Does the turbo downpipe _length_ matter?

[Buddyworm]

Although this is no proper substitute for an actual test, I run some simulations with Vannik's software. The simulation results say that the downpipe length makes very little difference to the simulated pressure traces at the exhaust ports. The waves are almost completely broken up by the turbine. The wastegate pipe length has some impact on the pressure waves in the wastegate pipe, but the diameter and the overall energy in that pipe is relatively small, so it doesn't make much difference to the exhaust port conditions.

I can't comment on actual tuning results, but I'm not convinced that the turbines are so efficient at extracting energy from the exhaust stream that they damp the pulses enough to make them irrelevant.

Standing several feet behind a turbo drift car launching off the line with straight pipe exhaust you definitely feel discreet and powerful pulses.

On the dyno I've also had a big power turbo car blow my exhaust extractor pipe off its supporting jack stand!

On both the above examples the wastegates do not plumb back into the main exhaust pipe.

I think there's some energy to work with post-turbine.

[John Wallace]

I think there's some energy to work with post-turbine.

Could run 2 turbos in series?

[ptuomov]

I can't comment on actual tuning results, but I'm not convinced that the turbines are so efficient at extracting energy from the exhaust stream that they damp the pulses enough to make them irrelevant. Standing several feet behind a turbo drift car launching off the line with straight pipe exhaust you definitely feel discreet and powerful pulses. On the dyno I've also had a big power turbo car blow my exhaust extractor pipe off its supporting jack stand! On both the above examples the wastegates do not plumb back into the main exhaust pipe. I think there's some energy to work with post-turbine.

There's energy in the post-turbine exhaust for sure. There's heat, sound, other kinds of pulsing, velocity, etc. There's no question about it.

However, this leads to two questions. First, is there way to harvest that energy in general? I think there is. Diesels run compound turbochargers, like mentioned already. Some engines have a second turbine assisting the crankshaft. BMW uses heat to run a steam engine with a device that also acts as a muffler. Etc.

Second, is there a way to use the pulses in the post-turbine exhaust to impact the tuning inside the exhaust manifold. There I'm thinking that its going to be hard for a high-boost engine with the turbine blades close to sonic.

[Buddyworm]

I can't comment on actual tuning results, but I'm not convinced that the turbines are so efficient at extracting energy from the exhaust stream that they damp the pulses enough to make them irrelevant. Standing several feet behind a turbo drift car launching off the line with straight pipe exhaust you definitely feel discreet and powerful pulses. On the dyno I've also had a big power turbo car blow my exhaust extractor pipe off its supporting jack stand! On both the above examples the wastegates do not plumb back into the main exhaust pipe. I think there's some energy to work with post-turbine.

There's energy in the post-turbine exhaust for sure. There's heat, sound, other kinds of pulsing, velocity, etc. There's no question about it.

However, this leads to two questions. First, is there way to harvest that energy in general? I think there is. Diesels run compound turbochargers, like mentioned already. Some engines have a second turbine assisting the crankshaft. BMW uses heat to run a steam engine with a device that also acts as a muffler. Etc.

Second, is there a way to use the pulses in the post-turbine exhaust to impact the tuning inside the exhaust manifold. There I'm thinking that its going to be hard for a high-boost engine with the turbine blades close to sonic.

I was thinking in terms of a CSA change downstream of the turbine to generate a reflected wave. I figured you would be able to make the pressure delta across the turbine larger but the points that were made about sonic turbines not allowing waves to propagate upstream of the choke point gave me pause. Maybe it's only suited to lower backpressure setups that haven't maxed out the turbines.

Or maybe there's some gains to be found with those helmholtz resonators you see in lots of performance exhausts for cancelling out drone on the highway. I know precious little about this subject, but maybe there's some waves could stand some cancellation and enhance power.

[ptuomov]

I can't comment on actual tuning results, but I'm not convinced that the turbines are so efficient at extracting energy from the exhaust stream that they damp the pulses enough to make them irrelevant. Standing several feet behind a turbo drift car launching off the line with straight pipe exhaust you definitely feel discreet and powerful pulses. On the dyno I've also had a big power turbo car blow my exhaust extractor pipe off its supporting jack stand! On both the above examples the wastegates do not plumb back into the main exhaust pipe. I think there's some energy to work with post-turbine.

There's energy in the post-turbine exhaust for sure. There's heat, sound, other kinds of pulsing, velocity, etc. There's no question about it.

However, this leads to two questions. First, is there way to harvest that energy in general? I think there is. Diesels run compound turbochargers, like mentioned already. Some engines have a second turbine assisting the crankshaft. BMW uses heat to run a steam engine with a device that also acts as a muffler. Etc.

Second, is there a way to use the pulses in the post-turbine exhaust to impact the tuning inside the exhaust manifold. There I'm thinking that its going to be hard for a high-boost engine with the turbine blades close to sonic.

I was thinking in terms of a CSA change downstream of the turbine to generate a reflected wave. I figured you would be able to make the pressure delta across the turbine larger but the points that were made about sonic turbines not allowing waves to propagate upstream of the choke point gave me pause. Maybe it's only suited to lower backpressure setups that haven't maxed out the turbines.

Or maybe there's some gains to be found with those helmholtz resonators you see in lots of performance exhausts for cancelling out drone on the highway. I know precious little about this subject, but maybe there's some waves could stand some cancellation and enhance power.

Along the same lines, I'm thinking that one could potentially use the downpipe to get a favorable pulse tuning effect to the turbine before the turbine is fully spooled and "choked". The problem with this is that for the downpipe to generate that sort of strong pressure wave, I think it would also have to be pretty small diameter. Am I correct about this? If I'm correct about this, then the downpipe that would generate a strong pressure wave would totally choke the turbine at higher rpms and loads. This gets me back to square one that pulse tuning with the downpipe length is hopeless in setups that have well matched turbines, and all one can hope the downpipe to really do is to not restrict the flow out of the turbine.

[ptuomov]

viewtopic.php?f=15&t=43234&start=45#p648711

[Dust Buster]

I believe that the down pipe should taper from little over turbine size to the required exhaust size with a taper no greater than 7 degrees included angle...

So, if I understand correctly, a seven degree taper on a 3” pipe going to a 2 1/4” (a drop of .75” for side A-C) would take 6.155” (side B-C). My silly question is, “how do you taper a pipe over that distance?” Are you preparing a cast mould?

Taper_7_degrees.PNG

[dannobee]

You'd use a slip roller to make a cone shaped tapered pipe.

[pcnsd]

Flat sheet to truncated cone layout. Try here: https://www.cmrp.com/cone-calculator

[Dust Buster]

Thanks for putting up with my silly questions, but I am learning something here.