Karpellarpy

iTrader: (3)

exactly as the title says, when building a motor for boost, is it better to lower the compression a little with heads that have a larger combustion chamber, or with dished pistons? ... and why.

Inspector12

iTrader: (29)

Always a little better to do it with pistons usually. As most pistons unless custom ordered are dished when designed with FI in mind. It also keeps you from having to do anything fancy with maybe exsiting heads and last but not least you can only drop you CR so much with the heads.

Karpellarpy

iTrader: (3)

I'm only wanting a small drop to 9.2 or so in order to preserve the street-friendly low end. I thought dished pistons were purely to drop the compression ... are there more benefits to having a dished piston? ... better flame propagation or something? Is the piston any weaker due to the dish? I know a lot of people lower compression with the 317 heads, but those are for a 4" bore correct? ... wouldn't there be shrouding problems? I guess if you drop compression with the pistons, then down the road you could lower compression further if you wanted with heads.

Inspector12

iTrader: (29)

You can lower CR with 317 heads on a 346 as I did that I dropped mine to about 9:1 with a set from TEA who opened up the chambers for me etc... The dished pistons are very strong that is how they are designed there are some other benifits. Stock pistons aren't very strong though so becareful. When useing the 317 heads you just need to run a 6L head gasket.

Beaflag VonRathburg

iTrader: (176)

The best thing you can do is call JE pistons. I was talking to one of their tech guys a while ago and he answered every single question I had about dish design, flame propogation, structural integrity, and what I should use and why. They were the only piston company I called that answered the phone and helped me out big time.

mterveen

iTrader: (2)

ttt - any more comments on this? was planning on doing the head route but since i need to put in forged rods/pisons anyhow was second guessing whether to go around 9.5:1 using 317 heads and xxx dished pistons or keep the lowly 241 heads (maybe port) and get deeper dished pistons.

ddnspider

iTrader: (26)

it honestly depends on how low of a compression you want to go.this has been beaten to deal awhile ago and the concensus was heads first then lower with pistons as the ls1 piston skirts are thin to begin with and then you throw a huge dish in them makes them even thinner.

mterveen

iTrader: (2)

thanks for the reply. i did search but the only other post that really addressed it that i found was your post from dec 05. then i saw this post here that seemed to indicate doing pistons first. apparently my search technique is poor! if you can point me to a good thread i'll stop taking up people's time. thanks.

blu00rdstr

If you demod and sell the car later, it's easier to swap heads.

Karpellarpy

iTrader: (3)

What about quench? Does a dished piston screw up the stock quench area? ... or is the dish only under the actual combustion chamber? We're lowering compression to avoid detonation ... but quench avoids detonation as well. If this is the case, it would seem we'd end up with the same detonation resistance, but with less throttle response due to the lower compression.

Pro Stock John

iTrader: (34)

Great question, let's see if we non-engineers can figure this out or come up with a rule of thumb.

I don't think that anyone makes a production LSx cylinder head, OEM or aftermarket, that is more than 76cc's. In fact, most stuff I think is between 60 and 72cc's. Just like the SBC Chevy, there are only so many choices when it comes to heads. If we say that 72cc's are typically what's available, that means that with flattop pistons, stock stroke, we might be limited to 9.5:1 compression. Now that's just for stock cubes. So folks wanting to drop their compression further run dished pistons. My current turbo car's longblock is around 8:1, with 68-70cc Allpro heads. I run a big dish.

Stroker FI engines typically have to run some dish because they are more compression due to the longer stroke. So a 70cc head won't yield 9.5:1 with a 4.000 crank, though I don't know offhand what the compression works out to.

Another point to ponder is whether the LSx engines are so efficient compared to SBC technology that you can run more compression with pump gas. I tend to think that is the case, given the power levels that folks are making with with boost plus stock bottom end LS1's and LS2's on pump gas. Hitting over 650rwhp on pump with a 10.1:1 SBC seems problematic unless the heads are amazing, but I digress.

So my response is that it depends. If you are trying to get below 9.5:1 with a stock cube LS1 you will have to run dished pistons. assuming you can only source up to 70-72cc cylinder heads. If you are trying to achieve less than (10.5:1, not sure???) with 70 cc heads with a 4.000 stroked LSx you need to run a dished piston.

blu00rdstr

I had the same question when I received my Diamond pistons. The catalog picture showed a reverse dome (flat where the heads were flat), and they are dished pistons. I called Diamond up, and they assured me their .300" ledge did provide enough quench to avoid detonation. Still skeptical, I told them of my Mahle experience, and they did say in testing they experienced detonation with .200" ledge pistons, but .300" would be fine.

Beaflag VonRathburg

iTrader: (176)

Quench distance is simply the amount the piston is in / out of the whole combined with the compressed head gasket thickness. You're really referring to or wondering about burn characteristics and flame propogation. That typically breaks down to the simple factor of (dome < dish (largish) < flat top and small reverse dome) in those characteristics.

blu00rdstr

Yes, that is the correct definition of quench distance.

The quench area prevents the areas of the combustion chamber furthest from the spark plug from igniting prematurely, and it creates a squish effect to create turbulence. The larger the quench area, the bigger the squish effect.

http://www.chevyhiperformance.com/te...ide/index.html