So I’m starting a build on an ls 5.3 and wanted to boost but am a beginner. My engine has
9.9:1. It makes 310hp at flywheel stock. I want to run pump 91 but also want 500whp from a stock engine. How much boost and should I switch out the pistons, which are flat tops right now, to something else to lower the compression a bit. Thanks for any help

 

the lower the compression the more boost you will need to make up the loss in power

 

Loss of hp from compression drop is small (~3% per full point) so I'd look to drop comp ratio if your goal is strictly pump gas. If you were open to running some water/meth then I'd leave it.

 

I am certain you can make 500 whp on a turbo 5.3l on pump gas, without dropping the ratio. Lots of stock bottom end coyotes at 11:1 running boost and pump gas. What are you using for engine management? Is E85 an option?

 

It probably can be done with some careful tuning, but I don't think the coyote is a good comparison with its rpm potential, dohc, and variable cam timing.

 

That the most extreme mainstream example I could think of. You're right, not apples to apples.

 

an Iron block 5.3L LM7 engine from 2002 is roughly 9.5:1 compression iirc and in my experience will support at least 600rwhp on 93 octane pump fuels without limitation to mileage or durability.

The caveat being that you are an exceptionally creative and experienced tuning expert. If you have any qualms associated with the tuning of such a delicate engine (cast pistons) I would simply add a 50/50 shot of methanol/water injection to make up for lack of tuning ability, as it will dramatically widen the tuning window for timing and a/f ratio, again with a caveat that the system never fails. Which, it will, one day, as those pumps have a life cycle and must be replaced as maintenance, and the lines can clog, and so forth.

 

No you don't need to lower the compression to hit 500whp on 91. Run a decent intercooler and keep timing around 10-12*. Shouldn't take more than about 10lbs of boost. Water meth helps as mentioned, but not needed at that power level.