I've been studying up on my '72 351c 4v - designated as a cj... 290 ponies and I'm confused as to the science behind the build.

An old performance artical done in '71 claims they re-tuned vs de-tuned the '72 351 cj...the 71 had 10.7:1 cr and put out 285hp... now the 'tune' language is fine...refers to the point that all the motors were being decompressed and losing power and this one gained 5 ponies somehow in this twighlight zone.

So they take the '72 cj and run 8.6:1 compression, .491" lift both intake and exhaust, put in a pretty wild cam 270, 290 with 46* overlap and intake closing at 72 freaking degrees. put on a 800cfm carb....with 219, 171 valves and max out hp at 5400rpm. 360ftlbs at 3600.

the '71 is simlar except had a ton of compression, a bit milder cam...268, 280. and a 600cfm and puked out only 285hp


makes no freaking sense to me.... the '72 must have a dynamic compression of what...6.8:1??? enemic even by out 87 pump standards....

So someone pahlease straighten me out... did they fudge numbers due to insurance? heard that was a big issue back then...kind of like torque and the diesel monsters of today. and less than .5 lift? even the boss' were down under .5....

I thought I learned some things and am questioning it bout now.

 

there was never a real good dyno in 1970 so it was more just guessing estimating and how they felt. The first vehicle dyno wasnt invented until around 1930, and never a really production one until 20 years later they never gained much popularity until recently.

Thats what id have so say about this.

 

1972 was when they went to s.a.e. ratings at the rear wheels, right?

 

1972 was the first year for NET horsepower ratings but those were not taken at the rear wheels. Instead, the factories were to start testing eninges with full accessories, stock exhaust manifolds, and stock type exhaust systems in place. S.A.E. also dictated what the correction factors would be so that the ratings were more or less equally obtained regardless of maker. Comparing HP ratings is a fruitless exercise. Any numbers posted from the "Muscle Car Era" are highly suspect.

 

still begs the building philosophy of a very low operating or dynamic compression (for a relatively light car) due to late ivc and then a raudacious cam fed by 800cfm carb

i dont know if i could get an engine to run much at all with those specs... it certainly leaves much to be desired and a few parts can onviously make these things scream.

but what really interests me is how they would have reported net sae in '72 against rear wheel hp in '71????

Now that would make sense given the results and having a '71 that has 10.7:1 compression and a good cam behind the '72 model.... and perhaps preserve my sanity ...or whats left.

 

roger,

1971 and earlier didn't use RWHP either. Before 1972 the auto makers used GROSS HP numbers which were pulled from engineering blueprint engines that were run in optimal settings, with no accessories, no air cleaner, and no exhaust. Many times the engines were run with some type of dyno specific branched tube manifolds---headers. The auto makers made good use of their dyno rooms and used analog, water braked dynos. By the later 1960's, the Big 3 were using early computers to help develop camshafts, intakes, heads, etc.,.... The processes just were not as refined and the computers were not nearly as fast so the rate of development was much slower. Also, they didn't have the shear amount of data that they have today so many things we take as common engine building fact was cutting edge technology back then.

 

darn...back to wondering... did some research on cams and wow ford did use some big ones...quite a few on the 289-302 had reverse splits in that intake duration was 10* more than exhaust...not often as most were the same but when split on the larger engines they obviously increased the exhaust.

but the builds and cams seem to make sense given the high compression ratio. the '72 appears as an anomoly.

 

I once watched a show on tv where a 1971 Boss 351 Mustang was dynoed in stock form. They came out with 430 hp... Don't know how true this was, but I do know many engines Ford used were under rated for insurance purposes...

 

As well as pontiac.

 

I hear you on the insurance crapola...the '71 boss was rated at 330hp and 370 tq..by 2 publications. 330hp is nothing to get excited about and the boss was exciting. I've been studying Bill Carroll's book...Fooorrd v8 performance guide "stocker's bible".... great reading...but he copied specs as they come from ford. I'll likely dyno my '72 cj before deciding what to do with it. i haven't run it yet but it's supposed to run well...untouched 78k.

leaking tranny fluid outof the front seal...might take the opportunity to rebuild the engine and do something very special to it while keeping it fairly stock...at least stock looking..will likely add 4v closed chamber heads, 9.5:1 ...and voodoo...
Duration @ .050 IN/EX: 219/227; Lift IN/EX: .540"/.552"; LSA / ICL: 112/108; RPM Range: 1800–5800
Hydraulic; Torque Master for 351C with 4BBL heads on daily driven street performance vehicle. Good torque and horsepower production with heavier emphasis on the lower mid thru upper RPM ranges. Use 3.55 rear gears. Choppy idle!

or could go crazy and upgrade to chi heads and equivalent intake, 10.5:1 pistons plus voodoo cam:

Duration @ .050 IN/EX: 227/233; Lift IN/EX: .552"/.564"; LSA / ICL: 110/106; RPM Range: 2000–6000

want to get the body work done and painted and running first...then motor it.

 

Roger,

I have a pile of parts for a 351 build that uses stock-ish parts. The block is a standard 4-bolt motor that needs bored. Heads are a set of Boss 351 service replacement heads. These were NIB when I picked them up and had never been run or installed. Compression should be in the 10:! range with a set of reverse dome pistons. The intake I'm looking at comes from Australia and is an exact alloy replica of the factory squarebore cast iron intake on the outside. On the inside the intake has been completely redesigned. It makes more HP and torque that the old Buddy Barr intake and puts out similar numbers to the Blue Thunder/Shelby intake. Carb is a Ford/Holley 735VS and the cam is a custom ground Boss 351 mechanical with extra lift. Instead of .477/.477 on both sides, this cam has .491/.509. I know that isn't much of a change but I'm not building this motor for anything other than shear wierdness. The headers are a set of H-M Falcon GT-HO Phase IV headers. I'll run a set of Harland Sharp roller rockers. Not sure what the power level will be but it'll be period correct and look bone stock. If I can make 300rwhp, I'll be happy. It'll go into another Aussie Falcon that I have my eye on.