Plenums and runner tube length?

[T0nyGTSt]

One of the things of the original RB20/25 intake plenum is that the tubes are thin and long.

This apparently helps with low end power and torque.

The Greddy plenum has it's own runner tubes and they are big and short ie. making good high end power at the expense of low end.

One of the complaints of the various aftermarket plenums is that they bolt onto the OEM runner tubes.

Is this a good thing?

They make for neater IC piping and all the apparent benefits, a bigger plenum volume for high end and the OEM runners for low end.

Is this 'new thinking' right?

T.

[DoughBoy]

One important factor of intake runners is determining the velocity of the flow. Smaller diameter runners give high velocities, which fills the cylinders alot quicker. The problem is that at higher rpm and flow levels, the small diameter actually restricts flow and hurts top end.

Longer runners work on a similar principle, that the air speed is high and contributes to cylinder filling. The "ram" effect from the force of the high speed flow assists, and when design to work in conjuction with pulsing from the "echo" of the flow off the back of the closed/closing valve can fill a cylinder more efficiently than if it were an independant runner type manifold - similar to your old type Weber manifolds.

There are so many variables to calculate in designing an intake manifold that it's impossible to properly design one without alot of R & D. I think that the modified standard lower/ big plenum upper is a bit of hit and miss - it takes away some of the restriction in the long standard upper plenum but the shape and volume of the aftermarket bolt on plenum could be a bit... sub optimal. Of course it isn't going to be as good as a built-from-scratch manifold (like the Greddy), but then again it's price should reflect that (and only be around $500-700).

[T0nyGTSt]

Price is not such a big deal really because whichever way it goes you're probably gonna take it up the arse in install charges, FMIC and all that jazz.

But the dyno charts of a Greddy intake sure is a sobering thought. I think a new plenum on the standard runners might be a good compromise but who's plenum to use?

T.

[TokyoTaxi]

DoughBoy,

Can you please contact me on 0414-862-626.

Thanks,

Scott.

One important factor of intake runners is determining the velocity of the flow. Smaller diameter runners give high velocities, which fills the cylinders alot quicker. The problem is that at higher rpm and flow levels, the small diameter actually restricts flow and hurts top end.  

Longer runners work on a similar principle, that the air speed is high and contributes to cylinder filling. The "ram" effect from the force of the high speed flow assists, and when design to work in conjuction with pulsing from the "echo" of the flow off the back of the closed/closing valve can fill a cylinder more efficiently than if it were an independant runner type manifold - similar to your old type Weber manifolds.

There are so many variables to calculate in designing an intake manifold that it's impossible to properly design one without alot of R & D. I think that the modified standard lower/ big plenum upper is a bit of hit and miss - it takes away some of the restriction in the long standard upper plenum but the shape and volume of the aftermarket bolt on plenum could be a bit... sub optimal. Of course it isn't going to be as good as a built-from-scratch manifold (like the Greddy), but then again it's price should reflect that (and only be around $500-700).

[TokyoTaxi]

DoughBoy,

Please diasregard. Just got your email.

Cheers.

DoughBoy,

Can you please contact me on 0414-862-626.

Thanks,

Scott.

[JasonO]

But the dyno charts of a Greddy intake sure is a sobering thought.

Tony,

Where can I see these dyno charts ?

What sort of improvement could be expected by replacing the pleanum alone ?

[KamikazeR33]

what we have done is shortened the std runner length, but increase the plenum size to compensate

we seem to be getting very good results with this setup