Hi guys, my rx8 finally blew its motor yesterday while driving on the freeway. Upon checking the dipstick; it was filled with metal shards and ✨ glittery oil. My question is if it’s better to have my mechanic drop in a JDM low mileage rx8 motor? I bought the car at 80k. The paperwork showed that the previous owner installed a JDM low mileage motor at about 70k. Doing some quick estimated numbers, the jdm motor made it to about 70-80k using the average 30-40k most jdm motors advertise. The car currently has 110,000 miles on the odometer.

 

Sorry to hear that. Wherever you get your motor, confirm compression. JDM and low mileage can still be low compression or other damage. Best solution is to have a fresh known-good build from a rebuilder, but you could probably buy several questionable JDM engines for the price of a rebuilt

 

do it right or twice. get a proper rebuild or mazda reman if you can find find one

 

You didn't exactly 'blow your motor' via the conventional rotary mode of failure-- for turbo engines (FD RX-7) it's typically detonation (broken apex seals and damaged rotors/rotor housings/turbine wheel(s)) and for NA rotaries it's typically low compression due to apex seal to rotor housing wear from age/mileage. All rotaries can have coolant seal issues after an overheating event.

You my friend have experienced the absolute worst and most expensive mode of rotary engine failure-- appears you most likely spun a bearing (probably stationary gear bearing, probably the front) due to lack of lubrication.

Had you checked your oil level recently? How many miles were on the most recent oil change?

you're going to most likely need an entirely new block, and you 100% need to remove the oil cooler(s) and have them professionally flushed along with all the ancillary oil lines. If you don't do this you're going to lunch your next engine as well.

 

Hi Goodfella, yes - I check it regularly and it was at about a 1/4 quart low when I checked it. I agree with you l; unfortunately seeing the metal shards and glittery oil made me realize the engine damage was severe. Some background on the engine and when it showed signs of issues. 2 years ago my uncle bought a used compression tester and we tested his car and mine. My engine came in at around 78-83 on both rotors. His 12a came in low as well so we assumed maybe the tester was bad. This Being that both of our cars were running great with no issues. I’ve driven the car as a daily for the last 2 years (15-20k)


. A half a year ago I bought a package deal locally that came with the 1st gen BHR coils and BHR motor mounts among other things. I installed them along with new spark plugs. I again checked compression during this install and my numbers came in lower this time. Front came in lower (60s) and rear rotor was similar as last time around. The car again drove fine with a better and faster startup with the BHR coils. It wasn’t until about a month ago I started experiencing hot start issues. The car would start when hot but just took more cranks than I was used to.

 

Sucks! I don’t know an exact number but I have read we should all consider rebuilding engines that we use regularly when the numbers reach below the 80’s for compression. It’s not necessary to have them rebuilt right away because even a low compression motor will run great. I have seen guys with original first engines in a series 1 run well past 100k. When my first engine went in my rx8 2020 I bought a reman from Mazda and it’s been pretty good now has close to 40k on it. Good luck and do the work yourself save some $$$

 

Yeah, the engine is not rebuildable and I will be making sure to flush out the oil coolers and lines to ensure there's no issues when the JDM motor gets installed.

 

I would not reuse those oil coolers and lines.

 

They are super expensive though whether you go with OEM or aftermarket ones. The goal was to take it to the local radiator shop for a full boil-out of the lines and coolers along with repairing the fins.

 

There isn't really an exact number, but as the apex seals get shorter the forces acting on the slot in the rotor start to increase exponentially. Usually most Rx8s get hotstarts around 120-130k miles. Ive only taken apart one 80k mile engine which looked considerably better rotor wise. I never kept good track of measurements though. At the price of rotors chances are you could find a beater jeep or something cheap to buy in the meantime while you rebuild early lol.

I did the math out and made a chart comparing the Rx8 vs Rx7 depth apex seals a while ago. From this you can see that the shorter seals get exponential in forces applied to the slot. For the rx8 mazda probably balanced out whether they wanted more forces+damage on the rotor compared to more wear centrifugal force on the housing from the heavier seals. The upper two lines are comparing the force seen at the apex slot tip. The lower two lines are the forces seen where the lower part of the apex seal levers down inside the slot. Protrusion is how much of the seal is exposed from the slot(this changes as the rotor spins around, 2mm is roughly at ignition timing if I remember right). You can get a general idea though by looking at a 2mm protrusion comparing the 5.3mm tall rx8 seal vs the 8.5mm tall rx7 seal. Just imagine how much the forces increase if the rx8 seal is half its height. Even comparing the 2mm protrusion the rx8 height seal sees roughly 30% more force.

 

I would think the important force for apex seal wear is the component normal to the rotor surface, not the lateral compression force. With a 2mm exposed seal face, you have 2/3rds of the lateral force of a 3mm exposed seal. Plus RX-7 turbos produce more chamber pressure at boost.

From the years of failed engine anecdotes here, the side seals is what gets you. Apex seals blow by into the next chamber, side seals blow by into the pan and you get oil barfing.

I wouldn't preemptively rebuild an engine with its own parts. Build one out of known good parts or buy a known-good one.

 

Yes for apex seal wear you are correct. What I was talking about was the prying forces wearing the rotor slots increasing the shorter and shorter the apex seals get.