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Discussion Starter · #1 ·
In the next month I will be installing the LT4 Hot Cam and LT4 Heads/Intke/RR's. My fuel delivery system is completely stock. Is it going to be up to the tast to support the ~400+ HP that this system will produce? I did replace my fuel pump about 1 year ago with a NAPA replacement (this may be my weakest link). I've been told the stock fuel injectors are good up to ~450 HP, is this correct?

Please let me know what I might need to change (fuelwise) to get the most out of my engine.

Thanks!
 
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Discussion Starter · #2 ·
That 450 hp figure can be a little misleading. That figure came from Lingenfelter, who runs the stock injectors on his 440hp strokers. But the problem is that a stroker will make that HP a lot lower in the powerband than a 350.

Most heads/cam guys find that their injectors are going static at about 6000 rpm. At a lower rpm, they have time to stay open long enough to add the required fuel, but above about 6000 they just can't keep up with the demands. They just don't have enough time to inject the fuel.

On your fuel pump, it will probably work OK. The Walbro would have been a better choice, but a fresh stock fuel pump will work fine until it begins to wear out.
 
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Discussion Starter · #4 ·
The 30 pounders will work just fine. You can even run the 36ers if you like, although that might be a little bit of overkill. My car was making 320 rwhp (about 400 at the crank) and the 30 pound injectors were seeing a duty cycle of about 15 milliseconds. Roughly a 75% duty cycle. The ideal duty cycle for optimum fuel/air mixing is considered to be about 80%, according to both Vizard and Lingenfelter, so I'm pretty happy with the 30 pounders.

At 6000 rpm, there are 20 milliseconds between closings of the intake valve; so the injector only has 20 milliseconds to inject enough fuel for each combustion event. If your injector pulse width is 20 mS or greater at 6000rpm, then you've gone static and the knock sensor will start cutting in real quick.
 
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Discussion Starter · #5 ·
If I can get the 30# and the 36# injectors for the same price, is there any reason I shouldn't get the 36# ones? Is there any harm to come from running an injector that is too big?

Thanks
 
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Discussion Starter · #6 ·
What are your final goals for your car over the next year or two, realistically? If you're adding a blower or turbo, then yeah, go with the bigger injectors. Otherwise, stick with the smaller ones. As I said before, both Lingenfelter and Vizard feel that an 80% injector duty cycle is the optimum setup for proper fuel/air mixing. In my case, my 75% duty cycle with 30 pounders would have been only about 60% with the 36 pounders. Not sure if that would have mattered at WOT or not, but I chose to go with the pros' recommendation.
 
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