S0 if 0.059 quench is bad and 0.040 is good, is 0.029 better?

 

Only until the piston kisses the head. Chevrolet reccommends a minimum piston to head clearence of .035"-.040". Large bearing clearences (to a minor degree) and loose pistons are reason to stay towards the high side of the range. The engine I'm building has a, whopping, .042" clearence. Good luck, and...

RACE ON!!!
1984 Crossfire
Member NHRA

 

Optimum quench is .040". Anything in between .035"-.045" is acceptable. Your engine will run with a .059" quench....but it won't run to it's maximum capability. The little things start to add up whenever you build a motor.

 

Mine is 39 thats about as close as I care to get for a street engine.

 

Well here's what I did, tell me if I effed it up (it hasn't been started yet).
L-98 with Ross pistons, 0- decked, balanced and blueprinted to 99 World Challange spec short block by Cottrell.
Ported and polished 87 heads.
0.029 composite head gaskets (to get the compression up to 11-1.
Sounds like I won't have enough quench? :eek:

 

I would use a 039 gasket

 

I agree. You could try it. Maybe you'll be OK. Specially if you don't wind it too high. If you do the math, though, I think you'll find that the .010" won't lower the CR diddly squat. Good luck, and...

RACE ON!!!
1984 Crossfire
Member NHRA

 

it all depend...... i built my 468 to .035" a true/uniform .029” sounds good

my opinion? you need to think about stretch, bearing clearance, etc........ as long as the piston doesn't touch the head, you're good. .029 may be just fine, if everything else it relatively tight and uniform (good machine work, and uniform pistons, rods, and crank). The quench thing is all about pre-detonation, and power production right ?. and by the way, .010” will make a difference...... yes, do the math, and you will see.... it may not be much, but certainly measurable, seems a shame to throw away free horsepower if you don’t have to.

i'm sure those with more expereince than i will speak up concerning this........ don't count out the .029" gaskets yet :-)



[Modified by 66427-450, 12:18 PM 6/8/2003]

 

Just about to break in a motor, I went with a thinner head gasket to get a little extra C.R. too. :lurk:

 

.029 is an impossible dream as far as I'm concerned. You'd have to spend zillions of $$ to get parts that were uniform and strong enough for this type of tolerance. I'm at .042 on my 383, and I'm happy with that. Just my $.02.

 

Well unless you measure all of your rods there could be a small variance there with on or more rods being longer than the rest and at high speeds rod stretch should be considered, for me. 035 is max.

 

Yep thats why I used a special fixture that a local automotive machince shop has that you put the rod on a pin same size as rod journal it holds piston straight and dial indicator on top of piston. you find the shortest then turn of the rest of the pistons till all are the same length. Yes this takes a lot of time so some will say it is not worth the trouble but I want to make everything as near perfect as I can rather than just throw it togeather and hope that it will be ok. All blueprinting amounts too is paying attention too details. :chevy

 

Pretty much anything short of smacking the head is best. My 540 was at .028 last time around. One deck of my motor is .0005 (that's half of one thousandth) shorter than the other and the pistons on that bank were just barely kissing head. I mean no real marks or ring land damage, but you could see slight discoloration marks on those four pistons. Just a case of piston rock and relatively short rods slamming things around a little. No big deal and just perfect!

Super Stock racers routinely run the engine and continue to cut heads/deck until pistons JUST BARELY touch. Of course that is extreme racing and doesn't really apply to us street guys.

.035-40 is a good avg. to shoot for just like everyone said. But if you can run .029 and not hit anything, I'd do it in a heartbeat.


JIM

 

For decades, sharp engine builders have known about “quench” or “squish.” These terms refer to the area in a wedge-shaped chamber designed to create turbulence in the chamber as the piston approaches top dead center (TDC). This squish effect can also occur in other types of chambers as well. We will limit our discussion in this story to small-block Chevys, but the basic facts relate to all wedge-shaped combustion chambers.

The quench area is the tight area between the flat portion of the piston and the flat portion of the combustion chamber in a typical wedge-style chamber. As the piston reaches TDC and the mixture begins to burn, the air and fuel located between the piston and the head is squeezed or squished out into the dished portion of the combustion chamber. Think of the turbulence that occurs when you smash a tomato with a large mallet and you get the idea. With a flat-top piston, this squish area can be very tight. This is also the tightest clearance between the piston and the cylinder head. Since mechanical contact between the piston and the head is not advisable, most production engines rely on a piston-to-head clearance of 0.060-inch or more in this area.

Unfortunately, this is not an ideal piston-to-head clearance for optimal squish. But because of production tolerances, factory engines usually fall on the larger side of the clearance for obvious reasons. But when it comes to optimizing a performance engine for more torque and horsepower, this is an area where a knowledgeable engine builder can squeeze out a little more power.


The Squish Effect

Since wedge-style combustion chambers rely on the squish or quench area to create turbulence in the combustion chamber, an intriguing effect occurs in the combustion process. To better understand this process, imagine that the intake valve opens and a rush of air mixed with fuel enters the combustion chamber area. The piston comes screaming up toward TDC at 5,000 rpm (almost 3,000 feet per minute) as the intake valve closes. As the piston reaches TDC, a virtual hurricane of fuel and air is squished out into the chamber from this tight area between the piston and the head. While this turbulence sounds bad, the opposite is true. This turbulence has the effect of more thoroughly mixing the air and fuel into a much more homogenous mixture that tends to burn much more quickly and efficiently.

One way to produce maximum power from an engine is to use the least amount of fuel necessary to create maximum power while attempting to burn all of it. Given this, if you can evenly mix the air and fuel into a homogenous mixture with an extremely fine mist of fuel, you will make outstanding power.

Unfortunately, the opposite is also true—varying pockets of lean and rich mixtures within the cylinder when the spark plug fires will cost power and the combustion process will not be as smooth. Excessively lean or rich pockets within the chamber directly affect the rate of combustion and the amount of pressure applied to the piston. Rich mixtures tend to burn slower, while lean mixtures generally burn at a faster rate than a “proper” air-fuel mixture. If modifications to the chamber or piston affect these rates, the ignition timing will also need to be changed to optimize power.

What is the proper air/fuel mixture? In the last few years, the answer has been changing as the area between the combustion chamber and the top of the piston becomes more efficient. For example, the classic air/fuel ratio has always been 12.5:1, meaning 12.5 parts of air for every one part of fuel. But many race and properly designed street engines can make best power with air/fuel ratios now approaching 12.8 to 13:1.

So now let’s introduce a tighter quench space into this equation. All of the respected engine builders who we’ve talked to are firm believers in minimizing the quench clearance. According to Ken Duttweiler, the tightest quench he recommends is around 0.050-inch. He has built engines with far tighter clearances than this, but much of this depends on the piston-to-wall clearance. All pistons tend to rock slightly as they transition through TDC and this rocking motion reduces the piston-to-head clearance. Smaller-diameter pistons with tight piston-to-wall clearances don’t rock nearly as much in the cylinder bore compared to larger-bore pistons with wider piston-to-wall clearances.

Since piston clearance plays such a big part in piston-to-head clearance, it is possible to run a piston-to-head clearance tighter than 0.040-inch if you feel brave. Noted horsepower hero John Lingenfelter says that clearances of 0.037 to 0.040 inch are possible, but you must know what you’re doing. The late Smokey Yunick also recommended a quench clearance of 0.040 inch as a safe but critical clearance.


Advantages

So what are the benefits of all this squishing and quenching? The benefits are small, but >> often important. Pump-gas engines that run on the ragged edge of detonation, for example, can greatly benefit from a tighter piston-to-head clearance to reduce rattle. That sounds contradictory since increasing compression should lead to increased detonation. All the engine builders we spoke to mentioned that tightening the quench (reducing the piston-to-head clearance) to get it under 0.050 inch will increase the static-compression ratio, but this tighter clearance also creates a more powerful squish effect. This additional turbulence creates a more homogenous “soup” in the chamber, reducing the harmful effects of lean air/fuel ratio pockets. With all other variables being equal, this contributes to creating an engine that is less prone to detonation.

We tried this on a recent dyno flog of a 383ci small-block. To keep the compression at around 9.5:1, we used a set of 0.050-inch head gaskets that created a wide piston-to-head clearance of around 0.060 inch. CHP engine flogger Ed Taylor swapped in a set of 0.040-inch Fel-Pro head gaskets and then tested the engine again. We witnessed only a marginal gain of around 2 to 3 hp (less than 1 percent), but it’s doubtful that the marginal increase in compression was responsible. Clearly, tightening quench with a thinner gasket had something to do with the increase in power. Tightening the quench area often results in the reduction of ignition timing requirements. This can then lead to a reduction in negative work (the cylinder pressure rising while the piston is still approaching TDC). This often is evidenced by a gain in low- and mid-range torque.

There is plenty of discussion about the net effect of squish and quench. While it’s doubtful that this will ever amount to more than a few horsepower in any street application, it does offer some distinct advantages when it comes to increased engine efficiency, better fuel mileage, and driveability. If you’ve ever wondered why certain engines run better than others, this could be one reason why.
http://chevyhiperformance.com/techarticles/94138/

 

I couldn't have said it better myself.
As an offshoot, the less total spark advance that produces maximum HP in a given engine, the more efficient that engine is. Many factors go into combustion efficiency, but with so many constants, such as engine basic design (sbc for one example) it pays to pay attention to the quench. That is ONE thing, we as builders, can control. Good luck, and...

RACE ON!!!
1984 Crossfire
Member NHRA

 

ld85,

What a great post! You have not "quenched" my thirst on the subject, rather you whetted it! I really never appreciated the importance of quench prior to your post. There is much for me to learn.

Thanks,

Chuck

 

AFAIC, optimum is zero clearance between piston and head at whatever rpm and operating condition you encounter.

With steel rods and confident that your (assembly) clearance is no tighter than the .029 noted, you'll be ok. Just don't go over 8 grand. :)

Years ago, with steel rods, and an actual assembly dimension of .025" was considered minimum. Unofficial reports (not substantiated by me) say high reving motorcycle engines have survived with less than that. :shrug:

 

I build my engine with 0.040" in mind. The actual result are between 0.039" and 0.046". I have 9.9:1 static compression, around 8:1 dynamic compression and I run gas at 93 octane without any detonation.

The extra cost of machining were worth it. I drive my car daily so the quench area was a priority to keep away detonation. The efficiecy of the engine was important too. I did 19 mpg on a trip at 70 mph.

Stephan

 

That's great, if it's a carbed engine. My ole worn out wheezer of a Crossfire always did get 23-24 @ 85 mph...still does. I has finally got to the point, where I now have to stop more often for oil, than gas. Goog luck, and...

RACE ON!!!
1984 Crossfire
Member NHRA

 

Yes it's a carb engine (quadrajet). I hope to get in the 20 when the headers are installed. The engine produce around 350 hp at the engine with the stock manifold. That's an estimate but with 13.6 sec 1/4, it should be close to that.

Engine with injection will do better than that. The LS1 does 34 mpg. Maybe some day....($$$$$$)

That crossfire needs a little rebuilt... ;) :D

Stephan