Exhaust piping on a turbo engine..

[RDSport323]

I need an impartial, unbiased answer regarding the exhaust on a turbo engine. I have read from various websites that the use of an X-pipe is beneficial for most engines. I am not sure if that applies to turbo tuned cars or not. So would it be better for an exhaust system to incorporate an X-pipe or will having a divorced pipe setup achieve the same results?

thanks

 

[RGUN]

Originally posted by: RDSport323
I need an impartial, unbiased answer regarding the exhaust on a turbo engine. I have read from various websites that the use of an X-pipe is beneficial for most engines. I am not sure if that applies to turbo tuned cars or not. So would it be better for an exhaust system to incorporate an X-pipe or will having a divorced pipe setup achieve the same results?

thanks

Is this car twin turbo? Otherwise all the pipes are going to go the the turbo already, and your question becomes irrelavent.

 

[mooseracing]

Originally posted by: RGUN

Originally posted by: RDSport323
I need an impartial, unbiased answer regarding the exhaust on a turbo engine. I have read from various websites that the use of an X-pipe is beneficial for most engines. I am not sure if that applies to turbo tuned cars or not. So would it be better for an exhaust system to incorporate an X-pipe or will having a divorced pipe setup achieve the same results?

thanks

Is this car twin turbo? Otherwise all the pipes are going to go the the turbo already, and your question becomes irrelavent.

They're still going to leave the turbo and go out the back at some point. It just depends on what kind of back pressure you have? And are you looking for more bottom end or top. It's only a couple HP/TQ at WOT vs straight duals/X/H.

 

[Kadarin]

You might post this question on a forum that deals with turbo cars, like e90post.com, or maybe an Evo or Supra forum.

 

[shabby]

No point for an x-pipe in a turbocharged car, this isnt an naturally aspirated v8.
In fact theres no point for an exhaust at all, the turbo itself provides any and all the backpressure thats needed.

 

[sjwaste]

Originally posted by: shabby
No point for an x-pipe in a turbocharged car, this isnt an naturally aspirated v8.
In fact theres no point for an exhaust at all, the turbo itself provides any and all the backpressure thats needed.

This is correct. On a turbocharged car after the turbo, use the biggest pipe you can fit. The turbo's job is to convert that wasted heat energy from the engine into mechanical force which is then used to compress the intake charge. After the turbo, you want as free a flow as possible, without regard to backpressure -- the turbo doesn't need/want it.

The only real discussion on downstream exhaust is whether you're going to use a single pipe mated to the turbine outlet and wastegate outlet, or if you're going to divorce the two with a smaller second pipe on the wastegate outlet, which merges further downstream in the main exhaust.

Now, the exhaust manifold/header will matter on a turbo. On a straight six like the Supra, the stock manifold got us pretty far, but equal length tube headers made a difference once you hit 500 RWHP or so.

 

[maluckey]

x3 for as big as you need to support the HP level you have. I ran stock a 2 1/4 inch exhaust until I hit 240WHP, then went to 3.0 inch piping which will support more than I will likely need with my SRT-4 setup.

I'm running at just shy of 280 WHP at the moment and when I recently ran without exhaust for a run, it made zero statistical difference. There's the answer. Too small hurts power, and too big hurts your ears for no extra power.

 

[Kadarin]

Originally posted by: shabby
No point for an x-pipe in a turbocharged car, this isnt an naturally aspirated v8.
In fact theres no point for an exhaust at all, the turbo itself provides any and all the backpressure thats needed.

I would absolutely love to hear a technical explanation for why backpressure is "needed" in any situation.

Backpressure is pressure in the exhaust system that acts as a force against the piston rising in the exhaust stroke. Backpressure = lost horsepower, period.

 

[sjwaste]

Originally posted by: Kadarin

Originally posted by: shabby
No point for an x-pipe in a turbocharged car, this isnt an naturally aspirated v8.
In fact theres no point for an exhaust at all, the turbo itself provides any and all the backpressure thats needed.

I would absolutely love to hear a technical explanation for why backpressure is "needed" in any situation.

Backpressure is pressure in the exhaust system that acts as a force against the piston rising in the exhaust stroke. Backpressure = lost horsepower, period.

Without getting too technical on it, where I'm likely to make a mistake, the short answer is that you never need backpressure, but you do have to tune an engine based on the backpressure.

When people suggest an exhaust is too open, not providing the right amount of backpressure, it really says the system isn't tuned to deal with that. When you alter backpressure drastically, changes to ignition timing might be needed, the A/F ratio will be off, valve timing might need to change, etc.

So basically you need a certain amount of backpressure if you don't have the time or money to tune for the new conditions. It can't exceed the conditions that the stock ECU can adjust for, or you'll start to lose power.

On a turbocharged system, most of that is based on the flow characteristics of the exhaust mani/header and the turbine/wastegate housings, which really provide so much restriction on the exhaust side that whatever pipe is on the other end won't change or contribute to things all that much. That's why it matters less/not at all on a turbocharged setup.

 

[Howard]

I hate when people say you need backpressure. What you need is velocity, which is gained in some situations where backpressure exists in others - e.g. an undersized exhaust makes for good velocity and therefore torque down low but increases in backpressure as engine speed goes up.

 

[Zenmervolt]

Originally posted by: Howard
I hate when people say you need backpressure. What you need is velocity, which is gained in some situations where backpressure exists in others - e.g. an undersized exhaust makes for good velocity and therefore torque down low but increases in backpressure as engine speed goes up.

Winner.

ZV

 

[Kadarin]

Originally posted by: Howard
I hate when people say you need backpressure. What you need is velocity, which is gained in some situations where backpressure exists in others - e.g. an undersized exhaust makes for good velocity and therefore torque down low but increases in backpressure as engine speed goes up.

You're on the right track here.

An oversized exhaust (including header primary pipe diameter) will reduce backpressure since it's less restrictive to total exhaust flow, but can hurt low-end torque because the exhaust gas velocity is slower. A higher exhaust gas velocity coming out of the exhaust port will assist in scavenging remaining gas from the cylinder on the exhaust stroke, and this improves torque. At higher engine rpm, total capacity becomes much more of a factor, and larger headers/exhaust can flow more total volume of exhaust gas, thus making exhaust system restriction less of a factor in the total horsepower an engine can produce.

There's more to header design than this, however. Primary pipe length is a factory, as is pipe placement. Exhaust gas from the engine pulses, because cylinders fire at different times, and it's not a continuous flow.

However, how all this affects a turbo engine is something I'm not really that familiar with.

 

[Howard]

You didn't need to say all of that, you know...

 

[Apex]

There's better info here than on most automotive forums.

 

[sjwaste]

Originally posted by: Kadarin
However, how all this affects a turbo engine is something I'm not really that familiar with.

It generally doesnt, unless you're talking about changing the turbine housing itself. In theory, moving to a housing with a different A/R ratio will change the flow characteristics, as will changing the trim or clip of the turbine wheel itself.

The issue itself receives very little attention for most modders, though, because you're spec'ing the turbine housing and trim on power characteristics that have been done by others in the past. That is, you're probably driving the car to Dinan or someone else, and having them bolt on something they've designed. Same can be said about aftermarket exhausts for NA cars.

From what I've seen, the choice of ignition timing, fuel delivery, etc are definitely related to what you've chosen as a turbo, but the fact that you're stuffing more air via forced induction into the engine plays such an overwhelmingly greater role than the backpressure characteristic of the turbo itself. I don't have any evidence to say that the difference in backpressure between a higher flow vs fast spool (say > 1.00 A/R with a T88 vs a .5 with a T3 40 trim) is all that great, but I suspect it just isnt -- that until you get into some really exotic turbos, backpressure isn't a a big deal because its not changing a whole lot on a turbocharged car on the mods that 99% of people will undertake.

 

[sjwaste]

Originally posted by: Apex
There's better info here than on most automotive forums.

I might be letting out a big secret here, but most of the knowledgeable car guys still use listservs. The Supra community fragmented to a degree when a lot of people started to hit the various forums instead of the lists, but our really knowledgeable people still use the listserv exclusively.

 

[TitanDiddly]

Originally posted by: Kadarin

Originally posted by: shabby
No point for an x-pipe in a turbocharged car, this isnt an naturally aspirated v8.
In fact theres no point for an exhaust at all, the turbo itself provides any and all the backpressure thats needed.

I would absolutely love to hear a technical explanation for why backpressure is "needed" in any situation.

Backpressure is pressure in the exhaust system that acts as a force against the piston rising in the exhaust stroke. Backpressure = lost horsepower, period.

Two stroke engines need backpressure to prevent the fuel/air mix from leaving with the exhaust. Hence tuned pipes.

 

[Kadarin]

Originally posted by: sjwaste

Originally posted by: Kadarin
However, how all this affects a turbo engine is something I'm not really that familiar with.

It generally doesnt, unless you're talking about changing the turbine housing itself. In theory, moving to a housing with a different A/R ratio will change the flow characteristics, as will changing the trim or clip of the turbine wheel itself.

The issue itself receives very little attention for most modders, though, because you're spec'ing the turbine housing and trim on power characteristics that have been done by others in the past. That is, you're probably driving the car to Dinan or someone else, and having them bolt on something they've designed. Same can be said about aftermarket exhausts for NA cars.

From what I've seen, the choice of ignition timing, fuel delivery, etc are definitely related to what you've chosen as a turbo, but the fact that you're stuffing more air via forced induction into the engine plays such an overwhelmingly greater role than the backpressure characteristic of the turbo itself. I don't have any evidence to say that the difference in backpressure between a higher flow vs fast spool (say > 1.00 A/R with a T88 vs a .5 with a T3 40 trim) is all that great, but I suspect it just isnt -- that until you get into some really exotic turbos, backpressure isn't a a big deal because its not changing a whole lot on a turbocharged car on the mods that 99% of people will undertake.

I used to know a guy who drove a turbocharged Fiat rear-engined car, and he replaced his cast iron exhaust manifold and turbo housing with a tubular one that he made himself. When he was done with this, the car was so much faster it was just plain silly. I remember seeing him hit 20psi on the boost gauge once.

In terms of theory, it seems that a tubular header going into a turbo housing will preserve gas velocity from the exhaust ports, and thus spool the turbos faster.

But yes, you're absolutely right in that most of us will just go buy parts off the shelf that someone else engineered. Often we don't even understand the theories behind them.

 

[alkemyst]

Originally posted by: Howard
I hate when people say you need backpressure. What you need is velocity, which is gained in some situations where backpressure exists in others - e.g. an undersized exhaust makes for good velocity and therefore torque down low but increases in backpressure as engine speed goes up.

and scavenging.


People got confused somewhere way back when and found smaller pipes lead to better power sometimes so thought it must be 'backpressure'.

Number of bends also makes a difference.

 

[sjwaste]

Originally posted by: alkemyst

Originally posted by: Howard
I hate when people say you need backpressure. What you need is velocity, which is gained in some situations where backpressure exists in others - e.g. an undersized exhaust makes for good velocity and therefore torque down low but increases in backpressure as engine speed goes up.

and scavenging.


People got confused somewhere way back when and found smaller pipes lead to better power sometimes so thought it must be 'backpressure'.

Number of bends also makes a difference.

Correct. Basically you need it in the sense that the stock tune is based on the stock exhaust. If you alter/reduce backpressure, you need to tune for that too. I think the fallacy that you "need" it was based on changing from a 2" to a 4" pipe on a NA car, losing power, and concluding that some restriction is necessary, all without having actually tuned for the new setup.

 

[alkemyst]

Oddly on my car (2.4L 240sx), the power curve keeps improving through 3" pipe. Probably due to the long stroke my engine has.

 

[Jessica69]

Originally posted by: maluckey
x3 for as big as you need to support the HP level you have. I ran stock a 2 1/4 inch exhaust until I hit 240WHP, then went to 3.0 inch piping which will support more than I will likely need with my SRT-4 setup.

I'm running at just shy of 280 WHP at the moment and when I recently ran without exhaust for a run, it made zero statistical difference. There's the answer. Too small hurts power, and too big hurts your ears for no extra power.

Turbo Dodge or a variant? Just curious..........myself, 1985 Chrysler LeBaron turbo convert...and love it!

 

[maluckey]

Turbo Dodge or a variant? Just curious..........myself, 1985 Chrysler LeBaron turbo convert...and love it!

It's a massaged SRT-4 swap crammed into a 1995 Neon ACR coupe. Factory reliability at just shy of 300 WHP and 34 MPG. What's not to love!

 

[halik]

I was under the assumption that backpressure was only necessary on carburated cars, for better mixture (the air moves back and forth in the venturi the instant the intake valve opens since the cylinder has some pressurized air left due to the backpressure)

 

[91TTZ]

You need a banana in your tailpipe.