For reference, we’ll discuss a stock/modified stock turbo on a 7.3 PSD.

I’m going to say higher boost = more HP almost with no exception. A ‘stock’ truck will make about 17psi. Modify the tuning on the same truck and you’ll see up to 30psi and the increase in power is linear with boost. ‘Hotter’ tunes make more boost.

Here is a stock turbo with Riffraff 4+4 compressor wheel being SPANKED to 38psi with 175/30’s.


Now is that ‘efficient’? Probably not. However, boost and EBP are pretty close to 1:1 up to about 30psi most of the time.

‘Normal driving’ will basically never see the boost I was generating in that video. That was a hot tune (probably PHP’s 100 or 120 tune?). This truck has probably not left 60tow tune since it left here 7 months ago. In that tune, it only makes about 30psi and that is really running hard. So, during ‘normal operation’ we are likely staying inside the map for this turbo setup.

So what happens when EBP begins to exceed boost? I think the biggest issue is laterally loading the turbo shaft. This causes accelerated wear of the journal bearings. I also think the overall efficiency of the engine is being reduced because the air charge is likely also hotter with the turbo outside the map. BUT, less efficient does not mean less power. It means you don’t get the same bang per gallon of fuel.

Turbo science is deep. I know enough to know I don’t fully understand it. The physics include pre and post turbine temperatures and many here already appreciate how many options there are regarding housing AR’s, wheel trims, blade configuration, housing design (ported, etc), and so on. All of these options exist because there are nearly infinite applications. In our case, International/Ford developed a turbo that fit the needs of a (stock) pickup truck running stock tuning.

 

Ok, so let’s unpack this a little.

The TURBO is concerned about a 1:1 ratio. Even then, the delta is a symptom of the turbo moving outside it’s effeicincy zone on its map. What I think is actually happening is the turbine/hot side is getting choked - resistance to flow. This resistance is causing the turbo assembly to get HOT, because the exhaust gasses are naturally hot when the engine is working that hard (burning lots of FUEL = BTU’s). This heat is being transferred to the charge air. I don’t think it’s as related to compressed air being hotter as people imply. Just spinning the compressor wheel to make 35psi doesn’t make the air that much hotter than 25psi. It’s the turbine side driving the heat. I may be wrong here and I may not be ‘splaining it right (it’s been past beer o-clock for some time now).

 

Possibly for a short period of time (very short...like a WOT highway pull), more boost may equal more power. But for an extended operating condition like pulling a heavy trailer down a highway all day, you want the turbo staying near or at it’s peak efficiency range and not spiking too much past that. It’s important to define the parameters you expect the system (turbo and engine) to operate in. Turbocharged diesel engines are obviously very versatile tools for getting work done, but usually people speak in terms of longevity (my engine lasted a million miles!) rather than how fast they went down they quarter mile or pulled a sled however many feet they pull sleds for.

 

Not to be the perpetual Duramax dropper, but if you guys have not watched the L5P videos Gale Banks has made over the last year or so. He has been thrashing an L5P to see just how far he can push the stock long block until it scatters. He goes over a lot of turbo science in the videos. Lots of talk about efficiency, air density, back pressure, etc. VERY educational, and could lend some fodder to this discussion.
One thing that is interesting...you can see where guys go to a bigger turbo, less boost and more power. So there is something to getting turbo efficiency correct. A turbo in its sweet spot can make the same power with less "boost" than a turbo maxed out.

 

We seldom generate high boost while ‘driving down the highway’. For the most part, we see higher boost for short intervals while accelerating or pulling a grade. While there are grades that demand higher boost for long periods of time - that is not normal for most of us.

I say that because even though the truck in my video above CAN make almost 40psi - it seldom (never?) sees boost outside the turbo’s efficient range in a modest tune.

You make a solid point about parameters. There is a world of difference between a compound turbo set-up that requires stages of nitrous to spool at the Christmas tree and then RIPS down the 1/4 mile in under 10secs seeing EGT’s that would make grown men here cry.

I want to say let’s keep this conversation about daily driven trucks that pull heavy trailers.

I’m interested in better understanding the relationship between exhaust housing AR, turbine wheel size and design and compressor housing and wheel options and how these things change BOOST and CFM as well as the way these options affect EBP.

 

This was exactly our point. It is possible to get outside of the correct performance parameters of the turbo. In these cases more can equal less.

What about when people try to do that with the stock compressor wheel? The ratio will be pretty far off from 1:1. I remember watching a video with Bill from PHP dyno testing the billet wheel when first coming out. He was impressed with how much boost it could make while remaining near 1:1. The billet wheels are a game changer for the stock turbo in my opinion.

 

Too late tonight but I would enjoy being educated by those videos. Thanks for mentioning.

 

We are already unraveling some confusion.

When you say ‘less boost made more power’ - you are talking about 2 different turbos.

Im going to say the SAME turbo making more boost will always make more power.

In the example where less boost made more power - we are talking about a turbo that can move more CFM with less resistance/lower boost.

So, while boost and CFM are related on a given turbo - I think a different turbo can move more CFM at lower boost numbers. But, I just finished the last beer and I might have (even more than usual) diminished cognitive capacity...

 

One more thing before my speech is to slurred to understand.

The eminent danger of a large EBP/MAP delta is reduced turbo life. I think we will also see higher EGT and obviously less efficient use of fuel - but spanking a stock turbo is not risking chucking a rod.

 

SkySki, you are correct, and it may not be fair to work turbo swaps into the equation. So let us chuck that one to the back burner for now...
So the discussion would lean towards increasing CFM capability of a given set up.

 

36psi with stock ADs and stock turbo.
And here 44psi with stock ADs and 63.4mm inducer "stock" turbo.

 

Same 63.4mm turbo with 205/80 injectors.

 

Boost is a measurement of restriction. If we compared a bone stock truck to a truck that is stock but everything smoothed out (ported but not taking meat away) in theory boost should drop while cfm goes up. More likely is that boost drops while cfm stays near the same. Less boost equals less heat, less heat equals more power and a much better chance of staying within the turbos map.

 

And here I am only getting 30-32 psi from a 363 or a 364.5 with my 160/0
get 38 with a 364.5 and 180/30
Guess my tunes are too reserved. Max pw of 3.6

 

May be worth noting that in the BitterRoot HEUI thread he mentioned of a brand new 160/0 only flowing 50cc.