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while i am rebuilding the stock tbi throttle body, I was wondering if any of the mods are beneficial with a completely stock engine?
I am rebushing the shaft, got an injector spacer and 14psi spring. I was considering unshrouding the base on one of the 2 throttle bodies, but i think that may be overkill on the stock engine.
The air scoop, air duct, snorkel on the air cleaner base, air cleaner lid, and air filter on a TBI F body doesn't flow very well. The air cleaner base is OK.
You can easily improve 3 of these buy using the air cleaner lid, wingnut, and offset air cleaner stud from a late 1980's to 1991 Chevy/GMC truck and a K&N E-1500 air filter. 1992 to 1995 same truck dual wing nut and dual stud air cleaner lid will work too but you have to use the two bolts/studs that hold the TBI to the intake manifold in place or the two bolts holding it down now. This will give you an air filter that is 0.938 inches taller and flows much better. This will fit under the hood of a 1992 Firebird with the plain flat smooth base Firebird hood assuming stock engine and trans mounts. Aftermarket or Poly mounts verify the hood clearance!!! I don't know if the under hood bracing is different on the offset power bulge hood or the Trans Am/GTA style hood so verify the hood clearance!!!
I have done this on my 1992 Firebird with the L03 5.0L TBI engine and the base smooth hood with the air cleaner lid, offset air cleaner stud and wingnut from an 1991 GMC Sierra C1500 with the LO5 5.7L TBI engine and a K&N E-1500 air filter. It clears with just barely touching the lid against the factory under hood insolation pad that is drooping some. I checked mine by dusting the air cleaner lid with baby powder and very slowly and carefully closing the hood until it latched fully closed making sure no increased pressure was needed as compared to closing it with the factory air cleaner setup and keeping a close eye on the center of the hood. There was just a very faint dusting on the under hood pad, circle shaped matching the taller truck air cleaner lid's highest point.
If you want to improve the air flow of the rest of the air induction system then look for some thing like this air cleaner setup off a 1977 to 1979 Chevy Caprice Classic with a 5.7L engine.
If you could find one of these and was crafty you could replace the snorkel, air duct, and air scoop going to the stock L03 TBI air cleaner base. Looks like it would flow better than the stock Firebird parts and seems low profile enough for the air scoop to fit on the Firebird's core support. If you was really lucky and found two of them you could probably make a dual snorkel air cleaner by flipping the air scoop upside down and doing them same thing on the passenger side.
Driver's side. Upside down for passenger side.
And after all that work freeing up the intake air flow you would still have to replace the entire exhaust system starting with the cast iron TBI manifolds and everything all the way back to the exhaust tips because the stock TBI exhaust system SUCKS in through a straw and blows out through a straw.
Don't even bother with running an open element air cleaner. It's just going to ingest 230+ degree under hood air and run worse than the stock single snorkel air cleaner assembly did. 3rd gens run hot under hood and they trap the heat under hood.
14 PSI spring is a good adddition I done same thing on my L03. Send the TB to Sean at SPR Performance I think he is on here as ( DC TRUMPET ) if you really want a nice one. He bored mine to 46mm and upgraded the injectors and put in an 18lb spring for my 350. He does really nice work.
Could be useful if the throttle body shaft supports are worn. Verify first. And if you do this, be absolutely sure to remove the staked part of the throttle shaft threads that hold the throttle plates. You'll break the screws otherwise.
Originally Posted by Chris 84TA
got an injector spacer
Total waste of time, money, effort, and enthusiasm. GM used spacers on the big-block throttle bodies because the throttle plates were bigger diameter. The raised injectors therefore sprayed a wider cone of fuel, matching the bigger diameter of the throttle plates.
A wider cone of fuel does nothing useful on smaller throttle plates.
Originally Posted by Chris 84TA
and 14psi spring.
Isn't that what's in there already? Perhaps I'm wrong. Most everyone wants the fuel pressure at the high-end of spec (9--13) or even a little above.
I just popped a washer under the OEM spring, between the spring and spring seat.
Originally Posted by Chris 84TA
I was considering unshrouding the base on one of the 2 throttle bodies, but i think that may be overkill on the stock engine.
If you mean "grinding the lips around the throttle bores", yeah, that's probably a waste of time. Max airflow through the throttle body isn't what holds-back the stock-ish engines, it's flow through the ports 'n' heads, and exhaust that's the bigger problem.
Originally Posted by Chris 84TA
I am also going to clean the stock injectors.
Again, if they need it, this is very wise. But these injectors flow far more than port injectors, the passages are bigger and not so prone to plugging. I wouldn't do anything "special" to clean them beyond a bottle of Techron Complete Fuel System Cleaner in the tank for the first couple of fill-ups, regularly buying gasoline from a station advertising "Top Tier" fuel, and a bottle of Techron in the gas tank at each oil-change time.
Don't let the fuel filter plug. Clean the screens on the injectors, and have some thought for the filter sock in the tank.
well the injectors had corrosion from being in a salt state.
I cleaned up the injectors and rebuilt the pod with the new spring. I think I may still need to replace the iac module. engine is running good, but I have not tried putting it in gear. with the 350 injectors it was not idling down and was like neutral dropping when putting it in gear.
It seems to be running better than ever. If I can get the front end aligned better tomorrow, I will take it for a test drive.
Could be useful if the throttle body shaft supports are worn. Verify first. And if you do this, be absolutely sure to remove the staked part of the throttle shaft threads that hold the throttle plates. You'll break the screws otherwise.
Total waste of time, money, effort, and enthusiasm. GM used spacers on the big-block throttle bodies because the throttle plates were bigger diameter. The raised injectors therefore sprayed a wider cone of fuel, matching the bigger diameter of the throttle plates.
A wider cone of fuel does nothing useful on smaller throttle plates.
Isn't that what's in there already? Perhaps I'm wrong. Most everyone wants the fuel pressure at the high-end of spec (9--13) or even a little above.
I just popped a washer under the OEM spring, between the spring and spring seat.
If you mean "grinding the lips around the throttle bores", yeah, that's probably a waste of time. Max airflow through the throttle body isn't what holds-back the stock-ish engines, it's flow through the ports 'n' heads, and exhaust that's the bigger problem.
I once thought the same way you did on the injector pod spacer and the lips on the TBI. Then I started datalogging the ECMs and noticed that even a 4.3L starts pulling a noticeable intake vacuum at WOT. The 4.3L, 5.0L and 5.7L use the same TB bore diameter. The bigger the engine the more vacuum it creates as the RPM increases. The pod spacer knocks a bit of vacuum off the intake tract, especially with the power bowl or a flat base air cleaner. Removing the lips allows even more airflow into the TBI. The TBI unit is only like 450 cfm @ 3 in/hg. With the pod spacer and the ridges removed it is about 550 cfm @ 3 in/hg. Once you start opening up the rest of the breathing, the TBI becomes even more restrictive in stock form.
A new spring is not a bad idea provided it is the right one. I have seen too many to count that have rusted into two or more pieces now even here in Texas. I have seen TBIs run with 6-7 psi of fuel pressure. The Firebird my brother had was beyond weak and it was running at 9 psi. I threw a pod on it that I had previously set up to run 13.5 psi. The car ran a lot better. At 9 PSI when you went WOT, the narrowband 02 sensor would peg out lean and show like 50mv. No driveability issues, just no WOT power. The spring seat cup was factory adjusted at a very low height on that one. My guess is that car ran horribly lean its whole 215K miles. It was not a weak pump either, as soon as I put the other pod on it the fuel pressure jumped right to where the pressure had been running prior.
it looks like the stock fuel pressure regulator is adjustable, but just has a cap over the head of the screw. I think I will see if I can mod the spare one to be adjustable and get a fuel pressure gauge to see where the fuel pressure is really at.
it looks like the stock fuel pressure regulator is adjustable, but just has a cap over the head of the screw. I think I will see if I can mod the spare one to be adjustable and get a fuel pressure gauge to see where the fuel pressure is really at.
There are several different styles, and they are adjustable. Until GM melts metal to make them "tamperproof".
Some guys have removed the melted metal to make them adjustable again. I've wrecked more than one trying to do that.
10-31-2023, 01:09 PM
