Hello Everyone! I'm new to the site and to "LS's" in general but I'm learning fast as I build an LM7 that came out of a 2005 Silverado, to put in my 03 ford ranger. So far, Ive had the block cleaned at the machine shop and the cylinders power-honed(stock bore) and I had the stock crank polished and new cam bearings installed and I purchased new clevite main and rod bearings. Ive already got the crank installed and torqued down with the new bearings installed while I wait on the machine shop to resize my rods after they get my arp rod bolts. I'm replacing the factory dished pistons with SpeedPro H1132CPA flat top hypereutectic ones to bump up the CR. I bought a set of stock 241 cylinder heads from a guy that have new PAC 1206X Springs, retainers etc.... I had the machine shop do a good valve job and they cleaned the heads and replaced the valve seals and checked everything out. I'm also going to have the heads ported and polished but nothing too crazy, just enough to make them flow better then 243s..... All I need to figure out now is how much I want to have milled off of the heads to bump up my CR while they're at the machine shop getting ported. This is going to be a street engine that I might end up throwing a little nitrous on in the future. It's going to be carbureted on E-85 with a single plane intake since the truck is so light its going in. I'm waiting until I get the comp. ratio set in stone before I go looking at a cam or having one ground to work good with the heads and CR. I was thinking about having them milled .040 and going with a thinner Cometic head gasket to try and get the CR to around 12.5-13.0:1....... If you guys need to know anything else please don't hesitate to ask. Ive done a lot of reading on this site and I know there's a lot of LS Pro's here so I'm excited to get this engine built and have fun with it! Thanks in advance for any help.

 

Yeah I dont think I you're going to be able to hit 12:1 with that combo. You will be lucky to get 10:1.

Try this Comoression Ratio Calculator.

 

Wouldn't the flat top pistons make it 10:1 with the stock 241s?

 

Check with the calculator. I get 9.8:1.

Even if you mill 10cc's off and get the chambers to 56cc, you're barely cracking 11:1, but you're sacrificing a lot of PTV.

 

I got 11.17:1 CR with .020" Cometic Head Gaskets. I'm pretty sure the pistons I'm getting have a higher compression height then the factory pistons do so that should raise it a little by itself and I plan on fly-cutting the pistons if need be.

 

You might blow up your engine with .020" head gaskets. The stock pistons come out of the hole about .005-.007", so a .020" gasket will leave you with about .015-.013" quench distance. .035"-.040' is normally considered safe. If the compression height in the new pistons is taller, then expect them to stick out more leaving you with less quench distance.

I'm not sure how you're getting 11.17:1. I put in the info for a stock 241 head and a GM LS1 head gasket and got 9.84:1.

Bore: 3.780"
Stroke: 3.622"
Head Chamber: 66cc
Valve Relief: 0cc
Head Gasket Thickness: .052"
Head Gasket Bore: 3.910"
Deck Height: -.005"
**The Compression Ratio is: 9.84**

 

I'm having the heads milled to make the chambers 60cc and the head gasket bore is 3.780....

 

Stock 5.3L gaskets are 3.910-3.920" like an LS1. What gasket are you planning on using that's 3.780" bore?

Again, I plug those numbers in and I only get 10.7:1. Still a long way away from 12:1, and without valve reliefs, milling 60cc is going to make PTV tight with a decent cam.

 

I'm going to run custom made Cometic head gaskets

 

That is with 243 heads. With 241 heads it would be less. The Lm7 with stock 706 heads is 9:5 to 1 and they are 61.15 cc where the 241 heads stock are 66 cc. If you put stock 241 heads on a LM7 you will have 9 to 1 compression to start with.

 

I think you missed the part where I said I was having the heads milled to make them 60cc and I'm going to run flat tops.....

 

I think you missed where he wasn't replying to you. He was talking about the other comment asking if flat tops brought it up to 10:1.

 

Be aware, most of the Speed Pro pistons aren't even zero deck, more like .004 below. You need to measure your deck height before you even try to figure how much to mill the heads, select head gasket.