Right, I'm bored shitless sitting in sainsbury's car park waiting for the mrs!
And I was just trying to work out why the 500 plenum is supposedly able to support 600bhp+ and a 4x4 430bhp ish?
Before stating the obvious of 8 injectors ie. More fuel, I was thinking more along the lines of restrictive properties, I was also trying to find a thread where I read that the 4x4 was good for near 600bhp.
Is there that much difference in size/volume or is there a massive difference in the elbow?
Also heard people say that the flutes/trumpets are more the restriction rather than the plenum it's self!

Any thoughts or proven facts?

I know it's a deeper plenum, has larger throttle body and better elbow, so it all adds up to better airflow. The 8 injectors are irrelevant in the flow rate of the plenum.

Throttle body aside, how much deeper is it if any? and wouldn't a spacer on a 4x4 surfice to make up for the gains of the 500?

Spacer would help flow in the plenum I would have thought but elbow design would still make a difference, although aftermarket plenums are available that flow better

IIRC the 500 plenum is 10mm deeper and there are people that sell the spacer like MK on here

4x4 manifold with elbow mod has been on 500+ cars if im not mistaken

ive got a 4x4 with 10mm spacer and ive just cut off the elbow and enlarged the hole where the air enters the plenum.
the area of a 60mm TB is 2828mm, the area of the hole where the air enters the plenum is 3000mm (120mmx25mm) so obviously more than the TB
HOWEVER
a 75mm rs500 TB has an area of 4418mm
so you might get more air in through the TB itself but not into the plenum.
also behind the TB flange on a 4x4 manifold the hole is only 60mm across too.
plus the actual elbow isnt very good for flow.

this is the hole i mean


i have opened the hole up on mine to 185mmx25mm = 4625mm area



so in theory i will be able to pass more air into the plenum/engine than an rs500 inlet, and with my new elbow it should flow in better.
not that i will ever push it lol..



im assuming an rs500 inlet also has the hole in the base of the plenum that is as big or bigger than the area of the throttle body and the elbow is less restrictive.



as you can see from the 1st pic the hole is off centre and more air is directed to the rear of the maifold, i have evened up the hole so it is in the centre so should direct more air to cylinders 1&2

ps you can also get 14mm spacers for 4x4 manifolds.

Stu you'll have to let me know how that works out as that was where my thoughts were heading, I've got a spare 4x4 plenum so wouldn't mind giving it ago.