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Old 13th October 2014, 01:50 PM   #506 (permalink)
mambastu is looking forward to fork lift powah ! seasoned Member
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Join Date: Feb 2002
Location: South, Uk
Cars owned: TH1 R32GTR, Ex-RB25DETT RS13
Posts: 1,980

Good to see you continuing with budget solutions. The GReddy replica plenums are a good thing if you're prepared to spend some time on them. With mine I found that the quality of the casting is as good as the proper GReddy RB25 manifold I've used in the past but the machining isn't great and they do need a bit of work to seal.

I found that all the flanges needed to be flatted. The worst face was between the plenum halves. On mine I struggled to get a local machine shop to take it on for a reasonable price due to the curved shape of the plenum outer so I bought a cheap glass table locally off Gumtree. Threw away the metal frame and G-clamped it to a flat wooden bench. Stuck a wide, cut up, sanding belt to it and spent a few hours re-facing the two halves.

The o-ring groove around the perimeter wasn't the same width or depth all the way around so I corrected that a bit and then used a 3.5mm seal made with a universal O-ring kit. Then made up a pressure test rig and blew 35psi through it and found that the three metal pipes that stick out of the plenum also leaked a little so they were sealed on the inside and outside.

The runners needed to be port matched at the throttle body face to correct the alignment but then my GReddy RB25 plenum also needed that when I used it.

I then spent a fair bit of time porting it and ended up with this:


Also checked the volume of the standard plenum against the GReddy replica and found the following:

Standard plenum 3750cc
GReddy replica 5750cc
GReddy replica ported 6750cc

Hope that helps



Last edited by mambastu; 13th October 2014 at 02:03 PM..
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