to the Ford HP line up. Remember the 428 CJ was rated at 335 hp, you cannt have the same rating with a 390. I think the stock 428 in 66 was rated at 345. Holley carbs, cams, and dizzys made the Horse power differance over the 390 4bbl z code engines from the s codes. Although the head change numbers each year there were not any major differances. the 69 Gt was a Z code engine with GT manifolds, std cam, dizzy and a 4300 autolite.
GM underrated their horsepower in 1966 and this caused strong-cammed 396 cars to be matched with 390GT cars, as NHRA rules matched up against "advertised horsepower". 1966 was not a good debut for the 390GT engine. Ford figured this out by 1967 and advertised a horsepower rating more in line with the GM HP "factor". Too late to help the 390GT reputation, but plenty early for the upcoming 428CJ.
The 1968 390GT was basically the same as the 1966-67, though thermactor became mandatory to meet the 1968 Federal emissions laws, as the GT/CJ cam had sufficient overlap to cause some raw fuel to exit the exhaust valve. Thermactor-equipped 390GTs got a richer carb calibration. Only California and NY 390GT cars needed Thermactor in 1966-67, though it was a $47.00 option for any car ordered. There was no 390GT engine for 1969, just the new 390IP (improved performance) engine which went into 390GT-optioned cars. The "IP" engine fell between the "standard performance" and "high performance" 390s, and became the 2nd-tier engine below the 428CJ.
Here is my opinion based on the "Gonkulator" in part
July 11 2008, 9:10 PM
I suppose there was a prototype 390gt somewhere on the dyno that made 335hp during development, probably super clean and smooth ports, careful blueprinting, etc. When I compute the most optimistic 390gt "S" code for 66, with c6ae-r heads and matching intake I get about 320hp. By 1967 a drop would be reaonable with the lesser ports of the c7 heads and the small port "S" intake. For 1968 the chamber got a bit better with the c8ae-h and the rating and reality went back up a few hp. So the changes roughly track, but I compute the 66-68 390gt at 310-320ghp depending on which year.
AFAIK the 390gt cam was the same 66-68 and the same as the 428cj 68-70.
The carb too was the same, 600cfm 4150 straight booster Holley 66-68.
I don't think the "Z" code 390/315hp made its rating in production either, reality about 285-300hp again depending on the detail, 1968 the best year with the c8ae-h heads and 2-1/4 log exhausts.
What happened on the 428cj?
My guess and somewhere I read text to this effect, a super-good well-tuned 428cj will in fact make 335hp if you're lucky. That is, 335 NET hp. Typical gross hp was more like 380 or so.
The 335hp rating of both the 390gt and 428cj is not near as ridiculous though as the later 429/360hp base Thunderjet, and the 429/370hp SuperCJ which in reality made almost 100hp more than the base T-jet, and it showed in the et and mph. When I was a youngster I thought, "big deal, 10 horsepower....". That was before I learned to go by car weight and trap speed!