Deputy_Dangle

iTrader: (1)

Long story short, I purchased a "new" 2nd hand LS3 block. After assembling the short block I noticed I have a larger window in my piston to deck heights than I was expecting. The block seems to have been decked before I purchased it, and my local machine shop says the deck is straight and true. The rest of the combo consist of: New OEM GM crank, Lunati H-beams (6.125,, ARP2000 bolts), and Mahle flat tops (1.314 CH). Engine is going in s 100% street car that I would like to turn 7000rpm.

The piston to deck heights are:

Cyl8 .017 Cyl7 .021
Cyl6 .018 Cyl5 .017
Cyl4 .020 Cyl3 .014
Cyl2 .021 Cyl1 .014

All measured from the middle of the piston, parallel to the wrist pin.


From all of my research on this topic, I have found .035 to .045 to be the general excepted quench distance for a street engine, while some have suggested .030 for race engines. Most of the information I have read gives those suggestion for iron small blocks.

My delima basically comes from the .007 difference between cylinder 7 and cylinder 1. I would like to run OEM head gaskets because of cost and availability, but while a .051 gasket will give me good quench on cylinder 1, it will be a super tight .030 on cylinder 7.

So, my actual question pertains to how tight of a quench distance can we get away with on an aluminum LS block?

How much does the aluminum grow once it gets up to operating temperature?

How much stretch and growth should I expect from the piston/rod combo?

I would like to think I could get away with .030 with the aluminum block, paired with a lighter piston and H-beams with ARP2000 rod bolts, and a reasonable max RPM. I also know there are many people on this sight with way more experience with this than I.

So what do you guys think is the minimum quench distance and a safe quench distance?

pantera_efi

iTrader: (1)

My Choice is .000" to +.007" Deck Height.

The +.007" with AL block IS used with the FelPro MLS .038" gasket most often when I order a RaceTec piston.

I don't often use Mahle "shelf" pistons though others I know report a TOP quality piston.

MY GUESS is measurement, if accurate, I would cut the "high" pistons, deck the block.

MOST here state the AL block growth with GM specification reported @ .007" deck increase at temp.

The "stretch" is NOT you concern, piston "rock" IS with a required report of ALLOY and Wall Clearance requested.

Lance

KCS

iTrader: (20)

I would go back and measure the piston protrusion correctly. The piston rock needs to be accounted for and there is plenty of info online to detail how to do it.

Deputy_Dangle

iTrader: (1)

I was considering taking the pistons back out and having them cut but then everything would need a rebalance, which is more of an annoyance than anything.

If the block does grow .007 at the deck I would think I could get away with .030 quench disance I have on the tight cylinders. Although I have not checked it myself, the machine shop says I have a piston to wall clearance of .003.

Deputy_Dangle

iTrader: (1)

From what I gathered onlined, I should measure the piston on the top side and bottom side, with the piston rocked either direction to get the highest and lowest measurement, then average those numbers. I remeasured using that approach and all cylinders measured with *\- .001 of the orginal measurments taken at the middle of the piston.

Is that how you are suggestion to measure? Or to use the highest measurement with the piston rocked?

pantera_efi

iTrader: (1)

FIRST, I most often "overbalance" +2%.

The "lighter pistons" will work FINE, though they MUST all weight MATCH !

My deck method, with piston present, IS to measure deck at the PIN sides.

I also "center" the piston EQUAL DISTANT on the thrust sides.

The .003" "states" a High Silicon Alloy piston with less growth than a 2618.

Thus, low "rock" will be helpful.


Lance

KCS

iTrader: (20)

You're supposed to average the distance then subtract from the starting point.

For example, one side rocks from .010" in the hole to .006" out of the hole. That's .016" of travel, which you divide by two to get .008". Then you either subtract that .008" from the .010" in the hole or .006" out to see where it is. Either way, it would be .002" in the hole.

Deputy_Dangle

iTrader: (1)

Using this method I have measurments that are way more consistant with each other amd more or less the numbers I originally expected to get, yet off a good bit from the peak height I measured at the center of the piston.

For instance on cylinder 2 measures:
High valley side -.030
Low valley side -.012
High outside -.030
Low outside -.013

So ((.012 + .030)/2)-.030= .009 out of hole. If I am understanding the math correctly. But I get a peak distance out of the hole of .021 on cylinder 2 measured along the wrist pin.

My measurements using the method you described look like: ( all rounded to the nearest thousandths)

Cyl1 .011 Cyl2 .009
Cyl3 .009 Cyl4 .009
Cyl5 .010 Cyl6 .010
Cyl7 .010 Cyl8 .010

So KCS, should I ignore the peak measurments taken at the pin and use the averaged measurments? The averaged measurments are very close to the .010 the setup should have came out of the hole and makes me feel way better about the setup.

pantera_efi

iTrader: (1)

OK, your measurement method may have improved !
My guess is that this fact = true due to the QUALITY manufactured items that you have chosen for you engine.

Could you repeat the "pin" measurements, both sides ?

The GM MLS/Fel-Pro MLS that I use measures .038'' compressed.
Thus a Piston/Crown distance of .028" = safe to 8K RPM.

Lance

Deputy_Dangle

iTrader: (1)

Lance, do you have any personal experiance with a set up that has .028 quench and turns 8000rpm? Do you have any more information you can share on that set up or explain how you concluded that is a safe setup?

I am leary of .030 at 7000rpm but have had trouble finding information on aluminum LS applications suggesting why .035 is the minimum safe limit.

If KCS explination of measuiring piston deck height is correct then I dont have an issue at all but he is the only person that has ever suggested to me using that math to aquire piston deck height, as most people say to use the highest measuremnt obtained. I hope he will chime in or PM me with a further explination.

Regaurdless, I am still very curious about how much these aluminum Ls blocks expand with temperature as well as how much a piston/rod combe expands/stretches.

KCS

iTrader: (20)

Yeah, I would ignore the pin measurements. I've never used them and I don't know of any reputable builders that measure that either. I was taught that method at SAM and every shop I've worked for used the same method. Most pistons use offset pins, so when you measure in the middle of the piston thinking you're on top of the pin axis, you're really not. That's usually why that method is inaccurate.

In regards your measurements, when you say -.030 and -.012, are those both negative values meaning .030" and .012" above the deck? That would change things a bit since that would be .018" of travel instead of .042".

Deputy_Dangle

iTrader: (1)

Yes, both are out of the hole. So, with a travel distance of .018/2= .009. Subtract the .009 from the .030 I began with or add it too the .012 and each give me .021 out of the hole.

What is your take on the variance in piston/deck height between cylinders. Would you take the short block apart and have it redecked to get more consistant measurments. This is just a street/canyon car that see's an AutoX every now and then.

What is your take on a .030 quench on an aluminum LS?

MaroonMonsterLS1

Here's some quick and dirty math. Very dirty math. But it should help you get into the ballpark so you have an idea of what's safe and what isn't.

Thermal expansion coefficient of aluminum is 12.3e-6 and steel is 6.7e-6 in/(in*R) we will call this "C-al" and "C-s" respectively

Guesstimate the piston to see around 380 (similar to what cylinder heads run) and the rod is at operating temperature of 210, call these "t-hot" and "t-op" then the thermal expansions are as follows:

Rod length * C-s * t-op=0.0274"
Compression height * C-hot * t-al =0.01375"

We will assume growth in the block to be about 20% of pure linear growth because it is much more constrained whereas the piston/rod is more free to stretch.

Deck height * C-al * t-al = .0152"

That's a total of .026" that the pistons will move CLOSER TO the head. .031-.026 is .005. That's what your effective quench is after everything warms up

Like I said, this is VERY quick and dirty math, but not too bad for a ballpark idea of what you're dealing with.

Also factor in connecting rod stretch at high rpm, and you'll see that .005 quench shrink even more. The goal of quench is to , at the closest point, get the piston to just Kiss the head. Not anything dramatic, just either barely skim it, or just miss it by .000001 for perfection.

To answer your question more directly, quench all comes down to what you are comfortable with. Personally, If it were my engine, I'd stay closer to about .035-.038 when assembling at room temp.
Guys with bigger ***** have successfully run. 031 down all the way to .028.
Less ballsy guys run .042-.045 or more just for the sake of being safe.

If you're coming up with .031, just keep everything cool, and keep it under probably 6600 for most safety and you should be ok based on other results. If you want more piece of mind open it up.

pantera_efi

iTrader: (1)

First, I agree with the latest statements.
I would like to add one statement about piston rock, the piston temp as stated above.
This statement concerns the expansion of the piston at temp, wall clearance.
When the piston/engine is at temp, the "rock" will be less.
This is a good reason to "warm" the engine before high RPM.

Now NHRA S/S rules :
1. The piston deck @ +.015" AND the GM .051" gasket are required for LS engine use.
2. Robbie Shaw, NHRA division champion S/S, MY ECU, Wegner assembled engine, my coils. (Empirical)
3. That 3.625" stroke engine , as stated by both, NOT a 4.00" stroke engine
Dangles case :
1. You must equal the piston height.
2. You should use the GM LS-9 gasket @ .051"
3. You need not worry about Bob Weight change, just equal piston weights.
4. MY measurement method may be different when the head gasket is measured, most state the GM gasket = .041" compressed.

This thread is in the "Advanced" tech area, good for discussion.
I thank the others who have posted for a good amount of knowledge.

I would add, I use a RacTec piston with a DLC heavy wall pin for this engine type.
The stated Mahle piston/pin I have little experience in use of that part.
Those who read this thread should take consideration with skirt design/pin flex.

Lance

KCS

iTrader: (20)

I wouldn't worry too much about the variance. It looks like +/- .001", which is pretty good. The variance could come from tolerances in the manufacturing of the pistons and/or rods instead of the machining of the block, but either way I'd say it's acceptable.

I'm not a big fan of pushing the envelope on the quench distance. The benefits of a tight quench diminish in higher compression ratio engines. It's not really worth risking piston to head contact IMO. I believe Tony Mamo learned the hard way a few years ago and ended up pinching a ring land in one of his engines. He was in the low to mid .030's I think.

If it were mine, I would probably take it all back apart and have the piston tops milled off in a lathe so I could get at least .035" with a cheap GM MLS gasket. You could take .010" off, and with 2618 aluminum weighing 2.8g per cc, that would be less than 6g difference in the balance of the crank. You wouldn't even notice that. Oil slinging around in the crankcase will be more than 6g.

Deputy_Dangle

iTrader: (1)

I'm sorry, I may have been a little confusing. I complete misunderstood how to measure like you explained and that scewed my measurments. After remeasuring my measurments are basically the same as the ones I posted I post 1 which gives me a variance of .007, on one bank at that.

MaroonMonster, thanks for that info. I am going to dig way deep in my brain and crunch those numbers.

Tony @ Mamo Motorsports

It was less than that....LOL (about .028)

Honestly my plan was to clean the carbon off every piston and I accomplished that with seven of them. One of the middle cylinders had a slightly longer rod by a couple of thou which snuck under the radar at the time which is the one that burned me. Every other cylinder/piston looked perfect on tear down or should I say looked the way I was hoping it would look (light to zero carbon on the quench pad of the pistons).

I knew it was tight....very tight in fact, but it was my personal motor (not a customer's engine) and I was willing and wanting to push it and see if it would fly (was encouraged to do so by some other knowledgeable folks based on their experience as well).

The window of error going that tight is very small....and the larger bore and shorter skirts didnt help my particular cause either (more rock at TDC).

My advice to the OP.....if your piston rock and piston skirt to bore clearances are in line, and you take the time to measure every hole and confirm you don't have a stray .029 mixed in there, go for it assuming you want and value a really tight quench.

For what its worth I typically build my customer's engines around .034 - .036 and I still measure every piston to confirm. If one came in at .032 and it wasn't a short skirt deal with more rock I would let that fly as well but in general I have been targeting .035 ish just to play it safe.

Hope this info is helpful!

-Tony

KCS

iTrader: (20)

Thanks for commenting. Can't fault you for trying to push the envelope. They say the fastest guys in racing often stand on the largest pile of broken parts.

pantera_efi

iTrader: (1)

Again, I love this thread.
My hand on a "mike" is strong, a bit different that a Caliper.
My .038" compressed 1040 @ +.007" deck = my .031", others here compressed .041" = .034" squish @ +.007" deck, which is the SAME.

There IS an effect NOT stated : Piston Pressure Growth.
This too effects "deck" distance requirement.
The 4" bore piston HAS a TWO hundred pound pressure "pushing" the piston into the head when the throttle is closed.
This pumping loss ALSO effects engine power.

The Pro-Stock drivers NEVER close the throttle at the "lights", they just switch off the Ignition.

The NASCAR Cup engine, as many Drag Racers, create a "depression" in the crankcase thus LOWERING Pumping Losses, an effect on the Squish distance requirement for safety.

The "better" drivers have a lower amount of broken part trophies.

Lance