Quote:
|
Would increasing the rocker arm ratio size from a 1.5 to a 1.6 be worth doing on a 305. Stock cam and springs.
|
That would depend upon your goal. It would also depend upon which rockers you use.
OEM Camshaft Specs - Lift -.359/.384, Duration@ .050 - 186.5/193.8, LCA - 110.3
With 1.6:1 Rockers - Lift -.384/.411, Duration@ .050 - 191.9/199.0, LCA - 110.3
As the numbers indicate, valve guide interference and coil bind will not be an issue - Not even close on stock heads until you get closer to the 0.480" lift area.
Using a stronger rocker will result in even better actual lift numbers and valve train compliance, since the OEM stamped rubber rockers are notorious for deflection under load. Using a roller-tipped rocker may provide even more gains by reducing valve train friction and imparting less side loading on the valve stems.
A word about using aluminum rockers in a street engine - Don't. I've yet to see
ONE street driven SBC with aluminum rockers survive intact past 60,000 miles. The aluminum rockers just aren't very durable in the long term. Moreover, the forged steel rockers are actually
lighter, as well as being much stronger. Unfortunately, they are also more expensive.
Unless you will be installing guide plates (very doubtful if you are not replacing springs and studs) you will need to use self-aligning rockers. That limits your choices. I'd suggest Comp 1418 or a similar rocker for your application. If you want to go fully forged, and double the price, there is the Comp 1318 to fill your needs and empty your wallet.
Further, your valve springs are "stock" which means that on the day the engine was built, the springs had about 85# of seat pressure. They are likely a lot weaker after 20 years of running, and are subject to valve floating at a lot lower RPM. Add the unnecessary mass of the factory exhaust valve rotators, and the engine will be self-governing at about 4,500 RPM due to valve float. At least you'll never blow it up from excessive RPM. If you are expecting performance gains from the rocker arm switch, you might be a bit disappointed. You can at least have the satisfaction of knowing that the rockers will be basically bullet-proof, and can concentrate on other areas of the valve rain in the future.
Additionally, for those who will argue that changing the rocker ratio does not change the valve duration (and there will likely be many), you need to complete the analysis of the valve train math. All modern hydraulically lashed valve trains specify camshaft duration degrees as a function of valve opening at 0.075" lifts (0.050" lobe lifts) in relation to crankshaft rotation. If you plot the valve curves with higher ratio rockers, you'll understand that the higher ratio rocker causes the valve to reach 0.075" lifts sooner than a 1.5:1 rocker, and holds the valve open above 0.075" lifts longer. That means the effective VALVE OPENING TIME (a.k.a., "duration") is longer. Opening the valves is what a camshaft is all about, so forget the conventions you've been taught, and understand the complete system. I'm fully aware that the rocker ratio does not change the lobe grind, but it DOES affect the way the valve reacts to that lobe. I can bore you even further with charts, spreadsheets, and pictorials, but you can probably grasp the concept without all of that.