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I don't know where it belongs in this beast of a thread but I did some turbocharging experiments on diesels when I worked for Diesel Power Magazine and found that boost for boost (in my case 60psi single vs 60psi compounded) a Single made about 30hp more. Why? Can't get a solid answer, but my guess is just the physics of having to drive two turbine wheels verses one. Once we pumped the compounds up to 75psi though, they made more. On a single that would be boom. Out of all the setups I've seen I like a blower setup with a bypass and a big turbo the most. Banks had a setup like that on their diesel boat, and they could get 800hp at 5000rpm and 800lb-ft starting at 1,200rpm! Anyways, neat thread.
 

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Your question is unclear.. what exactly are you asking?

I built a compound setup as my factory turbos have issues holding over 15-17psi by redline, now i can run 30+ without an issue.
What I was getting at was that compounding seems to come in where the large turbo cannot be spooled or spooled in a timely manner. I was curious why do it if your spool time actually increased. But I see you were having a flow issue.
 

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What I was getting at was that compounding seems to come in where the large turbo cannot be spooled or spooled in a timely manner. I was curious why do it if your spool time actually increased. But I see you were having a flow issue.
Ahh right. Yes, at the time it was cheaper to add a 3rd turbo than to buy an upgraded turbo kit for my car as they were about $4000+aud. The setup works however i am still at a loss as to why my factory turbos are so much laggier now. Previously i had 17psi by 3300RPM and now its closer to 9psi by 3300. The up side is, as mentioned the fact i can now run double the boost i could before.
 

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60psi or even 75psi is no longer the limit with single stage in either a single or twins if you use some of the new ProMod Gen2 turbos. They are still pretty efficient at 6bar+ and I have heard of guys running north of 90psi on both si gle and twins. Go to the Garrett site and look at a GTX-5533R or GTX-5544R or G57-3000 106mm map and check out the pressure ratios they can operate in. The Precision XPR turbos are similar as well as the new Hart offerings. I know this is predominantly an import oriented thread but some of the smaller turbos in the family could apply. Plus some of the lessons learned in ProMod and Radial racing has trickled down to some of the smaller models too.
 

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Discussion Starter · #1,667 ·
The main problem this solves for me is being able to spool an 80mm turbo on a 122 inch motor that otherwise can't spool anything bigger than 62mm without nitrous. I'm at full boost 0.5 seconds off the transbrake (45-60 psi depending on what I'm doing). I've not run more than 65 psi, but it doesn't matter what boost I'm running if it doesn't spool until 1000 feet.

I tested my previous compound turbo setup with and without the LP turbo installed and there was no change in spool time on the small turbo, so perhaps being rear mounted is indeed the difference.

I've run the same ET on the big turbo as a single, and as being compounded with a 61mm. Though I can't say if I'd notice a 30 hp difference or not. I lose many times that in my converter anyway, more tiny motor problems I haven't sorted out yet. :D

I did a little no prep testing today, still on WG spring.

 
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I thought that was one of the advantages of compounding? What is the advantage of compounding for your application?
Like Kevin said, in the import world compounding is used to spool turbos that have no righteous reason being in a motor that small. You spool faster overall than you would with just the big single, but you do lose spool vs just a single small turbo.

Why? Can't get a solid answer, but my guess is just the physics of having to drive two turbine wheels verses one.
This is exactly the reason why. You lose overall system efficiency vs a big single for the same power. Both in drive energy exerted on the turbines, as well as charge temperature and exhaust energy lost to radiated heat. Hp for hp a big single always wins. But only if you can spool up the big single. It's pretty boring waiting for a 72mm to spool on a 2l i4, even if you're making 1k hp for 600rpm before redlining.
 

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Discussion Starter · #1,669 ·
In my case, on both cars, there is no difference in spool on the small turbo, with or without the big turbo. But in both cases they are just inches away from each other.
 

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This is exactly the reason why. You lose overall system efficiency vs a big single for the same power. Both in drive energy exerted on the turbines, as well as charge temperature and exhaust energy lost to radiated heat. Hp for hp a big single always wins. But only if you can spool up the big single. It's pretty boring waiting for a 72mm to spool on a 2l i4, even if you're making 1k hp for 600rpm before redlining.
Oddly enough, the intake charge temps pre intercooler were actually about 80 degrees cooler with the compounds at 60psi....makes me think 60 for the single was pretty far out of its map! Intercoolers are so crazy these days (and big on diesels) though that the 120 deg temp probe never registered even under 12 second loaded pulls.
 

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60psi or even 75psi is no longer the limit with single stage in either a single or twins if you use some of the new ProMod Gen2 turbos. They are still pretty efficient at 6bar+ and I have heard of guys running north of 90psi on both si gle and twins. Go to the Garrett site and look at a GTX-5533R or GTX-5544R or G57-3000 106mm map and check out the pressure ratios they can operate in. The Precision XPR turbos are similar as well as the new Hart offerings. I know this is predominantly an import oriented thread but some of the smaller turbos in the family could apply. Plus some of the lessons learned in ProMod and Radial racing has trickled down to some of the smaller models too.
I dig what you're saying but you can still control and pedal and manage horsepower on a compound setup way easier. Once the big single falls off especially on a small cid engine you're pretty much over.
 

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Discussion Starter · #1,673 ·
Just when I get the compound turbos back on the car, I go back to the 16g record chasing crap. :D This is a 48mm turbo that came stock on early 90s Mitsubishi EVO3s. All passes were done no prep but the money pass was done PYOP. First day out I more or less tied my previous record of 10.04 at 137. Second day out it finally ran the first 9 second pass with a 9.89 on the dragy. I turned on the timing system for the next one, which was a bit slower but got the job done. 10.00 on the dragy and 9.92 on the track. I know it'll go mid 9.80s or perhaps even 9.70s on a perfect pass at this weight but I'm not sure I'll bother chasing it. I can swap turbo setups in a couple hours so it's easy to go back if I need to. I still really want to compound three of these 16Gs and try to make 1000 hp with them.

Video for anyone interested:

 

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I noticed a record breaking car using the GT55/33 as LP and GT4202 as HP,just wondering are the surge slots OK? or are they better filled with a compound set up.
Thought I would ask here.
Thanks
 

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Discussion Starter · #1,675 ·
I see no problem with the slots, they should function as they always would, just in some artificial mineshaft conditions. All of that stuff is volumetric.
 
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