I have a 91 Z28 and i am thinking of slapping a 58 mm throttle body on it. Will i have to port the openmings to the intake to make the openings flush? And also, how much horsepower can i be expecting? Thanks

 

yea you'll have to port the opeining....
the HP increase is/was an ongoing discussion...some belive that it does help, some belives you go slower, some belive it doesn't do squat...when I put mine on, I felt a difference, but unfortunatly, I didn't have any ET's prior the change, so it's hard to tell. On a stock motor, I'd say maybe 10hp. I think just porting you plenum (grinding the walls) would be the bette way for now. Spend that $$ elswhere...oh yea, you are not saying what mods you have so I'm going on the assumption your cam and heads are stock.

 

If you are running a stock motor it probably will not show any gains. It might even hurt a little.

But to answer your question...Yes you will need to open up the front of the plenum. You will have to radius it though, there is not enough material there to completely bore it to 58mm.

 

If you are looking for a good increase in power go with a 52mm unit. It costs less, will give you more power and less headaches. The 58mm is ovekill on a 305 and 350. It is best suited for a large displacement motor.(383-406 etc.) Expect 15-20 HP from it after the plenum is ported to 52mm. Make sure you get rid of the 2 base ridges at the opening of the plenum. You will make better power from the TB after you get an aftermaket base with larger openings and some high flow runners like the SLPs. I have had both on 305, 350 and a 406.

 

what size is mine now? stock?

 

48mm

 

even a 52MM TB on a relatively stock engine wont really help all that much...that $$ could be put to better use in other areas...

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Originating member of the SJNEP Crew
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Tony

 

I agree, what are you other mods? If you haven't done any your best mods will be to port the plenum to match the stock tb and runners, buy a set of good headers/catback, an afpr, & make up a ram air system. If you buy used parts you can do it for the price of a new TB.



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Dan
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87 IROC-Z, 5.7,auto, 3.27, leather, !cat, Holley fpr, K&N'S, SLP 1-3/4" Jet-Hot coated headers, Accel .219 cam, Comp 1.5 roller tip rockers, $uperPITAram, Edelbrock lower intake, Holley 52mm tb, Dynomax\Flowmaster catback. Coming Soon- DFI or prom burner, Dart Iron Eagle 200cc heads, 6" rod 383 or a 406

1987 Camaro Z28 - 5.2tpi, auto, 3.73s, ram air, airfoil, ported plenum, !cat, !tbcoolant, !smog, !4th gear

1989 Pontiac 20th Anniversary Turbo Trans Am - 161,000 miles, !cat, 9" K&N - SOLD

 

I tried a 58 mm TB on my car with the mods I have and when I did WOT it hesitates, so I'm going to stick with my 52 mm even when my 383 is ready...

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GTA -89:Free flow muffler,dual 3" HF cats, Edelbrock headers, Vette heads, 1.52 RR 7/16, ZZ-9 cam, Big Mouth, AS&M LTR, !plenum, 52 TB, MSD 6AL, TPIS level V prom, Superior shift-kit and all the minor mods, a Canfield headed 383 in the works!! ready May 2001...ok..june...
http://www.itv.se/tacs/members_cars/m279/m279.html (Swedish Trans Am Club)

 

I don't know. I did alot of mods at once (as recommended) but from what I hear and what I believe, a throttle body is not a friggin carb. You shouldn't be able to over TB a motor because the vacuum drop isn't something that couldn't be compensated with FI as PROM changes could compensate for the lower vcauum? that occurs at WOT that may cause stublage on an otherwise stock 305, but I definatly am happy with my 58mm on my 350 with alot of mods. Those that say with proper tuning and cam and gears... a 58 doesn't belong on a 350 because is will cause hesistation is wrong. Those that wip out a calculator and say a 350 will never use that much air I ask... Wanna race?

One drawback is it makes the trottle pedal more sensitive and you have to deal with it. Now someone gunna say 'yeah, more air and less trottle would make you think the car goes quicker' but I had to just slap the stocker back on after the flak and I could notice a small but significant loss at wot, although streetin' the stock TB 'feels' alot smoother.

 

I put one on my 86 with great results, but I had an SLP cam, and upper intake porting. This was combined with a 2000rpm stall converter so the little bit of torque that I might have lost down low was masked because of the converter. It woke my car up. That 48mm would not feed my car though I had lots of mods.

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89 Trans Am Turbo 3.8L All stock 43,000 miles #1053 of 1555

Past Thirdgen:
86 Trans Am w/ built 355TPI with SLP goodies and too much other stuff to List. One sweet *** car, wish I would have had a good enough Job to pay insurance on three cars so I could keep it, but for a 89 Turbo Trans Am w/ Low miles, I think I made the right choice!