Do-it-yourself (cheap) air horns for DCOE carbs

[RallyBob]

Just a quickie 'how-to' here, I'll try to keep it brief.

I was looking for some air horns (trumpets) for sidedraft Webers, but for the size I wanted they cost about $45 each. A bit of searching around my barn and I came up with these header reducers....they have a rounded flange and taper from 2.5" to 2". Quite rusty, but after about a half hour of polishing on a Scotchbrite wheel they cleaned up pretty nice.

I made a cardboard template from the inlet side of a DCOE Weber, and transferred the pattern to some 14 ga steel. I would have preferred 12 ga or so, but the thinner 14 ga will do.

A 2" hole saw was then used to drill the large center hole, and the smaller mounting holes were drilled at the same time before cutting out the outer shape with a steel bandsaw, and sanding the edges on a belt sander.

The last steps are to weld the flanges to the header reducers, and to either paint the air horns or have them plated. My total time to build these was about 2 hours, my material cost was zero.

Depending on the engine these could either be lengthened or shortened to suit the required powerband, or to fit the underhood space at hand.

Bob

[namba209 (R.I.P.)]

Another neat piece of work Bob. BTW any muffler shop that has a hydraulic tubing bender, and most have one, could take any length pipe, and taper it to whatever angle was needed, if they want to take the time to do it. I had the down tubes on my V-6 exhaust manifold expanded at the flange to be a snug fit before they were welded on. It took seconds to do each one.

[madhatterpdc]

It would be interesting to see measured differences between your homebrew and the $45 Weber part...

Okay so maybe I'm stirring the pot a little

[RallyBob]

I decided that these air horns needed to be extended a bit in order to compliment their intended engine application. I extended one by 1.25" and test fitted it, should work out okay.

In case you're wondering what happened to the DCOE Weber, it was an old one I cut up in order to use as a throttle body for EFI. They'll get injector bosses welded above the throttle plates, and there's a phenolic spacer between the TB and the intake manifold.

[nobody]

Ok
i'll bite what in the world needs a 6 or 7 inch air horn? this is to clear some physical obstruction right?

[RallyBob]

I borrowed a friend's engine building computer program some time ago and ran some specific engine specs through it (cam, compression, airflow, etc). The program allows you to change intake runner lengths and runs dyno simulations based on these changes. I found that the optimal intake runner length for that particular camshaft was 17.25" from exhaust valve to air inlet. So this is how I came up with those numbers! It won't make the best peak power, but it had the nicest power curve overall this way. In general the engine program seemed consistant and reliable, I 'built' four different engines on the computer before I built the actual engines, and all of them came within 4 hp on a real dyno, with the exception of one engine which made almost 20 more than it should have. So this is a little experiment to see if the runner lengths affect the power curve as predicted. We'll see.

Bob

[N61WP]

Would this be the mill for the "land speed record" GT?

Jc

[RallyBob]

Would this be the mill for the "land speed record" GT?

Jc

Nope, that induction system will be far more exotic, and very short...about 9-10" long overall to make power in the upper rpm range (9-10k).

Bob