This is not the typical topic about quench distance.

I am building a Cadillac Northstar for my Fiero. I have Eagle rods with stock crank and block. I am ordering pistons from CP and will have Total Seal diamond lapped gapless rings.

I need to give CP the exact compression height I would like my pistons to have, and to do that, I need to figure out exactly how much quench distance I will need.

Suffice it to say that I understand the math and engineering principles involved...

Quench distance is simply the desired quench clearance at redline RPM plus the amount of rotating assembly stretch at TDC at that RPM.

At my target redline (8500 RPM) with 3.307" stroke and 5.943" conrods the piston will see 4360 g's at TDC.

By estimating from measurements the cross-sectional area of the connecting rod shank, I can calculate that the shank should stretch about 0.0055".
0.0023 bearing clearance and 0.001 piston pin clearance add another 0.0043 for 0.0098".

Because of bearing clearance, the bore in the rod is 0.0023 bigger than the journal on the crank, which means (Pi)*0.0023 more circumference and the potential to oblong by (Pi/2)*0.0023 = 0.0036 for the big end and 0.0015 for the small end.
That brings the total assembly stretch to 0.0149", which is considerably less than the 0.040 dictated by conventional wisdom.

I have yet to take into account:
stretch (rather than simple oblonging) of the bearing end, and naturally of the pin end as well.
Piston stretch.
Crank stretch (negligible)
Relative thermal growth of the cylinder walls and the crank throw/conrod length (this should actually reduce the static quench necessary because the block will grow more than the crank/rod will.

Is there anything I am overlooking?
What should my operating quench distance at redline RPM be? I was thinking 0.003-0.005...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by The Dark Side of Will &raquo;</TD></TR><TR><TD CLASS="quote">This is not the typical topic about quench distance.

I am building a Cadillac Northstar for my Fiero.

I have yet to take into account:
stretch (rather than simple oblonging) of the bearing end, and naturally of the pin end as well.
Piston stretch.
Crank stretch (negligible)
Relative thermal growth of the cylinder walls and the crank throw/conrod length (this should actually reduce the static quench necessary because the block will grow more than the crank/rod will.

Is there anything I am overlooking?
What should my operating quench distance at redline RPM be? I was thinking 0.003-0.005...</TD></TR></TABLE>


Piston rock at tdc (both cold and hot)
Stroke variations (tolerance)

I have seen many small block chevys with 3.5" stroke billet crank,Carrillo rods, JE pistons with .030" quench,7200 rpm.At the freshen up the pistons have the machine marks from the head transferred onto the top of the pistons.Convectional wisdom often contains first hand experience.
Glenn

 

Endyn and alot of other people claim at least 0.037"-0.040"--but I have also heard even on high-rpm aluminum v8s running at 8000 and above that you can get away with less than 0.030". Aluminum blocks like Hondas expand when they get up to operating temp., giving you a bit more quench clearance. Also take it to account a loose fitting piston will give less distance. You could get a thinner/thicker hg to make up any small difference. I would say don't go below 0.030". My motor now is no more than 0.035" and its been fine.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by NJIN BUILDR &raquo;</TD></TR><TR><TD CLASS="quote">Piston rock at tdc (both cold and hot)
Stroke variations (tolerance)</TD></TR></TABLE>

The crank is pretty dam tight... journals were only off by 0.0001 from spec after 100K miles. Measuring the stroke differences wouldn't be too hard, but I'm reasonably confident such variations would be small.

Piston rock... that would be trigonometry: inverse tangent of (bore clearance)/(skirt height) would give me rock angle. Figuring 0.003 clearance and ~2.5" of skirt height would be 0.068 degrees of rock which is less than 1 thou of edge rise on a 3.670 bore. The skirt's going to flex a little bit and the pin is obviously down from the deck surface, but that's an estimate. That's cold of course... hot rock is going to be less because the piston will expand more than the block (both are aluminum, but the piston is forged and the block is cast)

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">I have seen many small block chevys with 3.5" stroke billet crank,Carrillo rods, JE pistons with .030" quench,7200 rpm.At the freshen up the pistons have the machine marks from the head transferred onto the top of the pistons.Convectional wisdom often contains first hand experience.
Glenn</TD></TR></TABLE>

Step that back just enough for the engine to last on the street and that's what I'm after. Builders of engines like that, of course, have built MANY the same way and can experiment to get exactly the right setup. I don't have that luxury, so I'm trying to calculate things out to the best of my ability.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by d16dcoe45 &raquo;</TD></TR><TR><TD CLASS="quote">Endyn and alot of other people claim at least 0.037"-0.040"--but I have also heard even on high-rpm aluminum v8s running at 8000 and above that you can get away with less than 0.030". Aluminum blocks like Hondas expand when they get up to operating temp., giving you a bit more quench clearance. Also take it to account a loose fitting piston will give less distance. You could get a thinner/thicker hg to make up any small difference. I would say don't go below 0.030". My motor now is no more than 0.035" and its been fine.</TD></TR></TABLE>

Yeah, the thermal expansion of the block gives me a little more room to play. There isn't much available for Northstar head gaskets, so I don't have a lot of room to play with fine tuning in that regard.

 

My $.02

If I am reading what you wrote correctly you have calculated that stretch will be approximately .015" at redline and you want to run an operating quench distance of ~.005" at redline. This would mean that you would want the quench distance at room temperature to be about .020". Correct me if I am wrong on that. I would open it up slightly to about .028" just in case. The last thing you want after building this engine is to have your pistons smack the head because you were trying to get quench as close as possible - unless you have all the money and time in the world.

To quote my mentor ( and this is why I said .028" ):

" .028" min to .040" max is the quench distance for best combustion chamber mixing. From about .050" to about .090" is a dead zone where the quench distance is too far for effective quench but too close for good flame travel. That dead zone is not good for power and can promote combustion knock. If you can't get the quench distance closer than .050" (or less) open it up to promote good flame travel.

Make sure the pistons are balanced evenly on each side of the pin to minimize rocking. Rocking pistons tighten the quench unpredictably. "


As far as thermal expansion is concerned, remember that the piston crown runs at a higher temperature than the block and will expand more because of that. Of course, the steel rods will expand at half the rate of the aluminum block so that will give you a little room too play with.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Scott_Tucker &raquo;</TD></TR><TR><TD CLASS="quote">My $.02

If I am reading what you wrote correctly you have calculated that stretch will be approximately .015" at redline and you want to run an operating quench distance of ~.005" at redline. This would mean that you would want the quench distance at room temperature to be about .020". Correct me if I am wrong on that.</TD></TR></TABLE>

Well... kindof... I have 0.015 so far, but I haven't taken everything into account yet. I'll try to find some time tomorrow to sit down with a conrod and some calipers and figure out what the bore stretch is going to be.

My goal is to run it as tight as I can run it without having to experiment. I'd rather not go with conventional wisdom because it's not necessarily geared toward the combo I'm using.
For instance, 0.030 can work in the above gentleman's small Chevy example, but that engine has a larger bore and heavier pistons (in general... a set of Bill Miller's is probably lighter than a corresponding set of CP's... etc.) than my engine, so rods of similar construction would stretch more and the quench would have to be left wider at assembly.
The Honda engine mentioned above might even be able to run tighter than 0.030 if it were limited to 7200 because the pistons are smaller and lighter and the rod is probably of similar construction.

However, if there's a company out there that can make a custom head gasket in whatever compressed thickness I want, I might try fine tuning using those. I'm going to have the engine out several times anyway...