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1989 Chevy K2500 TBI Setup - What's Your Problem?

TBI Power
Nathan Seabaugh, Northern Illinois:
I am building an engine for my '89 Chevy K2500. This is my daily driver/tow rig/off-road truck. I was hoping for about 300 hp while still retaining the factory TBI setup. I am flexible with going to a carb setup but would still like to use the stock heads. I have a good standard-bore, two-bolt-main 350 to start with that is assembled with a factory lower end and cam. I also have the carb intake to match the factory TBI heads if needed and a set of long-tube 15/8-inch headers. I know I need to swap out the cam to make some power, but what can I realistically expect while using stock Chevy K2500 heads and TBI? This Chevy K2500 engine is going to be in front of an NV3500 trans, an NP241 transfer case, and factory 4:10:1 gears. Can I make that much power with a $1,500 budget and the parts I already have? Or should I save a little more money and get a set of Vortec heads and a new intake manifold?

Jeff Smith: There are several ways to go about your Chevy K2500 setup, Nathan, by remaining with the original throttle body injection (TBI) system. To make 300 hp with TBI, the goal is to systematically eliminate breathing restrictions, but we have to say it will be tough with a relatively stock throttle body. Even on a Q-jet-equipped, small-block, 350 Chevy, the exhaust is the most critical point. You've addressed that perfectly with the 15/8-inch headers. Just be sure to include a 21/2-inch dual-exhaust system or a 3-inch single-exhaust and that part of the system will be acceptable.

Before we get to the electronic side of things, it sounds like you have a new engine ready to go into the truck. The best approach would be to go with a stock camshaft and a set of Vortec iron heads. These heads flow very well and would be a good choice to make 300 hp with a mild camshaft. However, the Vortec heads use a different intake bolt pattern that does not match up to the TBI intake manifold. There are ways around that by using a GM Performance Parts aluminum intake that bolts to the Vortec heads and features a stock mount for the TBI. The intake is PN 12496821 ($370 from Scoggin-Dickey). This gets expensive because you still have to purchase a set of Vortec heads, so we'll concentrate on retaining the stock heads for budget reasons. To keep the price down, have your machine shop perform a 30-degree back cut to the intake and exhaust valves. Stock cams produce relatively low lift, so the back cut will improve flow in that area. If you want to do more, spend about two hours opening up the exhaust valve throat to 90 percent of the exhaust valve diameter (1.5 x 0.90 = 1.35-inch diameter). This will help horsepower above 4,000 rpm or so.

The next restriction is the actual TBI unit. Stock small-block throttle bodies only flow around 500 cfm (compared with a stock Q-jet that flows 750 cfm). It's possible to make around 275 hp with a stock TBI unit, so rather than go nuts on a heavily modified TBI, I'd suggest you start by making sure the existing TBI unit and injectors are in good shape. The actual throttle plate diameters for the 4.3/5.0/5.7L TBI units are all the same at 111/16 inches (big-block TBIs are 2 inches). The difference is the injector flow rates. The 5.0L injectors flow 40 to 45 lb/hr, the 5.7 ones are 50 to 55 lb/hr, and the big-block injectors flow around 75 lb/hr. We would suggest sticking with the stock 5.7L injectors to begin your testing.

The big variable is fuel pressure. These late-'80s and early-'90s TBI injectors are intended to run at between 11 and 13 psi, although it's typical to see the pressure down around 10 psi. Generally, a 1-psi change in fuel pressure from 11 to 12 psi, for example, can be worth roughly an additional 4 percent fuel flow. That doesn't sound like much, but from 50 to 52 lb/hr can be enough fuel to make another 8 to 10 hp.

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