[n00b]

Quote: Originally Posted by IML337

yeah that's what I thought about the hitech system I had. The dyno certainly proved me wrong! Interested to see if you actually dyno'd yours before and after?

Same goes for the Miltek, didn't some Canberra dude (AndrewH?) lost 10kw or something?

[Off It's Guts S]

Yeah i did actually. Car is proven on the dyno at 193hp compared to 163hp stock. Other mods are 15% pulley and GIAC remap. CAI and the cat back exhaust. (plugs & hotwires, but there not giving anymore power)

[St00ge]

haha, wa%ker factor. I like that.

So micky you don't think it's a good idea? Surely a cat would be restrictive though? I can't understand why you wouldn't gain any power, but i'll take your word.

Yeh i think it's a 10k fine, but i also heard that you get a warning first.

[MickyD]

I don't think it's a good idea at all, apart from the environmental considerations and the risk of a fine, there are sensors both before and after the cat, and it comes back to my oft-repeated point that I'm sure BMW probably has half a clue about what it is doing when it designs a car, and those two sensors are there for a very good reason.

[n00b]

Try take the Borla off and slot the stock exhaust back on then see, you might be surprised.

And Elie, you get no warning. The chances of them catching you isn't high but when they do, mate you better pull your pants down and ready to be reamed. Also your vehicle will be on the 'vehicle of interest' list (yes there's such a list...)

[St00ge]

Quote: Originally Posted by n00b

And Elie, you get no warning. The chances of them catching you isn't high but when they do, mate you better pull your pants down and ready to be reamed. Also your vehicle will be on the 'vehicle of interest' list (yes there's such a list...)

i bet there is. I love the way you put things Evan. So convincing.

Hey micky, MTH can disable my cat's O2 sensor though...but anyway maybe you're right. I just cringe every time i look at the stock headers with there pre-cat and cat, not to mention the restrictive design. So do you think a high flow cat won't make any difference compared to my cat?

[Off It's Guts S]

I highly doubt that the standard exhaust is better or even on a even playing field than a proper after market exhaust. You just have to look at the standard exhaust and see how restrictive it is to understand that.. Look at even the JCW exhaust. Why would they change the exhaust to a less restricted exhaust if it was to give no power gain at all?? Or better still allow the car to "breath" better.

[n00b]

Quote: Originally Posted by etalj

So do you think a high flow cat won't make any difference compared to my cat?

Stop talking to your Skyline/Supra mates might be a good start the stock setup isn't as restrictive as one may think but yes improvement can be made. No one says a high flow cat won't make any difference but how big will the gains be? Unless you're chasing every last bit of hp/kw with a huge bank balance then why bother

Quote:

Why would they change the exhaust to a less restricted exhaust if it was to give no power gain at all??

Couple of things I've noticed over the years - how many people out there will dyno their car before and after an exhaust is fitted? How easy is it to rig a dyno? Let psychology into this and a lot of people thinks an aftermarket exhaust will be faster than the stock one? Sure some of these exhaust do gain a few hp's up the top end but I'd rather lose 5hp up the top end and gain 10hp between 3-4k rpm.

[St00ge]

Quote: Originally Posted by n00b

Stop talking to your Skyline/Supra mates might be a good start the stock setup isn't as restrictive as one may think but yes improvement can be made. No one says a high flow cat won't make any difference but how big will the gains be? Unless you're chasing every last bit of hp/kw with a huge bank balance then why bother

ha yeh, point taken. I definately don't have a big bank balance. And anyway, for now, no more engine mods (apart from repairs ). Need money

[MiniFr34k]

i thought it was a 7k fine on the spot? EPA?

Heres a tip ive learned from supra,nissan,wrx mates.

Drill a couple holes in the cat. No not legal. This will make ur exhaust sound full h3ktic brooOO (ive heard it personally). EPA wont detect this LOL (probably could but nfi if they will).

Cant say about HP gains, no one bothers with before and after.

[MiniFr34k]

Personally im a believer of highflow cats (when matched with highflow headers, catback).

No, its not about chasing every possible HP, but moreso allowing more flow and for car to 'breath' better. The overall result should lead to increase in HP (but meh, who knows if dynos agree.)

Trying to make the car 'breath' better is why i go for things like a CAI system as opposed to changing a 'high flow' hose only (and not the filter). The point is trying to free up as much bottle necks in the production line as possible.

That being said, i cant comment on how good the stock cat (+ pre cat is). Cars like S2000, and probably the Type S, the cats are so dam good theres no point changing them. But for us mini's ? Someone else can comment on that.

On NAM theres this massive long thread about the OBX header with stock cat, and probably Dynos in that thread, this might give a good indication of how worthy the stock cat is, when combined with some good headers. Probably a good cheap option unless u can workout a deal with a vendor to get some headers cheap

Cheers.

ps. Off Its Guts, can you email me when you get your headers? i really want to see what the Borla sounds like when combined with some headers.

[IML337]

Quote: Originally Posted by Off It's Guts S

I highly doubt that the standard exhaust is better or even on a even playing field than a proper after market exhaust. You just have to look at the standard exhaust and see how restrictive it is to understand that.. Look at even the JCW exhaust. Why would they change the exhaust to a less restricted exhaust if it was to give no power gain at all?? Or better still allow the car to "breath" better.

Dude, that's not really true. We all thought the standard exhaust was a piece of junk that was far too restrictive... In fact I remember being horrified when I first had my S on a hoist. This was until we played with aftermarket systems on dynos, and realised that the BMW engineers actually know what they're doing, and the engine in this car benefits from backpressure.

It was all ages ago now, but some dude did a test with one of those exhaust systems where youput the discs in the tailpipe to modify the level of backpressure... and he saw the most power at almost the highest backpressure setting.

The reality is, out of your 30bhp peak power gain, the 15% pulley is providing about 25-27bhp gain, and the CAI/remap/exhaust is providing the extra few bhp left.

[MiniFr34k]

Quote: Originally Posted by IML337

The reality is, out of your 30bhp peak power gain, the 15% pulley is providing about 25-27bhp gain, and the CAI/remap/exhaust is providing the extra few bhp left.

Couldnt agree more.

EDIT: Unless u go twincharge - right Evan?

[n00b]

Twincharge only makes its power from the blow off valve

[dsakko]

LOVE the discussions here. Some classic comments!

Watch your O2 sensors. You will need to rig the signals if you remove the CAT (I think JCW put out a kit in one of their latest packages to do that ???). Your car WILL register a fault, and might even drop into a limp home mode, if it detects your CAT ain't working!

And the other thing with the Siemens computers is that they are adaptive. Takes about a month to adapt to a free flowing exhaust before you get max HP. But after that it's good stuff.

Frequently use my rev limiter (not intentionally) cause it revs so much more freely over 5k. Driveability is a major plus even if HP figures aren't all that big.

[Baker]

Quick note before I head off to work - stock cat is fantastic, no gains at all from swapping it.

[MiniFr34k]

Quote: Originally Posted by Baker

Quick note before I head off to work - stock cat is fantastic, no gains at all from swapping it.

Well there you have it folks, stock cats are great

[Off It's Guts S]

Will do Pete. Should be in within 2 weeks.

[Tit]

In almost every conversation about exhausts that I've read or heard, backpressure is forgotten - thanks Tim, for bringing us back to reality...

Surely BMW has tuned the length and shape of the stock exhaust to the headers and engine exhaust so that it works as efficiently as it could. Not trusting BMW with after-sales service and support is understandable, but not trusting them to design a car mechanically well is another thing.

Sure, a phatter exhaust may give you more power, but how @#$#^ useless is an extra 3 kW at 7000rpm? C'man - the action is a lot lower in the rev range.....

Of course, I've expressed my opinion on this before; being that if you're after a cat back on your lightly modified street car for costmetic and aural reasons - then you're on the right track. There's no shame in that - just stop being a sheep and admit it!

[n00b]

Quote: Originally Posted by Tit

Sure, a phatter exhaust may give you more power, but how @#$#^ useless is an extra 3 kW at 7000rpm? C'man - the action is a lot lower in the rev range.....

That's exactly what I've said earlier on....