Miata Turbo Forum - Boost cars, acquire cats.

- Engine Performance (https://www.miataturbo.net/engine-performance-56/)

- - ACL RACE Bearings (https://www.miataturbo.net/engine-performance-56/acl-race-bearings-93567/)

ACL RACE Bearings

Hey, I'm one of the many lurkers on miata turbo.net

I'm currently looking for info on how i should build my engine, mainly looking for info regarding bearing clearances for a high revving turbo build up to 400whp range but limit torque to 300-330ish because 6 speed, Unfortunately Australia doesn't have many high powered Mx5 builds nor the engine builders knowledge of BP engine's that others in the states do because of sheer numbers.

The car's use is hill climb's / Targa rally and circuit so I'm after a safe rev limit of say 8200 not so much for power more for early downshifting accidents / over revving or ( not wanting to change gears )

This is my 1st forged piston engine build to this degree, I've been playing with rotary's for the past 11 years. ( and loving how simple these little engines are )

How its been explained to me is that a high revving turbo motor likes looser tolerances and higher oil pressure and oil flow, I have an external oil cooler and relocated oil filter so my thought was a Boundary pump High flow with 2 extra shims (72psi) to help with pressure drop for the larger oil system, I will also be keeping the oil squirter's.

So back to my original question, My crank measures within Standard wear tolerance with only (.0015 - .0018) difference 100k km or (60k miles) Use, no damage or marks on the journals, caps or bores , ACL have the STD X in .001 Undersized but my question is should i polish the journals for a more loose fit .0020-.0025 ? and have it even across all journals ? or just run with the standard size ACL X bearings.

Cheers

I'll venture addressing your question as far as I know based on personal experience. If your mains are standard size, get standard sized bearings and only a micro-polish on the crank. This has worked for me. Good luck.

When i built my engine (again) I had a rub in one of my crank bearing surfaces from when my oil pump failed. I ended up with the undersized bearings and the machinist kept the stock clearance, but towards the high. I was just in my engine (after only about 100 miles and a few dyno pulls) for more piston to wall clearance for higher boost other shit and it all looked good. I would think keeping everything as close to even across all bearings will help more so. I usually just ask my engine builder and go with what he says. Crank clearance is crank clearance, a good builder should be able to give you decent guidance there (oil pressure, oil weight, engine use). Generally from what I know alittle extra clearance helps in high HP motors.