Compression ratio decision

[Tom L.]

The guy who is rebuilding my '70 LS-5 has suggested pistons that will net an end compression ratio of slightly north of 9:1. The rationale is that I risk damaging the engine on today's gas with the factory 10.25:1. One of the things that drew me to a '70 was that they had the last of the muscle car era big compression motors. I'd hate to put anything back in other than 10.25:1 pistons.

A Sunoco up the street from me sells the GT 100 octane gas, but I'm concerned about having to fill up elsewhere if I'm away from home, or if the gas station stops carrying the 100 octane gas. How have other people handled this issue? Also, what about keeping a can of octane boost additive with me for an emergency? Are the additives junk or are they a reasonable alternative? Thanks in advance.

[Timothy B.]

Re: Compression ratio decision

Tom,
If you are using the stock cam go with the higher compression just like the engine was originally built and I would not even think twice about it. There is good discussion about today's gas in the archives that may help you. Build the engine to 1970 specs, if you want to improve performance, play with the timing advance curve some and pay attention to head work. I am sure others will respond with good info to help you.
Also, pay attention to head gasket selection as this will make a difference in compression ratio. Use the thin one!

[Paul H.]

Re: Compression ratio decision

Both my 65 L79 with 11:1 compression and my 68 L36 427/390 with 10.25:1 run fine on pump gas of 93 Octane with no pinging and no need to retard timing. I would rebuild it to stock specs.

[Duke W.]

Re: Compression ratio decision

As built most OE engines' actual CR is a quarter to half point less than advertised. If it didn't ping before the rebuild, it won't ping after the rebuild with the same CR.

You begin by measuring the deck clearance BEFORE the short block is disassembled. Then use the measured deck clearance, nominal head and piston volumes, and measured thickness of the original head gasket to compute the original CR.

If you failed to do this use OE replacement components and target a new CR range of 9.75 to 10:1.

"Managing" the CR along with links to an online CR calculator have been discussed extensively and are in the archives.

Duke

[Tom L.]

Thanks

I now feel better about putting it back together the way it was. Thanks.