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ryant67

Air box design - Weber/Mikuni w/horns

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Hey folks,

So, after seeing my ITG air filter go up in flames, I've now gone off the idea of running one of those.  I bought K&N's and found that my Mikuni air horns were too wide for the filter to squeeze over, and with things being so tight inside those little filters, it wouldn't be ideal.  I want to be able to run air horns on my carbs without being limited on size.  

The ideal solution seems like an air box that feeds cold air from a cone style filter in front of the rad, or at the extreme front left of the car by those 2 round holes in the front clip.

I've done a lot of searching, and there are a few of options out there to buy:
TWM - http://www.racetep.com/ztripdcoe.html
MJP - http://www.mjpshop.co.uk/index.php?route=product/category&path=155_78_113
Reverie (very expensive) - http://www.reverie.ltd.uk/product_detail.php?prod=zolder_macau_6_cyl_airbox_lh_carbon__to_fit_jc100_backplate_range&group=A%20AIRB&subgroup=A%20AIRB%206C&part_code=R01SE0561

My question is:  Does the shape of the air box really matter all that much for this application?  

I have seen others fabricate up aluminum air boxes that are considerably more square.  For $30 in sheet aluminum, and some beers to a welder buddy, I could have a basic air box up and running.  Do I lose anything by going this route?  

For example, 10 minutes and some board, and I have mocked up this.  Would this with a lid on top, in aluminum, be inferior to the TWM unit?

Any thoughts would be very welcome.  Thanks.

 

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There are some great threads on here about air box design. The one thing that sticks in my mind is how poor a basic retangular box works.

 

It is hard to create an ideal box without 3D flow modeling or following a pre-existing design. At the very least, if you have to have the inlet at the front, build a tapered box so that you have a 3/3 - 2/3 - 1/3 x-sectional area taper from front to rear to keep the rear from starving. Just think that at the front you have all 6, then after the first 2, you need 2/3's of the remaining air, then after 4, 1/3. And make it as large as possible. 3" inlet size minimum. Really crude flow design, but its better than a small rectangular box.

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I have Tripple Mikuni 44s which came with tall old K&Ns.  I then got Wolf Creek air horns which look to be at least as wide as yours, and they fit. It required a little squeezing to get over the horns, but they fit. There is a slight bulge in the middle, but that's all.  I can't tell you if my airflow is functioning ideally though, because my jetting is out of whack right now (wideband O2 and set of jets in the mail though  :D )

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Version #2 - firewall end tapers down to 4", instead of 5.5".  Box is now 5.25" tall instead of 6".  Adjusted the front entry angle to clear my oil catch can and allow a straighter pipe through the front clip.  

Going to try widening the entry side 1" more, which should allow a little more angle.

 

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Steve is correct about the volume compensation. The larger the box is and the larger the inlet is, the less it matters about shape. Looks like you'r on the right track. Remember whatever box you make has to be removable too...   Don't forget that there is lots of room DOWN not just toward the fender.

 

Ever seen Z race car engine bay photos where the ENTIRE and I mean ALL, of the engine bay from the vertical plane formed by the carb/intake flange all the way to the inner fender, from rad support to firewall, is boxed off, sealed to the hood and frame rail? One huge cold air a$$ box is formed that doubles as a heat shield too.

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Steve is correct about the volume compensation. The larger the box is and the larger the inlet is, the less it matters about shape. Looks like you'r on the right track. Remember whatever box you make has to be removable too...   Don't forget that there is lots of room DOWN not just toward the fender.

 

Ever seen Z race car engine bay photos where the ENTIRE and I mean ALL, of the engine bay from the vertical plane formed by the carb/intake flange all the way to the inner fender, from rad support to firewall, is boxed off, sealed to the hood and frame rail? One huge cold air a$$ box is formed that doubles as a heat shield too.

Good idea on using downspace, I could taper the inlet in from below, like the Reverie intake, should help with a more even distribution.  Trying to keep it simple for the welder though, so I can't get too carried away.  

 

I think I have seen a pic of the race car you mention, is this it?

 

coldairbox.jpg

 

Very hardcore setup, but impractical for my purposes.  It would be very difficult to get a good enough seal, and the air will probably suck in from wherever is easiest.  I notice the back end isn't sealed, and I am curious what the air filter set up looks like off the left side of this picture.  Very cool idea though!  

 

Take a look here - they are talking about turbo setups, but the theory still applies to NA...

http://forums.hybridz.org/topic/61549-turbo-intake-plenum/page-8

What a great thread, Monzter's post #157 in particular is enlightening.  I am curious if the same principles apply without forced induction though?  The air in my carburetor intake won't be pushed in nearly so quickly, so it shouldn't blow past #1 quite so easily.  The flow diverter fins are interesting though.  

 

Looks good! The taper should be a benefit. I didn't do that with my design, but I think enough volume can help make up for that.

Thanks man!

Edited by ryant67

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In ram air or NA applications a simple pressure diffuser at the inlet to the box changes the incoming airflow to pressure distributing ambient pressure equally to all air horns.

 

Study the SU entrance on the later carbs! or even better the Z432 Airbox. We try to,rethink the wheel when these devices have already had countless OEM dyno-testing hours and Cubic Yen Spent on development!

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The other post suffered from inadvertent postbuttonitis before I got these attached.

 

Yes, there are SU-Style Airhorns inside. Filter element is is secondary canister on the side.

 

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So let's not reinvent the wheel here, it's been done!

The Bling Factor of something fabricated looking exactly like something the OEM might have designed is not to be overlooked, either!

Edited by Tony D

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More food for thought there, thanks Tony!  

Came up with a few more ideas, will play around with making models of them over the next few days.  Aluminum is ordered, and I will be using dzus fasteners to attach the box to a backplate held by the air horns to the carb (same as how my ITG Megaflow attached).  Might try and have a lexan window on the top of the box, would be nice to easily keep an eye on what's in there, and it would look 'cooler' too.  

Torn between just using a cone filter, or paying a little extra for a canister style setup that I can force in into by redirecting air at one side at the front of the radiator.  Will probably just see how the cone performs first.  
 

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Version 3

Intake tube tapers in from underside, which should help even the air more evenly throughout the entire length of the box.  Angle adjusted at radiator side, just about lines up with the radiator support frame hole now.  

Pics:

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One final variant to mock up today, then I think I'm done.  

Gareth:
I do get a little bit of reversion, but it doesn't seem excessive - a small amount of black residue now and then.  Pretty sure the fire was caused by multiple things:
- My carbs are running excessively rich (working on fixing that right now, just got a wideband)
- I was having starting issues with a dying ignition switch, which meant that at times I would have to crank on the engine pretty hard while fully pressing on the gas pedal.  I've since replaced the switch and starting requires only a slight press of the pedal now and it starts instantly.  
- The ITG filter likely wasn't deep enough for the horns I was using.  65mm internal depth on 50mm air stacks.


Might as well take the opportunity to improve the set-up.  If I leave enough space, I can always get longer stacks down the road.  Clearance on stack #6 is currently 2.5" with airbox version 3.  I have bought a sheet of 0.125 lexan and plan to rivet (or screw, haven't decided yet) it on as a top panel to this box, so I can easily keep an eye on the horns and what's inside.  

The pipe in version 3 is 3 inch diameter.  I think I will go with 4 inch diameter in the final version.  This means that if I decide I need more airflow, I can enlarge the hole in the radiator support, and only have to remove the silicone reducer and buy a new cone filter.  

Should be moving forward with all this next week.  Sheet aluminum came in today, and I have the lexan and fasterners, so I just need to buy the pipe now.

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Based on dyno tests at a well-known shop in central CA, the TWM box with a cold air cone produced less hp than the same engine with a ITG filter.

 

I didn't put on the TWM box back on when the 3.2L showed up at my door.

 

If your filter caught fire you had other issues not related to the flammability of the filter media. I have over 20 yrs of service from ITG filters in my street and race 240Z without any issues.

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Odd.  Technically, it shouldn't work that way, since the engine will technically be getting cooler air, and even 10 degrees cooler should show a slight gain.  Hard to argue with someone's actual testing results though.  A fellow with a highly modified 240z on zclub (uk club site) has an airbox similar to the TWM (MJP Airbox) and he swears by it and has stated that tests showed gains for him.  He said, she said, stuff I guess.  

I do agree that the fire was due to my own poorly tuned set up, rather than the filter itself; but, at the same time, I have read of at least 5 other Z cars with ITG filters having them catch fire.   

Either way, I've spent most of the money required at this point, so will march on.  I plan on going for some dyno tuning time later this summer, so I will perhaps test with and without the box while doing so.  Understandably, without tuning for and without the box, it's not apples to apples, but it should be interesting all the same.

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You take notice of the shape of that plenum and how it differs from the S20/432 airbox.

Having owned multiple cars with factory Solex and Mikuni Multiple Carb Setups, non use the TWM Design, but are more or less permutations of the S20 Box Pictured above...

 

I think the velocity air, and diffused air inside makes a big difference - this was shown on Monzster's thread under pressure.

If you look at atmospheric pressure as just that, and not 'vacuum' then the plenum design of the S20 and the surge tanks makes sense. You want an area of diffuse pressure surrounding the inlet air horns, in a slightly positive pressure if at all possible... 

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There was one poor guy who coughed into his TWM, the plastic started on fire, melted, got sucked down the Weber and plugged the booster venturi letting that cylinder run way lean and "boom' there went his head gasket.

I ran the TWM Open unit for years. I think the open face helps with the draw into the barrels. Though ultimately it just crumbled from the SoCal Atmosphere. Ran it from 90-maybe Y2K on and off various vehicles with my Mikuni 44's and Weber 40's

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According to Dave Rebello, running one of those style air boxes is detrimental to max power. He recommends the ITG or another open air style filter. There was around a 10HP difference on the 3.2L he built for John Jeffery. John has now switched back to the ITG setup.

 

I run these RAMAIR filters on my Datsun 1200. They slip on and off easy, and stay on, even with rubber chunks flying around the engine compartment on the track:

 

http://www.ebay.com/itm/3-x-RAMAIR-Carb-Sock-Air-Filters-MG-MGC-MGC-GT-6cyl-Weber-40-DCOE-/390873883975?hash=item5b01e62547&vxp=mtr

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It's a shame that the TWM airbox proves detrimental to top end horsepower, but other box designs have been tested and shown improved midrange torque and no loss of top end horsepower.  

Thankfully I not attempting to emulate the TWM design terribly much.  

I don't mind pushing on with this project - worst comes to absolute worst, I still have my ITG backplate and can just buy a new filter and be out some money.  Might as well have some fun and see if I can build something a little different though.  

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