midrange

Its a flip flop situation. Too little back pressure kills off low end torque. Too much BP makes the engine labor too hard and kills off horsepower.

I'm not saying removing your cat will eliminate all the back pressure, but i agree with you, i guess the word NEED is actually an incorrect term to use as a NA engine will run just fine with zero back pressure.I'm just wondering why anyone would go through all this to eliminate a cat on a DD. If he uses this car strictly for racing, great.. but a street car, (cause race cars don't need to be emission tested). do you think its really worth it. I don't.

However, I would like to see if these catless downpipes will actually pass an emission test to get back on track. I guess I have some more reading to do, but I do know you will lose gas mileage with no back pressure.

I must say though, i HATE following a car that doesn't have cats or bad emissions... STINKS!

sakimano

on your DD...ok. It looks like your DD is a 1998 Audi A4 with an inline 4cyl 148hp 1.8T with an automatic transmission. I am talking about a 2007 S4 with a 340hp (stock) 4.2 L V8 and a manual transmission. I don't expect my goals with my Audi to be the same as yours...nor should you expect me to follow your path...or question why I'm doing the things I am to my car.

Hell I was excited to drop 20lbs of unsprung rotating mass when I changed my rotors! Clearly we think about cars differently.
flip flop indeed

A freer flowing 2.5" catless exhaust system on the B6/7 S4 will result in greater volumetric efficiency, greater torque, greater horsepower, and as a result, greater fuel efficiency. Again...you're tossing around old wives' tales as if they're proven. To do a certain level of work, if we're able to push the car LESS to do the same work, it's going to use less fuel. If we're choking the car with 4 cats (yes, the B6/7 S4 has 4 different catalytic converters in the downpipes), then choke it even further by running the exhaust gases through 2.125" exhaust piping...you're hurting all 3 (power, torque and fuel efficiency).

Check the b7 RS4 for a good example. quattro...Audi's performance division, gave the car a 2.5" set of downpipes and a 2.5" catback. Guess why? They didn't give a **** about emissions levels as they were only going to sell about 10,000 of these cars over 3 years. They wanted the most efficient exhaust system they could give it. They even included tri-Y headers design in place of the b7 S4's awful manifolds. All of this achieved 3 things - greater torque, greater hp and greater fuel efficiency. The tune, and the fuel stratified injection also helped the RS4's cause.

A great example is the B6/7 S4 owners who have JHM longtube equal length headers. These are even 'worse' according to you...as the flow is now improved even more than just good downpipes...now we're removing the restrictive stock exhaust manifolds and allowing the exhaust ports to spew the spent gases even more efficiently out the car (with even less back pressure) to the 2.5" catback.

The result? Even MORE torque...and about 15% fuel efficiency gains. Headers B6/7 owners report that they get an extra 2-4mpg (vs. 16-17avg for the B6/7 S4).

Again, you may be right talking about your car (whatever that is/was). You're just dead wrong when asking about the b6/7 S4.

After gutting the pre-cats, putting a 2.5" xpipe exhaust and getting a tune that can take advantage of the better flow...I improved my quarter mile time from 13.7 @ 102 mph to 13.2 @ 107 mph. Funny. I must have just been lucky. Cayuga is 50 miles from my house...and I now use less gas to get there and back

p.s. I'm not posting this thorough information for midrange...he'll never be convinced as he has his own opinion. Just thought I'd share some of what I've learned about my car over the years. It sure can't hurt the forum.

midrange

No need for the PS.. I don't know everything and never will. I'm not claiming that what your saying is incorrect.. I guess I look at it to the point of WHY? But you proven to me you obviously race your car.. For a street car, i don't see the point. Who cares how fast the car you use to get to work does in the quarter mile.

You are correct, and I do not know ANYTHING about the vehicle you are driving. I just don't see why anyone would try to get around the cat and break the law of ontario.. Who cares how fast a car is unless its trailered to a race track with a number on the side. My opinion.

I appreciate your posts and will always consider the thought if i ever need to.

2.8 though! not the 1.8T.

Cheers

cheeba

Oh really?


We get that your opinion is that you don't want to do this to your car, but please don't present your personal opinions as fact. There's no need to clutter up an otherwise useful thread by repeating your opinion on what a daily driver should, or should not, be. Furthermore, if it's all about efficiency in getting from point A to point B, you should be rocking a 1.2TDI Lupo - 100 impMPG baby!



Sources or calculations, please!

cheeba

Just to be clear, we're talking about post-header exhaust here. Headers should be tuned for proper scavenging/resonance/velocity/etc., but I need some sources/calcs for proving that the tubing beyond that does much of anything.

midrange

As a general rule, a normally aspirated vehicle will get better high RPM performance with a 2 1/4" exhaust system as opposed to 2 1/2" or above to retain exhaust gas velocity for street driving). The general consensus is that a 2 1/4" system is for mid to high RPM petrol heads.

I'm still learning a lot when it comes to this and i'm not stating my opinions as facts. My opinion is the why bother worrying about it and removing the cat.. Last time i had removing cats is better for your car talk was wahen i was 17 and thought having the faster car is better! I guess i'm past the stage in my life where I want tot take a good car and try to make it faster.

Going with what you said about if its economy I want, I should have taken a TDI.. Well in this case, If buddy here wants a faster car, why not buy a GT2 RS or Audi R8 if you want a faster car... Just views and points.

cheeba

Oh, so you actually meant "too little back-pressure in a fully-piped system"? You just said back-pressure, and that was incorrect. Either way, general rules don't mean much - there are plenty of N/A cars that benefit from >2.25" exhausts, the S4 being one of them.


Well, Saki never said that all he was concerned about was speed; he's concerned with making his car quicker. There's no need to obfuscate the situation with misappropriated arguments. What Saki does is irrelevant anyway, your arguments should stand on their own merit.

sakimano

interesting...but it's tough to say this as a general rule (this 2.25" rule) without testing it on each car. For example you're forgetting displacement...larger displacement expels more gas with each exhaust pulse. You could have a MASSIVE displacement engine, like an old 400+ cubic inch domestic that would get absolutely CHOKED by a 2.25" exhaust

Then there's engine speed...the B6/7 S4 will get choked at 2.25" (and further choked with cats). It is a 4.2 L V8 (not a big one at only 254 cubic inches) but revs to 7000rpm.

So, it needs to be determined what the most effective diameter is...and we (the B6/7 S4 community) seem to have found that 2.5" is the key diameter.

Saying things like 'why bother to make your car faster' etc...that just shows again...we must have different goals. I mean, why buy an Audi at all? You can end up on a slippery slope with that argument, and the only 'end' to that logic will have you walking everywhere as everything is 'too fast' or 'too expensive' or 'too fuel inefficient'. This is an enthusiast community and I'd definitely consider myself performance focused (otherwise I'd just drive my wife's B8 A4 2.0T

so...I should ditch my modest 4 door AWD sedan and get a $160,000 exotic instead? Yeah...that makes sense.

This logic of ditching the car in favour of a $160,000 exotic is the opposite (yet is an equally CRAP rationalization to prove your point) of your 'why bother getting faster' argument above.


p.s. here's some fun reading about the backpressure myth. I nearly spit my drink out when I read section I.

http://www.thumpertalk.com/forum/sho...d.php?t=659727

Backpressure: The myth and why it's wrong.

I. Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

the rest is at the link on thumpertalk

cheeba

Good link, Saki. I wish they would expand on it a bit, though - headers are a part of the exhaust and there are other factors - besides velocity - to consider. If a reader does not know any better, that information can be a bit misleading.

sakimano

well, it's a nice summarized post on debunking the 'backpressure myth' rather than being a symposium on exhaust. It's also just a post on an internet forum. As midrange shows, you need to do your homework with any forum information to make sure it's right/complete!