Courtesy of posts in ClubCobra by Byots

The engines installed in Cobras were generally as follows:

• Comp cars (very early production) - MR 1x4 side oilers with "C5AE-G" blocks, although there may have been some center oilers in there (?) and high riser heads.

• SC cars - Most, if not all, believed to have had 427 2x4 side oiler with "C5AE-H" blocks

• Street cars through approx CSX3200 - Center oiler "C5AE-A" 427 with 2x4, although a few cars had early installations of 428's.

• Street cars from CSX3200 to about CSX3300 - 428 1x4 (center oiler).

• Street cars from about CSX3300 on - 427 MR side oiler with "C5AE-H" block, 1x4

There were some exceptions to these rules, but the vast majority were this way.

Much of the determination of engine configurations delivered in 30XX cars can be correlated with what was available at the time of the car's completion. Ive "paid attention" to production casting dates on 427's over the years and the earliest production date C5AE-H side oiler block I've seen (the one most people associate with these cars) is around mid '65. There is also the factor of SA using up whatever engines they had on hand before ordering the newest configuration. Most of the early 427 comp cars were finished by around the 3rd quarter of '65 and would not have had the -H block, as it wasn't available or on hand. SA bought a number of lightweight MR 427's for comp cars in early '65 and used them up over some period of time. I have an original lightweight comp car engine with magnesium intake which has aluminum MR heads with Jan '65 casting dates and a C5AE-G cast iron sideoiler block dated around the same time (I think it's maybe Feb '65). The G block is the only sideoiler I've seeen dated prior to mid '65, with the earliest ones I've heard of like mine, Feb or maybe Jan '65 (the alum 390 was a sleeved 427 sideoiler and cast somewhere around mid-late '64, which is the earliest sideoiler I'm aware of). I'm not aware of even one early comp 427 car with the original -G block engine (most were blown up early on anyway).

Regarding the high riser, SA had some high risers on hand in late '64, evident from old SA pictures and ex-employee comments. I believe HR engines that may have been installed in a couple early comp cars would have had center oiler blocks, at least if they were engines assembled by Ford and not cobbled together at SA at a later time using side oiler blocks with the HR heads. Most of the HR engines also probably used cast iron heads, as the aluminum HR heads were made specifically for SA for the aluminum 390 with extremely small combustion chambers and 390 sized valves. I know of only 3 sets of aluminum HR heads and based on their serial numbers and comments from a number of long time FE gurus, I suspect only about 8 sets may have been made. At least one set had larger 427 valves installed and the chambers opened up some, which SA may have done to install on a cast iron block 427, but someone may have also just reworked it at a later time. FYI, flow numbers on the alum factory heads are about 285 MR, 305 HR and 325 TP, vs around 245 for a cast iron MR.

As for late 30xx cars (SC cars), most were finished in '66 when the -H sideoiler block, cast iron MR head, 2x4 Galaxy engine was readily available, so it all fits together that's the way they came.

More info (possibly taken from CSX Info)

More from ClubCobra by Nedsel

All of the 30xx cars were reportedly sold with side-oilers. Since they were intended to be race cars, and since the top- and center-oiler 427's had oil starvation difficulties with the main bearings, it would have been suicidal for Shelby American to sell these cars with engines they knew had a poor high-rpm record. The comp 427's were equipped with MR or HR heads and intakes depending on what type of racing they were ordered for, and once the demand for full-comp 427's slowed, S/C production started. Most of them had the MR heads and intake, but a few were intended for competition and were given the HR set-up.

The bulk of the early street 427's did come with center-oilers and low-rise heads/ intake. This has been confirmed by Marcos CSX3121 as well as many other early owners. Many of the later 31xx cars had side-oilers and MR heads/ intakes.

Minutes from the Shelby American weekly meetings show that the cost of the 427 engines vs. the new 428's, made for the T-Bird, was roughly $700 vs. $300. The swap to the 428 "Police Interceptor" was made at the start of the second contract for these cars, or around CSX3201, and was done for purely financial reasons. This continued into the very early 33xx cars, at which point it was decided to resume installation of the 427. By this time, FoMoCo was only producing the side-oiler, and that is what these cars got.

Interestingly, the switch back to true 427 Cobras was mandated by the hiring of a new (financial) controller at Shelby American, who they acquired through Ford. He felt the use of the 428 in what was promoted as a "427 Cobra" was likely to lead to lawsuits, not to mention ill will, and demanded the 427 engine be used again. Sadly, after the change back to the 427, production continued for less than 60 cars.