1.9L TDI exhaust size?

[TeachMeToMPG]

Looking to do exhaust on my 2000 VW Jetta TDI. Right now ot is stock with a hallow cat. I'm looking at a 3in from a buddy for $100, or keeping the stock 2¼ and straight pipe the cat ans muffler.

I don't care about the tone, just increasing MPGs. Last months average is 59.07 MPG thanks to a .622 5th gear swap. Before that I was around 53MPG.

So far the car is: VNT17, KermaTDI tune, SouthBend Daily clutch, new vacuum lines throughout, DIY oil catch can, ventectomy, Firestone Fuel Fighter 205/65r15 tires, gutted cat and the Cascade German .622 5th gear.

Thanks yall.

[Isaac Zachary]

Good question. If you don't rev up very high then bigger exhaust isn't going to help very much, and could even possibly have the opposite effect as pulsating fluids have a sort of happy medium or Goldilocks size where bigger could potentially cause more restriction. Personally, I'd focus more on muffler restriction before trying different exhaust sizing.

If you do rev up your engine to a high RPM a lot then a bigger exhaust might be warranted. But most ecomodders don't usually do that very often, if at all.

[bandit86]

Met a guy with an 03 beetle, manual tdi, claims 60mpg average but never driving over 2000 RPM. I have an 01 and commute 300 miles twice a week, eliminating exhaust drone to me is more important than the last mpg, with stock exhaust my ears sometime ring.

Can you go up in tire size? I think my beetle can go same 205/75 16 and still fit. Another inch taller

[Vekke]

Bigger pipe diameter than needed will hurt your efficiency.

Optimal size is going from certain diameter to smaller when moving towards the rear of the vehicle as gas temperatures go down. This has bee proved many times in NA benzin race engine cars.

So if you want to test use the start original diameter and make it smaller towards rear.

But there are many places where you will see bigger improvements with less money.

[TeachMeToMPG]

Bigger is bad for turbo car? I understand 5-inch pipe is excessive, but I would have never guessed it would hurt. Further, I thought the exact opposite for narrowing the pipe. I always thought youd want the pipe the same volume from front to rear. I say volume not diameter because the shape may change giving different diameters but the same volume.

I'm gonna try to get out to the car today and yank off the muffler. So far I'm running the stock size pipe.

My miliage bounces from 56-61mpg per fill-up. It all depends on PENDOT's construction, my flight schedule, and if folks car drive and not play bumper cars on I81.

[Vekke]

Too big pipe is not creating a vacuum so engine has to push the gases out to that "huge" space. That starts to create back pressure and it start to limit your power. Yes it first pushes the gases already through the turbo.

Same as in VAG TDI engines there is no benefits to putting aftermarket free flow air filters, unless you want to brake your turbo and engine.

Stock air filters and housing are being used up to even 400hp 1.9tdi cars.

Now if you pushing out close to 300hp migth be needed to put bigger pipe, but are you then worried also about efficiency or the max hp is the question.

[Logic]

You want to look for the tuned length (and diameter) calculators online.

Then you want to pick the rpm range at which you want to run the engine and work from there.

BEFORE you get to the silencer, you want to look up:
david vizard exhaust terminator box
that goes at the end of your ...into 1 pipe, so its also tuned...

NB that the camshaft profile should be optimized for the same rpm range.

Similarly; the intake can be optimized, but that's even more difficult than getting a properly designed exhaust to fit.
I have noticed that properly designed bell mouths:

• Into the air filter box (Ram air is worth a look here too)
• From the air filter box.
• Into the turbo mouth. (especially)
• etc; all the way back. (Velocity Stack)

Is not too difficult and makes a surprising difference.