I am thinking about adding a larger throttle body 52mm to my 89 IROC. I have heard both ways on replacing it. Anybody have any good or bad comments about them?

 

This works with a carb so it should apply.

Put a vacuum gauge with a long rubber hose on the plenum. Run the car WOT up to its shifting point. Note the reading of the gauge. If there is more that 1" of vacuum, the throttle body is starting to act as a bottle neck.

Ideal reading is 0.5" of vacuum.

 

What engine do u have??

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Looking For:

87 IROC-Z 350 TPI
84 TRANS AM 305 H.O.

 

You only need a larger throttle body if you've got different cam/heads, and you've modified your MAF sensor. Otherwise, the TB isnt a restriction and you wont gain by swapping it.

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West Coast GM Shootout 2001!
1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.25 @ 107.18 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
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I currently have a 305 tpi but will be soon installing a 350 10.5: cr .510 lift and 220/230 dur cam, dart heads, and more, possibly a mini ram?

 

A carb needs signal at the boosters in order to meter fuel into the engine.. fuel injection does not require this.

 

maybe someone who reads this can verify what I am about to suggest. I dont know if this is true or not but I have thought about it and have figured this much anyway.
ok, so you want a bigger TB, well there is only one real benifit you can see by going to a bigger one.
that is that your old one is not flowing enough to meet your engines demands.
this is the old kink in the garden hose theory.

the thing that I have come up with will tell you if you really need a new one.
I notice that my tpi (a 305) pulls harder and harder with the more pedal I give it --- to a point! --- after I reach about 80 - 90 percent throtle all the extra pedal does is make the tranny hold the gear longer.

now by assuming that the TPS has signaled for WOT before I reach this "wall" i am describing, we can extrapolate from this that once that certain percent of the throtle body is opened, and then exceeded, some other part of the intake system BECOMES the throtle (i.e. some other part of the intake is a bigger restriction than my partially open TB)
so this tells me that untill I find the other restrictions that limit or "THROTLE" the engine the stock TB is big enough and flows more than that other restriction in the intake.

I understand that this simplified discussion does not include rpm considerations. but I think that if I did this kind of test at the strip and fixed the tranny shift points or shifted manually at the same points, then I could acheive simmilar timeslips with ANY percent of throtle application above the "wall" or intake tract throtle. once I had reached the ultimate flow limit of the intake (at whatever rpm it occured) torque would level off and times would too. if times got better and better right up to 100 percent throtle, only then would I really consider the TB to be a "restriction", and accordingly try a bigger one until times started leveling out.
i dont think you really need a strip to do this though.
just find somewhere you can punch second gear safely and put your tranny in second.
then do a series of "runs" at varying pedal positions. idealy you would want every one of these runs to be with the ECM in WOT mode just to make things even, but I think checking that your TPS is over 4 volts at like 75-85% throtle should do it.
start from that % throtle on your runs and once you feel that the car doesent respond to any more pedal then you know your TB is big enough.

whew!

1000 apologies for the long post but I figured I had to write this stuff in an aplicable topic.

all in all I think looking at your mods a 52 would help, but you might want to listen and see if someone thinks my idea is a good one.
if not please disregard and call me an idiot.

good luck



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88 WS6 T/A 305 TPI--
K&N,Air foil,hypertech chip, thermostat+fanswitch
free mods, dynomax catback, urethane motor+trans mounts,
SSM SFCs, Bilsteins
very trick homemade ramair, Accel 300+ ignition system.
Planning: hedman 1-5/8 longtubes, NOS 150 plate, world 305 heads, replace the "peanut"! and other susp. mods.
check it out at:
http://www.geocities.com/transam617

 

With the 305 u have, no. With the 350 u are getting, with heads, cam and other mods, yes.

------------------
Looking For:

87 IROC-Z 350 TPI
84 TRANS AM 305 H.O.

 

ODB - a carb get its signal from the venturi of the carb. If you put too small of a carb on (or thottle body), the engine is bottle-necked. This can be verified with the vacuum gauge set-up I mentioned earlier. If you measure air pressure at the venturi, you would see a 3-5 inches of vacuum (WOT max RPM), but the intake manifold's vacuum would drop to 0.5 to 1.0 inch. That is as long as the carb (TB) wasn't a bottle neck.

I never said anything about a carbs signal at the boosters. I just mentioned an easy and simple trick that tells you if your throttle plates are big enough.