Ok guys after reading a bunch about compound turbo setups and some other factors I thinking about (someday) running a mild compound setup for DD/towing.

My reasons for compounds:
1. Lightning fast spool time to utilize the available fuel at low RPMs for fuel economy (think highway cruising in double-OD)
2. Lightning fast spool time for performance
3. Superior mid to late RPM boost and performance for everything else: towing, DD, fun factor, clean exhaust, etc.

Now my biggest purpose for compounds would be the spool time on the smaller turbo. Even the "fast" spooling turbos on these big engines run about 8 psi at 1800rpm from what I've read. That's wide open best case.
Im thinking lets find one that puts down 10psi at 1000 rpm. Maybe I'm crazy but that's just a rough goal I had.

This is basically for me the "ultimate" IDI setup.
Now you can gear it out or run a nice GVOD or big tires or all 3!
Part of my dream is that I'm currently living about 19 miles from work, 16 of which is a 70mph freeway. I drive 75mph every day and get about 13.5 mpg. Maybe 14. Cause I'm screaming at 2800rpm the whole time!
What if I could do 75 at 1500rpm with 10-12 psi of boost? Sounds pretty sexy to me.

Anyways this is only my vision and mainly theoretical.
I can cut and weld exhaust pipe. It's not easy, but a fun challenge.
The two things I'm not sure about are:
1. What turbos. A waste gate is a MUST for the first turbo as it will be WAY to small for this kind of volume. The second should be something that isn't just getting into its map at 2600rpm. I'm not looking for a top end setup, but rather something low to mid oriented. The nice thing with compounds is it shouldn't be hard to have it all! Low, mid, and top end. Probably something like an R&D stage 2.
2. How to plumb the oil supply/return. Supply is a little easier. But how would you do two returns? And how to fab/mount a second turbo. Do you use stock pedestals and fab from there or what?

This setup would be with studs and cam etc.
Any ideas? Thanks guys

 

10 psi at 1000 RPM sounds like a lot of stress on the engine. Maybe with studs and wider PSD rod bearings it's not an issue? I'm not experienced with turbo builds but I imagine you would run an external wastegate around the first turbo exiting before the second turbo so the turbo didn't become a restriction? For the drain, you just tap into the intake valley pan, oil pan, or possibly a fuel pump block off plate or the extra drain in teh block to the pan (dipstick provision for vans?).

It sounds like your truck is a daily driver. If that is the case you are probably better off with a good all around setup in the truck to meet your needs rather than adding a bunch of extra complexity. 75 MPH at 2800 is killing your economy. If you want economy you need to add another overdrive or change gearing. If your truck doesn't perform how you want it to with 3.55:1 gears you need more torque, so add power until the problem is resolved rather than swap to 4.10:1 and sacrifice economy on a truck you daily drive or pound out highway miles in.

In any event both my trucks are 3.55:1 gears and ZF5. The IDI doesn't quite have the torque of the 460 but I think it could pull a double overdrive at 70 MPH on level ground if not towing. Currently both the IDI and 460 turn about 2000 RPM at 70 MPH. I think the IDI could stand to drop 400-500 RPM and still perform adequately as that's where it starts making boost. It's a 6.9, so I also wonder if going to a 7.3 would help appreciably with down low torque from more displacement to bridge the gap before it makes boost. And of course there are far better turbos to run than the wee little Banks on mine. The 460 could probably stand to drop 600-700 RPM and still pull a load. The extra displacement probably helps a lot down low as the 460 doesn't lug or feel down on power at 900-1000 RPM as the 6.9 does.

So compound turbos are a cool idea but I would first focus on identifying your goals. Is it better economy? Is it more power across the board? What turbo are you running now? I'd imagine R&D has something to offer which would meet your goals.

 

With the cost of variable geometry turbos coming down, it almost doesn't make sense to run compounds now. I've been playing around with ways to make a Ford 6.0 turbo mechanically controlled rather than electronic over oil, although someone with stronger electronics skills could probably build a digital controller.
The one that Cummins uses on the 6.7 is easily adapted to mechanical control as well.
As a bonus these turbos also become exhaust brakes with some very minor wiring.

 

Yea VGT would be the way to go with what youre wanting, 6.0psd or 6.7ctd... but theyre a bit small for a 7.3 with any real fuel. That said, you can always drop a wheel in it. An S258sxe will spool "lightning fast" , and still flow enough on paper for a 110cc pump.

 

My 2c is... for highway cruising in double overdrive, you don't *need* a compound. Or even a special turbo, for that matter.

I've run my tan '88 for years with a GVOD, stock tires, 4.10 gearing and a zf 5-speed.
This gets me approximately 60 MPH at 1600. 75 MPH at 2000.
Before I installed a turbo, it would do 60 in double OD just fine. 75 was pushing it, especially any hills. Of course, that may have also been tuning - that was a long time ago and I've learned a lot since then.

Once I installed a Banks turbo, going 75 at 2K RPM isn't just doable, it practically /sings/. Feels great, and I've hauled probably a good 4K doing that without issue.

Now, sure, with the Banks setup, I'm not making a huge amount of PSI at 2000 RPM... but unless you are towing, you don't /need/ it. It's good to have the extra headroom, but that extra headroom really requires a much bigger pump and turbo to handle it.

If you are going to be doing an all-out heavy duty build, get yourself a S360 turbo, and put a .63 T3 exhaust housing on it.
The .63 makes it spool /really/ nicely, and it pushes a /lot/ of air easily due to the massive wheel size compared to a Banks turbo.
To run something like this effectively, you're looking at a 150CC pump. It cleaned up my 110CC pump without breaking a sweat.
Of course, due to the pressure involved with this, you are looking at:
1. Headstuds
2. Better valve springs
3. Strong turbo boots(I hit 35 PSI with 110 CC of fuel, I'm guessing 150 would make in the 45 range like Justin got).

Now, with a compound setup instead of this, perhaps you could run even lower RPMs... but honestly, this engine was meant to be running at 2K. It likes it there. 1600 RPM is 'peak torque' on a stock engine, and there is warnings not to run at full throttle below that, probably due to the extra forces involved.

Fuel economy wise, 2K with a turbo is just fine. I've hit 18 MPG on long highway trips(300 miles) going 75 in my truck before. Obviously, hauling a trailer or camper will be less...

 

Well that's pretty sick I forgot about those. Any real success stories of one on an IDI? I know of one guy running one but it's not really variable currently.

Thanks for the thoughts, I'll look into this more. I just Googled the S258sxe and literally nothing comes up. Even just S258 has very little info. Where can I go to get some specs/maps?

On a side note we just got a 2017 260HP Cummins 6.7 powered truck in our small fleet of shred trucks. So at least there's that, lol. It's nothing special to be sure, we have a 260 HP Mercedes powered truck that carries the same weight just as fast with 260,000 hard miles on it and tons of hours.
But hey at least I'll know what the turbo looks like in person a little bit haha.

 

Oh I agree with all of that, but a guy can dream can't he?

None of this is a *need* situation, just cool stuff and ultimate setup. The complexity of compounds isn't part of the equation either, just having fun with the idea and thinking out loud to a bunch of guys that know more than me about turbos.

I've *heard* also that dropping too low of RPMs with boost is bad but I have never seen hard numbers. We drive factory medium trucks all day with auto transmissions that do their best to run at 1400-1600 rpm whenever possible and Mercedes/Cummins seems to think this is just fine. There's even a mode button on the Allison AT control board that is a fuel saving mode and keeps the engine below 2000 rpm no matter the throttle position.
Maybe V8's are different?? I really don't know. If so, why? The International bus version of our IDI was governed at 2800rpm just like the class B trucks work, which suggests quite a bit of low RPM driving.

What is it that hurts the engine to boost it at low RPM?

 

Does it? I'm pretty sure it was expected that you'd be running it at governed RPM(or slightly lower) all day long. I've seen a couple of school busses from that era, and the setup was that it was geared to go about 65 MPH foot-to-the-floor with the engine at governed RPM.

I also have heard plenty of stories of IDIs running across the country and back going 80-ish at governed RPM all day long. For multiple hundred thousand miles. The efficiency certainly drops when you do that(NA at least), but the engine doesn't seem to care about the revs.

I'm pretty sure it depends on the engine size as to what the optimal RPM to run at.
Big ship engines run in the 500 or /less/ RPM range. But they also have cylinders that are big enough to stand in.

With a big straight-6 engine, yeah, the rev range is lower. Redline is lower, too.

On ours, one thing to be aware of is oil pressure. I'm not sure quite how this will actually affect the bearings, but often you'll only get 10-11 PSI at hot idle, but up to 35-40 PSI at 2k RPM. While our engines do run on a low-pressure-high-volume oil setup, I'm thinking that the higher oil pressure ought to help the "oil bearing" effect, especially when running at high levels of boost.

Beyond that, I'm not sure. I just know that V8s tend to handle higher revs better than other engines(due to crankshaft stresses?)

I just googled trying to find out more information: https://www.google.com/search?q=dies...utf-8&oe=utf-8
There's a lot of info out there, but what's valid for us?

It's a pretty well known fact that running a diesel engine at maximum load for very long will definitely shorten the life - boat engines are the best place to look for this, as they will be running at constant power/torque levels for hours on end.

Of course, this is probably more due to running relatively rich / high EGTs than anything else.

This is the same reason you don't generally want to run the engine too low - it's too easy to max out the power available and get the EGTs elevated. But if you are pushing a lot of boost in there, and therefor have a lot of 'headroom' HP wise?
Maby it'll work.


edit:
Here's an interesting link: http://www.truckinginfo.com/article/...l-economy.aspx
There's a bunch of manufacturer RPM ratings for various engines.
Lets take cummins for instance:
The ISX being a 13/15 liter engine, ISM being 10.8l, ISC being 8.3l, and isb being 5.9-6.7l
So, looking at those numbers... Bigger engine = lower RPM. Smaller engine = higher RPM.
Of course, the larger engines also have a lower power-to-size ratio, so that may be part of it. I'm guessing that lower power-to-size ratio = longer life, and lower RPM is probably a more efficient burn, if you can get everything else to handle it.

However, that page also talks about driveshaft and engine vibrations... which is probably true. I do know that my IDIs smooth out more at 1600 RPM, and run really smooth at 2000. It's probably an effect of designing the engine for a given RPM range(in an IDI, I'm guessing optimal power band being 2000-2800), and then all the balancing components are engineered to handle that range the best.

 

I think it has more to do with piston speed than RPM...

 

Interesting idea, lets see how it works out:
Cummins ISX, 15 liter. Stroke: 6.65", optimal RPM: 1450. Piston speed: 1607
Cummins ISB, 5.9l. Stroke: 4.72" Optimal RPM: 2250 Piston speed: 1770

IDI, 7.3l. Stroke: 4.18". @1607 piston speed, RPM is 2310
@1770 piston speed, RPM is 2540.

That... Makes one heck of a lot of sense, actually.
Here's the calculator I used: Piston Speed Calculator - Universal Entry

BTW, the ISX15 is governed at 2000 RPM. Piston speed: 2217
For an IDI, that would be 3180 RPM. That also... sounds right.

Also, peak torque on an ISX is 1200 RPM, 1330 piston speed.
for an IDI that would be 1940 RPM.

That's... also interesting. I suppose the actual peak torque numbers really depend on the turbo sizing and fueling - with a Banks turbo, it was more like 2800 RPM for peak torque, just due to the spooling characteristics.

 

More fuel at a lower RPM increases the EGT, and can also increase the effects of it as the hot gasses are in the cylinder for longer. This translates into higher load on the cooling system, more chance for damage to the exhaust valves, etc.

 

Also, the less boost you have while cruising, the better your mileage will be.

But yes, a small compound system can give you tons of low end. My 4BT will build about 8 psi of boost at 1,000 rpm. I have never seen a single turboed 6BT that will even begin to compare to the low end of my 4BT. As for the top end, it will go from 100 to 120 like my hopped up 416 goes from 50 to 70.

 

That's the point of the small turbo though. It will start building boost right away which will help keep the EGT's in check.

 

This is turning into a pretty good read already! Thanks for the thoughts guys. I read the article Rob posted and it's interesting for sure. Nor sure I understand why there would be "more torsional load" on the u-joints at this RPM vs that one. Pretty sure it's about ft-lbs and wheel speed. The U-joints don't know you've shifted...they are traveling the same speed if you're doing 45mph in 2nd or in 5th.

Does your 4BT have compounds then? That's sweet, 8psi at 1000rpm and fast up top!
I was thinking the same thing about the small turbo clearing up the extra fuel down low and keeping the EGTs in check. That's one of the reasons for I was leaning that direction.

 

Basically, if you take your current truck and go from 4.10 to, say a 3.55 rear end, the driveshaft will be spinning slower for every revolution of the wheels. This means that for the same power level, it requires more torque through that shaft and the U-joints.

Changing the rear end gearing also lets you run at lower RPM at the same speed, for better fuel economy.