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Thanks Nick,

 

I'm just planing an airbox for my SU's and might change the design slightly to allow it's use in a FI setup in the future.  My fabrication skills are purely theoretical, but I recon this looks like a good mix of achievable and ultimately worth doing.

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Hello Nick,

 

"injector angle is 45º.  Because it's easy to arrange - I don't think it's especially critical - works well on the Vitesse."

 

Kas Kastener did some dyno experimentation with injector location; he simply connected some shortish lengths of rubber tube to the end of the intake manifolds and just cut some slits in the tube to poke the injectors through. He found it didn't really matter which direction the injectors were, even pointing against the flow made no difference. He did find that having the injectors further away from the cylinder head gave an increase in power, how much I don't know. Perhaps you could find some information on his website?

 

Alec

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The limit on the size of a Pi plenum/airbox is the position of the wheel arch, and 6" diameter is the max.

The larger the diameter the better, as the air slows down and its pressure rises (Bernouilli)  so it's easier for the engine to draw it in.

 

A 2.5L enghine will draw in that much air, or a little less due to drag etc, every two revolutions, so at 5000rpm, 15,625 L of air per minute travel down the intake.

On the standard 3" diameter Plenum, that air will enter at nearly 40mph

Increase the diameter to 6" and that speed will halve, so it's worthwhile.

 

John

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Test fit today.

P1130571s.jpg

 

Not too bad.  Too close at the front.  

P1130569s.jpg

 

TB is angled slightly down and outwards at present but needs to move back about 30mm, point more down and less out.

 

Clears the wing -

P1130580s.jpg

 

if the throttle mech is on top, just fouls with it on the side, though I think will be ok with the mods mentioned above.

 

Will be ok.  In fact things appear worse because the test car has a full width radiator in the standard position.  Would be easier with a std rad or with a forward tilted rad.

 

Nick

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.

On the standard 3" diameter Plenum, that air will enter at nearly 40mph

Increase the diameter to 6" and that speed will halve, so it's worthwhile.

 

John

Surely double the diameter makes it 1/4 the speed? In which case it is very worthwhile. Downside is the "packaging". I am looking at my dcoe pattern ITB's, currently fed by a pair of dcoe style k+n's. Lots of induction noise (not great at all). So looking at a dolly sprint airbox, used empty, with a length of 3" pipe replacing the pair of std air feeds. I can't sensibly get a deeper airbox in, and I am working on the theory that the rather expensive pipercoss airboxes and others all use a 3" feed that all will be well. Fingers crossed. But it is making me think....

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Absolutely - it's the volume divided by the square of the radius!      My Doh!

 

Even better is the resistance to flow, which is related to the fourth power of the radius.

So doubling the radius reduces resistance sixteen times!

 

JOhn

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Flow along a rectangular section tends towards that in circular tube, as the corners contribute little towards flow.   

 

An oval section would be better, but the flow rate is then calculated using a special version of the Poiseuille equation.

Instead of the fourth power of the radius of a circular tube (r^4), the multiplier is (ab)^3/a^2+b^2 (where a and b are the radii along the major and minor axes of the ellipse)

That's a complex expression to evaluate, but would produce a lower flow than in a round tube of an equivalent size.

If you can keep the minor axis radius at 6", you could make the major greater than that, and then the flow could be better than a simple 6" diameter tube, but the question would be how much bigger than 6" to compensate for the lesser multiplier?

Flow in a rectangular tube would approximate to that in an oval tube with sides 2a and 2b in length, but I won't try to calculate Q = − ∇P/12η Ïwh^3 [ 1 − Sigma(Infinity, n,odd) (192/ Ï€^5) (h/w) (1/n^5) (tanh(nÏ€ w/2h))] !

If you really want to, see:  http://web-files.ait.dtu.dk/bruus/tmf/publications/3week/jun2004hydraulicres.pdf

 

The equivalent diameter of a circular tube (D) to a rectangular one (a x B) is given by the Huebscher equation:  D = 1.30 x ((a x B)0.625 / (a + B)0.25)    

 

The same for an oval tube is given by the Heyt & Diaz equation - D = 1.55 A0.625 / P0.25 where A and P are the cross section Area and Perimeter of the oval

 

John

Edited by JohnD
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I agree that at high air flow velocities, when it's acting as a duct, not a plenum, the rectangular tube will have limited benefit over a round one*.  However at lower velocities, when it is acting as a genuine plenum, then the extra volume of a rectangular airbox  will allow a smaller reduction in pressure each time an inlet stroke occurs and more central supply location should allow more even flow across all cylinders.

 

 

 

* although I'm not certain it will actually be worse, certainly in HVAC ducts (where flow tends to be much slower so could be a different kettle of fish) a 300 circular duct has a greater pressure drop than a 300x300 square duct

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I did stumble across some info (american fori on scoobies and hondas, but probably still applies) regarding diameters of ducts. This suggested that it can be tuned to suit particular engines, with some evidence that a 2 1/2" duct was working better than a 3" on a particular car (350bhp ish...) I guess this all comes down to the plenium and duct becoming part of the induction system and is either incredably complicated in terms of calcs, or is done by trial and error. 

Indeed, a friend who was into drag racing (street cars, 70's and 80's) was talking about tuning and top fuel stuff. However, for what we are looking at I suspect the KISS rule applies, and we would be better off just not eating as many pies....

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I think that you need to address the issue of changing static pressure above the intakes. Ideally they should be equal over each hence the reducing cross section . That Jenvy box may be OK for a carb setup but for a single throttle body would give a very uneven intake pressure above each intake and therefore uneven charge per cylinder. As each bell mouth should really have at last a 180deg semicircle.

 

No need to reinvent the wheel... copy the attached best as possible... no reason the TB mount should not be rotated 90 deg to fit better. This could be done with a simple sheet fabrication to approximate very well, no need to be cast and I haven't heard anyone talk of supercharging yet...

 

A couple of notes attached.

 

IMHO of course.... :)

 

N

post-785-0-48064600-1447148952_thumb.jpg

post-785-0-92707400-1447148974_thumb.jpg

Ideal ram pipe.xls

Edited by Millstone10
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Millstone10 that's a very nicely designed piece of kit!  It will act as a plenum at low flow velocities, yet when the flow increases the plenum acts more as a duct to direct airflow into the trumpets.  I believe the tricky part for DIY fabrications is getting this high velocity ducting to work effectively.  At high velocities you have to start thinking about the momentum of the air and if you have air traveling down a tube running perpendicular to the inlet runners (like the PI style systems) then it is possible to generate -ver pressure at the first runner (same principle as an airbrush) and large +ve pressure at 4/6 cylinder runner as the air hits the end of the tube and the momentum (dynamic pressure) is converted into static pressure.

 

As people don't tend to have issues with these systems this then I guess the theoretical problems I've mentioned above don't really have much of an effect at the sizes and flows we are talking about.

 

My proposal reduces the problems I've mentioned above by maximising plenum volume at low velocities to keep the pressure all inlets see as constant as possible, and at high flow I'm letting the air enter the airbox across the short dimension so I'm converting much of the momentum into pressure within the airbox.  Ultimately this won't be as effective as the cast item above, but it should be within my fabricating capabilities!  Also as I'm keeping the SU's for now I've only got to worry about 2 outlets to the airbox and I'm planning on smoothing the flow into them as per this design: http://mgaguru.com/mgtech/power/pp104a.htm

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egret,

Beautiful design for a bell mouth intake, but necessary?

 

I have a copy of Dave Walker's column in the old CCC magazine from 1994 (!).   He was using his flow bench, and left the bell mouth untightened on a Weber he was testing.  He was surprised to see that at full flow, it tried to slide outwards on its studs!   The velocity was so high at the surface of the bell that the pressure had fallen and normal atmospheric pressure behind it pushed it off!

 

This was a conventional tube type with the end belled out, so that there was a sharp edge at 90 degrees to the axis.   He wondered if a complete radius on that edge, as in the design you posted, would prevent this, so he built it up with, I think, Plasticene, and - it didn't.   But more importantly it made no difference to peak flow.  See pic.

 

John

 

PS Sorry! Upside down!  Either turn your screen that way, stand on your head, or take my word for Dave's!

post-690-0-41790700-1447176585_thumb.jpg

Edited by JohnD
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It was a while ago when I read up on the design and drew up some plans, but I think there are a series of those pages where they review the effects and I believe they found some boost in power, I could well believe that out didn't make much difference at peak flow as everything is likely to be turbulent, you might experience some low to mid gains though. Main reason I'm planning to do these is to get some tutoring time with my dad on his lathe and mill!

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Hi

 

First off, let me say this area is NOT my strong point. its been a while since I done any reading on the subject.

 

BUT as I am looking into fitting a Turbo to my GT6 mk1 . one thing I did read (Not interested in side tracking from this thred, so please no coments re the turbo.)

 

 was the need for a LACK of air behind the butterfly. (yes obviously, a turbo is producing boost) but surly the pricapals are the same.

 

YOU have a huge space behind the butterfly/metering device, plus the long intake pipes, 

 

when you shut of your butterfly, your enginge has all that air to use up no???????

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Urge,

 

Agreed, the single throttle body is a compromise which allows a simple throttle linkage and easier tuning.  The downside is that the plenum volume is between the throttle body and the valves. This air volume can expand and contract which will act to slow the response of the throttle control (the wave dynamics gif half way down this wikipedia article might shed some light on my point as it shows the response of a volume of air to a throttle opening and closing https://en.wikipedia.org/wiki/Cylinder_head_porting.  I hadn't really thought about this affect as my plenum is going infront of the butterflies, but I agree with Nicks setup there is an advantage to keeping the plenum down to a reasonable size.

 

This is the single throttle body vs Individual Throttle Bodies argument.  ITB's give a better throttle response because there is very little volume of air between the metering device (the throttle body) and the cylinder. The practicalities of ITB's are more complex meaning you have to link all the throttle bodies and balance the system correctly, which are steps you can avoid when using a single throttle body system.

 

*when you are looking at adding boost then the higher inlet manifold pressure will likely increase the lag although single throttle body boosted systems definitely exist.  My view would probably be to go the KISS route and go single throttle body if I were doing it from scratch, if I had a working setup of ITB's then that would appeal to the theoretical rather than practical engineer in me!

Edited by GT6MK3
Fixy linky
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The board can't have taken on your link properly, Urge, here's another go: https://en.wikipedia.org/wiki/Cylinder_head_porting

 

 

Many years ago, people realised that like an organ pipe, the length of the intake tube could be tuned to self reinforce and amplify the vibration of the reed or intake valve.

If that was done correctly, then the pressure wave would be higher at the moment the valve opened, rather than the reverse, assisting intake.

This diagram of intake length to revs comes from the 60s, when it was all intuitive rather than scientific, but none the worse for that.

The length is inversely proportional to revs, so that until you can achieve higher revs than an ordinary Triumph engine can safely do the tubes need to be infeasibly lengthy.

But maybe your twin-cam can?

 

For this reason, the diagram is more useful for exhaust design, so Helmholtz resonataor theory may help.

Here's one page from Stone's "Introduction" about it:

 

John

 

post-690-0-54854400-1447416406_thumb.jpg

Helmholtz resonance.BMP

Edited by JohnD
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Jhon I never intended to add a link.

 

The wave tuning you speak of (intake) is more reconised in two stroke engine exhaust tuning.

 

Four stroke both inlet and exhust wavery tuning is still unpredictable.

 

 

Not to be advised.

 

I take length is more to do with moving mass that's already moving.

 

Sorter inlet tracks suit high revving engines.

 

We're as long inlet track suit low revs more torc.

 

If you look at some of the modern production pellums.

 

You will note the have short tracks and the main pellums tappers down to lesson the total mass of the unit.

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Motorv8id

 

Are you referring to the content of my reply.

Or my spelling and grammar? ???

 

If it's the latter. Don't be so f ing rude!!!!

 

I am dyslexic. And if it is my spelling and grammar you are talking about.

Well that would be like having a go at someone in a wheelchair because they can't run.

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Motorv8id

 

Having re read your reply, AND spelt speed talk properly.  I have found the website you was talking about,

 

speech talk was probably not was you meant lol

 

that error was down to the dreeded spell check on my phone. sorry

 

 

 

I have not gone over the site speed talk. but from what I have lernt over the past.

 

I am not saying a wave is not formed, nor am I saying its use would not be VERY benifical in tuning.

 

But the wave is very unpridictable, it veries a lot through RPM.

 

laymen like us will never get it right and we would do more harm than good.

 

thats my undertanding of it.

 

I am happy to learn

 

being dyslexic make the subject of reading a real head ach.

 

VERY happy to learn about tuning.

 

perhaps share a direct link for all to take advantage of (thats not said sacasticly)

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