matLT1

This winter I am doing a heads and cam project. Over the weekend I got around to removing the cylinder headers to take some initial measurements of the shortblock, prior to deciding on head gaskets, compression, etc. Long story short, I measured the distance between the deck and the piston at TDC and came up with 0.045" This number seems very high and may place roadblocks on compression. Does anyone have suggestion regarding how I may increase the compression to 10.5-11.0 without tearing apart the short block? And how may such a high quench value effect the cars performance?
Thanks.

FYI-

This engine was rebuilt back in the summer of 2006 and has been bored 0.030. The pistons currently installed are as follows:

http://store.summitracing.com/partde...0&autoview=sku

They have a 5cc, four-valve relief setup.

The plan at this point is to install aftermarket heads with 58 cc combustion chambers.

RamAir95TA

If you have the heads milled .020" and use an Impala head gasket, you can likely get the compression up to about 10.8:1. I'm sure someone else will be able to chime in though.

1989TransAm

The compression distance is 1.540" on that piston. That is most why your piston is that far down the hole. When the rebuild was done it would be interesting to know if and how much they decked the block. I guess during the rebuild no one checked to see what the deck height was going to be with that piston. You need to do some figuring. Your best solution might be to change out the pistons.

ulakovic22

1.74 + 5.7 + 1.54 = 8.98 which still put you in the hole .020 even if your deck is zero'd and with a stock deck puts you at .045 in the hole, so you're measurements are accurate. With a stock gasket of .050 that's a huge quench, even higher than stock IIRC.

Even if you run an Impy headgasket of .029 that will still put you in the stock range of 10.5:1. Without touching the shortblock you are going to have to mill the heads quite a bit, enough to cause you to have to mill the intake as well. Ideally I would grab another stock shortblock for cheap, upgrade what you want if anything and swap them.

matLT1

Could I just swap the pistons for another set that are of a similar weight? I found a set on Summitracings website that are within 3 grams.

http://store.summitracing.com/partde...8&autoview=sku

The weight of the pistons currently installed is 695 grams. This includes the weight of the pin.

This different set would place the piston at a more reasonable 0.024" below deck.

1989TransAm

Yes you can swap the pistons. You would need to re-hone the cylinders and install new rings.

matLT1

I know a few things about building engines but am not familiar with the concept of how rings work. Could I just exchange the rings to the new pistons since they only have about 8k miles on them to avoid having to hone the walls? Or is this a wasted process? Please explain.

ulakovic22

Rings and cylinder walls wear together into a certain pattern, much like a flat tappet lifter and camshaft. If you replace the rings then you need to deglaze the cylinder walls so the new rings have a fresh surface to seat against (wear into). If you don't then you take a serious chance of getting a lot of blow by because the rings won't seal correctly. It is a necessity.

If you are going to swap pistons that means the shortblock will be completely torn down, old pistons pressed off and new pistons pressed on and at least toss it on the balancer to see if it's close just to make sure. A balance job isn't much. You would be advised while you have it torn down to make sure the decks are square and might as well zero deck the block. You can then run a Fel-Pro gasket at .039 and your quench issue is gone.

With new pistons, rings and machine work I still think you are better off just grabbing a take out shortblock for cheap, like > 500 and going from there. Unless you have something special done to your current block it's not worth it to mess with it. Just swap the cam with a new set of gaskets. Stock piston is about .020 -.025 in the hole already and an Impy gasket will get you where you want.

matLT1

Would running the deck height of 0.045 and the thin Impala head gasket cause problems with a cam around the size of the cc306? I calculated the SCR to be about 10.3, but the DCR would be about 7.5.

ulakovic22

With an Impy gasket that would make your quench approx. equivalent to a stock engine. A cam only (cc306) LT1 runs very well and most of the time makes about 340ish to the wheels. SCR and DCR looks good, I ran 11.3 and 8.9 on my old setup.