I have a bored 302 (.030 over) with AFR 165 heads using the stock Ford computer. I currently have 24lb injectors in it and have one injector that keeps cutting in and out. So, I need at least one new injector.

Here are my questions, should I drop down to stock injectors or keep the 24lb ones? Should I replace all of them or just the one? What are the pros and cons to going down to stock 19lb?

What would you do and why?

Thanks!
Steve

 

The first thing I would do is make sure the cutting in and out is not due to an intermittent electrical connection to that one injector. It might be misfiring due to an intermittent signal.

All reference here to injector size applies to naturally-aspirated motors on pump gas.

A 19 lb. injector will usually support about 290 crankshaft horsepower without "cheating" or pushing it. That 290 HP figures an 85% duty cycle, which is a good maximum for reliability; you can push 90% but the injector will run hot and you may be spraying fuel when the valve is closed. You can also raise fuel pressure to make the injector flow more than it is rated; a lot of people do this to get by 19 lb. injectors at ~300 crank HP.

With AFR heads, assuming the rest of your induction/exhaust components reasonably match the heads' capability, you are almost certainly going to make more HP than that, even with a stock cam. Using the same, standard 43.5 psi fuel pressure and 85% duty cycle, you will need 22 lb. injectors to support 325 crank HP; you'll max out the 24 lb./hr injectors at about 360 HP...which, by the way, is doable with AFR 165 heads on a slightly ratty but streetable motor. I think your 24 lb. injectors are probably the right choice.

What is the rest of your combo? What HP do you anticipate?

Here's a link to one of the many online calculators you can use to match HP with injector size: Fuel Injector Calculator | Fuel Calculators | Resources | DeatschWerks

 

In addition to the AFR165 heads, it has the Ford Racing X303 cam, roller lifters, roller rockers, PRO-M mass air flow meter (I just looked at the spec sheet for it and it is calibrated for 24lb injectors - so that really does answer the "24 lb injector" question unless I want to buy another MAF meter).

The internals were all balanced and the crankshaft was polished. The engine has about 3K miles on it since it was rebuilt.

 

Additionally, I just had a bunch of work done on it by a very experienced mechanic that has a lot of Ford fuel injection experience. I am confident that when he cleaned up all the wiring in the engine compartment that he tested all the contacts to the injectors.

 

Oops.. I forgot that I also have a flow ported port-matched intake manifold on it....

 

As the old knight guarding the Holy Grail in the Indiana Jones movie suggested, "You chose wisely."