Last month we began building the late-model Hemi engine for our '10 Dodge Challenger drag car by bolting together a 426-inch bottom end using a factory 6.1 Hemi block. Our intentions with this late-model Hemi are to show Mopar Muscle readers how well the latest version of the Chrysler Hemi can run when built properly using a combination of factory and quality aftermarket components. We've challenged ourselves to make big power from this engine, and we decided to build the engine to run on pump gas to reduce the weekly costs of operating our bracket car during race season.
Inside our factory block, we used a forged crankshaft and rods from Callies' Compstar line, Diamond forged pistons, a Comp hydraulic roller camshaft, and a Milodon oiling system with a Melling oil pump to keep things lubricated. With a bore of 4.090 inches, and a stroke of 4.050 inches, displacement is just shy of 426 inches. Of course you can see all the details of this engine's short-block in the March issue of Mopar Muscle, or read about it here. And while the short-block of an engine such as our late-model Hemi is important in terms of power and durability, the best components in the world won't matter if the engine isn't topped with the proper cylinder heads and induction system.
1. Last month we assembled the short-block of our late-model Hemi. This engine features fo
2. 3. All the late-model Hemi heads have the potential to make power, but we chose the lat
3. This head can flow 340 cfm on the intake side right out of the box, and with Indy Cylin
Chrysler has made several versions of the Hemi cylinder head since the inception of the third generation Hemi, and all are impressive in terms of power potential. All late-model Hemi heads feature dual spark plugs, a feature that improves both the power and efficiency of this engine. Dual spark plugs, located in the center of the combustion chamber, have been shown to not only improve reliability, as in aircraft engines, but have also been proven to increase power through a more thorough burning of the air/fuel mixture. Additionally, the dual plugs reduce emissions by burning the air/fuel charge more completely, allowing the engine to meet government emissions standards using less restrictive catalytic converters. Chrysler really did their homework when designing the late model Hemi head, and it shows in power these engines make right from the factory. With aftermarket companies such as Indy Cylinder Head further developing this technology for use in high-performance applications, the late model Hemi is in our opinion the best option for a performance build when power, weight, and efficiency are considered.
Since we're working with Indy Cylinder Head on this project, we knew that whatever late-model head choice we made would be treated to Indy's proprietary “Windjammer” CNC porting, and that we could make very respectable power with a set of the 5.7, 6.1, or 6.4 cylinder heads. The latest offering from Mopar, however, offers some distinct advantages when compared to earlier late-model cylinder heads. Utilized on the 6.4 (392) engines, the newest late-model Hemi head is called the Apache head, and is the cylinder head we chose to top our late-model bracket engine.
4. After CNC porting, the Indy crew assembled our Hemi heads using stainless steel valves,
5. The intake ports of the Apache head are square shaped, and CNC porting makes them into
6. Exhaust ports are D-shaped which is unique among the late-model Hemi heads. Equipped wi
The Apache head in stock form helps the 392 Hemi achieve its rated 475 horsepower and 460 lb-ft of torque in part thanks to longer valves and high-flowing square-shaped intake ports. The exhaust ports of the Apache head are D-shaped, and also flow better than any of the previous late-model Hemi head designs. The 6.1 head was no slouch, flowing more than ten percent better than the 5.7 head, and the Apache head has been shown to flow even better, up to 340 cfm on the intake side in stock form. These numbers are impressive, but with Indy's Windjammer porting, our Apache heads flow better than 370 cfm on the intake side, more than any other factory Chrysler cylinder head. Apache heads also offer a closed chamber design, with 74cc chambers, which combined with our Diamond flat-top pistons, will help us achieve a pump-gas friendly 10.85:1 compression ratio.
In addition to the CNC porting, we had Indy perform a multi-angle valve job, and install new 2.14-inch intake, and 1.65-inch stainless steel exhaust valves. To fasten our ported Apache heads to the block, we utilized a set of ARP head studs and Cometic MLS (multi-layer-steel) head gaskets. At 10.85:1 compression we aren't really pushing the limits in terms of cylinder pressure, but we want to make sure our heads stay in place for multiple seasons of drag racing and that our engine remains consistent and maintenance free. MLS gaskets ensure a proper combustion chamber seal and the ARP head fasteners are far superior to the factory head bolts, and unlike the factory bolts, the studs can be reused without the fear of stretching.