You know I can't recall seeing any Gordon Killebrew postings for quite awhile is he participating here anymore?

 

'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.'[/QUOTE]


I don't agree with this quote, I added a paxton to mine and didn't need larger injectors.

I also ported the plenum and matched the runners..... it was a noticeable performance gain. i say go for it.

 

I would guess that the two "dams" behind the throttle body bores are an aid to stop egr reversion and contamination of the throttle body/plates. Most of us are aware that when you get a build up of carbon and gum on these, the idle gets affected, to the point of cutting out with badly contaminated throttle bodies. With a high mileage engine, there is probably a lot of crap going through the egr system, especially at high vacuum cruise conditions where the egr would operate. Seeing how GM usually carries out exhaustive testing, maybe they came up with the dam idea to help better mix the egr and incoming air/fuel.
But anyway, I removed mine and didn't notice any adverse affects, I didn't seem to notice any significant seat of the pants improvement either, but it made me feel better that the inlet was (seemed to be)more free flowing.
I did the streamlining of the plenum to runner mods as well. I would have thought that the extra metal in there is for durability and strength and the modest reduction in flow wasn't an issue for GM. A lot of people who run siamesed runners cut most of this out anyway!
As for us enthusiasts, well we don't mind compromising durability in the name of performance do we? We do it all the time, porting heads, MAF's, etc, etc, all in the name of improving airflow in and out of the engine. IMO Just do it, if it breaks, don't do it quite as much on the next one. More air in, more power out with a proper tune. Just plan on making sure you clean the throttlebody more regularly than every 100 thousand miles owners tend to clean them and you probably wont have a problem

 

THANKS Davemc!
This has been the most comprehensive reply to date and your thoughts on EGR reversion hold merit with me; I cannot for the life of me see GM deliberately engineering the damns for NO REASON! B4 reading this I just posted a new thread to try and get more feedback.

Only thing IS; given that vacuum sucks (no pun) towards the windscreen, what conditions would you imagine would have the EG's blowing FORWARDS onto TB plates?

 

Smacko,
There are many unusual things that happen in an internal combustion engine, I'm sure there are many scientific reasons for this, it may be transients in vacuum/airflow when throttle blades are snapped open or snapped closed causing reversion, defective egr valves, passing when they should be closed, maybe poor sealing of intake valves allowing combustion gasses into the plenum, high overlap cams (not in the case of a L98 mind you) or just one of those phenomena's that just happen.
It happens to most engines, at least all that I have had, from carbed engines to fuel injected. The inlet tracts eventually get a build up of carbon/gum and combustion by-products.

Dave

 

I would think the restriction would be there to increase low rpm intake air velocity, thus low rpm,low speed throttle response. Anything done to increase airflow would only have an effect at higher rpm. At lower rpm slow airflow what difference would a restriction make, unless it had veturi effect to increase velocity.

 

Who knows for sure except the original design team! But I would have thought that the main 'venturi effect' if any, would be across the throttle blades if anywhere. Thats one of its main purposes after all. It wouldn't make sense to design two so close together. Without studying actual measurements and applying Benoulli's Theory, which I am familiar, but by no means an expert, we will probably never know the answer to this will we? It may be a combination of all we have suggested.
Dave

 

Hey, this is a great thread!

On my 85 I "HOGGED" the plenum and was very pleased with the effect.

Getting the L98 to breathe has been a mission for me!

I, like so many others here, opened up the TB to 52mm and all that extra material in the plenum had to get outta the way. I gasket matched the plenum to runner bores but haven't done anything to the intake yet. But in front of all of that I opened up the air cleaner lid and added a K&N filter, smooth TB inlet hose and then removed the screens from the MAF. I have LT headers and true duals (sans cat but with crossover) to 40 Series Flowmasters on the exhaust side. And that doesn't include the internal changes...

So, did any single mod make the difference? Probably not but GM engineered the car for the masses...the "put the key and go" mindless drones we pass on the street everyday. That's why we don't wave to them but only to each other! We re-engineer/refine our cars to our individual (albeit collective) tastes.

Was the car quick before all of this...yeah guess so.

Is it quicker now...HELL YEAH!

Point is have fun...hog out the plenum and see what happens. If you don't like the change you can replace it with one from ebay for around $50.

Last thing...gotta mention...

FUEL PRESSURE! Get an AFPR and set that puppy just a smidge under 50psi.