13:1 Compression Pistons in 340 Small Block

To the original poster/thread starter;

This is how I read everything so far. You want to reuse the old parts because there on hand. While I like that idea, if this engine specs out to an actual 13-1, then the size cam normally seen used effectively with that high of a ratio is starting around in the [email protected] with operating RPM ranges starting around 4,000+ RPM’s.

The trick there is to bleed of cylinder pressure at the lower RPM levels. Once the engine gets to spinning around, the cylinder pressure levels go through the roof and will require a very high octane level. While killing the domes off the top of the pistons will drop the ratio, it is not a route I would take.

It would be my opinion to save a headache and skip those pistons for readily available flat tops for a 340. With a set of open chambered J heads, the compression ratio should fall on ether side of 10-1.
(Actual head cc amount, gasket thickness and bore size, where the piston actually sits, will mean everything. And is variable at this point. But for the sake of this discussion..... )

At 10-1, premium pump gas will work and a decent size streetable cam that can perform at the track can be used.

The next issue are the J heads. are the stock or ported and with what size valves? The as cast heads didn’t flow very well for making power or racing purposes. Generally speaking, (very) the head flows around 200 cfm. On a laymen’s term on Potential power to be made, the math works out to CFM times two equaling potential is power on the a street strip machine. Exceeding this isn’t hard but the actual usage and friendliness of the engine /vehicle tends to drop hard.

Generally speaking, the difference in compression is 3% per 1 point of compression though this is general and not in stone since the more aggressive the build, the wider the percentage becomes. AJ above claimed 3% for the 2 points. I’d double that value without batting an eye if not more if the combination is wild or out of wack.

FWIW, @ 13-1, that is 110 octane level. For gasoline. That’s pricey!
E-85, not so pricey but the engine drinks heavier. But not double. Close though!