Larger airbox for big motors....fact or fiction?

[Jonny Hotnuts]

I have heard that bigger motors (1507 in my case); with porting cams and free flowing exhaust need a larger airbox because of the change in frequency generated.

I have found a larger airbox from http://www.motobody.com

Is it worth the money and do you have anything to gain by having a bigger box on a big mill?

[gazza414]

Good question..maybe Kent can answer that for you.

[Jonny Hotnuts]

Kent is out of town for 5 weeks.....I know he uses this on his own bike...maybe he will let me have one to "test"!

[Competition CNC]

It would have some effect but I couldnt say what excactly.

This is where you need Optimums engine simulation software....  its $35,000 for a one year licence.
What a deal!!!   

[396]

Depending on your motor combination (cylinder head), the stock airbox does become a restriction. On the dyno with no air going through it I have seen HP losses with a stock and large box mod, compared to no airbox and just stacks. I am sure a larger airbox will help, if you are at that point.

[crazybill]

I built a busa back in '99 with no airbox / just stacks . never could get thing to run quite right with no airbox...  So I added a huge wet kit 

youve probably all seen it before the pics been floating around since 99 ! wow I cant believe its been that long I been jackin with these... seems like yesterday I decided yeah its ugly but its fast so ill go get one . 

http://img.photobucket.com/albums/v174/crazybill1397/N2O.jpg

 I believe Bob Mason tried dynoing a busa without an airbox a year or two ago as well and it didnt work out well either .

[gazza414]

That's what ya call a fix Bill

[Steve S]

Fact along with more resonant tuning and noise reduction:



In order to implement ram air, the carburetors or throttle-bodies of our engine must seal to an airbox whose volume is large enough that the intake cycle of one cylinder cannot pull its internal pressure down significantly. Box volume is typically 10-20 times the engine's displacement. Then the forward-facing air intake is connected to the box. When this assembly is tested on the dyno — even without an external fan to simulate the high-speed rush of air past the intake — it is discovered that the engine's torque curve is greatly altered, with new peaks and hollows.

Why? The answer is airbox resonance. If you hold the mouth of an empty bottle near your open mouth as you loudly hum scales, you find that at certain “hum frequencies” the bottle reinforces your humming, which becomes louder. What is happening is that the springy compressibility of the air in the bottle is bouncing the slug of air in the bottle's neck back and forth at a particular frequency — higher if the bottle is small, lower if it is larger. Your humming is driving a rapid plus-and-minus variation of the air pressure inside the bottle.

The same thing happens inside a resonant airbox. The volume of air in the box is the “spring” in this kind of oscillator. The mass of air in the box's intake pipe is what oscillates. The “humming” that drives the oscillation is the rapid succession of suction pulses at the carb or throttle-body intakes. If the volume of the airbox and the dimensions of the intake pipe(s) are correctly chosen, the airbox can be made to resonate very strongly, in step with the engine's suction pulses. The result, when this is done correctly, is that the engine takes air from the box only during the high-pressure part of its cycle, while the box refills from atmosphere through its intake between engine suction pulses. This produces a useful gain in torque.

[gazza414]

Steve, where did you source the info above from?

[BRYAN]

Gazza, I am on Bryan's computer but when I get back to the office I will find the source and get it to you.

[osti33]

So would it be safe to say that on a large (1500+ cc's) motor that if you run the stock airbox a "large" airbox mod would preform better than a "small" box mod?? I am just wondering what everone thinks. Anyone done back to back dyno tests?

[glenn71]

A large airbox mod significantly increases the airbox volume on the engine side of the airfilter,i,m going to be doing some logging of mine soon to compare with small mod.early impressions without airfuel trimming were very smooth down low but felt a bit doughy,sounds like a gen2 and went the same up top.Will know better once air fuel is tuned to suit.

[osti33]

A large airbox mod significantly increases the airbox volume on the engine side of the airfilter,i,m going to be doing some logging of mine soon to compare with small mod.early impressions without airfuel trimming were very smooth down low but felt a bit doughy,sounds like a gen2 and went the same up top.Will know better once air fuel is tuned to suit.

Sounds good. Keep us posted on the results.

[Jonny Hotnuts]

I have consulted my friend and (other than myself) the smartest MF around Dr. Mayfield.
Most, if not all, have never heard of this guy but his knowledge of the physics of automotive engineering and HP is unparalleled.

Check out some of this old mans stuff:
http://www.mayfco.com/analyses.htm

Waaaaaaaaayyyyyy over my head.

My question to Dr. May was if the engineered (stock Suzy) box volume was (V=10000) for 1300cc could I take the difference between a 1507cc motor and increase the box volume directly proportionate to the increase of displacement, assuming that you could maintain a stock HP per CC ratio with the increase of displacement.

This would mean if the volume of the stock airbox was 10000cc (guess, just picked a number) with a motor displacement increase of 207cc (1507) the airbox would need to have 1592cc more (11592 from 10000) volume if CC displacement was the only factor (variables would include different port-valve, cam size and timing, exhaust…..).

Let’s just say that after his response I am fairly confident that a direct percentage increase is not perfect but should get you in the ball park. Another point is that the size of the ram air tubes should also be proportionately increased as well.

Kents bigger fiberglass box is looking better all the time.

[gazza414]

Gazza, I am on Bryan's computer but when I get back to the office I will find the source and get it to you.

thx Gents for getting back to me with the info

A large airbox mod significantly increases the airbox volume on the engine side of the airfilter,i,m going to be doing some logging of mine soon to compare with small mod.early impressions without airfuel trimming were very smooth down low but felt a bit doughy,sounds like a gen2 and went the same up top.Will know better once air fuel is tuned to suit.

I'm not sure from the post the info you are going to log ?


Back onto the air box and its volume..in the Suzuki blurb for 08's they mention a larger air box???..is this just blurb or fact?...

however if you have looked at tech articles over the years wrt new bikes they seem to be mentioning "larger air boxes" on a regular basis....is this to ensure a large reservoir of "stable" air or to reduce "cross talk" between cylinders or to aid resonate tuning or other factors that are beyond our knowledge base.....ie geometry, construction and rigidity... and materials to either aid or reduce heat transfer?

[DarkFalcon]

Gazza, they seem to offer a variety of benefits and the new Busa airbox does appear to be taller than its predecessor, suggesting larger volume. Resonant tuning, acoustical dampening and filling in voids in the power band can all be had with a correctly designed airbox. Unfortunately, though, I am led to believe shape plays some sort of role............so perhaps size alone will not purchase what we seek. The resonance characteristics are Helmholtz based and all of this makes for interesting discussion, particularly in the context of recent discussions of tuned port lengths.

[gazza414]

I thought you might pipe in DF.

The other thing I never mentioned as being a design attribute of the air box and that is noise reduction wrt pollution

[glenn71]

I want to log on my veypor(accelerometer) if when optimised the performance has improved or depleted.It definately feels less lumpy in its torque output down low but thats just my arse dyno.

[Jonny Hotnuts]

I may be wrong about this but I would think that while the large AB mod helps to improve air flow by lowering restrictions the fact that it is pre-filter may not increase the volume of the airbox as the motor sees it, unless you are running without a filter. But with the logic of saying “the volume post air filter is the only volume of air the motor sees as a chamber for resonate frequencies” means that a motor without the filter (and especially with the large mod) very well could exceed the motors desired airbox volume and could result in causing lower HP/FP numbers (this would be ironic because the motor would gain airflow and HP by removal of the filter restriction and could very well end up with a stagnation; gain some on the filter removal, loose some on a box volume being to large).

It would be interesting to see what would happen if someone dynoed without a filter and the large mod and then lowered the box volume (by putting something in the airbox like a attaching a clean wood block in the airbox and out of the air flow path). If you have better numbers with the lower box volume, you can bet that your box is beyond the motors ideal volume limits.

Just a thought. 

[396]

Let me try this again. Depending on your motor combination, you may or may not see a difference with a larger airbox. I HAVE SEEN A LOSS OF HP on the dyno with the factory airbox even with a large box mod, compared to no airbox at all. I have seen one pipe work real good on one motor and then not so good on a different combinatioin. It will all come down to your motor combination. There is no right or wrong answer. This is where the R&D ($$$$) comes into play.

[Competition CNC]

You know the harmonic frequency of each individual cylinders induction phase and you know the engine induction frequency.

If you have a reed tachometer and an electronic keyboard you can put a speaker in the airbox and find out its resonate frequency by watching the tachometer as you change the frequency.

Once you know the resonant frequency of the air in the airbox (which is not solely based on volume but shape also) you can determine the rpm that the sound waves will be positive sine and what rpms they will be negative.

By changing the airbox design you are merely changing the rpm at which the wave is positive.  I'll have to check a few formulas because I've forgotten.... but I believe you can not change the amplitude by changing the volume alone.

If you really want to start to engineer stuff like this you need to read this book...

http://www.amazon.com/Design-Simulation-Stroke-Engines-R-186/dp/0768004403

Or if you are an SAE member its $80 here....
http://www.sae.org/technical/books/R-186

If your an SAE member you should also read this paper....http://www.sae.org/technical/papers/2006-01-3653

And also part 2 of that paper.

Once you read those papers and the section in Blair's book on air boxes and the relationship to individual intake runners.... you can read this SAE paper.....http://www.sae.org/technical/papers/2006-32-0114  and design your own airbox or evaluate the design of the air box in question.

[whtrthanu]

The “humming” that drives the oscillation is the rapid succession of suction pulses at the carb or throttle-body intakes. If the volume of the airbox and the dimensions of the intake pipe(s) are correctly chosen, the airbox can be made to resonate very strongly, in step with the engine's suction pulses.

Steve, where did you find this info???
Im curious because an engine does not suck air in, there is a negative pressure inside the cylinder, atmospheric pressure outside the engine is higher so it is pushed into the motor........
i think you should research alittle more about this before jumping into it, I do believe you will see an increase in performance from a larger air box, but not much.  From my data I can tell you that an airbox should be no smaller than 10x the volume of 1 cylinder displacement. There are other factors that need to be considered also, but it is a good baseline to start.
1397 would be 21.3 ci X 10, so it would have to be no smaller than 213 ci... THe stock air box is bigger than this already, so i think your gains will be minimal at best

[Busa Quick]

 there is a negative pressure inside the cylinder



 Define suction....Suction is the creation of a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area.


Mark

[Busa Quick]

  Not trying to be a wise ass but, suction and positive pressure are more dependent on which way you are looking at it. Both terms are describing a difference between two pressures. One pressure is less or one pressure is more than the other.


Mark

[whtrthanu]

I hear ya mark, but I was trying to make a point, if you look at it like this, when the engine is running at higher rpm, when there is actually no vacuum in the motor, how does the air get into the engine??