Manifold Mania

[DavidVizard-GFN]

We test four popular intake manifolds to find which is best only to get answers already suspected.

Text, Photos and Drawings,

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David Vizard

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When Edelbrock introduced the current design of Air Gap Performer RPM dual plane intake for small block Chevy’s (and later for Ford) they really started a minor revolution. Not only were the runners designed from the outset to flow as much as possible but also, by making this an ‘air gap’ manifold, some thought had been given to thermal management. That first point about airflow might be worth expanding on. I used to ask myself why aftermarket dual plane intakes still had runners that were clearly less than optimal. The only answer I can come up with is that maybe they didn’t really need to go to town as the stock manifold was so poor in terms of airflow (Fig 1) that even a minor improvement showed big power gains. I also suppose that competition within the aftermarket manifold industry eventually drove Edelbrock to improve on what they offered. This leaves us with what can best be described as the third generation two-plane intake (Fig 2). This generation reflects a high degree of effort toward maximizing airflow. To that end the current Edelbrock Air Gap Performer RPM dual plane, introduced some 12 years ago, has set an industry standard by which all other ‘come lately’ intakes are judged.

This is the intake that really started the ball rolling as far as hi-tech two-plane intakes were concerned. Since it’s introduction in the 90’s it has been the manifold by which others are judged.

Since it’s introduction I have done a lot of serious testing on this intake and have sung it’s praises on numerous occasions in equally numerous paper magazines. Almost all these tests were far more than just taking the intake out of the box and installing it as per Edelbrock’s instructions. When I tested this intake the questions I wanted to answer included such things as ‘how much power will it support’ how big a carb is too big’ How many rpm can it go on a typical small block Chevy, what’s the fuel and air distribution like and so on. As what seemed like cubic hours on various dyno tests accumulated the answers to most of these questions started to emerge.

Fig 1

Fig 2

Edelbrock’s Air Gap Performer RPM.

The intent here is to do a comparative test of 4 intakes but let’s deal with the grandfather of all air-gap two plane intakes first – Edelbrock’s Air Gap Performer RPM. Pretty much everything that applies to this intake applies also to the other intakes in our test lineup.

First the purpose of the air gap between the runners and the base of the intake. This air gap is aimed at cutting the heat conducted into the charge by isolating the runners as far as is possible from the heat of the rest of the engine. This looks good but does it work? I have heard some rumors that after making the air gap prototype Edelbrock found no torque/power difference between air gap and non air gap versions of otherwise identical manifolds. (The story goes that Vic Edelbrock decided to produce it because it looked trick – what ever the case I am glad he did) If the air gaps lack of function, as tested by Edelbrock was so, and I seriously question such, then I can categorically say that it is not the case.

The point I bringing to bear here is well worth noting if you plan on getting the best from any of the intakes we are going to test. Doing a back to back test where the only change is the presence of an air gap (and the consequential reduction in intake temperature) or lack of, might seem like a true before and after test. The fact is that this is not so. For an engine to light off and burn the intake charge effectively what enters the cylinder must have a certain portion of the fuel content vaporized. Either too much or too little fuel vaporization hinders output.

When the intake charge temperature is reduced by the use of an effective air gap the amount of fuel vaporized can be reduced below the minimum require for an optimal combustion process. Result - the advantage of the cooler charge is negated by the reduce combustion efficiency. The way to fix this is to use a booster/air corrector combination that has a better atomization capability. To do a real back to back test of an air gap intake versus an otherwise identical non air gap intake requires that each intake has the fuel atomization optimized for each test. If that sounds like a major deal then you are correct – it is. And it’s what I did to establish that the air gap was in fact doing what it should. The bottom line is that an air gap intake is worth an average of about 5 lbs-ft in a nominally 350 hp 350 inch small block Chevy.

Next issue to deal with is carburetor sizing. Making the wrong assumptions here can really reduce the effectiveness of any of the intake we are about to look at. Here’s the deal. Unlike a single plane intake where every cylinder can draw on all four barrels a two plane splits the carb so that any one cylinder gets to draw on only two barrels of the carb. In effect that’s like reducing the carb size by half. To compensate a two plane intake needs a much bigger carb to make those big top end numbers. What limits the best size of carb is generally reached when the flow restriction of the carb drops to less than the flow restriction of the intake manifold runners. At this point it is the manifold restricting flow not the carb. For the intakes tested this occurs when the carb CFM is in the 1000 to 1100 cfm range. That, as most of you will guess, if far more carburetion capacity than any of the intake manifold or carb manufactures recommend.

Fig 3

It is one thing to run carb/intake combinations on the flow bench and altogether another deal on the dyno – so do these big carb and hi-perf two plane combinations work? You bet they do. Take a look at Fig 3. This shows the output from a 383 small block Chevy with a fairly big cam (Comp Cams Xtreme Energy solid flat tappet with 6059 profile (290 advertised 252 @ 0.050) on intake and exhaust ground on 106 LCA at 4 advance) and a set of basic ported Dart Iron Eagle heads. To make this combination work I would not recommend much less than an 11.5/1 CR which is what was used here. Note how, given the cam and carb an intake which is supposedly only good to some 6000 or so rpm made useful power all the way to 7400! The bottom line is that a good modern design of air gap two plane manifold can deliver a strong output over a wide rpm range – far more in fact than it is often given credit for.

The plan here is to check out the potential of four readily available two plane intakes and see how they compare. However a common denominator among all of these intake suppliers is that the carb CFM they recommend is far less than we are using here. Why? We will get to that in the next installment on this subject.

Read Manifold Mania Part 2

Read Manifold Mania Part 3