Calculating boost pressures.
 

Ok this is what I have, 6.2L LSA. I have a LSA blower for it and thats the easy part. Here is the hard part, I have to run a LS9 snout due to the fact I need to run the LS9 accessories in my frame set up.

This is where I run into confusion. I'd like to be in the area of 13 psi, which should put me in the area of 700hp with a ported blower and cam. How do I calculate my boost pressures based off this? I tried online calculators and they are all over the place in terms of numbers when just checking the stock numbers.

LSA stock pulley sizes 7.8" balancer 2.95" upper pulley = 9psi of boost.

LS9 stock size balancer is what I want to run and it is 7.33"

All are with a 6700 crankshaft RPM
Stock LSA upper pulley speed is 17,715 RPM.

7.33" balancer
2.6" pulley blower speed is 18,888 RPM.
2.35 Pullet blower speed is 20,898 RPM.
2.25 pulley blower speed is 21,827 RPM.
2.2" pulley blower speed is 22,323 RPM.

Max safe RPM for a LSA blower is 23,000 RPM.

What basic compression do you run ?.Do you run a boost gauge ? There are books fro SA and HP books on supercharging which may help you.

I've combed over dozens of builds and dyno pulls on Camaro5 forum this thread:
https://www.camaro5.com/forums/showthread.php?t=314663

Biggest variables are
Ported blower / snout
Ported heads
long tube headers
overdrive pulley

What are your cam specs? To hit your goal i think you'll need to be over .600 liff with 22.5% overdrive pulley.

The boost number is really "it is what it is" based the combination above. There really isn't a formula to use. My guess is your going to be 12.5psi - 14psi.

My LSA build has a goal of 700rwhp with 22.5% overdrive, ported heads, cam, welded and ported snout, 1 7/8" headers and good exhaust.

I'm just trying to get a idea of what pulley to get so I don't have to buy a bunch and try. I haven't picked out a cam yet. I'm leaning towards this comp cam 239/243 with 0.624/0.624-inch lift and a 114-degree LSA. Its designed for a LS engine with a roots blower.

It will have a ported blower, stock heads, stock lower end, 1 7/8" headers with a good 3" exhaust. I can't overdrive the lower crank pulley as it also over drives the water pump and power steering pump, which is a no go for track days. So I have to do all my over driving at the snout. My math on that works out like this. (edit: I didn't think about adding the percentage to the stock PSI)

Blower speed, pulley, % over stock, PSI
17,715 stock LSA 0% 9
18,888 2.6" 6.6% 9.54
20,898 2.35" 17.97% 10.62
21,827 2.25" 23.2% 11.1
22,323 2.2" 26% 11.3

So basically I should be safe with the 2.2" pulley and my rev limiter set to 6900 RPM as I'm below the max recommended blower RPM of 23,000.

Probably look at similar builds for ballpark. Your camshaft will play a huge role in boost also. The 114 degree split will help your boost. If there is too much overlap, which increases with duration and closer lobe separation, the supercharger will just be blowing out the exhaust. The more your heads flow, the more your exhaust flows and basically anything that increases volumetric efficiency will yield lower boost pressures.

Belt slip is a real concern for that small of a pulley. For road race applications most stick to 20-23% overdrive. There are a few 28% overdrive LSA surviving out there.

Who is going to tune it? They may offer a cam package and pulley ratio that they could recommend. That is the route I went with.

Thats part of my issue, Nobody has swapped a LSA blower with a LS9 accessory drive. The blower pulley is separate on the LSA and they can change that, the top, or both pulleys. The LS9 has a 14% overdrive available, but it runs more than just the supercharger so I can't swap it.

I thought about that too, weapon x has larger idler pulleys to help wrap the belt around the pulley more.

If I still live in Texas by the time its ready to tune I'll have PatG tune it. I've sent him 3 friends cars now that others have tuned and hes made more power and had better drivability. Otherwise I don't know.