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I have a 360 crank that I was going to put in a 390 block that has 390 rods and pistons but I was told this would really lower my compression. Any help would be appreciated, thanks in advance.
 

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Your friend is correct

2 scenarios depending on the parts you have

390 car pistons 1.76 compression height
You will be at least .160 below deck and around 7:1 compression

It will run but barely


390 truck pistons 1.67 compression height
These are really 410 Ford pistons, used in a truck to drop compression

You would be at least .250 below deck and around 6.2:1 compression

I am not even sure it would run, if it did, it would barely run

If you can measure the pistons you have, there are two options. 390 crank or 428 crank. The 428 crank is much more expensive, but if you have the truck pistons, you could end up at 410 cid, and approx 9.5:1

If you have 390 car pistons, with a 390 crank you'd end up somewhere around 9.3:1 and 390 cubes
 

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360 and 390 replacement pistons are the same. The only difference between the 360 and 390 are the crank and rods.

Installing 390 crank and rods into a 360 turns it into a 390. No other modifications needed, so there is zero reason to rebuild a 360... and not do the conversion.

As a bonus, the 360 has terrible quench, as the pistons come around a full 1/8" from reaching the top of the bore at TDC. This KILLS compression, power and efficiency. A 390 will actually get better fuel mileage than a 360... and run circles around it to boot.

Do NOT rebuild your 390 as a 360, or you'll be going backwards.

Good Luck!
 

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Could he not deck top of block to add more compression ? Then use dif kind of head gaskets to up or lower compression as well. Learning here myself. Gota goto work.
 

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Your friend is correct

2 scenarios depending on the parts you have

390 car pistons 1.76 compression height
You will be at least .160 below deck and around 7:1 compression

390 truck pistons 1.67 compression height
These are really 410 Ford pistons, used in a truck to drop compression

Correct. 390 truck piston is different than 390 car.
 

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Could he not deck top of block to add more compression ? Then use dif kind of head gaskets to up or lower compression as well. Learning here myself. Gota goto work.
That would be a lot of machining. Plus after that the heads would sit lower and closer together so the intake would need machining too. Need shorter rods now too... but basically, you are right, it could be done.
 

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The 360 has a shorter stroke than a 390. Since it uses the same pistons as a 390, it has to use longer rods. Only problem, is they aren't long enough, and the pistons come around 1/8" from the top of the bore at TDC. As was said, this kills quench, compression, efficiency and power.

360 and 390 blocks are identical. Installing 360 guts in one is going backwards. Really, really backwards. Using 390 rods with a 360 crank will have the piston WAY down in the bore at TDC. Bad. Really bad.

Rebuild the engine as a 390.

As for replacement pistons... All performance 360/390 pistons use the 'car' compression height, and deliver good compression. You might find 'truck' pistons in the bottom dollar sets, but you should be able to find the higher compression height 'car' pistons for a decent price as well.

360/390 pistons with the 1.76" compression height, along with a 390 crank and rods is BY FAR the best bet.

Good Luck!
 

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Could he not deck top of block to add more compression ? Then use dif kind of head gaskets to up or lower compression as well. Learning here myself. Gota goto work.
You'd have to mill around 1/8" off the deck to get decent quench out of a 360. You'd have to mill more than that to make a 360 crank work with the shorter 390 rods.

At this point, the deck would be too thin, and the intake wouldn't fit right.

Best bet is to get a 390 crank. Usually, regular auto parts stores like Advance Auto Parts and Autozone can get them for you.

Good Luck!
 
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