Hey all.

I am looking for some expert opinions for the possible compression ratio I can achieve with my setup. Here is the list of what I have.

Stock bottom end with stock pistons. The TDC looks to be even with the deck.

CNCed LS1 Heads with the larger valves. I am not sure what the current CC is on the heads, but I know they have been milled already to make around 10:1 with stock gaskets.

The cam is a 224/224 .581/.581 114. I have 7.400 push rods with stock lifters and rockers.

Can I mill the heads to make 11:1 or 11.5:1 without flycutting the pistons? What would be possible?

Thanks in advance.

 

WHY? Not much need to do more than 10-10.5 to 1 for a street engine. Buying gas for something in the 11.5 to 1 range would be tough too. Super/turbo charge if you want more, then you can vary the amount of added compression/boost without opening up anything. Better yet, for the money NOS is the way to get to "Grunt" only when you want it.

 

You could probably get to 11:1 and be ok on pump gas, but with that size cam, I wouldn't go beyond that.

A cam affects the actual running compression of the engine (called dynamic compression, or DCR). A car with a huge duration cam can get away with (and benefit from) a lot more static compression, because the overlap of the cam actually bleeds off some cylinder pressure when the engine is running.

For example, I'm at 11.7:1 compression, but with a 240s duration cam, the DCR remains fine for 93 octane. With your cam, the DCR would probably be too high, and you'd likely have to retard the timing significantly to run it on 91/93 octane without knock/detonation, or, mix in a little race ga$.

Do a search over at LS1tech for a DCR calculator. You will need to know ALL the specs of your cam, not just the .050" numbers. Use -.006 or -.007 values for deck height, then you can play with chamber cc and gasket thickness to best achieve optimum DCR....8.5:1 for 91 octane, 8.7:1 for 93 oct is good to aim for

Concerning your heads, porting and valve unshrouding usually enlarges the chamber a cc or two, and it makes sense than some light milling was necessary just to hold the stock 10:1 comp ratio, so let's assume your chambers are back to the stock 67cc size.
Even with a thinner .040" gasket, you'd probably still need to get the heads milled another .025-.030" to get in the 11:1 ballpark (~62cc chamber size).

I don't expect milling that much would cause any p/v clearance or manifold fitment problems for you, but if you're contemplating this, get the heads cc'd first so you know exactly how much would need to be removed to achieve the desired cc for the desired compression.

 

Thank you for the comments. The heads are at my machine shop right now getting hot tanked dis-assembled and inspected. They are doing their computations also, but I just wanted to get an idea from the forum experts.

Doug, to answer your question, because it will make more power. Additionally, I do not want to put any power adders on the car.

 

That's not a cam that will cause any PTV issues. With CA 91 octane, even 11:1 should be no problem. I doubt if you're going to have higher than 8:1 DCR with that cam, unless there's lots of advance ground in. Sounds like a Comp grind, with a not too aggressive lobe.

 

Ok so here is what I figured out with a DCR calculator and the cam specs:



What do you guys think?

Now I just need to figure out if there will be any PTV issues.

 

Looks like you're planning on using Cometic gaskets@ .040. I would back off that CR a bit for 91 oct. and use GM MLS @ .054-60 compressed. They're alot cheaper, too.

 

I would keep the gaskets and not mill quite as much.

 

So staying with .04 and 62cc=

 

For 91 octane, that should work very well.

 

Thats pretty much all there is out here in the Republik of California.

 

what Y2Kvert4me said
the .041 gasket will prevent knock better than the GM .053 gasket.
62cc minimum chamber. the shop can cc your head now and tell you what
you are starting with. adjust the chamber cc as needed.
8.58 DCR is great with a good tune you will have a nice setup