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Discussion Starter #1
I have read several articles that claim that Ford underrated the advertised HP ratings on the late 60 early 70's mustangs, amoung other things, for buyers to get lower insurance rates.

Some people say that the 428CJ was closer to 400 HP instead of the advertised 335 HP. I wonder if the same might be true of the mid-block 351's. Of course I am most interested in anyone's knowledge of what the actual HP ratings maybe from a dyno test using a stock 70 351C-4V, but I also wonder about the 302 Boss and the 351 Boss actual HP ratings compared to what they were advertised.

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Here is why that is a tough question to answer. Right around the 68-72, the auto makers went from using SAE HP to Gross HP, The SAE HP being without the accessories and such. And, yeah, the insurance companies were waking up to the high performance car "problem". And, some automakers kinda put "Ringer" cars in for the press to use, these being highly tuned, cars for the press only. Yet, the press was given the HP ratings to use from those same auto makers.
 

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I got a bunch of old car magazines from my father recently and just read an article where they dyno'd several old Mustangs. With one exception, they didn't even approach the factory rating. One caveat though is that several of the cars were not completely stock or were not properly tuned. That said, here's the results:

65 289 hipo auto: actual RWHP 141; est. Flywheel 176; factory rating 271
66 289 hipo 4spd: actual 144; est. 168; factory 271
70 boss 302 4spd: actual 179; est. 209; factory 290
65 GT350 4spd: actual 202; est. 236; factory 306
69 428 auto: actual 213; est. 266; factory 335
69 boss 429 4spd: actual 214; est. 250; factory 375
67 GT500 4 spd: actual 240; est. 281; factory 355
68 GT500KR auto: actual 275; est. 344; factory 335
 
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This is not what I expected at all! I assumed that the actual HP ratings would be HIGHER than the factory advertised. I think that prior to 1972 the HP ratings were measured at the rear wheels with all the accessories on. This makes it seem like there was a huge power drop, but not really. I always thought that the 69-71 Boss cars (302 and 429), the 428CJ, and the 429CJ were all underrated on their advertised horsepower ratings. Are you saying that you think that the factory advertised ratings were exagerated?

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Prior to 1972, automakers used Gross BHP ratings, measured at the flywheel without accessories. In 1972, they switched to NET ratings, measured at the flywheel with all accessories (including A/C) installed and operating. Along with that, due to EPA regs, most automakers also switched to open chambers, lowering compression and horsepower output. As an example, my early '72 Impala Convertible w/454 was shown in all the specs as 275hp, but the build sheet under the rear seat showed 365hp (RPO LS5).
 

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Bartl is right, In fact it is my understanding the gross BHP ratings were often measured without any accessories at all, meaning the waterpump, fuelpump, etc were driven exterally.Sometimes the engine were even driven as dry sump, without an oil pan to reduce drag.
The engines were usually run with open headers, and no aircleaner.
A lot of people think that from '72 on rear wheel HP numbers are used, but that number would be even lower.
 

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Way back when I seen an article on the big three's small blocks. The article had the NHRA's ratings for the Boss 351, 340 six pack, and the LT1 350. The boss stayed at 330 HP. the 340 was bumped from 290 to 300 HP, and the 350 LT1 was dropped from 360 to 270 HP. As for fords I'll say that a 1970 351 4v will run with all but the best tuned 428's. Now I wouldn't even think about racing a 428 in forth gear from 1500 RPM.
 

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Prior to 1972 HP was measured using SAE gross meaning without accessories and at the flywheel. Since 1972 HP is measure using SAE Net meaning with accessories and at the flywheel.
 
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