What effect does back pressure have on torque for the 6.5? Is it better to have wide open exhaust and let the engine breath? Or is there a finite amount of back pressure that is beneficial to the motor?

Diesel requires zero back pressure

If ya got zero exhaust back pressure and wide open exhaust, ya gots what is known as a normally aspirated Diesel - a not really great performer.
No dragstrip records, no world land speed records - but it'll getcha to the store and back, and tote a load, to boot.

If ya gots back pressure - say, thru a turbine wheel driving a compressor wheel, puffin' lots more air in there than a n\a could possibly hope fer - why, you could most likely stuff lots more fuel in there with all that extra air, makin' more torque and hp, and settlin' fer a happy medium on the ebp vs ipr (intake pressure ratio).

Secret is, the turbine is open-ended, where the compressor is a pressure source, and the engine is a boosted pressure source - volumetric efficiency is greatly enhanced.

The 45psi ebp is a 'regulated' pressure based on decreasing cylinder pressure exhausted out the manifold thru the turbine - exhaust is always flowing thru, creating horsepower in the exhaust turbine.

Every cylinder exhausts into the turbine, which forces equal volume from previous exhaust pulse thru the turbine - an excellent source of work to make the power to compress the air to burn the fuel to make the heat to do the work to......

But, I belabor the point, methinks.......

It is a dynamic, open-ended system - we can live with some backpressure.

We just need to strike a happy medium, deciding where we need to develop the most power -
- from standstill , to get the load moving, where low-rpm low exhaust gas volume requires small turbine to spool-up the compressor wheel
- at hiway speeds, where engine is developing greater exhaust volume at higher rpm, and a larger turbine would make power while creating less backpressure.

Small turbine is efficient at low engine rpm, with hi ebp at high engine rpm.

Large turbine is inefficient at low rpm, but with low ebp at hi engine rpm.

Might be helpful to think of it, simply, as exhaust pressure - pressure required to make horsepower in an exhaust gas turbine which drives a compressor.
Horsepower required to compress large volumes of air, as used to drastically increase volumetric efficiency.

How about a switch pitch turbine housing?

variable pitch? Like the VW TDIs and Powerstroke 6.0Ls have? One of those ideas that works great on paper, but the variable pitch vanes get full of carbon and sieze from what I've seen. They should use a different, stronger control method like hydraulics instead of vacuum IMHO. Even variable pitch won't remove back pressure, energy still has to be exerted on the turbine wheel to spin the shaft/compressor wheel to make boost, and that force means resistance which means backpressure.
From my experience scanning Powerstrokes, and I use this example only because they all have an EBP (Exhaust Back Pressure) sensor on them, is that their backpressure is always higher than boost pressure, whether the warmup butterfly flap is open or closed.

a properly sized wastegate works correctly also.....

For those that don't know - grape is experimenting with turbines, and compressors, and wastegates on an in-process built-up 6.5 in an in-process built-up old-style Blazer.

All that, mixed in with building - and I cannot think of one single street-use for such a vehicle - trucks that only turn left (unless 'bumped' from behind!).

Talk about yer Limited Utility Vehicle - cain't be much of a market fer a truck like that!

They do call'em LUV's, right, grape?;)


(talkin' 'bout them left-turners, here - not the Blazer.....................)

It will be a head turning race car hauler. His engine is just beautiful and he's my insipration these days.

I think grape know's what he's doing.

I think joispoi is asking about the acual exhaust, after the turbo. The exhaust pipe can be as large as you want, bigger is better.

3"-4" is fine but as joeyD says bigger is better.

Exhaust pressure in the pipe past the turbine is undesireable, as joeyd stated - the greater that backpressure, the more harmful, as overall exhaust flow is reduced, which means pre-turbine EGT cannot easily drop.

3.5" exhaust pipe is entirely sufficient for 400cuin and 3500rpm, but the market is driven by the 'mine's bigger'n yours' syndrome, so yeah - 6" looks better than 5.5", which looks better than 5", which looks better than 4.5", and etc.

Bend it, and they'll buy it...............

SSdiesel supply proves that.