There aren't too many things more misunderstood to many of us than fuel injection. Although carburetors haven't been used on new cars since the mid-'80s hot rodders haven't moved as quickly. There's no denying that a well-tuned fuel injection will perform better than a carb, but there's still a stigma left by some of the early aftermarket kits that left a whole lot to be desired.
One of the best ways to retrofit fuel injection is to still take advantage of the research General Motors put into it and find a used factory system. The Tuned Port Injection (TPI) units of the mid '80s to early '90s are still some of the best and easiest to do. In an effort to take some of the mystique out of upgrade to this "new" technology, we got together with the folks at Street & Performance to get some of the most common questions we get asked answered. We'll be following this article with an actual installation, hopefully, next month.
1. Why should I swap to fuel injection?
Swapping to fuel injection has many advantages. Your engine will last longer and have better low- and mid-range torque. The computer will keep the air/fuel ratio correct even when changing altitude, which not only provides better driveability, but improved fuel mileage.
2. Can I get a Tuned Port Injection unit out of salvage?
Yes, the only part to wear out is the shaft in the throttle body which can be replaced with a larger aftermarket throttle body or a new factory unit.
3. What vehicles should I look for as good donors?
The '85-92 F-bodies (Camaros, Firebirds, and Trans Ams) will be the most plentiful, but '85-91 Corvettes are also out there now at reasonable prices. The Corvettes were all 350 V-8s. The '85-86 F-bodies were 305s and the
'87-92 were either 305s or 350s.
4. Will the TPI intake manifold bolt onto my earlier heads?
The '85-91 Corvettes and '85-86 F-bodies all have the early bolt pattern. The '87-92 F-bodies have the late intake bolt pattern, but can be interchanged, or the two center bolt holes can be modified to fit early heads. Street & Performance will trade or modify them for you. If you have the new Vortec of Fast Burn heads, S&P can supply intakes to work with these as well.
5. Do I need to find a complete unit or can I piece one together?
Ideally you want to find one as complete as possible, but you can piece one together. Most parts interchange, but not all. There are lots of TPI units on ebay if you're looking to keep your hands clean, or you can look in salvage yards as the TPI-equipped vehicles are old now and are hitting salvage yards regularly. The S&P team can help you, or you can send your unit to them and they will interchange the parts with their own or new GM parts. S&P also offers complete units set-up for your engine (283-406ci or beyond) and transmission (automatic or manual) in three finishes (natural, polished, and chromed).
6. How much should I pay for a unit?
If you find a used but complete unit expect to pay from $150 on the low side to a high of $500.
7. How can I tell if it's a 305 or 350 unit?
The injector numbers are the only way to tell. A GM dealer or S&P should be able to tell you which ones you have. All Corvette units were 350s. All '85-86 F-bodies were 305s, but the '87-92 F-bodies could be 305s or 350s.
8. What's the difference between the Corvette unit and F-body unit?
On all Corvette units, the fuel rails come out on the passenger side. On all F-body units, the fuel rails turn and come out on the driver side. The fuel rails can be modified for rear exits with fittings supplied by S&P.
9. Can I reuse the old injectors, or do I need to get new ones?
The '85-88 injectors can be cleaned with S&P's ultrasound system and made like new again. They have new performance screens and all other parts to service the injectors. S&P recommends replacing the '89-92 multi-tech injectors as they have had a lot of problems.
10. Is there anything I can do about the dents in my runners?
S&P can straighten them and make them like new most of the time, or they can supply new or reconditioned ones for whichever year you need. Aftermarket larger runners are also available that will supply the engine with more air and allow you to pick up some horsepower.
11. Can the fuel-injection unit be too big for my engine?
An engine is like an air compressor. The easier it breathes, the more power it will make. Porting the intake, adding a larger throttle body and runners, and higher flow injectors will help a high performance engine. But, just as a high-
flowing carburetor can be too big for a mild to stock engine, so can the fuel injection. Go too big and the torque will start to drop.
12. What is a Mass Air Flow (MAF) meter?
The MAF is an air meter that is in the air induction tube leading to the throttle body that measures the amount and the temperature of the air going into the engine.
13. Can I run the old computer?
Yes, but the '85-89 computers were all mass air flow (MAF), which is expensive and is susceptible to damage by dirt or water (new GM replacement is about $300). Plus it can get in the way when trying to fabricate a custom air intake to fit a specialized installation. The '85s used the #1226870 computer only one year, and '86-89s used the #1227165 computer. Also the '85-88 used a ninth injector for cold start. The '89s used the same computer as '86-89 but fired all eight injectors for cold start like the '90-92s.
14. What is the ninth injector?
The '85-88 units have a ninth injector on the driver side of intake that fired one injector on the side of the intake for cold start only. The fuel then has to travel down each runner under the intake. The '89-up computer fires all eight injectors right on top of the valves so that you get better cold starting.
15. Can I disengage the ninth-injector cold-start function?
Yes, by using the '90-92 speed density computer, you will use all eight injectors for cold start.
16. Why should I change to speed density?
Speed density is preferable for many reasons. First, although the TPI units through the years remained about the same, the computers kept getting smarter. GM hasn't updated the '85-89 MAF computer in years. But they just updated the speed density computer in mid 2003. Speed density, uses the Mass Air Pressure (MAP) sensor because it's more dependable and only has a four-wire hookup (key, starter, ground, and fuel pump). Speed density also provides better mileage, performance, and drivability.
17. How do I get a speed density computer?
Speed density computers were used on '87-up four-cylinder and six-cylinder cars and '90-92 V-8s. Look for computers with these numbers #1227730, #16198262, #16196344, and #8889196 and weather-proof computer #1227727 and #16198260. You can also still get one through GM or some aftermarket companies.
18. Can I use a four-cylinder or six-cylinder computer?
Yes, they're all the same but will need to be upgraded with a chip designed to work with the V-8. Just be careful not to get a V-8 chip that has the Vehicle Anti-Theft (VAT) protection built into it. VAT will only let the vehicle run for two seconds then shut down the injectors--not allowing fuel to get to the engine. Most 305 chips don't have VAT, but all 350 chips do.
19. Can VAT be taken out?
Yes, if you have a V-8 speed density chip, S&P can reprogram it for your application.
20. Can I mount the computer in the engine compartment?
Only if you use the '90-91 Corvette computer (#1227727).
21. Why should I reprogram the chip and not just use it?
Because the vehicle will perform better and have better drivability because the chip will be programmed specifically for your vehicle's transmission, rearend ratio, and tires.
22. Why use a GM computer and not an aftermarket programmable one?
The chip in the GM computer can be programmed for just about everything you want to do. If you want to build a stroker 383-406, the chips can deal with it. GM spent a lot of time and money developing computers and updating them, and they know what they are doing. They use the latest technology and you can get a replacement anywhere if the need ever comes up.
23. Can I put 305 injectors on a 350?
If your engine is not over 250 horsepower, you can put an S&P adjustable fuel-pressure regulator on it and raise fuel pressure. Plus when you get your chip programmed, you can tell the computer you are running 305 injectors on a 350, which will lengthen the pulse width to let more fuel to the engine when needed. Engines with 250-300 horsepower need more fuel, so they'll need to run 21.9 lb-hr injectors.
24. What size injectors did the factory use?
The 305s came with 19.9 lb-hr injectors. The standard 350s came with 21.9 lb-hr injectors, and the LT1 350s had 24.9 lb-hr injectors. The 300-400hp engines should run 24.9 lb-hr injectors.
25. Can I use LT1 injectors in my TPI?
Yes, but be careful. S&P has had a lot of problems with LT1 injectors when they have sat for a while.
26. Is there an advantage to chroming the TPI unit?
There are no performance gains, but it depends on if you're looking for style as well as performance. The natural as-cast finish is hard to clean. The polished finish looks good when freshly polished, but you have to maintain it to keep it looking good all the time. The chrome units offer easy maintenance--just spray on some Windex and wipe it off.
27. How big of cam can I use?
You can run a high-lift cam, but it'll need a wide lobe separation and be able to provide at least 13 inch or more of vacuum.
28. Do I need to use a special fuel pump?
An in-tank fuel pump is best because the pump is kept cool by the fuel in the tank, and they run quieter. The in-tank units don't have to work as hard as the exterior pump because they just to have to push the fuel. A lot of the late-model in-tank pumps will fit into older tanks, (S&P can help you determine if yours will work) or you can check with companies like Rock Valley, Tanks, No Limit, and others who sell new tanks designed for the in-tank pumps. You can use an external fuel pump. They work okay if properly mounted--which is close to the tank away from heat.
29. How much fuel pressure do I need?
You need 38 to 48 psi for a 300- to 400-hp engine. High horsepower engines need 40 to 48 gallons-per-hour of fuel. You don't want to use a racing-type pump because they push too much fuel, and the TPI regulator cannot push it back to the tank fast enough.
30. Can I swap to fuel injection myself or do I need a professional shop?
It depends on how mechanical you are and how comfortable you are with your skills. The wiring harness only has four wires to hook up (ignition switch, fuel pump, starter, and ground). Most of the rest of the job is the same as
swapping any other intake manifold. CLASSIC TRUCKS will soon be running an install story showing a 17-year-old retrofitting his pickup with a TPI. S&P has videos available to help walk you through it as well.
31. What wiring grounds do I need to make sure I have?
The battery needs to be grounded to the engine or transmission, and the engine needs to be grounded to the frame. If a ground is bad, you will have poor starting and poor drivability. You'll also need a battery with at least 850 cranking amps and a 90-amp or higher alternator.
32. Do I need to use an oxygen sensor?
Yes, the oxygen sensor lets the computer keep the fuel ratio at 14.7:1 for good fuel economy and performance. You'll need to install one in the exhaust somewhere (single wire with short headers, three-wire heated sensor with full-length headers). The oxygen sensor needs to stay at 600 degrees, and the 12-volt wire of the 3-wire heated sensor will keep it hot so the computer goes into closed loop mode quicker--which will give you better drivability.
33. What distributor can I use?
Use the '85-91 Corvette or '85-86 F-body large distributor with the coil. If you need a smaller unit, the '87 and later F-bodies used a small distributor with a remote coil. If you are using the '85-86 distributors with a roller cam, you need to change the distributor gear to the appropriate GM gear (PN 10456413) or an aftermarket one.
34. Do I need special intake gaskets?
With aluminum heads you need the gaskets for the late-'86-91 Corvette if you are going to run the water-crossovers (the intake has outlets from the back of the heads to let water out to keep from getting hot spots). Cast iron heads can use the '85-92 F-body gaskets.