Stock actuator to close the wastegate, assuming proper vacuum to the diaphram and steady vacuum how much force in PSI would it take to open the wastegate? I'm wondering if one can get more than 20 PSI with the stock set-up, next problem with that is i'm also beginning to wonder what my Exhaust back pressure would be with 20 PSI of boost and rotating @ +/-2500 rpm? Will more than 20 PSI be self defeating?

Once you've been able to hit 20 PSI you want more for some reason....):h , -15C here today and i managed to make it to 21 PSI. (short lived though).

Well, if you get the measurement of the actuator diameter, we can figure backward from there. You've only got around 15-18" of vacuum which is maybe in the neighborhood of around -8 psig available to the actuator. There will come a boost level where that wastegate will blow open. At 20 psi of boost, dat's a bunch. Start getin' up there much higher, you maybe servin' up some crankcase hamburger.

So what would be the limit of boost on stock guts in a 6.5, anyone know? Whatever that limit is i would like to try and achieve it and sustain higher boost levels through-out the rpm range, it just seems that the more boost this truck gets the quicker it seems to get, an IC will be a must come summer but for now i'm good to go. If it'll take 25 PSI then thats what i'm going to try and get. I knew i bought a 35 PSI gauge for a reason... lol

2"hg vacuum = -1psi
30"hg Baro pressure is ~15psia
30" vacuum is -15psia, or zero (0)psia
WG servo diaphragm = 2"dia
Figure area of 2"dia circle x 15psia Baro = max force available to hold wg closed
pie are squared = 3.1414sqin x 15psia = 47 max pounds of force
20" vacuum = (Baro - 10psia) = 5psia on control side = ~16pounds on control side
15psia Baro on force side vs 5psia on control side = 10psia or 31pounds forcing wg closed against exhaust pressure

Increase control vacuum to 30" (-15psia) would give full 15psia force on diaphragm

Wastegate dia is ~1.250", so doing the same pie are square thing on area of 1.25" circle against exhaust pressure will give you amount of force trying to open WG

Exhaust pressure is heading up over 40psi at 20psi Boost @ 2500rpm.

Hmmm... my math isn't very good with the x-mas cheer. Am i blowing the WG open then? I tried to figure the above numbers but things just didn't seem to work right.

That's what's limiting your Boost to ~20psi, dude - need a bigger turbo.......

'Djoo verify my numbers, kn - they workable?;)

Anyone wanna buy a good used GM-8?

Too much boost ain't healthy in stock 6.5, if you are going over what the General wants you need to drop compression ratio, as well as lowering IAT, 18:1 CR is needed to go above what GM-8 can deliver.

OK, then can we maintain boost pressure and increase volume of burnable 02 more safely? I wish i knew if this thing was running out of breathing room or fuel to burn. I'm not really in the mood for pulling good pistons until they fall out the bottom. This unit should be getting all the air it needs for combustion with 20 PSI and should be delivering all the fuel it can with the current pump.

Another thought would be to introduce some propane to it and see what happens? For testing purposes what would be the best place to inject propane?

I think you showed dem pesky numbers a thing or two there GMCTD!:)

That's jd - 1, pesky numbers - 'bout a ka-jillion.

Maybe I'm improving, with age...........;)

I've been comtemplating the PS or cummins style turbo's swap. I've hit over 20 psi on the gm-4 but that's the end of the boost guage range and I don't know if it'd go further. I still havn't pulled the engine on my '93 but it's still in the works to shave the pistons to 19:1.

Too much boost ain't healthy in stock 6.5, if you are going over what the General wants you need to drop compression ratio, as well as lowering IAT, 18:1 CR is needed to go above what GM-8 can deliver.
Or just get water injection, the perfect answer to non-intercooled, high boost applications.

WMI or IC good for lowering IAT, but still need to lower CR on 6.5 when going above what GM-8 can deliver, 6.5 isn't as robust as other engines it does well when living to as designed margin, needs some help, but going too far with boost it needs some strengthening on bottom end and lowered CR to keep all things in perspective.

Will water alone (no meth etc) increase CR? Or lower it?

WMI or IC does nothing to raise or lower CR, it lowers turbo post compression temp, for a denser charge,(more air) also keeps temps cooler so you can use more of what is inherent in the engines available power. go to www.bankspower.com for more info about turbos and intercooling in general, WMI is just another way to lower IAT