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Making an Air Entry to a Triple Plenum


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Right those who know

I have a small issue with whats left of the original air box -

1 - Its beaten to death

2 - It was a replacement for the one Eales built, and isn't big enough

3 - I need one for the engine

The Plenum is a la piccie - 3 Butterflys, these are mounted on a 10mm ali plate, which I

can mount to but need to have on the engine in the normal place, with a 90mm entry hole for the V8 90

air box at the back. JE Has confirmed the V8 90 air box is fine, and no he oes make air boxes any more :(

So, on the engine it will look like this :

post-22-054245800 1283877825_thumb.jpg

And no, I have cleaned the inlet and trumpet base, not the plenum yet, whated to do this

before it comes apart into bits :)

So, I have enclosed a CAD (Crayon Aided Design) of the best idea I have had yet.

The 3 holes ali plate would have some means made where I can bolt on a curved plate, welded up radiator end, and the bulkhead

end, except on the bulkhead end this would have a 90mm exit to the V8 90 air box.

post-22-028063300 1283878135_thumb.jpg

So, question are

Would the curved bit be better / worse than say a square shape ?

The square shape would give greater internal area for air to enter

do I need to have a 3> 2> 1 type arrngement or is the 3>1 OK ?

I am hoping the above would be ok, I think maybe the square rather than rounded may be better

or would it ? would maybe a sqaure but just rounded off top and bottom rather than dead sqaure

be better etyc - not my knowledge are here, so rather than build one wrong,asking now to get it right

what do those that know think ?



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Not had any experience in tuning V8’s however with that said I have had experience with tuning motorcycles.. and the bigger the air box the better for instant response, snap throttle.

Really don’t know if one shape is better than the other but bigger is better..

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Hi Nige,

On a previous post, some guy's trying to use a twin plenum,

but suggestions came back that this was a pig to set up

and to maintain. Isn't this going to be just as tricky +1?

Or different idea?

Your airbox is a bit BIG. Is your house missing a Dyson now :lol:

Looking good tho'


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Ah :D

The BL Twin plenums had all sorts of probs, they were made by BL quickly and on a

tight budget (now theres a suprise :D )

IIRC they were for a race series, and built for that, they weren't expected to

make it to a road motor, the splindles and shafts suffer from massive wear and

the entire set up is BLs finest from the "Heath Robinson nice but Oh dear" Dept

I can't remember the engines they were used on, but by todays standards althought

tuned up to the eyeballs the cubic capacity was nothing special, and they

had a horrendous EFI Setup even with money thrown at them :lol:

Eales set up is hugely different, and the entire unit(s) are rebuildable but

uses quality parts etc, and design has less forces on the units as the sizes

are smaller but there are more of them. Also bear in mind mine is 2004 vintage,

the twin plenums (If genuine) are equally 20+ years old


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Hi Nige,

Your air box design looks fine, although a few comments worth a thought.

Looks like you have 3 x 75mm throttle bodies, and one 90mm outlet! If so then your 90mm outlet and hose are going to be your restricting feature.

I know from experience that you can get well over 600bhp from just two 75mm throttle bodies. And know that a short length of 90mm dia piping will flow more than enough air for any HP your engine is likely to develop so no real worries there, just seems a lot of trouble using 3 throttle bodies and then an outlet with less than half of the flow potential of the throttle bodies!

However, If you make a cover as your drawing but with an enlarging area as you go toward the inlet pipe, it will be perfect. As someone said, (within reason) bigger is better, but your scale also looks fine! And as you have done with the throttle bodies, Bell mouths with big radii work miracles!!! even in the air pipe entries!

I know I am not the closest location to you but if you would like to play on a flow bench you are welcome to use mine any time you like!


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not having much to go on (my fluid dynamics course was along time ago) but I would expect bigger is better and the extra size of square would be better than reducing the size for a smoother shape. Circles are better as you get maximum volume per surface area but I think the air box is slightly different deal. I'd go for big.

I wondered if my snorkel/air filter was restrictive on mine so I'm going to do a test with snorkel on and one with it off at full revs and see if the MAP value is different. If the pressure is the same I'm assuming it's getting enough air.

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Small addition to my last post as follows.

Air flow is massively important at the sides of a bell mouth / trumpet and you need to keep as far away from restricting this area as possible, (at least 1.5" on your size openings) Air flow directly above the Bell mouth / trumpet is not or hardly affected to as close as 1" from the opening on a trumpet with a well designed "bell" and a 2" throat, You can in fact have an injector right inside the trumpet opening and up to 1/2" above the bottom of the trumpet taper without affecting flow at all on a well designed trumpet with a 2" throat!


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Measurement of the throttle body unit is as follows

Length 246mm X Height 65mm x 15mm thick ali with raduis ports to 3 x 50mm Throttle Bodies

These are held on with 4 x resessed allen bolts 4 x to a body

A La :

post-22-090860700 1283943721_thumb.jpg


Hows about I add a Ali plate behind this, so that with 3 x holes in it to line up with the plate

and the 12 x screw holes this plate could be sandwiched btween the throotle bodies and the Plate

but be a bigger size all around, says adding 30mm to the top and the bottom and the sides ?

This plate would then be 125mm Height x 300mm Length, but then o add 90 degrees bends on all 4

corners so as to give a Lip" for a curved shaped unit to side over to bottom and sides and screwed up with

sealant, the far end (engine end) would be blanked off, the Gearbox End would have a plate and a 90mm inlet

tube, excuse the roughness but something like :

A LA : post-22-007788700 1283943921_thumb.jpg

Would the extra dims top bottom each end be enough ?

Reading you post should this have maybe more space underneath less on top or equal like my scrawly sketch ?

Thoughts please


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One thing to consider, is in effect you're going to end up with two plenum chambers? The main one sat ontop of the inlet manifold as usual, then the second one bolted to the throttle bodies?

Not sure how this is going to effect the pulse tuning effects that you get when using any plenum...

Personally, if i was going to start fabricating anything, i'd fabricate a new plenum box with a single large throttle body on it.

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I would taper it down, as you go further away from the 90 dia entry. The pipercross one is a pretty bad design in this respect. The fact that the intake length will be different for the 3 throttle bodies doesnt really matter I dont think, since the intake air comes back together in the plenum.


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Here you go Nige

post-1339-097102300 1284015588_thumb.jpg

does that make a bit more sense now ? You would need to think about how its fixed to the throttle bodies, bolts all the way through with tubes to stop crushing/distortion is one way, stubs and short rubber pipes - motorbike stylee could be another.



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To be honest I think Nige's drawing is probably the best so far on an "overall" assessment!

This last one looks nice but I am fairly sure that if made to the sort of scale in the drawing, there would be a disruption in flow on the end inlet (small dia end) caused by the proximity of the bell-mouth to the walls. This would need to be at least 100mm wide at it's minimum, otherwise it looks nice, but may be a problem to fix to the throttle bodies.

With the box that Nige has designed, it is big enough not to be effected by flow lengths etc as Daan has stated, and it is simple to make which is always a good thing! The air will be going in to those throttle bodies straight enough not to make any difference.

Tapering down is fine but try not to make it narrower on the width any more than 100mm

Your attachment method sounds good, or you could make a full box and have a simple screw on (with captives etc) flat "lid" covering a simple oblong access hole on the opposite side to the throttle bodies, to get to your 12 fixing screws for the sandwich plate.

a fancy converging 3 tube design would look great but would be one hell of a job to design well enough to tune the best out of it and overly complicated for what is more than likely overkill anyway! A bugger to fabricate too! But sexy ;)


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Would it be better this way :

post-22-002682700 1284020730_thumb.jpg

Rather than all 3 drawing air from the end ?

If so how much between the end holes and the "Front edge, would it be better made sloped (as the pic) or unsloped and

having an equal air gap from the holes on each ? Whilst I may be overworrying I can bild this anyway I want to

best seems the right idea and preferably just the once :D so importnat I consider before making


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Nige - just tack something together. By the time you've finished it'll be a tow point from 12mm plate anyway, and the weight gain will absorb your 800bhp and I'll be able to chug round you in my little diesel Ninety... :lol:

I expect the packaging context will dominate the argument anyway - no point having a perfect plenum and then six miles of trunking to get to the snorkel. Don't make it impossible to fabricate either...

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If you were talking fast flowing "wet" air then yes it would be better,

However the aim of your air box is to run at "Zero" under pressure and to run fairly slow air speeds, your airbox will flow only air and no atomised fuel, therefore your first design will work admirably! Extra trunking and bends in the air pipe will cause much more problems than any pitfalls in your design. Stick with your basic design, it looks fine and will work well! AND you can finish it this weekend rather than taking days of work to make a fancy box for no actual gain!

Good luck,


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