Ive got a 91 formula with a 5.7L and Ive got the plenum off and in my hand. I saw some instructions on here on porting the plenum at this link


http://www.gmtips.com/3rd-degree/do...n-port/port.htm

I see the blocky area on the bottom in the front, and thats it. The sides in the front are rounded, and all the areas between the runner openings are rounded, no blocky areas like in the pics. Maybe the 91's were different or maybe someone already modified it. the blocky area in the front on the bottom seems to restrict air flow to two holes in the bottom of the plenum that lead to a chamber and a port that has a smaller runner. Flattening out the blocky area in the front will affect those areas especially, I guess it will be better, more airflow etc right? Why does my plenum look different than the pics, meaning no square area between the runner ports? Is there not a reason for the blocky wall area on the bottom in the front? Anyway I want HP bu I don't want to screw up my car. Please help

Larry


Larry

 

The outside area can just be radiused a little, but if you're not sure the lower more obvious walls are the most important ones...that should be enough.

 

Those blocks on the bottom surface are there to create a low pressure area just behind them to allow the exhaust gasses into the plenum from the EGR runners that run underneath the plenum.

If you're not using EGR, knocking those blocks out won't matter. I'm not sure about what effect it'll make if you are using EGR, but most people think recirculating exhaust gasses into the combustion chamber is a dumb idea anyway, so what could it really hurt? It all boils down to emissions.

 

Emissions are gay....knock out those walls, and leave it at that, unless u plan on siamesing the runners/base, then u'll want to do a little more "searching" on the porting topic...search for base porting, or plenum porting, and your sure to find plenty of usefull topics.

Zach

 

Its a shame people think this, because I think their opinions are formed without having all the facts. Truth about EGR is this:

1)EGR does cause a small power loss, BUT ONLY AT PART THROTLE CONDITIONS!!!

2)EGR is NOT active during, idle, warmup and WIDE OPEN THROTTLE

The purpose of EGR is to reduce the formation and emission of nitrogen oxides (bad stuff).

I love working with cars, and I love clean air too. You really can't appreciate emissions devices until you have been to a country that doesn't use ANY....you can barely breathe during some parts of the day. Before everybody goes and takes there EGR valves off, thinking they are picking up who knows how much horsepower, they should consider two things: 1) Bottom line:your EGR isn't hurting your E.T.

2)There are a lot of LS-1's out there running fast times (some even faster than your modded third gens. I say "your" because my TPI is going in a Chevelle...but I am in the same boat), and are FULLY emissions compliant. If the EGR is turned off during WOT, I think the power loss is coming from somewhere else.

 

I opened my bores to 52mm with a clean shot straight through.
The EGR is still intact and seems to be working ok.

 

Here is a front shot.

 

The outlets just need to be matched to the gaskest and mabe
a tad more and clean up the inside around the openings as best
you can. Or open them up to match whatever runners you may
be running.

 

Larry,

Maybe this will help clear up a few items:

Plenum Porting Guide.pdf

You should see something like this:



and this:

 

so I've heard that if you take down the EGR walls your call will have problems with pinging(detonation) I assume this isn"t true then?

 

I have had no problems with detonation on my setup.

 

I finished it up over the weekend and got it back on the car last night. My plenum looks different from all the ones Ive seen on here as it doesnt' have the triangular indentation in between the two holes. The only thing I was confused about was according to one link on here it talked about square areas betweenthe runner openings and on the sides, I guess referring to the bolt housing areas. I didnt see any square areas on mine between the runner openings, the looked pretty rounded already. Also I didn't see where I could take alot off the bolt housings without jeopordizing the integrity of the threads inside. Anyway, I just did the EGR wall and a few minor areas around the runner openings that werent perfectly round. Seems to have come out ok.

thanks
Larry

 

by the way, when you do this don't forget to put back the vacuum connection to the brakes, I almost smashed into the garage wall trying to stop before I realized I hadnt' reconnected it.

 

Did u notice a pretty good increase in pull at upper RPMs????

 

Yes, I definitely noticed an difference. Its hard to quantify. I am going to take it to a 1/8 mile drag track near here next Friday. Unfortunately I don't have the before times to tell you exactly. I notice a definite slight increase in a throaty sound, and then the difference in the mid range speed (40-50 mph). Hell, I don't think its my imagination but I wished I would have ran the track last week when I was out there watching to have a baseline time.

Larry

 

This thread couldn't have come at a better time. I bought an extra set of castings on ebay a couple of wweeks ago so that I could do some porting and polish the outside without having my vehicle down. Thanks especially to Vader for the link and pix.

I was wondering about cutting a teardrop shape into the floor around the EGR ports. It would seem that it would improve the airflow by smoothing the transition of the rear wall of the hole into the floor and improve the EGR flow as well. What do you think?

 

As long as there is enough material, it couldn't hurt.

One thing to remember about the system is that there isn't that much air flow when the EGR valve is opened. As soon as the throttle angle gets opened enough to get some serious flow, teh EGR closes. At low and moderate throttle angles, the air rushing around the throttle plates has more than enough velocity to push the EGR gasses into the plenum and mix them thoroughly. When the throttle is opened to the point where the teardrop would come into effect, the EGR valve closes anyway.

Still, it couldn't hurt as long as there is adequate material.

 

It's a shame I think what? What I stated was fact.

1) Those blocks are there to help the EGR gasses into the plenum

and

2) most people think EGR is useless

Where in there did I state my own opinion? I never said that I think EGR is useless. I am building my 355 TPI at this very minute, and I'm retaining every bit of emissions related equipment (EGR, AIR, cat, etc.), even though emissions aren't required in Florida, because I want to, because I also believe in clean air, and because I believe it has *little or no effect on performance*.
I also summarized that the blocks are emissions related and did not state an opinion there either. So please read a little more carefully before you try to blow me out of the water. Maybe I should have said it this way: "...but most people think recirculating exhaust gasses into the combustion chamber is a dumb idea anyway, so what could it really hurt? I don't believe EGR hurts performance." Or maybe I should have said "...some people..." instead of "...most people...". Maybe you should have directed that at the guy that said "Emissions are gay...". That point-of-view is deserving of a rebuttal, so you're right to say it. But I don't have that point of view.

My mistake in not being clear.

 

I think your only mistake is in assuming that the blocks have any noticeable effects on emmisions. Plenty of cars running around w/o those blocks w/ no EGR issues whatsoever, and TPI most definitely has self-diagnostics to determine if the EGR is working. Bear in mind, EGR is only active at part-throttle / light load. Thus there will be vacuum in the plenum anway while EGR is active. I hate to always deer to carbed examples, but...carbed cars w/ EGR don't need obstructions to create low pressure on their EGR ports, why woud TPI be different?
Also, you shouldn't assume that most people think EGR is a dumb thing either. I'll always leave a working EGR system intact as there are no downsides whatsoever IMO.

 

Larry,

The blocks were put there as exaggerations of a restriction. They were made blocky to better show how to radius these areas. I should have been more clear but I drew that diagram real quickly while involved in an explanation to someone. Then put it up on my site.

The "radius" thing is hard to explain.

Sorry for any confusion........

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Let's review this line on my first post:

I never assumed anything..

And I did admit that I probably shouldn't have used the phrase "most people".

 

I am surprised that this is what these blocks are for. I always assumed they were some kind of casting block....... but hey it makes sense the low pressure area.

I noticed a definite difference in not so much "seat-o-the-pants-horsepower" but in a quicker throttle response.

But I have a theory for you guys. If the EGR only operates a small % of the time and every plenum that I have taken apart has the EGR passages 100% blocked with carbon and soot and since this doesn't seem to be a routine maintenance item I would venture to say that the majority of "stock" TPI third genners on the road have this function "naturally" disabled or soon will:

The majority of cars would benefit by having the blocks removed and the EGR passages cleaned out with a pipe cleaner, carb cleaner and air pressure. Then you would get at least "some" EGR flow. Further these emissions could be reduced by increasing coil voltage, widening the plug gap to burn off excess gases.

Does this hold water or is my boat going down?

 

It doesn't sound like its an issue anymore, but yes the plenums were different by 1991. They don't have anywhere near the restrictions as the earlier plenums and porting them does very little if anything. If you compare say a 91 plenum to an 85 plenum you'll see the EGR walls are about a third as tall on the 91 as the 85. All you can really do on a 91 is knock the walls down further, and clean up some casting marks. Porting the runners can't hurt either, but again its not going to make enough difference that you'll notice.

 

Thanks Drew. That's the first time Ive got confirmation of what I was seeing the pics of the older plenums compared to mine. Do you think going to a 52mm throttel body, SLP type larger runners, and an aftermarket ported base is worthwile. We are talking about around $1000 and up for this. Im planning on doing this and I sure hope Ill "notice the difference".

Larry

 

No, sinking all that money into base, runners and TB when you're still stock is a huge waste of money.

 

Not really...

I wouldn't waste money on aftermarket intake parts till you've addressed the heads, the cam, the torque converter, sticky tires, etc etc etc...

 

Ed beat me to it, but while I'm here... The SLP runners not only look like ****, but on stock applications people have actually had time slips show they lost horsepower.

There are so many things people can mod on their cars that will make a difference, but people still fiddle-**** around with little crap that never amounts to anything.

Things that matter:
Traction (sticky tires, boxed or reinforced rear suspension, adjustable torque arms)
more air in, more air out. (cam, heads, exhaust, intake)

Everything in the stock setup is pretty closely matched, the stock TPI has been proven to be able to support more power than the L98 makes, and it should be about the last thing a person changes. The exhaust its pretty good in stock form also, smooth bends, 2.75" tubing, dual cats, large manifolds... In my opinion start with opening up the intake ducting a bit, then start shopping for a cam.

 

Wow, this really changes my attack plan here. Since I really don't have much experience working on cars I was going to do easy things first, like bolt ons that I mentioned, and including headers and a cat back system. I thought I could do all that pretty easy myself, then see where I was with HP and THEN go into the motor if I wanted more power. Don't get me wrong, I appreciate the advice, its just that I may need to rethink my whole strategy here.
By the way when you say "intake" more air in and out, aren't you talking about the TPI air intake setup? I plan to do a cold air K & N setup along with the previously mentioned mods, plus the headers and exaust. This would be all the air in and out stuff I know about.

Where can I get info on what heads and cam to use, and how to do it too?

thanks for the advice.

Larry

 

Thats what a lot of people do, thats why you see so many people on this board with a signature 5 pages long, and a best time slip a tenth of a second faster than their car should have done stock.

The thing about working on cars is that no project is ever really easy. You're starting with a car thats already lightyears ahead of say an 87 L98 or even worse the L03's and LB9's that most of the discussion here is based on. A lot of people don't realize that. Unless your car has been patched up over the years to keep it driving it already has a relatively free flowing exhaust, a good decent air intake system, etc... I'm sure no one wants to read my ideas on what to do and where to start so I'll just send you a few things thru a Private Message.

 

I appreciate the benefit of your experience. So far I havent spent much money, just ported the plenum with tools I already had and bought a Jet airfoil. While I am not poor, I certainly don't like throwing my money away on stuff just to have a cool signature. I have already ordered the K & N 360 degree filter and bought a few items to fabricate a homemade cold air setup, probably $75 total for the whole deal including the filter. BUT, Im fixing to get into the pricier items and would like to get the most bang for my buck. I realize there are other things at stake besides HP and ETs, like a nice catback system that sounds good. That can add to a satisfying driving experience, but I really want to go fast more than anything else. My car already looks great. I don't want to seriously modify the car for only drag racing, Im looking for a balance between the strip and the street. The car is not an everyday driver, If I want to go on a long trip Ill drive my Crown Vic, heehee. Anyway, looking forward to recieving the info.

Larry