What did he say about the odd wear on the rod bearings?

 

You dont have 9.5:1 now. If you used 360 pistons (or the 390 4v which many 360's ended up getting) with the shorter rod of the 390, that would give you a deck clearance of .014 and a compression of just about 9.5:1.

You currently have a deck clearance of .062. Assuming you have 72cc heads and a pretty standard gasket. You have about 8.7:1 static CR right now.

If you want to see the math I can show you.

 

I have no idea.. I like Krewat's idea. I did wash the truck right when I last drove it and put the car cover on it, 4 months ago. It's possible that the rust spots were from it sitting with the exhaust valve open and moisture made it's way up there, hell I dunno.

 

I don't want to beat this horse, but did you run it for more than 30 seconds when you started it before the tear down? Did you have a dead miss?

If you ran it for even 30 seconds, and it was firing on that cylinder, you would have evaporated off any moisture, so the water we can see would have to have been coming in fairly consistently.

It really doesn't matter if you do a rebuild you will be replacing the gaskets, so the problem will be eliminated, unless you have a crack somewhere, but I think you know enough about what you are doing to check that thoroughly.

 

And not to jump in with something too obvious, but, based on that bearing wear I would have a competent machinist check the runout on the crank. Align honing will cure if it's core shift that caused the misalignment but if your crank is just a tiny bit too far out of true you'll only shift the wear pattern when you reassemble.

 

Well, there wasn't that much 'odd' wear on the rod bearings. Just much more wear than expected within 20k. Probably from the intake carbon sandpaper running through motor. Oh and I forgot to thank you for the offer on the block. I'm sure mine is fine. I will know more this week when it gets checked out at the machine shop.

THANK YOU! But, if I only have 8.7 right now with the LONG rods, then what in the hell would a normal 390 have with the short rods and the same pistons I have right now???? 6.1 comp, lol??

Why can't I keep the 352 rods and use a different piston? Lack of choices?

What is the highest comp cast (cheap price) piston I can get to raise my compression? The heads are C7AE, which are what, 72cc? Then milled a little bit. Normal head gasket .040ish.

No miss. It ran great. I even took a video of it running. But the lighting really screwed up the picture quality. I could post it up on youtube but not really worth it. It ran for about 2-3 minutes right before tear down.

Very good tip, I will have them look at the crank, too.

 

[quote=ceetwarrior;10028010]
THANK YOU! But, if I only have 8.7 right now with the LONG rods, then what in the hell would a normal 390 have with the short rods and the same pistons I have right now???? 6.1 comp, lol??

Why can't I keep the 352 rods and use a different piston? Lack of choices?

What is the highest comp cast (cheap price) piston I can get to raise my compression? The heads are C7AE, which are what, 72cc? Then milled a little bit. Normal head gasket .040ish.

[quote]



OK for the 3rd time...

Lets just do some math so you understand.
First off, some measurements:

FE block deck height = 10.17"
FE short rod (390/406/410/427/428) length = 6.488"
FE long rod (352/360) length = 6.540"
360 piston compression height = 1.776"
390 truck piston compression height = 1.676"
352/360 crank stroke = 3.5" (throw = 1.75")
390 crank stroke = 3.784" (throw = 1.892")

So, if we were to just plop a 390 crank in a 360, here's what would happen:

Deck clearance = deck height - crank throw - rod length - piston compression height. So:

10.17-1.892-6.54-1.776 = -.038". This means the pistons would be .038" above the deck (out of the hole) at TDC. Not good.

This is what you have, a 390 crank with 360 rods and a set of 390 truck pistons:

10.17-1.892-6.54-1.676 = .062". This means the pistons would be down the hole, or below deck, .062" at TDC.

It also needs to be noted that many believe 360 rods to be weaker than 390 rods. Not weak for stock rods in general, but weak enough to cause problems in a 390 if spun too fast.

So why do that when we have such an easy solution available to us? Use 390 rods with your 390 crank and 360 pistons (aka 390 4v high compression pistons).

10.17-1.892-6.488-1.776 = .014". This means the pistons would be below deck .014" at TDC, which IMO is about ideal.

Now figuring compression.
add the swept cylinder volume, this is bore raduis squared: 30 over 390 is:
2.015 x 2.015 x 3.1416 (Pi) x 3.78 (stroke for a 390)
then multiply times 16.387 to get it to cc's.
Then add that to the chamber volume, gasket volume (a .040 thick gasket is around 8-10ccs in most 4" bore range engines) , the piston top volume (valve reliefs plus any dish plus another 1-2 ccs for the area around the piston to the top ring) , deck clearance volume (less than deck you use the above swept volume formula but replace the stroke with the deck clearance value) If you have domed pistons, subtract that value. Add all the values, (minus dome value if any)
Then divide the result by the total sum of the deck clearance, chamber volume, gasket volume,& piston volume.
Here's what you have now: 790.1 (swept volume) plus 72 cc(chamber volume) + 10 cc (gasket volume) + 10cc piston top volume (again this varies somewhat, see your pistons specs) + 3cc's for a .014 deck clearance. So you have 790.1 + 95 = 885.1 Divide that by 95 cc's and you get 9.31 to 1.

Change the deck clearance to YOUR .062 that will equate to 12cc. Thats 9cc more than the previous mentioned correct 390 setup. Plug that into the formula and you get your current CR of 8.68:1.

Different pistons will have valve reliefs with different cc's. I used an average 10cc for the formula, if you know your piston's top volume you can punch in the exact number instead of using this average, and get a more accurate measurement.

Also I used an estimate on head gasket cc. If you know the cc of what you're going to use you'll know the true #.

To my knowledge there are no high compression pistons with the 1.676 height. It's rebuild time and time to get things right. Use the correct rods and you'll find you have all kinds of piston choice. The only other way would be using forged pistons ($$) and shaving them.

Make sense yet..?

Oh and to your question about what the compression would be with the short rod and your current piston. 8.097:1 so really 8.1:1

 

WOW. OMFG. I hate math. i hate math. i hate math. And this explains why. BUT THANK YOU for being so smart. That is all the information (and more) than I needed to know. Seriously.

So, just for BS sake: IF there were more pistons available (shorter) for the long rod without being "out of the hole", and IF the long rod wasn't said to be weaker than short rod, then it could totally be used with shorter piston and achieve the EXACT SAME THING as the short rod/tall piston thing. But since that's not the case....

So, sounds like I know what to do next.

I love assembling engines, but HATE all the math/problems that go with it. If only most engines were like a lego that I could just assemble with zero issues, I'd love it more. This situation stresses me out beyond belief.

Again, THANK YOU!

 

haha yeah it is a little expansive. But thats what a good forum is for. I used to not know any of this.
Yes if someone made a small dome of the appropriate height you could get the same CR. It's not like the 390 rods are hard to find or anything and they arent all that expensive either. So it's just the easiest choice.

So get the 390 rods and get some flat top pistons of whatever bore you'll be at and report back. Tell us the overbore and the piston top volume cc that the manufacturer will state, and we'll give you your static CR. Throw out some cam figures of what you're looking at and we can give you the dynamic compression ratio. Which is really more important than the static CR for determining what fuel to run.

They do make lego engines....they are painted orange and known as a SBC. LOL

 

I agree the 390 crank with 390/428 rods and the 360 pistons I did that combo along with the block 0'ed and holy craaaap on premium gas with the other goodies. I would not in a million years would have expected the kind of rip *** I get out of the old 390
my 3 cents.

 

I bet so. Looks like you have all the same stuff on the top end as I do. Birds of a feather! I love that cam too. Great idle note

 

Holy crap Sleepy, that's awesome. I'm definitely printing out that post and hanging it on my wall for future reference

 

United Engine & Machine Co. Incorporated

Here is a link to a good calculator if you lazy brains dont wanna do the math. lol
At least you know the math behind the calculators though.

PS. dont be afraid of that little rep button now, I like little green boxes... I dont think anyone can catch Art(Krewat) though.

 

I hit it for ya

 

It had been explained to me at some point by my machinist, that the short rod of the 390 actually HELPS low-end torque because of the change in acceleration of the piston as it nears and leaves TDC. Higher acceleration just after it leaves TDC helps get the intake flowing better. Or at least, that's what HE said. I can't say it really does that in the real world, but it IS something to take into account.

Of course, it might also hurt higher-RPM longevity, but the shorter rod is stronger anyway... and using forged pistons that's a non-issue.