rene-paul

I live on the Island of Hawaii, Kona side, not a lot of local options.

I am building a 327 from parts collected; here is where I am at:
327 block .040 over. Pistons are flat with valve reliefs. Later 327 rods with beams cleaned up. Crank is .020 under, mains and rod journals.
Piston @ TDC is .030 below the deck. Deck is milled as no numbers on the pad.
With the piston 1/2 inch down the bore I measure 115cc with a syringe through a plastic plate sealed with vasaline, piston sealed also.
Heads are 9917291 pocket ported and polished by me. The combustion chamber volume is 70cc.
I know I can plug in different compressed head gasket numbers and get different compression ratio's .. Please explain Quench and how does it apply in my situation. This engine is nothing fancy but I would like to know more about the engineering specifics of quench.
Explained equations could help too!
Thank You,
Rene

427Hotrod

You're dealing with a typical "rebuilder" type piston. GM typically had pistons .020" (give or take .002-.003:") below the deck. They also used steel shim head gaskets that were .018-.022" range thick. That put the pistons in the .036"-.045" range from the head typically.

Low cost rebuilder pistons are made short so the decks can be milled and the engine assembled with little need to measure things. In addition rebuilders will often use composition head gaskets with a thickness of .038"-.051" maybe. That can put the pistons .068"-.081" away from the head.

So assuming maybe a 5cc valve relief...your current combo has around 8.55/1 compression ratio with a .041" gasket. Could be 8.38/1 with a .051 gasket.

A good "nominal" combo with zero deck height, a .038" gasket and 64cc chambers gets you 9.82/1. So tightening things up makes a HUGE difference in compression ratio. Compression is good for just about everything. Idle quality, power, response, fuel mileage etc.

But back to original question...quench is a design feature of many wedge head engines to eliminate dead spots in the combustion chamber. It's the large flat area of the chamber that intersects with a similar area on the piston. Actually it sorta creates them if you think of it another way because it forces air/fuel out of certain areas of the chamber to the center so it is more apt to be burned. If you have a large quench area in the chamber you can get incomplete combustion in that area as well as lose out on the ability to put it where it can be burned more efficiently. For overall purposes a tight quench is good. Racers will build the engine where the piston literally makes light contact at high RPM with the head to ensure everything is out of that area.

Old hemi engines and many others often have no quench area at all. The chamber is just huge and open. Later versions often have quench pads built into them. Old hemi's do well when redesigned pistons are installed with quench areas designed into them...usually worth 20 HP or so.

Later design small fast burn chambers are designed without as much quench and the pistons are made to fit the chamber design a lot better.

There's a lot more to it all...but this gives you an idea. Shooting for somewhere in the .030-.040" total piston to head clearance will usually put the quench area in a good position on a small block. This is for a basic steel rod engine with normal piston to wall clearances and piston design.

JIM

SWCDuke

The first GM OHV "wedge chamber" V-8 was developed under the supervision of Ed Cole and showed up first in the 1948 Oldsmobile. Cadillac followed in '49 and Chevy and Pontiac in '55.

With the piston at TDC most of the combustion chamber is in the head and smaller diameter than engine bore. As the piston approaches TDC and the clearance between the piston crown and head surface becomes small, the mixture is forced out of the this narrow crevice and creates additional turbulence, which causes more rapid combustion propagation in a smaller chamber and allows a higher compression ratio on a given octane fuel than an "open chamber".

However, this design causes higher engine-out HC emissions because the flame is "quenched" in this narrow gap, which is why the open chamber big block head was developed in the late sixties.

Quench clearance is the sum of piston-deck clearance and the head gasket thickness. The Chevrolet Power Manuals from the seventies recommend a minimum of .035-.040" for road racing engines, but some successful drag racing engines have run with near zero quench clearance.

Production engines averaged .050-.060" due to increased piston-deck clearance (nominally .025") as the broach tools wore.

Some make a big deal out of quench clearance, but it's really not that important unless you are building a very high compression racing engine that is on the ragged edge of detonation with very high octane fuel. Taylor states in his textbook that experiments showed that very small quench clearance will increase detonation resistance, but not noticeably until it is less than .005 times bore diameter, .020" on a 4" bore engine, which is less than Chevrolet's recommended minimum.

A case in point are the early '62 340/360 HP engines. With nominal .025" deck clearance and the production .018" head gasket, quench clearance is .043", and with the 60-61 cc head chamber of the 461X heads, actual compression was pretty close to the 11.25:1 advertised value, but Chevrolet dealers got a lot of detonation complains.

The "fix", beginning about mid-production, was to use two head gaskets, which increased quench clearance to .061" and dropped CR about half a point, and the detonation complaints stopped. This practice continued through the '63 model year and a TSB was written to apply the same fix in the field to early '62s that were built with one head gasket.

The 64cc big valve heads in '64 along with the 30-30 cam that had a later closing inlet valve to reduce DCR allowed these engines to operate satisfactorily on premium fuel of the day with one head gasket, even with the much more aggressive spark advance map.

If you want to take a deeper dive into subject, search for threads started by me and download and read the tuning seminar and compression ratios pdfs.

Duke

cardo0

Sorry to disappoint you but I am under the impression quench characteristics are obtained empirically - not mathematically - meaning objective testing was done to measure the effects. And you have to read quite a bit to find that quench effects diminish with not only lower compression ratios but very high rpm.

When you think about it the lower the static compression the less static pressure there is when the piston to head distance is minimum at TDC. What would expect the squished pressure to be? The same or less? You can decide for yourself. So when does quench become effective? I have come across a c.r. value of 9.0 from David Vizard - take it or leave it.

Now for high RPM it is hard to visualize and really comes from proven hard data on someones dyno. But the assumption is the tight quench area produces an undesirable turbulence for the ignited flame front at the high rpm where the combustion chamber is filling and blowing down faster and faster. I recall somewhere over 7,000 rpm and maybe high compression contributes to this also.

Now in your build I see you are measuring volumes with liquid and if you measured the head chamber volume with liquid also you should have a very accurate c.r. calculation. So my recommendation is if your static c.r. is less than 9.0 I would be more concerned with a good sealing long lasting gasket like an MLS rather than a shim steel thinner gasket. But if good compression ratio is obtainable with a steel shim gasket then I would go with that. I currently use the FelPro steel shim gasket (I believe is a 1094 but maybe wrong here its been so long) and will swear by it as good. I see now that same FelPro gasket is no longer rubber coated as mine was but it is still a great gasket and only 0.015" when compressed.

Hope this can help.

68hemi

And all of the above is why I never use the term "quench" it is way to complicated for most laymen (like myself) to understand or explain. If you don't understand it and want to increase your engines performance I suggest you find an engine builder/machine shop that DOES understand it and let them build your engine based on what your goal is regarding the fuel you want to use or have available to you.

63 340HP

Everything posted is valid information based on the understanding of combustion in a chamber at the time the research was published (by Taylor, Vizard, etc.).

The layman's analogy of quench benefit is a number of techniques combined:

Combustion in a piston to head gap under 0.050 is suppressed as the gap closes to zero. The shape of the tight quench gap area can be designed to direct the remaining combustible charge to an open chamber area where it benefits power and/or emissions performance from directing a rich mixture near the spark ignition, and from promoting turbulence to gain a shaped flame/pressure wave acting on the entire piston face area in the dynamic chamber volume as the piston travels up and down the bore.

Modern techniques to gain the benefits of the combustion chamber shape are not obvious, and were not always achieved by chance in older engine designs. GM was lucky with the SBC chamber design quench area, the true wedge of the W-series (409) is an example where GM's luck hit a dead end. The Chrysler Hemi chambers had no quench area (and needed two spark plugs to optimize the combustion ignition), and Chrysler polysphere chambers had misunderstood benefits that did not merit the production cost (benefits later rediscovered in motorcycle engine designs, leading to modern 4-valve/cylinder pent roof chamber designs).

The chamber in the first generation small block Chevrolet that we enjoy was designed using the Taylor research and GM's own testing. As compression ratio's climbed in the early 60's the detonation problems SWCDuke noted revealed the limitations of the initial chamber shape. GM simply lowered the static compression with thick gaskets, and lowered the dynamic compression with long duration cam timing. In the 60's these bandaid fixes addressed the immediate complaints.

In the late 60's and early 70's pollution emissions took priority, to an extent the static compression fix of thicker head gaskets did not solve the detonation and emissions concerns, compounded by the poor octane of the era's new blend of unleaded fuel. Redesigned heads with open chambers that minimized quench area prevailed to lower NOx emissions, with the resulting low compression and poor low rpm power output. Good performance could still be found at high rpm, as predicted by Taylor and confirmed with the LT1 static compression change from 71 to 72, but limitations of the primitive emissions controls and fuel economy demands ended the consumer market for large displacement high winding engines.

In the late 70's and early 80's an Engineer assigned to improve power and emissions of the Jaguar V12 rediscovered the benefit of chamber turbulence. Combustion flame/pressure wave technology advancements lead to the demise of domed piston shapes to gain compression, with designs using flat top pistons that prevented unwanted turbulence and offered the pressure wave to act on a flat surface that was easier to control for wear and noise as the piston traveled through the stroke.

Honda Engineers rediscovered the benefit of a combustion chamber shape employing a rich mixture chamber near the spark ignition to light off a larger main open chamber above the piston (the CVCC concept). Honda's "revolutionary" head design was much like larger displacement late 1930's B & S stationary flathead engines that were designed to run on gasoline, Diesel, or kerosene fuels (my son has a single cylinder 25ci Model Z engine built in 1942 with the separate spark plug chamber design). Improvements in the technology deleted the need for a separate spark plug chamber, as designed quench areas were employed to direct the precise fuel injection mixture near the spark plug

These rediscovered design benefits to improve pollution emissions controls and power combined with better quality unleaded fuels and exhaust catalyst technology to gain interest in revised combustion chamber shapes. GM Engineers parallel research resulted in the Bowtie head designs with angled plugs, improved chamber turbulence, and the evolution to the Fast Burn chambers with computer assisted designed-in quench areas and improved performance. GM's Engineers were so successful that they were able to control chamber temperatures for good pollution emissions, to the extent that they were able to reverse the coolant flow direction through the engine in the later generation LT1 SBC to further improve power.

The advancement in combustion efficiency is easily confirmed with the best power in the later chamber designs reduced need for spark ignition advance. Old 461-462 style chambers needed 38-40 degrees of advance to make their best power, and modern designs only need 34 degrees to burn the charge for full power without detonation. The direct result of the extra four to six degrees of crank rotation and piston travel is higher static and dynamic compression, and more power (as predicted by Taylor's research nearly a century ago).

History is good, but what about improving the old 327 and 350 without a power compromise with low static compression and the older chamber design?

Hot rodders and racers reduced detonation hot spots in the chamber during combustion by eliminating sharp edges in the chamber. The areas with a piston to head gap in the 0.060 to 0.120 range were opened up and polished to eliminate secondary compression ignition spots.

We now have better quality unleaded fuel than what challenged GM Engineers in the 70's, so we can return to the initial GM head design clearance in the quench areas of a ~0.040 gap. The 0.015-0.018 head gaskets can be used (as recommended above), or you can cut the deck height to locate the piston crown at or above the deck surface to achieve the piston to head gap with a modern MLS gasket that range in the 0.039 to 0.050 thickness (again, as recommended above).

The improved performance from modern aftermarket heads is not only from greater airflow, as the modern chamber shapes and machined surface quality is far better at controlling the combustion ignition pressure than any GM casting.

rene-paul

I want to say Thank You to; 427 Hotrod, SWCDuke, cardoO, 68 hemi and 63 340HP.
My understanding of quench has improved. Thank all of you again.
My plan was to use the smallest compressed thickness composition gasket.
I believe FelPro makes a .036. I will look into other gasket options. With what I have the .036 to .040 quench is out of what is economically available.
A side note: my previous understanding of quench was actually the volume of the compressed area of the space within the circle of the bore of the gasket. This plays a part but there is much more detail. Thanks again!
Aloha,
Rene