Weber Air Horn question

team-gpracing

team-gpracing

Well-known member
So I have been running a K&N filter on my Weber 48IDA for a couple years now and I just made a new custom, ducted intake box to fit over the Weber filter assembly. When I installed it, I started thinking...

I know that the air horns on the Weber serve an important purpose that basically cleanly channels the air straight into the carb, but should I remove the horns when used in conjunction with the assembly and air box? The cover of the airbox/K&N assembly only sits about an inch above the top of the horns, and all of the air comes from the ducting, into the airbox and through the filter to the side of the horns. Again, I know they serve a purpose, but I also think that the flow of air would be much less restricted if I removed them. Especially at speed when the two 4"d hoses are really feeding the airbox.

I plan on trying it the next time I go to the dyno, but I'm not sure that will give me accurate results (fans only do so much). Any thoughts from the engineers (with degrees and otherwise) on the forum?
 
Not an engineer buti worked on something similar. I was told by engineers that the 1.25 inches I had was enough. Using the wheel dyno was difficult. I could measure an improvement with the airbox removed HOWEVER it ended up being attributable to a difference in intake air temperature, not restriction from the lid.
 
How did you determine it was air temp? Were you running with the hood on and a warm engine?

Anyone else care to give your opinion? It probably isn't much either way, but these are the types of problem solving improvements that I enjoy.
 
Initially it was a hood on hood off thing but we were able to negate all the difference with direction of additional air directly to the body air intake instead of just to the whole front of the car as you do for oil and water. Was a challenge.

Also I had 1.15 inch clearance for a 125hp engine breathing through a 35mm 2 barrel. Not sure my experience is relevant to your question. Just thought I would throw it out there.
 
If you spend about an hour searching on the internet you can find an doctorial thesis on the use of fluid dynamics to analyze air flow thru an engine. The test subject was a 2 liter rally car engine developing a lot of hp. The testing was done (I think) in Sweden. They moved the air horns in and out to discover the optimum configuration and air box size and shape. You definitely want to use the air horns and they should reach to approximately the middle of the air box. Sorry I can not remember the link or help more in finding the article.

Jim
 
On the Moser HP CRX, Mark Meller discovered that running the factory rubber intake hose was disrupting the air flow into the throttle-body. I think that you won't be able to tell if there is a difference unless the car is moving at speed.
Hard to do on a Dyno.
Mark speculated that the 90 degree turn in the intake hose was the cause. The smoother the flow through the air box the better. I suspect it is more important at very high revs. If the horns are close to the outer edge of the air box, I think you may have the same effect. The sharp turn in the flow causes turbulence and restricts the flow.
 
Hi Sterling,

I'm heading to the Majors at Buttonwillow this weekend. I'll take some photos and get info for the air boxes that are on the rotary 48mm IDA cars (RX3, 1st and 2nd gen RX7s). From my own experiences with my 48mm IDA (40 mm chokes) on a 12A bridgeport, I found that the air box size, the air horn length and the diameter at the mouth of the air horn can have profound effects on HP.

Best,

Guy
 
David Vizard's book "Tuning BL's A-Series Engine" has considerable information on Weber carburetors, a section on Ram Charging (velocity stacks), and a section on air filtration (with velocity stacks inside). Much of the ram charging section deals with SU sidedraft carbs, but I'm sure there is considerable crossover, as they only have an effect on air flow thru a venturi.

Mr. Vizard data indicates the short, solid, elipitical flare velocity stack has the greatest improvement in air flow. Many configurations have a negative effect.

I have seen the short, solid ram pipes mounted inside air cleaners and cold air boxes. It was my understanding that there must be 2 venturi diameters clear in front of the pipe opening for it to have full effect.

Interesting reading. YMMV

Ram effect of a cold air box may skew this data considerably.

FWIW

RJS
 
RJ, thanks for the book tip, I'll look into it. I have the Weber bible as well, maybe I'll crack it open.

It's been ages Guy, hope all is well. Thanks for doing some research for me.

I haven't installed my setup yet, but I've included some pictures for reference. As you can see, I am feeding with two, 4" inlets and I'm hoping for a swirling effect with the placement of the inlets. Regardless, the K&N filter cover sits only about an inch above the stack, which is where my concern lies. Of course, my new airbox hasn't changed that fact, it just got me to thinking.

Has anyone ever tried one of those on-board dyno contraptions? http://www.dynolab.com/on_board_dyno.htm
 
I hate to do much selling on here, but you'd do as well or better for less money using one of our GPS enabled data loggers and the included PC software, see:

http://www.race-technology.com/content. ... &cat=31363

With regard to your 1" clearance, my gut is that you are OK. There's a pretty big area if you look at the circumference of the OD of the air hown times the one inch clearance. I did a very informal test on a dyno years ago, on a Datsun Roadster engine with 2x SUs with air horns. In the car, they were closish to the inner fender. I held a sheet of aluminum closish to the air horns and saw no power decrease 'til "really close". 30 years later I can't put a number to really close but it was less than an inch I'm sure.

Al Seim
HP VW Scirocco 1.6
 
Thanks for the response Al. My question isn't so much whether I am reducing power by having a surface too close to the entry of the horn, it more goes to whether removing them will give me more power. The largest volume of air will be going directly into the side of the horn. So by removing it, I wonder if it has less resistance to it's intended location. Buuuut, does the lost funneling function of the horn negate any benefit from the better flow.

Does that data logger show power?
 
The PC software will, using data from the in-car box.

Like anything similar, it will be better at comparing A vs B than giving an absolute number, but even dynos are like that.

Al
 
Considering what the goal is here- more power with or without air horns- an A/B comparison would be just fine.....
 
I would add a domed cover to the lid. Look at the Moroso aftermarket air filters for Holley carburetors. I had one that I attached to a lid similar to yours. You may be able to find a used one on the web.

Adding air space above the air horns improves their air flow capacity. And reduces the turbulence over the horns. It was mentioned several comments earlier.

Insure you leave air box pressure open to the screen over the jets between the air horns. It affects cailbration of the jetting, it needs the same air pressure as the carb is seeing. And use the screen, I had grit clog a jet, discovered it as we were going to the grid. The engine wouldn't run properly til we cleared the jet of the grit.

The Can Am cars used 2 lengths of intake air horns. They had the advantage of huge 427+ cubic inch motors, but found they could improve the torque curve using the different lengths. The engine HP and torque will respond to the length of the air horn. It will also move it up and down in the RPM range. The optimum length can be ball parked on the dyno. Air pressure at higher speed may require some additional adjustment. But, I think the dyno will get you in the ball park, and improve your understanding of the affects.
 
Al/Matt - yes a comparison is fine. I don't care about the actual number/accuracy as long as it is consistent.

John, I definitely don't plan on messing with the screen.

I guess I'll just have to wait until my next trip to the dyno. Unfortunately I thought of this the day after I was there.
 
On my DCOE side drafts (I know you are working with a downdraft) any air horn, long, short, or in between is better than no air horn at all on the dyno. I would run them until you can test.
 
I'll add a little bit of anecdotal data to this discussion. In my GP/FP Scirocco, due to the then prod firewall rules the induction system was very space limited. Using an ITG air filter on top of my Weber DCOE carbs resulted in a very short stack clearance inside the filter housing. Even the off-the-shelf 'shorty' air horns were very close to the top of the filter. I did try running with and without air horns, and using the DL1 data acquisition I could see a measurable difference in wheel hp. So the air horns were definitely better than none. And I could see a difference between various off the shelf air horns that I tried.

What ended up working for me was having Walt Puckett machine a set of 'stubby' airhorns that were about 12mm tall and basically were just a flanged lip that rounded the entry into the carb. Those were even better than the off the shelf air horns, which I attributed to the increased clearance to the top of the air filter housing.

I never made an actual sealed airbox, but instead shielded the area around the air filter housing and directed cool air ducts at it.

Hope that helps,

Mark
 
JohnMcNaughton":dev96053 said:
I would add a domed cover to the lid. Look at the Moroso aftermarket air filters for Holley carburetors. I had one that I attached to a lid similar to yours. You may be able to find a used one on the web.

Adding air space above the air horns improves their air flow capacity. And reduces the turbulence over the horns. It was mentioned several comments earlier.

JohnMcNaughton":dev96053 said:
The Can Am cars used 2 lengths of intake air horns. They had the advantage of huge 427+ cubic inch motors, but found they could improve the torque curve using the different lengths. The engine HP and torque will respond to the length of the air horn. It will also move it up and down in the RPM range. The optimum length can be ball parked on the dyno. Air pressure at higher speed may require some additional adjustment. But, I think the dyno will get you in the ball park, and improve your understanding of the affects.
Click to expand...
Click to expand...

The Can Am cars were attempting to equalize the port lengths of the big block Chevy's. They appear to be staggered, but really they work out to be the same length when measured from the valve seat. The sprint car racers found they started to make more power when placing an air filter over velocity stacks that terminate in the air... blow over a straw and see what happens. An upright velocity stack is no different... you want to increase the pressure at the stack, not decrease it ;) .

You see these on fast 4bbl Holley engines:
59210615_L_1796f25c.jpg


It also allows for more air filter element area for a given clearance height.

Here are modern velocity stacks for an injected sprint car:
2012-12-08%2014.09.571.jpg


Think about how either one can help you, possibly.

The one thing V8 racers don't have to really worry about are harmonic waves coming through the carburetor barrels like an individual runner system. That's what the plenum below the carb(s) is for (the single Holley guys use carb spacers partially to tune this) and why it's volume is very sensitive (those guys spend a lot of effort tuning this when they can).

Think about how far you see stand off on an individual runner system and think about how far away the airbox face is from it. Anything less than the stand off and you'll have an effect on the 'tune length' and it's strength.

All velocity stack shapes aren't the same. You need to work with someone with a flowbench and try out as many combinations as possible. Any flowbench guy will tell you the importance of a radius plate in getting repeatable numbers and smooth the airflow through the system (think of the velocity stack as a radius plate for the carburetor - at least from an airflow perspective). The other is jet signal, which can be measured on the flow bench as well (basically, looking for the pressure drop across the jet). The better the signal, the less jet you'll need and the more responsive the engine will be. People have shared data of both velocity stack flow bench work and jet signal measuring on Webers elsewhere on the web.

Re: velocity stack length:
http://www.swartzracingmanifolds.com/tech/index.htm
(he was building manifolds for top NHRA Pro Stock teams).
 
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