Air Horn length question

[Grover]

I've been running a MegaSquirt managed EFI system in my car for the couple of years using the fairly typical 325i TB and 320 intake system. However, I've always wanted to switch over to ITBs and just recently picked up some lightly used TWM units. Where I am continuing to struggle is in deciding on whether to run air horns, and if so, what length. Two places I have researched have given me VERY different answers, so I'm hoping some of you all with some real life experience might be able to weigh in with your experiences.

On the one side is the very nice work done by a fellow on an E21 with Jenvey ITB's. He is a strong proponet of longer airhorns. On the flip side, I have a very nice conversation with Steve at Top End Performance, and he recommends either no airhorns (mated to RamFlo filters) or just 35mm airhorns. His opinion is that longer air horns provide virtually NO benefit whatsoever, at least on a street application. (Mine will be mostly street, with 2-3 track days per year.)

Would love to hear some other knowledgable insights from folks like Lee, Jim K, etc... Thanks so much folks.

http://77e21.info/megasquirt.htm

http://racetep.com/bmwmainframe.html

[vinceinIL]

What I've heard is that a good general rule of thumb for air horn length on sidedraft carbs is they should be about as long as the diameter. So of they're an inch and a half wide, they should also be an inch and a half long/high.

How much similar this is for your application is anybody's guess.

Hope this helps a little,

vince

[allbim]

For use on the street the total length of the TB and air horns (velocity stacks) should be about the same as the curved runners (not including the plenum chamber) on a 320 intake manifold.

If I remember correctly they are about 17" long, measured from the cylinder head. I don't know how you're going to do that with the brake booster in the way unless you bend the rear stack.

There is hardly a car on the road today without a tuned intake system. If it didn’t work nobody would bother.

There has been a good book around for some time on this subject, entitled "The scientific tuning of intake and exhaust systems".

[aa/fd]

Everything I have read indicates proper calculated longer runners are a big deal.

From the posts you linked, and reference texts available from Jenkins, Riccardo, Yunick, McFarland, Jenvey, and Bettis all concur.

Even Side-drafts would benefit from a longer intake.

[shauer]

For use on the street the total length of the TB and air horns (velocity stacks) should be about the same as the curved runners (not including the plenum chamber) on a 320 intake manifold.

If I remember correctly they are about 17" long, measured from the cylinder head. I don't know how you're going to do that with the brake booster in the way unless you bend the rear stack.

There is hardly a car on the road today without a tuned intake system. If it didn’t work nobody would bother.

There has been a good book around for some time on this subject, entitled "The scientific tuning of intake and exhaust systems".

That is a great book, I read it years ago and tuned intake theory works regardless of if the car is a "street" or "track" car in the same way that tuned exhaust theory works.

I can speak from 1st hand experience that swapping 50mm air horns for the 320i curved intake runners increased volumetric efficiency on my engine by 20% in the 3500 - 4500 RPM range while having a negative effect over 6000 RPM. This data is based on the change in my megasquirt VE fuel table before and after the change.

Unless you are planning on driving your car at 7000 RPM all day long, you want to go with an intake runner length in the 15 - 19 inch range as measured from the center of the intake valve to the end of the air horn.

- Steve

[Harv]

What Steve said.

He's very knowledgable in this subject.

John

[///mChris]

Also you might want to check out a myriad of online calculators for this.

http://www.rbracing-rsr.com/runnertorquecalc.html

http://www.velocity-of-sound.com/velocity_of_sound/calculator2.htm

Steve,

Can you quantify the difference between multi butterfly setup and the e-21 intake?

[shauer]

Steve,

Can you quantify the difference between multi butterfly setup and the e-21 intake?

I'm not quite sure if I understand the question but I'll give it a shot.

I cannot quantify the difference partly because I have never personally used the e-21 intake for EFI. I went from k-jet to dual Weber DCOE to EFI with ITBs.

The e-21 intake is not really the best choice for EFI anyways. The simplest solution is the E30 318i intake complete with injectors and fuel rail. It makes a very nice daily-driver setup that can handle moderate engine modifications.

The difference between a common plenum intake and multi butterfly is the same for EFI as it is for going to dual side-draft carbs. You can get faster throttle response and better high-RPM performance. ITBs are however much more difficult to tune correctly than a common plenum.

Hope your answer is in there somewhere.

- Steve

[TobyB]

Just to add sherbert to this sandwich-

the radius of the 'bell' on an intake's also a big deal...

heh

t

[Grover]

Thanks guys, this is all good stuff. Looks like I've got some homework to do, but also probably need to come to terms with the fact that I probably won't get it "right" the first time, it will have to be a journey...

[c.d.iesel]

GEE! I got excited! I thought this was going to about " AIR HORNS " ?

INSTEAD , it's about air horns