I know there is a good reason, and I may order my next set this way again. Does it affect quench or what? What is the specific reason to zero deck instead of using standard "out of the hole" pistons?

 

The only reason i can think of for zero deck is for rod stretch clearance. It will affect your quench in a negative way, you want tightest quench possible to prevent detonation and maximize combustion efficiency.

 

I did zero deck pistons to keep the the rings a little lower so they are run at a little lower heat. The quench can be set by the head gasket thickness. This can also give you room to deck the block in the future. Zero deck was also recommended by the sales man at JE.

 

Supposedly running a zero deck height offers better quench and helps promotes combustion/prevents detonation. I guess if people are selling zero deck pistons this is assuming that you have a standard deck block.

When I have heard of ppl running zero deck in old school SBC's, I usually see them put the rotating assembly in, measuring how far the pistons are below deck surface w/ dial gauge, then decking the block that distance.

 

I will actually have the block checked before ordering. If the deck needs milling, then I will order the pistons for that application. This will be a 15psi 408 motor, so the pistons will already have about a 28cc dish. I would think this would affect quench more than the deck height.

 

I agree with LS1derfull.

The only reason to run zero or negative deck is to compensate for conrod stretch if you plan on running HIGH rpm or aluminum rods. If you are going to be running regular stock-bushed or forged steel, then I would run the stock rise or even more.

Squish is not a factor, it is THEE factor when you talking of building a detonation free motor. Squish is the biggest contributing factor for detonation. You should always try to maintain at a minimum the stock squish. Piston rise on a stock LS1 is 6 thou.
On a FI build it is a good idea to run even more of a rise because of the gaskets that you have to run in a FI setup (they usually sit the heads higher off the deck).

I run a 10 rise on my pistons. I have had very good luck with this setup (and others that we have done) and have NO detonation problems on pump gas.

D1 on a 383 all-bore at ~8 psi. Total timing advance is 26 deg at WOT (this is alot for a FI motor). Most other builds (home grown or whomever) I have to back the timing down to 19-22 deg to avoid detonation.

Just my input.

Good Luck

 

Thanks Kevin...good input

 

I have found that small block chevs start contacting the head at about.027 squish
with steel rods.Does anyone know if that number is similar for ls1 engines?

 

I am running .020 out of the hole with my custom Diamonds. I am also using a .062" copper gasket, which will put quench height at .042 theoretically. I expect to lose 4 or 5 due to rod stretch however. Minimum quench height in my book is about .040, so it will be close.

 

Here's my question. LOnSLO, why go .020 out and have a .062 HG when you can go .006 out and use a .048 gasket (which is probably a stocked item)? Something with the compression height of the piston.

 

I did 0 deck which is really about 1-2 thousandths out with a .041 head gasket and have zero knock problems with a 11.2 compression. I run 28.5 timing with 0 knock and don't find it necessary to run more, with cali 91 octane. Running a little more out of the bore is supposed to give you snappier throttle response.

 

Exactly, they are 12.25 compression pistons. I wanted to avoid having to use a domed piston, so we decided to run them out of the hole a little more. They were built with a .062" gasket in mind. Also, this is a 4" stroke motor.

 

I'm doing 0.001" out with a 0.041" gasket which should give between 0.040" and 0.035" depending on how much the rods stretch

 

I run .010 out with a .041 (Fel Pro 1041) gasket ... With the smaller cam (224/224) it had more throttle response than I could realistically use on the street. With the bigger cam (238/240) I have an off-idle stumble that's driving me crazy

 

Try more timing and play with A/F tuning, if you have around 11 to 1 compression you should be able to run that cam w/out stumble.

 

From z-ya:

"I have found that small block chevs start contacting the head at about.027 squish with steel rods.Does anyone know if that number is similar for ls1 engines?"

I posted in another thread on this topic, and got no response from the tuner community. But, most of the parameters which control piston/head contact in an SBC are similar in an LS1 - bearing clearances, stroke (and therefore piston speed), rod cross section, rod length, piston weight. The only differences I could postulate are the block material - aluminum has greater thermal expansion than iron, and *might* tend to increase the deck height more in a warm engine - and the piston material, which is hypereutectic aluminum in a stock engine and therefore has lower thermal expansion than the forged pistons used in most aftermarket SBC applications.

In any case, I wouldn't bet on being able to run much tighter clearances

To the general topic of the post, I think the zero deck vs. out of hole piston doesn't matter much as long as quench clearance is minimized by tuning gasket thickness - as several posts have pointed out. Although I don't know for sure, I suspect the reason for the out-of-hole design of the factory piston has to do with emissions reduction. Modern combustion chambers are so efficient that most of the unburned fuel (HC emissions) comes from crevices where fuel/air hides out away from the direct flame front - valve clearance pockets, valve/seat crevice, even plug threads. Anyway, the biggest crevice is the piston/cylinder wall gap, with the compression ring as the floor of the crevice. By popping the piston out of the bore a bit at TDC, some of that fuel in the crevice is expelled, while only exposing the ring to extra combustion heat for a brief period at TDC. Very clever, actually. BTW, crevice volume also explains the OEM fondness for hypereutectic pistons - they expand less under heat, and therefore allow closer piston/cylinder wall clearance. And, the pure flat-top piston with no valve clearance pockets.

 

True. Crevice volume is also why emissions are a problem on larger bore motors.