390 build swapping crank, rods, and pistons

[coletrain777]

Hey guys,
Here's the rundown and then a few quick questions

I have a complete low mileage 65' Merc 390 Z code engine that has scratched cylinder walls (i believe from the pistons rings sticking from sitting). I also have a 1974 360 in great running shape that I am going to do the 390 conversion on. The 360 does have the partial skirt pistons just like a 390 piston so I should be good if I want to use them. So my questions are:

1. Should I just use the 390 crank and rods with the partial skirt 360 pistons or... should I use the 390 crank, rods, and pistons out of the 10:1 Merc motor and put them in the 360 block. The reason I ask is that I will be honing the cylinder walls and re-ringing anyway, so which option would make the better/stronger motor. By the way the 390 pistons have no carbon build up on them and look absolutely like new.

2. I know that most people say the 360 partial skirt truck piston is exactly the same as the 390 high compression car piston, but is this 100% true? I am wondering if the 390 car piston would offer even the slightest bump in compression or is it a better casting than the 360 piston.

I want the best compression ratio possible out of the two as I would like the motor to be as efficient/powerful as possible. I don't care about having to run premium fuel as this in not my daily driver, but it will tow from time to time and cruise as well.

Just wanting some factual info before I tear the blocks down and make the best combo possible.

[fordman]

i am no experrt in this area. but from what i read the partial skirt on a 390 piston. is so the piston doesnt hit the rod when its running.

[Brokenarrow]

I wouldn't advise going to a longer stroke without first boring the block. With the 390 rotating assembly the pistons will be slightly higher in the bore at TDC than the 360. (Hence the higher CR with the same piston) So at the very least the ridge must be cut out to keep the pistons/rings from hitting it. Even then the cylinders are worn to that stroke so bad things would probably happen. There is a book, "Rebuilding bigblock fords by Steve Christ", it has a lot of valuable infomation on FE specs and parts interchange. I highly recommend it.

[fordman]

oh that does remind me. the wrist pin on the 360 pistons sit differently than the 390 pistons. so the 390 longer rods will make the 360 pistons sit up higher in the cylinder and interfear with deck height to the head space inside of the cylinder. it will make the piston slightly stick out of the block i think is the idea i read about.

[Brokenarrow]

Since you obviously have made up your mind to do this I'd say use the 360 pistons in their original holes.

[coletrain777]

Brokenarrow - Do you think that it is a bad idea? I just ask because I have seen a few on here state that they have done the same thing with good results so i thought it would be OK. Please correct me if I am wrong, I don't want to break parts out of ignorance.

[Brokenarrow]

I'd be concerned about the longer stroke in worn cylinders busting rings. If it has very little ring ridge as you say, then it should be OK. Wrist pin fit is another issue that might cause problems. There's lots of information on the 'net about FE specs. Do your research and figure out what piston to deck height and static compression ratio this combination would give you. Then choose your camshaft. The 390 I built for my truck has 10.4/1 SCR, but the cam that's in it should make for a 8.4/1 dynamic compression ratio. I'm thinking she'll scream on 91octane.
Now, all that said, you either love FEs or you don't. Sounds like you don't, I've got a real good low-mileage 302 I'll trade you for that Merc 390.

[coletrain777]

I like FE's just fine (I own 6 of them), but at the end of the day they are just iron and parts. I say this to mean that I don't have any engine or even brand loyalties when it comes to vehicles, I will build and like every one of them. I appreciate the offer for the 302, but that is going the wrong direction as far as displacement and towing capability (now if it's a FI 302HO roller block motor then I am interested:)).
That sounds like a great a great cam choice you made, I wish more people understood the difference between static and dynamic compression and how engine setup relates to it.

Anybody else have any opinions? I just started the topic to get some discussion going, lets hear what you think!

[BobbyFord]

Lack of trans options? Why would you want anything less than a C6 behind an FE?

[Brokenarrow]

Lack of trans options? Why would you want anything less than a C6 behind an FE?

[Vinniesempire]

you can put t-5s, c4s, aods, c6s, tremecs, etc... all comes down to money

[coletrain777]

"why would you want anything less than a C6 behind an FE" - Because to me overdrive is almost a necessity. I don't like to tow (or cruise) at 60mph down the interstate for hours and most of the time when I go get a vehicle to haul it is several hundred miles round trip. Now, if I don't have to put hardly any money into the drivetrain and it will be a great puller then I don't mind the drawbacks of 1:1 (hence this low buck build), but if I am going to have to spend serious money on engine building then there are different engine options that I will go with.

Vinniesempire - yes you can put anything you want behind an FE, but I just couldn't justify the money spent.

I hope you guys don't think that I am arguing, I really appreciate the info. I just like to hear other people's opinions and love to talk to other gearheads about "options" whether they are low-buck or high-buck builds.

[Brokenarrow]

You say you want an engine you can depend on to tow heavy loads for long distances down the highway, correct? You want my opinion? Don't half-a55 build it. Which is what you're doing . You throw a bunch of used, mismatched parts together and it may or may not last. But, if you build a 390 the right way, and it's practically indestructable in a work truck. FWIW, That's my opinion.