Exhaust bore/width for 325i M20

Wed Aug 29, 2007 9:05 pm

Can anyone suggest a good bore/width for each pipe on mine. The standard bore is 44mm/1.75'' am i right? From i've seen a lot of off the shelf systems use the same width.
I'm getting a new custom exhaust system fitted right up to the manifold and in the future i'm probably going to be looking at other engine mods, like a 6 branch manifold, cam, chip etc. 190 bhp would be nice.
So would it be best to increase the bore slightly to allow for this, maybe 50mm, or is the standard bore fine?

Thanks in advance
Max

Wed Aug 29, 2007 9:23 pm

Max standard bore is 44mm yes.

The best M20 system out there was the Alpina system which was 50mm so with good silencers and this bore i would of thought it could help you in the grand scale of things

Wed Aug 29, 2007 10:14 pm

Thanks for your advice mate.

I thought it would be a good idea to increase it slightly, I don't think there's anything in between 44-50.

Max

Wed Aug 29, 2007 11:42 pm

well alpina used a cam and this exhaust with their 6 branch with a bespoke chip to get 190bhp from a normal 2.5 lump. So theres no reason why you couldn't get this if you went a very similar route. plus an aplina cam is only 268/268 which is still pretty mild compared to whats out there. 2.5 standard spec is something like 256/256 which is lawn mower spec

Thu Aug 30, 2007 12:48 pm

I have just finished cocking about with my exhaust...get yourself some digital calipers, 4.99 from lidl or screwfix are selling them in their sale for a bit more, my fettling has been a royal pain in the butt. If your doing a full system then you'll have nothing to worry about, but my desirer to retain the stainless back box and replace the middle section lead to a mine field of problems.

what i would say is measure IDs and ODs, Internal and Outer diameters, in several places before getting under the car and hacking the old system off.

Thu Aug 30, 2007 1:22 pm

Has anyone have more info in the Alpina system, it should not be hard to get a exhaust shop to do a replica. Would just need the dimensions and what size center and back box they used.
any ideas?