I'm trying to figure out the exact size fuel injectors I need. On RC engineerings website they have a worksheet but I have no idea what I'm supposed to put in... anyone have some advice or can someone work this out... Hope this helps out some people too!

I'm looking for 600WHP... no more.. so putting the injectors closer to full duty cycle isnt a problem..

http://www.rceng.com/technical.htm#WORKSHEET

 

According to their worksheet you need 751cc min size injectors to reach 725 crank horse power(minus 17% drivetrain loss for 600rwhp)

The values inputed are

725 hp

6 injectors

.60 BSFC (The mass of fuel required to burn to produce X hp)

70 psi fuel pressure. 50psi base plus 1 to 1 rising guessing 20 psi for 600 whp.

Max duty cycle 80% ( what percent of the time the injectors are open. 85% is often quoted as the feel good safe level)

From experience this worksheet has cushion built into it. So a set of 750cc's and you'll be sitting well. You could stretch a set of 650's at 100 percent or more fuel pressure or a combo of each to work but It's up to what you and your tuner feel like running.
Gary

 

I think most people for that HP are going with the 550 injector.

 

thanks gary..

But that does sound high... I was trying to figure out the fuel pressure thanks

I'm wondering though... if I was to get the APS clutch and flywheel and a lightened crank pully... that should reduce the drivetrain loss shouldn't it? I have no idea how much...

Are there any downsides to running fuel injectors over 80% duty cycle? Thanks for the help

 

The Duty Cycle value is what is causing it to recommend a larger injector. Which is fine.

But if drivability is an issue, than plug in 100% as the duty cycle and you will likely get a 550'ish CC injector..maybe 650cc at most.

Since the injector will only see 100% duty cycle near redline, and only intermittmently if its a street driven car, you can probably get away with a higher duty cycle that optimal.

In a PERFECT world, you would shoot for 80% duty, but they you sometimes end up with too large of an injector for driveability. Hope that makes sense.

I know some people have gone 750cc with TS reflash and emanage scalling and they were OK, but others had issues even with 650's. I think its a bit of trial and error.

 

Yeah that makes sense. I just didnt know what specific numbers went where.

I was thinking there was some reason why 80% was where it was... some kinda safety or limiting factor. I also didnt know how to figure out the fuel pressre. I understand the 1:1 ratio for boost... 50 to 70 for around 20lbs of boost.. im guessing 50lbs is from the stock fuel system.

GQ you said that there were people who used 750's and were fine but 650's had some issues... i heard that before too. Do you think that has something to do with the scaling in the e-manage... interval wise is 750 easier to adjust to than 650... just a guess..

I'll probably rarely hit redline anyway lol... now how do they figure out when the duty cycle will be at a certain % is it HP based or RPM based... RPM would make more sense... I was just wondering because you mentioned redline would be near 100%and every car has a different redline.

 

Question, which ones will work for our car? Also, these will require some modifications to the stock harness, right? If so, does it come with the required hardware or does someone sell it as a kit?

 

I just meant that you will approach the 10)% duty cycle level as your approach your redline. now if you plan on increasing your rev limit to say, 7100rpm, then you might have more fueling issues than if you stayed at 6700rpm.

The thing with RC injectors is that they are Cheap...around $500 for a set, and they are easily sellable on EBAY or whatever, since they work with many different cars. So you can always swap them out as your needs increase.

For me, I am going to try and max out the stock heads and 550cc injectors and see what happens.

 

well, other thing to consider is whether you are using the stock MAF or MAP sensor.

Also keep in mind these are high impedance injectors...you dont have nearly the amount of control you have over low impedance ones.

If you do your calculations at 80% IDC, 20% drivetrain loss (782 crank hp), and base fuel pressure of 53 psi and BSFC (the great unknown) at .65 (I'd estimate higher rather than lower, especially if you are not mapping with a full standalone), you'll likely need injectors closer to 900cc to make that power at the wheels with a safe tune and room left on the table.

Also consider this is with an RC injector, which as stated, are cheap. Many of their units are simple "modded" Lucas units, which frankly, I would not waste $100 on, let alone $500. If they are their Bosch units, they are much better quality - not sure if they allow you to specify or not. I think the great limiting factor here will be your Emanage, not your injector size.

 

Lucas= aka prince of darkness!
an old joke... why do they have warm beer in England?
the refrigerators are made by lucas.....

 

OK, what's your recommendations on injectors for 700-750 flywheel hp?

 

SGP Racing used 750cc injectors an emanage and a 380cc TS relfash and pulled close to 600whp with PLENTY of room for more fuel.

You might want to ask Kyle about the setup.

As a general rule of thumb, with this VQ35, 440cc should be good for 440whp, and 550cc for 550whp, and 650cc good for 650whp...etc..etc..

The assumes you high rising fuel pressure, and a return system, and dont mind running close to 100% duty cycle near redline.

In a pefect world, and if I had better tuning options at my disposal, I would run 750cc injectors for 550-600whp, which would give me a nice cushion of excess duty cycle.