Back in the 1960s multiple carburetor induction systems were common place. Six Strombergs, then three Deuces, and finally dual four barrels. It was the era of the wide open induction systems. But by the mid to late 1960s various race sanctioning bodies were adjusting their rules to limit all sorts of racing classes to the use of only one four barrel carburetor.

Holley introduced the list 3916 carburetor in the mid-1960s which stretched the air flow limits of the 4150/4160 size carburetor to 950 cfm for Chrysler's new 426 Hemi NASCAR motor. The new 950 cfm carburetor was a THREE barrel carburetor; it was equipped with a single, oval shaped, secondary butterfly. Holley designated it a 3160 series carburetor. Before the introduction of the 4500 series "Dominator" carburetors in 1970, the list 3916 three barrel carburetor was the biggest single four barrel carb you could buy, and it became quite popular in drag racing. Intake manifold plenum dividers required a notch in order for the oval butterfly to be capable of swinging open. That's why the automakers and the aftermarket began selling their dual plane, single four barrel, performance intake manifolds with notches in the plenum dividers. That small notch was not large enough to affect the performance of the manifold, that was not the original purpose of the notch.


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Original Post
All V-type dual plane intakes use a 'signal notch' in the plenum divider, always have and still do, of varying lengths and depths depending on design/manufacturer. The main purpose was the carb tending to lose it's vacuum signal between R & L cylinder banks intake valve openings causing erratic operation. Some folks may have needed to modify the notch back in the day of the 3v carb, but that definitely wasn't/isn't this opening's original purpose.

Originally posted by Rob F:

... All V-type dual plane intakes use a 'signal notch' in the plenum divider, always have and still do ...

I respectfully differ, that is not my experience. Consider these 3 manifolds.

40 year old Shelby manifold with Dominator carb mounting base, no notch

Modern Scott Cook manifold with design input by Darin Morgan, no notch

40 year old factory manifold, no notch


Originally posted by Rob F:

... the 4-hole dominator which has no divider ...

The Shelby dominator manifold is a two plane manifold with a full height, un-notched plenum divider. You just can't see it from the angle of my picture because the holes are spread so far apart. Its identical to your Shelby manifold except for the different carburetor mounting base.

The Edelbrock Performer 4V is another manifold lacking a notch in the plenum divider.

While you guys debate over how many angels can sit on the head of a pin, the simple answer is that just about every popular engine design, i.e., I don't think the International Harvester 350 v8 qualifies, has manifolds built for them both ways.

It all depends on the design of the manifold was intended to do.

Back in the days when electricity was a brand new thing, companies like Edelbrock and Offenhauser built manifolds to market to the masses of the budding aftermarket industry to give you A LITTLE more power, less weight and pizaz then the factory manifolds, WITH NO OTHER CHANGES TO THE ENGINE.

Here is a picture of the Offe 360 for the Cleveland. It certainly has a notch.

It should look at least vaguely familiar to SOME of you in this debate, right? Wink

There are a set of instructions that come with that manifold from Ollie Morris himself (Goggle him if you don't know who he is, I do, you should) that say, "don't touch it". It is what makes his design work.

The fact of the matter is that there are absolutely enough manifolds around to support both sides of the argument.

To me, the debate is a waste of time. The notch is a tuning device in the sense that absolutely every other factor in induction on a naturally aspirated engine is.

You are balancing idle quality, throttle tip in, low, mid and high rpm power for the best combination. "Best" being determined by the manufacturer, NOT YOU.

Edelbrock is notorious for building disappointing manifolds in the past. Disappointing, at least to me. I think that is simply because their intended market is not one that I fall into.

I am not putting their manifold on a big heavy car with an automatic transmission and 3.00:1 gears. To me, that is what their manifolds are tuned to. At least until recently.

In this day and age when seemingly everyone has a chassis dyno, none the less a CNC laser cutter, a TIG welder, a MIG welder and God knows maybe a former Soviet block nuclear device in the basement, everyone is a critic.

Just like selecting a camshaft, a carburetor, header tube size and length, these items are all intended to tune YOUR PERSONAL set up as best you can.

To me, the last thing that should be modified on a notched intake manifold is the notch itself. The problem is, once you went too big, the manifold is junk. Aluminum scrap here is $.23 a pound. That doesn't get your purchase price back.

Personally, if I was going to mess with that divider, I'd leave the Shelby script manifold alone and grind on the Blue Thunder.

Long before Dan Jones started his own dyno project, that manifold was played with by altering that divider.

Unless you are a serious race team looking to squeeze the last drop of horsepower out of a 40 some odd year design to compete now, what's the point and what's the difference? A 40 year old design competitive now would just be pure luck?

There is absolutely enough current information, i.e. DYNO DOCUMENTATION, by Dan Jones to indicate the differences with THAT manifold and the advantages or disadvantages of each configuration.

Exactly who is responsible for the original design of the ORIGINAL "SHELBY" manifold that Blue Thunder re-introduced, to my knowledge, has not been credited.

It DOES NOT appear to come out of the Ford, Kar Kraft, Holman-Moody or Bud Moore camp. It was thought to be cutting edge in 1970 when it was designed but that was with 1970 expectations for the 351C, i.e., improving an advertised 300 hp Mach I production engine in the Mustang and Torino. That's about what you got from it. Another 10 or 15 hp.

I personally doubt it was ever concieved of as capable of 500hp and 500 ft-lbs out of the 351 configuration?

Unfortunately it arrived at the end of the Ford's direct involvement in racing and their funding performance/racing parts program because of the increasing involvement of the auto industry in complying with the new California and US emissions standards, and their racing success at Le Mans in beating Ferrari in '66 & '67, which was all Henry really wanted to begin with. It probably could have used more development, BUT IT IS WHAT IT IS AND THAT BATTLE IS LONG OVER.

Detomaso himself was a victim of this Ford withdrawal leaving him to fund his own 351c engine racing development for the Pantera, which apparently he was not able to do very well without the American/Ford involvement.

I will point out also that AT Francis, aka Blue Thunder, has the reputation for "improving" original port designs and leave them original looking on the exterior.

I would expect the BT to be just a little bit different, a little bit "better" than the originals? Wink

This is like debating the US Civil War and who should have won? The war is LONG OVER. Don't fight it again. Get over it and live with the outcome.

Yikes. You guys are like 9th graders that won't give up the argument you started in home room? Roll Eyes


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Oh, and here is a shot of the current Pantera version of the BT manifold.

I could be mistaken but that sure as heck looks like a notch in the plenum divider?

Huh? 'magine that?

To use a Holley three barrel carb on this manifold not only would you need the notch but the entire secondary side of the plenum would need to be re-machined to take advantage of the combined secondary venturi.

That carb would probably be most beneficial on a single plane, single plenum manifold.

Nickey Shevre-olay and maybe Yenko-zes were the most notorious abusers of that carb on their ugly orange Chebby 427 installations.

Couldn't they at least spring for the CalCustom chrome valve covers?


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'Get over it and live with the outcome.'(?)

I'm a hot rodder, and modifying and 'improving' things is what we hot todders do. We don't just accept certain things and live with them, we work to improve them and make them better than they were originally. I'm not talking blindly doing things willy-nilly. I'm talking about studying the situation, and coming up with a better solution. That's the whole trip of being a hot rodder!

So if I hear of someone who has come up with a better mousetrap, I'm interested- enough to research their idea. In that regard, Mr. Smoky Unick was one of my hero's. That man had more 'common sense' in his little finger tip than most folks have in their entire body. I don't just bolt stuff onto my motors and 'live with them', not if they can be improved upon with a little imagination and common sense- and in this case a flow bench to sort out the manifold runners flow rates.

I've been in touch with the gentleman who modified Mike Drew's B/T intake, and I personally believe he's legit and knows what he's doing. In the case of my 'little' 357" motor, he's going to do a bit of porting to even up the runner flow, but pretty much leave the divider and its factory notch alone. He adjusted this divider on Mike's intake to 'match' the requirements of his 408" motor. A perfect case of pure, functional hot rodding!
Rob, I am not suggesting any such thing.

What I am saying is the the Blue Thunder is a reproduction manifold and is in plenty of supply.

The Shelby script manifold is long out of production and is likely a good deal more valuable than the Blue Thunder.

To me logically a non-modified Shelby manifold is going to be worth a lot more than a modified one?

In addition to that, there seems to be pretty good data from recent dyno tests that the BT doesn't come anywhere near the power of the Holley Track Dominator manifold, ported or unported.

I personally have run the original Shelby version of that manifold. I went through three of them back when they were pretty cheap and easy to get used.

I tried it against the Ford aluminum stock manifold and the Edelbrock Torker.

The Edelbrock ran circles around both.

Porting out the BT to me is just a waist of time and money. Been there...done that.

Start with something that is proven to work and try and get more out of that, but you don't need to do anything to the Holley. In fact, you can probably just make it worse?

There is a saying that you can't get blood from a stone. If you insist, no one will try to stop you. To me, life is short. I hate wasting time and energy trying to reinvent the wheel. That is just pointless to me. Other opinions vary.
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