Over-revved the Engine - Poof! (Update: 2017 engine swapped into 2015 car)

[nickels]

Little late to this topic, but my obd recorded 94xxrpm on a blown 4-5 shift on the highway.

No damage, but dear god I know it was ****ing close. No more absent minded racing anymore. Was doing like, 125 on the highway and went to pass a truck. Didn't realize I was climbing a pretty steep hill and was already near redline in 3rd gear and assumed I was in 5th.

So yeah, did a 5-4 shift but actually did a 3-2. Caught it quick but I had already kicked the throttle to revmatch so the clutch engaged at 6500rpm, and I pretty well dropped the clutch.

[Loren]

Hehe. I read "I was doing 125 on the highway", and was instantly thinking, "this guy is so full of crap!" Then I noticed the Canada flag.

For reference, 125 mph would be about 200 kph. I'm not sure the Mirage would even do that unless being towed by a Veyron. But, 125 kph... yeah, obviously 3rd gear territory.

Anyway, yeah... I'd say you were lucky. Or I was unlucky. Or both. Will be interesting to get this engine torn down and see what actually failed. I'm betting on slightly bent valves... at least 3 of them. But, we'll see.

[flatbroke]

Did you say you are putting a 2017 motor in a 2015? Is the harness and all the connectors the same?

[dspace9]

125 mph would be about 200 kph. I'm not sure the Mirage would even do that unless being towed by a Veyron.

The Mirage will do over 100 mph (165 km per hour) but I'm not sure much more than 103, the Mirage is buzzing at 5500 rpm steady there. But obviously it's dangerous at that point.

I'm thinking the Mirage will do around 105-10 max mph or 180-185 km/h... car isn't really designed for that sort of thing, but it isn't shake rattle and roll like an old Chevy Corsica etc. at that speed, either.

[Loren]

As far as I know, there should be no difference in wiring between 2012 and 2018. The changes in 2017 were all mechanical hardware: cams, piston rings and timing chain. I expect no compatibility problems putting a 2017 into my 2015. But, at this point, I'm not sure I'll find all the wires when it's time to plug them in, anyway. There are a bunch of them!

Got the engine out and did a quick post-mortem on it.

1. The 3A92 engine IS an interference engine.
2. The 3A92 valve springs are comically soft (reducing friction and all, I get it), something on the order of 35-40 lb/in.
3. Due to the soft valve springs, an over-rev can easily cause the valves to float. As soon as a valve touches a piston, it's all over!
4. All 3 of my pistons have clear marks from contact with all 6 of the exhaust valves.
5. Removed two exhaust valves, they are twisted!

This doesn't look good.


Yeah, definitely tweaked.


Wow!


What else?

Well, you can't fully remove the timing cover without removing the oil pan. The oil pickup tube comes off of the bottom of the timing cover and hangs up if the oil pan is in place. Remember that, should you ever be thinking about replacing your timing chain. This only slightly hindered our removal of the head.

Otherwise, I've found the whole car to be very pleasant to work on. Just thoughtful little design touches throughout. For the most part, the car was designed to be easy to work on. And the parts that aren't so easy to work on are things that you should almost never have to mess with, anyway.

Some reference data to the nearest half pound:
Transmission (including upper trans mount and bracket) = 73 pounds
Engine (all accessories removed, no manifolds, fuel rail or coil packs) = 112 pounds
Intake Manifold & Fuel Rail = 7 pounds
AC Compressor = 9 pounds
Alternator = 10.5 pounds
Flywheel = 15.5 pounds (better scale to the tenth = 15.1)
Pressure Plate = 6 pounds (better scale to the tenth = 6.0)
Clutch Disk = 2 pounds (better scale to the tenth = 1.6)

And since I'm not sure if this is readily available or not, I took some quick camshaft measurements:
Intake = 36mm base circle + 8.9mm lift
Exhaust = 36mm base circle + 8.0mm lift

Curious that the exhaust valves were the ones that crashed, given that the intake valves have more lift. I wonder if the intake valve springs are stronger than the exhaust?

[Daox]

Great info. Thanks Loren!

[AssassinTime14]

The 3a92 engine is theoretically able to hit 125mph, i tested mine stock and it tops out at 116 in fourth, doesn't really get any further than that man ����, not yet anyways..

[MetroMPG]

Late to the party. Replying for auto-subscribe and updates.

I'm somewhat surprised the 2017 motor is plug'n'play with previous years.

Anyway, bummer about the missed shift, but make lemonade from those lemons, eh?

[Loren]

Nobody has ever documented any changes in wiring or sensors with this car, so I'm pretty sure there aren't any. As for the other changes... they're friction-reducers. Should make little or no difference to the tuning. Even though the 2017 engine makes 4 more hp... that's largely due to the reduced friction components. The biggest potential difference could be the cams, but even then, I'm 99% sure they didn't change the cam profile, they just made it into a roller cam.

But, I look at it this way: if aftermarket performance cams were available, I wouldn't hesitate to install them and run them with the stock ECU. All they're going to do is allow more air into the engine. The ECU can handle that.

So, yeah... I'll document if I'm wrong, but there shouldn't be any issue plugging this engine into my stock harness.

I'm getting pretty stoked about the lemonade! More power, lighter flywheel, a little more weight reduction (smaller battery and battery tray). Should all add up to a noticable difference.

[Daox]

I've added some of the engine info you've provided to our engine specs thread:

3A92 1.2L 3-cylinder Mirage / Space Star engine info (specs, specifications)