I posted this as an email but I also wanted to list it here just in case some of us don't read the emails and also good topic to look back on.
I just purchased a motor 1972 400 with 7J2 heads. The motor is supposedly rebuilt time will tell. I do have plans to tear it down and verify.
My question the valves are 1.94/1.66 96 cc. The published rated HP is 200.
Now I looked at a 1971 with 99 heads valves are same but 98 cc. This motor is rated at 265 HP
A 1970 with #11 heads has the same valves but 80 cc.. This motor is rated at 265 HP.
Maybe some one can explain the big difference. Could the cams make that big of a difference?
Now one more question would it be worth the increased hp to change the heads out and if so which heads would I go with.
A little history I already have a built 455 4 speed 69 coupe. I am running 6x 4 heads and this car flies. I am not looking for the same power but I want more then my 350 is putting out.
Published HP spec changed to net hp for '72. There is no direct comparison between gross and net HP. Ignore those numbers if the engine was rebuilt, as carb, intake, exhaust, heads, cam and compression all factor in.
Check your math, 75-80cc will yield higher compression than you may wish for a pump gas 400. Don't forget to account for overbore.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
I have not open the motor up yet the owner claims it was rebuilt but did not need boring (That is one reason why I am opening it up) If it is a stock bore 75 cc will net 9.5 and 80 cc will net 9.1 both would run on pump gas unless like you said my math is bad. I have been wrong before why stop now
1) GM top brass wanted to restict the HP ratings for both cars to 10 Lbs./HP. Since the Firebird was a lighter car, the HP was reduced to 325.
2) To achieve said difference, Pontiac modified the linkage between the primary and secondary throtle shafts. The secondaries were not supposed to fully open.
The carbs were not actually the same from the factory.
Now ... in reality, both HP ratings were misleading, just like the RA IV HP rating of 345/370 HP.
There are well known published facts that the HP and compression ratios were speculative.
72 had 2 bbl. and either a 555 or 66 grind cam 71 had a 555 grind only 70 had a 254 grind
You could say 10 cc's difference between 70 (11) and 72 (7J2)could net you 65 HP.
Other than a few CFM difference in 2 bbl carbs, the heads and CC of them and the cam would be the difference.
My main point is the use of these heads and expectations. Unless you own or have access to a machine shop where these heads can be modified will the benefits be worthwhile.
Since is is unlikely to buy or find a servicable 2bbl. cam, I suggest careful exploitation of the "HP" numbers you seek. Too much of one and not the other is not good.
Not technically "back"...Thanks for the honorable mention Q. Just spent 16 hours installing Koni's and Hotchkis suspension, brakes and new bushings on the T/A and needed an outlet.
Si Vis Pacem Parabellum
1967 Starlight black PMD Engineering 400 Auto 1968 Alpine Blue 400 4 speed 1968 Verdoro Green 400 HO 4 speed 2013 1LE 2SS/RS Inferno Orange Camaro.
Hey Tom, good to hear you again! How's the TA feel now? Should be "tight like a tiger"! And thanks again for your help and hospitality when I was there! (Great guy but I'm not supposed to reveal his secret!)
On the 2 bbl stuff, one of the classes that I've run mandated 2 bbl carbs. The typical Chev 350 set up for the class dynoed at 260-280 hp with a "non-streetable cam", just for reference. Cam selection was close to what a 4 bbl engine would use for the same rpm range with the exception that the 2 bbl favoured a single pattern design to help the intake side. And had I continued in the class, I would have built a Pontiac 350 to take advantage of the better heads and manifolds - among other things!
