I rebuilt my 400 with the original #48 heads (D-Port, 10.75 CR) and the pistons that the previous owner had used (forged TRWs). What octane is this motor going to require to run right? I'm guessing its going to be somewhere above 93 but i'm not sure. Please help.

You'll probably be singin' the blues ... Turbo Blues ...

Don't go by the factory advertised,
But what's the actual compression ratio?
If it's 10:1 or less you can run premium but if it's much over that you'll probably have to run 104

I think you will have to figure out what deck hight you have, the more the pistons are in the hole the higher the octane you will need.

Jim G

my compression is 10.5 with my motor bored .40 over, and i have been putting nothing but 91 in it. my car doens't ping, unless you force it by putting it in 4th gear going 30 mph and floor it.
I think you should be ok

Steve

alot of it depends on the cam too. high compression with the right cam can run on pump gas. it might not idle well though

my brother's mercury has over 11:1 compression and runs fine on pump gas... but it doesnt like anything below 1500rpm. it idles, but with a thumpitythump. makes it hard to drive in a crowded neighborhood.

sixstarved is right. It's all in the cam. Static compression ratio doesn't tell the whole story. Dynamic compression ratio is the true measure. If there is a lot of overlap in the cam, the valve closes late and you do not see as much CR as the static number. So an engine with a 10:1 SCR can handle lower octane fuel than a 9.5:1 SCR engine, if it has a lower DCR.

A well tuned carb and distributor will figure in to.

True, a carb and distributor are key for running good, however when it comes to determining what octane fuel an engine must have, it is all based on Dynamic Compression Ratio (DCR). It is actually the cylinder pressure that is a result of the DCR that causes the issue. As any fluid is compressed (in this case the air/fuel misxture), it heats up. The more it compresses (higher cylinder pressure) the hotter the fluid gets. When this temp reaches the flash point of the fluid, combustion occurs. The higher the cylinder pressure the higher octane you need since higher octane is fuel with a higher flash point. Here is a web site that explains in detail and also has a DCR calculator that you can download.

http://www.empirenet.com/pkelley2/DynamicCR.html