Need Help!

Thekid1957 · May 28, 2013

I recently bought a 5 speed 90 notch. Stock intake and throttle body, 24lb injectors and a A9P ecu. It has a pro flow mas model # pft-77 calibrated for 24lb. The car idled very high when turned over, then normal when warm. The car would also cut out when driven for a while and you could smell fuel. I would have to trip the fuel pump from the trunk, crank a couple of times, reset the pump and the car would turn over. I am now installing an Edelbrock performer, any advice which throttle body to use, and was this mas any good to begin with?

Bill Gordon · May 31, 2013

I am running a BBK 70mm TB and Pro-M 76mm MAF, calibrated for 42# injectors and cold air. Runs awesome. You may want to check your FPR for the flooding issues, hook a fuel pressure gauge up to the Schrader valve and check fuel pressure. Good place to start.

jrichker · May 31, 2013

There is no need to run 24 LB injectors with stock heads, and stock cam. You need to pack more air into the cylinders before you add more fuel. Even an aftermarket intake manifold doesn't add enough airflow without free flowing heads and a matching camshaft to require 24 lb injectors. Swap back to 19 lb injectors and a matching MAF and most of your fuel related problems will go away.

Fuel injector sizing & injector photos

Revised 11-Dec-2011 to add larger injector sizes to injector table

Injector HP ratings: divide flow rating by.5 and multiply the result by the number of injectors. This uses a 100% duty cycle. These ratings are for naturally aspirated engines at the flywheel.

Example:
19/.5 = 38, 38 x 8 = 304 HP
24/.5 = 48, 48 x 8 = 384 HP
30/.5 = 60, 60 x 8 = 480 HP
36/.5 = 72, 72 x 8 = 576 HP
42/.5 = 84, 84 x 8 = 672 HP

The preferred duty cycle is about 85% maximum, so for a safety factor multiply the final figure times .85.

19/.5 = 38, 38 x 8 = 304 HP x .85 = 258 HP
24/.5 = 48, 48 x 8 = 384 HP x .85 = 326 HP
30/.5 = 60, 60 x 8 = 480 HP x .85 = 408 HP
36/.5 = 72, 72 x 8 = 576 HP x .85 = 490 HP
42/.5 = 84, 84 x 8 = 672 HP x .85 = 571 HP

Remember that the above ratings are at 39 PSI. Increasing the pressure will effectively increase the flow rating. Example: a 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.

See http://users.erols.com/srweiss/#jcalc to get the calculators used in these examples.

Here's the duty cycle explanation. Duty cycle is how much of the time the intake is open the injectors are turned on. The 85% figure means that for 85% of the time the intake valve is open, the injectors are spraying. The idea is that you want some percentage of the duty cycle left over so that you have some room to grow the process.

If you are at 100% and you need more fuel, all you can do is turn up the fuel pressure. That means the whole fuel curve from idle to WOT is affected. Maybe you are already too rich at idle, and turning up the fuel pressure makes it worse. If you had some injector duty cycle left to play with, a custom tune could use that where it is needed. That would not over richen the whole range from idle to WOT.

If you did turn up the fuel pressure, you might be able to change the injector duty cycle to get the air/fuel mixture ratio you want since the injectors will have extra fuel delivery capability.

With larger than stock injectors or higher that stock fuel pressure, you will need an aftermarket MAF that matches the injector size. The MAF “lies” to the computer to get a fuel delivery schedule that meets the engine’s needs and isn’t too rich or too lean. The best strategy is an aftermarket MAF and a custom tune to insure the best air/fuel ratio over all the RPM range.

Don't forget to increase the fuel pump size when you increase injector size or significantly increase the fuel pressure

Diagram courtesy of Tmoss & Stang&2birds

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