during the last two weeks i disassembled two FD motors. both motors were in pretty good shape however:

both owners are faced w buying two new rotor housings...





the "WHY" is simple... they were running the OE spec spark plugs. 9 and 7 heat range spark plugs should not be in your engine. this is the inevitable result:




i realize this is not news to many, but it was news to these owners so let's put it out there one more time.

run 10, or colder, heat range plugs in all four holes.

i happened to be talking to Dr Iannetti a few days ago and asked him what was the primary result of cracked spark plug bosses and he said that they eventually connect the compression with the coolant. both motors had coolant problems and neither had significant tells on their coolant rings. all four rotors spec'd out perfectly as to parallel and general surface wear but all four will not be going back into the motors.

in addition to leading to eventual coolant loss, notice the flanking lateral carbon on the housings. the spring loaded apex seal is being lifted off the rotor housing by spark plug mountain(SPM). all the relatively cool compressed intake charge is in communication with the rotor face that has previously ignited and is driving out the exhaust port and powering the turbine wheel. way more compression in the forward rotor face than the following intake face. intake pollution and moving the needle towards detonation.

SPM pushes the apex in and starts a repeating oscillation that results in CHATTER marks on the housing. take a good look at any used housing around 11 o'clock and you will see lateral chatter lines.

lower SPM and reducer chatter.

what's in your wallet? what's in your motor plugwise?

another quickie while i have your ear... if you smell gas on your dipstick, probably, you have leaking fuel injectors. you'd better fix it or:

gas just isn't as good a lubricant as oil. the fix isn't changing your oil more often. the fix is fix the fuel leak. and, yes, the other bearing looked similar.

 

what is the actual difference between the "front" and "rear" rotor housing?

 

My engine builder told me the front and rear housing differences are for the stock twins, if you're running single then you can just get 2 fronts and save some money (hopefully he was right because that's what he had me do, lol)

 

I don't even think it's necessarily a 'stock twin' difference as much as a 'stock' difference.

There's an orientation difference on the EGR ports between the housings. But if you're running full emissions delete, it won't matter.

When Chris rebuilt my engine and I wanted new housings he asked if I intended to delete EGR. I suspect this is why. If I did (and I did), he got to buy 2 front housings and saved money on the rebuild.

As for Howard's original point, I can't say that I've heard of it being common to run 10+ plugs unless you're pushing, like, 450+hp. I'm not opposed to the idea though and will give it a shot once the engine is back in.

 

Hmmm, interesting.
Ive ran my engine with the stock spec plugs forever. Never had an issue. Mind you, I’m not making over 450HP nor do I run emission hardware. She’s been running like this for over 2 decades now.
I guess it’s how you use it and how you put it away??

I’ve always heard if you’re going to run the car track wise, then yeah, gotta run 10’s. Since you’ll be running it to redline most of the time. However, for a street use vehicle, I doubt the OEM spec plugs would do any harm.

 

it is the "Port Air" passage, Mazda only drills it on one side so they face the center iron. it is possible to just use a front housing and drill a hole on the correct side of the one going in the rear
i know at this point keeping the air injection and "Port Air" passages seems silly, but it will never pass an emissions test without them, and rebuilding the engine just to pass an emission test is annoying.
if anyone remembers a yellow FD that Pettit did in the early 2000's, it came to CA, and it needed an engine to pass smog, dude was not happy.

the mystery is why there is such a big price difference, if you look at prices in Japan, it is not as big of a difference as it is in the US

 

Thanks for the info. The big price difference seems to be more recent. I had to buy new housings last June and the price was difference was about $100 (the front housing was about the same cost as shown above).

 

oh pics of said passage.

 

JDM manifold clean casting, no port. USDM has the EGR pickup port (circled). Same story as the rotor housings: the front and rear housings differ because Mazda only drills the Port Air passage on one side You need the matching manifold too, without that pickup port, the EGR system has nothing to feed from.

 

Can someone explain to me what's wrong with these rotor housings? I feel like I am missing something.

 

Both housings have demonstrable "Spark Plug Mountain" where the overly hot temperature around the spark plug causes the steel to expand. And it expands VERY slightly 'upwards' and puts more pressure against the apex seal. This causes that wear area you can see where the area right around the plug, where temps are highest, is wiped 'clean' because it contacted the apex seal while the areas to the sides have a small amount of carbon build-up. And Howard is positing that, subsequently, that contact causes the apex seal to move and 'chatter' against the housing elsewhere as the springs do what springs do and try to move the apex seal back into proper position. Colder plugs maintain cooler temperatures around the plug boss so should mitigate this to some degree.

Harder to see, but I think Howard is also referring to what appears to be hairline cracks emanating from the plug boss about the 12'oclock position on both images. At some point if these propagate far/deep enough, this could allow combustion pressure and gases to enter the coolant chambers (or vice versa)

 

Obviously cold plugs is the first go to, I know some guys like Barry Bordes and maybe Lynn chamfered the leading hole further and even added a flame path to rotor tub, has anyone done relief cuts before the working threads in the plug hole?

 

I've never seen bearings that.... Um.... Terrible.