Originally Posted by cl55amg
I know I am probably opening a can of worms with this question!
I will be rebuilding my 1968 327/350 HP L79. I have read a lot of different takes on this. I am looking to be set straight once and for all.
As you know the L79 has 11:1 CR. and the L79 (151) Cam. Some say it will run fine on todays 93 octane because todays 93 octane is the same as yesterdays 98 octane. The system of rating the octane was changed back in a day and the new system results in a lower rating. Now I do remember this happening back in the seventies I think. Is this the case? Some builder seem to default to lowering the CR to around 10:1 and others are OK with 11:1 CR. Please set me straight on this. I am sure others have this same concern and would like to be set straight also.
Thank you in advance.
Depends a lot on the cam timing. The point where the intake valve closes with respect to crankshaft degrees has the effect of shortening the stroke. This calculation is referred to as the Dynamic Compression Ratio (DCR). This calculation is used to alter the Static Compression Ratio (SCR) which is all the volumes of the cylinder divided by the volumes above the piston at TDC.
You need to have the SCR volumes, the intake closing point and the rod length to run the equations. A decent one can be found at the Keith Black web site:
Generally we've found that for 91 octane pump gas with a cast iron head you'll want the DCR to run from 7.8 to 8.2 for aluminum 8.2 to 8.5 to one. With an open chamber head and pre-Vortec style closed chambers stay to the lower side while a modern "Vortec" style closed chamber will go to the higher side. An engine with circular dish pistons needs to run to the lower side regardless of head chamber style while flat tops and D dish pistons can run to the higher side. Steeper gears let you go higher say 3.5's and up while higher gears like 3.08 to 2.73s need lower DCRs. You can play in the spaces with ignition timing rate and amount, faster and more lowers the DCR slower or less allows raising it; intake air and engine operating temps where lower allows more compression; and mixture ratio where richer allows more compression and leaner less.
93 octane will let you push the ratio a couple tenths but unleaded fuels don't respond the same to compression and heat as leaded fuels did, while 93 by calculation of averaging Motor and Research may numerically equal 98 of leaded fuels, unleaded is more unpredictable as to operating outcomes. Depending upon the chemistry selected to manage octane by the various blenders the response to your engine's operating temperatures and pressures can be rather different brand to brand for the same rating of fuel.