Recently picked up a 390 crank to convert the 360 in my truck. I already have edelbrock heads (72cc), intake and a cam. Planning to use my stock 360 pistons with 390 crank and rods for higher compression. My question is what compression should I expect to get running this setup? Also any tips for the conversion or things to do while I have the motor out would be appreciated.

 

Opinion...with the high dollar heads and intake, I would not reuse the factory pistons and rods. Look into new after market H beams and forged pistons. Flat Tops will get you much higher compression if needed. Go to SCAT's website and shop around.

 

Surest way is to measure and calculate. Old parts may have been cut at some point and throw off compression estimate.

As for "while you're in there"...consider stroker kit (as mentioned), custom roller cam, custom pistons to dial in compression and get more of a modern ring setup.

 

Have you checked your bores and pistons for spec & out of round etc? High mile used pistons could spoil your efforts.

Are your stock pistons flat or dished? How far below deck do you end up? How thick is your compressed head gasket?

Once you have that information, you can figure it out here:

Engine Compression Ratio (CR) Calculator

 

Not looking for a high performance motor and budget is pretty limited as Im in college. So roller cams and aftermarket rods are out of the question. Pistons are just stock 360 truck pistons dont remember if they are dishes or not. They probably have 130,000 on them. I was just planning on putting a new set of rings on them and running a ball hone down the cylinders. Just looking for a ballpark on compression so I can figure out what plugs and octane I would need to run.

 

Your compression will be between 9.5 and 10.2.

IMO you are making a foolish mistake here. If it the 360 is running, leave it running until you can do a proper rebuild with new pistons.

 

Can I ask why that would be a mistake? What is wrong about reusing the factory pistons if I put new rings on them? From some other forums I got the impression thats its pretty common thing to do.

 

The piston to wall spec is .0015 to .0035". If you are much outside that or out of round, you risk an oil burner or piston slapper with worse results to follow with the harder running it will get with the fancy heads, intake, cam etc.

Do it right or wait until you can.

JMO.

 

There's absolutely nothing wrong with factory Ford 390 connecting rods. The Mod motors ? No. Pushrod Fords ? They all had forged rods.

 

Depends on how much wear is on them. Skirts were knurled when new, this wears off after a length of time. Nothing wrong with 390 rods. Just get new bolts for them.

 

i am in the middle of a 352 to 390 with 390 crank and rods. my piston options were pretty limited because i had it bored to 4.050 and most modern options are for 390 over-bored, not original bore.

I ended up choosing Silvolite S116.STD pistons because my first choice was no longer available in 4.050. I had my C8AE-H heads rebuilt and the shop said they cc'd at 66.5 ccs. on the low side. If I use 40 mil head gaskets, my CR comes out to 10.22 and the pistons are spec'd at 9.7, so i will use a thicker gasket. Will make final determination after assembly and i see what my piston deck clearance looks like.

as i understand it, the original 360 pistons sit about 0.100" down in the hole and produce a CR in the 8s.

i agree with the other people, if you are going to disassemble, go all the way and have the block magnafluxed and machined, but i understand budget. they tend to be a money pit once you open them up. if you have 130,000 miles on the engine, it needs a refresh. but wth, you are young and will have more money when you get out of college. if you don't mind doing it all over when you are making money, you do you. best of luck.

edit: i went ahead and had my 390 rods resized on the big end and new bushings in the little end with new ARP bolts. i'm not worried about the rods. i'm worried about remembering how to put it back together after a couple of years apart!

 

To the OP. Your idea would work fine. You might be leaving a few horsepower on the table by not having a perfect bore, but it would be a great improvement. There is a reason that it will not work though. The piston will come slightly higher in the bore (according to my math, which should always be questioned), which means that if there is any lip at the top of the bore at all, your top ring will break in short order.