SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: eliminator18 on July 14, 2015, 10:26:12 AM
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Has anyone ever tried using 1.7 rockers for intake and 1.5 or 1.6 on exhaust? Just wondering if you can mix them up like that if say you have a piston with only 1 valve relief
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Yep. It's done often.
Had a 355 small block that had different ratios on the corners.
GT
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Yes, that is common to run different ratio rockers, but for different reasons than piston to valve clearance.
Most likely you will not need an exhaust valve relief cut in the piston. The intake valve is substantially closer to the piston at TDC as it is (or should be) aggressively opening at that point. This means that the piston and the intake valve are quickly approaching each other.
Along with a smaller diameter, the exhaust valve is also located much higher in the chamber relative to the intake valve. To put this into perspective, while placing an un-surfaced BBC head on a flat surface, combustion chamber side down, a 2.19 intake valve will hit the surface at approximately .070" lift and a 1.88 exhaust valve will hit at approximately .520" lift.
Another thing to keep in mind is, advancing a cam decreases intake piston to valve clearance but increases exhaust clearance.
Retarding cam timing increases intake piston to valve clearance and decreases exhaust piston to valve clearance.
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A 1.6 rocker, on a BBC, will effectively decrease lift.
Standard rockers are 1.7, and 1.8's are available to increase lift.
With a cam with .500" valve lift for 1.7 rockers, would be .470" lift with 1.6's, and .529" with 1.8's
It can make a cam feel a little larger than it is, with 1.8's
Dan'l