The bit about piston speed is an interesting take for sure, and is relevant im sure. Bottom line is you dont want 10 psi at 1000 rpm the stress involved would destroy the bottom end in short order. IDI's are stout, but dont forget they were designed to run at the governor under load, and without a turbo. The cylinder pressure and stress on the rotating assembly is massive just lugging an NA engine, now double that with a turbo. Have a look at tractor pullers and such, theyre not spinning 5.9s to 5k just because they make more power there, its equal parts engine preservation. If thats not enough, compare the bottom end of a modern "de-speeded" OTR diesel to one from 20 years ago. The 13L in our volvos pull as hard at 1000 rpm as they do 1600, but i seen a rod out of one, and it is MASSIVE.

 

But the question is: why is it more stress to run 10 psi at 1000 rpm than 10 psi at 2000 RPM or 10 psi at 3000 rpm? Assuming similar relative amounts of fuel. The cylinder pressures should be the same, right?

 

For the 5K 5.9's - to get 2K or 3K horsepower out of 5.9 liters takes a lot of air - way more air than turbos can provide at 2k or even 3k RPM.

For the big rig engines - I would be willing to bet that the rod out of an old Cat 1693 is bigger than the one out of your 13L. And while 1K to 1600 rpm seems like a "low" RPM operating range, take a look at old D9 cats with a MAX rpm of 1200 - or the John Deere R that governs at 1K. RPM isn't really what matters. Look at piston speed.

 

If you go compounds, I would go with something are a 57-60mm for the high pressure turbo. Justin is running a 57mm BW turbo iirc and a 75mm BW for the atmosphere turbo. For what you want I would stick around a 72mm on the big turbo. That would make a decent spooling setup.


Also a guy on the Cummins forums makes a He351ve VGT controller for a decent price using an Arduino controller. I think it is called A Banshee Controller. Could always go a compound setup with a VGT and a big turbo on top that would be cool. I've seen a few newer trucks with them and they spool like crazy! It would be harder to tune though for sure!

 

How much power are you aiming for? That's the biggest question. You have to size the turbos according to how much fuel you have for power. I have a calculator for all of that, but the power goal has a lot to do with it.

 

Its all relative to what the engine is designed for, which, like you say im sure has to do with rod / stroke ratio, and bore vs. stroke.

Macrobb, im not a physics engineer. All i know is the faster something moves, the less stress / pressure there is, be it rods, transmissions, or gearboxes. If i had to hazzard a guess, its because the piston cant "get away" from the combustion pressure. Like redneck says, piston speed. Amount of fuel doesnt control how fast it burns, just how big of boom there is. To get the most out of a given amount of fuel / air, the piston needs to be moving at given speed to accept that energy to convert it into motion efficiently.

Not sure if that makes any sense or not.

 

Pretty sure hairy is spot on in this last post. In cylinder pressures can go sky high with big boost at low rpm. In large part because of the amount of time the forces act on the piston at low rpm. The torque the engine produces is related to the force applied to the piston by combustion x time. At low rpm the time that the combustion has to act on the piston is significant like maybe 9-10ms. At 3000rpm the piston is passing outside of the 90 degree window within 3ms. The effect is multiplied at low rpm with the piston not racing away from the combustion event the combustion occurs more completely and at higher pressure. Sending combustion pressure spiking even higher.

Now I would take anything I said with a grain of salt, it's been ten years since took thermo dynamics and I haven't used it in work for five. And to boot I had my ankle reconstructed yesterday and I'm dopped up on my couch with nothing better to do.

But it all sounds right in the fog right now.

 

Well that's helpful! I wonder how we could determine the "limit" is before putting the engine at risk. Some kind of cylinder pressure vs. RPM calculator with a general idea of a safe limit.

Haha that's pretty funny. Hope the healing goes well! The good thing about your advice is that thermodynamics hasn't changed in ten years so what you know is still good.

 

After I wean myself off the opiates and can be up right for more than 30sec I might try and dig out my thermodynamics of internal combustion engines book. The problem isn't wether thermo has changed or not. It's my lack of practice and relative stoned-ness at the moment. I think estimates of cylinder pressures can be calculated but it's probably more work than is worth. And if I recall the difference between calculated and reality in this could be substantial.

Any way the best way to figure out what they can handle is probably to ask one of the Justin's. Pretty sure they've both blown apart a few idi's in the quest to eliminate the weak links.

 

Well the upside, as long as you dont have studs, the headgaskets will go before anything else.

 

typically, the vee configuration of our engines shifts potential torque UP the rev range compared to an engine built with the same number of cylinders in a straight line. Also, air flow through camshaft and valving, the porting and volume of manifolds and cylinder heads promotes torque at designed rev ranges.
Note, to modify air and fuel flow and delivery so that higher pressure mixtures are available at low rpms will require other engine mods as mentioned, these mods will probably narrow down the useful range of power at other engine speeds.
It may also effect operating temperature.

 

Bump. Any more thoughts on your setup?

 

I'm just going to interject in here that forced induction destroys traditional torque band, valve timing, porting rules for an NA engine. This is why if you look at torque curves for turbo diesel tractors or the 6.7 powerstroke you see that the torque skyrockets low in the rev band and plateaus right up to redline. Otherwise stated, you get nearly max torque from the word go, until you let up on the throttle.

On an related point, this is why the 7.3 IDI has a beautifully designed intake plenum, and very good, for a factory setup, porting. When switching to the powerstroke it became a steel plate bolted over a channel. With the only (loosely) important factor left being plenum volume. To little it'll choke it, way too much will just induce lag. Compared to a NA engine there are miles of wiggle room here.

With the powerstroke being designed from the ground up to be turbo charged the money simply didn't need to be spent on costly castings, and machine work. This is a big part of the reason that an IDIT with a leaking turbo system still chooches along pretty good. A powerstroke with a bad turbo leak is gutless, impotent, falls on its face, and burns out self out with rediculous egts.

I think if you want to do small compounds, because you want something different and fun. Do it. But be aware that it will add a lot of complexity to a beautifully simple engine. And probably won't buy the same bang for buck as small modern turbo like an S257sxe or S364.5sxe from r and d.

FWIW. My dream is to have the 257 on my 89 idi along with a moderate sized pump from Justin. And a 364.5 on my 02 7.3 with 160cc injectors. They both need more than what's mentioned to support this mods but this isn't my thread.

 

No power numbers have been mentioned, so until a goal is set, proper turbos cannot be recommended. With a number I can calculate what sizes of turbos you would need if you indeed want to go this route. As previously stated, you're going to be putting a lot of stress on the bottom end and I would recommend a main girdle in addition to head studs. The powerstroke rods may be necessary as well.

 

I've had some. Forgive me for my seeming lack of interest in my own thread... Ever since Android went 7.0(nougat or somesuch) the Garage Talk app for my phone is pooched. And apparently all other 10,000 users too. So since I'm not on the computer much I'm about a week or two between replies.

I did some independent reading on the boost vs low end torque thing. Didn't get as far as I'd have liked, but came across some good info. Lost the link of course...

The biggest thing that stuck out to me was, all things being equal:
Basically for low RPM torque, it all comes down to stroke length. This totally satisfies the suggestion of JoeF250 with piston speed, Hairy's thoughts on engine stress, as well as others helpful explanations.

Long stroke? RPM is limited due to piston speed. Too much inertia to "turn the corner" and eventually wreck stuff.
Jamming a cylinder at low RPM full of boost with a relatively short stroke and you put a ton of pressure on the piston and rod, as the lever(crank) is short and thereby doesn't get out of the way as easily as it would with a longer lever.

So I'm sort of soaking in the info and that's that. I have a growing family, full time job and a small side business too. A house that has lots of chores and projects and no fancy heated garage big enough for my pickup, so it doesn't get a lot of my time right now.

Why do people keep saying compounds are complicated? We're talking mechanical waste gates... The piping may look complex but that's about it. The wiring diagram for my wife's hairdryer is more complex than compound all-mechanical turbos.
(fabbing the whole thing certainly won't be easy, I understand that 100%)

Also any info on those VGT turbos mentioned earlier? I googled a bunch and came up with nothing on the one Hairy mentioned, the S258sxe.
I was able to find a bunch of info on the He351ve, as it's got quite the popular following it seems(no maps though). Found 3 different stand alone controllers too, though "stand alone" seems to include 3-6 different sensors, I'd have to dig deeper into that to see how much it would take to satisfy the minimum input senario.
It would surely be an easy way to go, assuming you get a healthy turbo and good controller and whatnot.

Another thought I had tonight after re-reading most of the thread is:
Couldn't we take Justin's engine failure numbers and do the math backwards?
I can't remember his numbers, but let's say it was 45psi @2800rpm. Let's say a safe limit for any of us would be what, 15% lower boost than that? (on the exact same setup).
45-15%=6.75. let's call it 7psi less boost for a safe limit.

Which leaves:
38psi @2800rpm safe limit. To take this number and try to equate it to 1000rpm, can't we divide 38psi by 2.8 to get a ROUGH safe limit at 1000rpm?

38/2.8=13.57psi @1000rpm(with all of Justin's mods, of course)

Now I'm actually quite bad at math. Please forgive me and correct me if need be. I think in percentages, which I'm ok at. Not so much with the hard numbers.
The reason I suggest the math this way is because of the piston speed theory: piston speed is perfectly mechanically matched to RPM. So safe max BOOSTxRPM should be a roughly static number, eh?
I know there are volumes of books about this stuff, I'm just trying to suggest a basic way of learning a safe low-RPM boost level on a given platform. And before when I threw out 10psi @1000 rpm as a goal it was just a number...who knew it would get so much traction?

Oh, as for my projected max HP/TQ, I don't have any specifics as of yet. I was more going for low-mid range power with available top end for fun or when I need it. I've never even been on a dyno...yet!

Thanks for watching and let the comments commence... and thank you guys for keeping this forum constructive. That's the main reason I love FTE for information; the general lack of BS and arguments, etc.
-Joshua