Since I can't seem to find information on this topic, I'm hoping the Experts on this forum may be able to share some insights before I make something to test.
Impacts of Compressor Wheel and Turbine Wheel TRIM on turbo performance.
Lets assume all blade configurations, and housing A/R's remain as close to constant as possible, in order to narrow the focus.
My limited general idea is: (feel free to expand on or correct my assumptions)
1. a bigger the TURBINE INDUCER diameter, will impart more shaft torque with a given exhaust flow. Lower Trim = smaller Exducer. A smaller exducer limits the max flow capability, a larger exducer will hurt spool but flow more.
2. A smaller COMPRESSOR TRIM (exducer diameter increasingly larger relative to the same inducer diameter) will tend to yield a slightly slower spool (due to weight), be max rpm limited due to tip speed, but will produce higher compression ratios with the same shaft speed.

AM I CLOSE?

I'm trying to configure an atmosphere turbo in a diesel compound setup.
I've found out some things through experimentation with triples with the same turbo:
1. all stock = good down low but Hi Pressure (manifold) Turbo exhaust chokes at some point and drive pressure spikes, egt climbs, while boost barely increases.
2. Bigger turbine in Hi Pressure turbo (same housing bored out) = significant spool loss, better top end and egt once lit..... turns out stock atmosphere exhaust side (Low Pressure turbines) were choking the bigger turbine down low. Verified by opening low pressure wastegates....overall performance was down, but pressure ratios across stages and low end spool improved.

What I'm considering is leaving the hi pressure turbo as is (stock 60/86 compressor wheel and 67/76 10 blade turbine ), and building a pair with 64/93 compressors and 64/76 ten blade turbines. I don't want pressure ratios over 3 or so in both stages. Trying to free up the manifold charger to spool and then have the low pressure feed it cooler air without dumping exhaust around the low pressure stage.

IF I'm in left field I can change my experiment to 62/86 or 64/86 compressor side with 64/67 turbine. I also have 67/76 turbine, but thought incremental changes might prevent buying more exhaust housings...lol.

Thanks in advance

 

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You are correct in your 2 statements, but as usual there's more to it than that. #ad

What your looking at is as if you were looking at the turbine wheel from the back, without the exducer blades on it. The thin black lines is the exhaust gas flow. And what it is showing us, is that at high gas speed the gas essentially "skips over" the blades.
The turbine doesn't get a full "gulp" of air and flow/ power is reduced. This happens to all turbine wheels, but the lower the trim, the worse it gets. This is why most manufacturers are going to high trim turbines, garrett is typically around 84trim.

There's ways around this, and is how I got a 78/75 turbo to gain another 200hp, but like anything else it comes with compromises as well.

In case I haven't been clear in your other thread... There's. More. To. Turbos. Than. Wheel. Sizes...

your turbine housings are your bottleneck on your atmosphere turbos. You could put a 2 blade 100mm turbine in and you'd still hit choke, because your housings won't allow anymore flow through them. Get the biggest cm housings you can on them, take a die grinder and hog out the dividing wall, install the stock turbine on the high pressure turbo and you'd have a setup with great spool and flow up top. Stop wasting your time trying all these wheel sizes when the wheels aren't your problem. You won't see much change in the wheels until you get the flow opened up to them.

 

I was hoping you would reply to this. As always I do really appreciate the feedback & advice. Your advice on opening the low pressure gates did point to restriction in that stage turbines holding the hi pressure turbo spooling back. It did result in an overall boost loss I was still unable to clean up the exhaust at full fuel up top.
I currently have clipped stock (60/70-12 blade) turbines in the low pressure stage. I do see a movement in the right direction. Quicker overall spool, less smoke, and better P/R across the stages. I haven't been able to do a full fuel test due to weather, but driving and tweaking fuel settings, throttle & load, are pointing to it will still choke under full power. With the fuel turned down it seems like it would be a great towing setup.

I noticed a big difference in drive pressure after boring the 9cm open housing for the 67/76 - ten blade (from 60/70). In my pea brain I can't help but think there might still be an atmosphere turbine/compressor combination that will move enough cool air to use up the fuel I have available, sticking with HE351cw platform. It likely won't be a 4 digit horsepower setup, but my pump is currently only calibrated to 450cc.
This is just a learning exercise, but I'm trying to use advice from experts like yourself. I will start digging around for housing options. The good thing is any improvement I make is doubled on the atmosphere.
Also if I go too far,it will be twice as bad ...lol.

What turbos would you recommend on the atmosphere pair? It seems like there is a limit to what will spool and when. A pair of S475's with 96/88 turbines and 1.32 housings seems too big, but what if it had 64mm turbines or 1.15 housings? I've considered 467.7/83s or 464's as well. A buddy of mine has an ON3 107mm sitting on my bench. I've considered putting that on as a single atmosphere charger in compounds just for kicks.
All of these combinations would act differently, and probably work to some degree. I'm not familiar enough with compounds to mathematically evaluate the performance characteristics of the various combinations ... just gut feel that is apparently skewed.
Thanks again.

 

And i thank you for taking to advice so easily! Alot of people think they know it all about turbos already and have a hard time accepting that what they think they know, isnt true/off/ect.
Theres 2 components to the hot side of the turbo, the wheel, and the housing. The housing is the "water nozzle" that controls how much it speeds up the exhaust gas to hit the turbine wheel. I keep saying that because you keep talking about this turbine and that turbine, thats only half of it, and its the "back half" that is being controlled. The controlling part being the housings. Change the housings, change the whole system.

You also need to stop accepting the general idea of trying to spool the atmosphere turbos. This comes from people not knowing how to size the high pressure turbo, so they try to do it all through the low pressure turbo. Not their fault, but its become so common that everyone else accepts it as correct.

Sized right, your low end spool will be the high pressure. As exhaust energy increases the low pressure turbos will come online, the whole system becomes one system. boost rises and falls together, no crossover point, no switching, no weird phenomenon. This only happens if your low pressure turbos are big enough, they will spool automatically but they HAVE to be capable of flowing the exhaust after its been expanded after the high pressure turbo.

That on3 107 is junk, i have a 98mm gt55 here that would work great. The 98 is big but gobs better than the 107, and ball bearing so alot easier on your oil system, and spool. Has a gt42 cover on it so its more compact than a normal gt55, but it is heavy, vband flange not t6, so even more compact. for a decent fee you can try it, its just sitting here.

 

After the experiments I've done to date (using advice from guys like yourself) I'm in agreement with your statement regarding system operation. The more free flowing the low pressure exhaust is the better pressure drop across the Hi pressure stage, increasing spool by letting the hi pressure charger work for driveability.
I can't help wondering how "free" I can make my atmosphere pair relative to my Hi Pressure unit.
Trying to quantify & visualize the hot side temperature and flow across stages, is it enough shaft horsepower to drive X compressor to X pressure ratio, in that configuration's efficiency, is fascinating. Obviously trying to tighten up the low pressure stage so it will spool right along with the hi pressure turbo is bad. Now it comes down to how loose it too loose? All I need from the low pressure stage is a pressure ratio of around 3-ish when at full load full throttle. Then if I have 3+/- out of the hi pressure unit without a bad drive pressure ratio or excessive egt, I should be good. My head isn't currently ported so boost will have to be higher than if it flowed better in order to make the same power. Thanks again for your advice and patience. If I had a dyno I'd be dangerous testing all of the changes.

 

"As loose as you can get" is where the lp turbo makes the required boost with no wastegating needed.

Just making sure you realize 3.0pr across each stage is a 9.0pr, or 120psi of boost.

 

Above is what I am now shooting for. As far as boost I am hoping to hit the amount of air "density" needed at 90-100psig, but leaving some room for inefficiency.
Thanks again ... I'll report results of testing, including full fuel testing of current configuration (which I expect to see low pressure p/r too high.

 

I would suggest some additional boost cooling if your aiming for ~100psi worth of density. The best is cooling in between the turbos, helps with drive pressure and there's more energy leaving the high pressure turbine to help power the lp turbos. But anything would help.

Right now at 100psi of boost, on a 60° day you'll have 200° inlet air temps.

 

I have 5 temperature sensors planned. 3 thermocouples are mounted, displays are not mounted or connected yet. Planing on 1.Outside Air Temp, 2. Temp at the air filters, 3. At inlet of Hi Pressure turbo, 4. At inlet of Intercooler, 5. At intake horn just before the manifold shelf.

Even though they are cheap sensors, I'm hoping to get an idea of what's happening.
I don't have external wastegates but have considered adding progressive Water/Meth and Nitrous.
Water/Meth for general cooling over a set boost or egt, and nitrous for being stupid or on the Dyno.
I've considered putting one jet of W/M between stages.