Manifold Mania-Part 2

[DavidVizard-GFN]

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

Text, Photos and Drawings,

By

David Vizard

Shown here are the selection of intakes that are to be dyno tested. The brands are (from R to L) Professional Products, Weiand, Dart and, at the back, Edelbrock.

So why do all the carb and intake manufactures recommend a smaller carb than our dyno tests show to be optimum? Looks like a heck of a good question but before we go there let’s continue our investigation as to why the current generation of hi-perf two plane intakes seem to thrive on greater carb cfm values than earlier designs. To do this lets go back and take a look at the runners on Edelbrock’s Air Gap Performer RPM intake as it is representative of all those that follow. Rather than cut up a number of intakes, which at the end of the day still doesn’t give a really good view of the runners, I got the guys at Edelbrock to supply the CAD drawing which really does show the free flowing forms involved. At this point you need to check out Fig 4. The points to note here are the smooth curves from the plenum all the way to the manifold/cylinder head face. These runner shapes are a far cry from not only the stock factory two plane intake but also from the older design Performer and other intake brands of that era. Just eye-balling those runner forms one would expect to see a big increase in flow efficiency and that proves to be just the case as Fig 5 demonstrates in no mean fashion. But, if we are to understand why these new intakes like more carb cfm than previous designs of two plane intakes, then a little more analysis of the situation is needed other than just plain flow numbers of the intake alone.

Fig 4 This CAD illustration clearly shows the more streamlined ports used in the third generation type two plane intakes.

To develop meaningful intake manifold flow values a few criteria need to be set to what can be considered realistic values. First the assumption was made that the heads on the engine would be better than stock but not up to race spec. In other words a good set of street heads (i.e. ported stock or aftermarket ‘as-cast’). Next assumption was that the engine would be employing a reasonable street cam. That would be something in the region of 275 degrees of off-the-seat duration and about 0.520 intake valve lift. To establish what a change in flow will realistically do the flow tests were done with the intake manifold on a street ported head with the valve lifted 70% of full lift. It was under such conditions that the results shown in Fig 5 were developed.

Let us consider the light blue bars first. These show the loss of cylinder head flow with a manifold in place compared to no manifold. A good race manifold runs zero to maybe 2% loss. As you can see the stock GM intake caused an average reduction in head flow of 25%. It hardly needs saying but that is not good if power is the goal. By comparison the old Performer caused an average flow reduction of 9.5%. This certainly was a great improvement but it came about partly from the use of bigger ports rather than increased flow efficiency. Finally we have the Air Gap Performer RPM. With smaller but far more efficient ports this intake dropped cylinder head flow by an average of only 6%!

Fig 5 The light blue bars show the flow loss when the just intake is installed on the head. Not how much is lost by the stock intake #1 compared to the Air Gap Performer #3. Distribution of the air is shown by the green bars. Again the latest Performer does well compared to stock The red bars show the effect that installing a carb has. Because the manifold is so much better the carb looks like more of a restriction for the Performer style manifolds than for the stock intake. This is part of the reason for needing a bigger carb on the later hi-efficiency intakes.

On to the green bars. These show the percentage variation of the runner flow loss – that’s how much each runner looses compared to the best runner. On the stock intake there was a 27% variation between the best and the worst runner. This was, for the Air Gap Performer, cut almost in half to 14.5%.

Now lets consider the flow lose when a carb is installed. For all these tests a carb of some 750 cfm was used. Note that such a carb on the stock intake cut overall flow by just 7% while the same cfm of carb on our Air Gap manifold dropped flow by 9.5%. So why the bigger loss when a carb is installed on what is purportedly a better manifold. Now here’s the kicker. Because the Air Gap intake is so much a greater proportion of the intake ports flow demand is passed through the manifold (because it flows better) This means the head/manifold combination makes greater demands on the carb. To see this more clearly imagine that the stock intake was so bad it cut a bare port flow of 220 cfm down to 50 cfm. In this instance having a carb that flowed 750 cfm would be like having no carb at all present as four such poorly flowing ports could not tap into anything like the carbs full flow potential. On the other hand having the same four ports that, with the intake manifold on, still flow 200 apiece, is likely to warrant every bit of the 750 cfm the carb has. So the better the intake manifold flows the more the carb needs to flow to allow the full potential of the intake/cylinder head combination to be realized. At this point you may now begin to see why these hi-flow Air Gap style two plane intake really warrant some revision in terms of the best sizing of carb to choose.

Runner Development.

Fig 6 Here is a pressure map of an intake port drawing on the manifold runner. High pressure is red dropping through orange, yellow green and blue. Note that the two barrels of the carb do not flow evenly. The one nearest the runner is supplying more air as indicated by the lower venturi pressure drop. The port runner shows quite a drop but some pressure is recovered on the outside turn of the port due to the port wall having to turn the air.

When Edelbrock originally designed the Performer series of intakes I am sure, even with their airflow guys vast experience, that a fair amount of trial and error went into the runners before freezing the design. Getting the best out of a two plane street manifold is in fact harder to do than it is for a race manifold. With one exception I am sure that the other manifold manufactures went through a similar exercise to arrive at their designs. Of course each of the subsequent iterations by other manufactures of Edelbrock’s original design had the benefit of seeing what had already proved itself to work. Interestingly Holley’s manifold the ‘Air Strike’ (marketed under their Weiand division as a Weiand manifold) was the result of Computational Fluid Dynamics aided design. If you are not familiar with CFD then all you need to know is that it takes a lot of computer and some pretty fancy software to do. Here a computer model of the intake is made and ‘mathematically flowed’ by the software. This allows the user to see where the air is going and what the points of restriction are. The computer model of the intended intake manifold is then manipulated until the best results are seen. The real hardware is then made to the form indicated by the computer.

Fig 7 Here is the velocity map of the same runner seen in Fig 6. Blue is the slowest moving air. This progresses through green to yellow then to red. Notice the highest speed (and potentially the greates source of flow loss) is on the short side turn of the runner.

Is CFD worth the considerable effort involved? The answer to that is it looks to be so now and in the future, when programs and computers have become more powerful; those companies not using CFD will almost certainly fall behind. The animated airflow effectively renders air as a visible medium and that, to a great extent, will change how we will design intakes in the future.
Now the need for a bigger carb, if power is a prime consideration, is justified we can look at why the both the carb and manifold manufactures all recommend smaller CFM carbs than we have so far found to be the optimal power generators. In simple terms it comes under the heading ‘Street Drivability’. Those big CFM carbs will make the power but any time you decide to use a carb bigger than those recommended you had better be prepared to do some pro level part throttle re-calibration. In the past I have been guilty of understating this situation probably because I have a dyno and mixter analyzer etc. with equipment to analyze what is going on and a full machine shop at my dsposal to fix it I suppose it’s all to easy to marginalize any problems that may come up. I’ve got into hot water with the guys at Barry Grant over this on several occasions and they have asked that I do make it clear that to use the big carbs I do you need to have a really good handle on carb design. If you are intent on educating yourself to this extent then my book ‘How to Build Horsepower Part II – Carbs and Intake Manifolds’ might be what you want. Now we are up to speed on what the extent to which this current generation of two plane intakes is all about it’s time to hit the dyno. That we will do in the next installment.

Read Manifold Mania Part 1

Read Manifold Mania Part 3