Race Engine Forum UK

Hello Guy,
I have just got back from the rally where I was servicing my friend's Seicento Sporting. The car only was able to finish one special stage after full rebuild made right before the rally.

Here is what got broken
- fire rings on 3 cylinders have a hole of about 10mm to coolant
- core plug from coolant jumped out of the block(it was not rusty)
- head has burned grooves deep for about 3mm in place of fire ring failure


Pistons crowns do not show any sign of melting or detonation- look brand new.

The engine was not built by me and I was only a part of a service crew so I cannot give you exact spec of the engine. The engine is using 2 throttle bodies just like Variant Kit FIA homologated engines in that seicento have been using. Ported head, rised compression ratio, some kind of wild cam from a reputable polish manufacturer and a rechipped ECU with a program written to use with that cam and throttle bodies.
Fuel Pump was brand new installed one evening before rally.
What i think that happened is that the engine was running lean as the chip might have been faulty, I assume that from the colour of the spark plugs but this in not 100% sure as it was running for a while with headgasket already blown. This could make the engine to detonate for a while and then the headgasket failed. This could make the whole pressure to go into coolant galleries and then pressed out the core plug.

The other option might be the core plug jumped out from a bit more pressure in coolant lines made by water pump runing a bit faster(engine revs to 7800rpm) and then all engine was overheated and this caused the headgasket failure.

What do you think might happen in this engine?

I will use megasquirt next time instead of that rechipped standard ECU for sure so that the fuelling will be set up correctly.

That's detonation. I saw the witness marks on the squish band on the very first photo.

Only detonation can cause this kind of damage with it's unique 'footprint', regions affected look as if they have been grit-blasted, the hole bored thru the head (to relieve the huge pressure) is absolutely conclusive.

Over-lean, ignition timing over advanced/retarded, too hot (engine and intake air), octane too low, all the usual suspects.

Sorry! Pity you pinned it on the headgasket, everyone blames the poor old headgasket. No gasket on the planet can survive that and even if it could, the heat and pressure would punch thru the piston instead.

GC

Thanks for help, this only means that the chip that was used in the ECU is bad and we will replace it with Megasquirt. I hope that the pistons survived. Of course the engine will be stripped down completely and all broken parts will be replaced with new ones.

I have not meant to say that the headgasket was the problem. It failed because something was wrong but I was not sure what caused it(overheating from core plug that jumped off the block or detonation from over lean running) but you made it clear now and this will help for sure to avoid this problem next time.

The other cinquecento(1,2l 16v dyno plot is here viewtopic.php?f=6&t=609&start=15 )that I made to run on Megasquirt works very good at the moment and this is what I am going to stick right now with that seicento.
I will let you know the results after rebuild with a dyno run in about 4-6 weeks(after it is all done).

Best regards
Tomasz Cecot

Don't worry, I sound harsh at times but I don't mean to be.

Yes you MUST strip the engine because you may also have fractured rings, ring lands etc.

G