Passing thought...... Has anyone experimented with changing the size of the intake manifold plenum when making other changes?

I've heard that as we increase throttle body size, we should also be shrinking the volume of the intake manifold. Has anyone experimented with that?

How about when we change cams?

Thanks for our thought!

John


[Modified by Racebrewer, 3:03 PM 3/30/2002]

 

The smaller the intake manifold the quicker the throttle response, generally.

Caveat!!!! There is so much wave dynamics going on which effect spark timing that ANY generalizations are fraught with errors. The manifold volume determines power delivery and must be matched with intake runner geometry [length and cross sectional area] and the throttle body diameter. Also, this effects the cam timing which must be tuned with all of the above. This can effect valve diameters and exhaust manifold geometry as well.

Few have access to the tools needed to perform such a Design of Experiments. Complex modeling software is used to determine much of this.

 

oh yeah? that's a new one to me...

 

yep big plenum-> small TB and small plenum-> big TB

you are balancing flow velocity for flow quantity.

I checked out Chris Rado's 8.91 et RPS turbo IM. HUGE plenum for HUGE flow capacity.
http://www.turbomagazine.com/archive...atures02.shtml



Us N/A types have to find that fine line between getting enough flow velocity for good mixture quality at rpms below peak torque and enough flow quantity or flow capacity to ensure we max out on the inlet side of volumetric efficiency.

I always found this graph by Jim McFarland to be a good teacher of when volumetric efficency is optimised, what do you need to work on...




if volumetric efficiency is maximized you get a curve that looks like the torque curve. There is a point when flow reaches a max. speed and it saturates the conduit. Any further increase in flow speed past this saturation point and you don't see a further increase in torque. So let's say we optimise flow speed. What you expect to see is the volumetric efficiency curve overlap and become identical to the torque curve. What you put in should come out as power.

But as we can see these curves don't overlap exactly or identically. Where they don't align means that any more air you stuff in won't gain you any more torque.

As the caption below the figure states, at rpms under peak torque, mixture quality as it enters the chamber (i.e. keeping the fuel atomised and getting a stratified charge in the cylinder) is the area you need to focus on improving if you have adequate flow capacity.

At rpms above peak torque, the limiting factor to work on is expelling as much of the burnt gases out of the cylinder becomes the limiting factor to getting more power.

One last tidbit: since the integra has a low rod ratio, the piston speed away from TDC on the intake stroke is faster than say the B16a for example. The piston speed away from TDC on the b16a surpasses the B18C piston speed at a later crankshaft degree on the intake stroke. So the initial suck or pull to draw in air is greater in the B18C due to it's piston geometry. This means that the initial low-mid rpm flow velocity on the intake stroke is pretty good compared to a B16a.

So from a IM plenum design point of view, the B18C's don't need help in generating flow velocity at low-mid rpm. They need help getting more flow quantity coming in. This is reflected in the design of the N/A aftermarket IM's for the B18C. The Skunk2 IM plenum is larger than a stock GSR plenum for example.

We didn't even get into tuning IM runner length and diameter....which is another ball of wax.

The bigger plenum has enough flow capacity and needs a little kick start to ensure enough flow speed is generated initially. This is why they prefer smaller diameter TB's. Narrower orifices generate more flow speed, simple enough. So you get enough flow energy into the plenum with a smaller TB and the plenum delivers the needed amount of flow quantity.

funny how one thing relates to another and how design of one part has to be made compatible to another....

slapping on a 75 mm Infiniti Q45 TB or BBK TB onto a big plenum will kill flow speed and affect mixture quality.

It's funny that the Rado 38 psi turbo uses only a 62mm TB with that humungus plenum. Big plenums deliver the volume for big peakier power but where you need help is the flow speed at low-mid rpms and that's where the Tb size and intake head port size comes in.

yes valve timing and spark ignition comes into play wrt where the peak cylinder pressure will assist in making power but when it comes to IM plenum and runner design, the TB size and head intake port volme/shape/ valve seat angles have a bigger relationsip and influence.

okay not writing any more.....or else Chad will say I'm writing another book and say "my God, Tuan...sheesh"