Limitations

What are the limits of the various fuel system components? That is what this page is all about. I hope to show at what rear-wheel horsepower (RWHP) values you need to upgrade each piece of the system.

Disclaimer

The values on this page are educated guesses. I take no responsibility for how you use this information, and I make no claims that it is correct. It is up to you to make sure you are providing the proper amount of fuel to keep your engine healthy.

Fuel Pump

The calculation for how much rear wheel horsepower you will get at some fuel delivery rate is simple:

RWHP = (1 - DL) * GPH * 6.2 / BSFC

The tricky part is figuring out what all the variables mean and choosing good values. Here is what each variable means:

Variable	What it means
DL	driveline losses
GPH	Gallons Per Hour of fuel delivered
6.2	The weight of one gallon of fuel (in pounds)
BSFC	Brake Specific Fuel Consumption (HP * lb / hr)

Driveline losses are probably about 12-15%, so we'll use the average of 0.135.

The stock fuel pump puts out 35 GPH at the stock fuel pressure (36.3 psi). That figure was verified by SR Motorsports testing the pump installed in the car with the engine turned off. However, the pump must supply that pressure plus match the boost pressure when fuel pump output is most critical. With boost pressure at 10 psi, the pump will need to supply fuel at 46.3 psi. I don't have any data at that pressure, so I am going to guess it will flow 4 GPH less at the higher pressure. This guess is based on some maps of another, probably similar fuel pump from a DSM car. The stock pump probably puts out about 31 GPH under boost conditions.

BSFC is a little tricky. RC Engineering has a nice explanation of what this variable means on the Technical Information page: "B.S.F.C is brake specific fuel consumption - How much fuel you are using per horsepower per hour". They suggest that turbo cars have a BSFC of about 0.60 or higher. 0.60 seems to correlate reasonably well with the RX-7 based on dyno runs and calculations, so let's go with it.

Here's where that leaves us:

Fuel Pump Limit: 277 RWHP = (1 - 0.135) * 31 GPH * 6.2 / 0.60

What? I know my car makes more than 277 RWHP with the stock pump. I suspect that the fuel pressure drops as the engine revs out and peak power is made. Most people with the stock pump do not have a fuel pressure gauge, so I don't have any confirmed reports of that phenomenon, however. You can still make more power by holding the injectors open longer to make up for the lost pressure. I suspect that many of the popular computer upgrades take this pressure drop into account in their tuning. For reliability and consistent performance, this is probably the limit for the stock fuel pump.

Here is a little form so you can play with the numbers:

Stock Injectors

Coming soon!

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