Greetings gentlemen,

My Mark II (sorry Mk 2) exhaust system (downpipes & mufflers) are shot.

I have a XJ6 4.2 lump which I am considering fitting later on, so the question is, what size pipes would be required given engine capacity may go up from 3.4 to 4.2 litres.

I am thinking 1 & 7/8" would be the go.

Standard pipes ( complete with rust) seem to be around 1 & 5/8" at the tail.

Any suggestions welcome!

 

I cant comment on what specifics the Jags have been using, but both my E34 BMWs have been running a twin 2" system from factory, being a 3.5L straight 6 and a 4L V8.


Both have very large and heavy cats, resonators and box mufflers.


There is calculations out there if you know your engines power/torque and max RPM limit.


Larger diameter is better for higher RPM use, less restriction up high, but can also affect low rpm torque due to limited back pressure

 

I removed both exhaust systems in my XJ and S type, and then reinstalled brand new stock systems in both, all by myself with the cars on blocks, one piece at a time, but I never measured the diameter of the pipes.

Having said that, I believe the XJ pipes are indeed 1 and 7/8" diameter, and the S type's are 1 and 5/8", so you might have it correctly. However, the stainless steel Tips are narrower in the XJ 4.2 than they are in the S type 3.8, which contradicts the measurements of the pipes.

Here's a picture of the complete exhaust system for the S type before I installed it, and a picture of the European 'Y' pipe I installed in the XJ-6. (the European 'Y' pipe does not have the second catalityc converter found in US cars).

 

Many thanks for the advice. Think I will go with the 1 & 7/8", although to my disappointment , my local exhaust guy (early 20's) says "get with the times, we now use metric measurements"