I have a 2004 RCSB Joe Gibbs Silverado with a 4.8 5-speed manual with the supercharger, Corsa exhaust and custom Gibbs tune of course. I know the fuel pump was upgraded with the supercharger option but I don't think the injectors were (someone please correct me if you know otherwise). Currently the truck is making 5-5.5 lbs boost stock and I want to change the pulley to a 3.4 or 3.2 (3.9 was stock for 4.8) to up the boost to around 8.5lb or 10.5/11lb debending on the pulley. Would you recommend new injectors stepping down to 3.4 pulley? I know going to 3.2 would most likely require it, but then a custom tune would probably be needed too. I don't wanna get too wild with mods, probably going to stop with pulley for now. Currents mods are shorty headers and Airaid CAI, on top of the factory Gibbs mods of course.

 

Don't know what injectors came in your truck, but I'd bet a good custom tuner would tell you whether you were at or over your injector's capabilities. If you're running the stock tune, you're probably leaving quite a few ponies on the table even if you don't pulley down. A radix blown truck with a 5sp manny tranny has to be a lot of fun. I'm jealous.

 

I am pretty sure it would have 42's so you would be ok, pulley it to 10psi and have it tuned. Should be fun till the 5sp breaks!

 

bye b ye tires.
who ever tunes it should keep it safe.

 

The injectors were upgraded, the fuel pump was not. Gibbs ran the trucks on LOW BOOST to keep from dropping the tank to replace the pump.

 

I Had a 4.8 5 speed gibbs (non-supercharged) and I put a Magnacharger 112 on it. My first mod was to pulley it down to a 3.25" pulley I belive and custom tune. It was a whole new truck. I can't remember if it ended up with 11.5 at that time. Can't recall for sure. I think my supercharger came with a 3.8" pulley maybe and around 8 psi boost.

 

Your truck was a freak of nature.....the 4.8 trucks all had 3.8 inch pulleys.

I think the dealer installed a 5.3 kit on your truck.

 

LOL! It was fun. It is fun now, but I would be perfectly happy if I had the 4.8 & 5 speed back. The mp112 is the perfect size for a streetable 4.8. 5.3 it's ok and the 6.0, its too small IMO. But works well at stock boost.

 

He bought his truck NA from the dealer then bought a magnacharger kit and put it on there.

 

I did the same shortly afterwards. In fact you were the inspiration. The 3.25" pulley made 10 psi boost. My maggie came with the 42 lb. marine injectors, and they were fine, but at their limits (about 85% duty cycle IIRC). The stock maggie tune was pretty much OK as well. I only had to tweak the power enrichment cut-in to avoid a lean spot where I would have 1-2 psi boost but still stoich AFR. BTW, still have the maggie (almost 6 years now) and it still runs great.

EDIT: Should mention, this was for a regular truck with an aftermarket maggie kit, not a Joe Gibbs truck.