When I first did my heads/cam, I left the stock injectors in there, because I didn't pull the fuel rail when I pulled the intake, so I figured I'd see where I was with the stockers.

CC306, ported heads, bone stock LT1 intake, stock TB. A hacked together fairly rich tune, stock fuel pressure, but the car was maxing out the injectors at 5800RPM. The car was running so rich that watching the O2's (not real accurate, but that's what I had to go by), and it ran safe a/f ratio's up to around 6300RPM.

Right then and there, I knew I had to sell the 24# SVO's I had laying around, and bought a set of 30# SVO's. I sure as hell didn't want my injectors running at 100% duty cycle, especially since I have spun this motor pretty damn high before (up to around 6800, though I've went up to 7200 once or twice, but it doesn't make any power up there, and pretty much is done by 6800).

 

Great link Josh. Excellent information.

 

Thanks!

It's definately not common knowledge, but the link explained it well.

 

The fastest run that I have seen so far on 24lb injectors was 9.98 at 130 mph. That was in a 3000 lb car (LT4 N/A). I spoke with the owner and he feels that he has gone to the limit of the injectors.
I have run both 24lb and 30lb injectors and feel that the 24's come out of the hole a little harder. Maybe it's because the spray pattern is not so good when the injectors are not close to max? Something to think about.

Daren

 

i think you dont have to get new injectors...
should you? yes
did I? yes

 

Duty cycle is calculated by dividing the pulse width by the number of milliseconds it takes for the crankshaft to make two revolutions.

For example, at 6,000rpm it take the crankshaft 20.0 milliseconds (mS) to make two revolutions. If you had a 17.0 ms pulse width, the duty cycle would be:

17.0 / 20.0 = 0.85 or 85% duty cycle.

To save you the calculations:

RPM---time

3,000 - 40.0mS
4,000 - 30.0mS
5,000 - 24.0mS
5,500 - 21.8mS
6,000 - 20.0mS
6,500 - 18.5mS
7,000 - 17.1mS

How high a pressure you can run depends on the type of injectors.... the stock ball/seat injectors do not like anything over maybe 60psi. The pintle style (e.g. - Bosch, FMS, SVO) are OK at much higher pressures.

 

In a perfect world with perfect everything I would calculate the time in milliseconds the intake valve remains open at Maximum (safe) RPMs then I would calculate how much fuel needs to be delivered in the window of opportunity for the amount of estimated power to be delivered by that one cylinder.

Offcourse this does not take into consideration the fuel weight and innertia and all other minor things (that add up) that can be factored in the equation and then I would give it another 25% headroom (I call it safety room) to the whole equation.

Nothing is perfect so I think I would just keep on dreaming


Marvin