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Turbo Backpressure (exhaust sizing)

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Quick question,

I've seen a lot of discussion on manifold pressure pre-turbo, but haven't found much on turbo backpressure post-turbo, other than too much will damage the turbo

In my understanding, the least backpressure post-turbo is best for efficiency/spool when it comes to making power

Just hoping to see any real world experience or testing

Thanks for any input!

Galen M.

The turbine works on a pressure differential, as a rough rule of thumb, the greater the pressure the greater the torque to drive the impellor, the greater the flow the higher the rpms.

In theory the less pressure at the outlet the better, but I have wondered whether one could increase the efficiency by the design of the transition to the rest of the exhaust? While each exhaust pulse is dampened in the turbine as part of the expansion, my thinking is there may be some benefit if the scavenging could be improved? Pipe diameter for inertial scavenging and length for optimising the timing of the pressure pulse differentiation across the turbine.

There seems to be a fixation on 'the bigger the better', but that can introduce packaging issues, especially for those of us who have to run full length systems and meet noise limitations.

Perhaps matching the outlet inner diameter with a smaller cross section transition to a larger diameter, or have a simple stepped/tapered section to the final diameter? Other options may be something like keeping the inside diameter, running it back to the mufflers, but using a larger inside diameter muffler with a transition cone?

That said, I'm not really a "turbo' guy", so may be quite mistaken.


I may be conflating a couple reading resources I've used but I believe both Corky Bell and Hugh MacInnes touch on this in 'Maximum Boost' and 'Turbochargers' respectively with their suggested 'optimum scenario' exhausts as bellmouth transitions from turbine outlet diameter to essentially as large as one could package.

Taking that idea I've found similar optimal results with turbo rotaries using over large downpipes that transition from turbine outlet diameter to 3.5-4" then converging back to a standard 3" diameter for packaging. So far its tended to net 90% of the upsides of the larger diameter and very little downsides.

I once spoke with Owen development guys trying to find out maximum boost that they would recommend on their thrust bearing turbo - they said 1.8 Bar. Then I asked about ball bearing turbo - same thing, 1.8 Bar. I got very frustrated and asked them why there is no difference ( as I thought ball bearing turbo would withstand more boost). The answer was that it's all about differential pressure between hot site and cold site. According to them most turbos wouldn't survive much more than 1.8 differential pressure for a long time...

That also surprises me, Shota, with a plain/sleeve axle bearing the thrust takes the full load, but there are many applications elsewhere where ball or tapered roller bearings provide excellent thrust control.

I see there are some companies offering "ramp" designs of plain thrust bearings, intended to aid the formation of the dynamic wedge that actually carries the load.

I don't particularly think that boost is a reasonable metric for a turbo and its bearings lifespan, though for whatever reason it's taken the same life as the 'horsepower' game on drivetrain components rather than torque. I'd be way more curious about RPM and flow redlines/characteristics for divining longevity.

Little off topic from my main post at this point..

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