Air Box Design

[lancer360]

I'm trying to get myself up to speed on the concepts of air box design, both from a ram air stand point and as a Helmholtz resonator. I know the ram air effect is pretty small but from what I have been reading the general concept is a relatively small opening with a smooth and gradually increasing cross sectional area up to the throttle bodies. What I'm unclear on from the ram air aspect is where is it best to put the filter. My engineering brain is telling me it is better to put it close to the throttle bodies as the air flow has reduced so you will have less pressure drop across the filter. It will also be much easier to do a large flat filter. I do understand you need to keep the filter far enough away from the throttle bodies as to not cause turbulent flow into the velocity stacks.

As a Helmholtz resonator, I have read that they can provide a pretty significant power boost if done right. I have read some articles taking a more simplified approach of a spring-mass calculation using the air box volume, and the intake length and cross sectional area to get a ball park idea of the resonate frequency. I have also seen numerous references to trying to make the air box volume as large as possible for motorcycle engines. This is part of what is confusing me. The spring-mass calculation has the resonate frequency as proportional to the cross sectional area of the intake tube and inversely proportional to the length of the intake tube and the air box volume. Considering that the 600cc motorcycle engines peak power is at 13,200 rpm the initial calculations are telling me that the air box volume needs to be very small if you size the intake ID about 25% larger than the ITB ID. Is this because of the long intake tube (relative to a stock motorcycle)? How does the air filter and its location play into the design of a Helmholtz resonator?

I know many are going to say just run the stock air box, but that will not work on my new car without butchering the bodywork.

[Will Velkoff]

I bet some many Formula SAE teams have beaten this exercise to death. Might try searching those forums for any tips

[DaveW]

Way back when I had my Z10 FF, I tried an air box that got its air from a duct over my head. The air-flow coming in was good, but the air-box volume was way too large, and we could never make that work correctly. With it, the engine always felt "flat." Going back to a more normal sized intake scoop resolved the issue.

So, you are certainly correct that the design is important. However, IMO, it will take a lot of work to optimize, and I don't think simple mathematics will be able to give you the "correct" solution.

[Purple Frog]

Not an engineer, and knowing very little, and can't afford to stay at H.I. Express.... My experience is based with multi-cylinder engines fed by one carb. In the 70's we had a layman's term called "standoff".
The street theory of the time was that because of firing pulses there could actually be a certain amount of valuable "stuff' hovering above the top of the carb. The goal was to prevent that stuff from getting blown out of place by air the vehicle was passing through, and you didn't want it condensing on the bottom side of a filter housing lid.
Thus, if you look at all sorts of 70s race cars from Can-Am monsters to formula vee, you always saw a stack above the carb/throttles, or at the least the roof of the filter housing being at least as far above the carb as the height of the carb itself.
No rocket science, just "follow-the-leader" copycat stuff. If McClaren did it on a Can-Am car, then it must be good for a VW based autocross machine.
Since you a dealing with engines that have one throttle per cylinder, the science may be completely different.
Now, back to posts written by someone who might know something...

[SEComposites]

The best place to start would be the stock suzuki airbox in my opinion. Its easy to get bogged down by the science of airbox design, and perhaps you don't have the space but bike manufacturers do all the hard work to make them work, albeit within the confines of a motorcycle frame.

[lancer360]

I don't have room for the stock air box which is what is forcing the issue. Even with the stock air box many of the same questions still apply for understanding it's design principles. Since the filter is divides the air box into two chambers do you use the entire volume up to the dual inlet openings minus the volume of the filter or do you only consider the upper chamber? Does the filter shift the resonate frequency?

[Hardingfv32]

For some reason the Helmholtz resonator is not not always used in the plenum design. The volume also seems a bit nebulous with recommendations from 2X to 4X displacement volume.

I have a very good paper on a F1 plenum. Very tricky reducing the velocity of the flow efficiently to maintain the largest pressure gain inside the plenum. Also have a few FSAE papers on the subject.

PM me with your Email so I can send PDFs.

Brian

[Stan Clayton]

About 10 years ago I started collecting books and papers on intake design, and for the 2007 Runoffs we designed and built this one for our Ralt-Toyota CSR effort. The bottom line? No clear improvement over the stock Ralt intake. YMMV.

[Stan Clayton]

PM me with your Email so I can send PDFs.

Brian

Would love to see them, Brian, but ApexSpeed says your PM is turned off. You can send them through the email link in my profile. Thanks!

[bill gillespie]

About 10 years ago I started collecting books and papers on intake design, and for the 2007 Runoffs we designed and built this one for our Ralt-Toyota CSR effort. The bottom line? No clear improvement over the stock Ralt intake. YMMV.

Hey Stan.....I don't know if you recall, but you and I exchanged some thoughts and ideas on ram air horns back when I was hell-bent on my Ralt/Cosworth FA project.

We came up with our best guess and went to the chassis dyno.....established our baseline, and did a pull without the airhorn installed , and one with the airhorn installed. Astonishingly, we gained 10hp at the rear wheel with no other changes except installing the airhorn.

I know it was beginner's luck, but 10hp with no Ram air is huge. On track the Motec plenum sensor data showed around a 3% ram rise at speed. Using leaded fuel I was a little nervous to use active lambda because of my fear of sensor unreliability .

YMMV,
Bill

[lancer360]

My first order of business will be to run the calcs on the existing air box to try and identify what rpm it was targeted at and also what rpm's the intake runners are targeted at. The stock velocity stacks have a staggered length with two short and two long. The difference in height in the 2011+ velocity stacks is less than the height difference of the 2008-2010 stacks. Overall the 2011 stacks are shorter than the 2008-2010 stacks.

[bill gillespie]

Here ya go....can't see the plenum

[Stan Clayton]

Hey Stan.....I don't know if you recall, but you and I exchanged some thoughts and ideas on ram air horns back when I was hell-bent on my Ralt/Cosworth FA project.

We came up with our best guess and went to the chassis dyno.....established our baseline, and did a pull without the airhorn installed , and one with the airhorn installed. Astonishingly, we gained 10hp at the rear wheel with no other changes except installing the airhorn.

I know it was beginner's luck, but 10hp with no Ram air is huge. On track the Motec plenum sensor data showed around a 3% ram rise at speed. Using leaded fuel I was a little nervous to use active lambda because of my fear of sensor unreliability .

YMMV,
Bill

Hey Bill, I do recall the conversation, but had forgotten you got such good results. We did not do dyno testing, relying instead on back-to-back track testing, where there were no unambiguous improvements. Glad to see yours worked, tho!

[Steve Demeter]

There is a guy named Pete Vorum, who in the 70's did extensive research on Helmholtz resonator concepts for the USAF, mainly related to exhaust system design. Not sure if he did anything with intakes.

Had him speak at engineering seminar not for credit course back in my college days.

I would try to find some of his work.

He was quite impressive but also sort of a cockeyed genius.

But his dyno sheets do not lie.