Exhaust Sizing on Turbocharged Engines...??

[NewbVetteGuy]

I'm curious if there are any good rules-of-thumb that can / should be used for exhaust sizing on turbocharged engines.

For fun, I used a traditional NA Exhaust Size Calculator I randomly found on the interwebs which says that a single 3.15" diameter pipe will support / become a restriction beyond 407 BHP.

-I know that lots of people are making FAR in excess of 407 BHP on turbocharged B58 engines using a 3.15" downpipe exist size, so it's pretty clear the traditional rules don't apply to turbocharged engines, but which rules DO apply?

The higher pressure levels in the exhaust directly following the turbo in a turbocharged engine means you can physically push a much larger volume of air through the same diameter exhaust???.. (Is that how it works / why?)

Real example on 2020 Supra B58s: Downpipe inlet 4.5", downpipe exit 3.15", Y pipe inlet 3.15", outlet dual 3.0". (So the 3.15" restriction/bottleneck only exists for a short period of time.)



Adam

[gunt]

to me exhaust on turbo cars also has to with cc and rpm as in , if you are only heading to 6300 or past it , you will also be amazed how much tq and spool you will pick up ,

[NORSK]

rule of thumb is,the exhaust pipe AFTER the turbocharger can really never be too big

[modok]

Traditional rules are NOT at all x size = xhp
in an NA application the collector size has to do with the exhaust system up-stream of it, how many pipes come together and what volume that all adds up to. It's a wave tuning thing.
In half of a v8 for instance that gas speed in the collector is still far from constant. How constant the gas speed IS would be a big factor in how much HP you can flow per size. And that's not a hard limit. In most cases it would be still far below any speed that flow would choke.
For instance a half length collector might NEED to be larger than a full length, because....like I said gas speed. And then after the x pipe or y pipe, the flow may be even closer to constant speed, so the size can be smaller.
IF the flow becomes nearer to constant downstream, then the pipe size can be smaller. You get it?
So, a straight six is a lucky case becuase that's fairly smooth flow anyway.
IMO most cases the turbo exit would be larger than downstream.
Even tho it is a single pipe the flow will still be becoming more constant as you go downstream..
Unless utilizing some kind of resonance, which is possible, but almost never done on purpose.

[4vpc]

rule of thumb is,the exhaust pipe AFTER the turbocharger can really never be too big

^ This. The only reason you fit an exhaust to a turbo car is to quieten it down, from a power perspective it doesn't need one.

[modok]

Yes but lets pretend for the sake of each others ear drums that there is a decibel limit.
And if there is that changes the game. Mufflers more or less "need" airspeed to work.
it is then more efficient to keep the speed UP, change the speed gradually, than it is to slow the airspeed down, and then speed it up again later in the muffler. Besides the fact that changing the speed suddenly often CREATES harmonics, which make it LOUDER, ironically

[ptuomov]

To first order, I think the turbo back exhaust should be sized the same as the normally aspirated exhaust of the same peak power.

One wants a lot more area right after the turbine outlet because of three reasons. First, in a twin turbo car, the pulses are still separated. Second, the exhaust is hot and has therefor a high volume. Third, the exhaust from the turbine has swirl which increases flow velocity and thus losses that needs to be offset with a larger cross sectional area.

For car factories, engineering a normally aspirated sound from a turbo car is a much more difficult and interesting problem than designing the smallest exhaust to produce a given x% power loss.

We had a long thread titled something like “making a muffler” on these topics.

[ptuomov]

Yes but lets pretend for the sake of each others ear drums that there is a decibel limit.
And if there is that changes the game. Mufflers more or less "need" airspeed to work.
it is then more efficient to keep the speed UP, change the speed gradually, than it is to slow the airspeed down, and then speed it up again later in the muffler. Besides the fact that changing the speed suddenly often CREATES harmonics, which make it LOUDER, ironically

I’m not sold on mufflers needing air flow to work. Designing a muffler to work with low flow velocities is in my opinion a lot easier than defining them to work with high flow velocities, if the cross-sectional flow area is held constant.

[4vpc]

Does an N/A engine need an exhaust system (after the manifold) for power purposes? Serious question as I see performance engines with no exhausts so presume they are just for noise reduction.

[modok]

Yes but lets pretend for the sake of each others ear drums that there is a decibel limit.
And if there is that changes the game. Mufflers more or less "need" airspeed to work.
it is then more efficient to keep the speed UP, change the speed gradually, than it is to slow the airspeed down, and then speed it up again later in the muffler. Besides the fact that changing the speed suddenly often CREATES harmonics, which make it LOUDER, ironically

I’m not sold on mufflers needing air flow to work. Designing a muffler to work with low flow velocities is in my opinion a lot easier than defining them to work with high flow velocities, if the cross-sectional flow area is held constant.

Sound, at a basic level is changes in the rate of flow.
What the exhaust system does is take pulsating flow and convert it into continuous flow.
IF the flow out of the turbo WAS constant, then a exhaust system would be a lot simpler.
But it isn't, instead, what happens is is if you put a length of pipe on it, you get resonance at certain speeds, as the interval goes in and out of phase with that' length's natural frequency, oops.
How would you design a "muffler" to prevent a length of tube from resonating as a length of tube?

[modok]

And...yeah the way I put it isn't really the right way to say it, it's not merely velocity. Velocity, lengths, AND volumes.
but, certainly the pipe and the muffler aren't separate elements.
You can have them work together, or they can fight eachother. it is more efficient if they work together.

[peejay]

rule of thumb is,the exhaust pipe AFTER the turbocharger can really never be too big

^ This. The only reason you fit an exhaust to a turbo car is to quieten it down, from a power perspective it doesn't need one.

On a naturally aspirated engine, the exhaust needs to be a certain size so that it keeps momentum so it doesn't need the engine to have to push it through.

On a turbo engine, the exhaust needs to be just small enough that you're not an asshole. since all exhaust energy gets used by the turbo and everything after that is wasted energy to push it out.

That said, there are a lot of Diesel truck driving people who are assholes.

[peejay]

Does an N/A engine need an exhaust system (after the manifold) for power purposes? Serious question as I see performance engines with no exhausts so presume they are just for noise reduction.

Not strictly speaking, no. However, a naturally aspirated engine with a too large exhaust will suffer excessive backpressure as the large exhaust won't have enough velocity to maintain enough inertia. Open header works better than a oversized exhaust.

[cjperformance]

On a turbocharged engine you can easily make way more Hp/csa than on a N/A engine.
You have a smoother exhaust flow due the the turbine acting as a pulsation dampner which makes the same pipe CSA 'look' larger to the turbo setup, you also have less reliance on scavenging post turbine than post collector in a N/A combo.
Yes pipe restriction post turbine makes a difference to spool time, boost and HP but not to the same extent as N/a Hp V pipe CSA.
In my experience even with a reletivly ineffective turbo setup you can get away with around 20+% less pipe CSA/Hp @.014"²/hp than you can N/A with a reasonably efficient pipe setup @.018"²/hp.

[ptuomov]

On a turbocharged engine you can easily make way more Hp/csa than on a N/A engine.
You have a smoother exhaust flow due the the turbine acting as a pulsation dampner which makes the same pipe CSA 'look' larger to the turbo setup, you also have less reliance on scavenging post turbine than post collector in a N/A combo.
Yes pipe restriction post turbine makes a difference to spool time, boost and HP but not to the same extent as N/a Hp V pipe CSA.
In my experience even with a reletivly ineffective turbo setup you can get away with around 20+% less pipe CSA/Hp @.014"²/hp than you can N/A with a reasonably efficient pipe setup @.018"²/hp.

My experience is similar, one can make a lot of power with turbos even if the turbo-back exhaust is somewhat undersized.

Another way to think about this is weight, noise, and hp. Turbos work as mufflers, so one _can_ run a larger diameter exhaust with fewer and/or lighter mufflers while meeting the same noise requirement. This would, on the face of it, probably lead to larger tail pipes on turbo cars than NA cars at a given hp level, if one is at the corner solution at the noise limit -- not certainly but probably.