I am about to convert a 360 to a 390 and I was wandering what the difference was (if any) between the pistons. I have heard that the two pistons are the same and I have also heard that the 390 pistons have a thinner skirt than the 360. also what about the pivot point.
also for the cylinder heads which ones will give you more compression? ones with high cc's or low cc's
thanks
74'-F-350

 

390 4 barrel pistons are basically 360 truck pistons. the difference between the skirts is early 360's had a full round skirt, and the 390's had the skirt cut away to clear the crankshaft counterweights. Small CC heads will give more compression. Compression ratio is the size of the cylinder when the crank is at the bottom versus the size of the cylinder at it's highest point. so the smaller you can make it at the top point the higher the compression is, try to get your compression from having the piston even with the top of the block before you go to smaller heads.

 

Just as a side note to that, some trucks got the low compression 390 pistons (also known as 410 pistons) Either way you go zero deck is important.

The design of the FE head has a squish area on it, this is where the pistons comes closest to the head and the air fuel mix has nowhere to go so it's forced toward the spark plug (this is a good thing) If you build the engine so that the pistons sits down in the hole while at TDC, you'll loose this effect and loose power.

 

You said if you build the engine.... how do you make the piston come closer to the head?
the 390 engine that I bought still has standard bore, but with a lip in the cylinders. so instead of having that block bored i am going to use my 360 block which is in good shape and bored .040. I would like to use the pistons that are in the 360 now. if possible.
and just change the crank and rods. The 390 engine is out of a police car and is a 4bbl.(with a holley 750) and the 360 is from a truck

 

A trial fit will tell you quickly if your piston is coming up above your deck height. It sounds like you have some testing to do. The 360 uses a 6.540-in center to center long connecting rod. The 390 uses a 6.488-in center to center connecting rod. Using a 360 connecting rod to make a 390 would put the piston top closer to the top, but you are going to want to check things like valve clearance. This may mean notching your pistons with valve reliefs. The other thing to consider is that the 360 piston is slightly taller from the 390 piston (wrist pin to piston top). This will also tend to put the piston closer to the deck top. Lastly, you will need to make certain that your piston does not have a full skirt at the bottom or you are likely to have problems clearing the crank. As you can see there are a few variables (just looking at stock pieces) that you can play with to try to get the piston going to the deck top. Once you start looking at after market pistons, there likely will be more choices, mostly dealing with changing the height from the wrist pin to the piston top. I hope all this doesn't confuse the issue. So.... can you just swap the crank and pistons.....maybe..... but you better measure twice to make sure you have all the proper clearances. You may also want to calculate what your compression ratio will be so that you can still put pump gas into it.

Bob

 

I am also planning a 360 to 390 conversion. I looked at Summit's online catalog. They list Sterling cast pistons for 390 truck (8.2:1 CR but they don't say which head) that have 1.660" compression height for only $10.69 each. They also have TRW forgings, 10.1 to 9.3:1 depending on head volume, with a 1.776" comp ht for $32 each.

Since the TRW is supposed to be a flat top design with zero deck, doesn't that mean the top of the truck piston is a whopping 0.116" down the hole which will eliminate the squish?

If I use the 6.540" long 360 rod instead of the 6.488" 390 rod, that would add 0.052" which is still 0.064" in the hole. Shouldn't the squish area be within 0.030-0.040" to be effective?

Would I expect more detonation problems at 8:1 with no squish than at 9.5:1 with proper squish? Cam will be small for max low end torque (probably Crane HMV-248). Any thoughts welcome.

 

C-65-f350, you are thinking correctly on the quench issue, 8 to 1 without will rattle worse than 9 to 1 with. I like your 360 rod idea, but as you said it is not enough. Quench pads lose effectiveness above 050. One of our posters mentioned 360 flattops on the 390 rod, I am going to do some measuring and see if this might work, I have some old 360 slugs laying around. I think 035 clearance would be ideal. DF

 

Don't forget the 360 rod is not as strong as the 390 rod. If you get 390 flat top car pistons, you should end up with zero deck, but I'd have all this ready for the machine shop and have them deck the block to get zero deck.

DF is right, you really need the squish that the zero decking offers even if you end up a bit higher in the CR.

 

Heeeyyyyyy, did a little digging through the left over parts piles and found some interesting stuff. We all know that ford truck FEs have the piston too far down the hole. The question is what combination of factory parts would get us closest to zero deck? I miked a 67 390 2V galaxie piston and found the height to be 1.7655, which leaves you about 030 down the bore at TDC. Then I miked a piston from a 76 360 4V pickup engine and found a height of 1.7848, which works out to .100 down the bore in the 360. But if the 360 pistons were on the 390 rods and crank, you would only be .0095 away from zero, pretty darn close for not much work. And, the 390 car pistons have a dish in the middle, but the 360-4V truck pistons are flat with 4 eyebrows for the valves. I have seen some 352/360 pistons with full skirts but these 76 truck slugs have the same style skirts as my 390 galaxie pistons, so no fear of hitting the counterweights on the crank at BDC. I think this might be just the way to go! What do the rest of you think ? DF

 

I know ford used those pistons at least as early as 72 because the 360 from my 72 (never torn apart before I did) had the flattop pistons like that with the cut skirt, and when I ordered my 10:1 pistons for my 360 they sent 10:1 390 pistons and it worked out to be the 360 truck piston at 8:1 or so

 

It seems that it should work. My book might be wrong, but it indicates that the 360 had a piston height of 1.776. This would still indicate a great improvement. I would caution about using a 352 piston as my indications are that it is taller still at 1.825. This would put the piston just above the deck height. I still think that compression ratios could get pretty heigh, but I guess it depends on still more variables. One wants to comsider the cyclinder head volume and the shape of the piston (ie dished, flat, or domed). There is a neat little spread sheet in the end of the Tech Articles section that allows you to play with these variables and calculate your compression ratio. From looking at the program, it sure seems to make a huge difference as to the shape of the piston. If the 360 piston were truely flat, it would seem that you would have a compression ratio of 10.6:1!! This may send you looking for a set of aluminum heads. Just a thought, the 360 piston is designed for a low compression engine, how well would it stand up to the higher compression? If you could get the pistons from a 65 390-4v they are .7919!!! ;-)
Bob

 

Can anyone find the compression heights and volumes for TRW pistons? I've done some online searching and can't find the specs. Looking for L2291, L2143, L3043.

What does it cost to mill piston tops or deck the block?

I may just use the $10 Sterling 381P (1.660 ht) without the quench, as a stock replacement. Worked for Ford, anyway

 

[font size="1" color="#FF0000"]LAST EDITED ON 06-Jul-01 AT 01:48 PM (EST)[/font][p]This is merely my supposition, but we already know that ford used some 410 4v pistons in 390 truck engine's to get lower compression. So it seems equally so that when Ford stopped using 390 4v's in cars they used those pistons in 360's to get their low compression ratio. This would also explain why some 360's have the skirt cut away for crank clearance when they dont need it and other 360 pistons dont have it cut away.

 

 

Is there an obivious external way to tell a 360 vs a 390. I live in Calif and when I did my final smog check, it was tested fisrt as a 360 then the final was a 390.
Or are the external parts interchangeable? I see most question are bore, pistion and stroke related, but I hate to have ask the dumb question and the external differences.
Thanks.

Mark