Air Horn Shape Matters

[sptcoupe]

There have been many posts and I ahve had many conversations with folks regarding the viability/usefuleness of air horns on carbs, and IDFs in, particular, and what shape works best. But unless you have a lot of dyno time playing with different horns, or a flow bench, the discussions are just our opinions and speculation. The common wisdom is that horns are better - and indeed, the right shaped horns do increase airflow, and thus, power output. But not all horns are created equal, and in fact, some of most common shapes that come with our carbs actually decrease air flow! I finally dug up this chart that I have used for years in selecting air horn shapes, and it is the only air horn flow comparison I have seen. It was done by an engine builder in the 80's using a flow bench, and he reportedly then did a dyno pull with each set of horns on the same motor to see if the flow incereases/decreases translated to power increases/decreases - and they reportedly did.

What the chart will show is that the way in which the horn is radiused determines how much it increases or decreases air flow. I am sure you will see an air horn shape you recognize, and you may be surpised at the results. For sure, you will see that the horns that came with the early IDFs are really very restrictive and seem to reduce flow.

This is not to say that a horn is not better. Even if your horn only provides a small percentage increase in airflow - that is an improvement nonethelsss, and good on ya'. But in some cases, you are actually better off running no horns at all.

Of course, how much room you have over the horns is also a factor, and the rule of thumb is that you need the same amount of clearance over the horns as the carb choke's diameter to get the full benefit from a given horn. So in 40 IDFs with the common 32mm chokes, you need 32 mm clearance over the tops to get max benefit.

So, in selecting a horn, get the shape that maximizes air flow (the chart), and get the height that allows you the max amount of clearance over the horns. A short horn, with the proper radius design, will be more effective than a taller horn with less than optimal clearance, and even more effective than a taller horn with the wrong radius design, no matter what kind of clearance you have.

I have always had to cut my horns on our TC applications, as there is never enough over-horn clearnce to get max benefit. In recent years, I have had them custom made to get the best design and enough room at the same time.

BElow are links to the chart and a pic of the horns on the coupe's 44 IDFs, which illustrates how you might combine the best design and maximize the over-horn clearance on a twin cam application.

These horns are enclosed in an air box that takes cold air from in front of the radiator, and with the top on it, I only get about 33mm of clearance over the horns - just shy of the 36mm I need to get max performance, but better than I would get with taller horns. This design proved to be the best among three shapes/heights we tried over many dyno runs when we were running the FI system

I hope this is useful as your consider what you want to do with your induction set up. I use the chart to help select the horns that optimize performance given the room restrictions we have in our FIAT engine bays and each application. Even small air flow improvements add up to better performance from the your investment in carbs/FI systems.

http://i721.photobucket.com/albums/ww21 ... 4-1845.jpg

http://i721.photobucket.com/albums/ww21 ... RMANCE.jpg

[majicwrench]

OK I thought this post was about our "Air Horns" that we blow at otgher motorists.

[sptcoupe]

I suppose shape and air flow matters there, too, especially on our italian air horns. In fact, in case you didn't know it, you can "tune" the sound of our car (as opposewd to carb) air horns by shortening or lenthening the horns. Just loosen the nut at the at the base of the horn and screw it in ot out to raise or lower the pitch.

[fiatfactory]

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[sptcoupe]

Correct. He also did a companion study on heights of carb air horns. I had that at one time, but have lost it somewhere along the way. In both studies, he did dyno pulls to confirm that the theory was backed up by real testing. On the engines he was using, the horns made a difference that was actually measurable in pretty big chunks of increased HP.

[BEEK]

could you post a clearer and larger image of the velocity stacks, or even e-mail it to me, i am very interested

[engineerted]

Here is a good read if you are looking for some more science around the bell mouth shape.

http://www.profblairandassociates.com/p ... h_Sept.pdf