I have a 01 trans am (mods in sig) and my tuner said they are in the 90th percentile. I am thinking about heads soon and would need new injectors, but i have no clue on what size to get?

 

Are the current injectors stock ones?
How many hp is ur car?
What is your current rail pressure?
Are u going to replace the fuel pump with an aftermarket one?
Are u planning to add things more than heads soon?

 

PM me and we can talk about all the details. I have a sticky in this section. Please read it and go to rceng.com and check out their technical page.

I have spent a lot of time trying to organize things and get it to where people can read my sticky and know what they need to do, so let me know if I need to change something or add something

WHATEVER YOU DO, DO NOT BUY 60# INJECTORS and do not dare anyone recommend him to get them

 

my car made 407rwhp/381rwtq. i'm not sure on the fuel rail pressure. yes they are stock injectors, yes i plan on installing a fuel pump, and plan on doing more work afterwards.

....so what do you guys think

 

To make u know how to get a quick result, do the following:

f:
Naturally aspirated = 0.9
S. charger = 0.75
Turbo = 0.72

lb/hr=hp/(no cylinders*2f) = 500 / (8 * 2 * 0.9) = 34.78 lb/hr at the rail pressure. Therefore if you are planning to upgrade your engine to 500 hp, u need to select the closest rate to that no.

Please note that, there is a difference between the rated pressure of an injector and the rail pressure of a pump. Usually, when an injector is being manufactured it is rated at X pressure, this X pressure isn’t equal to the rail pressure of car’s pump. Usually, the rail pressure of car’s pump is higher so it makes the injector gives more rate.

Use this formula to correct for pressure; new flow rate= old flow rate x sqrt(rail pressure of the car’s pump/rated pressure of the injector)

Here is a good reference you can rely on:

http://www.rceng.com/technical.aspx

I guess this way u can do it alone, let me know if more assistance is required

 

I'm getting ready to install a MS3 with the 2.5 5.3 heads and am going with 36# injectors which should be more than enough. I was thinking about 42# because I was going to spray a little but not yet. A buddy of mine has a MS3 type cam and FAST/NW 90/90 with a set of Darts and he bought 42#ers and said they were a little big for his set up. I don't know all the math for it, but trying to help out a little

 

To support your current horsepower requirements, you would need an injector rated at 29 lb/hr @ 3 Bar running at your current fuel system pressure of approximately 4 Bar. The LSx engine is very efficient as pushrod engine's go, so this assumes a BSFC of .46 which is very realistic. All calculations are based on 80% max duty cycle. See calculation below for this scenario:



Here's the same calculation adjusted for 500 flywheel horsepower. The minimum requirement is an injector rated at 31.12 lb/hr @ 3 Bar. Here's the calculation for this scenario:



Now, you have to look into your crystal ball and estimate what horsepower levels you will need to support in the future to help determine proper injector size. A very common fuel injector that works well is the FAST 303608. This is the same injector GM uses in the ASA motors and the PCM injector tables are readily available. This injector is rated at 36 lb/hr @ 3 Bar (43.5 psi). This injector would easily support your present horsepower levels and reasonably support most NA combinations up to 600 flywheel horsepower, depending on the ultimate BSFC (efficiency) of the engine combination.

Finally, the see where these injectors would max out. I adjusted the horsepower level up until the minimum injector rating hit 36 lb/hr @ 3 Bar. Here's the calculation for this scenario:


In conclusion, you need a minimum of 29 lb/hr @ 3 Bar to support your present requirements and stay below 80% duty cycle. Fitting an injector into your present and future horsepower level requires some estimation of future engine state and availability of an injector in a size reasonably close to your estimation. I tend to recommend the FAST 303608 because it's very efficient, reasonably priced, easy to tune, and easily supports most naturally aspirated applications without going way overboard with an oversized injector like the Siemens/Deka/Mototron 60 lb/hr injector that many recommend.

Good luck and happy motoring!

 

thank you Aaron for taking the time to figure that out. But now i have another question....if i am in the 90th percentile now and injectors max out in the 80th percentile...am i losing power with the stock injectors at the horsepower level i am currently at?

 

Losing power, no. Best practice maximum duty cycle for injectors is generally around 80%. This keeps the injector coils from overheating and keeps you out of the inconsistent fueling zone of the injector. As you reach closer to 100% duty cycle, the injector loses the ability to accurately meter fuel. Does this mean you can't run an injector above 80% for some period of time? No it doesn't and many do it, but it's not recommended.

 

I agree. It also gives you room for when it's colder and the fuel demand increases.

 

FMS 42's all the way, easy to tune, readily available at a decent price, extra room to grow, and best resale value.

 

Do you have the correct offset table's for these injectors in an LS1? If not, I'd bet you will have a hard time tuning these in the ldle and decel areas. I haven't seen a car with these injectors that didn't run rich at idle and decel without overly lean or rich transitions.

 

Sounds like a tuning issue to me, I wouldn't necessarily degrade a part on account of certain tuners lack of ability. These injectors are probably the most widely used street car injectors in the world and yes I know everyone jumps to them everytime someone asks "what injector", but realistically think about why people jump to them. Prolly because they are "easy to tune, readily available at a decent price, extra room to grow, and best resale value."

 

I don't think a tuning issue. An aftermarket EFI system would be easier to tune with any injector, but the factory PCM is much more difficult. These are popular injectors and I see them for sale all the time, mostly because they are DIFFICULT to tune properly with a factory PCM. Just because they are popular doesn't mean they are good or even mediocre for that matter. Using that criteria, the Siemens/Mototron/Deka 60 lb/hr would be king and look at all the issues tuning these for low speeds and idle. Large injectors don't like small pulsewidths and when you run an injector horribly large for an application that's what you get.

Do you have the correct offset table's for a factory LS1 PCM?

 

Don't keep that kinda stuff in my back pocket lol! I see what you are talking about but just because your pcm can't easily adapt to the two best sets of street injectors out there doesn't equate into the injectors being crappy. Sounds to me like more research needs to be done on the tuning side of things like I said before. Do you know how many cars run around with the siemens 60's and fms 42's that idle flawlessly and make 5-600+ hp? If they can do it then why can't the ls1 pcm do it? If the pcm is tuneable then the issue can be worked out...just a matter of tuning. I guess what you are getting at is they may not be the best selection for an ls1 pcm powered car correct? What injectors would you suggest and what is the main difference in comparison tuning wise?

"Large injectors don't like small pulsewidths and when you run an injector horribly large for an application that's what you get."

So you are implying that a 42# injector is horribly large for his application correct? You see this is where I get thrown off. If the idle/pulsewidth issue was as big as you make it out to be everyone would be running 15 lb injectors with a 120 psi fmu. I understand exactly where you are coming from but realistically it boils down to correct tuning. Take a ford lightning in comparison realistically you have a little less cubes, and probably about the same airflow as the original poster's engine. What is the difference between the two at idle? I know the lightning head flows a lot less and is still sucking through a blower therefore the injector pulsewidth would probably have to be less for a lightning than it would for an NA bigger cube motor but yet the 42's work perfectly on the lightning. That is the beauty of the 42's, they can handle the small pulsewidths. Why would these be considered horribly large for the application?