Kinavo

iTrader: (1)

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:


Thanks in advance.

Kinavo

iTrader: (1)

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

Cylinder 4 rod:

18psi

iTrader: (76)

hahaha that'll definitely make a sound

Kinavo

iTrader: (1)

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

Erat

iTrader: (5)

Overboost at 14psi did that? **** 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.

Kinavo

iTrader: (1)

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

What supporting mods are you referring to?

Erat

iTrader: (5)

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

Kinavo

iTrader: (1)

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.

Erat

iTrader: (5)

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

18psi

iTrader: (76)

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.

240_to_miata

iTrader: (2)

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.

Erat

iTrader: (5)

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

codrus

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).

Kinavo

iTrader: (1)

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.

18psi

iTrader: (76)

yes

myrando

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

Kinavo

iTrader: (1)

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.

18psi

iTrader: (76)

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

supercooper

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

supercooper

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