Hi Guys, I am wondering if my 24# injectors are a little on the small side. I've done many searches on here and see some people state that 24# on a 355 HSR are fine. Others say they are too small and are running 30# injectors. I tried some of the online calculators, and they say I should be running around 27#? My question is this - Would it be worth it to upgrade from 24# to 26 or 30# injectors? Am I missing out on some extra HP by running too small injectors? Any info would be appreciated. Thanks!!!

I think if you're going to up the injectors by nearly 30% you need to be ready to tune or have a tune done. I wouldn't have figured 24#'s are too small for a 355 but then again I've never run a 355.

Only if you are maxing out your injectors at present.You need to datalog and see what your injectors are doing
Some are running 24's in 383's with the correct tune.
If you have the $$ put them in now for future use but you are going to need a retune

How can I tell if I'm maxing out my injectors?

I think 350-360whp is the limit on 24's. I thought 1989GTATransAm posted his results with 24 lb injectors and he was in the 350whp range with the injectors above 80% cycle duty and most say to keep cycle duty no higher than 80%

Increase base pressure to make them act like 26-28 lb injectors if needed. I'd get that car tuned and running the best you can before you swap injectors. Need to check them like above said. I think with right base pressure, you should be fine with that setup

Thanks for the replies!

When TPIS burned my chip, they made the mixture more rich and I believe they said they programmed the PROM to think it has 26# injectors in order to richen the mixture.

I keep thinking of the old formula of More Air + More Fuel = More Power. Is this incorrect?

Thats kind of a crap way to tune, to lie to the computer to make it think it has larger injectors than what it actually has. Proper way to tune is to change the MAF tables and PE enrichment tables.

For a small changes in injector size, like 24 to 27 lbs, increasing the base pressure is a cheaper alternative.

This site has a fuel pressure to flow calculator, http://www.csgnetwork.com/fiflowcalc.html

From their calculations a 24 lb injector at 43.5 psi flows 26.9 lb at 55 psi.

I would have thought a 26lb injector would not have to open for as long a time to flow the same as a 24 lb, so a prom "thinking" it has 26 lb injectors would not leave them on as long, thus flowing less (leaner). Maybe TPIS said it would act like it has 26 lb injectors? Did they tell you a base pressure to run at? Their "book", Insider Hints from 1993 states all of their testing is done from 46 to 55 psi. Anyways TPIS may be expensive but they have been at it long enough they should know what they are doing, but who knows.

It seems everyone has a different opinion on injector sizing with some people liking to run at 100% duty cycle (static operation). There is one guy on another board running 600 flywheel hp with 24 lb injectors.

Like Orr89RocZ says the most common accepted maximum number seems to be 80%.

I ran Ford racing 24#/hr injectors on my 395 cu.in. stroker with the SuperRam for several years. I ran the fuel pressure at about 54 psi. The car ran as fast as 12.12 sec at Milan, Mi. See my sigs for my current stuff. AFPR is a great tuning tool.

On your 355, 24s are plenty of injectors. Its all in the fuel pressure.

When I last spoke to TPIS, they told me I shouldn't run more than 50+ psi? Is it safe to do so?

Also, they said they programmed my chip to run at 43psi, so I have it set around there now.

How do you determine the duty cycle of your injectors?

Thanks guys!

I just posted that today on another post. The duty cycle (DC) is calculated as below:
DC in % = (engine RPM) * (PW in msec) / 600

You are going to have to do some sort of datalogging to determine your duty cycle by seeing what the pulse width is and to see if your programming is optimal. It can be handy to see if your timing is being retarded by the ecm.

It's pretty hard to get a mail order tune 100% right. TPIS used to have a deal where they would rent you a datalogger to help get your tune right. Some other tuners offer this.

It all depends how serious you are about wringing out the last few horsepower. You could spend a few hundred dollars to gain 5 or 10 hp and you probably would not feel the difference. But if the car is surging, detonating(pinging),missing, running like crap or hard to start you should look into doing something.

If you run way too high a fuel pressure you could get an injector to stick open or get too rich a mixture, either can damage the rings. My guess is TPIS is concerned anything above 50 psi will make your car too rich. Most injectors will handle at least 55 psi.

A quick check of your tune would be to look at your sparkplugs. If they are not black(sooty) or white/grey but some sort of tan/brown your tune is in the ballpark. A proper check is a lot more involved (how the car is run, shut-off, etc.) but this will tell you if the tune is grossly out. You can get better info by Googling Sparkplug reading.

Thanks for all the info guys! Sounds like my 24# are good enough for now. I'll have to take the car out when the snow melts and see what the pulse width is so I can calculate the duty cycle.

Since TPIS programmed the chip to run at 43.5 of fuel pressure, should I accept that and leave it? Or should I bump it up a little and see if it responds better?

Thanks again!

It sounds like fuel pressure is your only tuning tool. When the closest dragstrip opens for you, take your car over there. Make your first two runs at 43.5psi. Then bump it up to 46psi - 47psi and make more runs. Dial in whatever fuel pressure gets you your best trap speed. The ET does not mean anything here because it is too dependent on the 60ft time. Also, here is an equation for you which relates RWHp and trap speed:
RWHp = wgt * (trap spd / 234)**3 where **3 means cube it or raise to the power of 3
The wgt is the weight of the car plus all occupants.

Thanks doc! I've never been to the track but have been looking forward to taking my car there this spring. I'll adjust the fuel pressure as you recommended.