The bigger the downpipe the better?

[Moschris]

Im seeing different sizes for downpipes.. 3,3.5

Is the bigger the size the better the performance? I also read on one of the threads that downpipes may cause a turbo to blow, is this true?

Should I get a new downpipe if im going to tune?

I also plan on having a custom exhaust from the downpipe and so on, will this affect the turbo?

 

[ExcelerateRep1]

In some cases bigger is better but there are many other factors such as flow and VME that are very important. There is solid data by various manufacturers that there is such a thing as too big. 3" in many applications is too large and isn't optimal in all areas of performance such as back pressure, throttle response, flow etc. 2.5" on a 1.8T or 2.0T up to stage 2, 2.5" is really all that is needed.

 

[Moschris]

In some cases bigger is better but there are many other factors such as flow and VME that are very important. There is solid data by various manufacturers that there is such a thing as too big. 3" in many applications is too large and isn't optimal in all areas of performance such as back pressure, throttle response, flow etc. 2.5" on a 1.8T or 2.0T up to stage 2, 2.5" is really all that is needed.

So 3.5" would be too big for the 1.8T? What are the effects of back pressure and flow?

 

[ExcelerateRep1]

So 3.5" would be too big for the 1.8T? What are the effects of back pressure and flow?

In my opinion as well as those who market them say yes it is overkill. Largest I would go is 3" down pipe. Mate it to a 2.5" catback post downpipe and that is more than sufficient for the 1.8T

On turbo cars, you want to eliminated any post turbo (downpipe/exhaust) restriction. But the 1.8T doesn't exactly have a large amount of displacement, so in this car, size would matter.

 

[Moschris]

In my opinion as well as those who market them say yes it is overkill. Largest I would go is 3" down pipe. Mate it to a 2.5" catback post downpipe and that is more than sufficient for the 1.8T

On turbo cars, you want to eliminated any post turbo (downpipe/exhaust) restriction. But the 1.8T doesn't exactly have a large amount of displacement, so in this car, size would matter.

So 3" downpipe then 2.5" catback?

 

[ExcelerateRep1]

That's what I would go with.

 

[Moschris]

What would be the difference of just having 3" all the way until after the muffler? what would a .5" difference make?

 

[Sandman GTI]

What would be the difference of just having 3" all the way until after the muffler? what would a .5" difference make?

From his advise it sounds like the .5" would matter.
Do you read it that way?

 

[toledospeed]

What would be the difference of just having 3" all the way until after the muffler? what would a .5" difference make?

Well, you could actually miss out on some potential power out of your upgrade across some of the powerband and driveability will suffer due to decreased velocity at lower RPM's. It is actually better to size an exhaust system slightly smaller than the max power you want to create than it would be to size it way too big. This is doubly true for a street vehicle. Right at the point the tube becomes oversized for the power you can make, you will lose some power as you continue to go bigger. I know it goes counter to how folks think, but turbo or not, airflow out of the engine is affected by both the size of the path area (pipe diameter/length in this case) and the velocity (speed of which it flows at a given RPM).

Specific Port Flow (cubic meter/sec) = Flow Velocity (m/s) x Average Path Area (m2)

When you increase the average path area (pipe size in this case), it can actually decrease the exhaust velocity, thus decreasing the overall flow. If you size your exhaust correctly (don't oversize it), the decrease in velocity will be as small as possible across the low and midrange.

For your 1.8, his advice to use a 2.5 exhaust is very sound. It would be a mistake to go bigger. Honestly, from what I have seen on other turbo cars, my guess is that you can get up to 300 HP (crank) with the 2.25" stock exhaust diameter for the 1.8. The 2.5" will likely get you up to 400HP with a single pipe.

 

[Moschris]

Well, you could actually miss out on some potential power out of your upgrade across some of the powerband and driveability will suffer due to decreased velocity at lower RPM's. It is actually better to size an exhaust system slightly smaller than the max power you want to create than it would be to size it way too big. This is doubly true for a street vehicle. Right at the point the tube becomes oversized for the power you can make, you will lose some power as you continue to go bigger. I know it goes counter to how folks think, but turbo or not, airflow out of the engine is affected by both the size of the path area (pipe diameter/length in this case) and the velocity (speed of which it flows at a given RPM).

Specific Port Flow (cubic meter/sec) = Flow Velocity (m/s) x Average Path Area (m2)

When you increase the average path area (pipe size in this case), it can actually decrease the exhaust velocity, thus decreasing the overall flow. If you size your exhaust correctly (don't oversize it), the decrease in velocity will be as small as possible across the low and midrange.

For your 1.8, his advice to use a 2.5 exhaust is very sound. It would be a mistake to go bigger. Honestly, from what I have seen on other turbo cars, my guess is that you can get up to 300 HP (crank) with the 2.25" stock exhaust diameter for the 1.8. The 2.5" will likely get you up to 400HP with a single pipe.

Good stuff to know. I plan on getting a custom exhaust pretty soon and I wanted to get the sizing right from the downpipe and so on.

 

[toledospeed]

Good stuff to know. I plan on getting a custom exhaust pretty soon and I wanted to get the sizing right from the downpipe and so on.

Cool, good luck! Look forward to seeing what you come up with. I would like to do a downpipe, but I really want to be able to self-tune before I do so (see my other reply LOL). For now, I have the NS PM. I have been running on the +7psi setting and have been doing some logging. Not sure I'll keep it there for good though. This plus meth injection could be interesting, but not sure about that on this car and I don't have any experience with that in general. I just hope Eurodyne comes along with their 1.8 solution soon! I'm sure it will be self-flashing first and then Maestro to trail behind.

 

[Jlungo]

It is actually better to size an exhaust system slightly smaller than the max power you want to create than it would be to size it way too big. This is doubly true for a street vehicle.

Your entire post was super helpful (came across this much later than you wrote it)
I have a 3" downpipeto go with my stage 2 tune. I have a cat system on its way but it's a 2.75" pipe system and i noticed it and got worried about the mismatch. Is .25" something to rly worry about? Or is straight 3 best option?

 

[Jlungo]

That's what I would go with.

Came across this thread and saw your advice. I was curious though. Different from the OP im on the 2.0 gti with a 3" dp and stage 2 flash. The exhaust system i have coming in is sized at 2.75". Would that quarter inch cause any issues with driveability? Performance? Installation?

 

[PLF8593]

Came across this thread and saw your advice. I was curious though. Different from the OP im on the 2.0 gti with a 3" dp and stage 2 flash. The exhaust system i have coming in is sized at 2.75". Would that quarter inch cause any issues with driveability? Performance? Installation?

Nah you could do a full 3" turboback and the 2.0 would make good use of it

 

[Jlungo]

Nah you could do a full 3" turboback and the 2.0 would make good use of it

3" all the way was what i thouggt i was going to do. But im going from 3" dp to 2.75" exhaust. Im just worried if that would cause any bottlenecks or driveability or performance issues

Sent from my SM-G920P using Tapatalk