In a very general sense, you can go by compression ratio. 8.5 to 1 MOST OF
THE TIME will use 85 octane at sea level. 9.2 to 1 will require 92 octane
at sea level. At higher altitudes, octane requirements will be less. BUT IF
YOU HEAR IT PING, ADD OCTANE UNTIL YOU DON'T. Olds engine design is old
school.
More modern combustion chambers with tight squish areas and 4
valve heads with swirl promoting port designs are much more tolerant when
it comes to octane requirements. Just what I was taught. Your experience
may vary. Chris C. and Bob De. may well be able to add some engineering
lingo here.
Jim Hupy
Salem, Or
78 GMC ROYALE 403



Gee, mine sounds like it's Pinging all the time
Hal



On May 26, 2016 12:48 PM, "Gary Worobec" wrote:

> Interesting, I've been running our 455 on regular 87 octane gas since we
> have had it 10 years now. I'm surprised that 91 octane is called for. I've
> not seen that before.
>
> thanks
>
> Gary Worobec
> 73 23 glacier.
>
>
>
>> On May 26, 2016, at 12:05 PM, Wayne Rogewski wrote:
>>
>> 455: 212 HP @ 3400, 344 ft-lbs @ 2400, CR 8.5:1 91 Octane or higher
>> 403: 200 HP @ 3600, 330 ft-lbs @ 2400, CR 7.9:1 87 Octane or higher
>>
>>
>> Pilfered from here.
>>
>> http://gmcmotorhome.info/engine.html#403_VS_455
>> --
>> 77 Royale, Rear Dry Bath. 403, 3.55 Final Drive, Lenzi goodies.
>> Mid Michigan
>>
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> On May 26, 2016, at 1:47 PM, Gary Worobec wrote:
>
> Interesting, I've been running our 455 on regular 87 octane gas since we have had it 10 years now. I'm surprised that 91 octane is called for. I've not seen that before.
>
> thanks
>
> Gary Worobec
> 73 23 glacier.
>
>
That because Wayne and many others are living in the past.

One thing that is often overlooked in discussions of Octane numbers is
that the method of calculating Octane numbers that are posted on
gasoline pumps has changed

When our GMCs were made the method posted was the Research octane method. The research octane number (RON) is determined with a test engine running at a low speed of 600 rpm.

There is another method of calculating Octane. That is the Motor Octane method. The motor octane number (MON) is determined with a test engine running at a higher speed of 900 rpm. That is about 10 points less than the Research Octane method.

The number posted on gas pumps today is the average of the Research and the Motor methods. If you look closely at pumps they will often say "(ROM + MOT) /2".

So, what used to be called 91or 92 octane is now called 87 octane.

Today's regular gasoline is just fine to use in your GMC.

Torque is a measured value. Horsepower is a computed value using the relationship between hp and torque. The peak torque occurs at a different RPM than the peak HP. More info on that is available in an article that I had written way back in 2001 for the GMCWS site: http://www.gmcws.org/techcenter/01-03tc32.htmlT . Hope its not too long winded for you.

I had once gotten some info from Dick Paterson and passed it on to Billy Massey who posted it to his site. http://www.bdub.net/OldsEngineSpecs.pdf
Page 2 has the hp and torque figures.

Emery Stora
77 Kingsley
Frederick, CO