Is higher octane fuel in anyway linked to bore scoring?

Put a little more in the budget and get a Gen 2, even if it's a 2.9, or go high spec 2.7 Gen 1..

You'll always be listening out 'what's that noise' or is it using too much oil!!!

Plus with less Gen 2's around will hold its money well too..

Just my thoughts..

Dan

Givent hat bore scoring overwhelmingly occurs in the hottest cylinders - ie 5 & 6 - it's hard to see how that maps with fuel.

But if anything I think hotter temps are associated with lower octane fuel.

But I doubt fuel is implicated either way.

Given ignition advancement leads to over-heating, I suspect LOWER octane fuel is linked with bore-scoring.

That would appear to be a simple mechanical relationship, given bore-scoring is an excess heat (and under lubrication/side load etc) dependent issue.

Cheers GT4, have got me a lovely Cayman S now. Yet to fill it but it will be Vpower.

I am not aware of anything that can relate high octane fuel to bore scoring.

If you are a reader that cannot wait to criticise my lengthy detailed technical answers or wants to live in a dream World where Porsche's can never go wrong - stop reading now - it is an involved subject.

We have run different engines with different pistons, cylinders, C/R and capacities and looked inside many engines over the years and always bore scoring is on bank 2 first (a long time before bank 1) and always associated with plastic coated pistons.

Pre-ignition shows signs that we are well aware of (usually associated with low octane fuels) and we have not seen any evidence of that and anyway there are knock sensors that adjust ignition advance to compensate. If low octane cheap fuel is in use there may be associated omissions from the specification that could be harmful but then you would expect it to occasionally affect bank 1 - which it does not.

We think bore scoring is a function of the degradation of silicon particles that become free from the cylinder bore and travel up and down with the piston until they escape in the oil.

This became clearer to us after testing engines with several different piston coatings – one of which was shiny black DLC and revealed tiny score marks up and down the otherwise completely untouched piston face.

Some cylinders may be better than others at retaining the silicon particles longer and the size of the particles varies as well.

Oil that washes around under the piston and up the bores when the piston has risen can wash away the Lokasil over time (in much the same way running water can eventually wear away rock) and we have seen evidence of that some Lokasil areas are prone to this but most are not and this suggests that the distribution of silicon and alloy in the matrix varies and in turn this could allow some cylinders to release more silicon than others.

It is what happens next when the little pieces of silicon are running up and down between the piston and the cylinder that can cause scoring.

First it wears the plastic coating much faster than it would the old hard ferrous coating. Then it can either stick on the piston and rub up and down the bore (knocking other particles out to join it) or it can remove areas of plastic coating so it leaves alloy in contact with the cylinder wall - which is too soft to run in a silicon based alloy bore. The damage can also cause micro-welding that is more typical with a seizure on both sides of the piston resulting from the piston growing larger than the bore. However the scoring is always on the thrust face (not on the other side of the piston as it is when the piston has overheated and grown bigger than the cylinder diameter) so it is clear that scoring is caused by the load pushing the piston against the cylinder wall and therefore the amount of load (or torque) influences the rate of damage because the higher the torque the higher the pressure and the nearer the piston face comes towards the cylinder bore and the smaller the space to keep the parts apart with the oil film and to prevent the silicon particles being larger than the oil film thickness. Therefore the oil film thickness has another influence.

Better thicker cooler oil will protect the piston face from the silicon particles impinging into it longer than thin weak hot oil - hence the advice to use a LTT etc.

We have tested and proven that under normal running conditions the temperatures in both banks are sufficiently similar not to cause such a one sided failure but after a hot rest our tests show they increase more in bank 2 than bank 1 and we assume this means that the oil is hotter and the oil film thinner and that it does more damage after a hot rest period – especially if the driver sets off with high torque.

When analysing faults like this – small businesses like ours have very limited resources to work out the causes of failures and many just guess or use their previous expertise to zone in on an explanation that for them fits the evidence.

For some - having seen or heard of failures in other engines due to fuel content in Nikasil bores they quite reasonably consider it a possibility with these failures. Almost all our rebuilt engines are fitted with Nikasil bores and none have had any subsequent scoring issues – so for us – that adds further evidence that fuel has nothing to do with bore scoring.

If you have seen more engines than others - and can work out correlations between the evidence you see and other potential causes – some fit a different explanation better than fuel additives.

No one really knows the answers and it would take a huge test programme costing a small fortune even for a major manufacturer to find out for sure.

The fact that Gen 2 engines have reverted to an Alusil bore (that theoretically should retain the silicon particles better and for longer) and a different harder piston coating supports what we assume they put the scoring failures down to.

The fact that in the Gen 2 engines they also have increased the coolant flow around the cylinders and converted the design to a closed deck (which we also do during our repairs) also should provide confidence that what we originally attributed various failures to were indeed influencing factors and that the changes we make during our rebuilds are along the right lines.

You can see from this that if an owner happened to have an engine with 6 good Lokasil bores, that drives modestly, regularly has top class maintenance done and frequent oil changes – perhaps a LTT as well – then he may get huge mileage out of his car before failure.

Similarly an owner that by bad luck has a car with less secure Lokasil bores, that thrashes the car everywhere, minimises maintenance and uses thin cheap oils – could expect on average a much shorter engine life.

Unfortunately we think that the variation in the Lokasil may also mean that the first more careful driver may still experience a premature failure while the second may be lucky enough to get a reasonably long life out of his car – and this correlation also explains and fits the evidence we see – that many fail around 50 to 70 K, some much less – others much more.

Unfortunately I think it would be impossible to analyse or even believe the driver styles and maintenance care of all the owners anyway and so we have to look at the evidence and come up with explanations that fit all the circumstances we find and our conclusions and explanations do fit all the evidence we see as does the subsequent reliability of the engines we build with our modifications. This has traditionally been accepted as the right answer until and unless someone one day comes up with a more compelling answer – which fuel types does not even start to justify.

So to conclude - I agree with many of the points made by other contributors and in our opinion bore scoring has nothing to do with high octane fuel but venues supplying cheap fuels often are also less careful of the cleaning of storage tanks etc and so you should always use the best fuel you can obtain from reliable sources and if you are too tight to do that I would suggest you may experience other issues shortening the life of your engine and are driving the wrong car for you.

Baz

And so speaks the master, thanks for that Baz. Read with great interest/trepidation all your posts about the infamous bore scoring.

It has the LTT and one of your sump extensions, not sure what oil it has in currently, will put some Millers Nanodrive in there, which viscosity do you recommend please?

I have read about not booting after having come to a stop after some high load driving.

For the record, I have never used anything other than Vpower in my cars. Bar Momentum when I can't get to Shell.