L68 Engine �run on� after shut down

[Mark F.]

Correct. The advertised compression ratio is 10.25:1, but "as built" could be up to half a point less due to higher decks as the broach tools wore. (My broach marks are distinct and will hook your finger nail if you run it across the pad and there's no disturbance in either end on the stamps, so I'm thinkin' whether mine was ever rebuilt or not, I don't think it was decked) Also, if the engine has been rebuilt it may have lower compression than OE. ...I suggested he try some pump premium, which is only 91 PON in California, and at worst we might have to retard the initial timing a bit, which was set at the OE spec of 4 degrees, well below the 6-10 range required to achieve optimum WOT advance. We had to drop it down to 2 degrees to eliminate the very slight detonation, but there was no SOTP loss of performance.
We also converted the ported vacuum advance to full time to reduce the occurrence of hot running in low speed traffic.
A common myth is that vintage "high compression" engines need high octane race gas or avgas or some kind of "octane booster" many of which are snake oil. Part of the problem is the difference between the RON method used back in the day versus the PON method that has been in use since the seventies that few understand. Plus, few owners know the actual CR (Agreed) of their engine whether it's original or rebuilt, and some engines have been rebuilt over the decades whose compression is so low they can run detonation-free on 87 PON regular unleaded. The bottom line is that, given all the variables, each engine is unique, and you can either believe the myth or get educated on octane ratings and do your own experiments to determine the minimum PON necessary to keep the engine out of detonation. Duke

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[Duke W.]

[Jeffrey S.]

[Duke W.]

[Jeffrey S.]

[Duke W.]

[Jeffrey S.]

[Duke W.]

[Mark E.]

[Duke W.]

[Skip N.]

[Duke W.]

[Skip N.]

[Duke W.]

[Joe R.]