Tapered twin plenum design.

**Steve Webb** · 06-02-12, 10:25

I was reading up about tapered twin plenums yesterday, quite an interesting idea, but I was wandering just how much of an advantage they were compared to a 'normal' well designed single plenum.

This is what I'm talking about:

The join between the tapered plenum and main plenum acts as a long thin entry to the main plenum, meaning that the distribution of air into the plenum is much more balanced.

**mpampis_** · 06-02-12, 11:48

The advantage of this, is that is stocking air. So I guess when you need air at WOT you already have until the turbo spool up.

**1NRO** · 06-02-12, 12:37

The problem usually on that type twin plenum manifld is the dimensions of the slot, this is hugely important and the whole concept doesn't work if it's wrong which it usually is, often by a very big margin.

Two main (there are others too) traits are seen with a twin plenum, one is even flow distribution and another is control of localised pressures.

I keep meaning to do a decent write up on the manifolds I've recently made, just never knuckled down to do it justice. I'll post it on here when I get it together. Couple of pictures below of the manifolds for interests sake.

By fabricatefast at 2012-01-08

**1NRO** · 06-02-12, 12:45

As far as improvement is concerned over a decent single plenum it hinges on how well the twin plenum is dimensioned but presuming the twin plenum is also good I believe the difference is night and day.

**Steve Webb** · 06-02-12, 13:18

Whats the second rail in the second pic for?

I was curious about the twin plenum as normally there are loads of rules and guides concerning the various inlets and runners, radius' and such. But then there just seems to be a square cut slot, slap bang in the middle of this supposedly better solution.

**1NRO** · 06-02-12, 14:08

The second rail is another fuel rail, for four extra injectors or the option to just run those ones. Out board injectors if you like, good for atomisation as rpm rises and to give the charge air an extra cooling just before disapearing into the cylinder. Means also you can run eight smaller injectors rather than just four big ones, again better atomisation as the injector gets to open properly at lower rpm rather than a big injector trying to supply a small amount of fuel. Basically loads of different reasons why it's all good

The slot is a pretty crude way of doing the twin plenum, it's not whats inside mine

It'll work fine IF it's the right size, very very rarely the case.

Yeah, there's a lot of little extra details that can make some inlet manifolds better than others. Radius entry into the runners is just one area that can be tailored for different objectives. Actual runner length is a tuning tool if the runners are sized to target a rpm point (actually the manifold above bridges two orders of harmonic fequency so benifit twice through the rpm range) that will benifit from sonic pulses assisting the incoming charge, this can amount to quite a few psi of FREE assistance at different valve opening points. Another coule of ictures below, one showing the radius entry to the runner and one that shows the manifolds out flowing Guy Crofts flowbench! To that in perspective, Guy works the Kappa to flow low 150 cfm's @10 and the same runners as I used to 166 cfm @10", the flowbench maxs out at 180 cfm. It managed to pull about 8" which allowing for some error (not the right way to scale it up) would be showing up around +200 cfm

Flow isn't really the focus though (in that picture they actually are flowing more than that but a change was made to reduce the flow but enhance other features, this was mid fabrication and hence the taped on plenums), for me the main effort is the efficency, even distribrution and harnessing the sonic/harmonic pulses. There's a lot you can do to make some very big differences, inlet manifolds are important IMO. The manifold in the pictures above is actually going to be bolted onto a Q4

By fabricatefast at 2012-01-10

By fabricatefast at 2012-01-15

**1NRO** · 07-02-12, 09:09

I tried to find this link yesterday but couldn't, just spotted it now. It's a decent read on the concept of the slot style and I'd hope shows that there is merit in the concept. Audi don't do so bad at Le Mans with it.
http://www.bufkinengineering.com/intake%20manifolds.htm

**Juan AR155Q4** · 07-02-12, 13:53

Great reading.
Are you using some software to simulate or just a try & error design?

**1NRO** · 07-02-12, 15:59

Some software, some hand done calculations, some engine theory, some assistance from others, some common sense, some hand skills, some flow testing, some long hours,a lot of influences is the truth. Most of all it's a case of one part influencing the next, a balance if you like between all parts around the engine. As good as the weakest link IMO, that's the problem when you start messing with what Fiat/Lancia/Alfa provided, they had a balance that achieved their targets and it's very easy to upset that balance when upgrading power.

**1NRO** · 07-02-12, 16:18

Mpampis brings up a good point about "stocking" air. This is something I call pressure recovery. It is linked to the inlet manifold in that it helps a lot if the engine can take a gulp when the throttle is cracked open, picturing the plenum as a lung I relate it to an athlete being able to have a big breath rather than a full time smoker/slob that struggles to breath. The big problem though is that the lung is only as good as the airways feeding it so if the athlete suffers from asthma and suffers an attack midway down his 100 metre run he'll fall way off the pace. This is why small intake pipes on an engine that is being improved is a big no no.

**Steve Webb** · 07-02-12, 18:06

Just thinking outside the box here, and this by no means take into account the practicability of runner length or location of inlets, but wouldn't the best design of a plenum look something similar to an exhaust system.
A lengthways cylinder being the main plenum, with the inlet runners branching off at one end in a similar style to a well designed exhaust collector. The inlet to the plenum being at the other end of the main cylinder.

Just another one of my wacky ideas, but it would minimize the flow imbalance between inlet runners, wouldn't it?

**1NRO** · 07-02-12, 19:43

good to think outside the box.

Two problems spring to mind with an inlet manifold like you describe.

With the cylinders being spaced a fair distance apart on an average engine and so to the ports that feed then there'd have to be a mixture of runner shapes to reach to different cylinders. Say the manifold was central and the two centre bores were fed with straight runners the outer runners would need to bend to reach so there'd a variation in flow across the runners and quite likely in length too. Two things that need equalised.

The small area feeding the collector would create a problem in the ability to breath, like the lung description. The engine would only be able to draw upon a small area, all runners gathered in that small area wouldn't be able to keep up with demand, a time/area situation.

**1NRO** · 10-02-12, 15:44

A relevant picture I thought.

**Steve Webb** · 10-02-12, 17:17

Is that from a simulation? Can't think of anyone making a straight 8 engine anymore.

None the less it shows up the huge difference in air velocities between cylinder #1 and #8

**1NRO** · 10-02-12, 17:47

It's just a general impression of what happens in a single plenum. Clouded a bit in that it's a steady state flow example whereas on an engine there's valves in the way. It needs to be looked at as a distribrution simulation rather than a velocity simulation, velocity is very different in a running engine.

**Evodelta** · 10-02-12, 21:46

Originally Posted by Steve Webb

Is that from a simulation? Can't think of anyone making a straight 8 engine anymore.

None the less it shows up the huge difference in air velocities between cylinder #1 and #8

Fancy CFD pics are all over the 'net, you must remember they are only as good as the people who wrote the programs. If they had no practical experience of what is going on then what they show isn't of much use - it's easy to be taken in. What is it they say? Rubbish in, rubbish out...

**1NRO** · 11-02-12, 05:57

Oh dear, you summed it up Martin, rubbish in rubbish out or as more usually stated GIGO, garbage in, garbage out.

If your going to post a "fancy" CFD picture at least ensure that the velocities shown are relevant to the application. With that picture showing at best 139 m/s (456 fts) down the runners it's NOTHING like a running engine, barely much more than a flow bench test of 28" h2o