Please help identify this sound from my motor.
 

Hey guys,

Well I have managed to it again. The newly rebuilt 1.8 motor (94 bottom, 99 head) I just put in went pop the other night. I am pretty sure it was because of overboost (~14psi on a gt2560) at part throttle and after several similar incidents over the course of a couple of months something just gave out.

The car was still able to running but it sounded like a tractor. Compression test results are 190, 190, 180, 130. Can you tell what is broken from the sound?
Here is a sound clip:
https://www.miataturbo.net/attachmen...ine=1366310402

Thanks in advance.

I did some poking around today and this was all I found.

Cylinder 4 rod:
https://www.miataturbo.net/attachmen...ine=1366511500

hahaha that'll definitely make a sound

Forged rods are in my ery near future. I'm just glad I didnt vent the block this time.

Overboost at 14psi did that? Shit tune?

My stock 94 engine has seen 20psi a couple times, and my overboost protection is at 18psi, with the boost set around 15psi. From a gt2560, and a nice intercooler... Perhaps you just don't(didn't) have good enough supporting mods, and tune.

I've had several folks look at my tune and have found nothing wrong or dangerous.

What supporting mods are you referring to?

Well, a big enough intercooler, better cooling system, and injectors would be a start.

I actually think my intercooler is slightly too large but it helps keep the temps down very well.
The cooling system is good but not great. I have the Mtuned reroute kit and a 2" aluminum rad. The reason its not great is because I dont have good shrouding around the rad. That is a work in progress but the water temps seem to be stable even after a couple of back to back AX runs.

I am using RX8 440 injectors that were flow tested.

Anything else?

My power goals are moderate. 200rwhp.

Shouldn't have bent a rod at 200hp... Something else happened. It must of knocked or something.

Not all engines are created equal, or are in equal conditions guys.

The 250whp "limit" we all refer to is an arbitrary number we all came up with based on experience. It in no way shape or form is a RULE.

I've seen stock motors blow at 200. I've seen stock motors hold down 350whp.

Mine popped w/ 120 miles on the clock (about 5k boosted) at the 200-225 rwhp range on a T25. It had a fairly safe tune too.

250 hp might be the average, but some do bend rods well before that.

The engines that blow around 200hp must of been built on a Friday.

IIRC, Keith at FM once said they'd taken apart a < 10K mile bone stock motor (no boost) and found a bent rod.

Mine did 83K miles, 50K of them boosted at 220+ rwhp and the rods were still straight when I took them out (ring wear in #4 is what drove the rebuild).

Can a bent rod be the cause of the knocking sound? The cylinder wall looked fine and the rod wasn't hitting anything. Valves seemed fine. Not sure what a spun bearing is supposed to look like.

yes

When i bent my rods the pistons made contact with the crankshaft. Rods got shorter so piston hit the weights on the crankshaft.

Everyone said the sound from my engi was a bad bearing.. so yes.. bent rods can make that sound.

Bent my rods at ~220rwhp

Thanks. Is it safe to assume that the valves are okay if in this case? There is no apparent damage and they all seem to close evenly.

Why in the world wouldn't you check them? It takes like 20 minutes if your head is off. I'd definitely do the leak test with fuel/alcohol/whatever

i would think that the bent rod, would shorten the upward stroke, even if only a small amount... when the spark went off, being that the piston wasnt where it was SUPPOSED to be, it would almost act like pre-detonation, slamming the piston down against the continual rotating mass, causing the knock against the crank.... even the smallest inconsistency can sound bigger than it is.

OR the slight bend put lateral torque on the piston, and it slanted in the cylinder, knocking off the cylinder walls... BUT, you said theres no wall damage.

im leaning more towards the first possibility

i would definitely check everything out though, and do all the testing you can, especially if its all torn down anyways