I've seen lots of Turbo customizers making changes to turbos. "Hybrid and Upgrade" are terms used a lot.
I'm making changes myself and documenting the results the best that I can. I have a list of questions for turbo experts regarding what is different besides the component modification required to facilitate wheel swaps.
Lets refer to HE351cw 60/86 Comp, 60/70 Turbine (73.5 trim) , 9cm housing.
Popular exhaust side changes are modifying the 9cm housing to accept either:
64/76 Turbine (70.9 trim) or 67/76 Turbine (77.7 trim).
I don't see where boring the housing affects the A/R yet flow increases due to the bigger exducer area.
It "seems" the turbine shaft power output would increase due to less exhaust flowing around the Inducer blades, and the larger radius of them (76mm vs 70). Some leverage is likely lost if switching to 10 blades vs 12.
Reported effects of turbine wheel Trim (low trim = higher flow, and high trim = higher P/R ) seems to only hold true if considering the same exducer dia. A 64/76 still flows more than a 67/70, and vendors say maintains similar spool to stock. It stands to reason a 67/76 will flow even more, than the 64/76 but will definitely spool later than the stocker or the 64/676. The higher trim theory holds true in this case.

Now going to the cold side... Switching to 62/86, 64/86. or 67/86 compressor wheels follow the flow increase, spool decrease, and trim effects. Bigger inducer = bigger bite at the air = harder to turn etc.
What if you throw in a 64/93 instead of a 64/86? Bigger wheel = heavier, yet it has a lower trim. Will it spool slower than the 64/86? Will it hold max shaft speed down? Will it make 3.0 pr with less shaft speed?

Only identical dyno testing can objectively answer these questions. I'm just trying to wrap my head around the basics to visualize general rule of thumb impacts when modifying turbos.

 

Simple questions don't equal simple answers.

They're good questions, but its getting more into principals of how turbos work. The turbine is the most important part of the turbo, but also the least understood.

The hot side will flow more with bigger turbines, but only up to a point. After that the housing is what bottlenecks the flow.

Boring the housing doesn't change the a/r. The nozzle, that the a/r refers to, is up in the housing farther. If you Google the breakdown of a turbine housing, you'll see why it isn't affected during the boring process.

Getting into turbine trim gets complicated and very lengthy, I'll try to keep it short...

The exhaust gas doesn't "swirl" around the tips. The nozzle in the turbine housing speeds up the exhaust, the speed of the gas determines the angle it hits the turbine wheel. A small a/r speeds the gas up alot, creating a shallow angle which is good for spool. A big a/r is the opposite, steep angle, good for flow.
That affects turbine trim. Despite popular belief, the turbines inducer isn't what powers the turbine. Remember the wheel is spinning with the gas, so the tips are going nearly the same speed as the exhaust (except for spool). What ACTUALLY powers the turbine wheel is a combination of the gas deflection that happens in the wheel when the gas hits the exducer blades. And when the exhaust expands as it exits the wheel, which causes the gas to expand. Every passageway between the blades turns into a "jet" of air.

While a low trim turbine will spool better, what can happen when coupled with a small a/r housing is the turbine wheel tips exceed the speed of the gas trying to get into the wheel. Because again the tips don't power the turbine, the exducer blades do.

So typically, a large a/r housing would have a low trim turbine wheel, and a small a/r housing would have a higher trim turbine.

Plus don't forget low trim turbines can limit your turbo from overspeed of the tips, where a high trim wheel has room to go.

I feel it's important a good turbo builder should know this stuff. The compressor, turbine, and housing should be matched. The last thing you want is a small a/r housing on a low trim turbine with a high trim compressor. There is gas speed to blade tip ratios, exducer throat area, and more to work out to really have a good match.

Switching to the compressor, keeping shaft speed the same, a low trim compressor will have less flow but more pressure vs a high trim wheel. A good example is the 2016-2020 Chevy Colorado diesels. Stock wheel was 37mm x 52mm, 50 trim. Garrett came out with their powermax "upgrade" turbo that had a 44mm x 52mm compressor, a 72 trim. Everyone of them installed had terrible compressor surge because the wheel didn't make the pressure needed to shove the flow it was trying to move.

If you look at the compressor maps of garretts gtx55 turbo, you'll notice no matter the trim, max rpm is the same. If you keep the inducer the same but increase the exducer, max rpm will be lower.

 

The turbo hot side sizing regulates the amount of total exhaust that can be flowed out of the motor which defines the total max power the engine can make. The turbo cold side sizing is meaning less in my opinion because the turbo will spin to the rpm that provides your set boost pressure. I feel Turbo rpm’s are meaning less and who cares if it spins to the moon as long as your intercooling is sufficient. I believe that it is best to utilize wastegating exhaust from the motor over depending on the turbo hot side sizing, I like to think of the turbo as just a performance part to the masterpiece of the engine.

 

It's been proven and shown over, and over, and over, and over again that an engine makes the most power when the turbo hot side is sized to deliver the boost needed to make power. 0% wastegating.
However that means it's a laggy setup. So you trade power for spool. The more wastegating you need, the less power your engine is going to make compared to its potential.

 

“The more wastegating you need, the less power your engine is going to make compared to its potential.”

Well then the problem is you didn’t start with enough engine or can’t turn it up anymore.

I want power force targeted all over the rpm range at any load any gear any rpm, not a certain high or low rpm’s rpm target. In my opinion if your targeting your build solely on turbo spec sizing that would not be correct because we want the outcome to be more than that plain Jane, we want to control the turbo properly via our own settings to do the most all over.

If you have a pure drag strip build then I understand because all you care about is high rpms and long runners.

 

I'm not saying to size the turbo hot side for max power.
I'm saying that the earlier you try to make a turbo spool, the more power your giving up on the top end. Turbos are always a compromise between max available power and spool.
But the answer is never "just slap a turbo on". Drive pressure combats boost, to small of a compressor creates high iats from high inefficiency. High inefficiency causes even higher drive pressure. High iats reduce power, the list just goes on.
If that's how you want to size and run your turbos then go ahead, but don't tell others to do the same. There's turbo "recipes" just like there's engine recipes for making power, or hitting someone's goal.

 

Its a game of knowing where you want to be at how you what it to be, when, and which turbo specs accomplish that for your own personal setup. It’s a matter of opinion. For me I’ve found my spot, I’ve had to big and to small already. Also yes higher air temps are a bad thing power wise, but you just add more boost and it no longer makes a difference (if your build allows this). I’m not telling anyone to do anything, I would prefer all that I say is taken into a persons consideration to find/meet their goals. I’ve done so many things in my life just like all of you here have. Where the money lies is in experience.

 

I'm fascinated with how the responses here are aligning with my experiments to date. I still haven't found the opportunity to Full Fuel test the current configuration so I can make the next single change of several I have planned. I did however recently drive 600 miles each way to attend Florida Truck Meet and watch the ODSS Season opener. If I were just daily driving this setup I would make a couple minor changes back the opposite direction, but I'm still in the hunt to be able to use up the fuel I have available now. On one little 30 mile "run" with a Mustang and a Challenger I was able to watch gauges under lite load. RPM 2600, (105mph), EGT 800-900, Manifold Boost 20psi, Drive psi 24psi, LP Boost 7 psi, LP drive 8 psi. A couple of my experiments have resulted in driveability of a big single without the power on top. The system was fighting itself. At least it (and BBI) started pointing me in the right direction. Currently I start hearing the turbos waking up at around 1700 rpm. Accelerating below that, (especially when in OD and locked) is a smoky mess. Following the turbo spool with the accelerator keeps exhaust fairly clean. During the run above it felt as if it was "Chomping at the Bit" for more, but I couldn't afford to get a ticket at near double the speed limit. 4 stops for fuel netted 17.5mpg, 18.5mpg, 16.5mog, and 17mpg. That's with 15.5" wide 35"od tires on 20" x 14 wide wheels on all 4 corners.

 

You reallyyyyy need some bigger housings on those lp turbos. You'll see so much change once you free up the lp turbine stage. Then you'll start to be able to play around with stuff.

 

Your recommendations are absolutely taken into consideration, and are appreciated. Each change I make that increases the LP turbine side flow is confirming it is sound advice. I have 3 more incremental changes left to make on the LP turbine front and then bigger housings or bigger turbos are next. Thank You.

 

Cool, I'm sure I sound like a broken record, keep us posted

 

When I tried your suggestion to open the LP wastegates and test It proved you know what you are talking about. Now I'm incrementally changing and testing one thing at a time in an effort to find the balance between capability and driveability.
Thanks again.