Good morning everyone,

Recently, I've started to feel the fuel pump in my 1995 Z28 show its age. Occasionally slow to start, occasional low power/stumble, and consistently low power in higher RPMs. I've purchased a (close to) OEM Delpi fuel pump and intend to install it over the winter break. I figure while I do so, I should also clean the tank (yes, I will be pulling it out from the bottom - I've decided against the trapdoor method), flush the lines, replace the fuel filter, and have the injectors serviced. The injectors at the moment are stock from factory, and to my knowledge have never been serviced. Sending the injectors out to be serviced would leave me without a car for a week or so though, as this is my only car.

About a month ago, I found a full set of brand new 32# injectors that my dad bought for the car back when he owned it, but never got around to installing. I'm now wondering "gee, can I just replace the original injectors with the 32# injectors and not go without a car while the OEM ones are serviced?". Conceptually, I think that the PCM should account for the new fuel pressure in the injectors and adjust accordingly. The only potential hinderances being too weak of a pump (though I think an OEM pump should be strong enough), or the air:fuel ratio being so out of whack the O2 sensors can't detect enough to tell the PCM to adjust.

Does anyone have information on what's required to do this injector modification? I don't have any experience with this just yet, so any input is valued!

Thanks gents,

Yazz

 

[EDIT]

Changing the injectors will not change the fuel pressure. The fuel pressure regulator closes off the return flow to the tank to maintain 43.5 PSI (less vacuum compensation) at all times. There's probably not enough fat in the stock pump capacity to supply 32 #/HR injectors.

The injectors will flow 28.5% more fuel. The O2 sensors will pick up the rich condition. OVER TIME the PCM will reduce the long term fuel trims (LTFT) to attempt to lean the engine out. The maximum the LTFT’s can reduce the fuel flow is 15.6%. The 24#/HR injector flow rate in the stock program will result in the LTFT's being pulled down to the lower limit of 15.6%, but the engine will still be receiving 12.9% extra fuel. And the injector programming has other data beside the #/HR. There are offset values to accommodate changes in the voltage supply to the injectors. If the larger injectors respond faster (opening and closing) it could result in even more excess flow, beyond the ability of the LTFT’s to reduce the excess fuel flow.

Run rich for any length of time - foul the spark plugs, fuel bypasses the rings and dilutes the oil, causing excessive engine wear.

I wouldn’t do it, without reprogramming the correct flow rate and offsets in the PCM.

 

Injuneer, thank you for that detailed response - that provided much more insight into the fuel system and how it operates. I think given that information, I'll send the OEM injectors to be serviced while I do the pump/flush job. The car won't be running anyway, and if I pay for quicker shipping hopefully, they'll return just in time to install them.

I may look into experienced LT1 tuners at a later date, as the 32# injectors are of interest to me (who doesn't love more power?), but I think the juice isn't worth the squeeze at the moment, especially given this is my daily driver.

Any tips on the fuel pump job? I've never done it the proper way before (I owned a 97' Firebird 3.8 in high school I used the trapdoor method on). Tips on cleaning the fuel tank once it's out?

Thanks again!

 

Changing injector size, whether you tune for it or not, won’t increase horsepower (If you don’t tune for it, it could even reduce power). It just eliminates one of a few bottlenecks in the way of adding more power some other way

 

OP. larger injectors on an otherwise stock motor s no gain, even with tuning. When you get into a heads and cam build, larger throttle body then they are beneficial but certainly need to be tuned for that combo

 

He claims he has a “LT1 383 cui”. Odd that the injectors weren’t upgraded when that engine was built. Hopefully it got a “tune” even if just to correct for the displacement.

 

ah, I didn't read his sig Fred...IMHO if using the larger injectors with the 383 which I would think has a cam other than stock....the 255 :PH fuel pump would be in order.

 

Ahh, I can see the confusion here. The motor IS a 383, and DOES have a performance cam, along with ported heads and throttle body. You would think that would mean larger injectors too, right? Why build the motor to accept that much additional air without introducing more fuel to actually make the power?

Growing up, this car belonged to my dad. When I bought it from him when I went to college, he told me there were 32# aftermarket injectors in there, leading me to believe he had increased the fuel delivery along with the 383 build. BUT - a few months ago I had to remove the injectors for an intake manifold gasket job, and I discovered they were the stock injectors. I'm assuming my dad used the stock injectors and had the car tuned to meet the new requirements, but I can only speculate as he can't remember.

At the moment, though, it does sound like this may be more trouble than it's worth. A friend of mine recently sent his injectors to 'Injectors RX' to have his serviced, and he's happy with their work. I think I will (for now), replace the pump with an OEM (or close to it) replacement, change the filter, clean the lines and tank, and send out the original injectors to be serviced. The larger injector modification is of interest to me, but I'll need to do more research of my own before committing to anything, and I only have a week to get this job done!

Thank you all for your input - I will post back on this thread with any notable moments from the fuel system job. I'm expecting this one to be memorable, given that I'm opting for the long route and not cut a trap door

 

I gave you bad info in my first post. I used 28 #/HR for the stock injectors, and that should have been 24 #/HR. In the stock PCM program the stock injectors show 24.9 #/HR. I will correct my original post.

Key point = running 32 #/HR on a PCM programmed for 24.9 #/HR will result in 28.5% excess fuel, and that cannot be corrected by O2 sensor feedback. Plugs will foul, excess fuel will wash the oil off the cylinder walls, destroy the rings and dilute the oil.

If we assume a typical brake specific fuel consumption (BSFC) number, and limit injector duty cycle to a reasonable value we can estimate the HP capacity of the fuel injectors. Over the years, I've simplified this process to "divide the injector rated capacity by 0.070 to get max HP". So, a 24.9 #/HR injector will support 24.9 / 0.070 = ~355 HP. If your 383 is capable of producing over 355 HP at the flywheel you will run lean on fuel when the engine HP exceeds 355. You will run lean, and encounter detonation (knock).

Using the formula, 32 #/HR injectors will support 457 HP. 36#/HR will support 515 HP. Again, these are based on typical numbers. If you have a good tune, good ignition, efficient combustion chambers, etc. you might make a bit more HP on any given injector size. My results are not as conservative as the typical online calculators, that tell you to use 0.500 #/HR/HP BSFC, and maximum injector duty cycle of 0.80.