Twin Sequential Turbos...how would you set it up...

[CobraSix]

Just curious how most twin sequential turbos are set up. I keep thinking about it, but I can't figure out how they do it to best utilize the system.

I was thinking:

Exhuast -> Small turbo->Larger turbo->exhaust system

but what about the charge air? Do both turbos feed the manifold individually? I guess this would decrease any lag, but how do you sync the turbos so that the larger one doesn't reverse the smaller one? Or, do you feed the smaller turbo's charge air through the larger turbo?

I've driven twin sequential turbo cars (never really looked though) and they are a nice way to get higher boost levels without the lag, but just curious how they are set up.

Thanks.

Slade

 

[BB6Mustang]

Sequential (or compound) turbo systems usually have a smaller turbo feed a larger one (in series). That way the pressure ratios of the turbos are multiplied, but the flow through both is the same. In a conventional twin turbo the turbos are in parallel and the pressure ratios are the same for both, but the flow is doubled. I've only seen sequential turbos on high power diesels (like quad turbo pulling tractors), because gas motors usually can't handle the high pressures (over 200 psi sometimes).

I'm not quite sure how the exhaust is plumbed, but the system you described sounds reasonable. Although it wouldn't surprise me though if they just ran each turbo off a separate cylinder banks just like conventional twins.

What cars have you driven that had twin sequential turbos? Are you sure the turbos were set up in series and not just in the conventional parallel arrangement?

 

[CobraSix]

Volvo actually uses a sequential turbo on the T6 models of the S80 and XC90. Was just curious.

Slade

 

[Anonymous]

If one turbo is feeding another, the larger feeds the smaller.


-=Whittey=-

 

[CobraSix]

Wouldn't that cause the smaller turbo to overboost when the big turbo finally start reaching the upper end of it's boost (obviously depending on turbo size).

Slade

 

[Anonymous]

No. A turbo doesn't know what boost is, just what the pressure ratio from inlet to outlet is.

Say you want a 4:1 ratio and want to run a pair of turbos. Say your engine will actually inhale 100cfm of air. So you end up ingesting 400cfm of regular air.

*****

15psia, 400cfm --> turbo(big) --> 30psia, 200cfm --> turbo(small) --> 60 psia, 100cfm.

*****

A larger turbo doesn't mean it makes more boost, but moves more air. The first turbo has to move more CFM (400) than the smaller one (200). Both have a 2:1 ratio. If you're looking to size the compressor, you have to change the lb/min of air (X axis on the map) in your head to ignore the fact that the second turbo is taking in already pressurized air. Basically you size it as you would a turbo taking in 200cfm of 15psia air and pushing out 30psia.


-=Whittey=-

 

[Anonymous]

just one more thing...1993+ RX7s ran senquentials. Quite fun to drive, and a very fascinating motor.

 

[BB6Mustang]

Compressor maps have pressure ratio (P2C/P1C) on the Y-axis and air flow (lb/min) on the X-axis. Air flow in lb/min is not the same as cfm. While the first turbo will have to flow more volume (cfm) both turbos will flow the same mass of air.

 

[CobraSix]

in that case, would you really want to run a larger turbo? Why not just two same sized turbos?

The reason I am asking these questions is something is not clicking in my brain on this matter. I don't know why...but I'm not visualizing it right. Thanks

Slade

 

[Anonymous]

They are flowing the same mass of air, but the second turbo doesn't know it. It doesn't know what pressure the incoming air is at.


-=Whittey=-

 

[sefus]

love the wankle. yea the 3rd gen Rx7s have them and the 3000gt/stealth

simple way of thinking about it: smaller one is used to spool the larger one quicker than if it was on its own. there is an exhaust comming off of each turbo. its a neat idea but a properly sized intake and exhaust housing is just as effective... provided you are able to digest the pressure and move the air (porting is good) to spin it in the first place.