So I am going to be rebuilding a motor for a surprise for my mother and her rx-8 since it has been sitting for 4+ years and they have a hard time paying for a new motor or getting one rebuilt. I was able to get an almost perfect engine (only rear iron is out of spec from oil control rings scraping, assume from the rear bearing going out as it looks rough, haven't measured it though. replacing regardless) for a fair price and a rebuild kit and ready to get it back together.

I know that the exhaust ports in particular are prone to carbon buildup and am curious if there is a recommendation aside from polishing them? I know there are porting templates but from what I read there isn't much to be gained so I would rather try to polish them the best I can and MAYBE open them up just a tad sooner?

I guess my main question is buying a template for 63-80 dollars worth porting or should I just polish and close this thing up?

 

Exhaust ports are a complete waste of time imo. Carbon buildup is more forum fairy tale/misdiagnosis than reality imo. Putting crap chemicals through it playing mad scientist or burning oil excessively are the more likely scenarios.

on the intake it’s a dangerous game doing anything more than cleaning up and smoothing rough spots. The chances of doing more harm than good with the original resonance design is quite high. I took a chance trying a few things, but haven’t run it yet to see if it’s zoom or bust. Not looking forward to swapping out the LIM if it’s a bust.
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I doubt carbon build-up on the port is a significant issue compared to carbon build-up locking the side/apex seals.

 

There isn't enough 'meat' on the irons around the exhaust ports to warrant porting. You can clean up the closing edge to reduce carbon collection, but even that isn't a major concern. The factory 13B-MSP intake ports are essentially what would have been an aggressive street port on previous 13B-RE engines.

There have been no proven gains to porting the Renesis stock location ports.

Save the money for maintenance items which will yield improved drivabiltiy and reliability:

-Test/replace as-needed oil injectors
-Replace the OMP lines
-Send fuel injectors off for cleaning
-Clean and service all intake valves
-Test/replace as-needed the intake control solenoids
-Test and replace as-needed intake vacuum lines
-Test and replace as needed intake coupler/accordion
-Use Mazda OE engine gasket set if not already included with motor purchase
-Replace all coolant hoses with Mazda OEM lines (if not done in the last five years)
-Replace coolant expansion bottle with Mazda OEM bottle (if not done in the last five years)
-Replace radiator with Mazda OEM radiator (if showing damage or not done in the last five years) -- ideally, replace with a quality all aluminum radiator, such as CSF.
-Use Mazda OEM pre-mixed FL22 coolant (Not to be confused with traditional green coolant)
-Inspect and replace/repair/reinforce as needed the radiator ducting to assure air is fed through the cooling unit instead of around it.
-Inspect and replace as-necessary the factory catalytic converter pipe (big ticket item, but it is best to remain emissions compliant)
-Replace oxygen sensors (if older than 10 years / 100,000 miles) -- Generally not needed unless a code is present.
-Replace MAF sensor with Mazda/Denso (if older than 10 years / 100,000 miles) -- Generally not needed, but is relatively cheap insurance to drivabiltiy concerns.
-Replace ignition coils if Mazda revision A or B units. If revision C, consider replacement if they have been in use for 30,000 miles. GM/AC Delco/BHR coils are an option, if you have an ECM remap to adjust dwell.
-Replace plug wires with Mazda OEM or MSD/BHR, and spark plugs with OEM spec.
-Consider an external oil injection reservoir, like the Epitroch/SOHN kit. -- Also pre-mix 0.5oz per gallon of fuel is sufficient for a road-going car and should not impact the fuel injectors or catalyst to any significant amount.
-Replace clutch assembly with Mazda OEM/Exedy and all hydraulic fluids (If automatic, ignore, but inspect torque converter for visual damage or rattling)
-Replace all driveline fluids

Having sat for a few years, I'd be critical of:
-Brake fluid
-Rubber bushings
-Balljoints
-Motor mounts
-Transmission and differential seals
-Axle boots
-Wheel bearings
-Fuel remaining in the fuel tank (and in-tank fuel pump and filter)

 

Nobody mentioned smoothing out the ledge at trailing end of the exhaust port . That is where the carbon builds up and can actually choke up that entire end of the port.

 

Irrelevant imo and only there for EGR carryover purposes. It’s too shallow and narrow to provide any performance flow potential.

 

Wasn't mentioning it for performance reasons .... but it's silly not to do it and avoid that big slug of carbon from forming.

 

I chose to port & polish my exhaust ports (see below) going into a turbo application. FWIW on a NA rebuild...probably would just "clean up" the poor exhaust port corner castings & runner transitions on irons going into a NA application.

Edit: Btw... During my tear down & rebuild I did find chunks of carbon deposits in the narrow upper trailing corners of some of my exhaust ports.


OEM Exhaust port ...poor corner castings

Post porting & polishing...1

Post porting & polishing...2

 

Well my only point is that it really doesn’t matter one way or another. Have seen quite a few high mileage teardowns where it wasn’t even that bad.