On the stock valves when they are ground on the 45 for the wide seat they are prone to cracking. This is from Metric. I had stock valves ground on the 45 on my high boost motor and burned a exhaust valve in the first 1,000 miles. This was found when I bent a intake valve from a destroyed DISA valve metal rod getting stuck in the valve seat. Tune was good since I ran the same tune for 15K miles with no issues after we got the new volvo valves in there that were thicker and cut the wide 45 seat.
On the ARP head bolts the shank is smaller then the M3 bolts. I don't know about stretch and all that during torqueing but I go with what they tell me works from R&D. Metric has blown a few head gaskets on boosted motors to get it right. They swear by their head gasket and bolt combo and it has worked good for me so far.
Valve springs. The stock springs are good to 7,000 rpm but after 10K miles if you take them off and measure them they will have fatigue and slight different heights. The Busa springs have not shown this after 10K but we don't have any motors with high mileage yet for a test of like 50K. Everything is lighter with the busa springs which is good.
That is all I can offer.
John S |