3.9L LZ9 engine internal upgrade & performance?

[NateD4]

After some more checking.... Here are some more pistons that are 'close' to stock that may fit. More research is obviously required. After all the machine work cost to fit them to the 3.9L they may not be any cheaper than a custom pistons. But here's the list anyway:

Most of these will require rebushing the rods for the .927 pins and a thicker head gasket to keep the piston to head clearance down. Additionally the valve reliefs may not be at the proper angle and dish may not match combustion chambers either. Piston to valve clearance may or may not be an issue.... all will require honing or boring to the right diameter as these are slightly larger than stock bore.

But... here's the list so far:

Probe SRS 14222-003 13cc dish
Manley 596207C-8 10cc dish
Wiseco K392X3905 25 cc dish
Wiseco K444X3903 11 cc dish 3.930" bore
Mahle LS1314905I16 16 cc inverted dome (dish??)
Diamond DRP 11503 8.6 cc dish

Not sure any single one of them would work. They either give a compression ratio thats too high or a piston to head clearance that is to tight without a thicker gasket.

Feel free to comment.

[NateD4]

There are actually a few you could try. I recommend picking your piston material and then looking for the bore size you need.

I found one that is actually fairly close to the stock size for a 3.9L.

Probe SRS Forged Pistons 14222-003 (no idea who they are).

deck height: 1.315 (vs the stock calculated value of 1.32)

Bore: 3.902" (.003 over stock size ), or 99.11 mm (vs 99mm stock)

Dish volume: 13.20cc

Pin diameter .927" (stock pins are .9425 nominal)

When I throw that combination in the Summit racing compression calculator with the following numbers:

3.31" Stroke
33cc combustion chamber volume
13.2 cc dome volume (dish in this case)
.015" deck clearance (may want to figure .010 depending on how you run them)
.055 MLS gasket

I get a 11.8 compression which isn't TOO bad for a street engine. Not something turbo capable but I think it could be run on 93 octane with perhaps some octane boost.

If you use .075" thick head gaskets instead of .055 you end up with a 11.15:1 which is actually in the range I'm looking for...

You'd also have to get your stock rods re-bushed to fit the .927 wrist pins.

Anyone else see any flaws in my math or specs?

I simply searched by material and deck height until I found something close.

[NateD4]

By my numbers you need a compression height around 1.32"

That is base don a 3.31" Stroke, a 5.9" rod length and a .055" thick MLS steel gasket (stock) and a 8.8" Deck Height.


That should point you in the right direction. Anyone else have similar numbers for a stock 3900?

[ericjon262]

Instead of my reading 3585830278476 posts, does anyone know which size / deck height LS1 piston I can order from Summit in stock that'll work in a 3900 with minimal / no extra work ? Around 8.5 : 1 and super tough.

What rods do you plan to run?

[Mars]

Instead of my reading 3585830278476 posts, does anyone know which size / deck height LS1 piston I can order from Summit in stock that'll work in a 3900 with minimal / no extra work ? Around 8.5 : 1 and super tough.

[SappySE107]

...ok

[paramount]

I remember this thread being longer...

[robertisaar]

so, while looking at a site filled with various engineers (eng-tips.com, LOT of smart guys there), i saw an interesting quote that makes sense in the case of the 3.9 not having an EGR....

Close the exhaust valve later and open the inlet later, and suck exhaust gas in on the first part of the induction stroke.

that concept works fine with the cam phasing method of the LZ9, i mean, doesn't it allow something like up to 50* (in crank movement) of cam retard? that should be more than enough to create the desired effect. essentially the piston will push out a lot of the spent air/fuel just by traveling upwards with the opened exhaust valve, then as it starts coming back down, if the cam is retarded enough, it should draw at least some of it back in as the piston starts traveling back down for the intake stroke, then the exhaust valve will close and normal events happen.

internal EGR.

not sure how much overlap there is with the stock cam, but i imagine there's not a lot if this is indeed how GM planned on NoX control.

[bearbear3_4]

The LZ9 does not have an EGR. The cam controls the emissions stuff.

[robertisaar]

they don't, from all of the wiring diagrams that i've looked at. they seem to indicate that it's all managed by the cam design and the ability to advance/retard it to meet whatever NoX standards.

[NateD4]

EGR, exhaust gas recirculation... does exactly that... pipes some exhaust back into the engine and burns some of the unburnt mixture/oil.

I don't think the LZ series have EGR. I found no indications of it when I tore down the 3.9L

[gectek]

Does anyone understand what egr does? And I did not say the VVT engines had the ability to change overlap in the ohv engines. Now it def will on the DOHC ones.

[ericjon262]

I am sure that is not the reason. At all. If it were, all the original LS6 engines would have VVT. Cam design will produce its own "egr" action with overlap.

overlap doesn't change on the high value engine's VVT, so this wouldn't be the cause, now changing valve events could give an EGR action, but it's not overlap.

[NateD4]

That won't happen unless you plumb in the exhaust into the intake.

the overlap will help with scavenging from the cylinders.

[gectek]

I am sure that is not the reason. At all. If it were, all the original LS6 engines would have VVT. Cam design will produce its own "egr" action with overlap.