Rocker Ratio Redux?
Frederic Giroux, somewhere in French Canada: I am the proud owner of a '67 Dodge Dart GT equipped with a '69 340 engine. The engine is 0.030-inch overbored, and for the goodies, it has a Mopar Performance cam (PN P4452992; 280/280, 0.474/0.474-inch lift, 110-lobe separation). It also has an Edelbrock Performer RPM intake topped with a Holley 750-cfm vacuum-secondary (which needs a total tune-up), TTI step headers, MSD 6AL ignition (6,000 rpm cutoff), and a Proform electric fan. The rest of the engine remains stock. The tranny is a 727 equipped with a B&M Transpack and a 2,500-rpm stall converter. The 831/44 differential runs an Auburn Gear LSD Pro series with a 3.55:1 gear.
The car runs 13.74 at 103 mph with some old, crushed Hooker headers (the TTI headers are brand-new), street tires (BFG Radial T/A), and steel wheels. The car is intended for street and strip performance (80 percent street, 20 percent strip). My objective is to run 13 flat in the quarter on street tires and street trim.
I plan to do a complete head job (port and polish with intake port-match and bigger valves), and I want to change the old rockers for some brand-new roller rockers. A friend told me I should use 1.6:1 rockers on the exhaust side and 1.5:1 rockers on the intake side to "stimulate" my cam a little bit and to make more power. Does it make sense or should I go with another formula? Which rockers are best for my application?
Terry McGean: Your friend is suggesting a way for you to offset the grind of your cam without actually changing the cam. Your MP cam has the same specs on both the intake and exhaust sides, which was standard practice for performance cam grinders for many years. But the intake valves and ports on your engine are not the same size as those on the exhaust side-they're larger, which is typical of American V-8s. For that reason, many modern cam grinds have additional lift and duration on the exhaust as a means of crutching the exhaust side to make up for the reduced flow.
Using the 1.6:1 rockers on the exhaust side of your engine would turn your 0.474-inch lift to 0.505-inch; duration would be increased by a very slight and insignificant amount. If you really want more cam timing, consider swapping the cam itself. The MP grind you're using is good, but you could step up to a split-pattern grind with a little more profile to get even more of the benefits your friend is trying to get for you with the offset rockers.
You said you were planning to port those heads, but be careful-an inexperienced porter can easily make heads, even old stock castings, worse. Simply making the ports larger isn't necessarily the path to increased power. Look into techniques for reshaping and blending the bowls just under the valve job-that's where most of the power is hiding. The old gasket-match usually doesn't yield gains on par with the amount of effort it requires, and again, you could screw the castings up.