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MG MGF Technical - Exhaust Tuning??!!.

Am having a "Sports" exhaust+de-cat fitted in the next week or so, so a couple of questions if anyone can advise?.

Down the years have had many "sports" exhausts fitted to my bikes, but never cars. On the bikes was always told that as the exhaust gases are exiting quicker (less restriction)with a race can, the fueling needs to be re-mapped to compensate for this otherwise the engine will run "lean" and over time will cause damage.

To get the full/safe benefit from the exhaust will the fueling compensate automatically (if required) or will the car need to be set up on a tuner/test book etc?.

I know most people dont bother, and just fit the new exhausts, but would imagine that the car was set to run with original standard exhaust, so a new "easier breathing" exhaust must have some effect on the running/tuning!.

The ECU will adjust for the small amount required. However I would recommend going for a sports cat rather than a no-cat pipe as the exhaust is then fit and forget and no problem at MOT time. Also I am sure you lose power with a no-cat pipe (see Rob Bell's site).
Blue Pocket Rocket

A sport exhaust will give you a little bit of a performance increase (2-3bhp) and the decat pipe really only gives benefit in high powered engines nearer 180-200 bhp, however the main restriction in the sytem is the convoluted exhaust tract, a 4-2-1 exhaust manifold will give better breathing if you really want to release a few more horses and torque, but without going down the expense of a new manifold you could remove the internal welds from inside the existing exhaust manifold this will give a noticable difference.

I assume that you already have a decent air filter, this is a mod which really will give an increase in performance.

tim woolcott

Yup, as the guys say Mark, the exhaust is "fit and forget" ;o)

IMO the cat bypass is fairly pointless, but if you're after the noise, then go for it baby, yeah!

Hmm, not sure where the Austin Powers reference comes in?!? Better have some more coffee.
Rob Bell


Can you give me some informations about the removing of the internal welds from the existing exhaust manifold?


Techspeed did this job on my exhaust manifold. The job, in principle, is very simple Titou: you use a Tungsten Carbide burr mounted in a decent quality drill, and grind the internal welds off, so maintaining the same internal diameter all the way down the length of the manifold pipe.

In practice, this is noisy, messy work that requires some skill in not removing too much weld material that the manifold joints fall apart - for the latter reason, many add an additional circumferential weld on the outside of each joint for additional strength.

Rob Bell

Rob have 2 but add some Kahlua - just what the doctor ordered !!! Just thought I would add that you get enough noise with a sports cat, so much so that future trackdays are in serious doubt!
Blue Pocket Rocket

So if I have well understanding, the removing of internal welds of the manifold is better than the fitting of a new exhaust if I am looking for performance increasing.

Honest answer to that Titou is that I haven't done the testing to find that answer out! I *think* that a sports exhaust would be better than a fettled standard exhaust manifold though, but a pukka 4-2-1 manifold would be better still (especially when used in conjunction with a decent sports exhaust)

>> Just thought I would add that you get enough noise with a sports cat, so much so that future trackdays are in serious doubt! <<

Uh oh! How loud is yours now? Most events are good for up to around 100 dB
Rob Bell

Last time I was at Castle Combe the level was 99db. Dave Morris was behind me on the grid and thought I wouldn't pass the test! With the sports cat it's now a lot louder! Will have to get it tested at the next action day at Combe.
Blue Pocket Rocket

>In practice, this is noisy, messy work

its not so bad if you use a die grinder. Be aware that tungstant carbide bits leave a million metal splinters which will penetrate your skin at the slightest oppertunity (as my fingers and thumb show!).

The trick is not just to remove the welds, it is to create a smooth surface from the port to the pipe.

First start by making a card 'manifold', and hang it on the studs, then push (just enough to make an impression) thru the card onto the ports on the head.
Now cut out the impressions with a sharp knife
Fit the card to the manifold, everywhere there is a overlapping lip, mark it with OHP pen (you may have to clean the surface first!)
remove the card and use the die grinder to remove the marked areas and NO MORE, smooth the taper down to the pipe.

This has the oppertunity of making bigger savings and is virtually free.
If you remove more metal than is required then you will actually make matters worse as the lip you create will add turbulance - BUT a lip which is bigger on the exhaust side is better than a overhang into the gas flow.

Will Munns

This thread was discussed between 21/03/2005 and 23/03/2005

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