Berliner Bel Air said:
Are these heads at all a 'fast-burn' type or not? The Vortec 350 should be timed initial + mechanical at about 32 degrees maybe less, while the older ones are supposed to be at around 35 degrees. Where should the timing for an engine with these heads be? I was under the impression that these were a 'baby brother' to the normal Vortec heads and timed the engine accordingly. Now I'm not sure that that is correct...
The combustion chamber is more like the cast iron version of the L98 head, the aluminum being the same but because of thermal properties can sustain another point of compression which the factory took advantage of in 'Vettes and Z-28 Camaros. The cast iron version being saved for passenger sedans. Advance is like compression in that an aluminum head can take more of it.
But of course the whole point behind fast burn combustion chambers is being able to reduce advance without a loss in power. Actually this often shows a gain as there is less back force on the crank from early ignition and a longer smoother power stroke. But the real advantages are lower emissions since the peak cylinder pressure is less spiky, which also reduces the tendency to detonate (another power eater). This also lets the factory get away with a leaner mixture producing better fuel economy. A good deal all around.
A word about detonation, probably the root cause of more engine damage than anything. As soon as it occurs power falls very quickly from the disorganized and explosive burn. The forces are well reputed to destroy valves, pistons and spark plugs; but it's seldom connected as the root cause of damaged to destroyed rod bearings which often leads to rod failure that everybody blows off to excessive revolutions or poor oiling. Not to say that excessive revs aren't a contributor, but what detonation/preignition does is to place so much load down the rod that it forces the oil wedge out of the bearing with predictably, screechy result. Many a blue to black bearing is blamed on the oil system when it was "pinging all the way up the hill" (form an old Texaco ad) that did it. Back in the 1960s this was a big problem that went undiagnosed for a long time with the 406 and 427 FEs. These engines have an open chamber plan the plug is way off to one side opposite a fairly small squish/quench pad. This configuration really made them ping prone (had a few of these and you really learned the difference between 100+ octane gas and the wanna bees not so hundred octane swill). Like Ford the Oldsmobiles and Pontiacs had similar chambers, rod ratios and high power, high RPM problems. Combine the detonation problem with Fords proclivity for large diameter, narrow width rod bearings and you had the perfect storm going on the the oil wedge. Rods came and went, cross bolts came, then the side oiler, then the Le Mans crank. All in an attempt to control a problem that originated in the combustion chamber. Oh well, so much for history lessons. But if you lived thru the era, one had to wonder why SBFs and SBCs didn't have the problem, neither did BBC Rats, the W motor certainly had it, and BBChs were pretty safe with their long rods and huge bearings covering their butts with those really open wedge chambers they used. Everybody else suffered from some to big degrees.
The difference in the 305 chamber shape and that of the 350 Vortecs also gets into the fact that burn time is related to bore. The 305 has a smaller bore so it isn't as sensitive to burn time as the larger bored engines which need more burn speed since the time is RPM dependant. So GM took advantage of this by eliminating the beak between the valves and save a fraction of a penny per engine on the 305. This isn't new, Ford when they released the Y block Lincoln in 1952 used a fastburn chamber, looks just like these modern "fastburns" with the plug toward the exhaust valve, a little relief in the chamber wall from the intake valve to the plug. The wall moves in a bit on the exhaust valve from the plug into that sides cylinder wall, and a beak protrudes from a half chamber squish/quench pad between the valves. The smaller Ford/Merc Y block that came out in 1954 also had this feature in the chambers. Then in 1955 it was gone from both engines not to return in anybody's production engine till the mid 1990s. However, it's an old concept for a vertical valve engine discovered by Sir Harry Ricardo (The High Speed Internal Combustion Engine) back in the late 1920's early 1930's. The Gurney/Westlake Ford heads of the the 1960's took it racing along with a 9 degree angle on the valves. It showed up again on the Doug Roe modified GM Vega head in the 1970s. It probably showed up in other racer built highly engineered engines from time to time as well, but the factories didn't get on the band wagon till the competing needs of low emissions and high mileage forced them to back to it. Then they discovered that not only did these chambers do good things for emissions and mileage, but they made power; just like ol' Harry said they did.