I have a b6 1.8T quattro and I'm thinking of either installing:

1) a full APR system with their high-flow cat, or
2) using a test pipe with another system.

What kind of gains am I losing by choosing the high flow cat over the testpipe?

 

I wouldn't go with the APR exhausts. I heard a lot of bad stuff about their exhausts.

I don't really know much about test pipes, but if you live in Cali, its illegal because of the emmissions,

sorry i can't help you that much

 

I was just talking to 1 of my customers that only deals with cats. He told me that the honey comb inside of cats are all the same and the only difference between a stock and HF is theactual size of the pipe its made of. (HF being larger in diameter)

 

Nope, that is totally not the case. HFC have a bigger cells. There usually 400 or 500 CPI.
If you ever look at a stock cat, and HFC you can see the difference is size between the cat substraight.
That's why they flow more, there is less of an internal restriction, and thus less of a pressure drop, do they flow more.

Turbo, and SCC did some Dyno testing of a straight pipe & HFC, they only saw around hp less, but showed about the same torque.
However, the straight pipe turbo spooled a little faster. In the end it was like a 2% difference.

 

I don't mean to get up on my soapbox, but I think of it like this:

There are a large number of people outside our community that think of performance cars as a polluting waste of natural resources. These people and the lobbyists that represent them are very vocal about the fact that laws should be written that nearly, if not completely constrain us from being able to modify our own cars. One of the touchpoints they hit often is that after we are done making our modifications, our cars polute more than when we began.

Well, we are all smarter than that, and know that a well setup and efficient motor is likely to burn more cleanly than one that needs work done on it. But if we succumb to the temptation to do things like remove the catalytic converters on cars that are going to be run on the street, we are playing into the hands of those misinformed lobbyists.

So if the threat of being ticketed or fined for being out of compliance with emmissions laws is not enough to make you keep cats on your car, then at least be considerate of the rest of us and don't provide fodder for the environmental lobbyists please.

IMHO, FWIW...

 

ya hi flow is the way to go...

 

high flow has a pipe through it ( unrestricted ) but catches most of the unburned gas same as the cat with filters, test pipe has no filters because it a straight pipe replacement

 

That's the same thing I heard in the Z car world too - dyno testing using a set of test pipes vs a set of high flow cats from Random Technology resulted in a difference that was negligible, in the realm of about 2hp. For that result, plus the fact that the cost was more or less similar, I'd have done the high flow cats rather than the test pipes. No issues with a CEL as you'd have with test pipes (although I know ECU programming can get past this), and no issues with police either.