[zipphreak]

I've been researching downpipes and full exhausts for the best hp/$ gain. I noticed that most cat-back/axle-back exhausts out there provide relatively small gains < 8 HP, < 10 Ft lb torque for equivalent dollar value $1000-$2500. That the single most gainful mod is tune piggyback or flash, and then catless or race cat version of downpipe, $400-$1200. Intake mods are claiming 10-15 Hp, 10-20 ft lb torque, for anywhere $200-600. There are no dyno graphs I could find with a stock car just showing intake swaps.

Sorted from lowest to highest:

N55 JB4 Piggyback ($530)
60 HP / 100 Torque = $8.83/HP, $5.30/FtLbTq ($11.48)

Race Catless 4" Downpipe ($500)
21 HP / 36 Torque = $23.81/HP / $13.89 FtLbTq ($18.85)

N55 F-series ECU Flash ($1500)
60 HP / 100 Torque = $25/HP, $15/FtLbTq ($20)

Burger Tuning N55 Intake ($230)
10 HP / 8 Torque = $23/HP / $28.75/FtLbTq ($25.88)

Magnum Force Stage 2 Intake ($450)
14 HP / 18 Torque = $32.14/HP / $25/FtLbTq ($28.57)

Race High-flow Downpipe 200cpi ($1200)
21 HP / 36 Torque = $57.14/HP / $33.34/FtLbTq ($73.81)

The combination of a piggyback and catless downpipe seem to be the best bang for Hp/Tq/$$$.

[JibHonk]

catless downpipe and jb4 are your best bang for buck. A full exhaust is useless except for noise. That was my last mod since it costs 2k. Don't cheat yourself the extra 10 hp and $400 going catted. Catless DP with a stock exhaust sounds great.

[zipphreak]

Yah If i wanted to change the sound more, you can change out the resonator pipe to straight pipe and throw in an axle back of your choice.

[mike@x-ph.com]

Best gains JB4 + Catless downpipes .
Exhaust will not give you any gains, but if you do a full exhausts you will be saving weight which will make your car go faster.

[Alpine F31]

Originally Posted by JibHonk

catless downpipe and jb4 are your best bang for buck. A full exhaust is useless except for noise. That was my last mod since it costs 2k. Don't cheat yourself the extra 10 hp and $400 going catted. Catless DP with a stock exhaust sounds great.

"Useless" is a bit harsh. I got a modest power gain, lost some weight and improved the appearance of my car with a full exhaust system. I do agree 100% that it is not the best bang for the buck.

I did not want to go catless because I don't want to hassle with California emissions testing and I don't want the potential bad smell you get by not running a cat.

[JibHonk]

california emissions, I totally get running catted. We don't even do federal emissions test where I'm located. The BMW exhaust is not restrictive. There isn't a single cat or anything after the downpipe (which surprised me under the car). The power gain any of these overpriced shops (Which honestly they are all overpriced because of the car they are going on) are making is due to ECU remapping or Downpipe replacement.

[zipphreak]

The emissions thing is what i've been thinking about. In Mass we have similar requirements and they test yearly to get an inspection sticker. Although the test is all electronic via ODB2, if they noticed the smell they could put the old school test on the exhaust pipe and figure it out

[G-Mann]

Originally Posted by JibHonk

catless downpipe and jb4 are your best bang for buck. A full exhaust is useless except for noise. That was my last mod since it costs 2k. Don't cheat yourself the extra 10 hp and $400 going catted. Catless DP with a stock exhaust sounds great.

10hp vs the hassle and potential cost of removal. Say $40 a pop to remove twice a year for maintenance and inspection x 5yrs there is your $400 savings gone poof along with lost time. If you do it yourself you are still out the time which can equate to $$

With the above said the JB4 and catless DP are the best bang for you buck period.

Originally Posted by zipphreak

The emissions thing is what i've been thinking about. In Mass we have similar requirements and they test yearly to get an inspection sticker. Although the test is all electronic via ODB2, if they noticed the smell they could put the old school test on the exhaust pipe and figure it out

Yeah if you don't have the JB4 to clear the cell your not going to pass that inspection. Wonder if they still look for cats and if they even know what they are looking for?

[Dattebayo]

Originally Posted by G-Mann

10hp vs the hassle and potential cost of removal. Say $40 a pop to remove twice a year for maintenance and inspection x 5yrs there is your $400 savings gone poof along with lost time. If you do it yourself you are still out the time which can equate to $$

With the above said the JB4 and catless DP are the best bang for you buck period.



Yeah if you don't have the JB4 to clear the cell your not going to pass that inspection. Wonder if they still look for cats and if they even know what they are looking for?

If you have JB4 clear CEL, it won't pass either as their OBDII scan won't read. They will continually get the message that the OBDII isn't ready. The JB4 may help with the visual inspection of the car, but it won't do anything in regards to the OBDII check, as all it is really doing is constantly reset the computer to remove the engine code and block OBDII readers. In all honesty, from what I have read and researched, in the sense of smog checks, really the only way is replacing the stock downpipe and turning off the JB4.

You can go with a Catted downpipe, but that won't even guaranty you won't get a CEL as over time, I have read that aftermarket cats aren't that reliable with the BMW sensors.

So from my research conclusions, I believe the best route is just get the catless downpipe with the JB4, and swap it out after 7 years (atleast here in CA) when the first smog check is required. Every other option either will be left to chance that the cat still works well with the sensors or the smog shop guy is lax or incompetent at his job that day.

That goes along with hopes that nothing else in the vehicle doesn't start to deteriorate before that time which may require additional maintenance. From my experience, about that time would be when the EVAP sensors and lines start to go as well as the oil pressure sensor or pump. Then again, I drive probably a bit more than your average person, putting easily 20k miles on any given car every year. YMMV.

[G-Mann]

Originally Posted by Dattebayo

If you have JB4 clear CEL, it won't pass either as their OBDII scan won't read. They will continually get the message that the OBDII isn't ready. The JB4 may help with the visual inspection of the car, but it won't do anything in regards to the OBDII check, as all it is really doing is constantly reset the computer to remove the engine code and block OBDII readers. In all honesty, from what I have read and researched, in the sense of smog checks, really the only way is replacing the stock downpipe and turning off the JB4.

.

Interesting info. so with JB4 itself you must go map 0 or remove for inspection.

[zipphreak]

Yah that's good info, safer anyways to replace DP with OEM and pull fuse to jb4.

[Dattebayo]

Originally Posted by G-Mann

Interesting info. so with JB4 itself you must go map 0 or remove for inspection.

Yeah, you can just run map 0 and it should be kosher, albeit, they could potentially nick you from visual inspection if they want to be a PITA. When you do run map 0 after replacing the downpipe though, be sure to run the car for a bit as the OBDII will need to re-calibrate which is why with JB4 on, will cause the reader to say the computer is not ready. Once the OBDII is calibrated, it should properly give out any CEL lights.

This is also the same reason why you cannot reset your CEL using a OBDII reader and go in for a smog. It will give the same outcome as the JB4. The smog shop will either let the engine run idle for a while, or tell you to pick up the car and come back tomorrow or later in the day with it.

This is also why JB4 doesn't work well with P3 gauge. Due to the fact P3 takes their data out of the OBDII socket, it won't be able to get a read if JB4 is constantly resetting the computer, causing it to kick out the computer is not ready, which gives the P3 no information to display on the gauge.

[zipphreak]

It's too bad the P3cars gauge becomes useless with the JB4, but i did read i can run a wire to the MAP and then i can atleast have it as a boost gauge which is fine with me. But i lose my precious oil temp and coolant gauge