Andy916

iTrader: (8)

Well, after 5 years in 3 Imprezas and loads of trackdays I've suffered the big-end death rattle. Had to happen at some point I guess, but I'd like to try understand more precisely what caused it...

Background: STi6 RA Lim with Cosworth baffle plate, breathers going to RCM can returning to sump, no oil cooler yet. Road tyres (Marangoni Zeta Linea). Standard 2.0 block, Zerosports hybrid VF28, Samco inlet tract, hybrid FMIC, ported headers, 550s, Apexi PFC, around 340bhp at 1.5bar.

Suffered a big end failure after 7-8 sessions at Mallory on Saturday, engine still runs (idles fine) but has the characteristic rattle as revs die away. Exact cause not clear to me at moment, maybe someone here can advise. This may read like a catalog of errors, but bear with me.

In reverse cronological order:

1. made aware of bad mehanical noise by the detalarm, all LEDs lit at slightest boost, but no significant knock count on the PFC hand commander; retired to investigate problem.

2. couple of laps earlier, filled up with half-tank of 97RON on top of existing half-tank of 99RON plus booster. Although car is mapped for 99, we were monitoring carefully for det and didn't want to risk fuel surge. Don't think there's any evidence of det on that fuel.

3. one session earlier, stayed out a bit longer than earlier whilst reeling in a 911 GT3, and reached 130deg oil temp. Whoops. Earlier sessions were less than 10mins, reaching 110deg. Millers CFS 10w60 which has done one previous trackday and 4k road miles. Datalogit trace shows 2 bouts of 'knock' count of 15-22 at 7k rpm, whereas engine normally runs a count of around 10.

4. couple of sessions earlier, reduced boost to 1.3-1.4bar on AVCR after seeing knock count around 20 on hand commander after a session, although detalarm remained quiet (green). Started logging to see what was happening during session.

5. error of judgement led to a spin exiting Gerrards. One wheel onto grass at top of 4th gear (RA gearing, 110mph?) and 720deg spin narrowly missing barrier. Foot on clutch and engine still running. Back to pits to check and calm down.

6. first time at Mallory and realised it's v.quick except for the hairpin. Fuel consumption through the roof. Gerrards long right-hander (like Coram or Chris curve but faster) could see oil pressure drop from 6.0 to 5.5 bar on some laps. Had fuel surge once (I think) at Devil's elbow, a 4th gear lefthander that drops away.

7. before the trackday had various short bouts of det due firstly to dodgy crank position sensor (now replaced) and to compressor surge (Paul at Zen's assessment, from the logs). Boost reduced in midrange to avoid the surge.

8. Oulton trackday a few months ago, before compressor surge problem was realised, with brief bouts of knock count 30-40. All apparently ok subsequently, but perhaps the seeds of damage were sown back then?

Apologies for going on a bit. Anyone care to comment on relative importance of the possible causes?

Andy

jon 928

Andy what rods you using matei

Bob Rawle

The key piece of info is "and reached 130deg oil temp", the normal limit to back off is 115 deg C. So you probably started the problem then. Sounds like you picked the bearing up at high rpm's leading to a quick rise in temps and the inevitable death rattle. Quite often this can happne when you flip onto over run from high revs. The reverse loading on the bearings can cause them to pick up and spin.

Bob

madmooro

oops sounds painful
from what bob says i did just that raced the **** of my car a few years ago and when i see the competition was lagging come of the throttle

hope it all goes well bud .it was a blast why ya enjoyed it .

Andy916

iTrader: (8)

Thanks Bob. I have Datalogit trace for that 130deg session, but that doesn't include oil temp and pressure. Will watch the gauges more carefully once back up and running. Will address cooling issues during the rebuild and re-install.

Jon - engine internals are standard STi6 RA Lim.

Andy

scoobyc

iTrader: (2)

Hi Bob, Could you explain this a little more for an idiot. i.e. me.

Are you saying from high RPM, when you lift off the reverse loadig ont he bearing can cause them to spin. If thats the case, what is the actual issue. Is it the high RPM, the sudden shift in loading on the bearing, or a combination of both.

Should sudden throttle lifts in gear be avoided to help deter this, or is lifting just the straw which breaks the back kinda thing?

Cheers
Chris

scoobyc

iTrader: (2)

bttt