Fuel Quality
Thread Starter
Drifting
Joined: Mar 2009
Posts: 1,461
Likes: 361
From: Monson MA
Fuel Quality
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.
Peter B.
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.
Peter B.
Last edited by twinpack; Nov 9, 2010 at 06:45 PM.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
It's not the holy grail of engine design, but theoretical DCR (based on the relationship between SCR and intake valve closing) is most definitely worth taking in to account, keeping in mind the higher is SCR the more margin ought to be left for peak cylinder pressures. Inversely, the milder the cam the closer to the limits you can safely go. And, aluminum heads also allow higher CR. Here's a good primer on theoretical DCR which should help, and includes a fairly comprehensive calculator which you can download...
http://www.empirenet.com/pkelley2/DynamicCR.html
FWIW, I refer to DCR as discussed above being "theoretical" because true DCR = CR x Volumetric Efficiency, which isn't so easily projected.
TSW
http://www.empirenet.com/pkelley2/DynamicCR.html
FWIW, I refer to DCR as discussed above being "theoretical" because true DCR = CR x Volumetric Efficiency, which isn't so easily projected.
TSW
Last edited by TheSkunkWorks; Nov 9, 2010 at 06:56 PM.
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
It's true that some of yesterday's gas was rated (R) research octane, while today's is a division between the old research and new (M) motor octane. Not sure today's 93 octane quite measure's up, but 11:1 compression on a carbureted engine is a lot to ask of it, as the outer cylinder bank's tend to run a bit leaner. Probably wouldn't hurt to rebuild using slightly thicker head gasket's and a more modern cam grind, as these have really improved over the year's. One thing's for sure if running pump gas, you'll be wanting to keep the tank fresh as possible.
CF Community Team
Joined: Oct 2004
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From: Cape Cod, Mass.
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My 65 L79, built to stock specs, runs fine on 93 Octane and most everyone I know who owns one runs fine on 93. The 11:1 factory compression rating was a little optimistic. They are actually a bit less. I would rebuild to stock specs.
Burning Brakes
Joined: Jun 2009
Posts: 840
Likes: 4
From: Syracuse IN
If you have a modern electronic ignition, you can install a knock sensor with a timing retard option, When the sensor sees any type of preignition it will retard the timing 1 or 2 degrees until preignition is gone. This happens extremely fast usually the signal is sent every 5 seconds. On some I believe they are adjustable. I have used these trouble free on Marine engines which are supercharged. I do not see why they won't work for your application. Al
