Plenum Porting - EXPERIENCED ADVICE SOUGHT
Thread Starter
Burning Brakes
Joined: Jul 2003
Posts: 922
Likes: 1
From: Brisbane Queensland, Australia
Plenum Porting - EXPERIENCED ADVICE SOUGHT
I'm in the midst of getting to my injectors on the '85 (good fun) - Have cleaned the plenum while it's off but here's the question :
WHY does each port (behind throttle body) have metal in the way of the airflow, at sides and bottom?
http://forums.corvetteforum.com/phot...cat/500/page/1
My ONLY 2 thoughts are:
1. Time saving on G.M.'s behalf
2. It actually helps 'swirl' the air before it meets the intake port...
I'm biased towards the second reason, but only because I don't want to grind it away and find my idle quality or performance/economy suffers!
I've searched the forum but only find mention of grinding it away once...Can someone who's DONE IT, PLEASE give me a learned answer on this ASAP before I put it all back together?
The setup as it IS, sort of takes away the theories of TB airfoils, smoothbore MAF -> TB etc when there's a dirty great blockage at the plenum!!!
Many thanks!
PS; just found this interesting statement via:http://www.chevyhiperformance.com/techarticles/90759/
'Be aware that as engine airflow increases, additional fuel must be supplied. TPI 305 and 350 injectors come from the factory with their flow capabilities nearly at their maximum. If you plan on modifying your engine, be prepared to swap in some larger injectors.'
WHY does each port (behind throttle body) have metal in the way of the airflow, at sides and bottom?
http://forums.corvetteforum.com/phot...cat/500/page/1
My ONLY 2 thoughts are:
1. Time saving on G.M.'s behalf
2. It actually helps 'swirl' the air before it meets the intake port...
I'm biased towards the second reason, but only because I don't want to grind it away and find my idle quality or performance/economy suffers!
I've searched the forum but only find mention of grinding it away once...Can someone who's DONE IT, PLEASE give me a learned answer on this ASAP before I put it all back together?
The setup as it IS, sort of takes away the theories of TB airfoils, smoothbore MAF -> TB etc when there's a dirty great blockage at the plenum!!!
Many thanks!
PS; just found this interesting statement via:http://www.chevyhiperformance.com/techarticles/90759/
'Be aware that as engine airflow increases, additional fuel must be supplied. TPI 305 and 350 injectors come from the factory with their flow capabilities nearly at their maximum. If you plan on modifying your engine, be prepared to swap in some larger injectors.'
Last edited by smacko; Apr 21, 2005 at 10:21 AM. Reason: Adding new link
Thread Starter
Burning Brakes
Joined: Jul 2003
Posts: 922
Likes: 1
From: Brisbane Queensland, Australia
Originally Posted by joalyons
I got rid of it all. I think the TPIS book suggests to remove it as well. The way I look at it anything that obstructs air flow cannot be good.
Thread Starter
Burning Brakes
Joined: Jul 2003
Posts: 922
Likes: 1
From: Brisbane Queensland, Australia
Originally Posted by joalyons
I got rid of it all. I think the TPIS book suggests to remove it as well. The way I look at it anything that obstructs air flow cannot be good.
ANY changews to economy or HP?
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
I took an air grinder to mine and got rid of them on my 85. I had already changed to a 52MM TB earlier. It only took me about 45 minutes to grind these away. I matched the opening to the gasket griding everything to match. There was no change at idle, nor did I notice any negative things because of this removal. I felt (seat of the pants) that there was a small gain in performance.
Pro
Joined: Feb 2005
Posts: 658
Likes: 1
From: Centerville OH
The projections in the floor of the plenum are part of the EGR system. The idea of the projections is to create swirl in the incoming air to help suck the recirculated exhaust gasses out of the passages and into the air stream. I completely removed the obstructions in the floor. I have a 52 mm TB, and I also opened up the bores on the intake flange to match. You can also blend the sidewalls of the plenum just behind the flange, but you need to be careful here because the walls are pretty thin.
I didn't notice any difference in idle quality, drivability or emissions. Performance wise, it will make a pretty noticeable difference assuming you have done other breathing mods like large tube runners and baseplate porting (or high-flow base).
I didn't notice any difference in idle quality, drivability or emissions. Performance wise, it will make a pretty noticeable difference assuming you have done other breathing mods like large tube runners and baseplate porting (or high-flow base).
Last edited by Z51L9889; Apr 14, 2005 at 11:01 PM.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
It actually helps 'swirl' the air before it meets the intake port...
It seems to help 'bounce' the air up toewards the port. In the process, it creates some swirl. I imagine this might enhance the off/part idle snap. I competely removed mine and as one above poster did gasket matched the runner holes and worked the inside of the plenum where the bolt bosses are. I imagine there are gains, albeit small.
It seems to help 'bounce' the air up toewards the port. In the process, it creates some swirl. I imagine this might enhance the off/part idle snap. I competely removed mine and as one above poster did gasket matched the runner holes and worked the inside of the plenum where the bolt bosses are. I imagine there are gains, albeit small.
Thread Starter
Burning Brakes
Joined: Jul 2003
Posts: 922
Likes: 1
From: Brisbane Queensland, Australia
Originally Posted by Z51L9889
The projections in the floor of the plenum are part of the EGR system.
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Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
Yes, (although your picture didn't come thru) the metal humps at the base of both bores and on each side. Grind off the humps and open up the sides to match the gaskets. You will see a difference at wide open throttle.
If you do a search you will find several posts that include pictures of the final product.
If you do a search you will find several posts that include pictures of the final product.
Thread Starter
Burning Brakes
Joined: Jul 2003
Posts: 922
Likes: 1
From: Brisbane Queensland, Australia
latest thoughts...
Originally Posted by John A. Marker
Yes, (although your picture didn't come thru) the metal humps at the base of both bores and on each side. Grind off the humps and open up the sides to match the gaskets. You will see a difference at wide open throttle.
If you do a search you will find several posts that include pictures of the final product.
If you do a search you will find several posts that include pictures of the final product.
Actually, I think I'll leave this bunny alone afterall; took another look and the sides can't REALLY be trimmed much as the rear of plenum indents and it's only the x-thickness of metal there. The BOTTOM where both ports have a 'ledge' also my well have been engineered that way as below both ledges there is a hole that channels off to each runner, so maybe they're there to act as a venturi of sorts...oh well, I'll put away my grinder...
PS; do you know what keywords to search for? I can't find anything...
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Originally Posted by Z51L9889
The projections in the floor of the plenum are part of the EGR system. The idea of the projections is to create swirl in the incoming air to help suck the recirculated exhaust gasses out of the passages and into the air stream.
Instructor
Joined: Sep 2004
Posts: 118
Likes: 0
From: Texas
Thread Starter
Burning Brakes
Joined: Jul 2003
Posts: 922
Likes: 1
From: Brisbane Queensland, Australia
Originally Posted by KAZ2
I was looking for some documents on it...
Burning Brakes
Joined: Nov 2003
Posts: 914
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From: Redding Ca
The TPIS book, "Insider's Hints" has a complete section on porting the plenum, complete with some dyno differences. I followed their instructions to the letter, installed their Large Tube runners and Big Mouth manifold, 52MM TB and the car ran just fine with absolutely no change in idle quality. The car hauled ***, but I did a lot of changes at one time so I don't know which changes did what, but the end result was very good. It is a simple job with an air grinder. Have no fear. I think it took me about an hour. 
Just my .02, why would anyone want swirl before the air gets to the combustion chamber? Seems to me that would reduce total air flow.
Just my .02, why would anyone want swirl before the air gets to the combustion chamber? Seems to me that would reduce total air flow.
Thread Starter
Burning Brakes
Joined: Jul 2003
Posts: 922
Likes: 1
From: Brisbane Queensland, Australia
What gets me is, WHY did GM leave the ledges at the bottom?
This is probably what concerns me most; Surely they would've bored straight thru otherwise...
Anyone?
This is probably what concerns me most; Surely they would've bored straight thru otherwise...
Anyone?
Le Mans Master
Joined: May 2004
Posts: 5,386
Likes: 534
Originally Posted by smacko
What gets me is, WHY did GM leave the ledges at the bottom?
This is probably what concerns me most; Surely they would've bored straight thru otherwise...
Anyone?
This is probably what concerns me most; Surely they would've bored straight thru otherwise...
Anyone?
Hmmm, this is just a shot in the dark but back before the release of the TPI, GM realized that performance from the 350 V8 wasn't dead. Unfortunatelly, the TPI system that comes on all stock L-98 vettes is a system originally designed for a 305 size engine. Other than that, I'd say the casting molds left over where items they figured wouldn't affect or hurt airflow on a 305 engine but a 350: guess that's another story. Just a guess on my part but there may be some truth to it.
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
I'm sure gordon killebrew knows the answer.
Maybe the 305 had a smaller bore throttle body? And the ridge wasn't in the way of it's smaller bore?
Maybe the 305 had a smaller bore throttle body? And the ridge wasn't in the way of it's smaller bore?
Thread Starter
Burning Brakes
Joined: Jul 2003
Posts: 922
Likes: 1
From: Brisbane Queensland, Australia
Originally Posted by CentralCoaster
I'm sure gordon killebrew knows the answer.
Maybe the 305 had a smaller bore throttle body? And the ridge wasn't in the way of it's smaller bore?
Maybe the 305 had a smaller bore throttle body? And the ridge wasn't in the way of it's smaller bore?
Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
Actually the TB & plenum are the same for the LB9 (305cid TPI) engine in my '86 IROC as for the L98 in my '88 Vette.
Both cars now run a 52mm TB, ported plenum, etc with fine idle and performance.
As mentioned, I don't think you will see any real performance gains from porting the plenum till you have done substantial air flow mods elsewhere.
Both cars now run a 52mm TB, ported plenum, etc with fine idle and performance.
As mentioned, I don't think you will see any real performance gains from porting the plenum till you have done substantial air flow mods elsewhere.