Need some TBI interchangeability help please, 5.7-7.4 IAC TPS regulator etc.

**kilohertz** · 02-03-2023

Hi guys,

After a 1 year break I am back to work on my '68 Firebird TBI conversion for the Pontiac 350 (which may become a 400 or 455 one day). I have a complete system from a '95 C-1500 4.3 but I need (I think from my research) a 7.4 TB unit. I have punched the numbers into the excel spread sheet and for 300HP it shows I should be using the 7.4 and I think the flow is 670 cfm for the 7.4 TBI which I think is close to the small QuadraJet.

I have a couple of 305/350 TBs and have found a 7.4 from an '87 and the base from a '92, missing the injector pod/regulator plate.

So my question is, what can I mix and match between the 350 and 7.4 TBI units? Is the IAC the same? TPS? ( I know over the years the plug changed) Fuel regulator spring? How about the injector/regulator holder, same?

Some of the units I am finding are quite a drive and I want to make sure before I commit that I can make them work. I know I will need the 7.4 injectors and will probably buy new.

So, any help will be greatly appreciated.

Thank you.

**1project2many** · 02-03-2023

Hi,

Sounds like you're going to have fun with this. I believe "early" 7.4L TBI units run from 87-90 while "late" ones run from 91-95.

The IAC units are swappable. I have found it's usually easier to start with an IAC that matches the calibration you're using. If you're starting with a 5.7 Chevy cal, it's probably easiest to use an IAC for a 5.7 Chevy. Or copy all IAC values from a cal that matches the IAC you want to use. Also, maybe someone else can confirm this but I believe some of the 454 IAC's are wired differently and will operate in reverse of what's expected without wiring changes.

As mentioned, throttle position sensors come in two different styles. Early sensors have a large, oval connector and are attached with two screws. They are adjustable. Later sensors have a small, round connector and are attached with three screws. They are not adjustable.

Injector pods can be swapped. Throttle linkage is different on early 454 TB's. Early 454 was mated to a THM400 or manual transmission and does not have connection point for 700R4 detent cable. If you are using a transmission that does not need a detent cable this won't be an issue. Fuel injector springs are interchangeable although they are not the same. At some point GM raised fuel pressure in the 7.4 TBI. Watch for the taller spring to tell you it's from a higher pressure system.

**kilohertz** · 02-04-2023

Cool, thanks for the quick reply.

I think I should probably just pickup everything I can

then I can mix and match as required, as long as people aren't asking the moon. My harness is from '95 and uses the small round 3 pin TPS, pretty sure I have a spare TPS 3 wire flat here somewhere, switching is no problem. I "slotted" the holes in my 2.8 TBI TPS to give me some adjustment, it worked pretty well, just enough to get the reference to the right value after adjusting the butterfly set screw (which we are not supposed to touch but since TBIs didn't come with stock flatheads I figured I could break the rules) and the IAC to get the desired 20-30 count.

So what I have found so far, complete TBI from '87 (not sure what truck) TPS, injectors and IAC, and from the wreckers the bottom portions of '92 454 TB missing the top plate and injectors/regulator but complete with IAC and TPS on one of them.

I finally found the adapter for my EMP-20 EPROM programmer so now I can program the 27SF512 (YEAH!) for the newer 7427 ECM.

Heading out shortly to pickup the complete TB, more later.

Cheers

pics from the wreckers

**stew86MCSS396** · 02-04-2023

Originally Posted by 1project2many

Hi,

Sounds like you're going to have fun with this. I believe "early" 7.4L TBI units run from 87-90 while "late" ones run from 91-95.

The IAC units are swappable. I have found it's usually easier to start with an IAC that matches the calibration you're using. If you're starting with a 5.7 Chevy cal, it's probably easiest to use an IAC for a 5.7 Chevy. Or copy all IAC values from a cal that matches the IAC you want to use. Also, maybe someone else can confirm this but I believe some of the 454 IAC's are wired differently and will operate in reverse of what's expected without wiring changes.

As mentioned, throttle position sensors come in two different styles. Early sensors have a large, oval connector and are attached with two screws. They are adjustable. Later sensors have a small, round connector and are attached with three screws. They are not adjustable.

Injector pods can be swapped. Throttle linkage is different on early 454 TB's. Early 454 was mated to a THM400 or manual transmission and does not have connection point for 700R4 detent cable. If you are using a transmission that does not need a detent cable this won't be an issue. Fuel injector springs are interchangeable although they are not the same. At some point GM raised fuel pressure in the 7.4 TBI. Watch for the taller spring to tell you it's from a higher pressure system.

http://www.gearhead-efi.com/Fuel-Inj...o-7-4-tbi-swap

Another thing don't know if it's worth mentioning...the BBC TBs had a spacer under the injector pod. Whether it adds flow this may be subject to debate.

**kilohertz** · 02-04-2023

Thanks for the link, some good info in there.

So I got some pics of the '87 TBI and I have never seen anything like this, it looks like it has an adapter plate made for a quadrajet as part of the body. Have a look at the pics. Is this GM's earliest TBI before they tooled up for the new manifolds?

Anyone seen this setup before? It's cool if it will fit my Pontiac Q-jet intake. I wonder what the donor manifold looked like? I think mine has 4 individual ports but I may have one kicking around that might have these oval ports...cool.

cheers

**stew86MCSS396** · 02-04-2023

That's purdy much the oem adapter that sits on a q-jet intake. Keep in mind that these came in trucks and hood clearance is not an issue. I'm still contemplating the Pontiac 350 swap into my '85 Caballero but I'm going to opt for a shorter 1/4-1/2" adapter plate due to hood clearance.

**kilohertz** · 02-04-2023

Mmmm, ya good point. I won't know until spring when the snow melts and I can get the car out of the shop and put the hood on. I painted it and it still needs some reassembly. I think there will be room...if not I can machine a plate like I did for the flathead. The center to center bore of the TBI is only 1/8" narrower than the Q-jet secondaries.

cheers

**kilohertz** · 02-05-2023

Picked up the TBI with the adapter today, 100,000 miles on the truck, really nice.

I took a picture of the manifold it came from and now I am wondering if it will work with a standard Q-jet manifold with 4 ports, I would appreciate hearing from those of you who know more about this than I. If it won't flow properly into the 4 separate ports then I have several options. Machine a flat adapter plate like I made for the flathead, just using the 2 large rear ports to mate up with the TBI, buy a new manifold with similar oval ports, or mill out the area between the primary and secondary on the stock manifold to match the adapter.

So what do you think?

Thanks!