GC99TA

iTrader: (30)

I'm just wondering about the LY6 piston tops. I've read that they have flat tops with no valve reliefs, but I also read that the SCR is only 9.67:1.

Doing the quick math with the info above and assuming the pistons are at least .005 out of the hole, I come up with about 10.33:1.

So, I have to assume the pistons either have about 6cc's worth of valve relief, or are down in the hole (.025 or so), or a lesser combination of the two.

I have a 2008 LY6 sitting in my garage for my conversion project and I'm curious to find the truth about the SCR/pistons......just not curious enough to pop the heads off and look/measure myself if someone else has already had one apart. Any pics or actual measurements taken would be great. Thanks!

FWIW, here are the "assumptions" I used to calculate the SCR:

Chamber volume: 70cc
Piston head volume: 0cc
Gasket thickness: .051in
Gasket bore diam: 4.083in
Cylinder bore diam: 4.000in
Deck clearance: -.005in
Stroke: 3.622in

Casimirblue

I'm interested also if someone has a parts and specification breakdown. The junkyard ly6's are fairly reasonable and I've seen new crate ly6's at $3000 to $3500.

Thanks
James

GC99TA

iTrader: (30)

Yeah, the junkyards really don't expect to sell many of them just yet because most vehicles orginally equipped with them are still under warranty. They don't realize that these are a great swap candidate for someone like me since they're an iron block, 4 inch bore motor with factory L92 heads. FWIW, I picked mine up for $1800.00 with less than 3000 miles on it. Came out of a 2008 Fed Ex van that got totaled out.

GC99TA

iTrader: (30)

Well, I just refined my search terms a little better and it appears most are saying the LY6 pistons do have exhaust valve reliefs, but not intake valve reliefs.

That pretty much answers my question on why the compression is what it is, but I would still like to hear from anyone that has actually cc'd the valve pockets and/or measured how far the LY6 piston is out of the hole, just so I know if my other assumptions/calculations are correct. Thanks a lot.

PwrdbyLS1

iTrader: (5)

I had the same question because I will run my LY6 1-2 years before I build my 408 and i've read 9.9:1 compression. But I have remove one head and the pistons are dish and have 2 notchs...
here's picture I've take


the dish have a diameter of 3" and a depth of 0.050 so .35cu in.. in cc(cubic centimeter?) 5.78cc
So around 6cc + 2cc for the notch... -8cc piston.. we are far from the flat top we have read...

And my piston are a bit over the deck... maybe .005"

Hope its helping you !

BES Stroked Nova

iTrader: (24)

same pistons in my short block,just mine have a notch for the intake valve, I just used a 06 GMPP block, and put one of these rotating assemblies in it. I didn't feel like dealing with a LY6 block... but having the exhaust valve notch there makes it a little eaiser to shove a monster cam in these, and mill the heads and make some decent power for cheap.

anyways, I have milled L92's, 242/246 cam, little port work, out of the box GMPP L92 intake, ultra low profile elbow. Made 585HP on the engine dyno, and it Rev's forever! I shift it at 7800.

Car manages High 10's, Low 11's full weigh(3800) 6 speed car.

I just love this little motor.

GC99TA

iTrader: (30)

Awesome information. Just what I needed! Thanks a lot for the pics too. It always helps to see just what you're dealing with.

GC99TA

iTrader: (30)

Thanks a lot for the info. Sounds like you've got a healthy motor there. I think mine's gonna be a pig for sure if I don't do some milling and maybe put in some thinner cometics.....just not sure if I'm ready to do it just yet or wait to ultimately build my 408.

Can I ask why you didn't want to use the LY6 block? Have you heard of specific issues with these blocks? My guess is just because of needing to relocate the cam position sensor to the front and get rid of the VVT stuff maybe?

pillagenburn

iTrader: (2)

I kept my VVT stuff. I know people are tempted to gut these things and stroke them out, but it really isn't all that necessary.

For the cost of a 408 you could effectively have a ZR1-style engine with one of these LY6 engines. FWIW it's a 9.6:1 compression motor, so it's basically ready for ~10 psi. You don't need a $1200 crank/rods to go fast.

BES Stroked Nova

iTrader: (24)

yeah, cam sensor, Knock sensors. and VVT stuff...

Plus I didnt have access to the blocks, but I did get some VERY low mile rotating assemblies(like 2 to 3 miles max) and used those in a GMPP 6.0L block.

Beaflag VonRathburg

iTrader: (176)

I just found this on google and it is a really helpful thread. Does anyone by chance know the OEM L92 piston valve relief size? It has the same two notched as the LY6 piston, but without the central dish.