Moparts Tech Archive

Motor General

A Lesson on Quench

this subject seems to come up from time to time and i just wanted to share some experience with anyone that cares.
the other day i disassembled an engine i had laying around that alway's had a problem with detonation. i've had this in a car for about 20 years. here's the combo.
383 bored .060 = 394 cubic inches
standard stroke
edelbrock dp4b intake W/ holley 4150 750 cfm race prepped
ported 906 heads flowed approx. 255@ .500 intake
crane CCH-302 cam 232/242@ .050 112 lsa 504/528 lift
ductile iron rockers
TRW flat top pistons with reliefs cut by me

actual C/R 9.25-1

even with that low compression when we were mandated to use unleaded fuel only this combo wouldn't take any more than 32 degrees total timing on 94 octane. with the old leaded gas it would run on 94 octane at 38 degree's with no problems.
the point to the story is this. just because you have a low compression ratio or are planning on building a combo with one, don't expect it will not detonate if you don't have the proper quench in the chambers. 25 years ago i knew little about building a quench engine i guess mostly because the gasoline was better and didn't really need to know. now i know better and i hope you can take something from this experience...
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Dark Horse
New Member
Posts: 61
From: Oxnard,Ca USA
Registered: Feb 2002
posted 04-26-2002 10:04 PM

being a non engine builder, my question is what is quench?how do you get it?how much do you need?
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DusterDave
Moparts Member
Posts: 2595
From: Tampa, FL
Registered: Mar 2000
posted 04-26-2002 10:13 PM

Let me try to answer it, and DRAM can take me to school!
As the piston nears TDC, it causes a near collision with the flat part of the combustion chamber, creating an extremely high mixture velocity resulting in a swirling effect of the air/fuel mixture. This accomplishes several benefits. One, the air/fuel mixture becomes more homogeneous, and two, the swirling effect tends to reduce or eliminate hot spots and averages the surface temperatures across the combustion chamber and the top of the piston. These are the reasons why quench engines have a reduced octane requirement over similar non-quench engines.
Quench is a term to describe the distance between the top of the piston and the flat part of the combustion chamber. It is generally agreed that this dimension be less than .060 for there to be an effective quench. As this dimension is reduced, the quench becomes more effective. Less than .035 is not recommended for a steel rod engine.
DRAM, how'd I do?
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DRAM
Moparts Member
Posts: 4164
From: West Palm Bch. Fl.
Registered: Jun 2000
posted 04-26-2002 10:17 PM

Dusterdave, you did a great job, thumbs up unfortunately the older mopar heads don't lend themselves to getting a lot of mixture movement in the chambers. that's another reason quench is so important.
Dark Horse, simply stated, quench is the amount of distance between the piston and the head/combustion chamber at TDC. if you had an open chambered chamber like a 906 head and the piston was at TDC you would have a quench in the area of .100 which would be ineffective.

the general accepted standard is between .035 and .055 but the numbers that i would recommend are anywhere from .040 to .045
IMO .035 is too close and .055 is too much to very effective.
either flat tops with a closed chamber or quench domed pistons made for either open or closed chambers will do the trick. as stated in my original post and echoed by others even a low compression engine without quench will likely detonate under load.
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Brian_wo
Moparts Member
Posts: 10564
From: Omaha Nebraska wo23dodge@cox.net
Registered: Nov 1999
posted 04-26-2002 10:20 PM

well the way I always explained it was that the close collision between the top of the piston and the bottom of the head squeezed the air fuel mixture over in to the open side of the head,this re-excites the mixture or in other words breaks up any fuel droplets and returns it to more of a mist,the fuel then burns better and to some extent is even cooled by this process.
The KB site has a write up on this also.
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Tony
Moparts Member
Posts: 170
From: Whittier Ca, USA
Registered: Dec 1999
posted 04-26-2002 10:39 PM

I have a question that I've wondered about for a while:
either flat tops with a closed chamber or quench domed pistons made for either open or closed chambers will do the trick (Dram)
If you use flat tops for quench with a closed chamber head, and the clearance is say .040...does that mean that the compression ratio will always be high (11:1?) because of the small clearance between the piston and the head... in a quench motor. Does this mean you cannot build a 9.5:1 motor with quench clearances? Or can you have a part of the piston come up to the head so that you have a lower compression. Can someone explain?
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Fast One
Moparts Member
Posts: 3072
From: Australia
Registered: Nov 2000
posted 04-26-2002 11:06 PM

Don't they call good quench, artificial octane ??
The squish(?) is around the spark plug & burning mixture area in the chamber.

A good quench design has a big effect on the combustion, can get away with more compression.
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DRAM
Moparts Member
Posts: 4164
From: West Palm Bch. Fl.
Registered: Jun 2000
posted 04-26-2002 11:16 PM

Tony, you may be surprised how little a difference .010-.020 in deck height will make in actual compression. there are several options i use as an engine builder to achieve the desired results. a lot depends on the actual combo of parts being used like cam, heads, available pistons and so on. i don't want to make this sound the wrong way but building engines is more than just checking a few clearances and bolting them together. head gaskets can be changed, chamber size, cylinder wall valve unshrouding, piston mods and a whole list of other things. the best place to start is to work the basic numbers and get an idea where you are compression wise and then look at available options if it's not the desired amount. it's not rocket science but alittle experience sure does help. the answer is alway's there, finding it is the hard part
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badasmopar
Moparts Member
Posts: 284
From: Portland
Registered: Dec 2001
posted 04-27-2002 06:15 AM

I have 11.2/1 compression and Iam runing 94 octane pump gas with 38* advance with my 440 with 915 heads on them and have no problems with detonation. Guess that quench works.
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71Cuda
Moparts Member
Posts: 1333
From: Circleville, Ohio, US
Registered: Jan 2000
posted 04-27-2002 06:21 AM

My old Challenger should be in the quench handbook.....stock heads, KB pistons 3700+me 12.00 @112.
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Tim Moffett
Moparts Member
Posts: 1245
From: Rochester, NY
Registered: May 2000
posted 04-27-2002 07:46 AM

I was lucky enough to hear about quench before getting my engine together. I was just a little in the hole w/ 11.1:1 compression. I run Ultra94 all day long no problems...
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Sargon
New Member
Posts: 85
From: Long Island NY US
Registered: Feb 2002
posted 04-27-2002 12:35 PM

A major reason for knock is ignition from a second source, shortening the time to gas pressure spike (like an un-intentional dual plug), so that the piston is too close to TDC when the pressure goes up. This source can be gas on the far side of the chamber (opposite the plug), being set off by a shock or heat wave caused by the primary ignition. With .060" or less static quench, the mixture contained between the 2 surfaces does not reach ignition temperature - it loses heat to the piston and/or head surface too fast to self-propagate, so the knock caused by a second flame front is eliminated. This is why smog motors are not quench - the unburned gas (volume between the surfaces) contributes to emmissions.
BTW: Harley-Davidson started making quench motors in 1923, and used .045" piston to head clearance in closed-chamber high performance motors in 1937.

Low-comp quench motors typically have a "D" cup in the dome - the D is exactly under the open (valve seat) chamber area, and the non-D surface is below the quench surface of the head.
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DRAM
Moparts Member
Posts: 4164
From: West Palm Bch. Fl.
Registered: Jun 2000
posted 04-27-2002 07:16 PM

just as a footnote to the original "lesson" LOL. next time you plan your combo start thinking "outside the box" just alittle. don't overlook all the things reqired to get the most out of your combo. it's not alway's a matter of dollars. do the math.

building a quench motor isn't hard. there's a variety of different pistons on the market just for that job. some are reverse domed to keep the C/R down a little and others are just a flat top with a raised pad. when looking at pistons use the compression height, the rod length and half the stroke to find total length. subtract that from the deck height. if you don't know the actual deck height use the factory spec to start
out with. you'll almost alway's be within .020 after assembly. in the beginning it's more about getting everything in the ball park so to speak.
the way i do it is to check the actual deck height and then do the math. even with rod length variations it get's me with .002 of the actual every time. after that it's minor machining to complete the job.

never2fast, most of chryslers low compression engine's that i know of do have an undesireable quench. many of them have pistons as much as .090 in the hole. typically deck heights from mopar are also .020-.030 more than the actual spec too.

just as an example, take a 1970 440hp with the advertised C/R of 9.7-1. that figure was actually very close but at .050 in the hole with an open chamber head and a .016 steel shim the "quench" would fall somewhere in the .110 range. not even close.
now put a quench style piston in there at zero deck and a .040 gasket and the quench can be .040 with a 10.45-1 C/R and less chance of detonation. with proper quench even though the ratio may be higher the chance of detonation is still decreased. naturally if you want low C/R it's still not hard to get that too and still have the proper quench.
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Sport
Moparts Member
Posts: 258
From: Canton, Ohio. USA
Registered: Jan 2002
posted 04-27-2002 08:00 PM

I agree you cant promote a quench motor enough. Not only does it help prevent detonation it creates more HP vs a non quench motor with the same comp.ratio. A quench motor creates a very violent combustion chamber atmosphere, a mini tornado of sorts,Very good for a nice nearly complete violent explosion of the incoming intake charge. Got Quench!
Unfortunatly I do not have Quench. Right now at .075 in the hole plus .020 gasket, A non quench of.095 But watch out when I do Sport!
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Unlawfl
Moderator
Posts: 4519
From: bloomfield hills, mi, usa
Registered: Jan 2000
posted 04-27-2002 10:56 PM

Yep, would have been a better idea for me to deck the block to achieve "zero deck" and then take less off the heads to get to my desired 10.7:1. I'll have to make do with the .054 quench which is acceptable but DRAM makes such a good point that these are the "little" things that often make a "big" difference in the end.
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440shorty
Moparts Member
Posts: 819
From: Annapolis, MD
Registered: Apr 2001
posted 04-27-2002 11:15 PM

Very good topic. For those who are a little confused about quench and CR -- only a part of the piston is within 0.060 of the combustion chamber. The combustion chamber arches away from the dome of the piston (especially when the piston is a flat top) where the valves are. On a closed chamber head (like SB Chevy heads, or the extreme case -- diesel heads) the part of the combustion chamber that does not contain the valves is the same level as the gasket surface of the head, which gives the chamber a heart shape, or teardrop shape. Open chambered heads, like 452 BB Chryslers, have chambers that are not level with the gasket surface -- the chamber looks round, like the piston.
Either way, the volume of the combustion chamber can be increased to compensate for the increase in compression caused by near-zero deck. Usually, several cc's are gained by cleanup and port work anyway.

Funny thing -- my combination, which I THINK is stock '77 pistons and stock 452 heads, does not detonate with 40 degrees total timing all in at 1200 RPM. However, I do run the mixture on the rich side, which would help explain the lack of detonation -- it USED to detonate until I fattened the mixture.

440shorty

I hope I helped, and not confused everyone. If I only had some pictures.
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