I'm wondering if my car might be starving for fuel on the top end. My 02's range from .860-.920 at WOT. I'm currently using the factory 24# injectors with my heads/cam combo but I'm wondering if they are being overtaxed. Is there a specific rule to figuring injector size requirements for an application?

 

There is a formula, but this chart works well. Use .55 bsfc for a blown engine and no more than 90% duty cycle.

http://www.smokemup.com/auto_math/fuel_injector.php

Jody

 

Thanks for the reply. I guess I'll be throwing in the 30#s after all.

 

Is that max HP they use at the crank, flywheel, or rear wheels???

 

Most likely flywheel. And I've been told to use .57-.60 for SCed cars.

-Alex

 

Joe:

Use flywheel HP, as measured on an engine dyno. If you have an engine dyno sheet, it should show BSFC along with torque and HP for each point on the curve, so you may have to look at points other than "peak" HP to calculate peak fuel flow.

If you think about it, using flywheel HP and the "elevated" BSFC for a blower motor is in part due to the fact that the blower needs a slightly richer mixture, and in part due to the fact that the engine is making a lot more "crank" HP than flywheel HP, with the excess going to turn the blower drive. But the engine dyno can only measure flywheel HP.

Big Dave:

Just be careful using the OEM narrow band O2 sensors. The are not intended to function acurately at 12-13:1 and are not necessarily accurate. The curve of lambda (A/F ratio) vs. millivolts is extremely flat at the A/F ratio you want for peak power or torque, and a very small change on mV can represent a very large change in A/F ratio. And, at those levels, the sensors are so sensitive to operating temperatures, that a small change in op. temp. can completely cover up a change in A/F ratio.

I would recommend a chassis dyno pull, with a wide band sensor, if you are at the point of maxing out the 24# injectors. Alternatively, use something like DataMaster, that calculates injector duty cycle (although I'm not convinced there isn't an error in the calculations???), or just record the pulse width vs. rpm for a full WOT pass, and I can give you the formula for calculating duty cycle.

The online calculator cited above is using the formula:

(flywheel HP x BSFC) / (8cyl x duty cycle) = #/HR

where BSCF is brake specific fuel consumption in #/Hr/HP.

If you read some of the literature on injector sizing, they make a very strong case for not going beyond 80% duty cycle.

RC ENGINEERING Injector Tech

 

Thanks guys and thanks Fred. I'll try to find someone who can log some runs for me.