Weber carbs - reversion plate

Chuck

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What height should the reversion plate set above the top of the velocity stack?

I am wondering if there is an optimum space for air flow to the webers.

There is easily 1 3/4 of space on our set up, but it looks too high.
 

Randy V

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Chuck,

Do you want one to maintain the original look or do you think you need one that is functional. The reason I ask is that I am not sure your engine would necessarily need one.
 

Chuck

Supporter
Randy

I want the reversion plate for a couple of reasons. First is originality. Second is to protect the polycarbonate from discoloration from the fuel vapors. Third is possibly the most important. Function.

I have not found a good explanation as to the theory. Suspect it has something to do with capturing the "pulse" in a confined area where it can aid the intake cycle on adjoining velocity stack.

In any event the plate is fabricated and ready to install. The support brackets will be made from stainless. The plate can be easily removed by popping off four screws, should we want to run without or show off the webers.

Just need to known the height above the velocity stacks. I suspect it is critical to achieve the intended air flow effect.
 

Randy V

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Hi Chuck,

Reversion from the Webers is caused by a lot of valve overlap. The originals had solid lifter cams with overlaps on the order of 106 degrees and there was a constant cloud of fuel hovering over the tops of the carbs. This would cause a possibly explosive situation and most certainly will collect on the surface of the rear window (discoloring as a reaction to the fuel).

Your cam is likely the same as mine as a Jim Inglese design cam specifically made for Webers at 112 degree lobe centers. We observed no reversion while on the dyno so I am not anticipating that I will have that problem and I would expect the same of your setup.

That all said - you may want to build one anyway.
I've seen two different designs. One is flat and the other has a crease running front to rear dead center in the plate with roughly a 15 degree angle.. In the pictures I've seen, the distance between the tops of the trumpet bells to the Reversion plate are roughly the same distance as the trumpets are in diameter. I have no numbers or measurements for you unfortunately and hopefully someone else here will.
 

Chuck

Supporter
Thanks Randy. Your are correct as to the overlap and the absence of that cloud over the new carbs.

I believe the angled plate was used on webers mounted on the canted manifold. Mine is flat and the reversions plate is flat as well.

I had spaced it about an 1.5 inches above the velocity stack and it just seemed to look a bit to high. I will look for some old pictures.
 
RAndy, Chuck
what is the overlap on your cam

I run this cam
31-336-4 - Magnum

and thinking about convertig from my 4BBL Carb to a 8stack (48 IDA Weber look alike) system. Will this be as sensitive to big overlap as carbs ?

Eight Stack Injection- 289 302 347 Systems Still waiting for them doing a 48 lookalike system with at least 50mm or better 52mm throttle plates.
With the 48mm i think they still will rob some topend power, although they dont have a flow restriction like the carbs with the venturi inserts.

THanks
TOM
 

Chuck

Supporter
Tom:

As I recall the overlap is 112^. The recommended range is 110 to 112^. I am at my office but can check and confirm if need by. My cams were custom ground / Comp Cam. I can probably dig up the specs if you like.

Cam and timing are critical.

Jim Inglese is an excellent source of information: 203-623-0659
 
Tom,

I'm no expert, but as I understand it, the modern high-pressure EFI systems are considerably less prone to reversion than webers due to a number of different reasons, including:

1. High pressure - better atomization via high pressure (typically 80-100psi v. 3-5 psi for webers) makes the incoming charge less prone to condense on the walls of the throat and the runners, keeping that area leaner and cleaner - less fuel to blow back up the throat.

2. Direction/angle - high pressure fuel squirting more directly into the runner right at the back of the valve tends to keep the intake charge better contained within the runner and CC rather than blowing back up the throat.

3. Atomization/efficiency - because the atomization is so good with high pressure EFI systems, the mixture can be a bit leaner overall, making the intake charge less prone to reversion (less fuel to revert), and the mixture is better maintained across the entire rpm range (again less fuel to revert). As you probably know, webers struggle with maintaining a proper AFR across the entire rpm range.

4. Due to all of the above, modern engines with high pressure EFI systems tend to run less aggressive cams - less need to, and the buying public is less tolerant of lumpy running engines these days. People are most accustomed to an engine that runs like a Honda or Toyota, more like a sewing machine.

If you think about it, fuel dripping (or lightly pressured to be precise) into a venturi high up the intake is a pretty poor way to get fuel atomized and into the CC. It's not horrible, but it's certainly not the most efficient. And, there's no surprise some of that fuel is going to come blowing back up the intake when there's both intake and exhaust valves open all the while combustion is going on in the CC and the piston is moving up and down creating pulse waves of pressure.

I've learned/heard of the above (1-4) only from heresay and some reading on the topic - have no real first hand knowledge with it so take it with that as context. The above does make sense to me however, and I have a lot of experience with webers (and their shortcomings). While I appreciate the looks and the legacy, I have a Holley on my GT40 which says something about me. I'm also looking into an EFI weber look-alike option.

Just my $.02.
 
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Chuck

Supporter
Cliff:

Which God fearing power hungry GT 40 owners want their engine to idle like a sewing machine? Surely not on this forum.
 
Mmmhmm

i think we have mix of terminologie here. Randy is talking about valve overlap and than later abount Lobe separation angel ( LSA).
As far as my understanding goes the issue in the original cars with hot cams with big overlapping ( Randy did they realy overlap by 106° which would be realy a lot). means that the exhaust and intake valve are open at the same time for that crankshaft angle. This cause a lot of reversion right?

THere are basically two ways of reducing the overlap angle. First is a shorter duration cam, second on a given duration cam increase of LSA.

With less overlap the reversion effect will be less.

Compcamps is selling specific cams developed together with Inglese.
you can find them here
Inglese: Inglese Hydraulic Roller Series Cam, Ford Windsor 5.0 Firing Order, Stage1

they feature a mild duration and a wide LSA, therefore much less overlap.

effect of this are:

Technical Specs & Information

115° is about as much as is commonly perceived as optimum. better would be less, but that does apperently not work perfect in combination with webers.

CLiff i think you nailed it and the issue is prone to webers and there concept which relies on velocity of gasses streaming by to make the mix. EFI will much less prone to the reversion issue if any.

TOM
 

Randy V

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Tom/guys I have no Internet other than my cell right now. Chuck and I are running the same cam I think from Comp Cams and the profile was designed by Jim Inglese. If you look at the Lemans 289 cam spec you will see that the LSA was on the order of 106. This was a totally non-street able cam. Individual runner intakes really do not work as well as a shared plenum intake as far as carbs and streetability go. The pulsing of the vacuum signal really messes up carbs which is why they are best suited for mid range to high RPM operation. If IR injection system has the injectors below the throttle plates reversion laden with fuel will be reduced. If injection is above the plates, I would expect almost the same fuel cloud as with a weber. Sorry for lack of links and info on this post. iPhone's are only so good.
 

Randy V

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Tom, I was able to click on your link to the cam and it is one of the two that are offered for the Windsor engine. The cam I have is the stage2 version I believe. My memory may be off on the 112 LSA and it should be 115. I will double check my info on that.
 
If I may, with respect...
My C30Z, supplied by Holman & Moody, worked amazingly well on my 11/1 289 with Weber IDF's.
It was however a flat tapped which is something Chuck may not want, but man it ran good.
On another note I think the Le Mans grind would also work well in a 10.5 ish to 1 or so, 363 ci SBF with Webers. However, on a smaller mill it would mandate high compression and a steady diet of avgas or Sunoco 100. It would definitely sound old school though!
 
Lemans 289 specs .... Can't do the math right now to calc LSA but would expect to be very close to kß

http://shelbytransam.com/walt/ShelbyTA/images/road289gt40.pdf

Randy that cam in the shelby informations you posted would have a 94° ( at the posted duration timing at 0,0125" valvelift") overlap and a LSA of 108,5°.

Still interrested what your cam has on advertised duration timing ( i only found the datas on 0.50" lift) in order to have comparable datas.

Scott your post contents a lot of information about the timing of the cam you used, thanks.

Thanks
TOM
 
Weber IDAs on original Gurney Weslake heads, 548 BHP/540Ft/Lbs, no "reversion " plates, no stand off,
 

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