Pretty Forged Piston Pics
07-27-2017, 01:22 PM
#16
Developer
Thread Starter
All that aside, FWIW, I have said it before and I'l say it again. For a build situation that doesn't require the maximum strength attainable I think that your service and procedure is the most cost effective and realistic approach to the Cayenne engine while being much more than adequate.
07-27-2017, 04:05 PM
#17
Developer
Thread Starter
As to why this happens... been through several scenarios and there seems to be more validity in some theories than others. The one that seems to fit the symptoms best is this:
Porsche contracted out the casting of the blocks, and the Cayenne blocks were cast to spec. From there, the raw cast part needs to go though initial milling operations to create the head gasket, front, rear, and bottom gasket surfaces. And of course, initial boring to set up the cylinders.
Of that cylinder boring operation, I have heard that they sub-contracted that to two different shops, and also that they bored them internally. This doesn't matter much - but what does is the difference between the two (or more) boring operations.
Porsche would call out a bore dimension of something like 3.780" plus or minus .002". That means that cylinder diameters from 3.778" to 3.782" would all be within spec.
Boring bars and honing mills all wear, and where they may start their cycle making cylinders close to max bore, they will finish their tool life making cylinders closer to the minimum diameter spec. Variables between machine setup also play a part - it would seem that one of the machines was producing bores to the tight side of that spec, and the other was producing bores to the loose side. Both within spec, but at the outer ranges of what was acceptable.
Our experience indicates that those engines that were on the loose side, when also experiencing wide temperature changes (parking outdoors in cold winter climates which will shrink the piston even further in the bore) - those engines experienced excessive piston slap and skirt movement that results in the galling.
Many Cayenne engines do not report this problem at all. According to the theory, they were either a) bored on the tight-tolerance boring machine, or b) bored on the loose tolerance machine, but the car is kept garaged in winter or in a warm climate. For example, a Cayenne kept in FL, TX, AZ, NV, etc can be parked outside all year and it still will not have a temperature extremes that ruins a WI, MN, MN, NYC Cayenne.
The stock pistons are cast, and have no lubricity coating on the pistons skirts. Should they shrink in the cold, they are going to slap in the bore until the engine warms up. Most of the damage we see occurs during that cold-start period.
Caveat: Sometimes we DO get a bad Cayenne engine in here from a southern state, and we have also gotten a bad Cayenne Turbo in here with the same problem - they have forged, not cast pistons. So...this hypothesis has some problems and does not always work. Special cases will always exist.
However, our solution works very well. We addressed the probable cause of the galling several ways:
Porsche contracted out the casting of the blocks, and the Cayenne blocks were cast to spec. From there, the raw cast part needs to go though initial milling operations to create the head gasket, front, rear, and bottom gasket surfaces. And of course, initial boring to set up the cylinders.
Of that cylinder boring operation, I have heard that they sub-contracted that to two different shops, and also that they bored them internally. This doesn't matter much - but what does is the difference between the two (or more) boring operations.
Porsche would call out a bore dimension of something like 3.780" plus or minus .002". That means that cylinder diameters from 3.778" to 3.782" would all be within spec.
Boring bars and honing mills all wear, and where they may start their cycle making cylinders close to max bore, they will finish their tool life making cylinders closer to the minimum diameter spec. Variables between machine setup also play a part - it would seem that one of the machines was producing bores to the tight side of that spec, and the other was producing bores to the loose side. Both within spec, but at the outer ranges of what was acceptable.
Our experience indicates that those engines that were on the loose side, when also experiencing wide temperature changes (parking outdoors in cold winter climates which will shrink the piston even further in the bore) - those engines experienced excessive piston slap and skirt movement that results in the galling.
Many Cayenne engines do not report this problem at all. According to the theory, they were either a) bored on the tight-tolerance boring machine, or b) bored on the loose tolerance machine, but the car is kept garaged in winter or in a warm climate. For example, a Cayenne kept in FL, TX, AZ, NV, etc can be parked outside all year and it still will not have a temperature extremes that ruins a WI, MN, MN, NYC Cayenne.
The stock pistons are cast, and have no lubricity coating on the pistons skirts. Should they shrink in the cold, they are going to slap in the bore until the engine warms up. Most of the damage we see occurs during that cold-start period.
Caveat: Sometimes we DO get a bad Cayenne engine in here from a southern state, and we have also gotten a bad Cayenne Turbo in here with the same problem - they have forged, not cast pistons. So...this hypothesis has some problems and does not always work. Special cases will always exist.
However, our solution works very well. We addressed the probable cause of the galling several ways:
- The block is Ni-Com plated. This produces a superior finish that is much much more scratch resistant than the original bore, and has a finer, smoother finish. Galling is far less likely for these reasons.
- Our pistons are forged, not cast. This allows use of a superior alloy that is stronger, more abrasion resistant, and expands and contracts less than the aluminum alloy they use in a typical cast piston. Our pistons further defend against the galling by having coated skirts.
- Because of the controlled piston expansion do to heat, we are also able to tighten the piston-to-bore gap a bit, again reducing piston rocking and galling.
Last edited by Carl Fausett; 07-27-2017 at 04:31 PM.
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