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How to decide runner taper?

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How to decide runner taper?

Postby JRL » Sun Jan 08, 2017 1:56 am

Been thinking of making my own sheet metal intake for a street strip motor. The motivation is cost and availability of intakes.

Motor is a 505 Chrysler, max wedge heads with a six pack setup. Carb. flow will be 1500cfm @ 3.0inHg. The motor is a street/strip deal with estimated peak power @ 6500. For this rpm range pipemax is telling me to make the total runner length 11", which will limit the plenum height/volume to abut 40% of eng. volume to be able to run stock hood. As I understand a low plenum volume might not be critical for this low rpm combination, but that's just my personal opinion.
As for the runners I'm thinking of making them rectangular just to make fabrication easier, but I'm really not sure how much taper I should go for.
My guess would be 20-30% increase in CSA.

Anyone got an opinion on this?

JR
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Re: How to decide runner taper?

Postby The Radius Kid » Sun Jan 08, 2017 4:10 am

Safe bet? They say somewhere around 7-9 degrees.
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Re: How to decide runner taper?

Postby JRL » Sun Jan 08, 2017 6:10 am

Thanks, but wouldn't that much taper result in a shorter effective runner length?
To lower the rpm range I need all the runner length i can get.
I was thinking somewhere between 1-2 degrees would give me the right CSA.
-But I could be wrong...
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Re: How to decide runner taper?

Postby mag2555 » Sun Jan 08, 2017 6:51 am

If in your heads the pinch point is the push rod buldge then from that point to the valve do not use more than a 2% expansion rate per inch, and the same from the buldge on back to the plenum.
You will leave some HP on the table with this, but you will not be wondering where the bottom end went to on the street and you can get away with less than a 4K rpm converter also!
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Re: How to decide runner taper?

Postby MadBill » Sun Jan 08, 2017 8:58 am

7°-9° is the kind of taper you can get away without major flow breakaway issues for say a grill to radiator diffuser box, not for an intake runner. Individual cases may vary, but 1°-2° is in the range for most apps. Note that this is the 'half angle' that corresponds to the wall taper of a cylindrical runner. You get to do the math by converting the resultant areas to the shape of your runner.

Example: You want a 2° taper per side on a 10" long runner with a starting area of 3.14²", equivalent to a 2.0" round port. The Sine of 2° is 0.0035, so the taper will be 0.0035 x 10 = 0.35" per side. Thus the plenum end will have the area of a 2 + .35 + .35 = 2.70" diameter circle, or 5.72²".
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Re: How to decide runner taper?

Postby Newold1 » Sun Jan 08, 2017 10:12 am

Most push rod V-8 engines have about 4-6" of intake port in the head to start with so to get to the overall best production of power and torque combined the remaining intake tract is going to be in the 6-10" range. If your remaining runner is in this range then I use 7% and I taper to the intake port opening in the head itself. If your runner length in the manifold is less than lets say 2-3" its not going to make a big difference straight or tapered as the port in the head will determine most of the performance characteristic but a proper entrance to each port opening in the manifold is very important. Think end bell whether it is opening to a plenum or extending into a plenum. Obviously boosted applications are not as constrained by taper, but it helps somewhat here also. Flow is flow.
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Re: How to decide runner taper?

Postby F-BIRD'88 » Sun Jan 08, 2017 12:09 pm

Any Big block Mopar engine tunnel ram that has short enough runners that allow it to fit under a stock flat Cryco car hood is going to have a problem in that the short runner tuned length will not become resonent at a low enough rpm to give a result on a 6500 rpm limited motor.
To get the runners long enough to be resonant in the needed rpm range up to 6500 rpm will need quite long runners much like the existing cast BBM tunnel Rams like the Weiand and the Mopar M-1 intakes. These work GREAT. These do not fit under a stock car hood.
The way to get the runner length + a low height is to build the tunnel ram as a cross ram.
There is at least 1 Big block cross ram intake for that motor already.
If you want to use the 3x2bbl carbs you can make your own custom top for it.
The cross rams are very effective in that mid range up to 6500 rpm range.
2x 4bbls are better than 3x 2bbls, ultimatly but......

If you really want a tunnel ram that works (makes effective torque on a 6500 rpm ish motor) I'd look at a hood scoop and look at the existing
cast tunnel rams and make a custom top/plenum for the 3x2BBL carbs for it.


A short runner tunnel ram (anti tuning design) will run good but it will be less torquey as the runners are too short length to be resonant at modest (6500rpm) engine rpm. The shorter the net runner length the higher the tuned frequency (rpm) that the intake becomes resonant (ram induction tuning)
You'd really have to rev it up and the cam duration would have to match the rpm.

Short runner --- higher rpm...... Longer runner --- lower rpm where torque is boosted by the 'ram" action.

The short runner, short height type ("anti tuned") manifold make very good intakes for supercharged motors as the blower does the work.

What you are planning to build can run and drive very nicely with the 3x 2bbl carbs but will lack the big torque of a tuned induction. The runners will be shorter than optimum. (6500rpm)
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Re: How to decide runner taper?

Postby F-BIRD'88 » Sun Jan 08, 2017 12:17 pm

Didn't they call the old Big Block Mopar cross ram intakes "Rat Roaster" 's.

Your sheet metal short runners under hood 3x2 carb Tunnel Ram will certainly have a strong
visual impact.
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Re: How to decide runner taper?

Postby JRL » Sun Jan 08, 2017 4:39 pm

Thanks for all answers, really appreciate it.

Building a street/strip motor there will always be compromises. In this case I want the best result I can get using a stock six pack hood and stock appearing carbs.
If I'm not mistaken there are only 3 MW tunnel rams ever produced and they would all need a lot of work on this combo. The cross rams are just too expensive and leave a lot of HP on the table, other manifolds will need to be cut and welded to fit the MW port.

So according to pipemax I need the runner being about 11" which means the actual manifold runner will be about 7" tuned to 4th harmonic. As I said in the first post this will result in a small plenum volume to keep the total height of the intake down to about 9"+.
With the relatively small taper on the runners and a small plenum my guess is that this actually might help low end torque and streetability. On the other hand I'm a bit worried if this could give me problems with reversion. This will the first time I've ever done something like this so I'm on thin ice here...
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Re: How to decide runner taper?

Postby F-BIRD'88 » Sun Jan 08, 2017 5:23 pm

Six pack carbs on a "Modman" intake. Height unknown. Short anti tuned runners.
http://www.moparshop.de/component/optio ... emId=12576

What is the height and the intake runner length of the Mopar M-1 Tunnel RAM base.
Just the base. not the top plenum which looks quite tall.
Just the base. Make your own low height custom plenum top for 3x2 carbs.

Is this on a B block (400) or a RB block?
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Re: How to decide runner taper?

Postby F-BIRD'88 » Sun Jan 08, 2017 5:28 pm

This one. The base does not look that tall. Potential to mill it down even shorter
and add a custom low profile carb top.

Wood is a very good material for prototyping a top to get it right.
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Re: How to decide runner taper?

Postby F-BIRD'88 » Sun Jan 08, 2017 5:49 pm

If you go this route. I RECOMMEND you first stick the M-1 tunnel ram on your car with 2 4bbl carbs
and TRY IT. DRIVE it. Take it to the track. Minus the hood, if necessary.

2 x Edelbrock Carter 750cfm AFB carbs work Great on this manifold. Now you know what you Should
end up with. Now you can plan and build a custom 2x3 carb plenum top and A-B it.

And consider just making a taller "OutLaw" version of the mighty Six Pack hood scoop.
Cross that bridge when you get to it.
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Re: How to decide runner taper?

Postby nhrastocker » Sun Jan 08, 2017 5:57 pm

It's not a big deal.
Mopar sheet metal 440 CID, 6-pack intakes are currently in use by many NHRA Super Stock racers.
Since the class requires they retain the factory hood scoop configuration, the runners are short and optimized for the OEM cylinder head configuration since OEM valve size must be used and they are also limited on the port runner size to the NHRA specs.

Dave Hughes at HRE Racing Intakes, PPM Racing Engines in Chandler, AZ and Jones Engine Development in Quartz Hill, California would be good sources of guidance and information on the intake requirements.

HRE Intake:
Image

PPM Intakes
Image
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Re: How to decide runner taper?

Postby Erland Cox » Sun Jan 08, 2017 6:10 pm

I use 90% of the valve curtain area at full lift for the area in the runner just before the radius to the plenum.
Make the radius 1/2" to 5/8" not greater.
You can change the tuned length with the radius.

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Re: How to decide runner taper?

Postby F-BIRD'88 » Sun Jan 08, 2017 6:15 pm

Be aware that a NHRA IHRA Superstock motor is VERY HIGH RPM.
They launch at a higher rpm than 6500 rpm. Thats why the short(er) runner works.

Just saying. ... But beauti looking intakes.
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