I got a 73 F100 4x4 with a 360 and a c6. For anyone who knows about these FE motors, if I take a 390 crank and put it the 360 block (I know the blocks are the same), do I need to put 390 rods in it too? My guess is no, since the 390 crank would lengthen the stroke.

Also, because the stroke is longer, should I bore out the block or just hone it? Will it even matter?

Thanks for any help!

turbo

 

You need the rods from the 390, shorter for the longer stroke. 360 rods will put your piston into the heads. But your pistons will work from the 360. The 360 pistons are suppose to raise the CR. You will need to make sure you measure your deck height, you want close to flush for higher a CP. All truck 390 & 360 were low compression ~8.5:1 There were a few years in the cars that CR was over 10.5:1 with the 390. You probably want a CR in the 9.5:1 range if you still want to run pump gas. Of if you have E85 you can go higher.

 

All you need is the 390 crank and rods. BTW the rod length does not change the stroke. As far as boring or honing... it kinda depends on the condition of the cylinders or if you are looking for more cubic inches, etc.

 

The rods are shorter? That makes sense. How do I check the CR? Can I do it by simply measuring compression on each cylinder or just by the deck height?

I would like to put a mild cam in it, and run 9.5:1 CR, to raise my bhp's.

Thanks for the info!

 

compression ratio is determined by measuring your pistons CC's, depth below the deck, thickness of the head gasket, and size of the combustion chamber.

http://www.csgnetwork.com/compcalc.html

 

The compression height is different on original 360 and 390 pistons. The rods and crank are also different. The after-market piston manufactures sell (sold) one piston for both based on the 360 (taller) height.

Despite what the guy at the auto-parts store said (who had built hundreds) the 360 rod is longer, not the same as 390. When you put the tall pistons, long rods and 390 crank in, the piston is about 0.010 out of the block. The head gasket, crushed, is 0.050. But I was assured it was ok.

It will start and run strong, but the pistons expand enough to start tapping on the head. It also has enough compression to run without any ignition what so ever...

I ended up fly-cutting the pistons to 390 specs and running it with the long rods. It liked 100 octane fuel.

The old "Rebuild your FE" book has all of the numbers and you can calculate the CR from there.

 

so I should rebuild it with 390 crankshaft, 390 rods, and 390 pistons...do you where I can find shorter pistons?

 

The std 390 truck pistons will give you about 8.6:1 CR. There was several years around the 1968 that the cars had over 10:1 CR.

There was a thread about a month ago, that got in a discussion on where 360 piston would work. Was was determined was that the 360 piston (don't recall mfg, this is key) the piston was ~0.040" below the deck. All the formulas indicated around 10:1 CR. Numbers were determine by measuring pistons, pin height, rods, etc .....

If you go for the 390 pistons you need to decide on what CR you want. Then have one of the users run it through desktop dyno to determine approx what you will have.

As far as shorter pistons, most are short to start off with depending weather there dished or flat tops.

Most good machine shops should be able to tell you what pistons will give the CR you want. But it's your job to double check them.

 

turbo, Don't know if this will help you. The 360 is a destroked 390, it uses the 390 bore block with 352 crank and rods. This was an attempt to deal with early emission standards. One good thing came from it, the 360 heads are some of the better flowing FE heads and came with hardened seats.

John

 

so, I should just find out which cars used the higher CR pistons, and buy them(new)...buy 390 rods and crank...measure everything, and then put it all together...

I think I got it...
thanks!

 

I used car 360 pistons in my truck 390 after much research on this forum. The 360 piston is flattop with just the cutouts for valve clearance....usually around 10 cc. For my engine, the top of piston ended up about .012 below the block deck. I used D2 heads, ported, polished, and surface decked. I didn't cc the heads, however with a compcams 33-238 cam I've had to run premium fuel to prevent detonation. Even with the premium it's close. I wish that I had calculated the the compression #'s closer. From the info I got, I assumed I was around 9.5 to 1....maybe up to 10 to 1. I also used the steel shim head gaskets which will raise compression even more....however I've since changed them as they were not reliable long term for me. Here's my take on this...you should use the 360 pistons....you will most likely need to bore the engine as the rings will go higher in the cylinder....and you should cc your heads, look at head gasket selections, and really calculate your static compression ratio. Shoot for around 9 to 10 to 1 static compression. After that, choose a cam that will bring the dynamic compression ratio down to where you can run without detonation at whatever fuel you intend to run...be it regular or premium or race...and get the valve springs necessary for the cam you run.. Of course along with this you need to think about headers, your intake...I used an edelbrock peformance RPM....carb....and for good engine longivity I'd think about a windage tray and at least a rear deep sump oil pan....and then there's the rocker arm issue....these 390's are short changed stock with with a nonadjustable valve train....l also rebuilt the C6 and put in a shift kit and I put in 4.1 gears with a tractionlok rear. For me, I've been extremely happy with the performance....I've taken down a couple of new mustangs no problem....however I pay for it at the pump.....I'm at maybe 8 mpg around town....it's a blast but an expensive one. One more item....flat tappet cams have issues with breakin these days....it seems even more so with the FE's but that's probably because the roller options are just so expensive that there are more flat tappet builds....you can get by this....use good diesel rated oil at breakin and beyond....use the cam mfr's breakin stuff....it's the zddp that you need....and consider initial startup and breakin with a lower spring pressure that the cam requires....remember that at breakin you won't be pushing the rpm's....maybe 2500...so it's ok to have a lower spring rate. I've probably mentioned way more that you want to hear....lol. I can say this....an FE requires more attention to detail to build than your run of the mill engine....but when you get it right it is amazing....I used to be a chevy guy but I've never had as much fun as I've had with the FE and the sound of an FE just blows away a chevy imho.

Good Luck!

Tracy

 

wow! thanks a lot! that's a lot of great info!

i don't know if i understand everything yet, but i'll figure it out, and i'll be sure to use it...

 

You can use a 390 crank, 360 rods and short pistons, I forget the exact size but the short car pistons will work, do the math (stroke+rod size+piston height), I know because I did it on my first rebuild when I was on a budget. Either way if you are short on cash and must use the 360 rods it can be done. If you can afford the 390 rods, do it because the 360 rods are real scrawny.

If you do change the crank you might as well rebuild the motor as long as it is out of the truck, here are a few links to help you out:

Complete engine kits w/cranks:
http://www.larrysperformance.com/data/enginekits.html

Rocker arm assy:
http://www.shastacounty.com/rockerarm/page5.htm

FE parts, check out his rods before you buy any, or anything for that matter:
http://www.dscmotorsport.com/About/index.asp

Good luck!

 

Tracy, I have to ask, where did you find your car 360 pistons?

thanks

John

 

i think i got a good idea for internals...what about a high performance cam? any thoughts?