FederalMogul L2380 v. L2381 Pistons...

[kris]

These are the FM forged pistons for the 401.

Ive found a dish spec for the 2380's of 27.5cc's. I dont think its correct.

I havnt found one for the 2381's. I remember them being a lower compression piston though, somewhere around 8.5-9:1. They have a dish that resembles stock, but cant find any specs online anymore.

I have a 401 block that is currently .030" over. Im thinking about going .040" over with the 2380's but want my cr under 9.2:1.

Ive been reading that the 2380's are a +9.5:1 piston. I did the math with a piston at .040" over, 27.5cc dish, and a .020" deck clearance. If I'm correct this combo nets a 9.14:1 cr.

Is the dish volume for the 2380's correct ? Because the math is not making sense...

And BTW ...
Im asking because I have a new set of the 2381's at .030". I dont have a problem going .040" with the block, but am concerned about the compression ratio.

kris.

[AMX69PHATTY]

Stock 401 deck clearance is only .002 and think replacement pistons are made with a .020 shorter compression height than stock and they assume the heads will be milled .020 and the block decked .020 when doing their advertised CR calculations.

Is the L2380 Compression Height 1.488, which is .020 shorter than 1.508 stock ?

So if the CR calculations are re-done with the heads chamber volume reduced 3.2cc from the heads being milled .020, and a .002 stock deck clearance from the block being decked .020, and use the 1.488 Comp. Ht. and 27.5cc dish volume for the L2380 pistons, does it come out to be the advertised 9.5:1 CR ?

Working backwards and using this method, L2380's and a
27.5cc dish results in a 9.87:1 CR.
Values used to get the 9.87:1 CR with the L2380's:
1.488 Comp. Ht.
27.5cc dish volume
9.188 deck ht.
.002 deck clearance
4.195 bore
3.68 stroke
5.858 rod
54.72cc chamber volume
.048 thick head gaskets with a 4.275 dia. hole.

Working backwards and using this method for the L2381's their dish volume comes out to be
44.5cc's to result in 8.5:1 CR, or
37.5cc's to result in 9.0:1 CR.
Values used to get these CR's with the L2381's:
1.488 Comp. Ht.
9.188 deck ht.
.002 deck clearance
4.195 bore
3.68 stroke
5.858 rod
54.72cc chamber volume
.048 thick head gaskets with a 4.275 dia. hole.

[kris]

Thanks, some of my math was off...



Is the L2380 Compression Height 1.488, which is .020 shorter than 1.508 stock ?

Compression height on the 2380 is 1.505, its the same on the 2381.



So if the CR calculations are re-done with the heads chamber volume reduced 3.2cc from the heads being milled .020, and a .002 stock deck clearance from the block being decked .020, and use the 1.488 Comp. Ht. and 27.5cc dish volume for the L2380 pistons, does it come out to be the advertised 9.5:1 CR ?

I just cc'd the 2381 and it came out to 38cc's, so 37.5 sounds just about right on...



Working backwards and using this method for the L2381's their dish volume comes out to be 44.5cc's to result in 8.5:1 CR, or 37.5cc's to result in 9.0:1 CR. These are the results of using 1.488 Comp. Ht., 9.188 deck ht., .002 deck clearance, 4.195 bore, 3.68 stroke, 5.858 rod, 54.72cc chamber volume, and .048 thick head gaskets with a 4.275 dia. hole.

ok...
9.188 deck is with .020" shaved right ?

Using the following numbers...
4.270" gasket bore, .048" gasket thickness (just checked the used FelPros in my garage), 9.208 deck height, .005" deck clearance, and a 58cc head (Ill be generous there).

Heres the 2381...
38cc dish = 8.69:1.
27.5cc dish = 9.51:1


Thanks,
kris.

[AMX69PHATTY]

Yeah, 9.188 deck ht. is shaved .020, this was included in the calc's based on the
assumption that the compression height was 1.488 which is .020 shorter tha stock.

So if the Compression height for both the L2380 & L2381 pistons is 1.505
which is pretty much the same as the 1.508 stock comp height
the block deck would not need to be shaved because
the 1.505 comp. height results in .005 deck clearance
with the stock 9.208 nominal deck height.

+ 1.505 = Comp Ht.
+ 1.840 = 1/2 of 3.68 stock 401 stroke
+ 5.858 = stock 401 Con Rod
= 9.203
+ 0.005 = Deck Clearance
= 9.208 = Stock nominal 401 Deck Height

Re-doing the calcs again with new numbers:

4.195 bore, stock 401 +.030
3.680 stroke, stock 401
5.858 connecting rod, stock 401
1.505 compresson height for L2380 & L2381
9.208 deck height., stock 401 - no deck milling
0.005 deck clearance
0.048 thick head gaskets with a 4.275 dia. hole.
57.92cc chamber volume, stock - no head milling

comes out as:

8.73:1 CR with 37.5cc L2381 dish volume
9.52:1 CR with 27.5cc L2380 dish volume

Pretty much the same as the results you got, which is the CR you're after right ?

Guess I was incorrect in thinking the pistons were .020 shorter and the block needing decked.

#-o

[MoPedal]

STOP! STOP! My head is going to explode! #-o
I need to go back to school! :mrgreen:

[kris]

NO NO ! (its not that bad, really) :mrgreen:




Im gonna stick with the lower compression.
Went ahead and checked the DCR with the 2381's and ends up its at 7.42. I thought it was going to be much lower than it ended up being. Cam is a custom ground Engle that Ive been running for about 8 years. I want (more like need) to run 87 so these will work out great.

So.
The question is now do I go .040" with a new set of 415's, or sleeve to run the pistons I already have ?
Sleeving to go .030" seems like a waste of a sleeve, but Im concerned about going .040" without testing.

415's or sleeves, cost is about the same...

kris.