Edelbrock's RPM Air Gap for Gen 1 SBC.
Nosed around the internet looking for a number but I could find nothing.
Speier's website has some listings but they're all single plane.
http://speierracingheads.com/manifold-lengths.html
Any direction would be appreciated.
Thanks.

 

I found some info at musclecardiy.com
https://www.musclecardiy.com/perform...ake-manifolds/

Now for the length required. For an engine turning 10,000 rpm, a plenum intake runner length of about 7 to 8 inches is required. For every 1,000 rpm below that, increase the length by 13 ⁄4 to 2 inches.

Intake Runner Lengths and Diameters

We now get onto the tricky subject of runner dimensions. To appreciate why these two dimensions are so important, you need to understand a few basic facts. First is that air is heavier than you may think (an average school gym contains about 40 tons of air!) A suitably high port velocity helps ram the air into the cylinder at the end of the induction stroke. As the valve closes, the air piles up, creating a positive pressure that, during the last few degrees the valve is open, helps push the last few CC of air into the cylinder. On a well-tuned system, the pressure just before and at valve closure can reach 7 psi above atmospheric pressure.

Also (if the exhaust is tuned right for the RPM involved), a very strong negative-pressure wave can arrive at the exhaust valve during the overlap period. This low-pressure wave is communicated to the intake through the combustion chamber. This low pressure wave can also be very strong. A well-tuned exhaust can pull 4 psi of vacuum here, and an optimally tuned race system, as much as 7 psi. This, on an all-out pushrod V-8, can result in the intake charge moving into the cylinder at speeds up to 80 mph before the piston has even started on its way down the bore. To make all this happen at the desired RPM, the intake needs to be a certain length.

The following formula gives the required length:



Where:

L = Intake Length (from the intake valve to the open end of the intake runner)

ECD = Effective Cam Duration

V = Pressure Wave Velocity (about 1,300 ft/sec)

RV = Reflective Value (usually 2 but, for a tuned length for lower RPM,

can predict an impractically long intake length. In this instance, a less-effective but more-convenient RV of 3 or even 4 can be used.) D = Diameter of Intake (at the end of the intake tract just before any entry flare)


Here is the finished engine utilizing the induction system on page 38— 402 ft-lbs and 443 hp from a totally street-drivable 302. Along with that went good fuel mileage too.

ECD is an assessment of when the valve is open sufficiently for some useful activity to be occurring. For a typical engine, subtract 15 degrees from the 0.020 tappet-lift duration to arrive at a good approximation. (By the way, cam manufacturers’ catalogs have all these). As an example, let us assume we want to tune the intake length to 8,000 rpm for a fuel-injected four-cylinder engine.

ECD at 0.020 = 285 degrees

V = 1,100 ft/sec RPM = 8,000

RV = 2 D = 2.25 inches

Inserting these numbers into our equation, we have:



This equals 15.47 inches.

From this, we subtract half the inlet diameter, which is 2.25 inches in our example.

So, 15.47 – 1.125 gives us the final length of 14.34 inches.

 

Regarding the above, although DV talks specifically about the RPM Air Gap, he doesn't state it's length.
Combine that will the typical port length of the cylinder head (5.5" +/-) and the total length can be ciphered. I need that manifold number.

 

I sent an RFI to Edelbrock regarding the length.
Their reply in full:

Hello Kevin, No Sir, Sorry Edelbrock does not have this information anywhere, as I have been asked this before, and after working here for 28+ years, the Engineer's tell me why do they need to know. Even so, it is really hard to measure the runner length, especially in a dual-plane, where as a single-plane is a lot easier, sticking a tape in each runner. ...there is cross sectional area to deal with and the longer the runner the more torque it will generate...

It ends there.

 

Now you have me curious.
I have a Performer RPM manifold (non air-gap) on the shelf.
I have a roll of foam backer rod (the stuff you put in a concrete joint before filling). It's about 5/8" diameter.
I will try to run it through the center of a long and a short runner tonight.
I will post the lengths.

 

I'd do the same if my stuff wasn't squirreled away in storage. It's only the length from the manifold face to where each runner ends at the open plenum that is of concern.
It may not amount to a hill of beams but still useful from my data crunching point of view.
As stated in the article you posted (above), get the exhaust tune right and the intake tuned in a similar way and it should result in an output increase. For free essentially if you think about it. But as I mentioned in the last paragraph, it may not amount to much. But worth exploring as far as I'm concerned.

 

OK, not all the way to the carb mounting flange. Got it.
I will mark inches on the backer rod and try to get some pictures with my crappy digital camera.

 

I have some results for you. You may not care for the numbers, but the measuring method you might find quite clever. I hope so. It took 2 hours to do. Performer RPM (non air-gap). 1/2" foam pipe insulations was used, and it just squeezed through the ports.


Performer RPM (non air-gap)

#1 to #7

Plenum oval measures 3+5/8" long

underside

1 to 7 marks

15+1/4" long - port to port thru plenum












continued...........

 

delete - double post

 

Here are the measurements. You will have to decide for yourself at what point the plenum ends and the port begins.
I went from port-to-port through the plenum. Subtract the plenum in your calculations.
#1 to #7 measures 15+1/4"
#3 to #5 measures 16+1/2"
#2 to #8 measures 16+1/4"
#4 to #6 measures 15"

Here's one that crossed from side to side as I was pushing the pipe insulation through: #2 to #5 measures 16+3/4".

If you look at the pic showing the carb mtg base with the scale, you will see where the divider in the ports measures 7".
Bottom side measures 6+1/2".
I will let you do the math.

I would probably choose 6+3/4" for the plenum. Therefore, (1 to 7) 15+1/4 - 6+3/4 = 8+1/2. Divided by 2 gives 4+1/4" for each port.

If you need any other manifolds measured, I have a few.

 

Now that's thorough!
So, when it's all said and done (and I'll revisit this when I'm sitting in front of a decent sized monitor) you reckon the runner is little more than 4" long?
I can see now that observations by others conclude that due to the irregular runner shape (where it enters the plenum) the runner behaves as if it's longer. Not sure how all that plays out though.
As runner length increases, peak HP RPM gets lower. Using 11.5" as the induction length, which would be an average single plane intake plus the 5.5" for an average SBC port, we get HP peak at x RPM. Add an inch to the length and peak is roughly (x - 500).
You'd think from that then the AG intake would have functional length greater than a typical single plane.
This may ultimately be a tuning tool when (if) I get this lump built and am testing at the drag strip.

 

Everything I have read says that a dual-plane's runners are LONGER than a single plane.
But it's not that simple. The single plane has a common plenum.
And if carb spacers are involved, then that must assume that the "runner" goes all the way up to the base of the carb.

Now you have me thinking - I seem to recall that I saw some runner lengths published somewhere. But I bet they were all for single plane manifolds.
It was probably in the SBC Jenkins or Yunick SA Design books.

Remember, Edelbrock makes "packages" - cam & intake manifold & carb at the very least. Designed for a particular rpm range. It's what most people bolt on.
They have the time and money to do tons of R&D. I believe they had their camshaft, and kept casting intake manifolds until they found which one works best with the rest of their combo. We can't do that. I'm sure that the shorter ports of the dual plane have a larger cross-sectional area to compensate for the length differences. We could only try to "CC" the ports to get some kind of measurement. I don't think any of us regular guys could cut the manifold apart and computer model it.

 

Also remember, the design of the dual-plane manifolds has changed over the years.

 

Sometimes the science isn't so simple.
​​​​​​At the end if the day, we all know the dual plane operates better in the lower RPM ranges than does a single plane. The purpose of this thread was to try an establish a length and use software to determine a peak HP RPM point. This exists for single plane manifolds in Torque Master. It's interesting to experiment with virtual tuning. Taking a cam spec and seeing the peak RPM numbers and then manipulating the induction length to coincide with the RPM target looks like easy money (and free HP) to me. It would appear though that the dual plane is excluded from that kind of exercise. I had read where Joe Sherman kept on adding open carb spacers up to the point where they were stacked 4" tall and power, he said, kept on climbing. Obviously this has nothing (much) to do with runner length and it's the plenum volume that was affected.
There is a small library of single plane runner lengths on Speiers website. Link in post #1.

 

Nice read, thanks for sharing.

The old rule of thumb was single plane for peak HP and dual plane for peak Torque.

• Use a single plane for a high RPM track car to make peak HP at higher RPM at expense of giving up low RPM Torque.
• Use a dual plan for a low RPM street car to make peak Torque at lower RPM at expense of giving up high RPM HP.
• Combined with the right camshaft for the application you are simply shifting the Torque/HP curves.
• Obviously you want the correct bore/stroke ratio and valvetrain to take advantage of high RPM application.

Back in the 20th century my version of Desktop Dyno had the performer air gap RPM dual plane intake, but that was a long time ago when I had my 383 which was a great street engine but it was all done by 5800 RPM. (Performer RPM air-gap dual plane)

 

Being all done, or nearly so by 5800 is about on target for this build. The heads are maxed out at 254 CFM with a guesstimate of the pinch being around 1.7x sq. in. Theoretically, this limits peak HP RPM to around 5700 for 383 CID.
This I would think is starting to approach the limit of the dual plane intake. That said, what I was attempting to do was calculate the total induction length to see where it contributes the most to where the peak HP and TQ numbers occur. That, it seems, is all but impossible as the Air Gap intake (and other two plane manifolds I would guess) have irregular port shapes without a clearly defined entry into the plenum. The only real way to find out where the induction pulses fit in is to test on the dyno.
When it's all said and done a Vic Jr intake (or similar) could be swapped in and track tested. But that's a long ways off yet. (The car itself is a few months away from coming home).

 

You will be happy with the dual plane Performer RPM Air gap.

My 383 was a very nice balance between TQ and HP. 472 LB FT and 465 HP

Personally speaking you want the peak torque to come online sooner on a street car and HP will be strong from 3,500 to 5,800 which is plenty of RPM for a street SBC.

Get a good quality torque converter with enough stall speed for your cam, and if you plan to use the 700R4 get it re-built/upgraded to handle the torque.

 

I can't say I have an application exactly. Or rather, how much track vs how much street. I'm tending to lean towards the former. As it is, last years package wasn't really suited to drag racing. Too little torque, not enough stall and the gearing was off as the 355 RPMs were quite a bit higher and revs at the stripe were easily 1000 less than ideal.
If this comes off, I've a looser converter for this year (about 500 RPM in the difference) and with the increased torque from the additional cubes as well as the lower peak HP RPM, it should come into it's own. My 4L60 is very well built. Easily in the 600 ft/lb range. And it shifts supremely.

 

Now this may prove enlightening.


Weingartner does some pretty interesting stuff and this video happens to fall right in my yard.
Haven't watched it yet (but have it rolling on another tab) but it may provide some insight into what us dual plane users deal with.
Que the video.

 

FWIW, what I was really after was how of the port flow is killed off by the intake. And specifically a dual plane.
Now I have a pretty good idea.