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Cosworth 2.0L knock/pre ignition? Pictures inside

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hi. This is a friend of mines ford cosworth engine. 2.0L 4cyl.

Comp ratio is 8.2:1

clearance between top and piston is 1.7mm

It’s running 3 bar absolut pressure on 99 octane vpower gasoline. (We don’t have e85 here)

I haven’t had anything to do with the tuning of it. But it was tuned on a dyno where the tuner used a Plex knock monitor.

the tuner said he had a hard time hearing if the engine was knocking.

He has only been driving the car for a short time before taking it apart.

the piston shows sign of knock/preignition?

Problem with the ignition table ? Problem with too low squice area ?

Best regards


Attached Files

Hi Kent , possibly a little to much timing maybe , depending on how the rest of the ecu is set up . Looking at the pistons they dont look like Mahle [please correct me if wrong] if not which brand ? and did the builder follow the guide line for ring gaps . the det is predominantly on the edge of the piston , its possible the ring gaps are to tight causing heat when on boost and igniting the trapped fuel between the piston crown and cylinder wall.

Have you got any pics of the bores , are there any marks / scores in the cylinder that would point to the rings binding ?.

HTH cheers Keith

ROSS pistons and wiseco ring

Ring gap is as 0.45mm 0.50mm and oil ring is 0.25mm

piston to cylinder clearence is 0.1mm

nothing really to see on the bores, a mild hone and they are fine again.

You dont one think that too little squice area have something to say ?


Aside from the obvious detonation damage to the piston on the crown and the side above the top ring, there's the damage on the skirts which I have seen once before. My understanding is that could possibly be caused by debris during assembly, or possibly by some burrs on the bottom of the cylinder bore. When the piston reaches BDC on some engines, the piston skirt protrudes through the bottom of the bore and as the crank and rod rotate, the piston rocks in the bore at an angle. If there are any machining burrs or a sharp edge on the bottom of the bore, this can rub against the piston skirt and cause galling on the aluminium, which creates a rough surface that breaks through the oil film on the bore. This then causes the piston material and bore material to rub together which results in the dark grey abraded surface in your pictures.

The damage on the skirt is not as bad as it looks on the picture. If you look at it closely IRL, it looks like it the aluminium from the crown of the piston has moved down to the skirt.

You've got reasonably bad signs of damage from detonation (not pre-ignition just to be clear) which I'd guess should have been obvious during the tuning process. It's hard to give you a specific answer as to why unfortunately. The majority of the problem most likely comes down to the fuel you're using. If your 99 octane fuel is a pump gas that's anything like our local 98 octane fuel, I'd almost guarantee that you're not going to be able to support 2 bar of positive boost on it, even with a relatively low CR. On our local fuel there are very few turbo engines that I end up running past about 1.5 bar boost as I get to a point where the engine knocks when I add boost and hence I need to retard the timing so I go around in a circle adding boost, reducing timing, and making the same power with more heat and stress placed on the engine.

The other issue is that the detonation damage is predominantly visible on the top ring land, but not really on the crown from what I can see in those pictures. I'd be willing to bet that this may be caused by the excessive piston to cylinder head clearance. This reduces the squish effect which is used to help force the fuel/air mixture towards the spark plug. With a good quality steel rod you could reduce the clearance down to 1.00 mm or less with no concerns. I'd be inclined to couple this with a reduction in boost if you aren't able to run on a good quality race fuel with an octane of 105+

Thanks for the reply.

i have always thought that you should open up the squis area some when raising the boost. That you should need around 2mm from head to piston crown. ?

There is like 5% ethanol in our 99octane pumgas here.

The car have been running around 2 bar boost pressure and 610flywheel hp

There's multiple schools of thought on the squish area. the current trend is to use what's referred to as 'slant squish' where the squish area is still retained to some degree but machined on an angle as you move towards the centre of the combustion chamber. This helps move the combustion charge towards the centre of the cylinder but helps prevent mixture becoming trapped in the crevice space.

Having tuned a number of Ford YB Cosworth engines, I'd assume that 13-14 degrees at 2 bar of boost is pretty optimistic on pump gas (without knowing your engine of course). We have 100octane pump gas with 5% ethanol here as well, but usually at around 1.6-1.7bar boost I go around in circles with timing, like Andre mentioned earlier.

The squish area might have an impact, but I'd strongly suggest to revisit your tune after the rebuild and use better fuel for that boost/power goals.

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