Formula for calculating injector pressure
Superchargers require supercharger goodies. Check out the boost gauge from Auto Meter; it&
The shift light is activated by a module that we hid under the shifter console in the Cama
The MSD 6BTM comes with a boost-retard adjuster that can be mounted inside the car or unde
We destroyed the gaskets while pulling the intake plenum and runners, so we needed an uppe
There is a four-digit code written on the ECM that is hidden behind the dash on the passen
The injector (on the right) is from TPIS. It is considerably larger than the stock units a
In the story entitled "Blower How-To," we bolted a Vortech V-1S to our 305-powered 1991 Camaro and hit the track. The supercharger produced 10 psi of the good stuff and ran a not-so-healthy 14.48 at an equally gutless 93 mph. Considering the baseline for this car was 14.90, we were a little concerned. Fortunately, the TPI Chevy is a well-researched and easy-to-tune platform and there were plenty of guys who came to our aid. We ended up changing the injectors and the computer chip and performing some standard drag race-type tuning before finally breaking into the 13s--just as we predicted (hoped). Heres how:
The first step was determining why the engine was backfiring and lurching around 5,000 rpm. The answer was in the fuel-pressure setting. As this is being written, it is still illegal for Californians to install an adjustable fuel-pressure regulator on a late-model car. This meant that we were stuck with a measly 43.5 psi at idle and about 50 psi at full go. The supercharger needs about 40 percent more fuel flow to feed the extra 100 or so horsepower it is designed to produce. To provide this much needed fuel, it is necessary to either install larger injectors and reprogram the ECM or add additional fuel pressure. Vortech decided to take the fuel-pressure route with the use of a fuel-management unit (FMU) to make the kit less expensive and easier to install.
The FMU is calibrated by Vortech to deliver 4, 6, or 8 psi of additional fuel pressure to the injectors per pound of boost produced by the supercharger. The V-1 S-Trim supercharger for the 1988-1992 Camaro responded best to an 8:1 FMU on a 350-inch TPI engine with 6 to 8 pounds of boost on the Vortech dyno. The 8:1 FMU was, therefore, supplied in the kit.
Our '91 Camaro has a smaller-displacement 305-inch engine that allows the supercharger to produce more boost than it would with a 350 or 383, all other variables being equal. The result was not 6 to 8 but 10 pounds of boost in first gear, wide-open. The FMU is referenced to manifold vacuum and boosts fuel pressure by restricting the fuel-rail return line. At 10 pounds, the FMU boosted the fuel pressure to slightly more than 100 psi (8 psi x 10 pounds + approximately 50 percent of the static pressure [20 psi] = 100 psi).
The factory injectors are rated for a maximum fuel pressure of 72.5 psi before the injector cycles off and cannot cycle on again. According to our fuel-pressure gauge, the stock injectors withstood about 90 psi before they shut down. This occurred at exactly 5,000 rpm and was causing the problems.
We were trying to keep this project at a low cost without performing extensive modifications to the rest of the engine. Unfortunately, because of the high boost numbers, we had to upgrade from the stock injectors to units that could handle the high fuel pressure. Injectors are size-rated by pounds per hour (lb/hr) of flow and maximum fuel pressure. The 305 Camaros use 19-lb/hr injectors from the factory, and the 350 uses 22 lb/hr. The lb/hr rating shouldn't be confused with the maximum fuel-pressure rating mentioned earlier; they are not the same. The lb/hr calculation is useful in picking the appropriate fuel-flow rate for an engine, and the fuel-pressure rating determines the maximum fuel pressure that the injector can withstand. When it came to selecting new injectors, the maximum fuel-pressure rating was our primary concern.
Tuned Port Induction Specialties (TPIS) offers Bosch injectors that can withstand well over 100 psi of fuel pressure in any size (lb/hr) you'll ever need for a small-block. Be careful of stock-style injectors that are rated to 450 hp; they are designed for pressures below 72.5 psi and won't work with this combination.
Remember, if you're using a 350 engine, it will have 22-lb/hr injectors and the supercharger will likely produce 6 to 8 pounds of boost pressure because of its larger displacement. The same FMU on a 350 at 6 psi will boost fuel pressure to 68 psi (8 pounds x 6 psi of boost + 20 psi = 68 psi). This will work with the 350's stock 22-lb/hr injectors if the fuel pressure does not exceed 72.5 psi.
We chose the 23-lb/hr injectors from TPIS. According to our calculations, they will feed 315 hp at 50 psi of fuel pressure, plenty for the stock 305. Here’s the math: pounds per hour x 8 x 0.8 x 2 = hp. Here’s an example: A 23-lb/hr injector on an eight-cylinder engine with an 80 percent duty-cycle multiplied by 2 equals 294 hp. To get the 315hp figure, you must use a conversion factor to determine the pounds per hour an injector will produce at a greater fuel pressure. The same injector will flow 24.6 lb/hr at 50 psi instead of its rated 23 at 43.5 psi. This is the formula (see mathematical designation above right):
The square root of the product of the new fuel pressure divided by the old fuel pressure times the original injector size equals the new injector size. We used this formula to determine if the injector will match the fuel demands of a stock 305 with no boost (about 200 hp). At 50 psi, the 19-lb/hr injectors can handle about 250 hp but cannot handle a significant boost in fuel pressure. Our first test runs with the stock injectors were very poor as a result.
Keep in mind that changing injector sizes on a naturally aspirated engine is different than the considerations for a supercharger. The stock ECM will accept injector changes that are no more than 20 percent larger than stock. In our case, the 23-lb/hr injectors required a calibration change. We called Superchips in Longwood, Florida, for help.
According to Superchips engineers, changing injectors and producing 10 pounds of boost requires recalibrating the fuel and timing maps in the electronic control module. Yup. Our stock 305 ECM is calibrated for an engine with 19-lb/hr injectors, and the injector-pulse widths (duty-cycle) are based on these calculations. Larger injectors will deliver more fuel at the same pulse width and cause a rich condition. The solution is to have an electronic chip-burner tool that can access the fuel maps and tell the ECM the size of the new injector to use in its calculations. All the calculations for the ECM are stored in a programmable read-only memory chip (PROM) in the ECM. The PROM contains information stored in hexadecimal (base 16) notation that is read by the computer as binary (base 2) code. Binary is the language of all computers and is merely a combination of ones and zeros. Hexadecimal can be read by converting it to a decimal (base 10) number that is familiar to us. Using the hexadecimal code and a digital chip burner, the engineers at Superchips were able to access the fuel map for the injector-calibration change. The timing map was also accessed and tuned for additional advance at lower rpm and retard at high rpm to avoid detonation.
Superchips performed the hard work for us; all we needed to do was ask for a custom-chip order form and fill out all the vital information about the car. The engineers used their knowledge, experience, and chip-burning tool to properly modify the PROM. See the Speed sidebar to see what a difference it made.
So now what? We have stretched this 305 with a supercharger without so much as a fouled plug from the 100,000-mile mule. We haven't changed the cam, intake, or heads, so we think there is a bit more power in it. If we changed these items, we could probably get into the high 12s. The only problem is the valves would have to be small to fit on the small-bore 305. The best plan at this point is to build a 350 or 383 and get a good set of heads. The beauty of the supercharger is that we can build a small-block with an emissions-legal cam and good idle qualities--like something that would make 325 hp naturally aspirated--and then throw boost at it. We already know that with the 23-lb/hr injectors, we can feed about 315 hp at stock fuel pressure, so a mild motor will run fine until the supercharger spooled and the FMU boosts the fuel pressure. Using the fuel-pressure and horsepower equations, we know that the 24-lb/hr injectors will safely feed 450 hp that will get us into the low 12s. Or we could build a monster 420-inch mouse and get 37-lb/hr injectors for $80 more and feed 737 hp at 100 psi. Elevens here we come! CC
Auto Meter Products Inc.
413 W. Elm St.
134 Baywood Ave.
Autotronic Controls Corp.(MSD)
1490 Henry Brennan Dr.
4255 County Rd. 10 East
Borla Performance Industries
5901 Edison Dr.
Vortech Engineering Inc.
P.O. Box 1966