Hi Guys,
1998 M52 owner here
Upgrades that can be donw that i know off
M50 Manifold
M50 (M50B25) manifold which is larger than the M52B28 manifold. Relatively easy to fit and will release 15-20bhp. budget for £150-300 for the manifold and fitting kit.
Some pipes and spacers are needed but kits are widely available online.
BBTB (Big Bore Throttle Body)
bolt on replacement available from a few suppliers, larger bored out TB with matching plate. shown to deliver a few more hp
Chipped and raised rev limit ecu (7000 rpm)
Not a massive increase in BHP as the dyno shows that the power is dropping off even at 6000rpm but gives more room to play with the revs
ASC TB delete
Deletion of the ASC throttle body (before the main TB) to smooth out airflow, adds a couple of BHP
CAMS
Lots of CAMs out there based off of the USA S52 cams that are the biggest power adders to these engines but will cost £500+
Intake
Induction kits can show power increases (mine did) but is a contentious subject. i sell these to forum members, the noise is ridiculously good!
Exhaust
Better flowing exhaust for more bhp
Underdrive pulleys
reduces parasitic losses
Rad fan delete
reduces parasitic losses
Lightened / single mass flywheels
reduces parasitic losses
see the power board here for some more
https://zroadster.org/threads/the-zedshed-power-board.20242/page-2#post-371083