It Could be the cat but I think you would have other problems.
Get your bypass valve checked out this will cause low power normally the thing that fails.
Check your intercooler boots make shore there not leaking or split.
I'll get the items checked that you mention.
I dont think there are any mods, certainly the guy i bought from didnt mention. It was services recently, but not a BMW place, a local garage. I'll get it checked out by a decent mechanic.
The rattling at idle is most probably your clutch but to check this just push the clutch pedal down see if it goes.
Also a thing to check is you supercharger belt is ok, you can check this quickly by looking at the tensor locking holes you should be able to see the top of the top hole. If you can't see the hole you should replace the belt
if it's slipping you will have low boost = lower power see picture below.
Get your air and fuel filter changed first if you don't know the history of them. Also the spark plugs. Basically a proper service. Get the easy, cheaper stuff ticked off the list before you start delving deeper.
Also I wouldn't read too much into your 0-60 time. To get anywhere near 7.7 sec time you are going to have to really hammer it, like dropping the clutch at high revs and giving it zero mechanical sympathy.
Sounds like my car when I first got it! Bypass valve !! My car wasn't modded when I first got it and the bypass valve wasn't closing or evening opening fully when I checked it! It ain't hard to get out either, plenty of walk throughs
You can check it befor removing it. If you know where it is you can get your hand down there and push the spring type thing and see if it goes in and springs right back out.
About a hour work all in all I'd say,
BTW, given that the engine is supposed to be around 160bhp, what kind of effect do you think a bad set of IC boots and/or bypass valve could have on power? I honestly think i'm down by around 30-40% of power. Is it possible to have some much loss from these items?
Not sure about the boots but bypass valve I had about 30 to 40%
The differance when changed was amazing.
My car didn't really go past 90 mph
But it was really bad damaged.
How is the car in acceleration ?
Get the intercooloer off and then the throttle body and bypas valve. It's very easy to do if your okay with a DIY
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