Packard Forum Index » V-8 (1955-56)

Wondering what the community consensus is on octane of gas and whether to add lead substitute for the 56 V8 engines with 9.5:1 compression?

My '55 352 runs just fine on 87 Unleaded. Previous owner of 10-plus years used the same.

Quote:

Quote:World's of difference between a 7.8 compression '55 352" and a 10:1 '56 374". The '56 with 9.5 is in-between. I've seen some with piston tops showing severe detonation effects from running cheap gas. No more miles than hobby cars get, go with the good stuff and keep a close watch on ignition initial timing settings.

jack vines

Agree 100% with Jack. Even with 91 octane gas my 1956 Caribbean required the timing to be kicked back a few degrees to keep spark knock down to a tolerable level.

Thanks for the responses so far, I have a couple questions:

Is there any sense to use octane booster with normal gas instead of buying premium gas? Or a booster/lead substitute combo?

Why does a 10:1 ratio need 90-something octane when new cars with 11:1 ratios run fine on 87?

Wow, I didn't know that there was that much of a compression jump between '55 and '56. Too bad really, because I've been in the situation of having to figure out what 'mix' is right for a car and it can be a real pain. It's nice just pulling up to the pump and reaching for regular.

"Why do new cars get away with 87 octane with 11:1 compression"

There are really 3 reasons:

1) Direct injection

2) Variable valve timing

3) Infinitely-variable ignition timing

All of them, in combination with computer control and knock sensors, allows the engine to run on the edge of knock all the time. In fact, many cars will detect and compensate for better or poorer gasoline - so you can get better performance with higher-grade gasoline (because of more advantageous timing), but the car will still run with lower-grade gasoline.

ScottG

Yeah, what's odd (and annoying) is that regarding fuel, the 55 and 56 owners manuals have the exact same phrasing to use "high octane fuels" without suggesting an actual number... unhelpful when there is easily a 5 point difference or more between the fuel needed for a 8.5:1 55 engine and a 10:1 56 Caribbean engine.

IIRCF< octane rating numbers changed sometime in the 70s, so any number suggested in factory literature would have gone the way of the dodo.


My bikes are 10:1 and 10.7:1 respectively. They both get 93 octane from Shell. They both have EFI and computers, so they likely could make do with other fuel.


My truck book recommends 89 octane but it runs fine on 87. I tried 89 once and noticed not a bit of difference. And I ain't spending the money if it doesn't care. It has EFI and a computer, too.


The '46 Deluxe gets 87.

I don’t recall octane even being mentioned in the ‘50s or ‘60s. The cars used either Regular or Ethel.