OK I know everyone wants to hog out their plenums for more power, but I have my reservations. I know GM designed TPI around the 305, but wouldnt they put those restrictions in the plenum for a reason?

They spent millions of dollars developing this, so i'm sure if there was some kind of performance gain from porting (or casting them differently) wouldn't GM do that to keep the camaro ahead of the mustang?

What are your thoughts?

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1980 Camaro, '88 Police 350 engine, Headers, full exhaust, completely redone, my baby!
1988 Iroc-Z L98, MSD 6a, Accel coil, K+N's, Airfoil, Coolant Bypass, Flowmaster, Gears, Late model Bose, I like the 80 better

 

If you really think GM cared all that much about performance, why didnt put a bigger cam in the engine ? Why didnt throw the Corvette L98 heads on it ? Why not some tubular exhaust manifolds or a higher stall converter ? A better exhaust system, a bigger throtle body.. etc etc etc

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1992 Z28 5.7 TPI

ZZ4 longblock
Harland Sharp roller rockers
Accel base and SLP siamesed runners,
52MM SLP Throttle Body
Hooker Headers & Flowmaster catback
MSD6A ignition
aluminum driveshaft
World Class 5-speed

 

LOL, well I guess that takes care of that question!

AMEN! BTW

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j.wolfe
_ ____ _
91 GTA L98 MSD K&N 195,000
Best time: 14.389@95.49 (VMP) Done with 193,000 miles and no engine work what so ever.
"...Deception..." Brandon Sloan * ...wounds heal and chics dig scars...

 

Question #1. Why didn't they put a bigger cam in the engine?
Answer #1.
This engine was designed to meet emissions standards. The bigger the cam, the more stuff gets blown up in the cylinder, the more unburnt hydrocarbons show up in the exhaust
Camshafts that have larger durations typically operate in higher powerband. If the TPI can't flow air after 4800 rpm, why give it a cam designed to make peak power at 5500 rpm. Also high lift numbers require expensive valve springs and increase the likelyhood of valvetrain failure.

Q#2. Why not the corvette aluminums?
A#2. Why should they? They were reserved for the corvette. The corvette has to be better at something than the f-bodies.

Q#3. Better Exhaust?
A#3. Durability issues. Look at the thin walls of the TES system. Imagine driving through alaska on salted roads 9 months of the year. How long do you tink the TES would last? My old Formula had a 2 3/4'' pipe from the factory, and that was good enough for a 14.7@96

Q#4. Higher stall converter?
A#4. Driveability issues. Why give it a higher stall when you've already decided that it has to have a tiny cam.

Q#5. Bigger Throttle body?
A#5. Marginal performance gains over the 48mm.Not worth the time, development costs.

Horsepower sells cars. If it were cost effective to produce drag strip terrors, GM would still be doing it. When GM wants to build Horsepower, it does it. (Z06=405 HP)


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355 c.i.
Dart 180 Heads
Lunati 224/224 cam
Harland Sharp 1.5 rockers
Performer RPM Manifold
Holley 600 cfm double pumper
Hooker Super Competition Headers
Flowmaster Exhaust
Competition Engineering Sub-frame connectors
Tremec TKO
Pro 5.0 Shifter
GM posi 3.42 rear
Hurst Roll Control
13.9@102
http://www.geocities.com/thetpiguy/index.html
chemjoker@aol.com

 

My guess on why the EGR walls are there: As air passes over them, it helps draw EGR gasses up into the plenum.

 

If you've driven anything that GM has produced in the last 15 or so years, it's pretty obvious why they didnt put more performance stuff in these cars. It's all about costs. Why should they put all the goodies in their cars? They figure if people are serious they'll do it themselves, so they may as well just build a cheap car and get them off the showroom floors. But thats not the issue. I'd argue cutting costs and how cheap GM builds alot of things, but all i'm concerned with is why they put those restrictions in the plenum in the first place.

 

Here's my $0.02. I still beleive porting the plenum on a 350 engine is worth it, and effective. My resoning is, GM simply slapped the TPI off the 305 onto the 350, thus not flowing efficiently for a 350. Also, to answer a question, Why didn't GM port plenums from factory? LOL... imagine that on the assembly line, either CNC or a machine operator grinding the inside of a plenum... hmm... can we say add 100 bux extra, their cost (if not more) times say 150,000 engines? thats 1,500,000 dollars on porting a plenum, as opposed to simply machining mating surfaces,grinding flash and slaping it on the engine.

Also...are you saying the nice step in the runners and intake are designed into it? I think not, this removes efficiency, resulting in an ineficient engine. Efficiency is hand in hand with emissions.

GM can make huge power out of anything they build, but, why? Will you have themoney to pay 100,000 bux for a F-Body? Or will you pay 20,000 (in 1988 obviously) for an F-Body? Its all cost of production, not design. I am sure the engineers designed the perfect setup on their nifty computers, or the drafting tables.. but then the manufacturing/production crew gets their hand on it and starts to hack it up saying this is cost effective.

So... I stick to my answer, porting the plenum is feasible and effective

If you want performance engineering from factory, look into a ferarri.. LOL

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1988 Pontiac Trans Am GTA L98 5.7L, Black on Black with 114,000kms.

Best ET 14.559 @ 95.25MPH
Custom 3" Stainless Exhaust Y-Pipe back (High flow Cat and Flowmaster Mufler)

 

Crucial,
I noticed you have a ZZ4 motor. I was wondering what you run in a 1/4. I have the stock TPI and wanted to get a better intake. How much of a gain did you notice w/ the Accel pieces? You can email me if you want.

Sorry about that.

GM made the cars to be mass produced. They make cars to make $$$ just like everyone else. The cheaper the better.



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Mods: ZZ4 engine w/stock TPI, adj, fuel pres. reg., MSD 8.5, HI6S ign. w/ PS-91 coil, K&N filter, TPI air foil, Hooker comp. shorties, American Thunder flows. 160* stat., TB bypass, 3"cowl, bat. relocate, AC delete, smog delet, cats gutted, stock T-5 w/ 3.42 rear, ported plenum, 24# inj., dual friction centerforce.

 

I think that the castings are there to 'mix' the EGR with the oncoming air. Just as a carburator works. The sharp reasonably sharp edge feeds the EGR into the plenum with greater velocity. What is being done now is the use of longer duration cams, which have a certain amount of EGR built into them. Therefore, the 'EGR' system is not required, resulting in less production costs. Also porting the plenum intake ports flat instead of angled will help. The idling characteristics will change somewhat as well. This coupled with an adjustable FPR gave me the largest HP increase on a stock 305. I then ran a 14.2 1/4. stock with maf screens rem etc. Also look elsewhere in the plenum, it is full of restrictions, especially # 7 outlet. It has the MAT sensor and the screw boss from the plastic HEI cover sticking down. Anyway, the porting of teh plenum will result in more throttle responce, and more HP/TQ=more fun. My 2cents+exchange

 

I didn't mean sit there with the dremel on the assembly line lol
I meant cast them with wider passages, like how they cast the new Vortec heads as if they were already ported. And look how cheap they are and how they flow

 

Okay smarty pants...

Question #1. Why didn't they put a bigger cam in the engine?
Answer #1.
This engine was designed to meet emissions standards. The bigger the cam, the more stuff gets blown up in the cylinder, the more unburnt hydrocarbons show up in the exhaust
Camshafts that have larger durations typically operate in higher powerband. If the TPI can't flow air after 4800 rpm, why give it a cam designed to make peak power at 5500 rpm. Also high lift numbers require expensive valve springs and increase the likelyhood of valvetrain failure.

The cam that I have in my ZZ4 engine passed with flying colors. A slightly bigger cam will do that also. Unless you go with ridiculous duration numbers it WILL pass.

Q#2. Why not the corvette aluminums?
A#2. Why should they? They were reserved for the corvette. The corvette has to be better at something than the f-bodies.

They were RESERVED for the corvette ? Now there's a well educated answer. So was the TPI !! Why did that make it on the Fbody ??

Q#3. Better Exhaust?
A#3. Durability issues. Look at the thin walls of the TES system. Imagine driving through alaska on salted roads 9 months of the year. How long do you tink the TES would last? My old Formula had a 2 3/4'' pipe from the factory, and that was good enough for a 14.7@96

I said tubular exhaust MANIFOLDS. As in... cast iron manifolds like the vette's have. That would be a lot more durable then headers yet flow a lot better.

Q#4. Higher stall converter?
A#4. Driveability issues. Why give it a higher stall when you've already decided that it has to have a tiny cam.

A 1000 rpm stall increase would barely be noticeable to the average driver. Why do the corvettes have a higher stall with the same "tiny cam" ??

Q#5. Bigger Throttle body?
A#5. Marginal performance gains over the 48mm.Not worth the time, development costs.

[b] Development costs ? What's to "Develop" here ? A bigger drill bit ?? LOL

Horsepower sells cars. If it were cost effective to produce drag strip terrors, GM would still be doing it. When GM wants to build Horsepower, it does it. (Z06=405 HP)

NOW they do it because somebody in GM said they have to. Back in the 80s it was aaaall about economical ****boxes so HP took a backseat as it's obvious in 30-40% of the stock thirdgens outthere



------------------
1992 Z28 5.7 TPI

ZZ4 longblock
Harland Sharp roller rockers
Accel base and SLP siamesed runners,
52MM SLP Throttle Body
Hooker Headers & Flowmaster catback
MSD6A ignition
aluminum driveshaft
World Class 5-speed

 

Cmon kids, let's play nice.

I believe Steve's original question was really addressing the EGR wall specifically, not the details of 'touching up' the casting imperfections/details in the rest of the plenum.

The EGR wall has a pretty simple purpose. As the others have stated in a more round about way, the air flowing in from the TB will cause a low pressure area on the other side of the wall. This low pressure area was probably theorized to aid in EGR operation (delivery and mixing), thus why GM included it in the casting. To those of us in search of all the performance you can get from the stock TPI set-up, removing this wall simply eliminates that small about of turbulence, w/o significantly impacting EGR operation.

As to the other questions....look, GM obviously had their heads in their asses in the 80s. look no further than the 86 TPI. In 85 the LB9 had the L69 good cam, and they ran great. Then in 86 to do whatever they could to improve emmisions/economy they slapped the peanut cam in the TPI under the cover story of better low end torque. Yeah, better low end torque, at the expense of the making any power whatsoever. A clear case of the general going backwards in performance, on purpose...

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Ed Maher - Moderator @ The TPI & Carb Boards
92 Z28 Convertible - Quasar blue / Tan top
305 TPI A4 2.73 - 14.7 @ 93.6
Stock except ported plenum and dual cats
-=ICON Motorsports=-

- Definitely prototypes, high powered mutants of some kind. Too weird to live, too cool to die

 

I woul dhave to agree with "crucial" on his post... although GM always made sure the F-Body was "less powerful" than the Corvette, in theory.. And it is very costly to get a smooth cast... we're talking investment casting here, now thats expensive. Imagine the die costs... phew

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1988 Pontiac Trans Am GTA L98 5.7L, Black on Black with 114,000kms.

Best ET 14.559 @ 95.25MPH
Custom 3" Stainless Exhaust Y-Pipe back (High flow Cat and Flowmaster Mufler)

 

I plan to have my plenum off this winter to polish the outside. What can I remove from the inside to get a little more efficiency out of my motor without sacrificing/risking passing emissions and making the engine run incorrectly?

 

relocating the MAT sensor to the air plastic 'y' intake like the V6, and putting a plug a threaded plug in the hole. Make sure the threads match, and nothing protrudes in the plenum. If you want to get fancy, remove the boss from the top screw by #7. This is only there for the plastic HEI cover, and I don't think it needs to be so long, or even there. If you want the screw to be there, cut off the head and glue it to the plastic. I'm going from memory, but I remember that these were restrictions that didn't need to be there.

 

If you compare a later model plenum from a SD car to an older MAF plenum you will see that GM did some work in this area. The EGR wall is smaller and the runner openings are radiused on the SD plenums.

Compare the two and you will see there is a difference.

 

Um, I'm [SHAMELESSPLUG] porting plenums for sixty bucks. I can do a thorough job with my Dotco and Ingersoll Rand grinders.... It'll be good for as much flow as you can fit through a 52mm throttle body... 48mm throttle bodies will work nicely as well.[/SHAMELESSPLUG]






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Daniel Burk
View my third-gen hobbyist performance page!
View my reader's ride!
'84 Trans Am WS6/L69
KB SFC, Moser axles, Torsen Diff., Spohn Adj. torque arm,
Ported 305 heads w/1.94"intake valves, Comp Cams XE262H, Griffen alum. radiator,
Turbine Technologies 2500 stall converter, underdrive pulleys, Crane Hi-6 & more.
1.05g skidpad verified.
Best of 14.039 at 100.82 MPH in Stanton, MI
New!Full 1LE brake upgrade! Details at website above.

 

Damn dan, you reach for free advertising whenever you get the chance on my board. You ought to siamese my spare base for me as an advertising fee. And then i get double bonus points if i help bill davis get his car in the 13s with your stuff on it, lol.