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D Series 426 Race Hemi Heads

During 1969 and 1970, Chrysler did some experiments in refining the Hemi cylinder head. Namely working with the various aspects of the design that affected the intake and exhaust flow. The most feasible ideas were designated for prototype or limited production use on the racing engines. These became know as the D heads. Here are the various D heads:

D1

Alternate cylinder heads for the 426 race Hemi. Initially developed in 1969 with the same intake and exhaust valves, but with larger ports. Each intake port had 3.00 sq-in and the exhaust port had 2.10 sq-in. Intake valve angle was 35 degrees; exhaust valve angle was 23 degrees.

D2

Alternate cylinder heads for circle track racing. Initially developed in 1969 it was essentially the same as the D1 but was overbored 0.02-in to create 429 ci (as permitted by NASCAR). This program was cancelled after about a month since performance gaines were not sufficient.

D3

Alternate cylinder heads for the 426 race Hemi. Initially developed in 1969 using larger intake and exhaust valves. Intake valves were 2.38-in with a 3.65 sq-in intake port, and the exhaust valves were 2.00-in with a 2.53 sq-in exhaust port. Valve angles had to be changed due to the new intake valve size. Additionally, the cylinder bores had to be notched for intake valve clearance. Ironically, the power was less than the D1 program when power runs were made on May 9, 1969.

D3.5

Alternate cylinder heads for the 426 race Hemi. Initially developed in 1969 using the larger intake valves and ports from the D3 and larger and straighter exhaust ports of the D4.

D4

Alternate cylinder heads for the 426 race Hemi. Initially developed in 1969-1970 using the same intake valves and ports as the D1 but with larger area, higher exit and straighter exhaust ports. The first D4 head power runs on May 1, 1970, and the 426 Hemi generated 641 hp.

I have been told that Mullen made an aluminum version of the D4 head, called the D4A.

D5

Alternate cylinder heads for the 426 race Hemi and probably the most famous of all of them. Initially developed in 1970 with all of the improvements of the D4 but cast in aluminum alloy using two spark plugs per cylinder.

We believe the D5 Hemi heads had 2.35 intake valves and 1.94 exhaust valves, slightly larger than previous Hemis and round.

D5 Hemi Head - Top D5 Hemi Head - Bottom

D6

An aluminum alloy head with two spark plugs like hte D5, only now included a raised exhaust port.

D20 Magna

Alternate cylinder heads for the 426 race Hemi. Initially developed in 1970 with substantially larger ports and a wider iron head to house the ports. The intake valves were 2.50-in with a 4.00 sq-in intake port, positioned 35 degrees, 45 minutes from cylinder bore center. The exhaust valves were 2.20-in with a 2.6 sq-in port, positioned at 36 degrees. The larger head design required larger cylinder head covers.

D21 Magna

Alternate cylinder heads for the 426 race Hemi. Initially developed in 1970 as an aluminum-alloy version of the D20 Magna.

Last Updated: 2002-09-06

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