3.9L LZ9 engine internal upgrade & performance?

[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.

[bearbear3_4]

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

[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.

[paramount]

I remember this thread being longer...

[SappySE107]

...ok

[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.

[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?

[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?

[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]

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.

[Mars]

Seems like enough work and research will need to be done yet.

I'm just half an hour from having mine installed into my Camaro. We shall see what a cam and spring only 3900 can do next summer.

The intake manifold ports on this engine are HUGE compared to a 3500. I hope it's as reliable / powerful as my 3500 was.

[Fireball]

Seems like enough work and research will need to be done yet.

I'm just half an hour from having mine installed into my Camaro. We shall see what a cam and spring only 3900 can do next summer.

The intake manifold ports on this engine are HUGE compared to a 3500. I hope it's as reliable / powerful as my 3500 was.

Deard lord you always have the most bad ass cars lol

[Fieroghini]

My 3900 with cam and springs did a 13.3 with an automatic at 103mph. With the 4 speed manual it did 13.4 at 106mph. I'm switching to the F23 with the hopes of clearing up my shifting issues. I've been told that my cam leaves a lot on the table so you may do better. Good luck.

[Mars]

My 3900 with cam and springs did a 13.3 with an automatic at 103mph. With the 4 speed manual it did 13.4 at 106mph. I'm switching to the F23 with the hopes of clearing up my shifting issues. I've been told that my cam leaves a lot on the table so you may do better. Good luck.

what were your cam specs?

That's pretty fast...

[Fieroghini]

You'll have to adjust for the 1.7 rockers. This is for 1.5's.