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I want a Larger bore TBI

6K views 6 replies 6 participants last post by  Astronaut87 
#1 ·
Anyone know of a direct fit TBI with more cfm than the factory 4.3 TBI. I think i can get more hp out of my v8 conversion if i upgraded the tbi unit. Does anyone know what the cfm is on these factory 4.3 tbi units.
 
#4 ·
Can anyone tell me, why, When doing a V8 swap do i have to use the 4.3's TB, Why can't I use the 5.7 TB, the TP sensors is the same the IAC is the same the linkage is the same
 
#5 ·
You are either going to A)need to enlarge the opening of the intake to use a 454 TBI or B)use a spacer with larger openings. The way I would check if you need a bigger TBI unit is with a vacuum gauge. If you are pulling more than a couple inches at wide open throttle with the TBI you need more flow. Besides this the 4.3L and the 350 TBIs both use the same diameter throttle plates so it's not going to give you any gain at all.
 
#6 ·
FROM WHAT I have recently read in my car craft magazine is that the only diff between the 4.3, 5.0, 5.7 tbi is the injector flow rate where the 5.0 flows around 40-45lb per hour and the 5.7 flows around 50-55 lb per hour. Where as the big block tbi has 2 inch bores compared to the small block bore of 1 11/16 inches and they flow at around 75lb per hour. The big variable is fuel pressure, the late 80's - early 90's tbi's are intended to run at 11-13 psi but typically they will degrade to 10 psi. generally speaking a 1 psi improvement in fuel pressure from say 11-12 psi is worth about a 4% increase in fuel flow. This in turn could be enough fuel flow to make another 8-10 hp. One of the easiest ways to increase fuel flow is to install an adjustable fuel pressure regulator and set it at 13 psi. Remember that stock fuel pumps are only capable of 14-15 psi so dont ov er do it.
Also if you wish to simply increase air flow through the throttle body you can buy an injector spacer through trans-dapt that raises the injectors about 1/4 inch.

I hope this helps somebody out there and I didnt just ramble on for nothing :D
 
#7 ·
First of all, you don't have to use the 4.3 TB, as long as all of the sensors on the V8 TB will plug in...there's no difference in the TB itself, only the injectors...as someone pointed out above. On my 350, I put the 5.7 injectors on the 4.3 TB so that all the sensors would hook up. I'm currently building a 383 to replace the 350, and since I have access to the CNC machine shop, I bored out the manifold to accept the Holley 670cfm TB that I just scored off craigslist...which is the same size as the stock 454 TB, except the Holley unit came with an adjustable fuel pressure regulator. I don't think you'd see much of an increase in power from upgrading to the larger TB, if you're still putting it on a stock manifold. The manifold itself would still be a restriction. So you could try the adjustable regulator and see how much power you can get from that before you run out of fuel flow from the pump. The guy that sold me the Holley TB said that the stock pump was making the 18psi that he had the regulator cranked up to, but at that pressure it ran out of volume...so he said I'd have to upgrade to a higher volume pump. If you really want to upgrade to the larger (Holley or stock 454) throttle body, I'd recommend swapping to a square-bore carbureted manifold, and then buying the right TBI-to-square-bore adapter plate. Or if you know a machinist... you could trade, barter, or pay him to bore out your stock intake. The reason I went this route, other than having access to the machines and wanting to save some money...was to gain flow through the intake, while still having all of the sensors and accessory brackets bolt right up to the stock intake, and not causing any other issues there. I might consider boring out stock intakes for other people, but it still wouldn't be super cheap...considering the shipping costs, and the time each one would take.

By the way, from what I've found on the internet, I'm pretty sure that the stock 4.3, 5.0, and 5.7 TB's flow around 500cfm, and the big block and Holley units are about identical at 670cfm. I'm personally really curious as to whether a set of stock 454 injectors would bolt right onto one of the smaller throttle bodies, or not...

Wow...I really wasn't expecting to write that much...but hopefully somebody finds it helpful...

CK
1987 Astro RWD
TBI 350/700R4
 
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