My Fiero is an 88gt that is stroked to the 3.1L, the rest of the engine is built-up as much as you can build the 2.8. My original plan was to use the stock fuel injectors, but to increase the fuel presure to supply the nessesary additional fuel...

I thought I read somewhere (back when I started the build 15 years ago) that I could use an adjustable fuel pressure regulator to increase the flow of my injectors. But som other PFF members tell me that this will not work in my situation...

So my question is what size injectors should I use? Should I get some new Accell injectors from Summit Racing or can I get some out of a donor car (what car, make/model/year)?

This is basically the last item I need to address before I begin to prepare for her first start...

Thanks,
AL

17 lbs.

out of any GM 3.1 model??

quote Originally posted by ALJR: So my question is what size injectors should I use? Should I get some new Accell injectors from Summit Racing or can I get some out of a donor car (what car, make/model/year)?

quote Originally posted by TK: 17 lbs.

quote Originally posted by ALJR: out of any GM 3.1 model??

Assuming your final question above is "Will I find 17-pound injectors in any GM 3.1L engine?" --- the short answer is "No."

As you likely know, there are numerous GM cars that have employed 3.1L engines. However, the general pattern I find is that older GM cars with a 3.1L tend to have 15-pound injectors (possibly because of their lower horsepower), whereas newer GM cars with a 3.1L tend to have 17-pound injectors (possibly because of their higher horsepower).

After I analyzed some Accel data on standard GM injector sizes for 3.1L engines in 1980s and 1990s cars, it appears that 3.1L GM cars that were 1992s or older had 15-pound injectors, whereas those 1990s cars which were 1995s or newer had 17-pound injectors, but the 1993-1994 cars could have either.

Remember that the data I analyzed cover 1980s and 1990s cars; I don't have injector size information for any 3.1L GM engines built after 1999.

I hope this helps, but if you prefer to avoid any hassles in finding used 17-pound injectors, I don't foresee you having a problem if you just purchased one set of new 17-pound Accel injectors (their part number 150617).

Good luck to you with your 3.1L project!

the ideal answer is of course to size the injectors based on HP
but, overall - the stock 15, or the next step 17 pound injectors will work just fine
adjustable fuel pressure regulator is another approach

if your build actually does hit 200HP's - 19 pound injectors maybe in order

Don't know who told you that. With a 3.1 you can run stock ones fine just with the FPR. I used them in a 3.4 with all the mods I could get except nitrous with no problems at all and made 149rwhp. So save your money and just get the FPR.

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Red: TPI V8 + 6-Speed Yellow: Nitrous 3.4 + 4 speed Auto
304rwHP/366rwTQ

Sounds like I will give the stock injectors a try... The motor is prety extensivly built and I am hopping to get (based on the stock 2.8 hp number of 125-130hp) as close to 200hp as I can get. Ya, I know I am probably dreaming and expect to get at least 150-170hp...

Quick rundown on my mods:
2.8 stroked to 3.1
+10 over bore
272 cam
ported heads, ported intake, gasket matched intake and ported exhaust
ss undercut valves
1.52 roller rockers

I think that are all the mods that would relate to air/fuel flow... With the above being said, can I get away with the stock injectors still??

Below is the information another PFF member posted on one of my other threads regarding the adjustable fuel regulator, it made sence but I thought I cound increase the flow by pressure:

"The fuel injectors are a constant-flow-rate design, so they are relatively insensitive to small fuel pressure variations. You can't compensate for the increased engine displacement by simply increasing fuel pressure."

"As I stated above, "the fuel injectors are a constant-flow-rate design, so they are relatively insensitive to small fuel pressure variations." I've actually done the experiment, and Fiero fuel injector flow is relatively constant over a pressure range of +/- 20% from nominal.

Your assumption would be correct if the fuel injectors were simple pulsed orifices. If the injectors were simple orifices, you would expect the flow to vary directly with roughly the square root of the pressure differential across them."

"I have the newer 3.1l but with the stock injectors it seems to be running a bit lean so im thinking about bumping up to either tpi or mustang injectors which are 17 and 19lb injectors."

https://www.fiero.nl/forum/Forum2/HTML/103654.html

Thanks guys! This is the last item on my list before I begin to prep her for the first start since the project began over 15 years ago!

quote Originally posted by ALJR: My Fiero is an 88gt that is stroked to the 3.1L, the rest of the engine is built-up as much as you can build the 2.8. My original plan was to use the stock fuel injectors, but to increase the fuel presure to supply the nessesary additional fuel... I thought I read somewhere (back when I started the build 15 years ago) that I could use an adjustable fuel pressure regulator to increase the flow of my injectors. But som other PFF members tell me that this will not work in my situation... So my question is what size injectors should I use?

I don't know if this where you read that comment about adjustable fuel pressure regulators for a 3.1L some 15 years ago, but essentially that is what it says on page 29 of the First Quarter 1995 issue of the now-defunct magazine, Fiero Owner.

quote Originally posted by ALJR: Below is the information another PFF member posted on one of my other threads regarding the adjustable fuel regulator, it made sence but I thought I cound increase the flow by pressure: "The fuel injectors are a constant-flow-rate design, so they are relatively insensitive to small fuel pressure variations. You can't compensate for the increased engine displacement by simply increasing fuel pressure."

I also saw that statement and admit to being taken aback somewhat by it, because that is the first time I'd heard of such a limitation in regard to our Fieros. More specifically, like you, I too thought an adjustable fuel pressure regulator could be used as a tuning tool.


However, let me address your question about injector size, since that likely is the more important one for you anyway:

quote Originally posted by ALJR: Quick rundown on my mods: 2.8 stroked to 3.1 +10 over bore 272 cam ported heads, ported intake, gasket matched intake and ported exhaust ss undercut valves 1.52 roller rockers I think that are all the mods that would relate to air/fuel flow... With the above being said, can I get away with the stock injectors still??

IMO, your list of modifications isn't as extensive as it may seem to your wallet.

Because of that, however, I believe you can still use stock-sized 15-pound injectors.

Here is why I made the two statements immediately above.

Perhaps the primary limitation presented by your Fiero's current set-up is that instead of something like a Trueleo intake manifold ( http://trueleo.com/Fierointake.htm ), it still employs the Fiero intake manifold's inherently restrictive design (even though the latter has been ported somewhat). Similarly, but secondary to that, I also didn't see any reference to headers, a free-flowing Y-pipe, or a free-flowing catalytic converter, for example. Therefore, IMO, you can still use stock-sized 15-pound injectors (and I hope yours still are in good shape).

I forgot to mention the most important thing. On top of the FPR you will need a custom PROM to tune to the required fuel needs of your modified engine. Without that don't expect too much. Only if after that you may detect a lean situation then you may consider other injectors. But I bet you won't.

LOL, ya, it was FOCOA that mentioned the FPR increasing the flow

There are more mode that were done, but I was trying to think of the ones that would be directly related to the air/fuel flow... Dut what was said makes sence, the intake is restrictive...

I am going to try the stock injectors for now and see what happens...

So I MUST reprogram the prom to get it to start? or just to maximise performance? I would have to assume a dyno shop can do this?

you tune it for maximum performance. it will start and run fine in the stock trim, your mods might only have an issue with higher rpms going slightly lean, tho from my experiences with the fiero styled ecm's, I really doubt it, as they do tend to run rich.
As for injectors, the 88 and later years used the newer style disc types, not the small pintley types in the earlier years. those are much better injectors, less tendency to clog. being you started with th 88, you should have the new style, and those are the same 15 lb injectors used on the 3.1's in the early 90's.

Even if you can;t afford to dyno tune it, a wide band o2 sensor would be a decent addon to verify that you are not toasting the motor.

quote Originally posted by ALJR: Sounds like I will give the stock injectors a try... The motor is prety extensivly built and I am hopping to get (based on the stock 2.8 hp number of 125-130hp) as close to 200hp as I can get. Ya, I know I am probably dreaming and expect to get at least 150-170hp... Quick rundown on my mods: 2.8 stroked to 3.1 +10 over bore 272 cam ported heads, ported intake, gasket matched intake and ported exhaust ss undercut valves 1.52 roller rockers I think that are all the mods that would relate to air/fuel flow... With the above being said, can I get away with the stock injectors still?? .......

yup. and, the big determining factor is: the intake manifold
if you dumped the "Fiero" intake, and used the "TrueLeo" intake, you may actually get to 200
with the "Fiero" intake, you are basicly "capped" at 160

quote Originally posted by tjm4fun: you tune it for maximum performance. it will start and run fine in the stock trim, ....

Mine didn't with the 272 cam but I have the Truleo intake and is a 3.4.

My 86 came with Multec disc injectors from the factory so they must have mixed them over the years.

He has a couple of options for fueling:

Adjustable FPR
17# injectors
Up the base pulse width in the ECM.

I recommend 17 lb injectors and then scan it and see if he needs to adjust anything. I am guessing the simple "2.8L to 3.1L" will escalate to other changes and require some ECM adjustments.

If I was building a new engine, I would spring for new injectors regardless of the size. 15lb injectors will flow plenty of fuel if the cals are changed but I prefer to get the injector size close based on a realistic HP goal (and not some wishful thinking) and then adjust if I have that option.

Well, I left the stock injectors in and set the FPR to 50 psi... We'll see how it works out... Worst case, I am pulling them out and going with the 17# injectors... But will look in to having the ECM tuned...

Do I need to take it to a dyno shop for that? Or can it be mail-ordered with the adjustments made?

quote Originally posted by ALJR: Well, I left the stock injectors in and set the FPR to 50 psi... We'll see how it works out... Worst case, I am pulling them out and going with the 17# injectors... But will look in to having the ECM tuned... Do I need to take it to a dyno shop for that? Or can it be mail-ordered with the adjustments made?

I know DarthFiero at GMTUners.com can make a chip based on your mods, but, overall - datalogging or dyno run w/wideband is the best way to go.