Heads to replace Pro-Topline

Well you can go to head with a 64cc chamber, lots of then in the aftermarket that use a modern Ricardo chamber similar to the Vortec L31 but will use your current intake. Summit, Jegs and others have very decent house brands. You can get these in iron which lets you keep a shim gasket to hold your squish/quench clearance on the pistons you have. You could go to aluminum heads but they will need or at least prefer a composition gasket but the thinnest of these are .026 to 028 inch which is going to open up the squish/quench to .051 to .053, however, all is not lost the slightly diminished Squish/Quench function is more than compensated by the thermal characteristics of aluminum. The GM L98 and LT1/4 ran .078 S/Q clearance with aluminum heads on these engines.

The Ram-Jet cam you sight is simply the old 300 horse 327/350 cam ground as a roller with the lobe centers squeezed together to reduce the LSA. This makes for a rougher, meaner sounding idle but basically leaves you a lopey 300 horse cam. In my opinion the go to flat tappet cam is the Comp XE268H. This cam with modern 64cc chamber heads be they iron or aluminum is an easy 370 to 380 hp engine around 5600 maybe 5800 RPM engine with about 390 to 400 ft lbs of torque around 3500-3700 RPM with a very long and high torque curve on either side of the maximum. This with an unmolsted head just bolt in pretty much standard parts, except for the cam kit, together. Milling the chamber down a little, beefing up the valve train with 7/16ths studs, 1.6 roller rockers, cleaning up the ports, just that kind of stuff makes a 400 plus hp engine that lets you know it’s there with an authoritative idle, but has enough manifold vacuum to run accessories and is just at what is comfy with a stock stall converter. Not that it needs be the comp cam but read the specs and compare to other brands that make similar cams. The Comp, if you go to their site shows about 350 hp or so but they used older (buy Darts current design) SR heads. The new SR heads use, you guessed it, a Vortec chamber. But they mixed this with more than less standard GM porting and they include an exhaust heat crossover just like the original GM heads. EQ and some others are using the, aaah, ahem, Vortec chamber. These days on the circle tracks that require an SR head if your not using these modern Vortec chambered SR heads the best your gonna do is run toward the back of the pack.

What can I say besides the only thing getting better with age is the parts for the SBC. Anyway check out the specs of zero lift duration to .050 duration, getting out the amount of degrees in the ramps between zero and .050 is important as is lift, look at the LSA 110 is about as tight as you can go before the low speed characteristics go totally wonky, this in terms of overlap and where the intake closes is a variable with total durations. A short timed cam with a tight LSA has the lobes twisted together to get more overlap, this can come at the cost of an earlier closing intake depending on which lobes got moved and by how much. These cams like the Ram-Jet are trading overlap for better low speed cylinder trapped pressure, but if your running a carb or TBI rather than timed port injection these type cams do throw mixture out the exhaust, that doesn’t cost power but it is a cost added at fill up time. This is a big reason why I’m not crazy about the R-J cam if your not running sequential, timed injection.

Another way to go at this is with a piston replacement with either D dish or flat tops which improves the burn for any head.

Bogie

Roughly the equivalent of import head’s in the same market, they are very good for what they are shooting for.

Aluminum head’s either need raised compression height pistons or zero decked or close to zero decked blocks. The problem here is the race between the desired .040 inch piston to chamber step distance for best squish/Quench and available composite head gasket thickness. The minimum composite gasket available being .028 inch which combined with the factory piston crown to deck clearance of .025 arraives at .053 inch which while not a disaster with aluminum it is going the wrong direction especially with factory style round dish pistons which add even more to clearance to much of this critical dimension so you end up throwing torque and power away while increasing detonation sensitivity and reducing burn efficiency.

Round dish factory style pistons are to be avoided for reasons stated above. They increase the squish/quench distance (clearance) over much of the available area of the step. This is a two part function useful to extracting energy from the burn and resisting detonation. Prior to ignition by the spark plug the rising piston squishes the mixture which rather forcefully shoves the mixture into the valve pocket, mixing and presenting it to the spark plug, this is the essence of “fast burn”. This reduces miss fires as well by standing most of a mixture’s molecules before the spark and makes them available to chemically react, this is one of the reason that modern head’s do not need so much spark lead another is a more centered spark plug so the burn starts close to the bore center spreading outward instead of starting on one bore side and burning across the bore. The second activity is the closed area under the step becomes a heat sink with little volume and great surface area. This cools what little mixture is on the farthest end of the chamber lowering the temperature pressure gain of the yet to be burnt mixture below the point of self ignition which is a secondary burn that explosively starts ahead of the flame front, Detonation is the ping sound that you hear of this event. It lowers power and is extremely destructive especially to standard alloy cast and cast hyper-eutectic pistons burning holes through the ring pack or just disintegrating the piston.

Aluminum head’s are more forgiving of these bad actors than cast iron because of aluminum’s faster heat transfer rate than iron. Here not pushing squish/quench and the higher compression ratios it tolerates to its optimization range does reduce power and burns more fuel in the process which also unnecessarily increases pollution that can otherwise be simply avoided. Getting this right is the difference between just another so-so performance mod and building a contender.

Getting to the balance of maximizing compression ratio for the fuel octane available typically results in a closed heart chambered 64cc head with a D dish piston, or using a flat top piston with a large heart chambered 74-76cc head. With modern heart chambered head’s either way will work.

Of course the traditional way of building hot rod engines is to skip the engineering, just stuff a huge cam in it, then wrestle the needed stiff axle ratios, high stall converters, difficult to impossible to tune carburetors or fuel injection and ignition.

Bogie