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I think that the stock 951 engine will run very well without squirters, but if anyone is looking to materially increase the rpms or the stroke of the engine, then one may want to change some things. First, one needs thinner rings to prevent ring flutter. Second, one needs a lighter piston to keep the conrod (and what's attached to each end) happy. These two changes together mean that the piston can't handle or shed as much heat as before, while the heat put into the piston increases with more power. By my logic, that heat needs to be pulled from the piston and the easiest way to do that is with a piston oil cooling jet.
By my understanding, F1 with almost 100mm bore and 2mm thinnest spot on the crown needed 19 or so spray jets aimed exactly right per cylinder from two nozzle systems to prevent piston cracking. This is not what I expect our cars to need, I am thinking that an oil spray jet anywhere on the piston will be helpful.
I think this explanation makes the most sense. It is flawed logic to say "because engine X doesn't have this, then our engine doesn't need it!" Each engine should be considered as a system of components, made of different materials having different properties. I would imagine that the designers of said BMW F1 engine carefully considered the material and design of the pistons and rings, and scientifically determined that they didn't need piston oil squirters.
Likewise, Porsche engineers determined a need for piston oil squirters in the 968. At the time, Porsche was a tiny company strapped for cash and were updating a dated car line, so if they invested money into tooling for this, there must have been a good reason! And maybe that reason is that piston oil squirters were cheaper than using better material for the pistons, who knows?
All of this said, I am not claiming that everyone needs to get piston oil squirters installed in their 951 block. But, it is something to be considered if building a high-performance engine that will see track use, with the tradeoff of piston cooling and the effect on the overall oiling system carefully considered.
With the 968 the factory probably got to a point where mean effective pressure was higher than on the 944 S2 to the point where piston crown expansion became problematic under extreme use, requiring extra cooling.
It was probably never a problem on the 944 turbo where pistons are "conical". The 100mm block is probably thermically more efficient as it contains 1.1L of coolant where the 104mm block contains exactly half.
With the 968 the factory probably got to a point where mean effective pressure was higher than on the 944 S2 to the point where piston crown expansion became problematic under extreme use, requiring extra cooling.
It was probably never a problem on the 944 turbo where pistons are "conical". The 100mm block is probably thermically more efficient as it contains 1.1L of coolant where the 104mm block contains exactly half.
If we are comparing 968 vs. 951, 968 has a longer stroke and larger bore, both of which put more conflicting weight requirements on the piston. The simple piston cooling formulas also prescribe a thicker crown for larger bore, and that’s a double whammy in terms of piston weight. It’s not surprising to me that these considerations lead to a more modern type piston for 968 compared to 951 which in turn lead to the piston oil cooling requirement.
Someone that knows what they are talking about, told me the factory explanation for fitting squirters was to control the expansion of the pistons during warm up.
Someone that knows what they are talking about, told me the factory explanation for fitting squirters was to control the expansion of the pistons during warm up.
That’s inconsistent with what Mahle’s piston design handbook says and inconsistent with what Mahle engineer said when we (really, they) designed the last set of pistons. That’s today, though, maybe the original rationale was different back then.
How does the logic work in this explanation? Help me out. Does the piston warm up so much faster than the cylinder tower? Also the jets only open when the rpm is pretty high because they require a minimum oil pressure to open, so in most warm up situations they do nothing. Is the operating mode of concern here one in which someone drives a cold car hard at high rpm right after the startup?
The theory was, some owners may drive the car hard from cold. And with the very small factory clearances it could cause the pistons to pick up in the bores. Don't shoot the messenger.
The theory was, some owners may drive the car hard from cold. And with the very small factory clearances it could cause the pistons to pick up in the bores. Don't shoot the messenger.
No plans to shoot the messenger, that's a useful piece of history.
Also the jets only open when the rpm is pretty high because they require a minimum oil pressure to open, so in most warm up situations they do nothing. Is the operating mode of concern here one in which someone drives a cold car hard at high rpm right after the startup?
IIRC, some factory documentation mentions that the squirters on the 968 open at a pressure of 1.8 bar, which means they are open pretty much all the time, at least on a healthy engine.
IIRC, some factory documentation mentions that the squirters on the 968 open at a pressure of 1.8 bar, which means they are open pretty much all the time, at least on a healthy engine.
I've seen some magazine articles on that. In any case, we're using 928 squirters which open at higher oil pressure.
There's also the question which I asked above about the location where the spray should hit, depending on the piston design. It may not be critical, but with a box-in-box piston the ideal location is smaller than with the older single box "evotec" piston design and much smaller than with the very much older full round piston design.
Do you know why the squirters were deleted on the 928? Perhaps someone thought they were overkill considering the "terrific" hp/L ratio of the S4 (and later) engines?
Do you know why the squirters were deleted on the 928? Perhaps someone thought they were overkill considering the "terrific" hp/L ratio of the S4 (and later) engines?
i think they just said the were dropped as unnecessary.
Yes, this is what is mentioned in the 928 WSM.
So considering all we have discussed so far it seems they were added on the 968 because the engine got to a level of power where they were really needed, as suggested by the extract of the factory documentation, and by their absence on the 944 S2 which is the closest comparable engine and which made 29 less HP.
if you want to adjust the spray angle and where it hits the piston, both the inlet and outlet hole coordinates need to be changed to clear the crankshaft counterweights and hit the piston in the right spot;
05-07-2020, 11:23 AM
