Quote:
Originally Posted by cyde01
it just sounds a lot easier to change the rod length and change the cylinder heads and pistons than changing the entire borexstroke ratio. also, do those crazy angled rods have anything to do with the short rod length? i wonder if having the rods at an angle may compensate for the low rod/stroke ratio.
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Although it "sounds easier"; clearance is a much bigger issue that way. If they were "just" going to toss in longer rods then they would have to make the block deck higher (the length between the center of the crankshaft to the top of the block where the head meets) and shorten the cylinder head height. The FB and specifically boxer engines in general don't have a lot of room to play around with side to side.
Extreme example: The FB20 stroke is 90mm, it's current connecting rod length is stated at 129.3mm 129.3mm/90mm is 1.43666 rod to stroke ratio. If you wanted 1.55 rod to stroke ratio without altering the crankshaft you would need 90mmX1.55 = 139.5mm connecting rod.
Compression ratio (CR) of stock FB20 is 10.5:1 and 90mm of stroke results in a ~9.47mm clearance between piston at TDC and cylinder head 10.5:1 CR=(9.47mm+90mm)/9.47mm. Can you already see the problem? Any addition to the rod length decreases the clearance between the piston at TDC and head. In this case I wanted to increase my rod length to 10.2mm when I only have 9.47mm till it nails the cylinder head, less till it meets a valve. And say I don't have the space to increase the block deck height and or shrink my cylinder head by more than 17.7mm to acquire a CR below 13:1. Higher piston wrist pin placement or less tall piston will put some compromise on the pistons ring lands and piston structural integrity so it's not a good solution either.
Fact is you could get a 12:1 CR on a stock FB20. You would need 8.18mm clearance which means you could safely add a whopping 1.29mm to the 129.3mm rod making the rod to stroke 1.451, that engine is still going to wear like nothing else at high rpm and won't like it up there...
I believe the angled connecting rod end caps are better on the set screws in terms of tensional loading when transmitting force to the crankshaft. I can't remember all the reasons.