48rat

Morning guys! I have some questions about my stock 1999 4.8! I currently have 55# inj. 320lph pump. 76mm turbo. Running 7# of boost with a great tune. I want to know what this girl will take for boost without making any mods to it?? How far can I go with boost and b ok! Thanks in advance

JoeNova

All of it. It should take all of it.

You'll top those fuel injectors out before that 4.8 has any problems. Just keep the timing low, these engines love low timing. I run 12 degrees above 14 PSI. I didn't gain any power when I added timing.

48rat

what are you running at wot for your AR?

48rat

Anyone feel free to post! I need all the help i can

JoeNova

No leaner than 11.0-11.5, 12 degrees of timing on a stock 5.3 at 20 PSI.

48rat

nice, im at 10.8-11.3 at 9psi. I did change it last nite! huge even from 7 to 9

JoeNova

You could easily get away with another 5 lbs with a safe tune and it wouldn't be any harder on the engine than the 9 PSI you are running.

SLOW SEDAN

iTrader: (13)

So far my 5.3 has seen 26lbs and 13 degrees timing... Going on a year of beating on her. Got another standing by just in case but she won't give up!

48rat

awesome guys!! thanks for all your input!!

67Firebird455

iTrader: (27)

My 4.8 loved 15psi when I was having a major issue with my wastegate and I was only running 10º timing and 11.0 AFR

forcd ind

iTrader: (7)

My 6.0 was running 115% duty cycle on 60 lb injectors and around 13-15 lbs of boost-the 4.8 might not be as hungry, but I would keep an eye on the duty cycle, depending on base pressure

48rat

my tuner told me that at 90-100% there pretty much peaked! I have to log some more data and check it out. I raised it to 14psi last nite but didn't get a huge hit on it as to my tranny isn't likin life at the moment!! lol thatll be next!

48rat

I just put a pic on my profile but it isn't showing up on here! how is yours?

JoeNova

Yes and no. Duty cycle is a calculated number that is basically (pulse width/cycle time) If your injector pulse width is 20 ms, and you are revving fast enough to do a complete 4-stroke cycle in 20 ms, your duty cycle is 100%. Your injectors are technically wide open until 100%. Most tuners say 80-90% as a safety margin, but its not technically an issue going over 90%.
If your fuel pressure regulator is boost referenced, you can still be adding fuel and running fine even at 100% duty cycle, or above. If your duty cycle is climbing because your fuel pump is maxed out and pressure is dropping, switching to a large pump or adding a second pump to hold constant pressure can also bring duty cycle down. So duty cycle is more of an indicator than an actual measurement of how far you can push your injectors.

48rat

very cool

67Firebird455

iTrader: (27)

I'm at 69% DC with my 60's at 60psi base with 1:1 reference at 10psi on my 4.8, no methanol yet, this is at 6400rpm and 11.4 AFR.

JoeNova

62 PSI Base, I'm pushing my 80s into the mid 90% at 20 PSI. I've had them over 100% on several occasions, and one overboost occasion where they were at 170%.

I can run 24-26 PSI and have them at 110-120% or so before they actually start to lean out. Turns out, my regulator isn't raising pressure at a 1:1 ratio so if I fix that, I should be able to keep them down near 100% at the same boost level.