Throttle Body Experience
Improving drivability and more power in the real world
The abundance of trick and cool equipment available for our cars is incredible these days, but no matter how the new product is engineered, you still have to invest a good bit of planning, strategy and time management to make things come together.
Many projects go awry simply due to frustration caused by unrealistic expectations. There was a time when basic fabrication skills, cunning and just good ’ole fashioned American ingenuity made cars better and faster and today is no different.
We installed a Holley electronic fuel injection system on a stout-running ’65 Corvette owned by Donny Grace. He bought the project in 1996 and began a body-off rebuild with the full intent of making a “matching numbers” restoration. Somewhere along the line, he switched directions and it turned into the killer street machine he always wanted to drive.
His current engine is a 550-plus hp 388ci stroker. It features Dart 215cc heads, a 248°/256° at .050-inch with .656/.623-inch lift on a 109 LSA Bullet solid roller cam, Doug Nash five-speed and 3.36 gears in the original IRS differential.
We selected Holley part number 550-412 HP EFI with the throttle body injection configuration since Donny was looking for a simple installation to replace the Holley 750 double pumper that had served so well. This kit came with a 900 cfm throttle body (which Holley claims can support up to 600 hp) to bolt right onto his 4150 flanged intake manifold.
The first step was to take inventory. Start walking through every component’s installation in your mind to make sure you have everything you’ll need. All of us on the project own midyear Corvettes, which gave us insight into the little quirks that we might encounter. The real advantage is to have people who have seen other configurations and can come up with alternatives to work well on your car.
The Holley kit comes with the electronic control unit, the throttle body assembly with four 85-lb injectors already in place, all the various sensors, complete wiring harnesses with OEM quality connectors, pre- and post-fuel filters and a ton of small parts and fittings. The kit we selected does not include the fuel pump (though some do). In our case we used a Holley #12-920 in-line electric pump.
While some EFI systems rely on a “returnless” design, Holley designed this one to utilize a return line. By constantly returning the excess from the throttle body, we eliminate the possibility of air bubbles and we keep the fuel much cooler. It’s not easier to install, but more reliable and trouble-free.
We concluded the biggest challenge was the installation of that fuel return line to the stock tank. There are numerous ways to do this. In some cases, there’s already a return line in place on the car. On most older vehicles, you’ll need to come up with a game plan. The Holley TBI uses only 21 psi of pressure instead of the 40 to 60 psi of other EFI systems. We decided to use a hard metal line. It’s possible to use the plastic lines, but getting those in place and routed could turn out to be as challenging as steel. There are also hoses designed to handle abrasion like the race-type wire braided style using A/N fittings, as well as some newer style fortified hoses. Do not run regular rubber fuel line the entire length of the car.
We decided to use a relay to power the fuel pump and trigger it with the ECU to take the load off the control wiring. Five-blade Bosch-type relays are found at any parts store. We discovered, for some reason, the neat little connector to cleanly wire them is not!
Due to the huge fuel cap opening on the Corvette tank, we opted to use a “pass-through” bung fitting to install without welding. Where space permits, you can also attach a return line to the fuel filler tube routed downward so returning fuel won’t impact the cap. On other “normal” tanks, you’ll need to weld a threaded bung fitting or tube into the tank or through the sender unit.
Corvettes of this era used a sending unit that mounts in the bottom of the tank and sealed by a special O-ring, which is notorious for swelling slightly once removed and a bear to get to seal again. Alternatives are to utilize a local radiator repair shop that could flush the tank and weld a threaded bung in place. If you need a new tank anyway, you can buy them ready to bolt in with all these fittings in place including an in-tank fuel pump.
The “pass-through” fitting we chose is commonly available through hot rod type warehouses, but not so common locally unless you know what to ask for. We found a neat little item designed for the installation of a drain plug in an automatic transmission oil pan.
We selected 3/8-inch for our return line and nabbed two five-foot sections. We grabbed a brass union to join the two lines and some fittings to adapt the flared ends to a barb-type end which allows short lengths of fuel injection-rated hose to connect the fuel pump and the throttle body. We added five feet of fuel injection hose (just in case), some clamps, an assortment of wiring terminals and solder to the parts counter. That elusive relay connector wasn’t available, but we found what we needed in the stereo section of another store.
We were fortunate to have a lift available that made it much easier to get things done. Paul Joyner jumped on removing the right header so we could weld the bung for the oxygen sensor in place. You can pull the header the night before and take it to a muffler shop first thing to be welded if you don’t have the equipment. Donny went about pulling the glovebox to mount the ECU. I removed the carb and bolted on the new throttle body assembly.
Paul was dealing with the funky alternator mounting system. Donny was using all sorts of measurements under the dash to determine the best way to mount the ECU between the heater box and the glovebox. A piece of sheetmetal was cut and drilled to make a mounting bracket. Paul removed the stock mechanical fuel pump and installed the supplied block-off plate, while I drilled the header and welded the oxygen bung in place.
Pulling the fuel tank on an early model ’Vette can be pretty easy if you have the spare tire carrier and mufflers out of the way. Donny’s exhaust system, however, was welded at each joint with no removable connections all the way to the headers. With the tailpipe tips protruding from the rear valance and the exhaust pipes snaking through the holes in the transmission crossmember, there just wasn’t enough movement available to allow the intact assembly to drop out.
We had to use a Sawzall! You’ll want to get the tank as empty of fuel as possible for safety and make it easier to handle. Cap the fuel lines and leave the fuel cap in place during removal. Whatever you do, think safety! The vapors created while working around gasoline will ignite quickly since they are already mixed with air.
Once removed, we took an extra safety precaution by filling our empty tank with water to displace any lingering vapors and easily drilled a hole to install the bung fitting. We then taped a socket to an extension (so we didn’t lose it in the tank), applied a little grease to the nut so that it wouldn’t slip out of the socket, then utilized Paul’s skinny arm to reach inside the large fuel filler opening of the ’Vette tank. I did the tricky part of holding the fitting from the outside while he installed the nut.
On most cars, the routing of the steel return line is pretty easy. You just make another line and route it with the stockers. The fuel lines on our Corvette are very cleverly hidden between the body and the completely boxed frame, leaving little room to do anything to them without jacking up the body. We voted to route the front section of our line outside the right frame rail and secure it up high near the body behind the removable rocker panel. The rear section was matched up with the stock supply lines along the frame and clamped into place. I just used a simple hand operated tubing bender to create the necessary shapes in a few minutes. While Donny and I installed the line, Paul went about reinstalling the fuel tank.
With the tank and return line in place, next was the electric fuel pump installation. Holley provides the pre/post filters along with the necessary mounting bracket, clamps and fittings. It’s best if the pump is kept below fuel level. Wiring was pretty straightforward with our relay install, but take the time to solder all the wires if you want to have a truly trouble-free installation, because 99 percent of all electrical troubles come back to poor and hurried wiring. Holley has made it as easy as they can for us with pre-terminated connectors that just plug in. Where you have to make a connection … do it right.
We thought we were ready until someone mentioned that the outlet line from the stock fuel tank sender seemed to wiggle a little bit as the original fuel line had been removed. It leaked like a New Orleans levee when we added gas. Out came the sender unit to braze the tube to the plate. Donny and I finally got that blasted O-ring to seal. It meant draining and adding fuel multiple times, which seriously cut into our expected timeline to complete this project.
With everything hooked, attached and plugged in, it was time to power things up and perform the initial configuration set-up. We loaded the supplied Holley software on a laptop computer, but Holley also offers a handheld LCD programmer for use with the HP Systems that is slick. Included in the software is an incredibly detailed set of installations, tuning and troubleshooting guides, as well as a long list of “base tunes” to get you started.
We selected the TBI85SB245HE file from the base tune folder and followed the instructions to program the ECU. This breaks down as: Throttle Body Injection with 85-lb injectors on a small block with a 245’ish at .050-inch cam. Holley has filled in most of the information necessary to get you running, but it’s a great idea to spend some time getting familiar with the screens and the easy navigation around the program. You’ll want to verify that the engine parameters match up and then look at the numerous dropdown windows of the features available that allow flexibility for things like rev limiters, fan controls, nitrous oxide, etc.
Once everything looks right, it’s time to turn the key to “run” to verify fuel pump operation and check for leaks. At this point, we couldn’t put it off any longer. It was time for Donny to finally join the ranks of the EFI world and crank that sucker up!
The engine fired right up and settled into a sweet idle. For those of us that have spent our lives in the comfort zone of carburetion, this is a big moment. It’s always a little intimidating to see what electronics can do to manipulate things in deference to Mother Nature.
Watching the computer screen, we saw that the oxygen sensor was reading the fuel mixture properly and that the computer was “learning” what it needed to do to make Donny’s engine happy. The throttle response was incredible, though we soon discovered that somehow the plug wires on #2 and #4 got crossed during the reinstallation of the header and that fabulous fiasco of an alternator mount.
The evening was perfect as we headed out for the first test drive. I settled into the right seat to watch the vitals on the laptop screen as Donny rowed through the gears. The Holley TBI system had no problem feeding everything that nasty small block wanted well past 7,000 rpm, and we had verification of the fuel mixture through the whole range. The data-logger function within the Holley system was able to keep track so I could review things. I couldn’t resist the ability to play with the live tuning functions as we drove to tweak the fuel mixture at specific points.
The magic within the Holley EFI system is that unless you just want to manually play with every field of the fuel mixture, all you really need is the initial base tune that Holley has developed. If your particular engine needs a little more or less fuel at any given point in the rpm band, it’s automatically provided by continuously self-tuning as you drive without touching a thing. You can save these final tunes for your current engine configuration, and if you make changes later, you can easily select a tune that is closer to your new combo (if it has radically changed).
After getting some extended seat time with the Holley TBI, Donny reports that while his old double pumper seemed to run great, the difference in drivability and fuel mileage is incredible with absolutely no loss in power. An added benefit is that his wife Debbie enjoys cruising a lot more since they don’t come home smelling like gas anymore (gentlemen, take note!).
You built your car to drive and the Holley HP TBI system allows you to enjoy all that new power with instant cold starts, on-the-fly tuneability and overall performance that a carburetor can never provide. While things might not always go as smoothly as we would like, just how bad can a good time in the garage with your buddies be? It’s just another part of why we do this stuff.