My dad wants to get a little more cheap power from his 390. We found a good 428 crank and have some good 390 rods. What pistons do we need to get the compression around 9.5:1 so it will run on bad gas?

 

This thread is on the subject, with links to kits and pistons:

https://www.ford-trucks.com/forums/9...-build-up.html

This won't be cheap.

Cheap power from the 390 would be 390 2v regular fuel pistons (also used in the 360), an rv cam, springs etc, a set of headers, and a stock S or T 4bbl manifold with a 600 cfm vacuum secondary carb.

Real cheap? Headers and a tune up.

REEEAAAAL cheap? Dual exhaust.

 

That's about exactly what he has now. He just wants more. (Can you ever get enough?)

410 pistons are low compression 390 pistons, correct? We have the crank, so the only thing we need is pistons. But we want lower compression than 10.5:1 like stock.

 

Stock pistons are 10.5, thus the link to Probe for slightly different pistons to get the lower CR.

 

You have the crank, but do you have the matching flywheel/flexplate? It's gonna shake if you don't....
Alternatively, there is the survival/scat 3.98 stroke crank, that retains the internal balance like the 390, so the current flywheel/flexplate could be retained. They also sell it as a complete stroker kit, with various CR pistons in the offering.....

 

You can also "cam it up" to lower the octane requirements with the 10.5 to 1's. I did this twice with 10.5 to 1 390's. One would run on 87, the other with 89. The 87 had a Cam Dynamics grind (now longer around) with a 290 something advertised duration/.523 lift. The 89 octane motor used a Crane 272* Energizer