1.85 Rockers VTP Clearance Issues?
02-18-2008, 07:01 PM
#1
Staging Lane
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1.85 Rockers VTP Clearance Issues? Hey guys i have a 4.8L truck with flat toped pistons. I also have 243 heads with a 02 zo6 cam. My current lift is .551/.547 with the rockers my new lift will be .600/.595. I was wondering if any one has ran 243 heads with flat toped pistons and a cam bigger than .600. The reason why i ask is i found this article today on car craft.
"The valve-to-piston clearance in swapping LS6 heads on another Gen III engine is also an issue because GM has used multiple piston dome/dish and cylinder head combinations. Going with the small LS6 combustion chamber cylinder heads, a nearly 0.600-inch lift cam, and a flat-top piston will challenge the accepted 0.080-inch clearance between the piston and valve at max lift and piston TDC. This is best accomplished by using modeling clay (not Play-Doh or similar play clay, as it "springs" back giving a false reading) between the valve and piston during the "mock-up" stage of the engine build"
Website http://www.carcraft.com/techarticles...ads/index.html
Thanks any help, opinions, or suggestions would be great i just dont feel like assembling my engine with that clay, rotating my engine and then disassemble it to see the clay.
"The valve-to-piston clearance in swapping LS6 heads on another Gen III engine is also an issue because GM has used multiple piston dome/dish and cylinder head combinations. Going with the small LS6 combustion chamber cylinder heads, a nearly 0.600-inch lift cam, and a flat-top piston will challenge the accepted 0.080-inch clearance between the piston and valve at max lift and piston TDC. This is best accomplished by using modeling clay (not Play-Doh or similar play clay, as it "springs" back giving a false reading) between the valve and piston during the "mock-up" stage of the engine build"
Website http://www.carcraft.com/techarticles...ads/index.html
Thanks any help, opinions, or suggestions would be great i just dont feel like assembling my engine with that clay, rotating my engine and then disassemble it to see the clay.
02-18-2008, 07:42 PM
#2
None of these engines would clear "at max lift and piston TDC." While they meant well, what the boneheads failed to mention in that article, is that "at max lift and piston TDC" never happens in a running engine. Furthermore, it's duration, not overall lift, that determines PTV clearance.
As the piston approaches TDC on the exhaust stroke, the exhaust valve is nearly closed, while the intake valve is just starting to move off the seat. At the point of TDC, each is usually less than .100 valve movement off the seat, leaving plenty of room. Since you are running the LS6 cam at just 204/218, it's next to impossible to raise the rocker ratio enough to put the valves that close to the piston.
As the piston approaches TDC on the exhaust stroke, the exhaust valve is nearly closed, while the intake valve is just starting to move off the seat. At the point of TDC, each is usually less than .100 valve movement off the seat, leaving plenty of room. Since you are running the LS6 cam at just 204/218, it's next to impossible to raise the rocker ratio enough to put the valves that close to the piston.
