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

No, you can't alter the angular position of TDC & BDC by bending the rod -- those are fixed by the throws on the crank. The piston's linear axis of movement is constrained by the cylinder, so a bent rod basically acts like a shorter and fatter rod. The rod can hit the cylinder wall or the piston skirt can hit the crank at the bottom of the stroke.

Here are a few more pics from teardown:

https://www.miataturbo.net/attachmen...ine=1366755561
https://www.miataturbo.net/attachmen...ine=1366755561
https://www.miataturbo.net/attachmen...ine=1366755561

Cylinder 4
https://www.miataturbo.net/attachmen...ine=1366755561

so, being that the bent rod acts like a shorter fatter rod, then when the piston is at its highest position in the cylinder, its still lower than if the rod werent bent, right?

so, would this JUST make a bigger space for combustion to occur, but still combusting when the CRANK is TDC?

could that create any kind of noise? because it doesnt look the the rod was bent radically enough for the piston skirt to knock on the rotating crank.

sorry, im just digging to find stuff out... its not easy to do, not having the engine in front of me, im just really curious where the knock could be coming from, so im running all kinds of possibilities through my head. lol

could the bent rod have torqued the metal in some way down to the base, making a loose fit on the bearing, creating a larger clearance, and therefore a knock against the crank???

You got a pm Kinavo. I want those rods.

Oh hai, any new updates on that lamp?

As far as the rod goes, i'm guesssing it could make that sound since the weight difference to the one side could theoretically slap the cylinder at the wall. I'm guessing it also twisted sideways so it was harder to turn. At the very minimum ensure you get the walls honed and checked for roundness.

Yeah, with a twisted and bent rod, plus missing chunks of piston, there's any number of reasons why that motor would make noise.

Personally I'm not sure I'd reuse any part of that bottom end.

--Ian

I dont think there are missing pieces of the piston. It looks intact but I will double check. Nevertheless, I will have the cylinder rehoned and checked for roundness. I will have the crank checked and will replace the piston. Forged rods are already en route. All Rings and bearings will be replaced as well.

I appreciate the advice guys.

Regarding the hypothesis of the sound coming from piston slap, would the noise be as consistent or would it be more random?

Your picture of cylinder #4 has what looks like a missing chunk at about 4 o'clock. Is that just oil or something?

--Ian

My experience has been mostly consistent.

Interesting.


What you thought was a missing piece was in fact a carbon flake pushing to the side making it look like a hole.

This part of the piston touches the crankshaft.
Attachment 74931


If you inspect your crankshaft you probably find marks on the "counter weights" dunno what you call it (I'm from non-English speaking country)

that is whats making the noise. My engi made the same noise when i bent my rods.

You are correct sir. :)

https://www.miataturbo.net/attachmen...ine=1367188626
https://www.miataturbo.net/attachmen...ine=1367188626


Here is a pic of the bearings on that rod. Is that normal wear? It doesnt look good to me.
https://www.miataturbo.net/attachmen...ine=1367188626

Wow, it's a scary thought that a rod can bend on stock or sub 200 bhp.

Might be best to replace the bearings, don't know if that's normal or not.