I'm building a Duramax and am targeting around the 1,500hp mark. I've built compound turbo setups in the past (by following what I've read 'worked well') but I'm wanting to take a little more of an engineering approach on this one. I'm wondering if someone can help me with the math and ratios to figuring out the best compound setup to run? I'm thinking about something like a GTX4202 and a GTX5544, but this is again - by reading what others have done. Is there a more scientific approach that will help me find two better matched chargers? This setup is externally gated and inter-stage intercooled.

Thanks in advance!

 

I can't help with any of the science,way above my head,but here's a vid I found very interesting:

 

I just went to school for 30 minutes. Great video.

 

I like the way he explains it all. I was researching compounds for a diesel project last year, and was having a tough time grasping some of it, but that was painless.

 

Great video and thanks for the post. Anyone else have any thoughts about a good turbo combo or how to figure out a good pairing? Also I'm wondering how to determine the best A/R to run? Anyone have any experience? For example I had a dodge that I ran compounds on and the larger turbo was a 75/96 with a 1.32 A/R - towing it was a little laggy, but the drive pressure was pretty low. About 102psi of drive pressure and it would make about 96 psi of boost. I switched to a 1.1 A/R and towing was much nicer with it. Truck lit damn hard then. I dont want to have to buy several turbos and a bunch of housings and test them all though - anyone have an idea on how to figure out what housings to start with?

 

That is an excellent video, it wasn’t that long ago that I thought you ran a small turbo into the big one!
I suggest asking this question in the Advanced Tech Section. The moderator is a retired engineer who has helped develop oe turbo systems and a lot more. He does not post except in that section, he should have your answers.

 

Will do thank you!

 

A few pointers on how to start calculating this.
1. Aim to run the same pressure ratio in each stage. eg if you want a 9:1 PR overall (130.5 psi MAP, 116 psi boost) you should aim for sqrt 9 = 3.0 PR in each stage.
2. Start at the engine and work back. If your engine flows 500 cfm at peak power NA, the volumetric flow into the 2nd stage compressor will be 3.0 x 500 = 1500 cfm (assuming you have a 100% efficient intercooler between the compressor and the engine). Similarly the 1st stage compressor will be flowing 3.0 x 1500 = 4500 cfm, again assuming you have 100% efficient intercooling between stage 1 & 2.
3. You need to use compressor maps that show volume flow not mass flow on the horizontal axis. Maps showing mass flow assume atmospheric air at the intake which is not the case for the 2nd stage compressor. You can convert the mass flow scale to volume flow yourself.
4. Intercooling will never be 100% effective as assumed in 2. above. The correction for this is the ratio of absolute temperatures. So if ambient temp is 27*C (300*K) and you estimate your intercooler is only going to get the charge temp down to 37*C (310*K), the final stage airflow will be 3.0 x 500 x 300/310 = 1451 cfm. (Do you like the mixed units - metric temp and imperial flow?)
5. Apart from all the above, choosing each compressor is the same as usual. Try to find a map that puts your peak-power operating point slightly to the right of the efficiency peak. The 2nd stage turbo is easy - it will be one that works well on this engine by itself at the relevant PR (3.0 in this case).
6. The video talks about running 3 wastegates. In theory you can run just 1, gating from the exhaust header. This assumes you can get the right turbine housing for the primary turbocharger to balance the two stages. Its not the end of the earth if the two PRs are not identical as long as both compressors are close to peak efficiency.

 

The best way is to find someone who has run at the levels you want and replicate their combo. Why fail when you can just cheat . But in all seriousness, there are sponsored trucks with huge cash backing them up, they have done real world research and spent time at the track and dyno. There is no reason to spend money figuring out what they already have. Some of them are secret squirrel hush-hush but as often as not car guys are cool and love to talk about the stuff they've built.

 

https://www.yellowbullet.com/threads/compound-turbo-charging.216811/

 

Just use °Rankine, then you can keep the chem professor off your back

 

Working on a very rough estimate of 1500 cfm to reach your 1500 hp goal, you only need about 5:1 PR (60 psig boost) ie 2.03 PR per stage at 3200 rpm. Both turbochargers you mention are on the large side for that operating point.

 

Great lesson on the compound turbo setup but I really like what he says at the end about making the engine survive 3000hp. Many people seem to overlook that. Rods or other parts are frequently given a hp rating for durability purposes. In reality most parts need a torque rating and an RPM rating as the loads are usually different. Especially critical with a torquey turbo vs high RPM N/A even if peak hp rating is the same.

 

GTX4202 paired with a GTX5544r is good for up to 2000hp. On our 2300hp pull with the Duramax we ran a 480/87/1.0 T4 and an S594SX-E. And with Shredder we run Triple 106mm GTX5544R Gen II’s.

But to answer your question, the primary charger should be close to double the compressor flow of your secondary charger (manifold turbo). And of course turbine housing sizing plays a big part.

 

Can you share any information on boost pressure and inter-stage pressure?
"Double the flow" would be right for a primary compressor running 2:1 PR and good charge cooling. If the charge is only cooling to say 50*C a PR of 2.2:1 would indicate a requirement for 2:1 compressor sizing.

 

Good questions. I should specify these are a “goal” scenarios, and real world scenarios often aren’t the case. But if I suggest double the flow of the secondary it tends to help guys understand the sizing spread needed to compressor maps happy.

Having a gate on the secondary (manifold charger) charger is huge. “Usually” you’ll want to see HALF the boost pressure from the secondary as you would have for the secondary as a single. (I’ll use diesels as an example)

Lets say a guy has a Common Rail cummins running a 62/65/14 as a single and it sees 45psi with the wastegate set at 40lbs of crack pressure. Generally we’ve seen up to 100hp gains buy dropping the gate crack pressure to 25lbs so the secondary sees 25psi and the primary ends up supplying the rest. As far as cooling, the cooler is going to have to be sized around the turbos and the turbos not sized around the cooler. If you’re understanding enough to calculate the Pressure Ratio for both chargers, I think you’ll be juuuuuust fine picking chargers. But, compressor wheel profile is the up most biggest factor in creating heat.... if you have a comp wheel (factory drop in turbo) that’s designed to draw its own air, tends to create turbulence once having pressure applied to the face of it. There are most definitely comp wheel profiles for compounds.


We NEVER see ratios down in the 2.2:1 range. On diesels they continually run in the 4.4:1-5.8:1 range... main reason they run such large charge air coolers.

 

Yeah so I am suggesting double the flow would be unusual. Essentially the
size ratio of the turbos should equal the density ratio (=PR if intercooling to ambient) of the primary turbo eg primary DR = 3 means the secondary turbo should be 1/3 the flow of the primary.

On control. When you run 2 WGs - primary and secondary, do you exhaust the primary gate to atm or into the secondary turbine inlet?

 

Hi I am busy with a compound setup. Toyota vigo 3.0l d4d. I am using the stock turbo vnt with a 4mm bigger billet wheel. My boost is 1.6bar manifold pressure. Exhaust pressure is 44psi. My secondary turbo is a hx30. That only boost 0.7bar.
What would the best sizing be. I gated between turbos. And thinking of gating on exhaust manifold to drop exhaust manifold pressure. What must the boost be on secondary turbo? Also 1.6bar. Really need some advise. On sizes. No one wants to help.

 

Your units are a bit confusing. It would help if you provided all 4 pressure readings and using the same units - preferably Bar absolute. eg
Compressor 1 discharge - 1.7 Bar
Compressor 2 discharge - 2.6 Bar
Tubine 2 inlet - 4.0 Bar
Turbine 1 inlet - ??

The pressure readings are given in the same order as air flows through the engine.
(I have assumed the 3 numbers you gave are gauge pressure and converted them to absolute)