Perhaps a bit different turbo question?

[Notorious C4]

Okay, here's the deal. I have a turbocharger from an early '90s 5.9 L. Cummins diesel engine that was in a medium-duty truck. My car is a '95 Corvette with a 388 stroker small block LT1, all forged but a street build and daily driver. Current naturally aspirated numbers are about 400 HP and 445 lb-ft of torque at the rear wheels. My questions about adding this turbo are this. Since the Cummins' maximum RPM is roughly 3000~ and my car makes maximum HP at roughly double that, would my engine spin this turbo too fast? Due to limited underhood space, I'd be mounting it in the rear of the car, behind the differential where the spare tire used to reside. I'm totally capable of fabricating the piping, oil lines and such so that's not a problem. And while I'm not overly concerned with a slight amount of lag, would an air cooler of some type be advisable?
Just as a bit of background, I've worked on turbo engines for years, but only in stock applications and mostly on diesel engines. I'm also a life-long hotrodder, build my own engines, etc. Thanks for any insight that may be forthcoming.

[KiwiBacon]

There are a couple of things to be wary of.

It sounds like you've got a few good ideas already (overspeed and all that). The way to know for sure is to identify your cummins turbo. It's probably a Holsett, sounds like earlier ones used the H1C series, later used HY or HX or whatever they're called.
Once you've found the relevant compressor map, plot out your maximum engine airflow on it and see if it fits.
If the max airflow and boost are still on the map then you won't overspeed the turbo.

Diesel turbos typically have a smaller exhaust housing than the equivalent airflow petrol engine turbo. This is because diesels put out colder exhaust. This will make the turbo spool up earlier than usual, but it will also be a restriction at higher rpm and flow. I know you can get a range of exhaust housings for those turbos, but I don't konw the details. Worth looking into.

If you need help calculating airflow, just ask. You need to know engine size, rpm, any intercooling and intended boost.

[Notorious C4]

Kiwi, thanks for the response and it verifies that I've found the right forum for my admittedly unusual quest for information. I'm going to go dig the turbo out in a little while, but I think it's a Holset too. Especially since that's what I've mainly seen on Cummins engines. I know the tag's still on it so hopefully it'll have some relevant numbers. Here's some preliminary info on the engine. The heads are ported factory LT1 castings with 2.02-1.6 valves. At my maximum cam lift of .525" they flow about 275 CFM on the intake side. I would have to find my sheet for exhaust flow specs, I can't remember the number. Here are the cam specs FWIW. It's certainly not a forced induction cam by any means and I would be more than willing to swap it to something more suitable. Just listing it as a point of reference.
218/227 int/exh @ .050" duration
0.525" / 0.525" int/exh lift w/ 1.6 rockers
112 LSA
TB is 52mm dual butterfly, or 4 mm larger than stock. The intake manifold is also ported, it has long tube headers w/ 1 5/8" primaries. I also have a set of 1 3/4" primary long tubes on hand. Bore is 4.030 and stroke is 3.80" w/ 6" rods. 11:1 SCR (I know, I know) with a .041" quench area. I know I'd need to run a low boost given the SCR but I'd prefer that to swapping pistons at this point. It's an extremely efficient combination that still nets 15/26 MPG w/ 3.07 gears and a 3000 10" converter w/ lock-up. Bottom end is all forged and reasonably light weight. Pistons are 400g. with 130g. pins and the H-beam rods are 445g. Crank is lightened to 46lbs and is a fully neutral balance design. It also has a 6.75" aluminum housing ATI Super Damper. Durability-wise it's easily good to 7000 RPM but peak power is currently right around 6000 and that's close to where I'd like to stay. I never have and have no intention of using nitrous. It currently has 30# injectors and I assume I'll have to go larger again. The cooling system has been well upgraded and an oil cooler added, as well as a 7 qt. road racing oil pan.
Although I haven't discussed this with my dyno-tuner yet, I know he's perfectly capable of making any necessary adjustments. He specializes in forced induction cars and has even tuned a blown Bonneville record holder.
I appreciate your offer to calculate airflow. I'll leave any recommendations for intercooling to your discretion. If I'm totally just be gentle.
Thanks again.

[KiwiBacon]

I hate to break it to you, but all of the head work you've done to help the engine breathe easier, is pretty much redundant with a turbo. But it should still work okay.
The port matching is still well worth the trouble. Your bigger oil volume and oil cooler are excellent upgrades.

So your displacement is 6354cc?
Max rpm is 7000

With 8psi boost and 50% intercooling (you'll need an intercooler with that compression) you're talking a max flow of 82 lb/min. Some people size turbo compressors by hp. 10hp worth of airflow is approx 1lb/min.

Taking VE into account, you need a turbo capable of approx 800hp at 8psi, that's a pressure ratio of roughly 0.5 when you're reading compressor maps.

[Notorious C4]

Here are some numbers from the tag.
Assembly# 3534923
Model# HX35W
The only other numbers are the serial# and a customer#. I would think that cust.# refers to either Cummins or Ford, which is the type of truck this engine came in. I did take some measurements that are as follows. The air intake has an ID of 3.5", the outlet an ID of 2" and the exhaust outlet has an ID of about 2 19/32". Would the exhaust inlet port's dimensions be helpful? I didn't think of that earlier when taking the other measurements. It's starting to seem a bit small but I don't know. It does have a wastegate, I wasn't sure if it did or not. This would make maximum boost pressure easier to control, wouldn't it? I'm beginning to wonder if this might be more well suited to a much milder 355 LT1, such as the one I'm using in a street rod project. It will be freshened up anyway so I could start with a more suitable compression ratio. Also the heads for it are pretty much stock except for port matching and a little pocket work. Valves are also the stock sizes, 1.94" and 1.5". Damn, I've got to use this on something! It's nearly new and was free! My only other option is a 2.4 Nissan Altima. Although it could use the help, not serious.....

[KiwiBacon]

The compressor map I have for an HX35 is very basic, but does show it's a high pressure turbo (happy providing 40psi boost) and can only flow 0.25 kg/second of air at 8psi boost.

That's only enough for your engine at around 3000rpm.
It'd be best on an engine of around 3L displacement. Not much good for your current application.

[KiwiBacon]

I should add.

Two of these turbos would do the job.

[Notorious C4]

Your obvious knowledge on the subject and the time you've taken is much appreciated. I guess I'll stick it on eBay for a tuner to snatch up. Thanks.

[KiwiBacon]

Happy to help.

[UncleBob]

little late but kiwi is exactly right. You need two HX35's.....at least

Nothing wrong with head work at all! Keep in mind, the more power an engine made naturally asperated, the more a turbo will add to it. The only stipulation to that is very high compression is bad, and very big cams are bad (unless the turbo you are putting on is VERY large and has very low backpressure, then a huge cam is no prob)