This will be dual purpose but mainly a street engine built with long life in mind. LT1 stroker w/ zero deck. 4.030" forged and coated pistons with .0025" clearance. Would you recommend .040", .045" or?

 

I think the only people who have actually have purposeful insite as to what the limits are would be experienced race engine builders who have taken it to the limit and back to learn what they know. And they aren't gonna be telling everyone else what they know.

.040 would be safe. IMO

 

John Lingenfelter recommends a piston-to-head quench area from 0.040-0.045" on page 48 of his book about modifying small block engines. For a dual purpose street engine, I think that 0.045" is a safe figure to work with.

I've had excellent results running as little is 0.039" on pure racing engines, but this leaves little room for any errors in measurement of any of the components, i.e., block, rods, pistons, and gaskets. If you go this route, be sure to measure the assembled components carefully to avoid accicently smacking the head with some portion of the piston if rocking in the bore occurs. (Been there, didn't do that, and paid the price in broken parts!! )

- Greg Smith, Perpetual Corvette Mechanic

 

Any recommended method for reliably measuring rocking?

Seems to me that there are just to many unquantifiable variables. Piston rock, rod stretching when the piston pulls on it, the rod lengthening due to heat etc..

 

My Chevrolet Power Book states, "Piston to Cylinder Head: .035" minimum." A few lines above that they spec a piston to cylinder wall clearance of ".005 +/-.0005" with forged pistons, which would rock more than yours with less than .003" clearance. Elsewhere I have seen Chevy spec, ".035" to .040", minimum". I have heard of people running .030" and less. With your intended mild usage and tight piston clearance, I would allow no more than .040". Again with your set up, you might get brave and shoot for something closer to .030".

RACE ON!!!

 

Definitely. I will say this, I've measured the gasket thickness after use with the Impala gaskets that I've been using and they checked right at .029" as they should. Obviously I won't be able to use them anymore. The gasket is where I'll get my final clearance after taking all measurements, custom Cometics may be in order.

I wouldn't worry about rod stretch unless you're running aluminum rods. Any other varibles due to heat, expansion, etc. is why you have a safety margin to begin with. To check for rock, just measure piston height at the pin and then 90 degrees in both directions while working the rock to it's limit. Whatever this variance is will only improve as the pistons heat up when in use.

 

I'd like to see you STILL use those Impala gaskets with your zero deck. *I* think that would be perfect.

RACE ON!!!

 

How would that get me the generally agreed upon safety margin though, boss? Do you know something I don't? Just sent you a PM on related matters, BTW.

 

After blueprinting and matching your components, i.e., pistons, rings, and rods, to a specifc bore in your engine, do a assembly check. Lubricate the bore with a light coat of engine oil to eliminate excessive friction on the rings cocking the piston. If you assembled the engine with the piston-to-bore clearance recommended by the manufacturer and gapped the rings per their manufacturer, you're ready to measure piston rock with each piston at TDC.

Use a dial indicator gauge on the flat, outside edge of each piston in two positions each perpendicular to the wrist pin while applying slight pressure to the opposite side. Record the readings to make sure that all the pistons exhibit the same relative movement.

Now the fun part, you must refer back the piston maker's specs to see if you are within the acceptable range for all pistons. (Be careful with offset wristpins. Each side will have a different value.) Hypereutetic pistons have lower movement values than forged pistons since they expand less as the engine reaches operating temperature. Forged pistons typically have the highest "rock" values due to their higher rate of expansion. If you are running a tight quence (anything under 0.040"), any piston rock over 0.006" means take it easy with the engine until it reaches operating temperature as rule of thumb. (Specific range of values must be provided by the piston manufacturer.)

If you're interested, I've found that Keith Black-Silvolite Pistons to have an excellent web site with many excellent tech articles and engine building calculators availabe for free. Their tech reps will anwser your questions via e-mail or phone.

Hope this info helps!

- Greg Smith, Perpetual Corvette Mechanic

 

PM answered.

It definitely pushes the envelope. Consider, however, that the specs are for looser racing pistons, turning much higher rpms, and frequently if not devoted to racing. It takes some ***** to go there, but if there were a situation that begged for it, and is likely to get away with it, it's yours. Think of the money you'll save on head gaskets.

There was a fellow that I was explaining quench to, that related having a big block that ran a little harder that all his other, similar racing engines. He recalled that when they disassembled it they found the part number that was stamped into the top of the piston, embossed on the quench area of the cylinder head. That one had a TIGHT quench distance. At the time he wasn't aware of the benefits of a close quench, and they didn't check to see what the clearance was. After he knew about, and understood the benefits of quench, he knew why that engine ran so much better than the others.

RACE ON!!!

 

LOL, nice.

 

Ross piston and 23 Degree Pro-Topline heads, ported "in house", mech. roller SB 406", tested and delivered this past weekend. Compression ratio is "dead-on" @ 10.95 and runs with no problems on 93 Octane @ 34 Degrees total timing. We did the initial test using 110 because it was in the fuel cell. We ran it out and tested on 93. Unit has a .070" quench area, .032" P/D, and .038" Head Gasket. (Again, how important is this number?) Final numbers=580 HP @ 6600 RPM and 523 Ft.Lbs. @ 5500 RPM. This one will be returned in 1 year to be converted to a "Blown" alcohol unit with only a cam change. The C.R. and all other components will remain constant for use with the blower. Thanks, Gary in N.Y.
P.S. This one also has a $15.00 "nylon" timing chain setup (TC499S), which we figure was worth some additional "upstairs" H.P. We use these on a number of mechanical roller, street units, to keep the engine/driveline vibrations from getting up to the cam/lifter area, as opposed to a "Jesel" belt setup.

 

Too bad.
The tighter the quench, the more effective it is. The effectiveness gradually decreases as the distance increases. It is generally considered that the benefit of the quench area is all but gone, at about .060"

RACE ON!!!

 

 

Kid....i'd shoot for .040 unless you're building a comp engine or something. 40 is safe and if there are some variances in the parts it shouldn't be a problem. As i understand it, combustion cannot be supported in such a tight space and it pushes the a/f mixture out into the combustion chamber with turbulence. Going to 30 or 35 isn't going to make that much of a difference so why push the envelope on a s/s application.

 

That is an interesting philosophy. On a competition engine with greater loading, stresses, and rpm, where a little extra clearance might not be a bad idea, you advocate pushing the envelope.

On the other hand, with the streeter, the daily driver, where the demands will be less frequent and of a lesser magnitude, you want to be conservative.

Interesting.

RACE ON!!!

 

Yes....on a street application I would be a more "conservative" than i would be on an all out race engine where squeezing every bit of performance is required to be competitive.

That doesn't sound logical to you Jim?

 

[QUOTE=ralph Going to 30 or 35 isn't going to make that much of a difference so why push the envelope on a s/s application.[/QUOTE]

I don't agree with that, and you have no way of knowing for sure. But IMO it will make a difference.

 

[QUOTE=I don't agree with that, and you have no way of knowing for sure. But IMO it will make a difference.[/QUOTE]

OK so what is your estimate? 2 hp, 5, 10, more? Is it worth pushing the envelope for a car that i assume will be more street than strip? Would the engine rev quicker with a big $ lightened rotating assembly? Of course it would, but i wouldn't recommend it for this application either. Would it make more HP down two qts of oil? yes.....I assume you see where i'm going.

 

What is logical to me includes defining "pushing the envelope" What is conservative? At .030" one may be pushing the "specifications envelope", more than the .040" race engine, but I'm willing to bet that the race engine, with it's higher rpms, piston to cylinder wall clearances, and likely, other clearances, will be pushing the "piston to head contact envelope" to a greater degree than the tighter clearanced streeter.

A daily driver that makes a little more power than it should, no pinging, better power and fuel mileage with less ignition lead, all sound like the DD I'd like to have. Because of of the difference in applications, .040" isn't necessarily more conservative than .030".

RACE ON!!!