dynamic compression ratio /octane requirement

rjay455

Member
What would be the likely DCR of a 1971 stage 1 iron headed ( basic porting gasket matching only ),455 ex a Riviera using stock 10 to 1 piston , stock bottom end , TA 284-88H cam installed 4 deg advanced with a intake closure at 37.5 deg as per cam card ? .Down in Aussie we use research octane rating, instead of your motor octane number , i am not sure of the conversion value but I'm thinking this may run on regular ?? Thank You Gentleman.
 
If vehicle “pinging” on Regular 87 Unleaded, try upgrading to the Mid-grade 89, high-performance vehicle may need a higher-octane fuel, such as 91 or 93.

When my 455 is done with the build (similar to yours I think) it will never see anything but the highest octane. The effort and expense involved is the justification!

What would be the likely DCR of a 1971 stage 1 iron headed ( basic porting gasket matching only ),455 ex a Riviera using stock 10 to 1 piston , stock bottom end , TA 284-88H cam installed 4 deg advanced with a intake closure at 37.5 deg as per cam card ? .Down in Aussie we use research octane rating, instead of your motor octane number , i am not sure of the conversion value but I'm thinking this may run on regular ?? Thank You Gentleman.
 
Ron,
Here's some factory data from 1970-71 on octane requirements for the 455. Obviously your setup will not be stock, so the usefulness may be limited.

Octane requirements eased considerably when the compression ratio went from 10.0 in 1970 to 8.5 in 1971. The '71 shop manual indicates all 455 versions that year needed 91 octane (Research method) regular fuel which equated to 82 octane (Motor method). Present-day pumps in the US are marked with an octane number which is the average of the Research and Motor method figures. (R+M)/2 is shown on the pumps in tiny print. So in stock form, the '71 455s would run OK on today's US regular rated 86-87 octane.

Every year the Buick factory would issue a Service Bulletin advising dealers of tweaks necessary to operate customers' cars in areas where octane was subpar. For '71 models, only a change in distributor springs was needed to operate on gas as low as 86 Research. Contrast that with the '70 Bulletin where octane requirements for a 10.0 compression ratio were much greater.
 

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Using actual measurements generally known to the 70 455 with a steel shim gasket, the static compression was less the the advertised 10:1. It was closer to 9.6:1, but varied some due to assembly tolerances with the deck and compression height. Putting the TA 284-88H with 110* lobe separation and 4* advance through the dynamic compression ratio calculator with a 9.6:1 static compression ratio yields almost 7.5:1 dynamic compression. 7.5 to 8.5 dynamic compression is generally acceptable for midgrade gasoline (our 91 octane). You may be able to get away with regular (our 87 octane) since you are on the low end of the dynamic compression ratio. You could try it and back off the ignition timing if there is any pinging.
 
Thank you all for your replies, I have posted a few questions to this forum over time, and I am impressed no end by the vast wealth of knowledge and experience and the willingness to assist , a pleasure to be involved cheers
 
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