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How to decide runner taper?

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Re: How to decide runner taper?

Postby rebelrouser » Wed Jan 11, 2017 11:00 pm

000_0265.jpg

My bad, posted two pictures of the same thing. This is what the completed manifold looks like. Our NSS rules stipulate that we have to run AFB's Notice I converted them to dual feed.
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Re: How to decide runner taper?

Postby MadBill » Wed Jan 11, 2017 11:17 pm

Impressive work! Do you have any shots with the lid off?
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.
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Re: How to decide runner taper?

Postby groberts101 » Thu Jan 12, 2017 12:03 am

That's awesome. One of my bigger regrets.. not cutting and welding up more parts like this. If no one makes what you need?.. make something yourself. Homemade ingenuity. Lots of mainstream speedparts came about like that.
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Re: How to decide runner taper?

Postby F-BIRD'88 » Thu Jan 12, 2017 1:21 am

Right on guy. I'd like to see a pic with the lid off also. Nice job.

Was thinking of a similar thing and sort of like the Box Ram too but for dual (AFB) carbs
like you did.

My plan , much like yours, but my box would also enclose the two (AFB) carbs in a sealed box
with a inlet duct (With difuser) for supercharging.
Your box ram could use a 3x2 carb top very well too.

You are right on the money. it looks "THE BOSS" too. :notworthy:
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Re: How to decide runner taper?

Postby F-BIRD'88 » Thu Jan 12, 2017 1:49 am

My (Rough) idea was to make the plenum about the same width as the 4bbl carb plenum but make it longer
to accomodate the Dual quad carb pattern and preserve more of the intake runner length.
Starting with a SBC Hurricane single plane intake.
Possiblely with the 2 4bbl carbs mounted back to back on the top. Allows 2x4 Qjets but Symetrical 8 venturi
orientation over the plenum. Can you imagine the howl of twin Qjets when you stand in it...

Again great job.
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Re: How to decide runner taper?

Postby JRL » Thu Jan 12, 2017 1:56 am

Nice work,
Kind of remind me of the Indy modman intake, but with a larger plenum.

@F-bird,
Indy sells tops for the modman that might suit what your thinking of.
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Re: How to decide runner taper?

Postby simos » Thu Jan 12, 2017 4:38 am

Some ballpark information from racing manifold builder I found some time ago.

http://www.swartzracingmanifolds.com/tech/index.htm

Runner taper

I quantify runner taper by comparing the cross-sectional area of the intake port (at the gasket) versus the area at the plenum opening of the runner (not including the entry fillet). This is expressed as a percentage of increase in area. I calculate the area at the intake gasket, and then I add the percentage required by the application to determine the runner entry area. Since most applications have round plenum openings, I simply convert the opening area back to the diameter of the required circle.

Obviously, the key is to know what percentage is needed for your application. Several factors seem to be important to determine this number. I can’t quantify the impact of these factors, as every engine combination is different. Camshaft events alone can severely affect how taper is tolerated. I can give you the following relative guidelines to determine where to start:

•Most drag racing manifolds require between 25% and 45% increase in area. There are applications that go beyond these limits, but this is rare.

•Engines that are built on production platforms tend to like about 30% increase in area. I’m referring to the typical bore/stroke/rod ratios derived from OEM engines. As bore/stroke ratios approach 1:1, the increase in area is likely to be 25% or less.

•Race engine platforms with large bores and short strokes like higher percentages – 35-45%.

•Short deck engines (and resulting rod/stroke ratios) tend to want more taper. The piston motion of a short rod combination will create adequate port velocity in large runners.

•Stick-shift applications tolerate more taper than automatics.

•Larger displacement engines like more taper than small engines of the same bore/stroke/rod proportions.

These guidelines should help you narrow down your taper requirements within a range of a few percent. The combinations are so varied, even within the same class of racing, that set numbers and fixed rules are difficult to derive. My goal is to minimize your testing iterations required to find the best performance.
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Re: How to decide runner taper?

Postby rebelrouser » Thu Jan 12, 2017 12:50 pm

unfortunately I did not take any pictures after the manifold was finished on the inside, several people have asked. I also used a pair of reversion plates from http://rehermorrison.myshopify.com//pro ... es-spacers
They lifted the carbs to gain linkage clearance, and are supposed to help with air flow. I made the plenum as large as I could, from my reading a large plenum sometimes helps with any distribution problems and helps with High RPM flow. The Indy cross ram manifold has a huge plenum, and even has issues with backfiring and blowing the top off, several racer in my group have had that issue, always give the engine some throttle when cranking with a big plenum. I built the manifold at school and a couple students helped with setting up and flowing the intake, one of the students said I should call it the tornado because of the swirl and turbulence in the plenum, when we were using strings to check flow direction.
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Re: How to decide runner taper?

Postby groberts101 » Thu Jan 12, 2017 3:52 pm

Damnit man! This thread's got me thinking about cutting my old Offenhauser 360° split plenum(side to side) in half all over again. :mrgreen:
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Re: How to decide runner taper?

Postby F-BIRD'88 » Thu Jan 12, 2017 4:26 pm

I think these "anti tuned" short runner (or too short runner length to be resonant at low- ish rpm)
with dual or multi carbs would be great for nitrous oxide injection racing as the typical longer tuned runner design manifolds that work great at creating big mid range torque and too much midrange torque when a generous nitrous oxide shot is used. Shifts the torque curve up in rpm and moderates the big torque spike hit on nitrous in the mid range that can bend connecting rods.

A more usable torque curve when on nitrous.

Wish Edelbrock would offer just the supercharger intake manifold and top for their SBC E Force supercharger kit.
Let racers play with the intake manifold for this purpose and for "blow thru" centrifugal supercharger/turbocharger applications too. Carbs or EFI
With some bolt in internal plenum mods it could be a winner. (Divider... fuel distribution director dams
Height spacers etc)
It has the desired short runner anti tuned design but is capable of high air flow.
The bolt on removable top makes internal plenum development easy.

Offer other SBF SBM BBM BBC versions too.
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Re: How to decide runner taper?

Postby JRL » Fri Jan 20, 2017 3:58 am

So I've finally come home from work and have been in the garage to take a few quick meashurements.
-And offcourse I've got a new question...

From where do the runner start in the head?

When I meashure the head I get 5" from the floor of the runner entrance to center valve stem, 4,75" center port to center valve guide/stem. Is this the correct way to meashure the total lenght of the runner?

JR
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Re: How to decide runner taper?

Postby Carnut1 » Fri Jan 20, 2017 6:05 am

Using David Vizard's program you need to measure the roof length and the floor length then average them together. Measured from the seat to the manifold gasket opening using solder to get the shape correct. Seems like a reasonable way to me. Thanks, Charlie
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Re: How to decide runner taper?

Postby JRL » Fri Jan 20, 2017 7:20 am

Thank you,

so the measurements was 7,3" and 4,5" which would equal a 5,9" port length?
Looks like it's doable to make i intake with 11" total runner length.
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