The squish, or quench thread..
12-12-2012, 12:18 AM
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The squish, or quench thread..
Quench, or what is commonly referred to as squish, has been a recent topic and was spread out on a few different threads that started going in a non productive direction so I said I'd start a separate thread because it is a very complicated subject. The theory isn't overly complicated but the number of variables and conditions that effect it are almost endless. I will try to keep it as simple as possible to begin with and we can get more in depth as the questions arise. I would encourage anyone who is going to contribute to the thread to use real world examples of what has worked for you, or you've seen first hand work for others.
I'll start with MY thoughts on N/A applications and limit this discussion to small, and big block Chevy's. I often see numbers of .035-.045 thrown around, or it has to be less then .060, or .025 is the magic number, etc. The only problem with people regurgitating these numbers on the internet is that someone might follow it without understanding some simple variables. In an N/A deal I like 'em tight, within reason. You need to account for a certain amount of piston rock at TDC, a bit of tolerance stacking, rod stretch if you're turning some rpm, etc. If you have a large bore with a loose fitting piston, like a lot of BBC marine engines, and went for a .015-.030 squish/quench height and ran into even a little detonation at 6000rpm that piston will rock enough to possibly tag the head. Another issue is carbon build up. A really tight quench area usually accumulates carbon and it doesn't take long to build up enough that it starts contacting. Circle track guys like to set them up so the piston is literally leaving a faint witness mark on the head. The risk outweighs the rewards for that kind of set up in the type of engines we run.
I feel a correct compression ratio for the octane of the fuel being used is more important then a really tight quench. However, if you're running slightly higher compression then optimal, the tight quench engine will be less likely to detonate. Engine building is always a compromise, even at the highest levels of motorsports. OEM's often compromise based on parts availability that is usually dictated by cost. There are a LOT of BBC and SBC marine engines running around from the factory with the pistons .025 down the bore and a .040 or .050 gasket without any issues. A good example of a GM engine designed this way is the ZZ4 crate engine. It is a spec engine for a class of Marathon jet boat racing and is very widely used. They get run for hours at WOT on pump gas, race after race, with amazing reliability (when tuned correctly).
In order to be legal after an overhaul they have to be reassembled with all original OEM components. When I did my first one years ago I was horrified. The pistons were .028 down and they used a .051 thick gasket. I almost couldn't bring myself to put it back together that way... Surely it was going to detonate itself to death!! But, it had already run two complete seasons like that so I figured it Might be ok.. I can only assume they went with the thick gasket to drop the compression a little because of the 58cc head and it seams to work just fine all over the world. I don't know if it would make more power with a tighter gasket 'cause I never tried it but I would be surprised if it made much difference. They already make decent power and don't take a ton of spark timing but, I bet if the compression was just a tad higher it would have some issues without tightening up the quench. There are lots of other examples like 502 Mag's, 500EFI's, etc. I personally set up my generic BBC marine stuff between .045-.055
On to forced induction app's...
I have, reluctantly, found that the importance of a tight squish/quench on a forced induction app isn't as important as it is on an N/A deal. It still has it's merits and benefits but there is generally enough turbulence to keep a good homogenous mixture and get a complete even burn. On FI stuff we are generally trying to slow the burn down through intercooling, water/meth injection, less timing, fuel type, etc. Under certain circumstances the fast burn induced by the squish action, and resultant pressure rise is counter productive and sometimes damaging. At lower engine speeds out of boost with a cold chamber, low comp, and a large quench area the squish velocity can actually extinguish the flame kernel resulting in a misfire. At higher boost levels the pressure in the chamber can rise so quickly that the squish area and end gasses can't escape resulting in blown head gaskets, pinched ring lands, and detonation. This is almost always at a much higher boost and power level then what most here are running but is still useful info. It can get very complicated and you can calculate Max Squish Velocity for your engine plus a bunch of other variables to try and determine where it needs to be but, the general rule is when running high boost, or lots of nitrous, and a piston/head design that uses quench/squish then you need to get the piston at least .090 away from the head to reduce any squish action. I don't now how much boost Merc is running in their bigger engines but there must be a good reason they're running .100-.120 quench height.. Piston design is another important factor.. Take a look at the picture of this 4.600 BBC blower piston. The piston design itself virtually eliminates any squish so you don't have to run a thick gasket or low installed height. Even at lower boost levels like 10psi I have started opening up the quench a bit, not extreme, but a little looser then I used to. I have found the pistons and chambers show a more complete uniform burn pattern and I've had no detonation issues, at least not related to quench..
I had a funny conversation with Bob M today about how I usually learn things by "testing" to failure.. He informed me that I'm only supposed to test up to the point of failure, not beyond. Well, I'm an overachiever 'cause I've failed the chit outta some expensive parts! I find the more expensive the "test" the quicker I am to find a solution. Anyway, this is what I've seen work and it's nothing new or revolutionary, maybe just a little different approach then some are using.
To sum it up: In my opinion - I've never seen a problem from a tight quench on an N/A deal unless you make contact and it's virtually always a good thing. (Not the contact part). A tight quench on an average blown BBC isn't going to cause any problems either and can be a valuable tuning tool. If you're really getting after it with the boost, then tight quench can be your enemy. If you're adding a blower to an engine that has too much comp already, then adding a thick gasket to lower the comp is a viable solution. It might not be the best way to do it but, it works and has been done hundreds of times. The tuning and timing curve often needs to be a little different then a tight quench engine but it's no problem.
This is an extremely simplified, generalized, overview. Instead of jumping all over a specific portion of it, please post your question/statement so this can possibly become an educational thread. Thanks, Alex
I'll start with MY thoughts on N/A applications and limit this discussion to small, and big block Chevy's. I often see numbers of .035-.045 thrown around, or it has to be less then .060, or .025 is the magic number, etc. The only problem with people regurgitating these numbers on the internet is that someone might follow it without understanding some simple variables. In an N/A deal I like 'em tight, within reason. You need to account for a certain amount of piston rock at TDC, a bit of tolerance stacking, rod stretch if you're turning some rpm, etc. If you have a large bore with a loose fitting piston, like a lot of BBC marine engines, and went for a .015-.030 squish/quench height and ran into even a little detonation at 6000rpm that piston will rock enough to possibly tag the head. Another issue is carbon build up. A really tight quench area usually accumulates carbon and it doesn't take long to build up enough that it starts contacting. Circle track guys like to set them up so the piston is literally leaving a faint witness mark on the head. The risk outweighs the rewards for that kind of set up in the type of engines we run.
I feel a correct compression ratio for the octane of the fuel being used is more important then a really tight quench. However, if you're running slightly higher compression then optimal, the tight quench engine will be less likely to detonate. Engine building is always a compromise, even at the highest levels of motorsports. OEM's often compromise based on parts availability that is usually dictated by cost. There are a LOT of BBC and SBC marine engines running around from the factory with the pistons .025 down the bore and a .040 or .050 gasket without any issues. A good example of a GM engine designed this way is the ZZ4 crate engine. It is a spec engine for a class of Marathon jet boat racing and is very widely used. They get run for hours at WOT on pump gas, race after race, with amazing reliability (when tuned correctly).
In order to be legal after an overhaul they have to be reassembled with all original OEM components. When I did my first one years ago I was horrified. The pistons were .028 down and they used a .051 thick gasket. I almost couldn't bring myself to put it back together that way... Surely it was going to detonate itself to death!! But, it had already run two complete seasons like that so I figured it Might be ok.. I can only assume they went with the thick gasket to drop the compression a little because of the 58cc head and it seams to work just fine all over the world. I don't know if it would make more power with a tighter gasket 'cause I never tried it but I would be surprised if it made much difference. They already make decent power and don't take a ton of spark timing but, I bet if the compression was just a tad higher it would have some issues without tightening up the quench. There are lots of other examples like 502 Mag's, 500EFI's, etc. I personally set up my generic BBC marine stuff between .045-.055
On to forced induction app's...
I have, reluctantly, found that the importance of a tight squish/quench on a forced induction app isn't as important as it is on an N/A deal. It still has it's merits and benefits but there is generally enough turbulence to keep a good homogenous mixture and get a complete even burn. On FI stuff we are generally trying to slow the burn down through intercooling, water/meth injection, less timing, fuel type, etc. Under certain circumstances the fast burn induced by the squish action, and resultant pressure rise is counter productive and sometimes damaging. At lower engine speeds out of boost with a cold chamber, low comp, and a large quench area the squish velocity can actually extinguish the flame kernel resulting in a misfire. At higher boost levels the pressure in the chamber can rise so quickly that the squish area and end gasses can't escape resulting in blown head gaskets, pinched ring lands, and detonation. This is almost always at a much higher boost and power level then what most here are running but is still useful info. It can get very complicated and you can calculate Max Squish Velocity for your engine plus a bunch of other variables to try and determine where it needs to be but, the general rule is when running high boost, or lots of nitrous, and a piston/head design that uses quench/squish then you need to get the piston at least .090 away from the head to reduce any squish action. I don't now how much boost Merc is running in their bigger engines but there must be a good reason they're running .100-.120 quench height.. Piston design is another important factor.. Take a look at the picture of this 4.600 BBC blower piston. The piston design itself virtually eliminates any squish so you don't have to run a thick gasket or low installed height. Even at lower boost levels like 10psi I have started opening up the quench a bit, not extreme, but a little looser then I used to. I have found the pistons and chambers show a more complete uniform burn pattern and I've had no detonation issues, at least not related to quench..
I had a funny conversation with Bob M today about how I usually learn things by "testing" to failure.. He informed me that I'm only supposed to test up to the point of failure, not beyond. Well, I'm an overachiever 'cause I've failed the chit outta some expensive parts! I find the more expensive the "test" the quicker I am to find a solution. Anyway, this is what I've seen work and it's nothing new or revolutionary, maybe just a little different approach then some are using.
To sum it up: In my opinion - I've never seen a problem from a tight quench on an N/A deal unless you make contact and it's virtually always a good thing. (Not the contact part). A tight quench on an average blown BBC isn't going to cause any problems either and can be a valuable tuning tool. If you're really getting after it with the boost, then tight quench can be your enemy. If you're adding a blower to an engine that has too much comp already, then adding a thick gasket to lower the comp is a viable solution. It might not be the best way to do it but, it works and has been done hundreds of times. The tuning and timing curve often needs to be a little different then a tight quench engine but it's no problem.
This is an extremely simplified, generalized, overview. Instead of jumping all over a specific portion of it, please post your question/statement so this can possibly become an educational thread. Thanks, Alex
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12-12-2012, 07:29 AM
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Fair and balanced Haxby and I appreciate the contrary viewpoint. I'm still a fan of the cooling effect on the exh stroke from quench on a blower engine, but I never build anything like you discuss in the upper psi ranges.
12-12-2012, 07:43 AM
#5
that looks a lot like the JE coated pistons I used in the last pair of 800's we built, nice piston, narrow quench ring, gentile dish, great flame travel.... BTW
I wasn't ripping at you for lack of knowledge my friend, just wanted to be clear as to what actual combinations we're dealing with in said discussion... sounds like you've been around the block a time or two yourself... lol
I wasn't ripping at you for lack of knowledge my friend, just wanted to be clear as to what actual combinations we're dealing with in said discussion... sounds like you've been around the block a time or two yourself... lol
12-12-2012, 09:13 AM
#6
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Again another great discussion! I am currently running my 524 at .040 with 10:1 compression, based on input from a number of sources. My question to the consortium is this, what is the consensus on squish verses induction type? you mentioned with a blower you run larger, what about carb verses EFI? In addition there are factors of port design and chamber design as it effects turbulence and mixture homogeneity? Carbs typically make more peak power than EFI, I believe due to several factors, one being the cooling factor and two the fuel has more time to mix with air providing a more homogeneous mixture. As you mentioned the ZZ4, the heads on that engine are a quick burn design requiring less advance. BBC chambers along with the large bores require more time for flame travel, therefore requiring more initial timing. It would be great to identify the factors that effect squish and how for those of us who can't afford your education.
12-12-2012, 10:08 AM
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This should be a great thread. I set my little blower engines up with around .050 quench. One block had the pistons around 0 deck. The other, about +.008. I have it written down somewhere, but anyhow, the engine with +.008 got a .060 Cometic, the zero deck got .050 cometic. My goal was to keep them the same quench wise. Would it really matter on a low boost engine like mine, probably not. But, we concentrate on so many other small details while we assemble these things, why ignore the quench distance.
Heres a question for you guys. What happens to the chamber on a typical BBC open chamber head, when the head is milled a bunch, making the chamber smaller in size?? My Dart Race Series heads had very thick decks, which were milled down a bunch in the past, my guess in a quest for added compression. Does doing this have any effect on the burn rate, like say in a fast burn style chamber?
Heres a question for you guys. What happens to the chamber on a typical BBC open chamber head, when the head is milled a bunch, making the chamber smaller in size?? My Dart Race Series heads had very thick decks, which were milled down a bunch in the past, my guess in a quest for added compression. Does doing this have any effect on the burn rate, like say in a fast burn style chamber?
12-13-2012, 02:03 PM
#9
I'll be putting my 489 back together in a month or so, and I was running Mahle flat tops with 119 cc 088 iron heads - .039 Fel Pro gasket on it. Estimated static CR was around 8.6:1. Pistons were .008 in the hole for a quench height of .047. I am considering going to a .027 Cometic head gasket to bump my CR up to around 8.8:1, but that will lower my quench height to around .035. From the sound of what Haxby is saying, I will be all right in either case. Would the added cost of the Cometic gaskets be justified here? Am I getting to tight on clearances?
12-13-2012, 02:40 PM
#10
MarineKinetics
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Tom,
The compression ratio will increase from 8.52 to 8.69 with the move to a .027” gasket from a .039”. That is an increase of .17 of a point and while that may seem insignificant, any increase is worthwhile at that low of a ratio. A .035” piston to head clearance is ideal in a sub 6000 RPM engine with a steel connecting rod. I would advise you to check the RA of both deck surfaces to verify the finish is compatible with a MLS gasket.
Bob
The compression ratio will increase from 8.52 to 8.69 with the move to a .027” gasket from a .039”. That is an increase of .17 of a point and while that may seem insignificant, any increase is worthwhile at that low of a ratio. A .035” piston to head clearance is ideal in a sub 6000 RPM engine with a steel connecting rod. I would advise you to check the RA of both deck surfaces to verify the finish is compatible with a MLS gasket.
Bob
