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kenikh kenikh is offline
3 restos WIP = psycho
 
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Join Date: Feb 2005
Location: North of Exit 17
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Quote:
Originally Posted by stownsen914 View Post
The engine I currently run is and oversquare 2.7L (93mm x 66mm). I shift at 7500, and the engine has never seen over 8000. The engine makes about 260 hp (this is an estimate). I would have thought a well-prepared mag case would have handled this, but my experience was different. My scenario is probably a rare one. But I think it's fair to say that aluminum is significantly stronger than magnesium, and I guess I just prefer to err on the side of reliability when it comes to a race motor's crankcase.

Scott
First you are right...Aluminum is much stronger. This is why Porsche chose aluminum to support 700HP turbo motors. They have lots of runway, where mag doesn't.

Your spec is definitely "at (or over) the limit" for any mag case, even a well prepped 7R. The short stroke crank was defintely to the motors benefit, but 93mm is in the "danger" category for P/Cs on mag cases due to the thinness of the spigot bore. 7500 RPM, while below 8K, is still pretty high, given this. Was the crank counterweighted? If not, the rocking stress from an non CW crank is also contributory.

A well prepped mag case is only one factor for a motor of this spec. Anyone running 93mm pistons should spend an inoridnate amount of time lightening the rotational mass of the internals and balancing the components to ensure that rocking stresses are mitigate as much as possible. R&R rods, JE FSR pistons all help here. Nickie cylinders help too, since they require thinner skirts than Mahle, so you can keep more meat on the spigots, too.

Steve Weiner and I have talked A LOT about the challenges of mag cases. He feels that specced correctly, they can live a long, long life without cracking. He said that it is when you push any of the above items, you can run into problems.

Net-net: keep the bore small, the case well oiled and reinforced, the rotating compenents as well balanced as possible and the stroke short, if possible and 300HP is well within these cases' ability to manage reliably.
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Old 02-11-2010, 07:54 AM
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