I'm looking at the Porsche 930 turbo cylinders here, and find it very strange that they ONLY have cooling fins on the bottom.
Why is it that the 930 turbo cylinders have significantly lower cooling fin area than the normally aspirated cylinders? The turbo engines by definition produce a good bit more power, therefore they're pumping quite a bit more heat.
Any ideas why the cooling area of the 930 turbo cylinders is so low?
The only explanation I can think of is that they run a lower compression ratio, so under non-boost conditions, they produce less heat, and Porsche reduced the cooling fin area to keep them from getting too cool?
Yup, these are nikasil, but All 2.4+ 911s are Nikasil, only the early 2.2 911s were cast iron.
The thing I can’t figure out is that the non-turbo ones of the same year had full circumferential cooling fins, and all the subsequent non-turbo ones also had circumferential fins. Only the turbo ones used these smaller asymmetrical fins.
Does Porsche know something about turbos that our air cooled VW turbo builders don’t?
It does make a lot of sense that the fins should be bigger on the bottom because of airflow, but I can’t figure out why the total cooling area looks smaller on the turbos.
Yup, as Slow 1200 pointed out, the 930s have a faster spinning fan. Head heat is big problem on air-cooleds, so maybe the increased airflow cooled the cylinders too much ???
It's not just about the cyls and fins. It's also about the ducting under them. Obviously, Porsche wanted to run the underside of the cylinders open to the air on the bottom, without extra panels making the air go further around. Here's an early motor with the ducting in place...
I can see how the bigger fins would have run down into the bottom of the ducts, trapping the air in the fins all the way down. I guess these ducts do pretty much the same thing for your VW. I've got them on my car.
Not to change the subject but I remember when I first built my turbo system back in 07' or so the big thing seemed to be oil squirters for the pistons. I haven't heard that mentioned in a long time. Did it turn out to be ineffective?
Oil squirters are mandatory IMO for any air-cooled turbo. Even virtually every water cooled turbo and nearly all high-perf N/A water-cooled engines have them.
They're especially important for an air-cooled turbo IMO because of how hot these engines run, and they're actually oil/air cooled. In fact on a 911, the oil plays a huge part in the cooling. Also, if you look at the VW service manuals, they recommend notching the rod big-ends to act as an oil squirter on the T4.
I'm currently planning a T4 turbo, and I haven't decided yet how I'm going to set up the oil squirters, I'd like to fit the VW 1.8 turbo oil squirters in the block by machining the inside of block, and setting up some oil lines to go to them. My second choice would be to go with the Porsche 930 style with a pressure valve in the rod big-end, where the pressure valve opens at something like 20 PSI. This I think is a better approach than notching the rod as it keeps oil pressure up at idle, whereas the rod notch approach could let idle pressure drop too low.
