Chris White

NZ – you need to change the way you are perceiving this stuff. Even thought there are waves don’t try and visualize it that way. Look at it as an acoustically tuned port – basically the same as a loudspeaker or a musical instrument.

You want the enclosure (manifold) to resonate at the frequency you are tuning it to.

The tricky part is that most tuning info you see is based on the fact that the other side of the throttle is open to free space. Not true with our cars when the manifold pressure is the same as the intercooler pipe pressure (Wide Open Throttle). You can get into the situation where you are dealing with two tuned systems – the individual runners and the throttle body/intercooler piping. My guess is that you can decouple these with a distinct change from the runners to the plenum (air horn) but a smooth transition may blur the separation.

This stuff is super critical in highly tuned NA motors but it is not as critical for turbo motors. The NA guys are trying to increase the VE of the system to 100% or greater, we can do that by twisting the boost ****.

To directly answer your question – the plenum shape (as long as you don’t get too small or too constricted) is not critical to the tuning of the system.

Since you want to contemplate this stuff –how about this question – If you do make a single throttle pre cylinder set up where to you measure the tuned runner length from?

Chris White

ehall

For a thread that started with a drwing of Special's Tool, this is getting good!

NZ951

Thanks Chris, I started changing my perceptions of the waves after Dan's post, its tricky to visualise! I actually decided against ITB's. For 2 reasons, 1, it would over double the cost of the project! 2, The throtle response gained is not THAT important to me at this stage. So I have not put head space into the runner length with single theottle per cylinder. But I would suspect the intake valve and the point where the space the runners feed into become a more common volume, in the plenum. Thats a total guess by the way.

johne

Man that dual plenum does look like a pretty smooth way of equalizing pressure across the runners.

I have also been wondering if intakes perform differently under vacuum and boost conditions. I wonder if there is a way to simulate boost pressure on a flow bench?

-John

NZ951

I dont see how the dual is better than a properly designed single plenum.

J Chen

I think you could look at the dual plenum
as a damper for air. It will smooth out
the turbulance which will aid in consistant
& even filling of the cylinders.

RKD in OKC

My home heat and air system has a plenum to help even out the flow to the different lengths of runners throughout the house. It keeps the output of the fan from forcing the flow to the vents directly in the flow from the fan and the shortest runners. In this case insead of even flow into cylinders, it is trying to acheive even distribution to the rooms of the house.

Again, if I were going to design my own intake for the 944 Turbo, I would seriously take a look at the lengths, sizes, and dual length runner design of the newest 928 models to see if there is any way I could fit that type of design on a 944. I think the only reason the newer Porsche models don't have this design is that they are using vario-cam to widen the torque powerband instead of intake runner tuning.

NZ951

Hmmm found a few 928 manifolds, I cant see how they could be made to fit easily, they have too much form around the plenum to runner section.

I have come across this though...

Design techniques for engine manifolds : wave action methods for IC engines /
Desmond E. Winterbone and Richard J. Pearson.

Seems to be the bible!