Size of 94-95 inlet plenum?

SeafoodSC

Registered User
How big is the inlet plenum where it meets the tb on a 94-95 SC?
Is there any benefit of installing a larger tb without modifying the inlet plenum opening?
 
I know it is smaller than the 70mm tb that I used to have on my car. I had to grind it out a little to make the transition between the tb and plenum smooth. Now I have the magnum powers inlet and their tb. Can you say big. Wow that thing flows some air!
 
The throttle body opening is definitely a little smaller than 70mm, I had to take off maybe a couple mm all around. I did some measurements of the cross sectional area when I installed this stuff.

Stock '91 TB: 65mm diameter (as measured), x-sectional area: 33 cm^2

BBK 70mm TB: 38.4 cm^2

94/95 opening to supercharger: 104mm x 37mm with rounded corners of 7.5mm radius: 38 cm^2

On my S-model, the opening on the supercharger was larger than the plenum opening, so I opened it up by a couple mm all around. Still, I think the internal size of that plenum is such that a 70mm TB is probably all you need. With a bigger Magnum Powers plenum you would see more advantage from 75mm or 83mm TB.
 
So I guess I should plan on grinding the inlet plenum down a bit to get a good match to 70mm.
Thanks for the info.
 
Just be careful doing your porting. the rounded portion of the inlet (behind the square boss that the TB mounts to) isn't even 70mm on the OUTSIDE! If you try to port match to the bigger TB too far into the inlet you will end up with a hole.
 
You can go nearly 76mm on early models for sure. Stock the inside measures 64mm and on the outside measures 75mm you have to gently taper it as you pass through the flange and blend it as you go.

Jeff
 

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You can go nearly 76mm on early models for sure. Stock the inside measures 64mm and on the outside measures 75mm you have to gently taper it as you pass through the flange and blend it as you go.

Are you saying the earlier plenums are larger or just have more material to grind off? :confused:

So really you just port the flange area and blend it into the inside. How far in do you take it until you stop grinding?

Thanks for the info everyone.
 
If you look closely at any intake plenum you'll see that they were hand-finished on the outer surface during manufacture. So...besides dealing with core-shift during casting, we have to contend with the quality of the finished product that wound up on our favorite cars, strip the finish off a plenum & you'll see what I mean.

As with any mod, measure twice/port once. The particular plenum that J57ltr posted a pic of was a very decent casting to begin with so it was transitioned with ease. It's transitioned about 1 to 1-1/2" into the bore, more than enough to create a somewhat smooth exchange from throttle body to plenum. Minimum wall thickness in some areas looked to be 3/32" in spots but overall it's still strong enough structurally to do the job of holding the throttle body against throttle cable action & the torque transfer from the MAF tube.

Concerning early casting quality versus late, think about the poor guy that had to make approx 40,000 of these look presentable enough for sale after they were cast. By about the 10,000th one I imagine he was gettin' a little tired of the job and could've slipped here & there with the grinders & sanders, probably hates Super Coupes to this day...

:)

'bird

ps: here's a slightly different angle of view.
 

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