Skoda 8v 136 engine - Detonation?

[djaychela]

I have just bought (as spares) a supposedly ex-works Skoda 8v 136 engine which was billed as being "Group A" - it was for spares (the crank had broken), and it was cheap.

The Skoda 136 is an aluminium block and head engine with wet liners, and has a 3-bearing crank. It's overhead valve with chain-driven camshaft, solid lifters, standard (31/37mm) valves and a non-crossflow head design with individual ports for each valve (arranged Ex-In-In-Ex-Ex-In-In-Ex). Standard it makes 68bhp, while the works engines supposedly made around twice this amount at around 7500rpm. When built, the engine is intended to go into my Skoda Felicia rally car (which, due to Group A regs will need to run the standard MPi inlet manifold, but which I will be running via a Megasquirt injection system, having used this hardware on a previous Skoda rally car).

The engine was partly dismantled when bought, and on full dismantling, apart from examining the broken crank (the crank had broken on the web by the rear main bearing, meaning that all 4 pistons would still have been attached to their big ends), the main thing I have noticed is piston damage - the skirt of No.4 piston had been broken. I had expected this, but I have another area of the engine I would greatly appreciate your opinion on.

The No.1 and No.4 piston edges both look like those in the attached photos - would I be right in saying that this is as a result of detonation? The marks around the piston edge and on the crown look to me to have occurred before the catastrophic engine failure, although I could be wrong in this.

Given that the damage had happened on the No.1 and No.4 pistons, I originally thought it may have been due to the broken crank, but as the crank was intact at the No.1 end, I think that it may just have been poor engine building in the first place?

The photos are of the No.4 piston - there is "fresh" marking on the piston crown too, from touching the valves (circular marks) and also from contact with the head - the No.1 piston doesn't have any of these marks, but still has the same detonation.

I had originally hoped that the head would be a good starting point for my new engine, and clearly I would be making careful measurements to ensure the correct compression ratio (I haven't made these measurements yet), and also the clearance between the piston crowns and the cylinder head.

If you could confirm that this damage looks like it's from detonation, that would help me greatly - I've ordered all the parts for the new bottom end (I have decided not to re-use any of the old rods as they have been subjected to this strain, and am currently preparing new ones, along with a new crank, pistons and liners), so should be able to accurately set the clearances to the new head. Any advice or comments are appreciated - this is my first post, and I hope I've not left anything out.

[Guy Croft]

Yes that is definitely classic detonation, so either too lean, overadvanced or too hot (engine/intake air).

I know those engines better than most oddly enough, note that those 'works' GpA cast pistons are, well (sorry to use slang on my own site) poor. As I know to my cost (literally) they're very weak indeed, note cracks in your already, they break up at high speed, get forged.

Don't know why Skoda ever produced a cast piston for GpA where forged could easily have been homologated.

GC

[djaychela]

Thanks Guy

I didn't know you knew the Skoda engine well, but I guess I should have known better! Any tips you could share or things to look out for/avoid?

Do you have any suggestions for manufacturers on the piston front?

Darren

[Guy Croft]

Forged pistons? Yes, though I rarely say so here, me.

I design by cad (week in week out) and have made by CP in Calif, superb spec, any ring pack u like, super light. I don't copy, most do ... but I need a pattern piston and some setup data (I'll call for that). I don't go into price here.

What to watch out for? Don't put the rocker gasket on upside down!

Shimming the liners is a horrible job. Apart from that, dandy little motor and way light..

GC