Vortec heads 12558059 305 5.0

Thanks for the pictures, you have made it clear what has been said several times in this column, "that the 305 Vortec head isn't the same configurationally as the 350 Vortec". It is in fact a close relative, design wise, to the 350 TBI Swirlport head casting number 14102193.

Inside the chamber is similar to the late 1980s-early 90s L-98 cast iron head, a feature the 193 also shares. Which is kind of inching toward the 350 Vortec at least on the spark plug side, but it does not have the beak between the valves of the heart/kidney shaped chamber of the 350 Vortec heads from 1996 to 2002.

The ports, as the pictures show so well, have those swirl vanes. These are really good for a bottom end engine that needs to grunt at low RPMs. But they quickly get into the way as the revs start getting over 3000 and totally plug things up by 4 to 45 hundred. They can be removed if one is porting these heads for running in a class that requires these heads and has no restriction upon port mods, otherwise using them is rather pointless except for a stock rebuild.

Bogie

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...

Click to expand...

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.

Bogie

Ok, I was going to try these heads for sheer trial and error on our test rig but a friend wanted to try his new mill out in it first. I have been waiting for my block to get done from the machinist for my rx7. I called him last weak and he said it would still be some time before he can get to it. I was itching to try out the rx7 and wanted to try these heads so I took a 350 I was going to use in my plow truck and tossed it all together.
The 350 is basically stock other then arp rod bolts, melling hv pump and 30 over flats. I had a cam that was an add on in a northern auto kit a while back. It was one of those $20.00 add on deals they offer all the time. I really didn’t think I would ever use it and thought it was a waste of time for 20 bucks. The cam specs are elgine: cl1786pk 3500-6500 302 /246 106lsa 500/500.
The intake, ignition, 3400 stall and everything else was all ready there and intended for the 409 going in it.
I have to say that I am very impressed at this combination working so well. Best idle is around 950 rpm. The cam is installed straight up and runs best at about 14 deg adv. The distributor advances mechanically and I haven’t messed with its stock setting yet.
It runs smooth after about 2000 rpm and on the highway cruises at about 3200 at 55 mph. I think anyways, no Speedo in the car yet so basing from traffic. Acceleration is amazingly strong up to about 6300 rpm. This is where I think the heads start to really constrict flow. I am no expert on this stuff merely giving my experience with them.
The rx7 is extremely light and geared very low around 3.90 with a 13-inch tire. This may have a huge factor with the combination working so well with these heads. I do not know why it works so well but it sure is a lot fun to crack the hammer. It would be relatively worthless to even try to get a time slip from it unless something was done with traction. I don’t want to attempt it at this point until I get the 9-inch into it. The stock rear end makes me leery of an explosion of scattered gears on the highway.
I also didn’t know how the dual 4s were going to work out with it being a 350 and such restrictive heads, but it works flawlessly and the motor exploded to life when the second carb is cracked open. At cruising speeds it runs only on the back carb.
They may very well be a huge pouch when on a heavier car? But they sure perform nice on this combination. I would not be afraid to run them again and I am sure I will use them elsewhere when the 409 is finished.