tmoney

who here knows if the plenum on a 89 mass air flow system and the plenum on a 91 speed denisty system are the same.i'm doing a tbi to tpi swap,bought a 89 maf system but want to go speed denisty!are they the same?

speed88

Yes they are all the same!!

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88 GTA
L98 350
700R4
3:27 axle
MODS,
Edelbrock TES headers, 3" straight pipe, Flowmaster 80 series, K&N filter, Accel 8.8mm wires, Bosch Platinum plugs, Wells cap and rotor button, Wells ignition module, MAF screens removed, Hypertech TB air foil, Thermomaster stage II chip, 180* Powerstat, Hypertech cool fan switch, B&M shift kit, B&M power clutch pack, B&M servo, GM fuel filter.

Omar

NO, they are not all the same! All plenums have an EGR passage, while 85-88 plenums also have a cold start passage that mates with the cold start injector used in some applications. 89 plenums eliminated cold start passage, and 90-92 plenums with speed density used a MAP boss. If you look at this pic, left is 85-88. middle is 89. right is 90-92 plenum.
http://home.san.rr.com/iroc/plenum2.JPG

#1 are the EGR passages
#2 cold start passage (left plenum)
#3 MAP boss (right plenum)

Also, 85-88 use an idle air control passage. An early throttle body cannot be used on a later model without the IAC passage; a late throttle body can be used on earlier plenums. One other way to identify the later plenum is by the cast fins on the top. Early plenums with the IAC passage have fins that extend farther forward than the later plenums. See here:
http://home.san.rr.com/iroc/plenum1.JPG

In your case, you cannot go speed density unless you tap and drill the #3 MAP boss hole. Sorry the pics are big and blurry. I am still trying to get used to this camera.

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1989 IROC-Z 5.7L

Omar

Here is a much better scan.

http://home.san.rr.com/iroc/plenum4.JPG


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1989 IROC-Z 5.7L