Miata 1.6 rotrex supercharged spark plug read (damaged plug)

**edit - it ate one of the throttle plate screws. New question - how can I know how much damage it did to the motor? I'm doing a compression check now. What's the best thing to secure these screws with? JB-weld??




Hey Guys,

I was shaking down the new motor at NCM (the new motor has forged rods, double valve springs, stock pistons, upgraded oil pump, etc) - after fighting with the supercharger belt shredding and popping off. I managed to get in a few hot laps and I **think I did a small mechanical overrev at one point BEFORE things went bad. I was coming out of the sink hole and the motor sounded like  SOMETHING went bad - note this isn't when I did the over-rev, that was a lap previous - I couldn't get a good read on what had happened but it sounded like I went down to 2 cylinders or something. I'm limping it around the track and everything seems to come back online but with a more pronounced miss. By the time I'm off the track everything sounds roughly regular (for a ghetto exocet with a huge hole in the exhaust). 

I take it home and check that it didn't jump time, check the igniter. Timing is good, igniter swap no obvious difference. I check the plugs and #2 looks like someone punched it, #3 looks rough as well - 1 and 4 look perfectly normal. I'm assuming this is from detonation? I've run this tune and this level of boost on the other engine for a full year, and I check the plugs after every event - usually a bit sooty, but no damage. I have a ton more injector than I have power and the tune is generally too rich in most places. I'm assuming my over-rev sent the supercharger over spinning and spiked the boost way high? 
 

Is there shrapnel in that cylinder?

Why are there pits on the electrode and the base of the thread that look like something has been impacting them?

In reply to SVreX :

That's what I was thinking, but I can see no marking on the top of the piston, no shrapnel that I can find. I'm not sure what could happen to have a piston smack a spark plug, but then still run fine? 

I don't think the piston hit it.  It would have smashed it to smithereens.

I'm thinking small metal filings which were like mini missiles under pressure.  They could have ended up in your oil, or exhaust.

... or crap that came in through the intake.

Are there similar dings and divits on the other plugs?

Just the middle two plugs, and the other one (#3) looks like the probe is bent, but nothing like #2

My first guess would be that something went through the motor.

I'd do a compression test and very thoroughly check the entire intake tract for debris and source.

Hopefully you can put a couple of plugs in it and your back in business.

Man I hope nothing in the supercharger disintegrated. There's the supercharger and the intake valves post filter.....

I will chime in as well that that is definitely not detonation. This ingested something. Probably a piece of supercharger that broke from the over rev.

What about option 3? I forgot to liquid weld them down on this new intake..... E36 M3. How can I know how much damage it did? 

In reply to accordionfolder :

That's a possible.

I'd be tempted to pull the head.

Compression test will likely tell if you pinched a ring or dinged a valve.

I try to NEVER remove butterfly screws, if I have to, I stake them so they cant fall out.

In reply to bentwrench :

This one was never removed, they're prone to breaking off with age unfortunately. Usually you epoxy them down, I forgot to.

Compression looks even across the board, 130ish. What says the hive?

But how did one screw take out two plugs...?

Me?  I'd stick some plugs in it and drive. 
 

But I can't make that recommendation in good faith if you are racing.  I'd pull the head. 

In reply to spandak :

Only one looked like it'd been beat, but I haven't the faintest.

In reply to SVreX :

I don't race, just pretend I'm a race car driver. I like to go in circles and try to be faster than last time. So far I think I'm going with "just run it." 

In reply to accordionfolder :

That's fine...

Can you afford to take the risk if there is more metal in there or the supercharger has disintegrated?

It sounds like you just built this engine. 

If there are dings or divots inside (cylinder walls, piston, valves, whatever), those spots are gonna burn hot, and lead to a premature failure if you are regularly running it near its limit. 
 

You might find no damage.  Or you might find something that needs a light re-honing.  Or you might find pieces of your supercharger with an engine that is not yet damaged. 
 

There are some very good reasons to pull the head. 

In reply to SVreX :

I've checked the impeller on the supercharger, it's all accounted for. I might get a scope rather than tear it down right now. I'm 100% certain the screw was there when the morning started, so it's definitely part, if not all of what was ingested. Send me them good vibes hive folk.

That screw will very likely be somewhere in the intake manifold, although the supercharger might possibly help send it out the exhaust.

The plug is folded because the screw hit it.  If you look in the cylinder, you will find a screw mark embedded in the piston.  If it is a brass screw, it probably didn't harm anything too much yet, but a steel one can cause some unhappiness- the most common damage is to pinch the ring, if the screw wandered around the outside edge of the piston for a while. That will show up as low compression.  Not a dead cylinder, but 25% or more low.

If you never believe me about anything else, believe this:  Metallic objects that get into an engines cylinder will almost always exit that cylinder back into the intake.  Close your eyes, and imagine the pressure in a cylinder, and in the exhaust as the piston travels and the valves open- We can assume the most likely time for a screw to exit a cylinder will be as the piston reaches tdc- On the top of the exhaust stroke, the exhaust runner is high pressure, due to the piston pushing the exhaust out.  However, the intake is just opening, and there is negative pressure in that runner.  Therefore, the screw will exit to the intake, and then it will head back into another cylinder the next time it feels like it.

Please believe this.  I have seen it many times.  I have had it happen to me personally.  Really.

You may need to pull the head, or not, but I would find that screw, even if it involves pulling the intake right off.

In reply to Streetwiseguy :

I'm convinced to pull the intake, I'm hoping to not pull the head!

jesse prather motorsport does a fix on the throttle screws, its a known issue on the FP miatas.  I've seen many a racer "just run it" after this happens.  If its made it into the combustion chamber and you still have compression then its probably worked its way through (leak down might be wise as well).  

If it was a stock motor I'd just say drive it and see what happens, but I have heard of people having rod failures a few hours after this.  Supposedly this is because the screw gets caught between piston and quench area and spikes the loads on the rod, weakening it.   For a built motor that I cared about, I'd tear it down and inspect it before running it again if it ate a TB screw.  Some photos from when mine (built turbo 1.8) did this:

As for a fix, I went with a Skunk2 throttle body, but I dunno if that fits the 1.6 intake manifold.