I got a blown up VVT engine from my friends. Plan is to use it for a motor build and/or parts mock up. But first I thought I'd conduct an autopsy on why this motor failed. It's a 1.8 VVT from their rallyX miata. Allegedly it had stalled a couple times during runs, and then completely died on course. Maybe it restarted once more to drive off course, or maybe they pulled it off, they don't really remember. Oil pan is full of glitter. The #2 rod bearing is the guy that quit, the bearing that remains there is basically paper thin now. The obvious theory is oil starvation, but nothing really looks discolored severely from heat so I'm not convinced. And while I've never personally had a VVT or any miata engine apart before, the piston faces look weird to me. Like the piston surface looks grainy, like a really bad sand casting or something. That's not normal, right? I think this thing detonated its brains out, the #2 bearing quit, and filled everything with glitter. Any thoughts or opinions? Just trying to learn from whatever mistakes were made here.


#4 rod bearing

#1 rod bearing

#2 rod bearing, the quitter

#2 piston, weird texture? Detonation?

 

Because you mentioned RallyX, there's a chance it ingested a fair amount of debris too. That piston does look like ****, possibly either bad injector or wiring etc.

I'd personally toss the rods, pistons, and crank. At the very least you got a great deal on a block and VVT oil pan.

 

A sandblasted look to piston tops usually is indicative of detonation, yes.

 

I don't think it ingested any chunks, but the intake was definitely coated in a thin layer of mud from the dust.

What about the piston's look leads you to bad fuel delivery? Just want to understand what is jumping out at you here.

The block does seem good, maybe minor cyl wall damage, but it should clean up on an overbore. The mains all seemed to still be well indexed to the block. I think the head is salvageable too, but the cam phaser is probably junk. Can a VVT be converted to non-VVT?

What's desirable about a VVT oil pan? I'm still learning about the finer details of the BP engine changes year over year. I have a working 1994 parts car that's becoming an exocet soon, so I'm interested in things I could frankenstein with that working engine.

 

VVT pan just had the stronger main bearing support plate, a lot of people argue about whether it's worth anything though. Higher horsepower miatas break main caps occasionally, and the bigger support plate wouldn't really help with that.

You can't convert a VVT head to non-vvt, the cam seal wouldn't touch anything. You can swap to a 99-00 or MSM head and cams though. The VVT phaser can be disassembled, cleaned, and rebuilt with a rebuild kit from miataroadster.com.

https://miataroadster.com/products/o...t-rebuild-kits

The detonation can come from bad fuel or debris or ignition issues. Can't tell you much without knowing more about injector history and current flow rate, tune, etc. If the intake looked like ****, there's a good chance it got sucked into the engine too.

 

Ahhh, the VVT pan is the MBSP that I see mentioned around. I agree that the plate didn't look like anything to write home about. It's not even attached to all the mains, so I don't understand the plan Mazda had there.

Thank you for the tip on rebuilding the VVT phaser. Question, was there a way to get the cam bolt undone without having it explode into a million pieces? I feel like that wasn't the correct process, and reassembling it seems like it would be tricky.

Well the engine was a stock motor. Factory computer, injectors, tune, everything. No boost, they were running in the stock class.

I think maybe you misunderstand my question about why you're asking about fueling. I think you're trying to figure out why it detonated, I'm just trying to confirm that it detonated. Just to clear, every piston looks like the #2 that's pictured, but nothing looks melted or cracked. Just this peppering pattern on all of them. If you guys are happy with a diagnosis of detonation overloading the rod bearing as the cause of death, that's all I'm looking to figure out as I have no logs or additional info to know the sequence of events of the actual failure.

If we want to speculate on the cause of the detonation, here's my theory. The knock sensor was all melted, I think the computer was just putting in too much timing for the conditions/fuel and had no idea anything was wrong. Your question about wiring and bad injector leads me to believe you suspect fueling was the cause, so I'm wondering if there's a visual clue you see here that leads you in the direction of a lean event being the cause? Given the natural of RallyX, a momentary fuel starvation event is not unheard of.

Just trying to get a feel for piston palm reading.

 

Aftermarket forged pistons have machined crowns, the factory Mazda cast pistons do not. They are left in the as cast condition. Being a sand casting they look like they were sand blasted.

 

Well that's what I'm asking here. People who have had a VVT motor apart, does this piston I pulled out look like a happy stock one, or does it look like bad things happened to it?

 

They look like your standard worn out OEM piston. The finish does vary from lot to lot and the pict is not very good. If they all look the same, then that is how they were cast.

 

See I believe that's more than OE casting marks, hence my concern for fueling or debris.

Here's an OE NB1 and +1 NB2 piston. The NB1 piston has almost no casting marks, the NB2 piston has, to my eye, less than OP pictured.



Piston to piston might change of course, so maybe I'm wrong.

The spun bearing happens on high stressed engines, especially if you're in a dirty climate, high revved, no baffle, etc etc.

 

Both of those NB2 pistons look about the same.

 

Yeah, if I look back and forth on the two photos, they do look about the same.

You win this round, newb.

 

I don't think there is det - the standard pistons usually get a little upset at the edge on one side of the engine if pushed hard. The edge looks good on the OP's pistons.

RallyX miata. The sump is terrible on circuit for starvation under braking. I doubt its ace at rallyX - could be simple starvation but agree you would expect something less localised unless this rod was over loose in the build.