It's been a while since I've installed the exhaust, but I wanted to write this to help anyone else with a MK2.5 (which seems to be a minority vs people with MK1s) that might have been able to snag up a Cobalt backbox from Moss Europe but also needed to change the midpipe as well.
In my situation, the flanges between the original backbox and midpipe had severely rusted to the point where there was a large hole. This would have failed my MOT, so I started looking for possible fixes.
I managed to pick up the Cobalt single exit backbox for £123, and then I opted for the MX5Parts midpipe to complete the replacement. To Cobalt backbox came with an O crush ring gasket, while the MX5Parts midpipe came with 2 flat universal gaskets, as well as 4 bolts and 4 nuts to bolt the system together. The other stuff I bought to fit it was all purpose grease (I should have gotten copper slip but was too lazy), exhaust sealant, and a can of WD-40 Fast Release Penetrant (there probably isn't any difference compared to normal WD-40, but oh well). I also bought a full set of Powerflex mounts which apparently solves the problem of the aftermarket exhaust system knocking on the diff.
This was the first mod that I have ever done to a car, and it was surprisingly easy. With the massive help of the forum post on MX5Nutz, I was able to complete this in just over 3 hours. The post is comprehensive enough for me to follow, bar a few variations to how I completed my exhaust swap.
- We hired a bay in a self service garage. We used their air powered impact wrench (rattle gun), which made undoing the bolts from the cat and chassis brace super easy. Plenty of WD-40 beforehand probably helped.
- When it came to removing the exhaust, we used a flat head screwdriver to stick into the old rubber mounts and using a prying motion to force them off the car's mounting points. Again, plenty of WD-40.
- If possible, try to remove the Lambda sensor (O2 sensor) from the midpipe ASAP. We unbolted the whole system as shown in the above picture, and then realised that the sensor needed to be unbolted as well. WD-40 and a 22mm wrench with a good length was enough for us to unscrew it.
- The Cobalt backbox had no fitting issues with the MX5Parts midpipe. The backbox flange looks idential to the MX5Parts backbox, and there was no issues with compatibility.
- We didn't come across any issues with how the MX5parts hanger washer was positioned. the powerflex mounts are quite narrow in comparison to the stock ones. We pushed all the mounts as far right as possible to force the exhaust to sit away from the differential housing.
- We used plenty of exhaust sealant on the gaskets and flanges, test fitted, ran the engine to check for leaks, and then re-tightened the bolts.
And here's the system fitted before the O2 sensor secured and chassis brace reffitted.
Hopefully these few pointers can help people with their own installs!
I have since driven around 150 miles in London, as well as 400 miles on the motorway. The exhaust note drones a bit at 3,000rpm 5th gear 60mph, but anything below or above 3,000rpm and the sound is not very obtrusive. Normal city driving changing gears every 10mph, the exhaust is very discreet, but once the throttle opens up it adds a nice classic sports car sound. Deceleration and downshifts have a nice burble to it. Overall, I think the combination gives the car a more sporty characteristic to the sound without being very chavy.
