C2 '66 BB compression ratio

[Tom Merkel]

In a '66 427/425, what should the compression ratio be?

My 1967 shop manual indicates "N.A" under the category 427/425hp. (Tri-power, however is listed as 11:1.) But this is for '67, not '66.

Anyway, I've got a mini-mystery unfolding with my neighbor, who owns the '66.

The point of this all is he claims the '66 BB won't run on today's pump gas. I thought the BB's had a lower compression ratio than my '67 SB L-79, which has 11:1, leading me to think that his '66 BB with a lower compression ratio would find 94 octane OK.

By the way, there probably is a whole lot more to this story, since he - being the 2nd owner- said the first owner had the motor "blueprinted". This fact, to my understanding at least, would not have affected the compression ratio.

Lastly, there was an interesting thread several months (?) back regarding octane, etc. I'll take a look.

So, all yee BB experts along the way, what do you say?

tom

[Clem Z.]

Re: C2 '66 BB compression ratio

if the engine was "blueprinted" the compression ratio was corrected to what the stated C/R was listed. non blueprinted enginges have a lower than stated C/R because of the manufacturing tolerances. the stated C/R for a 66 425 HP 427 is 11:1 which would have trouble on 92/93 pump gas if it was a true 11:1

[Joe L.]

Re: C2 '66 BB compression ratio

Tom-----

In addition to what Clem responded, big bore engines suffer greater sensitivity to detonation at higher compression ratios than do smaller bore engines at the same ratio. Most of the 454-502 big block crate engines which GM offers have 8.75:1 compression ratios and specify the use of 92 octane fuel even at that ratio. The "ZZ-502" has the highest compression ratio of any GM crate big block engine at 9.6:1. The aforementioned is the reason for this.

[Clem Z.]

Re: C2 '66 BB compression ratio

if 100 octane no lead is available in your area at the pumps he could try a 50/50 mix of the 100 with 93 to prevent the detonation. 100 low lead av gas will also work if mixed with 93 no lead but the airports will not put it in your car because there is no road tax on av gas so you will need to buy it in drums and add it to the car at home. that is the problem you have with these older hi compression cars.