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How to Make Power with a Chevy TBI V-8

Tips for adding power to a throttle body injected (TBI) Chevy 305, 350, or 454 V-8 engine.

Here's a question from a reader about adding power to a GM TBI engine. The principle is pretty much the same with any speed-density fuel-injection system, so check out engine guru Jeff Smith's answer to a question posed by Nathan Seabaugh of Northern Illinois.

Question: 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 1 5/8-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 transmission, 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?

Answer: 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 1 5/8-inch headers. Just be sure to include a 2 1/2-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 going from 50 to 52 lb/hr can be enough fuel to make another 8 to 10 hp.

One of the easiest ways to increase fuel flow is with an adjustable fuel pressure regulator. Companies such as JET, Turbo City, and CFM Technologies sell adjustable pressure regulators. The price hovers around $90 depending on the source. The approach is to first measure the existing fuel pressure. If the pump in your truck is already delivering 13 psi at the TBI, consider yourself lucky and don't bother with the adjustable piece. More likely, the fuel pressure will read 10 to 11 psi. Mount the new regulator and bump the pressure to 13 psi. You won't be able to go much higher because stock pumps generally can't generate more than 14 or 15 psi. Keep in mind that raising the fuel pressure increases fuel delivery across the entire engine operating range. At part throttle, this additional fuel isn't needed, but the oxygen sensor should sense a rich mixture and reduce the injector on time (pulse width), which will lean the mixture back to the ideal 14.7:1 air/fuel ratio. At wide-open throttle, the computer switches to its base fuel map and the additional fuel pressure will increase fuel flow and (assuming the engine needs the additional fuel) will increase power. You should test any fuel pressure change with a WOT acceleration run in Second gear from about 30 to 60 mph. If the truck accelerates to 60 in less time, the engine is making more power.

Another simple trick we've tested that works well is the PowerCharger (we call it the soup bowl) from Hypertech. This is a simple radius adapter shaped like a bowl and placed between the truck air cleaner and throttle body. Summit sells these adapters for $37.25 (PN 4001). We tested the PowerCharger on our TBI 350-powered Jake truck buildup ("Project Jake, Part II," July '04) and discovered it was worth 8 lb-ft and 8 hp. If you want more, multiple companies make swirl-inducing TBI spacer plates. CFM Technologies, Trans-Dapt, AirAid, JET, and many others manufacture spacers that, while we've never actually tested them, appear to be worth some torque. Expect to pay anywhere from $50 to $100 for one of these spacers.

You also mentioned camshaft requirements. The stock camshaft for your application is pretty mild. We found the specs for an '81 to '87 computer-controlled flat-tappet hydraulic cam that specs out at 194/202 degrees at 0.050 with 0.384/0.403-inch lift. Adding a longer-duration camshaft with more lift will drastically improve airflow and increase power at a higher engine speed. The problem is that it will require additional fuel that will mean changes to the base fuel map in the computer. We'll get to that in a moment. If reprogramming the computer isn't a deterrent, consider a cam like the Edelbrock Performer-Plus flat-tappet hydraulic camshaft and lifter package (PN 3702, $127.95, Summit Racing). This cam is designed specifically for the TBI 350 small-block and specs at 194/214 degrees of duration at 0.050-inch tappet lift with 0.398/0.442-inch lift. As you can tell, it doesn't deliver much of an increase in lift. The advantage is the additional duration that will move up the power curve slightly. The downside to this cam is that for a tow vehicle, it may hurt the low-speed torque slightly, which might not be a bad thing. The idle quality should still be close to stock.

Here's where it gets tricky. The GM TBI is a speed-density system, which assumes the engine ingests a given amount of air at wide-open throttle (WOT). Mass airflow systems measure the air entering the engine, but speed density systems like those used on these TBI engines do not. Instead, they assume a stock airflow value based on throttle position and manifold absolute pressure (MAP) sensor readings. The fuel requirements at part and full throttle are established by the factory based on the engine's air and fuel demands. Increasing airflow with a better intake, heads, camshaft, and/or exhaust increases the airflow through the engine and demands a change to the stock computer base fuel and spark maps. These TBI injection systems are controlled by GM computers equipped with chips that are burned or electronically imprinted with the necessary information. Later GM computers use erasable EE-PROMS that can easily be reprogrammed while still in the vehicle. However, the chips in the TBI trucks and cars have to be removed and a new chip is burned with the new information. This can get time consuming and pricey if you have to make multiple changes to improve drivability. If you plan to do this, call one of the companies such as JET or CFM Technologies to find out about a complete package it offers. These companies can supply the parts and a compatible chip as a package that has a much better chance of being correct on the first try. These systems cost a little more than what you could put together on your own, but that's because you're paying for their experience and tuning expertise. Trust us, it's worth the investment.

More Info
Airaid Filters
Phoenix, AZ
888/876-8984
airaid.com

CFM Technologies
Moultonboro, NH
603/238-6721
cfm-tech.com

Edelbrock
Torrance, CA
310/781-2222
edelbrock.com

Hypertech
Barlett, TN
901/382-8888
hypertech-inc.com

JET Performance Products
Huntington Beach, CA
800/535-1161
jetchip.com

Scoggin-Dickey Parts Center
Lubbock, TX
800/456-0211
sdparts.com

Summit Racing
Akron, OH
800/230-3030
summitracing.com

Trans-Dapt
Whittier, CA
562/921-0404
tdperformance.com