Injector Size, that is... I've been convinced that many folks buy far oversized injectors for their applications. I have been doing some informal calculations based off of REAL WORLD results and then matching them to the RC Injector Online Worksheet and things are adding up pretty close. I would appreciate it if others would contribute to this discussion.

We all know that there have been large numbers of Evos safely making between 410 and 420bhp on stock 550cc injectors (~370whp on a dynojet). I know there have been several evos that have made quite a bit more than 370whp on a dynojet on stock injectors, but to be conservative, I'll use 420bhp as a maximum safe number that stock injectors can reliably achieve. So, what kind of power do we make per cc and does it apply to applications with bigger turbos?

Case Study 1: Well tuned stock turboed evos:
550cc / 420bhp = 1.31 bhp per cc

Case Study 2: Evo with Garrett 3037 turbo and 660 injectors. Everyone who does the traditional math says the injectors are undersized but the car is raced regularly and has run absolutely perfect over time.

660cc / 540bhp = 1.22bhp per cc

Case Study 3: David Buschur quote: Lets just figure an 8% drivetrain loss, so:

660cc / 584bhp = 1.13bhp per cc (definitely maxed out...)

Case Study 4: Eric from AMS
880cc / 680bhp = 1.29bhp per cc

Please post up your supporting or contradictory evidence. I'll post more from the RC Injector Calculator later.

 

i thought stock 550cc's would be more than 420bhp,hmmmm......

 

+1

 

I run 680's on my GT3076 w/ AFR's in the 10's.

Injector selection should also be based on what ECU is being used. The stock ECU allows more injector overlap then most aftermarket standalones.

 

Now, there are two ways to use Worksheet provided by RC Injectors:

RC Engineering Injector Size Worksheet

This way seems to best match the above case studies:
For Brake Specific Fuel Consumption desired plug in .62
For system fuel pressure @ fuel rail plug in 44 + max boost psi (for a GT3076 turbo, that might be around 29psi which would equal = 73 psig). This method shows a very small, but realistic fudge factor that would be expected from a company that would like to sell their product.

The other method is to always type in 43 or 44 for System fuel pressure, not adding 1psi per lb of boost. This may be the right way to do it but it yields results that do not seem possible when compared to real world examples. This method would appear to have a massive and unrealistic fudge factor built into these injector worksheets. The first method seems more accurate when comparing known results.

 

JJ, if you have the time, could you explain this? I don't get it.

 

The stock ECU drives the injectors w/ a signal which stays open for a period of time and closes for a period of time. The stock ECU can open and close the injector cycle faster. Which is why stage 1+ cars can run fine on stock injectors. If a similar car has an AEM, for example, the injector cycle is slower and will show a higher duty cycle (90% or higher). I am not saying this is bad, but it is just different.

IMHO, I think the way the AEM, Autronic, etc does it is better, but it wouldn't work very well w/ the stock MAF at idle due to big injectors. Of course, if your running an AEM you wouldn't care since you would be running MAP.