You’ve got questions. We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re examining camshaft duration and piston-to-valve clearance.
D.W. Nova Scotia, Canada
Q: I have a 454 in a 1983 Trans Am. I would like to install a bigger camshaft with an operating range to 5,500 rpm, yet have safe piston-to-valve clearance. The cam I have now is a Crane 216-degree duration (@ .050-inch lift) .515-inch lift version that gives me .190-inch clearance on the intake and lots on the exhaust. Can I install a cam with the same lift but more duration and have enough piston-to-valve clearance? How does duration affect clearance?
Also, how do I determine if a GM 700R-4 transmission is a 27-spline or 30-spline without taking the transmission out of the car?
A: Duration is the number of crankshaft degrees a cam holds a valve open. If you use a cam with more duration, there will be less clearance as the engine turns and the piston hits top dead center. So yes, you can use a cam with more duration, but you will have to measure piston-to-valve clearance very, very carefully. As a general rule of thumb, minimum piston-to-valve clearance should be .080-inch intake and .100-inch exhaust. Check out our earlier posts on measuring piston-to-valve clearance here.
As for your 700R-4 question, there is a date code on the left side of the transmission case, just above the rear corner of the pan. Once you find the code, you will have to take it to a Chevy or GM dealer and have them tell you when the transmission was made. If it was made before 1984 1/2, the transmission is a 27-spline; after that date, 700R-4s were 30-spline units.