the IDA lovers on their GT40's & they might be able to keep up with the 4bbl guys

Jac Mac

Might be doable. What cylinder heads (and what deck height block) did you have in mind?

Stan

Russ Noble · 03-11-08, 01:44 AM

Slightly off topic, but out of interest, does one get a lean condition on some cylinders with a twin 4bbl set up on the normally available twin manifolds? I believe one does with a single 4bbl setup.

StanJ · 03-14-08, 11:30 PM

Quote:

Originally Posted by Russ Noble

Slightly off topic, but out of interest, does one get a lean condition on some cylinders with a twin 4bbl set up on the normally available twin manifolds? I believe one does with a single 4bbl setup.

Russ, there are many possible causes for why a multi-cylinder engine can experience uneven fuel distribution. The problem can reside with the carburetor(s) themselves, the "environment" in which they operate (air box/air cleaner assembly), the design (or even the manufacture quality) of the intake manifold, acceleration/deceleration/centrifugal forces exerted on the engine while operating, or some combination of these. As regards the design of most "street type" twin 4brl manifolds, typically they are dual-plane designs...which have inherent fuel distribution problems due to both the difference in plenum floor height from one side to the other, and also due to the rather convoluted path that the intake runners themselves must take from below the carburetor base to individual intake ports in the cylinder heads.

To a large degree, the "static" fuel distribution issues resulting from carburetor and manifold design...along with a single order and vector of dynamic forces...can be "tuned around", although the process is most often a lot more involved than just changing main circuit jets, air bleeds, and accelerator-pump nozzles...since one is often dealing with as many different lengths and amplitudes of the negative pressure wave as one has cylinders in the engine. If one is given a free hand to do so, it’s generally easier to reduce (or avoid altogether) these problems while at the design level rather than trying to band-aid them afterwards. Such a “free hand” is rare though; the packaging/styling/ aero people are usually standing in the way of the most efficient solutions…and what they don’t cut off at the knees, the "bean-counters" can reliably be counted upon to finish off.

Russ Noble · 03-15-08, 01:11 AM

Thanks Stan,

I'm surprised that any twin 4bbl is a 'street' setup, seems kinda pointless to introduce the sort of problems you refer to.

I ASSumed they would be performance orientated single plane and have a nice transition from carb to plenum to runners.

Silly me.

I understand the point you make about designing with a clean sheet of paper and no restrictions. Pity we have to live in the real world!

Cheers