What are the differences between early 383's and 1971.

[71 Bee Man]

Hi guys,

So, I read everywhere that in 1971 the 383s were "detuned" and only had 300 horsepower.

My question is......exactly what is different in a 1971 383 HP motor and the earlier ones with 335 horsepower.
Is it just the compression, or what ?

Peter

[XH29N0G]

From an old chiltons for the HP version

Advertized Compression  (69-70-71) 10.0 - 9.5 - 9.5
Cam (all .450 .465)

I believed they changed how they did the horsepower ratings (something like testing with things like alternater and without alternator etc...)
My impression is the compression and detuning did not occur until after 1971.

Others will surely have an answer

[c00nhunterjoe]

I beleive pistons changed on them. Sbould all have 906s in those years and possibly 346 in the later but ccs were the same. 

[Scaregrabber]

Low compression and different heads. The compression is the problem. The 68-69 383 Magnums were the best I think, Some of them really cooked in the Road Runners and Super Bees. Someone had the bright idea a 383 Magnum should run on regular gas in 1971 for some reason. I think they were rated at 8.2-1 or something like that, about 1.5 measured or advertised less than a 68-69 383.

Sheldon

[XH29N0G]

Interesting.  This implies the chilton's manual has an typo which is definitely possible. 

I see the 8.5 listed on the web and trust those who posted to know more than me. 

I don't have a factory service manual to double check.  Can someone confirm from the FSM?

Also interesting that the 440 apparently kept the high compression but had a slightly smaller HP drop. 

[71 SE3834V]

The advertised ratio in the dealer data info was 8.5:1 but my FSM states 8.7:1. My November built 383 HP has flat top pistons with 346 heads.