Hey Guys. Found a set of 6X Heads that have been rebuilt with hardened seats from a guy for $200 for both. The ID on the heads are as follows:
GM1 on the right side of the top of the heads.
Dates codes: D126 & D196
A Stamped "8" on the lip that goes up. I think it's a secondary stamping or something.
Can anyone tell me what the horsepower rating for these heads are and if I can swipe them out from a 7K3 Head to these? The guy I bought them from said they were from a 1967 Firebird but I don't tghink they did this kind of head in 67. Any heklp would be great. Thanks.
The 7k3 heads were used on a 400 in 72 and produced 250HP, the valves were 2.11/1.77 and the chamber size was 96cc.
The 6X heads were also used on a 400 in 77 and produced 200HP, the valves were 2.11/1.66 and the chamber size was 101cc on the heads marked with an (8). The 6X heads marked with a (4) had a chamber size of 96cc.
IMHO, the 7K3 heads with a larger exhaust valve would be the better set. I'm not sure if they changed from net HP to SAE hp in 71 or 72, If SAE was used in 72 I would keep what you have,
That's very interesting. I have been told in the past that 6X heads were better for some reason. I do know that the 6X heads that I have bought have hardened seats for the valves. Did the 7K3 heads also have hardened seats? Also, is there w way to know about the SAE HP for those heads in 72?
Here is the definition of HP and when it changed in 71/72, I should have called it SAE gross HP (BHP) for 71 and older and SAE net HP for 72 and newer. During this time the insurance industry was demanding less horsepower from the big 3 automakers, some was reduced by lowering the CR in engines and some was reduced by reading HP with the engines accessories connected. I still believe your 1972 heads are the better set of heads than the 6X(8) heads, It would be a toss up if you had the 6X(4) set.
Maybe Mitch, Rukee, Eric, Jeff or some of the other guys will chime in,
SAE gross power
Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower, because it was measured in accord with the protocols defined in SAE standards J245 and J1995. As with other brake horsepower test protocols, SAE gross hp was measured using a stock test engine, generally running with few belt-driven accessories and sometimes fitted with long tube (test headers) in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.
SAE net power
In the United States, the term bhp fell into disuse in 1971-72, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, the SAE net power testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold.
OK, so here is my question. Did the 7K3 from 1972 have hardened seats? And how do you solve the problem with the exhaust manifold bolt holes on the ends? There is only 1 on each end. not the typical 2 per mounting section? Is it possible to weld a threaded stud on these iron heads or tap a hole? And also what is the best cast iron headers to use for these heads? And what is the best diameter exhaust pipe to use for the exhaust? Do you recommend and "H" pipe? Also, can ya recommend a good muffler for performance?
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