Forged LS1 piston comparison to 3900

[Guest]

Got a great deal on a set of MAHLE pistons $381 for a set complete with rings:

There are friction marks in the upper part of the pin bore of the stock low mileage piston to indicate that the oiling hole on the new piston will be very beneficial. Side by side it really doesn't seem like one piston is weaker than the other considering design. It is interesting that the rings are thicker for the forged piston although they are the same diameter and similar in application except for stroke of the engine

Specs:

Bore 3.898" same as stock.
Compression Height, 1.314", stock 1.305
Dish, 9 cc, stock 18 cc.
Pin, .927, stock .942.
Weigth 402 g, stock 420 g.

[leong6gtp]

Got a great deal on a set of MAHLE pistons $381 for a set complete with rings:

I'll post pics in a minute, the system is making me wait for not doing it right the first time.

Specs:

Bore 3.898" same as stock.
Compression Height, 1.314", stock 1.305
Dish, 9 cc, stock 18 cc.
Pin, .927, stock .942.
Weigth 402 g, stock 420 g.

nice when do you plan on installing them? damn whered you get them for that price? do you have the part numbers for everythin?

[Joseph Upson]

nice when do you plan on installing them? damn whered you get them for that price? do you have the part numbers for everythin?

I had been watching them on Ebay until they were marked down from $449 to $381 and on top of that the owner agreed to free shipping so easily I suspect he would have sold them for even less if I had made a lower priced offer. They arrived in the original packaging from MAHLE complete with pins and rings. I priced them at around $640 everywhere else.

If I have the crank offset ground to .075" stroke increase and use a set of forged chevy 5.7" rods with the 2.1" crank pin it'll drop my compression to about 9.1:1 as well as increase the displacement. If I use the old V6 rods with the 2.00" crank pin I can have the crank stroke further increased by about .100 to 3.41 for a little more displacement and still have less than the stock 9.8:1 compression for upping the boost more as well.

I was quoted about $17 ea to have the bushings in the stock rods changed for the .927 pin, if I decide to stay with the stock rod and stroke. I'll have to have valve notches added which should bring the pistons dished volume pretty close to the stock ~18 cc.

In that scenario I would probably have about .020 or more shaved off the top of the pistons to bring compression closer to about 9:1 to have room for more than 15 psi of boost with an intercooler and water/meth injection although the intercooler should be enough to reach that point considering I'm already half-way there with 212 degree inlet temps.

You can't tell easily in the picture but compared to the forged pistons, the stock piston has thicker ring lands than the forged piston which may support the theory that they are strong enough to handle a considerable amount of boost pressure. The top portion of the piston above the ring is thicker on the forged piston and the skirt and overall piston is shorter by ~1.5/16".

Here is a link to other deals, some have more than one piston listed, with different dish and compression heights for longer rod combinations;

http://motors.shop.ebay.com/items/ls1-pistons_W0QQ_nkwZls1Q20pistons

[IsaacHayes]

So the LS1 are the used pistons in the pic?

[ForcedFirebird]

So the LS1 are the used pistons in the pic?

Seeing that the 9cc dish is the LS1's and the stock ones are 18cc, it seems that the LS1 pistons have the valve relief

[IsaacHayes]

both have valve reliefs in the pictures.

[Joseph Upson]

both have valve reliefs in the pictures.

Yes Isaac but the LS1 valve reliefs are in the wrong place for the V6 heads. Remember I just purchased the LS1 pistons brand new so the used piston belongs to my extra 3900. The LS1 pistons can be found in several different dish volumes and compression heights, I chose these because they are manufactured by the same company, are similar to stock and I really liked the price.

[ForcedFirebird]

Yes Isaac but the LS1 valve reliefs are in the wrong place for the V6 heads.

Guess I should have specified that as well. I was more implying that you could see the difference in dish size from the pics.

Now, to get a FWD block in the Firebird, perhaps the 3900 would be a better option :P

[IsaacHayes]

Whats up with the finish on the forged ones? Looks bead blasted.

I know the reliefs don't match up, that's apparent I was talking to FF.

3900 in RWD, you got the room to custom fit the cooling system and cam phaser has room.

[Joseph Upson]

Whats up with the finish on the forged ones? Looks bead blasted.

It's actually a very thin coating that is very smooth to the touch.


"Phosphate is a dry film lubricant that aids ring seating, helps prevent micro welding, and provides start up protection to the wrist pin bores."

[Nightingale]

I believe CNCguy already has rwd conversions for 3.9L in prototype form atleast.