Hey guys A friend recently got me a turbo off a big diesel engine 13+ liters. The setup that they use for these engines have two turbos bolted together. One smaller and one bigger. He got me the small one and it measured out to be 60mm and .42 a/r which I am asuming is not big enough for a 468. I was wondering how using both the turbos would work. It would be set up like a single turbo because they are actually bolted together. I have never seen this used, but was wondering if there has ever been any talk about it. any input would be great.

thanks
Tom

 

using a compound type turbo setup work well on a diesel but is to much trouble on a gas engine. if your worried about spool just get a properly sized turbo.the larger one may work better but you need to give more info on the turbo.

 

I am not worried about the turbo spooling I was just asking because I can get them for free. Why is it harder to use on a gas engine?(just curious)

thanks
Tom

 

On a diesel engine two stage turbos are used to make more boost. The boost output of the first turbo gets mutliplied with the pressure ratio of the second one. (i.e. if the first makes 20psi and the second one is operated at a pressure ratio of 2, the total boost will be 40psi).

Gas engines don't run this much boost normally, two staged systems are only used with one big one and small turbo to get better spool up. However the air goes either through the small or the big turbine, because the small one would be a restriction at higher engine speeds. Because of this a complicated system with two switchable air paths is required for the hot and cold side.

It is complicated but it works great. The 1980s Porsche 959 used such a system.

Simon

 

Simon,

You don't think that we will see compound gas systems in the future?

 

Not unless someone discovers a practical way to push more than 80+psi through a spark ignition engine. Single turbos have been used to do that for at least the last 25 years. Adding compressor stages multiples the inefficiency as well as pressure ratio. I'm not sure current fuel and engine technologies will allow much more than that. Diesels aren't limited by knock, so they can tolerate as much boost as the mechanical parts will hold.

Yes compound systems can be used on spark ignition engines, but it's not necessary with what single turbos can do these days in relation to what current fuels will allow.

No, you multiply the pressure ratios and then convert that to boost pressure. 20psi is a 2.36 pressure ratio. Multiply that by your second 2.0 pressure ratio and it comes to a total pressure ratio of 4.72:1. Multiply that by atmospheric pressure(I used 14.7psi) and you get 69.38psi absolute. Subtract atmospheric pressure and you end up with 54.7psi gage/boost, not 40.

 

No need for a compound turbo setup on a gas engine, and I really dont think we'll see them for much longer on the big trucks.. Most everything diesel now is going to variable vane turbos ( Volvo is one of them. They use Holest ( sp? ), Cummins uses them on the 6.7's and Navstar/International on the 6.0, but used them on the 6.4 for some reason I haven't figured out, I'm sure there are others, I'm just not aware of them ) .. I could see that being used on gas engines in the future as well, other than the electronics it's just as simple as a single turbo setup. It's the ideal way to go in my mind.

 

a little off subject but how bout the new hellion system for the 03 cobra, a twin setup compounded via supercharger?? makes impressive numbers at 40 + psi on stock long block though not as efficient as a turbo at that level, it has great low to mid power.