Unlike Wedge heads, the Hemi...
Unlike Wedge heads, the Hemi has dual rocker shafts and the design puts the exhaust valve so far to the opposite side of the head that significantly shorter exhaust valves are used. Another difference is that Hemi exhaust valves generally benefit from a tulip design instead of a flatter angle on the back side of the valve.
Part of making big power with the Hemi head is by cranking those valves open as much as possible. “They like all the lift you can get. With an OEM head, you get too big and you start risking valve edges clipping each other. That 426-1RA head that Indy did actually starts playing with moving the valves around a little bit.” In fact, when designing the heads, Indy shifted the exhaust valve 2 degrees toward the cylinder wall in order to gain valve-to-valve clearance with tight lobe separation and/or big cam designs. They also have big honkin’ shaft rocker bosses cast directly into them. Those 1.6 ratio Indy rockers fit perfectly on the supports and operate Ferrea stainless steel valves without issue. “The good thing about it is when you put your rocker shafts on those pedestals, it’s really easy to shim those shafts in a position to correct the geometry, or travel misalignments across the valve tip.”
Another goody that the Indy heads have is a raised exhaust runner. Rather than having the hot exhaust gasses blowing out the valves then making a sharp U-turn to find headers mounted down low, the “RA” heads move the port exit up a solid .5 inch for a smoother transition to the header. When mocking the heads up in an actual car, it is recommended that the header-to-floorpan clearance be checked since it will be raised with those heads.
The block pushrod hole is...
The block pushrod hole is seen between the upper head bolthole and the water passage. It is easy to see here how much dome is required on top of the piston to make an 11.4:1 compression ratio. High-compression (14-plus) engines would need a massive dome that almost mirrors the giant hemispherical combustion chamber in the head.
When asked for his advice on choosing a set of heads for a street Hemi, Corey told PHR: “The Hemi ports are big to begin with, and I would definitely pay particular attention to how big you go. I would be fairly conservative and stay a little bit closer to the smaller side, especially if you are going to be on the street with it. The [Indy] 426-1 heads will flow 430 at .700 lift and you compare that to an OEM head that flows about 330, so there’s quite a bit of difference.”
Because the Hemi heads are capable of moving such a large mass of air, it is quite common to see them with a big blower mounted on top. In the case of the Triple S Hemi, a naturally aspirated intake manifold was the solution. There are several options for intakes and the Shorts dyno tested three of them to find the best results. “The first time we ran it, I had an old Offenhauser tunnel-ram that had a single 4500-series carburetor on it, and it had a divider that almost split the whole plenum in half. The first time we pulled the trigger it started nosing over at 6,000 rpm abruptly, which was the first time I’ve seen any Hemi that I’ve built do that. So I had another intake manifold from Indy Cylinder Head, a 426-2, that I bolted on with a 4150-style 830 Holley, and it immediately picked up a full 100 hp over the tunnel-ram. Looking at the graph though, it was still nosing over pretty quick by about 6,800 rpm. Indy had another [426-4] manifold that was a little taller with a little more plenum volume and runner length that was set up for a 4500-series carburetor, so I tried that and it picked up another 60 hp,” Corey says.
During initial dyno tuning, the Shorts had an ignition problem to deal with on top of their intake trials. This was solved with the addition of a premium MSD dizzy that brought unexpected benefits. Corey says, “We had some issues with a distributor that I had in the engine, and so we bought an MSD Billet unit and tried it, and it liked that by 30 hp.”