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jluetjen jluetjen is offline
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Join Date: Oct 2001
Location: Westford, MA USA
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Quote:
since you're asking a lot of the cast iron cylinders if you exceed 20-ish horsepower per cylinder and still expect to be as reliable as it was in stock form. With 200 horses, I would expect an overall reliablity of maybe 50,000mi between overhauls, maybe less, depending on how hard you drive it.
I guess I'm awfully confused about this statement.

1) Where did the 200 HP limit come from? What's the 20-ish per cylinder??? That just doesn't add up! Iron liners have been standard issue for F1 engines up to maybe 4 or 5 years ago. So they were putting out something like 150-200 HP per cylinder.

2) Iron bores are standard on many engines (SBV8 for example) with very reasonable service lives. Maybe not quite up to the silicon impregnated aluminum as was perfected with the Nikasil cylinders, but I wouldn't expect it to be bad.

Quote:
If it was me I'd be worried about heat dissipation for anything over 200hp. I've got a modified 2.4 running iron cylinders, and heat management is an issue - especially as I am currently without an external cooler (it's going on soon).
Cast Iron has a conductivity of 58 W/mK at 27 degrees C, and aluminum is more then 3x that at 202W/mk. Great. But that may not be such a big deal when you consider that while the combustion area may have a temperature of 2000 - 2400 degrees C, and the head has a temperature in the range of 200 to 250 C, the tops of the cylinder walls (near the head) will generally have a temperature of about 160 - 220 C, and near the crank case a temperature of 100 to 150 C. The point is that the cylinders have a lot less to do with conducting heat away then the heads. BTW, this is why the fins on a cylinder are longer near the top and smaller near the bottom of the barral. If it was just a question of the ability of the iron cylinders to conduct heat, you would expect that they could have just put more fins on the barrels. But this would have added weight which appears to be a path that the 911 engineers generally avoided at all costs.

A good feature of the iron cylinders is that they expand less then the aluminum cylinders -- actually by a similar amount to the steel studs -- so I'd expect that you'll rarely find an issue with snapped or pulled studs with iron cylinders. Also since they don't expand as much, they'll hold the bore dimension better which means that the pistons will fit better in a wider range of situations.

I guess as I understand it the benefits of the two choices are...

Aluminum (Nikosil): Lighter, longer wearing, conducts heat better (but this may not be critical in the case of the cylinder walls).

Cast Iron: More dimensionally stable under a wide range of temperatures, stronger

Did I miss something?
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'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
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Old 05-06-2005, 11:53 AM
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