Why do some cars use bigger diameter tube in front exhaust sections? 1

[rcx194]

I've noticed two of my cars use bigger diameter tube in front exhaust sections. Eg my Vitara had a big down tube from manifold to chassis, slightly lower diameter cat section, and then lower diameter again for the silencer and tail sections.

Is there any reason for this? If I'm building an exhaust for a kit type car should I replicate the diameteers from the engine donor car? The engine is 100 BHP, so I'm tempted to use 45mm 1.75" tube all the way from the manifold to the tail pipe,

 

[EdStainless]

The issues at the front (hot end) of the system are usually related to oxidation and scale formation which result in wastage.
There is also the fact that this tubing runs much hotter and in order to have reasonable strength it needs to be thicker.
The corrosion attack is much greater after the Cat where the exhaust is cooler.
This is because condensation can form and the resulting acidic water is highly corrosive.
Often the hot end is aluminized carbon steel and the cooler end is SS (usually 409 or even 439).
I would stay with the larger and thicker tube down to the Cat, and then run the Cat outlet tube clear to the rear.
A manufacturer saves a little money by reducing size and gage as much as possible.
It won't matter to you cost wise, the work is where your cost will be.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[rcx194]

I found a couple of websites with another possible explanation, that exhaust flow is turbulent in the front sections of exhaust, and therefore requires larger diameter tube:

Some quotes:

Koracing have compared 2.5 inch downpipes to 3 inch downpipes. They observed power comes in sooner but peak power is around the same, although on the 3 inch downpipe boost was 1 PSI less.

http://www.koracing.net/viewarticle.php?article=7

Stav Tech has an interesting post here, the sum up being the larger the downpipe the better, because the gas exiting the turbo is so turbulent. The larger downpipe improves spool and power. The exhaust system itself can be a smaller diameter because the gases become more homogeneous as they go down the exhaust.

https://www.facebook.com/STAVTECH/p...ing-subject-and-it-feels-lat/985989948244194/

 

[LionelHutz]

The same volume of hot gas takes up more space than cold gas does.

The starting size is related to the engine and power goals.

 

[gruntguru]

Engine pipe diameter is critical on turbocharged engines. The flow exiting the turbine is typically rotating and a rapid increase in diameter aids pressure recovery.

je suis charlie

 

[GregLocock]

In my (second hand) experience, in modern cars the design of the downpipe to the cat is largely about early light-off for emissions. Thereafter external and internal noise requirements get a look in.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?