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Tweek's summer tweek....
Published: 10/22/1999  
Author: Rick Metcalf AKA Tweek
Yeah, he's back. Has this been a long, hot summer, or what? At least the A/C in the full size GM trucks is a real ice cube maker! How about just thinking about this modification until it cools off outside a bit. This one might take a while to finish, and

Yeah, he's back. Has this been a long, hot summer, or what? At least the A/C in the full size GM trucks is a real ice cube maker! How about just thinking about this modification until it cools off outside a bit. This one might take a while to finish, and of course it is for the 90 model, but you can also make this work on the 91 and up if you want. Hey, look, the 90 model needs lots of help!

Let's talk throttle bodys here. In fact let's not fart around. Let's talk four (4) barrel throttle bodys. Everyone knows the sound of that old Quadrajet 4 barrel when you tip those massive secondarys open. When you have a big-block under the old Q-jet, the sound waves coming out of that gas mixer can be sweet!

Common knowledge is that anything that you do that would effect the sonic signature of the engine also effects the power and efficiency of that engine. Louder isn't always better, but it is an indication that you have made a change in the tuning and operation of the engine. Not just volume of sound, mind you, but the tone of the sound as well.

Some good and bad examples are: Glass packs: Loud and usually pitiful for power production. Headers: Probably the best dollar per horsepower investment you can make. Both these things change the sound of the engine and also directly effect the power production capability as well. Every performance mod will change the sound of an engine. Imagine going from a 2 barrel throttle body to a 900 CFM Holley 4 barrel one. Sound different? You bet! Better horsepower? Read on.

Let me mention this one's not for beginners. You need a good working knowledge of engine and injection repair techniques.

1. Open hood. (Instructions are in your owners manual) Just act like you are showing a Ford Lightning owner what a real engine looks like!

2. You did let the pressure down on the fuel injection didn't you? Unplug the fuel pump relay and start the engine and allow it to idle until it cuts off.

3. Remove the factory throttle body and all associated plumbing. Unplug all the electrical connectors, and remove the water lines to the unit. Remove the fuel lines slowly, wearing safety glasses to avoid fuel spray if there is some residual pressure in the system.

4. Do not throw out the throttle body. It is rare. Only the 454's use it. Sell it as a core to one of the re-builders, or sell it to a friend that has the 350 engine truck as an upgrade.

5. (91 and up model) This is where it gets expensive for you guys. Install your new 4 barrel intake manifold. The 90 model has a Q-jet manifold under the throttle body, so skip this step.

6. Install the Howell spacer plate. This adapts the square flange Holley throttle body to the Q-jet, or spread-bore flange intake and also has the required vacuum ports in it. It comes with a diagram of the vacuum line hook up, so I'll skip that part.

7. Re-work the factory fuel lines to adapt to the new throttle body. Careful bending and some pipe adapters should be adequate.

8. Install the new unit, do not over torque. This will warp the throttle body and cause the throttle plates to bind. Look on the bright side, it will only get a chance to do this once if you mess this step up. A big-block will turn 6000 RPM.....once. Hook up the fuel lines, and all vacuum lines as shown on the chart.

9. Install the new throttle cable, working from under the dash. You will need to adapt the bracket to hold the cable on the throttle body end.

10. You will want to close off the unused water lines that were hooked to the old throttle body. You can just loop the line back, hooking the outlet to the inlet, or use some plugs to cap the lines.

11. Go buy a GM double return spring set. Install this so that you get full return to idle. Test this with the motor off. Duh.

12. Grind off the tabs inside the production air cleaner so it will fit on the Holley TB. The lid should fit if you hammer the center down a tiny bit to allow the stud to be exposed. Use a block of wood to preserve the painted finish. Watch the vent line to the rocker cover. Check to insure the linkage to the TB does not bind.


13. Ya gotta adapt the two connector system to the single connector required by the Holley TB. Open up the ribbed wiring cover on the right side over the rocker cover. Find the two twisted pairs of wires there colored blue, green, red, and white. Cut these four wires ahead of the MAP bracket about 8" from the rear of the rocker cover. Strip and connect the vehicle wires to the new harness for the Holley. Blue to blue, green to green. then connect red to pink (either one) and white to pink. Butt splices work great here. Tape everything up nice and use tie-wraps to hold the new wires in place. Hook up the new connector to the Holley. Tape the four pin IAC connector up, it is no longer needed.

14. Inside the truck, remove the glove box liner. The engine computer is behind the glovebox near the firewall. I know you already disconnected the battery, didn't you? What? Who cares if the radio looses the pre-set stations!

15. Unplug and remove the computer and remove the small access door. Using a small screwdriver, unlatch the eprom cover, and remove the eprom from the computer.

16. Install the custom programmed Howell chip and re-install all that crap you took out. The chip will only go in one way, you will have to look at the old one, and orient the new one the same way. Be careful not to damage the chip, its pins, or the computer.

The fun part:

17. Start this puppy up! Let the fuel pump run for a few seconds to pre-fill the new TB. It should run if you did everything right. If it starts, let it idle for a bit to smooth out and purge the air out of the system. Check immediately for any fuel leaks and correct them. Also, look over the wiring and fuel lines to insure that they are not where exhaust heat will melt them.

18. All emissions equipment remains operational. The only function lost is the IAC or Idle Air Control. The computer can not control your idle speed because the passages for this unit are not in the Holley TB. After warm up, set the curb idle to about 8-900 RPM. This should let the engine idle even in gear with the A/C on.

19. Just like some of the other tweaks presented here, the computer requires some reprogramming time. It will take a while of everyday driving to get the data fields full of the normal operating parameters. This adaptive learning usually is almost complete after about 20 start and drive operations.

20. Here is an odd note to add. Because there is no longer heated (power robbing) water flowing through the throttle body you can experience icing. Air gets cold when you expand it. Use an air nozzle from an air compressor. The air nozzle will get cold with use. When you flow air into the engine, the vacuum under the throttle plate 'expands' the air as it flows in. This will cause cooling of the throttle plates and the surrounding metal components. In certain temperature and humidity conditions, ice will form rapidly in the throttle bores. 30-40 degrees outside air temperature and high humidity will cause this. The ice can completely block the bores, or hold the throttle open. Be aware of this and if the idle is suddenly erratic on a day like described, just allow a longer warm up time to correct this problem. As soon as the engine warms up, this problem disappears.

21. Another note. You can adjust your fuel pressure on the new TB. 11 PSI is the setting the prom was written for, but up to 13 PSI should be available from the factory fuel pump mounted in the tank. Use caution here, monitoring any changes using an OTC analyzer looking at O2 sensor data while operating at full throttle. Too much fuel is just as detrimental to power production as too little. Also, this mod usually passes inspection if they just sniff and stick. The visual should pass as long as you do a clean factory like install. How many of those inspection geeks have ever seen a 454SS anyway? Oh yeah, the Holley is not progressive. All four barrels open up at once. Talk about punch! Still it is very driveable with the modified linkage as supplied by Howell.

Now, when you mat the throttle on this big-block, you are gonna hear that big ass moan from under the hood. You know when you hear that new sonic signature that you really did do something good for making horsepower, and bad for Ford owners in your neighborhood!


Here is a post-script for the article from one of the web site readers:

Cool article on the throttle body swap! As I read it, I thought of a couple things that I wanted to pass along based on some hard lessons learned:

1) The Holley/Howell throttle bodies can be ordered from Howell with an IAC motor that plugs into the factory connector, giving you the same idle control that the SS came with from the factory. Also, the throttle body can be ordered with drilled "ported" vacuum connections for the EGR line (S port) and the CAN line (A port) in addition to the other normal vacuum ports. This retains all of the factory emissions control strategy, and will help a person to pass some of the more sophisticated state emissions tests.

2) An easy way to tell whether the MEMCAL has been installed correctly in the PCM, and contains valid codeset, is to watch the SERVICE ENGINE SOON light as you key on. Make sure the ignition is off for at least twenty seconds. Turn the key to the run position without starting the engine and watch the SES light. It should illuminate, then go out, then illuminate again and stay illuminated. This is called a SES light bounce. If you do not get a bounce then something is wrong and damage could result. If you try to start/run the engine when a proper SES light bounce is not present, then the computer will default to backup fuel and spark mode because it cannot execute a valid computer program. This can happen if the MEMCAL is in backwards, or not seated, or contains a faulty program, or if the PCM has been damaged electrically.

3) You DO NOT want to ever run the Holley/Howell in backup mode, or you will damage the cylinder bores with excess fuel. Backup fuel mode is factory calibrated for two 75 pound per hour injectors. The four Holley injectors deliver 80 pounds per hour of fuel each, which is in total a little over twice too much fuel for the engine. Backup fuel is usually around 12:1 or richer from the factory, and at twice the fuel delivery rate, you will be trying to run at 6:1 or richer!

4) Forget trying to get more fuel flow through the Holley injectors via increased fuel pressure. When the Holley injectors are "paired" and driven by the factory PCM, they will only tolerate 15-16psig fuel pressure, with a fully charged battery and a good alternator. At 18-20psig under the same conditions, the injectors intermittently shut-down or stop flowing completely. Still, at 15-16psig, you will have enough fuel delivery for just about 600 Hp.

Hope this helps a little. With other program and hardware mods the Howell/Holley throttle body is an excellent foundation for substantial horsepower and "fun factor" improvements.

Walt Sherwin

And here are further items of information on the Holley/Howell throttle body modification:

If you park a Holley 4BBL TBI on top of a hot engine like your 502 Rat, it can sometimes be difficult to get radical top end power and RPM. This is mainly due to the fact that the big Holley B2 injectors simply don't like to turn on and off very quickly!

In fact, in a typical TBI mode, at about 5000 RPM the injectors can say "forget it", and stop physically opening and closing properly, even though they are being electrically commanded to do so. This usually limits horsepower to about 450-500 Hp. Even Holley rates its own Di System at 500 Hp. To push beyond this limit you have to pay special attention to a few things, but the rewards are substantial, with almost 600 Hp in return! Beware, these same limitations can also appear at lower RPM's if any part of the injection system is marginal, or if the system is not properly setup or controlled.

I would tend to perform the following health checks, in the order that they appear below, to first sort out your 502 Howell/GM system. At the end of these checks, you will know for certain whether your Howell/GM system is performing as intended and you can then make a decision about what other steps may be required by your project.

First check the electrical health of the injectors. Remove the black pods, to expose the injector terminals. You want to use a digital voltmeter to measure the resistance between the terminals. BUT, you want to take two different measurements for each injector. Measure from the large body of one terminal, to the large body of the other terminal. ALSO, measure from the very end of the small tip of one terminal to the very end of the other small tip of the other terminal. In both cases you should expect to see around 1.5 ohms. Much higher or lower indicates a bad injector. Holley B2 injectors are famous for loosing electrical connection between the large outer body of the terminal post and the small inner body of the terminal post, which interrupts the electrical connection between the coil and the computer. This will cause an injector to intermittently fail and eventually stop working. You can fix this by carefully applying solder (high quality TV & Radio) where the small tip meets the larger outer body in order to bridge the connection. Otherwise, you will have to replace the injector. Chances are that your 4 injectors are from similar build batches, and if you see this condition on even one terminal of one injector then you can bet that you will see the same on all of the others eventually!

Check your fuel pressure. Holley B2 injectors will work all day long at 15psig. At 18psig they will intermittently not open against the inner hydraulic pressure, especially when hot. At 20 psig, the injectors will shut down and stop. All of these pressure relationships hold true only if the injectors are driven by individual 4/1 amp current drivers, as Holley had intended. Your Howell/GM setup does not use individual 4/1 amp drivers (more on this below) so you should very seriously think about setting your pressure at 15psig! These relationships also assume a fully charged battery, and an alternator that is delivering at least 13.2 volts.

Next, I would flow test the injectors at your nearest fuel injection shop. It is a really cheap way to make sure that all is well, and it is also a good opportunity to check for funny spray patterns. The injector body shape is identical to that of a Chrysler/Bosch TBI injector, and as such will fit in most commercial injector benches (no, Bosch don't make the injector, nor do they have anything that even comes close in terms of flow capacity). Make sure that the shop uses the fuel pressure that you are running in your truck. Bill Howell can also perform this service.

Okay, so lets assume that everything about your Howell/GM system has checked out perfect so far. Here's how the base Howell/GM system works, and why it signs off at less than 500 Hp..........

Two of the Holley B2 injectors are paired in series with each other and are driven by one of the original factory 4/1 amp drivers found in the GM computer. The other two injectors are paired in series with the other GM 4/1 amp driver. This is a wonderfully simple and functional hookup that serves most "normal" applications very reliably. However, it also means that the "paired" injectors share the current of the 4/1 amp driver and that each injector experiences half of the opening force (current) that it would of, had it been driven by its own 4/1 amp driver. What's important to you, is that this really slows down the injector's response to being commanded on and off.

Each pair of injectors is commanded by the GM TBI computer to turn on/deliver fuel/shut off and be ready to do this again by the end of every second ignition pulse. To think of this another way, each pair is fired twice per crank revolution. While the injector is struggling to open and close it is not delivering its rated volume of fuel. What's important to you is that at 5300 RPM these series paired injectors are only fully squirting during about 66% of the time allowed!

Now at this point I would never think of scrapping the Howell/GM computer system. I would call him and discuss your project in detail, and let him offer recommendations. If memory serves me correctly, he may be able to offer an external 4 port injector driver box, or perhaps a faster 91-93 SS GM TBI computer, or even perhaps a Camaro computer. I like the idea of using GM computer hardware because it interfaces so well with the rest of the vehicle's systems, and has rock solid reliability. Also, you can buy replacement parts literally around the corner at your local dealer, and you can use common service tools, manuals, and built-in diagnostics to fix the darn thing if it ever breaks.

I mentioned the Camaro computer, and here is why it is significant. The Camaro computer is designed to "pulse" the injectors (ie: a pair of Holley B2 injectors) once per every crank revolution, and in some cases once per every two crank revolutions. By doing so, you can eliminate 3 to 7 of the turn on/off injector events as compared to what you would have with a TBI computer! This lets the Holley B2 injectors do what they like to do best.......flow fuel!

If after all of this you are still bent towards some other aftermarket system, then my only advice is to interrogate the manufacturer's technical help desk. If the company does not have a help desk, or if it is not very helpful towards you, then I would think twice about using their product! Ask the company for a description of how their system works, and demand documented proof that it is capable of what you want it to do.

If I had to pick just one aftermarket TBI system to control a Holley TBI on a 502 (aside from the Howell/GM system) I would, and have in the past, recommend the systems from Electromotive. They offer quality products and software, that are all supported by quality people.

Here is the contact information for:

Howell Engine Developments

24356 Sorrentino Ct.

Clinton Township, MI 48035

(810) 765-5100

[Send E-Mail to Howell Engine Developments]

[Link to Howell's web site]

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