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You don't see them much on gas engines, because they are not necessary at the boost levels most people run with gas.

The main purpose of a compound setup is to run very high boost pressure. The second stage of a compound system doubles or triples the already compressed air it gets from the first stage.

Most any properly sized turbo can make 30+PSI and still be in the sweet spot of the compressor map. IMO unless you require 50-150+ PSI on your engine, there would be no advantage to go compound other than possibly a quicker spool-up time and that would be better addressed with a sequential type system.
 

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They don't usually need such high pressure ratios. Search here there are a few people doing it, but they seem to be small engines.
 

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That makes sense. I don't know much about turbos on gas engines. I never thought bout boost levels not being in the 30's at a minimum like the diesels i mess with. I was guessing it had to do with over-speeding the secondary or something because of the high rpm's the gassers turn.
 

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The intake ports on gas engines don't suck like they do on a diesel. You can get way more mass in without heating it to hell like temps running 50+ psi. Low pressure with high mass flow is the most efficient path to power. If the intake ports flowed 300cfm on a dodge cummins, you could make the same power at way lower boost pressures.
 

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I was curious why compound chargers arent common on gas engines.
what's wrong with you guys? just answer the question.you can't run compound charger on gas,because gas will ignite under too much pressure.gas will not wait for a spark.it will ignite on it's on under pressure.
 

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what's wrong with you guys? just answer the question.you can't run compound charger on gas,because gas will ignite under too much pressure.gas will not wait for a spark.it will ignite on it's on under pressure.
Hey guess what genius, diesel does to.
 

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what's wrong with you guys? just answer the question.you can't run compound charger on gas,because gas will ignite under too much pressure.gas will not wait for a spark.it will ignite on it's on under pressure.
WOW. You really should know what you're talking about before saying something can't be done. It is not widespread but there are guys out there doing it and making good power on GAS.
 

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WOW. You really should know what you're talking about before saying something can't be done. It is not widespread but there are guys out there doing it and making good power on GAS.
my bad I shouldn't have said can't.but whats the point of over kill if you can't use all of the boost you are making.and my point was:this guy asked a simple question,and you guys came back with so much tech shit.when a simple answer would have surfice
 

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my bad I shouldn't have said can't.but whats the point of over kill if you can't use all of the boost you are making.and my point was:this guy asked a simple question,and you guys came back with so much tech shit.when a simple answer would have surfice
The point on smaller engines is to to be able to make big power out of a large turbo without it lazy. You basically get the spool characteristics of the smaller turbo while making the power of the larger one.
 

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my bad I shouldn't have said can't.but whats the point of over kill if you can't use all of the boost you are making.and my point was:this guy asked a simple question,and you guys came back with so much tech shit.when a simple answer would have surfice
I think you have grossly over-simplified things.
 

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The intake ports on gas engines don't suck like they do on a diesel. You can get way more mass in without heating it to hell like temps running 50+ psi. Low pressure with high mass flow is the most efficient path to power. If the intake ports flowed 300cfm on a dodge cummins, you could make the same power at way lower boost pressures.

X10!!

Also, now that there is such a thing as gasoline direct injection, I suppose you could work the boost up, but like you said, why.

Sequential low pressure units would seem to be a better idea for a gasser.
 

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I'm running a compound turbo 4 cylinder in the 8s. It seems to be working fine. :)

http://www.yellowbullet.com/forum/showthread.php?t=216811

The intake ports on gas engines don't suck like they do on a diesel. You can get way more mass in without heating it to hell like temps running 50+ psi.
There's also nothing wrong with 50+ psi when trying to make good power from 122 inches. ;)

The short version is that compound turbos on gasoline engines are done to reduce spool, not increase pressure. In my case it does exactly that. I'm spooling up a 96mm 1.32 AR S475 with a 2 liter engine. Each turbos runs at only ~2:1 PR, but that results in 45 psi in the intake manifold (4:1), with no more than ~48 psi in the exhaust manifold, and it doesn't even cross over until over 8000 rpm. The turbos are out for a sunday drive compared to a single at 4:1 PR.

If you have enough displacement to spool up all the turbo you need, this won't help you much IMO. There are other benefits in efficiency but with enough displacement you can probably just increase the hot side size and get the same results.
 

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^^^OK.

I have done the math on compound twins and I'm just wondering, can the overall efficiency of the compound setup exceed that of a single? Where you put your intercooler will affect it significantly, but I'm just curious (without spending all the time to do it) how much more efficient your compounds are versus an appropriately sized single (neglecting low end / spooling behavior).

For the same mass rate, I just wonder what the overall efficiencies are.
 

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The short answer is yes. I think it was fast82z that put together a spreadsheet that allowed you to explore the efficiency of various setups. Most of the gains should come from intercooling between turbos, which I haven't gotten around to yet.
 

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The short answer is yes. I think it was fast82z that put together a spreadsheet that allowed you to explore the efficiency of various setups. Most of the gains should come from intercooling between turbos, which I haven't gotten around to yet.

I'll have to dig it out, but when I looked at it, putting the IC after the HP turbo was a better deal, because all the heat added to the system from both chargers would see cooling.
 
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