Hello
I was just looking at some pics when i noticed the the grooves on one plenum extended much farther than the grooves on another.if you look closely on one the grooves end at the begining of the first two runners. and on the other the grooves go even farther than that.

I am not sure as to why this is. maybe it is a vette/camaro, bird difference or maybe they changed the look slightly one year. All that i know for sure is that both my 89 iroc and 90 gta have plenums with grooves like the blue one.

It isnt anything important but i was hoping just out of interest if someone could shed some light on the subject.

Speed

I don't see any differences in those pictures. But in the later third gen years GM was phasing out the L98 parts and switching everything over to the LT1. So when the factories ran out of L98 parts, some third gens wound up with LT1 runners.


*Edit*

I just noticed you were talking about the plenum.

Maybe it's the same story?

There's definitely a difference, probably something small between certain years.

The MAF systems have a different plenum than speed density systems. The difference in grooves made it possibly to quickly tell what year car it came from. The longer grooves were used from 85 - 88, and the shorter were used on 89 - 92. The other thing that made them different is the earlier plenums had a passage for the cold start injector, they also had a cut out in the front for the IAC. The later plenums did not have this, in 90 - 92 the plenums got a MAP boss for the map sensor.

Yeah, they also started using LS1 parts on the last couple years of the LT1 motors because they were running out of LT1 parts.

Yes, to tell the difference between the MAF and SD cars. Maf had longer ribs and were slower, and the SD cars were shorter ribbed but much MUCH faster.

I have GOT to get me some of those LT1 runners!!!

I didnt know thirgens came with LT1's

WHAT?

Those LT1 runners for the L98 are pretty rare. I have been looking for some for quite awhile now but they are hard to locate as so few were made.

Since when do LT1's have removable runners?

Anything's removable with the right tools

since when do LT1's have runners??? with the exception of the fuel rails, the intake on an LT1 is all 1 peice isn't it?

The runners are the actual rectangular ports and yes they are cast into the manifold.

well, that's what i was thinking but someone posted before seemed to imply that they seperated from the intake like the tpi runners do. didn't sound right.

Dude, read the rest of the posts. They've been ripping this guy up ever since he said that.

wait so is the MAF plenum the one on the left and SD the pic on the right?

Not quite. My 1989 MAF car is like the one on the right. So it well could be that the one on the right is for 1989 to 1992 cars.

I dont think the difference is a maf/map thing. MY 89 iroc tpi [maf] and my 90 gta tpi [map] both look the blue tpi pic. The plenums are slightly different because the is a hook up for the speed density on the rear passenger side of the plenum but the grooves are the same.

I side the 89 to 92 tpi plenum statement. I know for sure those look like the second pic .. but as for 85 to 88 i havent seen a tpi in one of those for a while so i couldnt be sure.

Speed

My 87 looks like the one on the left. Longer grooves.

so then really all we now need to know is what 88 has.

Speed

Although this has already been established, my 87 is also like the one on the left.

Cast into the underside of the plenum, there's an encircled number, which might mean something. From an '85 in a junkyard, that encircled number was 85. And one I have in my garage for porting has an 88 on it(though I don't remember what year car it was from).

But assuming that number relates to the model year, then the 88 looks like the one on the right, whereas my '87's, like everyone else's '87, is like the one on the left(the better-looking of the two, IMO). But I've never had my '87's plenum off, so I can't confirm that its number is 87. Maybe someone else can though.

So if that's correct, then that also would make the SD/MAF plenum comparison inaccurate.

There are several differences over the years of the plenums between the length of the grooves on top, which was more or less for astetics, to the different set ups such as ones equiped with the cold start injector or not, as well as MAF and MAP. I belive there are 3 or 4 variations, and the Corvette had another design if I recall.

There were three different plenums used through the TPI era-





Don't forget the three lower intake manifolds-



So, maybe it would be easy to confuse who had Vette parts versus F-body TPI parts. They weren't always the same!



From-
Identifying GM's Tuned-Port Injection Units

The maf to sd is mostly true, except for 89. Check out Chevy TPI swapper's guide on page 20. It gives you the run down. There were actually three upper plenums made from 85 - 92. 85 - 88, 89 and 90 - 92. 89 - 92 had shorter fins than the earlier plenums.

Here is a link that most of the ID storys seem to originate from:

http://www.hotrodlane.cc/PDFFILES/TPIStory.pdf

ScotSea