Test pipe - fuel consumption?

[rickjones]

On another forum I regularly visit I came across an interesting thread, when replacing the catalyst for a less restrictive hollow test pipe will increase or reduce mpg?. So which one is it, I am curious to hear your answers. Thanks

 

[GregLocock]

It should improve the fuel consumption slightly, due to the reduced backpressure.

In practice it seems daft to chase a few % improvement in mpg in exchange for a large fine... but that's your choice.



Cheers

Greg Locock

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[patprimmer]

It may also loose a little due to fuel being drawn over during the scavenging process at TDC overlap.

This might partly offset the gains from lower blow down pressures.

Regards

eng-tips, by professional engineers for professional engineers
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[NormPeterson]

I think you used to be able to gain about 10% with respect to the early pellet type GM converters. Maybe a little more if it wasn't in "like-new" condition. That was 25 or 30 years ago.

Relative to a modern high flow converter in good condition I don't think you'd be reliably able to tell the difference (your driving probably varies more widely). Aftermarket high flow units can flow over 500 cfm (some well over that figure), which indicates a level of backpressure comparable to (and quite possible lower than) a high performance aftermarket muffler.


Norm

 

[reidh]

One also needs to be wary of changing the effective length of your exhaust system by removing a restriction, assuming it is a tuned exhaust.

-Reidh

 

[GregLocock]

Nope. Acoustically the cat has little effect.

Cheers

Greg Locock

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[reidh]

Greg,

How can a cat increase back pressure but not have an acoustic effect? We don't actually deal with catalytic converters in our industry, so I am by no means an expert on the subject. I had assumed that if it has enough restriction to affect back pressure that it would change the effective length of the pipe.

Reidh

 

[GregLocock]

Sorry

I should have said no significant /measurable/ effect.

Yes, obviously it has some effect.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warmington]

The removal of the restriction will increase overall airflow into the engine, meaning more fuel will need to be added to compensate for the increased air.

You will have more power availiable, therefore the fuel consumption will go up. Of course in a steady loading condition, you will have the same or slighlty better fuel economy at less throttle openings.

 

[SomptingGuy]

A cat may have negligible effect on noise but some say a DPF can replace a muffler.