Choosing The Right Fuel Injector ????

[AmericanZ28]

I was wondering if there is an adverse effect of using a bigger injector than necessary. Seeing how Racetronix has the same price on all their injectors, I dont see why I shouldn't just go with the 42# units. I will be using the cam below, and the stock heads w/ 1.6 RR (due to lack of funds) on a 383 stroker (forged internals, 2 bolt main). I dont know how much power this combo will make, but Im sure it will be less than the initial goal of 500 FHP. Also, if anyone can recommend a better Off the shelf cam than that below, feel free to comment. Car must be street friendly. Thanks in advance guys!



CC-TN-227XR Comp Cams TN227XR Camshaft 227/233 duration @ .050", .560"/.569" lift with 1.6 rockers, 111lsa. Huge top end pull N/A cam, 3000+ stall converter recommended for auto cars. THE cam for street/strip cars looking for maximum power without sacrificing too much driveability.

[lsdengines]

Bigger injectors will lead to rougher idle and driveability. The formula I use to get in the ball park is: expected fwhp/ # of cylinders * bsfc( .5 for na and .65 for turbo/blower). This gets you a ballpark idea of what size injector @ 45 psi fuel pressure you will need. You can also raise the fuel pressure on an injector to make it act a little bigger and get better driveability than using an actual larger injector.

As far as cams, what are you looking for? I was running the old cc305 cam with nitrous, and it was an awesome street cam and VERY EASY to tune. The cam you are describing will be a little more difficult to tune at low rpm due to the tight lobe seperation. It is also a little small for a 383. For just an na engine with stock heads, I would probably use the cc xfi 07-4678 The specs are 230/236 @ .050, 280/288 advertised, .576/.570 lift(w/1.6), 113 lobe sep

[AmericanZ28]

Bigger injectors will lead to rougher idle and driveability. The formula I use to get in the ball park is: expected fwhp/ # of cylinders * bsfc( .5 for na and .65 for turbo/blower). This gets you a ballpark idea of what size injector @ 45 psi fuel pressure you will need. You can also raise the fuel pressure on an injector to make it act a little bigger and get better driveability than using an actual larger injector.

As far as cams, what are you looking for? I was running the old cc305 cam with nitrous, and it was an awesome street cam and VERY EASY to tune. The cam you are describing will be a little more difficult to tune at low rpm due to the tight lobe seperation. It is also a little small for a 383. For just an na engine with stock heads, I would probably use the cc xfi 07-4678 The specs are 230/236 @ .050, 280/288 advertised, .576/.570 lift(w/1.6), 113 lobe sep

Thanks for the help! Based on your formula, if my goal was 500 fwhp, I would need a 31.25# injector. Does that seem right? I thought about the cc305 but many people say it's an outdated cam, and that there are better ones out there. I'd really like to have a good amount of torque down low without sacrificing too much top end. I'm thinking a range between 2000-6000 rpm would be perfect. I did a search on yahoo for that cam (cc xfi 07-4678) but I didnt get any results. Where can I get it, and how much is it?

[lsdengines]

its comp cams new xfi series. you can get it from summit among other places for about 255. The 305 is a very good street cam, yes outdated, but good. It is too small for the 383 however.
As far as injectors go, yes 30-34 is a good injector for about 500 fwhp. Like I said that formula is just to get you in the ball park. If it says use 31.25, I would probably use the next closest larger which I think is a 36 just to be safe.

[sneitzel]

I too was looking at the injectors on Racetronix and was wondering if anyone has opinions on the disk style they feature vs. the traditional pintle style injectors?

[Injuneer]

I'll offer another point of view. As long as you don't get ridiculous with size, and as long as the PCM's drivers can handle the low pulse widths required to turn donw the injectors to the idle requirements, you will not suffer with oversize injectors. Numerous people have demonstrated that the stock LT1 PCM will support 42 #/HR injectors, and is capable of driving the short pulse widths required at idle. Many people have done this with no adverse affects. There is no downside to your plan.

My guideline for sizing injectors is a bit less conservative. The standard formula quoted above assumes a brake specific fuel consumption (BSFC) of 0.500 #/HR/HP. Then it is recommended that you divide by 0.80 to limit the injector duty cycle to 80%. Example:

500fwHP x 0.5 = 250 #/HR

250 / 8 = 31.25

31.25 / 0.80 = 39 #/HR

That is very conservative for a well tuned performance engine. I've achieved a BSFC of 0.44 with my setup. So I opt for a guideline formula that reflects a BSFC of 0.476 and an upper duty cycle limit of 85%. That will give you a pretty good lower limit for injector selection. Using those numbers, the rule-of-thumb that I use is to simply multiply the flywheel HP times 0.07 to get your minimum injector size.

Using that basis:

500fwHP x 0.07 = 35 #/HR.

My setup uses a Bosch 64 #/HR (rated at 2.7bar = 39.15psi) injector at 58psi. That gives an equivalent inector size of 78 #/HR. The 500HP engine idles smoothly at 800rpm, with a 230/242 114LSA solid roller camshaft. The engine has returned as much as 20 MPG when it was still the M6 w/ 3.73 gears. I suspect my aftermarket ECU has better drivers than the stock PCM, though. With the 300 dry shot, the engine is producing 800fwHP as a BSFC of only 0.54, and the injectors never exceed a 70% duty cycle.

[AmericanZ28]

its comp cams new xfi series. you can get it from summit among other places for about 255. The 305 is a very good street cam, yes outdated, but good. It is too small for the 383 however.
As far as injectors go, yes 30-34 is a good injector for about 500 fwhp. Like I said that formula is just to get you in the ball park. If it says use 31.25, I would probably use the next closest larger which I think is a 36 just to be safe.

The closest thing Racetronix has is a 37#, so I guess that will be it! So would you consider that new xfi cam to be conservative on a 383? Thanks again for your input!

I'll offer another point of view. As long as you don't get ridiculous with size, and as long as the PCM's drivers can handle the low pulse widths required to turn donw the injectors to the idle requirements, you will not suffer with oversize injectors. Numerous people have demonstrated that the stock LT1 PCM will support 42 #/HR injectors, and is capable of driving the short pulse widths required at idle. Many people have done this with no adverse affects. There is no downside to your plan.

My guideline for sizing injectors is a bit less conservative. The standard formula quoted above assumes a brake specific fuel consumption (BSFC) of 0.500 #/HR/HP. Then it is recommended that you divide by 0.80 to limit the injector duty cycle to 80%. Example:

500fwHP x 0.5 = 250 #/HR

250 / 8 = 31.25

31.25 / 0.80 = 39 #/HR

That is very conservative for a well tuned performance engine. I've achieved a BSFC of 0.44 with my setup. So I opt for a guideline formula that reflects a BSFC of 0.476 and an upper duty cycle limit of 85%. That will give you a pretty good lower limit for injector selection. Using those numbers, the rule-of-thumb that I use is to simply multiply the flywheel HP times 0.07 to get your minimum injector size.

Using that basis:

500fwHP x 0.07 = 35 #/HR.

My setup uses a Bosch 64 #/HR (rated at 2.7bar = 39.15psi) injector at 58psi. That gives an equivalent inector size of 78 #/HR. The 500HP engine idles smoothly at 800rpm, with a 230/242 114LSA solid roller camshaft. The engine has returned as much as 20 MPG when it was still the M6 w/ 3.73 gears. I suspect my aftermarket ECU has better drivers than the stock PCM, though. With the 300 dry shot, the engine is producing 800fwHP as a BSFC of only 0.54, and the injectors never exceed a 70% duty cycle.

Thank you Fred for the detailed explanation, looks like the 37# injectors will fit the bill! Smooth idle at 800 rpm? Talk about a well tuned performance engine! I know I'm not going to be at 500 fwHP with the stock heads, but atleast I'll have the peace of mind knowing that I wont have to worry about buying new injectors in case I ever swap out the heads and reach my goal. Do you have any advice on a cam? Im looking for a 2000-6000rpm range with plenty on torque down low without sacrificing too much top end. Thanks again!