Flow Restrictions in Intake Designs



Flow Restrictions in Intake Designs

tmoss

I’ve written some about intake runner design shortcomings regarding the Ford EFI lower intake. I’d like to write some more about these concepts in general and apply them to some current Ford (or any other actually) intake designs.

An intake is nothing more than a duct system to deliver air to the chamber. Well, Mechanical Engineers for buildings have been doing this for many years. They have done extensive studies on how to make both low and high velocity duct systems more efficient (require less fan HP to deliver the required amount of air – pumping losses in an engine). Well, an engine is very much the same. The piston in an engine is what makes the air move and the fan motor is what makes the air move in an air handler. The difference is simply which end of the air duct the air pump is on.

Air system design engineers have done studies on fittings and shapes that are used in air duct design to determine the coefficient of loss that those shapes cause. If you study them and think about how some modern EFI intakes are designed, you can see how the various shapes either help or hurt the basic intake design.

We’ll take a look at those system components and their loss coefficients. The arrows indicate the direction of air flow. Here are some that would apply to a plenum design. The Local loss coefficient number in the below illustration represents the restriction to flow, the higher the number, the more the restriction.

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The second illustration down is how air is delivered in many EFI intakes from a throttle body runner to an EFI plenum. Notice how its losses are much more than a design that would help direct half of total flow to both ends. I suspect the reason is that the plenum needs to be more common than the illustration above allows for. However, I think that the losses could be reduced by experimenting with some fashion of turning vane that did not reduce the ability of runners to pull from the plenum. When I have more time, I intend to experiment with this concept.

You can see in the last two illustrations that any cavity opposite the TB runner exit to the plenum would increase the loss coefficient dramatically. I have seen pictures of upper EFI intakes where someone put a new TB access to the upper plenum on the opposite side of the cast TB runner to give a “straighter shot” of the air into the plenum. The idea was that to some extent at least 90 degrees of turn would be taken out of the flow path. If this concept did not blank off the TB runner internally, I think it may have created more of a problem.

When the air leaves the TB runner, it hits the back of the upper plenum and does a 180 degree turn to access the entrance of the individual runners. When you simply open a TB path on the back of an EFI runner, you may remove some of the 180 degree turn, and if you blank off the old TB runner you will avoid the added restriction spoken of above, but the TB will be close to the runner entrances and the runners on the far ends of the plenum may be restricted by air moving across the entrances of the other runners, becoming turbulent. To do this right, the TB should be moved back and placed at the top of a sheet metal pyramid that spans the width of the rear of the individual runner entrances. THEN you have removed a nice chunk of flow restriction from the original design. Why these concepts have not been tried (or maybe they have and not been publicized or produced) I have no idea. Here is an illustration of what I had in mind.

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The illustration below would apply to runner design or any situation where air has to make a turn

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The above illustration can be applied to the Ford lower intake runners. As you can see, nice big smooth radiuses in curves lend to low losses. Here again is the Coanda Effect at work. The lower illustration is somewhat of a representation of the #1 and #5 front runners on an EFI Ford small block lower intake. The top corner is not present in the runner as the lower has a fairly good radius to it, but the short side has a sharp bend in it like the above lower illustration. So, the more you can port and morph the lower illustration to look like the upper illustration, the more efficient the runner becomes and the more air it will flow. Total flow is important and the quality (turbulence) is also very important.

NASA designed a low loss 90 degree air duct and it incorporates a wide short side radius (SSR) to reduce the centrifugal affects of the moving air mass tendency to “stack up” on the long side in tight turns. Below is a screen capture of the NASA paper. Now you see why head porters try to widen the short side radius (SSR) in a head port transition to the bowl and to the extent you can do the same in any inlet portion that has 90 degree turns, you will reduce flow looses in the duct.

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You may think about or come across situations where these same principals will apply. I was porting a Lightning 351 GT40 lower for a customer who sent me the intake and the Downs Ford box upper intake that he was going to use on it in a blower application. The 351 GT40 lower is a VERY nice piece – much less restriction in it than its little brother 5.0 liter versions. In stock trim it flow tested right at 280cfm average. We needed 300+ cfm out of it for the heads to be used, so I ported the lower and we got 302cfm average with a delta from low to high runner of 9cfm. Flow testing was done with a nice clay radius at the top of the runner and the injector bung was plugged.

I then bolted up the Downs Ford box upper to that ported 351 GT40 lower and flow went down to 260cfm average with a low-high flow delta of 59cfm (223-282 cfm). What happened? Well, the Downs has some design limitations too. The throttle body is mounted in the center of the plenum wall, but it is not very far above the entrances of the runner into the lower intake. The box needed to keep a fairly low profile to fit under the stock hood, so the plenum is not very tall in relative terms. The runners on the TB side just below the TB entrance had their flow drastically reduced. The reason flow was hurt is the air, despite not having a duct to interfere, could not negotiate the very tight turn radius into those runners, so flow over-shot and became turbulent which hurt flow. The sharp edge of the lower runner entrances hurts some too, but if the lower runner entrances were welded and/or the runner entrances were radiused, it would not change the spatial relationship of the TB runner floor to the entrance of the runners beneath it. I made the below illustration to show what I mean. This is on 2D but you can see that the radius and distance to turn is the shortest for those runners just below and in front of the TB inlet. I added a turning vane in the top view that may assist the turn and reduce turbulence. See the first illustration above and you can see this takes the plenum from the second design to the first and should reduce flow loss.

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One potential advantage the Box style upper has over its EFI cousin is that the flow path has less total turns and a shorter runner length which tends to increase peak power. In the stock EFI upper, the air must enter, turn 180 degrees and enter the runner entrances. In a Box style upper, the turn is now reduced to a 90 degree turn.

Trick Flow produces a Box upper that I think works better from a design perspective. The Runner entrances are raised in a pent style and the runners are radiused to help hold turbulence (vena contracta) down. The throttle body is set back to outside to help the air make the turn into the near runners. The intake seems to work well on large cubic inch engines or engines with pressurized inlets.

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I hope this stimulates your thought process when it comes to porting and intake or evaluating what modifications and/or intakes might be best for your application.

The next aspect of design is the intake runner entrance design. David Vizard has done testing of designs that show which designs are best and the percent improvement seen in the various designs. The second illustration below is an illustration of these designs tested by David. While these look like velocity stack designs, the principals apply to any entrance. The illustration after David’s is from National Advisory Committee for Aerodynamics that slows similar testing and results.

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