4" downpipe thoughts

[DA BIG ONE]

I see BMW and other euro brands are using a large oval (6"+-) cat attached to the turbine outlet on their turbo motors. Understanding that the big cat will give little restriction under boost situations and its oval shape is designed to clear areas available under hood.

So, I'm thinking a 3" or bigger oval downpipe into larger exhaust for our 6.5td's isn't such a bad idea!

Larger diameter exhaust as opposed to a tuned exhaust for a turbo diesel seems to be the ticket here.

Any thoughts?

[john8662]

w/o a bigger outlet on the turbocharger itself, I wouldn't throw a dime towards a bigger downpipe.

Some turbo's have a 4" outlet (HX40?)...

You're right though, oval (new trend) would be a neat concept to clear the frame and beside the exhaust manifold itself. I haven't run accross any pipe shop that can mandrel bend the stuff, could be tricky.

[DennisG01]

Keep in mind that 3" oval is smaller in area (square inches) than a 3" circle. I'm not sure how a 4" oval compares to a 3" circle, though.

[stingthieves]

I just always "assumed" that I would have to hack off the flange on the Flowmaster Pipe and flange it to the dBux 4inch down pipe I have - at the appropriate juncture, of course. But today I put the two pipes next to each other and ----> Boy they look similar. The Turbo flange is the same and fixed at about the same angle - the dBux down pipe is deeper - it will hang down just a bit more but in my application - so what? Am I dreaming here - will it simply bolt up? I have my ceramic coating on order so which down pipe should I paint?
Rick

[DA BIG ONE]

w/o a bigger outlet on the turbocharger itself, I wouldn't throw a dime towards a bigger downpipe.

Some turbo's have a 4" outlet (HX40?)...

You're right though, oval (new trend) would be a neat concept to clear the frame and beside the exhaust manifold itself. I haven't run accross any pipe shop that can mandrel bend the stuff, could be tricky.

SpinTech mandrel bends em soft turn, or hard turn.

http://www.spintechmufflers.com/spintech/spintechindex.asp

[DA BIG ONE]

Keep in mind that 3" oval is smaller in area (square inches) than a 3" circle. I'm not sure how a 4" oval compares to a 3" circle, though.

Called them (SpinTech) today pipes measure for hard bend as follows;

3" oval = 2 1/2" x 4"
4" oval = 2 3/4" x 4 5/8"
5" oval = 3 1/4" x 6"

http://www.spintechmufflers.com/spintech/spintechindex.asp

[DennisG01]

Now that we know the actual sizes, anyone out there know how to figure out the area of the ovals? If I did my math right (it's been a while!) the 3" circle would have an area of approximately 7.1 square inches. It'll be interesting to see how this works out!

[kaylabryn]

I got to believe that if you take a 3" pipe and squished it into an oval, without kinking or smashing it, the internal area of the pipe stays the same. Kind of like a chain, just because you take it from a circle to an oval doesn't change the length.

[JTodd]

i believe the area of an oval is the height/2 x width/2 x pi

3" circle = 7.1 square inches
3" oval = 2 1/2" x 4" = 7.85 square inches
4" oval = 2 3/4" x 4 5/8" = 10 square inches
5" oval = 3 1/4" x 6" = 15.32 square inches

[dieseldummy]

The Dmax has an oval shaped pipe off the turbo. People say that it doesn't flow well. Something about turbulence of the oval shape as compared to a round pipe. With that said I think a 4" round pipe downpipe could be made to fit. I don't think that it is worth it however seeing as how the 90 bolted to the turbo doesn't flow worth ****. Just my .02

[DA BIG ONE]

The Dmax has an oval shaped pipe off the turbo. People say that it doesn't flow well. Something about turbulence of the oval shape as compared to a round pipe. With that said I think a 4" round pipe downpipe could be made to fit. I don't think that it is worth it however seeing as how the 90 bolted to the turbo doesn't flow worth ****. Just my .02

I've heard this before and may apply more to N/A motors which rely more on scavaging/tuning/& even x-over for tq on dual setups. Thinking if oval area is larger it would be a non issue and lead to quicker spoolups, lower egt's, iat's and other temps.

The german, japs, and others are all going to crazy big and oval in this area right after turbo.

[DennisG01]

I got to believe that if you take a 3" pipe and squished it into an oval, without kinking or smashing it, the internal area of the pipe stays the same. Kind of like a chain, just because you take it from a circle to an oval doesn't change the length.

I'm not using anything scientific to figure this out, but my thought is that if you take a drinking straw and squish it down, you can't the suck the soda through it as fast.

[Craig M]

As a round pipe is made into an oval shape, the internal area does decrease. The perimeter of the pipe stays the same, while the area decreases. Area of a 3" ID pipe is 7.07 square inches. A 3 inch pipe squished down to 2.5 inch x 3.28 inches has an internal area of 6.86 square inches. The wetted perimeter is the same and area has decreased by 3%. For a 4" ID pipe area is 12.57 square inches. Squished to 3" x 4.57" area goes to 11.78 square inches, a 6% decrease. As stated in other posts a round shape is one of the best flow shapes. An oval will flow slightly less that an equivilent round area. The "correct" size oval, to clear the frame or other area can be figured.

[JohnC]

I got to believe that if you take a 3" pipe and squished it into an oval, without kinking or smashing it, the internal area of the pipe stays the same.

Not true! A circle is the greatest area you can get for a given perimeter length. Anything else is less.

[Patrick m.]

i must agree with John8662, you could put a 7 inch down pipe on if it would fit, and not realize any gain.
The object limiting exhaust flow is the turbine housing.
The turbine housing is too small for engine speeds over 2500 rpm.

I just swapped from the large Garrett, to a new GM-8 and the power difference is incredible above 2000 rpm. (pre engine failure) Eventhough the GM-8 was able to produce higher pressure.

[DA BIG ONE]

While turbine outlets can be ported to get better flow "and should be". In spite of this restriction with a stock turbine housing we see that larger diameter pipes, straight through mufflers, or no muffler do indeed give better performance, lower egt's, iat's and operating temps on turbo diesel motors.

No being an engineer I think the performance increase has to do with reducing/eliminating backpressure relating to the extra cfm flowing through the pipes of a turbo motor as opposed to a slower flowing n/a motor. Not sure if scavaging plays a major role in turbo exhaust, or?