Stock Iron block LMG 5.3 with OE Flat tops 0cc, 243 heads 64.45cc PRC porting. Piston out of hole .006-.008.

Was thinking about a .040 or .045 head gasket to get a better quench. I was told .035 is the best for a street N/A engine under 6500 RPM.

Now with a Cometic MLS .040 my quench would be .032-.034
Now with a Cometic MLS .045 my quench would be .037-.039

Which would be more ideal for 91 octane. Max RPM of 6,000 RPM, Stock Forged Steel I-Beam rods. Stock Static Compression is 9.95:1 with a .051 head gasket.

 

I wouldn't run below .040, I know Lingenfelter drops down to the .038 range sometimes but that's tight. I wouldn't trust stock rods at that.

If that's what your out of the hole I would just use a .051 for a mid to low 40's quench.

Given that the stock pistons are flat tops you achieve the same effect without dropping as low with the quench vs a deep dish that relies on the outer ring only.

 

I think stock I am roughly at .045 quench. When I talked to BTR and Patrick G they all recommended around a .035-.040 quench with one of their cams since I plan on running a high Dynamic Compression ratio, so that is why I was asking. You think I should stay stock tho?

Thanks for your reply's as well.

 

Best to measure to be sure as every block is different as well as sometimes the core shift is to 1 side and will affect out the hole measurement side to side.

I wouldnt be afraid to try the .045 but I probally wouldnt go any lower. You really don't want to be any less than .035 Quench.

As mentioned the stock .051 will probably be just fine as well as give some margin of error. Not to mention they're a fraction of the Cometics.

 

As Joseph mentioned you may have .05 variance between holes so I would check before picking a gasket.

 

i prefer to run quench as tight as i can. ive run ~032 on a sbc before.
recent trends in turbo cars has moved away from that to much wider quench distances, and people are successful, but i am hesitant to go that route.

 

I have measured everything but the Piston to deck clearance, will due soon. Everyone I talk to says .035-.040 is best with DCR above 8.5:1. I will be running a 8.6:1 Dynamic compression ratio and a 10.75:1 Static Compression. Texas speed said they prefer .038-..045. So I will probably end up going with a .045 head gasket and once I measure the Piston to deck clearance I will know exactly. I was told the LMG 5.3 is .006 above the deck but we will see in the next few days when I am back at home. From what I read the tighter Quench the better it handles the higher SCR and DCR in regards to detonation. I really would like to shoot for .038 which is the most common number I am seeing and .035 is optimal for 11:1+ SCR and 8.5+ DCR.

Stock is at .044 or so quench, so we know that is safe.

 

Every block is different. It's all in how it was machined at the factory.

From what I have seen .006 -.008 is average, How ever I have had blocks as low as .005 and seen it bad enough to one side that required 2 different gaskets thickness to get it even or to remove it and have the block decks leveled out.

I have one block that is shifted to one side and has an uneven deck. I think it was nearly .009 but more like .013 at the lowest point on the bad side. We were going to cut the decks to even it out and run a thicker .060 gasket. But my machinist thought it would be better to source another block.

I had him cut the new block so the piston set would be .010 out of the hole and I can run a factory head gasket.

 

hopefully that is not that case, I will measure all pistons and factor in piston rock.

 

So this is my ideal goal if everything figures out. Say the piston to deck clearance is -.007 above deck. Using a .045" head gasket so Quench/Squish is at .038", Mill the 243's .015" to 62cc and use the highest octane available which is 91 octane. PTV Clearance will be fine. The only factor that can change is the piston to deck clearance, -.006 Through -.008 according to the GM LMG 5.3 Build sheet I have, which this will only effect the Quench/Squish but still leave it in range (.037"-.039"). I have measured all other things.

Cam specs are 210/218 @ .050, .553/.553 lift, .325 lobe lift, 113+2 LSA, 111 ICL, Adv Duration @.006 264/270

10.65:1 Static Compression, 8.6:1 Dynamic Compression, 175 Cranking PSI at 0 ft, sea level.

10.65:1 Static Compression, 8.2:1 Dynamic Compression, 165 Cranking PSI at 2000 ft above sea level

I live in Vegas so it's at 2000 ft but I travel to the beach in Cali every few months. I know that at 2000 ft the DCR is perfect but at sea level the DCR is at the limit so I hope it will do. I am going to be talking to a lot of engine builders for advice on this tomorrow and get as much advice on the forums as I can.