I haven't made a decision on my motor yet and I have asked questions here before about quench. But does anyone know how much power you give up by not havin good quench? My pistons are down .025 and if I use the GM gasket that is .028 thick. This will give me .053 quench. How much better would a quench of .040 be?

GM stock gasket is 0.018" thick steel shim
Felpro #1094 is 0.015"

Quench is essential to a good burning flame front in a wedge headed motor (SBC and BBC)
lousy quench means knock, even with reduced timing. Both hurt power.

A composition gasket isn't needed on a rebuild when the heads and deck are reasonably flat. And a composition gasket won't fix a warped head or wavy deck.

depends if you have a full round dish or not on the piston. If so, you're wasting your time getting it tighter since the dish removes the quench benefits. I would gladly give up .015" of quench to get a composite head gasket in the engine and guarantee sealing for the long term. (I hate steel shims)

I was planning on getting a flat top piston and a GM composition gasket that is .028 thick. Damon has mentioned the gasket before.

A 4.03" b x 3.48" s = 355" stock, flat-topped 4-notch piston engine will be right at 10.17 CR if you use 66 cc double hump factory big valve heads and Fel-pro's #1094 .015" thick headgasket and a piston down-in-the-hole of a stock .025"..

Should be perfect for a 268, 270, 272 and 274 cam and good pump gas, imho!!

And I know GM rated the old 295 and 300hp/350 engines like this at 10.25 to 1 CR with 64 cc medium valve heads and .020" thick shims but I figure they are more like 9.9 to 10.1 CR. unless blueprinted...

My new '67 350SS was anyway..

Try here for figuring CR.

http://www.campbellenterprises.com/Race%20math%20calculators.htm

pdq67

go .015

I would deck the block and take a thin cut off the head if you want to run those thin ass gaskets. there is a reason felpros standard gasket is .039" thick.

but you wont know your deck height till you do a trail mock up

Right Doug,

GM used million's and million's of .020" thick steel shim headgaskets STOCK!!!

pdq67

The engine is my original and I would prefer not to deck it and eliminate the numbers on it. My car has been down over a year with a cracked block. This was a stroker motor. I guess I should consider other options.

***noob alert*** I pop in the engine section every once in a while to try to learn alittle more about engine building. I've heard the term before, but what is quench?

2 things here, not all flat top pistons are created equal, and busting your arse for a reduction of .013" in quench height for a street engine is not worth the effort IMHO. (that is to say that the difference between .040" quench and .053 quench is minimal) Make sure the pistons have a true 1.560 compression height if it's a 350, most jobber cast replacement's are 1.540.

Quench height is the distance between the deck of the head that cover's the cylinder to the top of the piston at TDC.

right pdq
GM used them on NEW freshly machined blocks and heads. not on used motor rebuilds.

DJ

I haven't found being a bit tall on quench to be some kind of disasterous problem. It's not like a "make or break" kinda thing. Tigter is always better than looser right up to the point of the pistons smacking into the heads. But it's not like the effect of a proper quench height magically disappears if you step over .040. It's a fairly gradual thing like anything else.

If you want to try some real thin head gaskets to get you under .040 then go for it. If they don't leak you will have made a good choice. But if they do you'll be tearing into the motor again. The GMPP .028" gaskets won't leak- almost a guarantee. You won't have ideal quench but there's not much possibility you'll be tearing into the motor again to replace them unless something went horribly wrong.

I'm NO pro-engine builder by no stretch, but every engine I've reworked, I've used Mr. Gasket .020" steel shims in with no trouble at all.

I do however very carefully go over all deck's with a fine toothed file to clean and remove anything that may be on them. I just go from bolt-hole to bolt-hole easy-like! it's a "feelie" thing here...

Then I use either, aluminum spray paint, spray and dauber bottle CopperCote or even good old Permatex, "Indianhead Shellac Gasket Cement" to seal them.

I then go over all head-bolts at least three time's over several days to make darn sure I have them torqued....

I've stacked shim's as well as hand fit 350 shims for use on my 406!!

FINE!!

pdq67

rustbucket79, I was going to use the piston you suggested when I posted before. Speed Pro STL-H631CP, they are supposed to be 1.560.

I plan on using the TRW 2304 forged piston with the single trough and .100" dome. What quench is ideal, and is less than that better, or more than that better?
Paul

Target between .040 and .045 quench, then adjust final compression ratio by milling the domes or cylinder heads.

Thanks. By the way, just thinking about this today - .040" isn't much room between the flat part of the piston and the flat part of the head. What happens when carbon builds up? I know for sure I have seen enough carbon build up both on the head and on the piston top to where the quench would be all but gone. Even on a new running engine.
Paul

You will be fine.

pdq67

I wanted to know what happens to the critical quench clearance when the carbon builds up.

It's never going to build up to the point of being an issue for the average "hot rodder" the carbon knock tends to be an issue when you have a gentle operator driving the car for several thousand miles then the teenager hops in the car and lays a beating on the car, which can cause some of the built up carbon to flake off the piston and get trapped between the deck of the piston and the head, causing a knocking sound. If you drive the car the same way all the time you'll never have an issue.

Your going through the trouble and fun of planning your combustion chambers characteristics before you build your engine. If your doing that, I would guess we can expect you to possibly tune it fairly well. Shouldn't take more than a few passes to get the soot down and keep it clean.

I've been running a tight quench (.039") with a good rotating assembly. Helps me to run 11.25:1, iron heads and pump gas.

What is the detremental effect of building a motor with too little quench to start with? Say a zero deck and the 1094 shim head gasket? Is combustion quality sacrificed? Detonation increased?

No, only the chance that the tilt/rock of the pistons in their bores might hit the heads at rpm.. Same way with rod stretch at rpm letting the pistons get closer to the quench flats.

pdq67

What is the detremental effect of building a motor with too little quench to start with? Say a zero deck and the 1094 shim head gasket? Is combustion quality sacrificed? Detonation increased?

Best case scenario, you will see serious witness marks on the pistons, more likely the contact will be severe enough that it will pinch the top rings and/or collapse the skirts.
On a tight cast piston engine (or the street TRW that need .003" or less skirt clearance) you can go as tight as .035", but you'd better check all of them because of piston, rod and stroke differences. .040" is pretty much the industry standard and will work in most instances. You will find witness marks on the pistons in a high RPM engine with a short skirt piston (6" rod style SBC) or any piston that requires a bunch of skirt clearance. Those engines I set the pistons at .005" neg deck for added clearance. Aluminum rods get at least an extra .010" clearance to allow for growth, and in fact many aluminum rods are machined .010" shorter for this reason.

depends if you have a full round dish or not on the piston. If so, you're wasting your time getting it tighter since the dish removes the quench benefits.

Um...rrrr, I've never heard of that before???

Quote:
Originally Posted by rustbucket79
depends if you have a full round dish or not on the piston. If so, you're wasting your time getting it tighter since the dish removes the quench benefits.

Quote:
Originally Posted by rustbucket79
Um...rrrr, I've never heard of that before???

it's true. during the mid 70's for example,, the 440 chrysler heads went to a chamber that gave an extra .050-.060 cutout area as cast,, in the quench area to "enlarge" the combustion chambers. that's why the 383 closed chamber heads are so desirable today. or aftermarket. because they got rid of the quench benifits to lower compression.

it's more important when trying to use more compression on lower octane fuels than with a stock engine at 8.5:1 compression and low cylinder pressures,, and pretty much why everyone goes to at least a flat-top piston (with sbc) at the very least to get those quench effects back for performance use as well as more compression.

if it can't squish the mix out of the "tight spot" because it's not there,, then you don't have "quench" or "quench area". jmo.

Those MOPAR heads back then didn't even have any quench!

pdq67

Take a look at the new, much ballyhooed "Hemi" chamber, its got two new small quench sections along the sides as well. A very "Wedge chamber" idea!

Same thing with the old -461 double-hump head with it's double quench and later 305HO, -601 head that is almost just like the earlier head except for being like 52+ cc's in volume vs 62 to 66 cc's..

pdq67

We may be confusing quench with clearance. I realize piston to head clearance is one issue. But my question mainly surrounds the issue of quench and its properties. Is proper quench essential to burn characteristics and if so it would seem a small amount of carbon build-up would easily ruin all the forethought.
Paul

Paul,

The deal is that if you putt around like an old Lady and let your heads build up with carbon and then say, your teenage Son takes it out one Sat. night and blow's the cobs out of it, there is a very slight chance he could knock loose a clinker and that it could lodge between the head and piston flat and f-- things up. But it is highly unlikely the way we drive our old cars...

I did this once with my Bosses wife's old Rambler 6-banger years ago.. They went to Pheonix one winter for about two weeks and he asked me to keep his wifes old car running due to the cold..

The first thing I did to it was VERY CAREFULLY take it out on the highway and leave it in second gear for like three miles at 60 mph!!

You shoulda seen the soot, clinkers AND red-hot coals blow outta the sucker!!

Anyway, they got back and on Tues. morning my Boss jumped me and asked me what I'd done to his wife's car. I didn't know what to say until he smiled and said whatever I did was fine b/c she said it ran better than it had in years!!

I then told him that I just blew it out and he thanked me......

Heck, his wife NEVER got it out of town much less above 35 mph!!!!!! What with hauling her old girlfriends around...

to add to pdq's and rust's story,, todays fuel injected engines are alot better at keeping chamber temps more consistant over the long haul. not as much buildup with them today in those. f/i and distributorless ignitions have done wonders at making engines last over 100,000 miles not washing rings down with fuel, carbon build up, etc...... and the wonderful "check engine" light today to tell you when something isn't right,, that it's going into open loop mode on the computer. it'll still run,, but not like it should. the majority of the fuels seem to have some kind of cleaning agents in them as well.

older carb engines get out of tune because of one reason or another,, like distributor mechanical advance weights sticking,, vacuum pod leaking vacuum or froze up on the pickup coil rotating plate where it won't pull in vacuum advance at idle/part throttle, etc..... when the timing is retarded and it's idle'd arround not making enough chamber heat,, it's then,, when the mix gets richer and/or chamber temps are more cool,, it slowly starts building up carbon in those situations. then a person gets in it and makes some heat in the chamber like pdq and rust said.

i always had the thoughts that as the carb'd cars got older and slowly built up carbon,, the timing chain also had stretched some to let the cam retard and the distributor timing retard hand in hand with the carbon buildup,, and kind of hide the "effects" for a little while.

another way of looking at it i guess. hope it helps.

That and unleaded gas and also oils that nowadays don't have the extreme pressure heavy-metals in them like of old that all, when burned, created the clinkers.

pdq67