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bazhart
Approved Trader


Joined: 20 May 2009
Posts: 1021
Location: Bolton Lancashire


PostPosted: Thu Jan 30, 2020 9:44 am    Post subject: Reply with quote

The original cranks for decades have been produced to a consistent tolerance of nominal size minus 0.01mm (0.00039") to 0.029mm (0.00114").

The damage to journals occurs when the shell wears through the white metal layer and the clearance increases.

The oil pressure leaks out of the journal and the more the clearance the faster it leaks out so eventually there is a loss of the pressure at the shell surface, an increase in clearance, a reduction in oil pressure and the two parts touch and the heat generated wears into the surface of the journal until friction damages it.

If the shell is wider it spreads the load/unit area so there is less pressure on the surface and it wears more slowly. At the same time the leak period for oil under pressure is extended - so it lasts longer by retaining whatever oil pressure is available for longer. Less wide shells have a higher load/unit area and spill out the oil under pressure faster - so do not last as long (but have a slightly lower friction coefficient).

Nutride (or Tuftrided) cranks are not hardened by quenching the red hot material so do not distort after hardening or re-hardening (because the hard material is infused into the surface and not quenched within it) but they do grow a thin white layer than is not smooth and needs polishing. You can allow for the growth of this once you have nitrided a few and learned how much they grow but small quantities always introduce a learning curve until you get used to the amount of growth and allow for it and grind them to a slightly smaller size in the first place to compensate for it.

Making new with a thin hardened layer reduces the proportion of the white layer so reduces variations and improves quality - but also reduces the skin depth.

Providing the oil is good quality, the running temperature does not get too high and the engines are not thrashed for too long - most crankshafts are still within tolerance when we measure them for a rebuild and so are perfectly OK to rebuild with new shells (and we recently had as customer in our reception talking to a prospective new customer and to put his mind at rest about a rebuild explained he had his from us 10 years ago and it is still running perfectly - and many comments and posts on here over the years have confirmed that).

The original Porsche engines were very cleverly designed to be too strong and so they either lasted for hundreds of thousands of miles or you could tune them, rev them higher, fit turbos or superchargers and the basic over design resulted in reliable performance. However that also meant that for most owners and applications they were over designed and cost to much to make. So as manufacturers learned more about design limits and production costs became more of an issue, the lower price range range cars were designed closer to the operating limits they expect and Porsche did a good job on these engines regarding the crankshaft etc.

However, because they are so close to the designed limit, racing these engines (which they were never designed for) puts them over that limit and crankshaft shells for top competitors are worn after one season and for less aggressive competitors perhaps after two seasons (and that is less than 2000 miles). The highest forces occur on the over run (maximum acceleration and therefore load is at TDC) and when you toe and heel with very fast braking and slick tyres you are forcing the engine to slow down at many times the rate it accelerates at and so the forces on the shells are much higher. For road use maximum loads are around 20 to 25 degrees after TDC (when combustion pressures are at their highest (after which they tail off as the clearance volume expands as fast or faster than the burning fuel) and this often leaves a tell tale wear mark just 20-25 degrees round form the centre of the shell - indicating the crank is probably still Ok (but was just caught in time).

Going racing helps those specialists like us involved in engine work to find the weak spots earlier, prioritise improvements, test them out and their road going rebuilds benefit as a result.

So - to get back to the reason for the post - used cranks are either perfectly OK to use again, very slightly worn and on their way out or damaged.

Because of the thin hard layer (which because the surface is infused with hard compounds - very abruptly changes to a much softer layer internally - unlike traditional case hardening which tails off progressively) once a small amount of wear has taken place the wear will progress rapidly to failure.

Regrinding through to the softer layer might work for some time as long as the engine is not revved to high or driven too aggressively but ultimately the surfaces need re-hardening for longevity.

To do this properly they need to be re-ground below tolerance size (to allow for the growth of the white layer), then polished back to a smooth finish and to end up within the original tolerance band listed above (most crankshaft grinders cannot realign with the ground crank to such a small accuracy to then re-grind the minute amount of white layer off afterwards).

We have made our own crankshafts that are also nitride hardened, but for longer and deeper and the result is that they CAN be re-ground and still be hard enough to work perfectly, but they are more expensive than new Porsche crankshafts and we had to make them in quantity (to get the price down) AND that tied up tens of thousands of pounds WE ALSO NEEDED TO ORDER CYLINDERS AND SPECIAL PISTONS FOR SEVERAL DIFFERENT STANDARD ENGINE SIZES AND OVERSIZED REBUILDS (a range covering 10 different rebuild options) and with similar manufacture of support rings, larger IMS housings and all the spares needed to rebuild several engines/week - there is only so much capital you can throw at issues like this - so with new cranks being still available we have chosen to offer these instead.

We do still however continue to research and develop better solutions for our customers. In the process (for example) we are currently testing oversized bearings and different bearings from two suppliers (just that alone involving several thousand pounds of investment) to a new design specification to find even better options in the future.

Since we cannot wait 20 years of customer use to find out the outcome - we use racing as a useful short cut to compare results.

Unfortunately this can result in a track failure (that our business competitors
will be only too happy to use against our reputation) but this is still the best way to quickly establish if a new solution will prove better or not.

Fortunately, as a result of this policy, we have had a lot of wins and Championships with engines at the highest level of strain at the front and lasting all season than others in several classes but this season (for example) will be testing out some of those crankshaft bearings in our Cayman S and it is entirely possible that as a result we will experience a failure that is extremely useful to us (and our future customers) but is embarrassing (can take a lot of time and money to repair) and looks bad to the uninitiated.

However this is what is behind our generally accepted success and quality and is a policy we will continue with despite that downside.

As new products and solutions become approved we pass them on to our customers and will inform the various forums.

I hope this answers your questions about the subject.

I should perhaps also add that for track use with slick tyres - the wet sump becomes an issue keeping enough oil under the pick up strainer not to lose oil pressure in corners. There are several different solutions to this some of which are better than others.

Ours has flap valves and additional capacity while feeding oil back into the pick up section in corners and we have a cambox scavenge pump as well but still in RH corners (where the oil slides up the camchain housing) it can flick on our oil pressure drop light for a few milliseconds while our ACCUSUMP recovers oil pressure directly into the oil pump housing.

To be frank it is a damned nuisance that it is not dry sump but the events we race in would not permit such a change - so we have to live with it and the issue is so near the limit that it is unfortunately only those competitors who are quickest that push the boundaries even closer to the limit tend to go over it.


Baz
_________________
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You can trust us to "CARE FOR YOUR PORSCHE"
www.hartech.org

Last edited by bazhart on Wed Feb 05, 2020 5:31 am; edited 3 times in total
 
  
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fot0
Newbie


Joined: 03 Apr 2018
Posts: 47
Location: Thames Valley


PostPosted: Thu Jan 30, 2020 9:04 pm    Post subject: Reply with quote

@baz Thanks for your time and explanation. After posting my thread I went away researching on crank hardening techniques and found some interesting articles, all of which ties in nicely with your response.

I fully undestand your R&D and the investment vs consumer return. Your whitepapers certainly underpin expertise on rear transaxle & watercooled cars. I will certainly be using your services in the coming months.
 
  
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