Helmholz Midpipe: 4” or 5” Cat?

[RBNetEngr]

I am almost ready to pull the trigger on a Helmholz midpipe for my 2008 GT PRHT 6MT. It currently has a Roadstersport SuperQ installed, and I purchased the 1.8” Roadstersport Max Power catless header during the 2023 Black Friday Sale. I was waiting to get the midpipe before I install the header.

When I added the midpipe to my Wish List I noticed that it now says 5” Cat for Manual, 4” Cat for Automatic. Given I have a 6MT, and the 5” Cat adds $50 to the price, would the larger Cat be a better option?

Are there any fitment or clearance issues with it? Is installation more difficult?

And finally, what would be recommended for a tune after the full header, midpipe and exhaust is installed, to get the most benefit from the larger exhaust?

The car is 99% street use, with a possible HPDE maybe once per year.

[Brian]

The 5 inch cat is massively better in every way. IT flows better, it will last longer, and the sound is much better with the 5 inch. The ONLY reason we still have the 4 inch cat is for customers with auto transmission because that box is so much wider that it can be hard to get the 5 inch neatly fit for those with auto. If you have the manual transmission, the choice is a no-brainer.

[RBNetEngr]

Thank you, Brian. I will be ordering it very soon.

What about suggestions for a tune after the new exhaust system is installed?

[Brian]

For tunes have a look at Xero Limit and Fab9.

https://www.xero-limit.com/mx5-miata/nc.html

[RBNetEngr]

Thanks again, Brian. I really appreciate the information, as well as your responsiveness to questions on this forum.