390 Pistons

[Dwight]

I JUST BOUGHT A NEW SET OF FEDERAL MOGUAL 390 PISTONS OFF OF EBAY.
The part number is 381P-020. What compression can I get out of these pistons? Are there different pistons for different compression ratios or different heads? We will use Edelbrock heads. I have a few books but none tell the differents in the pistons. web sites?? Help
Thanks
Dwight

[DuckRyder]

with a 1.66 compression height. It will give you around 8:1 with a 72cc head and a 390 crank. It will end up down in the hole quite a bit too. It could be used with a 428 crank to make a 410 and end up with 10:1 or so.

-20 is the oversize.

You can look it up on SummitRacing.com, just punch 381p in the search box.



It is the one on the end with the longer skirt, note the compression height.



A little closer, and a little out of focus

HTH

[Dwight]

THANKS
DWIGHT

[Dwight]

DUCKRYDER
I have a pair of C8AE-H heads ( 67-70cc). What compression ratio will these head give me if I used the 381P pistons and a 428 crank?

thanks DWIGHT

[DuckRyder]

Assuming nominal "blueprint" dimensions.

9.8-10.2. Of course a little higher if the heads and deck are surfaced.

[Dwight]

Using the H heads and a 390 crank, How much has to be removed from the head to increase the CR one point? Two points?
THANKS DWIGHT

[DuckRyder]

You kind of have me there.

Every cc will drop or raise it about .10, so about 10 cc for a full point. How much off the surface 1 cc is I do not know. I am guessing quite a bit.

I would think that it would be easier to polish the chambers adding head cc to get the 410 compression down, than it would be to raise the 390 compression. If the clearances stack up right, you might even end up in the usable range without any work. In addition, the cam might help you out, even at 10.0 or so a more radical cam would get the DCR down due to the intake closing time.

Speed Pro/Federal Mogul makes a standard 390 piston that gives more reasonable compression in a 390, so selling those to someone who wants to make a 410 might be an option.

You could search the FE Forum on network 54, and/or ask there. Many there know ALOT more than me.

http://www.network54.com/Forum/74182

I was going to build a 410, which is why I have these three pistons, after studying them I decided to go with the forged 390 ones, so quit messing with the others.

[Excaliber]

All this ASSUMES your block deck height is standard. If the BLOCK has been decked, that changes everything. I was astonished to find MY block HAD been decked and I had to be very careful ordering custom pistons because of that. Stock, even stock over bore pistons would not work.

[ted]

Originally posted by Dwight


Using the H heads and a 390 crank, How much has to be removed from the head to increase the CR one point? Two points?
THANKS DWIGHT

DuckRyder is real close on his amount as 10cc per full point in compression is indeed a good rule of thumb for that particular bore and stroke combination. The actual math for a stock 390 figures out to 9.12cc's to raise the compression one full point. As far as milling the heads, the H heads should need 0.006" milled for each cc removed. For 10 cc's, you'd need to mill the head 0.060" to get your full point in compression increase. I'd also suggest cc'ing the heads end to end though to insure the heads are equal across the chambers. It's not unusual to have one end having larger chambers than the other and this is easily compensated for when ultimately milling the heads as you'd just have the end of the head that's got the larger chambers milled deeper than the other end so that when you're through, both ends have equally sized chambers.

[Dwight]

OK, GUYS HERE IS ANOTHER FE QUESTION. HOW DO I CHECK THE BLOCK DECK HEIGHT?

DWIGHT

[Dwight]

SOMEONE HELP????

[blykins]

Dwight, the deck height I believe is the distance from the cylinder head deck to the centerline of the crankshaft. I hope I remembered that correctly...

There are tools available to measure it...I don't know how else you could do it down to the thousandths......you might wanna call your local machine shop and ask them...

[Dwight]

THANKS