[AlpineG05]

Not to familiar with going catless but how exactly do CL DPs make more hp? Faster spool times?

Reason I ask is bc in thinking of jumping in the VK dp group buy so I want to make sure they are worth it. Thanks appreciate the help.

[ZJP]

Less backpressure = more power.

Catless downpipes allow you to fully maximize your exhaust flow which will increase the ability of your car to produce more horsepower/torque.

[E82tt6]

Post above is right, also energy efficiency. Less back pressure means more energy spinning the turbine, and less "fighting" the back pressure. It's easier for the turbos to make boost, and the engine can pass more air at higher efficiency. More air=more fuel=more fire=more power.

[longodj]

Originally Posted by ZJP

Less backpressure = more power.

Catless downpipes allow you to fully maximize your exhaust flow which will increase the ability of your car to produce more horsepower/torque.

+1. They also marginally reduce weight in the front of the car.

[Kevin@AKMotorwerk]

cats hold alot of heat also, so ur taking away a major heat source away from your turbos

[cali_guy]

can you pass a smog test in California with catless DP's?

[theanhdy]

^ no cat = illegal in cali, hence the common disclaimer "for offroad use only"

[E82tt6]

Originally Posted by Railgun

All yes except for more energy. The power to spin the turbines does not increase due to the lack of back pressure. At a particular flow, the exhaust energy will be the same whether the cats are there or not.
I know...just nit picking, but that's how it is.

Efficiency is the key with the mod.

Yes, badly worded on my part. More flow = more energy to spin turbine would be a better way of phrasing it.

[badass335]

The other question would be, if you go with catless dps on stock exhaust, are we back to square one? Or are we still going to see an improvement? Also, if we go with an exhaust (say cp-e) how bad is the noise?

[scottp999]

Originally Posted by badass335

The other question would be, if you go with catless dps on stock exhaust, are we back to square one? Or are we still going to see an improvement? Also, if we go with an exhaust (say cp-e) how bad is the noise?

1) Nope, not according to people's opinion posted here.

2) CPE, AE, and HKS are all raved about. I have had Britalman and AE on my car. AE is better quality than the Britalman system in my opinion. If you are anywhere need AE on the west coast, then you should take a trip to their shop.

[Septro]

Originally Posted by badass335

The other question would be, if you go with catless dps on stock exhaust, are we back to square one? Or are we still going to see an improvement? Also, if we go with an exhaust (say cp-e) how bad is the noise?

Well I'm on AR DPs with stock exhaust and my dyno jumped 43awhp from the baseline if that gives any indication.

[badass335]

Originally Posted by Septro

Well I'm on AR DPs with stock exhaust and my dyno jumped 43awhp from the baseline if that gives any indication.

So stock exhaust with 1set of cats I assume. How loud is the car? The gains look good!

[jeho]

its not tHAAAT loud....

catless dp makes allows your turbo to spool soo much faster and freely...
car 'feels' lighter and acceleration is instant...I can't ever imagine anyone driving a turbo car WITH catted DP, its such a big restrictor.

[Septro]

The exhaust on startup is a definitely noticeable, once warmed up it is slightly louder than stock at cruise. But J86 is right - if you put DPs in with a DCI - the turbo wooooosh is very pronounced. Some love it, others hate it - it doesn't bother me much.

[badass335]

Originally Posted by J86

its not tHAAAT loud....

catless dp makes allows your turbo to spool soo much faster and freely...
car 'feels' lighter and acceleration is instant...I can't ever imagine anyone driving a turbo car WITH catted DP, its such a big restrictor.

But we're not all crazy speed demons like you, dude!
Just teasing..

I'm just trying to figure out if I want to replace my exhaust as well or not.

[Zeph]

Originally Posted by cali_guy

can you pass a smog test in California with catless DP's?

that is a good question, since the exhaust still has the 2ndary cats in there... but it might not be enough for the tree huggers

Anyone have catless DP's and stock exhaust that wants to go test it? We don't have emissions laws where I live

[ar design]

Originally Posted by Septro

Well I'm on AR DPs with stock exhaust and my dyno jumped 43awhp from the baseline if that gives any indication.

Chart or ban.

[Drakos]

If DP's are that much better than stock and potentially save on turbo wear, why does the dealer have so much issues with them for warranty? I really want to add DP's but I have read posts where people have had warranty voided or refused.

[JoeyFiasco]

Originally Posted by Drakos

If DP's are that much better than stock and potentially save on turbo wear, why does the dealer have so much issues with them for warranty? I really want to add DP's but I have read posts where people have had warranty voided or refused.

because it's not a bmw part that has been tested or bough from them. in general it is true that DP's offer better performance, but when BMW designed the car they counted on their parts' efficiency and craftsmanship to tune the engine, and to a point dictate the strength of the materials used. because they know what materials and the engineering that went into the vehicle they know the vehicle component tolerances and can issue a warranty. once you start modifying their design it gets much harder for them to pinpoint the cause of a fault. basically you're adding unknowns to their neat and simple equation. which costs them time and money.

[Septro]

Originally Posted by ar design

Chart or ban.

[scottp999]

Originally Posted by cali_guy

can you pass a smog test in California with catless DP's?

Don't they do a visual check in CA? If they look at the O2 bungs and see a straight pipe between them, I would think you would have some explaining to do.