I put a magnaflow on my 2000 jetta tdi instead of straight pipe because my longtime vw mechanic and the mechanics at the shop where i work said it's not good for the valve train or the turbo to not have some back pressure? my cat 3208 is running straight pipe but it's in a bus and never really gets reved like the vw??

 

Look at your exhaust like a garden hose. Unrestricted the water will pour out 3 to 6 feet from the hose end.
If you start to squeeze the hose water will spray way farther- up to 30 feet and more. Pressure rises in the hose until the point the diameter gets to thin. The rang decreases now and restricts flow.

The same happens in an exhaust. You want to support the scavenging process by a proper diameter and the appropriate backpressure.
So size does matter- thankfully diameter is a lot more uncritical than in a gas engine.
By choosing the right exhaust diameter you can support spool up time of the turbine( as you know exhaust speed is turbine speed)
So the exhaust is also an effective tuning method.

Yes, you can go to big or get to much flow. But a right sized diameter(even a smaller one) can support your turbo.
The difficulty is to choose and to find the right one. This needs a lot of testing on a dyno.
A good mechanic will have the experience to decide which exhaust works better in which case depending on the engine.
I hope this answer leads you in the right direction. You will find more information about this in the thermodynamic laws.

 

With all due respect, DONT listen to mechanics. They are dumbasses. Turbos ARE backpressure; so anything after the turbo you want ZERO backpressure.

This one dumbass mechanic YELLED at my buddy when he asked if he should seafoam his truck.

I looked to the mechanic and asked whats wrong with it? The mechanic says it's snake oil and will blow his motor.

I said "interesting..is the fuel you not put into your fuel-tank not petroleum? Because according to this can of seafoam, it's ONE HUNDRED PERCENT petroleum, and the can even says it contains upper cylinder lubricants".

He went off to say how a can, can say anything it wants.


moral of the story is, if I need my f*cking motor replaced, I'll go to a mechanic. If I want to mod my cars or maintain them PROPERLY, I'll consult an online forum.

 

so should i just remove the pipe after the turbo?

 

Ya why not? Just straight pipe it.

 

Ok not all mechanics are idiots, there are a fair share of them though. That being said this mechanic says N/A diesels are effected slightly by exhaust back psi, on turbo diesels there is no effect. Do you think every performance diesel truck out there would have a hood stack if it hurt performance. Lastly to the guy who said screw mechanics youre gonna consult on line forums I wish I could dump all the bull ship people have spread all over these forums right back on there heads. If you don't trust mechanics then other peolpes opinions who you don't know is even worse, why don't you do some of your own research, remember
those things called books.

 

Well yes, NA motors needs backpressure.. but not turbodiesels. We are talking about turbodiesels here though, remember?

I'd say MOST mechanics, when it comes to mods and modding cars, are dumba$$es. They dont know squat when a car is modded. They may know how to fix stuff.. but if I asked a mechanic most of the questions on I've asked on the forums, high performance questions, they'd be scrathin' them's heads stumped.

As far as spreading BS on forums, thats why you cross referance. You dont believe something because somebody says it; you reasearch that topic thru 100 different forums reading 1000 different threads, then you conduct your own hypotheses, then you put it to work and see what you come up with yourself. If it was good data, you continue spreading it around the forums. If it's poor data, you correct others that think they are onto something and inform them that they are wrong.

As far as books, sure for basic mechanical engineering. Most books wont cover half of what you'll find on these forums, and they will take months longer to come to the same conclusion that I can come up with in about five minutes on my BlackBerry.

Ahhh.. I love technology.

 

To anyone who thinks ANY engine needs "backpressure".....

Stuff a potato in your tailpipe and tell me how well your engine runs.



Yes, there are certain "rules" to follow, but having ANY "backpressure" at all on ANY engine is just wrong.

Oh, and as far as internet forums having more REAL information.....

Once you actually FIND some good information, you've already thrown $25000 worth of someone else's "guesses" at it.

 

^ at the last sentence.

Gassers DO need a degree of backpressure though. They need the 'scavenging' for cylinder evacuation (which isn't an issue with turbo diesels).

 

Tell me what "backpressure" has to do with scavenging.