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.
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.
