Free Horsepower

We found more than 60 hp and 23 lb-ft of torque just by switching to FAST EFI

Words and Photos: Brandon Flannery

We all need a little more useable power, don’t we? Well, with a little help from FAST, we found more lurking inside this small block. Years were spent transforming Shawn Brereton’s ’55 Chevy into the childhood dream car we all doodled with big wheels, dark paint, and a big honkin’ supercharger poking through the hood. As we all know, when building a dream car like this, it’s easy to get carried away making the biggest, baddest engine you can, and supercharging brings on its own set of complications.

Bearing that in mind, this engine was built to drive. With a sensible parts selection, it has reliably powered this car since 1992 and is still going strong. Lest you think this has been a fairgrounds queen, it racked up well over 6,000 miles last year alone on road trips and local driving.

To get a baseline, the engine is a 355 with 72cc Dart heads. Compression was set at 9:1 with a set of TRW pistons, and the cam was ground on a 110 lobe separation with a .550/.575 lift. The BDS supercharger was topped with two 650 cfm Holley carbs, and it was backed with an American Powertrain T-56 and a set of 3.73 gears.

While the car was sound, Shawn is a “normal guy.” This means he’s not an engine builder or obsessive tuner, nor a racer worried about wringing every last ounce out of the car. While some may argue the carb system could have been fiddled with to be improved, changing jets in hotel parking lots isn’t his idea of fun. It was taken to an acceptable level and driven.

Though it worked, it left a lot on the table. The tune left the fuel mixture rich at idle and lean at high RPM. The timing was “safe” for wide-open throttle, but didn’t finesse itself at part throttle.
Our best baseline testing with the carbs produced 450.4 hp at 6,400 rpm and 432.9 lb-ft of torque at 4,600. Respectable, no doubt. However, the Air Fuel Ratio swung from 15.2:1 at 2,700 rpm to a fairly low 10.4:1 at 6,800, with the timing only reaching 28 degrees of advance.

With a new XFI Sportsman system from FAST, specifically developed for Roots-style supercharger systems with dual quads, the idea of having modern EFI was now a reality. We were curious how much more power could be found without changing any of the mechanicals. Could we break the magical 500-hp barrier with more precise tuning?

The answer was yes.

Yes, it was free horsepower, no, it wasn’t FREE. We did have to convert to EFI, but no other modifications were made to the engine itself. It simply maximized the potential of the moving parts. In the end, our best dyno pull broke the 500-horse barrier at 516.0 hp at 6,200 rpm and 476.8 lb-ft of torque at 4,700 – 4,900. What’s more, the XFI log showed an even 12.5:1 Air Fuel ratio and 32 degrees of timing advance through the pull. Same components, more efficiently harnessed equaled more power.

In addition to bigger dyno numbers, driveability and throttle response improved significantly. The crisp acceleration off the line and during gear changes were a pleasant surprise. It’s one of those things where you don’t know what you’re missing until you get something better. This fun car became a really fun car.

The actual conversion process wasn’t very hard. It can be done with common hand tools in a few days, save for adding an oxygen sensor into the header. We did replace the fuel tank with the preferred in-tank pump setup, and ran new fuel lines. The throttle blades on the EFI throttle bodies sit a little higher than those in a carb, so the linkage needed to be tweaked slightly, and we did replace the old distributor with one from FAST. The previous one would have worked with a phaseable rotor and some love, but the FAST unit is plug and play.

Follow along in the assembly photos, and give FAST a call if you want to get their XFI Sportsman Roots Blower Dual Quad EFI System — and find more horsepower in YOUR supercharged engine.

Shawn’s take: Behind the wheel

I’ve owned 1PDQ55 for 29 years now and installed this engine in 1992. It has never given me any trouble that wasn’t caused by me (i.e. timing set wrong, didn’t set the valve lash correctly, etc.).

Only there were a few things I wanted to improve about the car, and EFI seems to have been the magic bullet for them all. Mind you, they weren’t big problems to me (I’ve been dealing with them since ’92), but more like annoyances — one of those “if I ever get the money” kind of things.

In the hot, humid Memphis months, the water temperature would start to creep upwards if I got caught in traffic, to the point I would eventually have to pull over. Since the evolution of ethanol, no matter what gaskets I used or how tight I turned the thumbscrews, the carbs would weep fuel just enough to be annoying but not dangerous. Lastly, I didn’t feel I was getting the gas mileage I should be getting with a 6-speed and 3.73 gears.

I thought I had the carbs dialed in pretty well. Yeah, it smelled a little rich at idle, and the plugs were pretty dark, but the car ran strong, and it never gave me problems starting or running.

Once I made the switch to EFI, I can’t believe the difference. First off, I didn’t know the water temp problem would be fixed by EFI, so that was a happy surprise. I rode around Du Quoin fairgrounds for an hour in the summer heat and it never went above 195!

The fuel weeping is obviously not an issue with no float bowls. The biggest change however, is in the drivability of the car. The throttle response is so quick and crisp that it makes it feel like a completely different car, which leads me to the only problem I have found.

I can’t give a good accurate mpg average yet because I have had my foot in it at every light. I can say this, however, despite stomping on the gas pedal every chance I get, I have improved so far about 2-3 mpg in the first two tanks. I think there may be another 2-3 in it if I quit acting like a teenager again, but I’m not sure when that will be. I will provide updates on how things progress on the PPN website, on my project car page.

To date, I think this change to EFI, which I was very reluctant to do, turned out to be the best update I have done on the car — and there have been a lot of updates in 29 years!