“Ensure a 0.070" piston to head clearance is maintained, measured at engine assembly. Most hot rodders are tempted to raise the compression by reducing the piston to head clearance down to 0.040" or less, resist the temptation”
Why? What happens with a smaller quench distance that doesn’t happen with the larger one or vice versa?
Seems like you’re recommending grooves in the 2x2mm range (about .078”) which is just large enough to hold a significant mixture volume but traditionally not enough to support combustion, so it would seem that the groove(s) are just directing a hot jet of gasses from the main section of the chamber to the area below the quench pad as the piston starts dropping in the bore. That being the case I would be surprised if getting the quench distance in the .035-.045” range wouldn’t improve upon the total situation UNLESS both quench and the groove are basically accomplishing the same thing making the groove redundant.
Post edited by: Silverback, at: 2005/08/25 03:34
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Re:Grooves and Quench Distance
Date: 2005/08/25 18:54
By: Automotivebreath
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I have run the grooved heads with .040" quench distance and .070. The reason I recommend .070" is because the engine is more resistant to detonation with the wider gap. I see no improvement in performance with the tight clearance.
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Re:Grooves and Quench Distance
Date: 2005/08/25 23:33
By: Silverback
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Traditionally, a tighter quench should reduce detonation tendencies even if it raises compression, which makes your answer somewhat confusing.
Are you saying that you didn’t see an improvement by going to .040” from .070”, or are you saying that it didn’t work? If nothing else that would give you the little bump in compression that he is suggesting without any major work on a lot of engines.
For example, I’m considering experimenting with this on the engine I my 92 K1500 full size blazer. It has the light duty 350ci from that year, rated at 9.3:1 compression (the heavy duty is the same but has 8.9:1 compression and slightly less power). Typically most manufacturers build their traditional engines with the pistons roughly .025” in the hole and use a gasket in the .040” range. That would put the stock assembly right around .065”, pretty close to the recommended quench distance.
But Somender also recommends raising compression to something in the 10:1 range even for heavily loaded engines in heavy vehicles since this modification is supposed to help control detonation. This makes perfect sense because if you can run more compression at the timing that the chamber wants for that combination with the fuel available you should increase the overall engine efficiency, lowering bsfc….
Now taking the engine that I mentioned above and reassembling it with a felpro .015” faced shim style head gasket lowering my quench distance right to what is traditionally considered the optimum quench distance of .040” I raise the compression to roughly 9.9:1, which would seem perfect considering the other recommendations for this grooved setup and considering that this truck is often heavily loaded towing a car trailer (in other words, slightly on the conservative side which I’m fine with on something that I pretty much expect to be reliable and run on cheap gas)…. Even more so, if the grooves prove to control detonation as well as advertised, being on the conservative side will allow me to experiment with boost for some additional power down the road.
This all leaves me with: why resist the temptation to go .040” or less? What happens? And the reason that I’m “worried” about it is that with the combination that I’m looking at it would be a simple way to get everything else in line with the recommendations by running .040” AND being a gearhead, instinctively, using traditional reasons going .070” leaves a lot on the table and shouldn’t be done unless you’ve got no other choice…
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Re:Grooves and Quench Distance
Date: 2005/08/26 14:18
By: Automotivebreath
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Let's discuss the sequence of events as the piston approaches TDC on the compression stroke. As the flat surface of the piston top approaches the quench area of the cylinder head the mixture in that area is forced to flow into the combustion chamber cavity. If the quench area of the head has been modified with a groove the fluid flow into the combustion chamber cavity is altered. When the spark plug ignites the turbulent mixture created by the altered fluid flow from the squish area, accelerated combustion races towards the squish area.
Now think about the shape of the cavity formed by the piston top, the quench area of the cylinder head and the groove it self. If the flame reaches this area with the piston at or near TDC and the quench distance is too tight the quench area cannot support combustion. The end result is unburned fuel in the quench cavity. Some people choose to run the quench clearance so tight that the piston almost touches the head. Experience tells me that if I set the quench distance to .030" at engine assembly the pistons will come very close (or touch) the head at RPM. If one piston happens to be closer than the others disaster will ruin the day.
I find that with the grooves the clearance can be opened to .070" and performance will improve in spite of the fact the compression is lowered.
I recommend milling the heads to obtain the desired compression and set the quench distance with the proper thickness gasket.
Post edited by: automotivebreath, at: 2005/08/26 15:22
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Re:Grooves and Quench Distance
Date: 2005/08/27 02:57
By: Silverback
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Automotivebreath wrote: Let's discuss the sequence of events as the piston approaches TDC on the compression stroke. As the flat surface of the piston top approaches the quench area of the cylinder head the mixture in that area is forced to flow into the combustion chamber cavity. If the quench area of the head has been modified with a groove the fluid flow into the combustion chamber cavity is altered. When the spark plug ignites the turbulent mixture created by the altered fluid flow from the squish area, accelerated combustion races towards the squish area.
Now think about the shape of the cavity formed by the piston top, the quench area of the cylinder head and the groove it self. If the flame reaches this area with the piston at or near TDC and the quench distance is too tight the quench area cannot support combustion. The end result is unburned fuel in the quench cavity. [/b]
Please don’t take this as being argumentative, I’m trying to understand what is going on here and trying to explain where my confusion is coming from, but I’m not sure what your point is here… whether we’re talking .070 or .040” quench, there is not really enough room to support combustion at TDC, even some of the mopar engines that came from the factory with quench distances >.100” don’t appear to have enough space in that area to support combustion. As far as what I’ve seen I don’t know of any evidence that shows any advantage to quench distances >.050”. As far as I can tell, the only reason that OEM’s did it is to always be on the safe side of the “tolerance stack” since for something like my truck engine, where millions were built blueprinting each one is not an option.
Some people choose to run the quench clearance so tight that the piston almost touches the head. Experience tells me that if I set the quench distance to .030" at engine assembly the pistons will come very close (or touch) the head at RPM. If one piston happens to be closer than the others disaster will ruin the day.
I’ve seen quench distances in the high .02x” range where there is clear evidence of the pistons touching the heads, but the additional turbulence created in the chamber usually still results in more power. I wouldn’t want to do it in anything that needs to be reliable…
I find that with the grooves the clearance can be opened to .070" and performance will improve in spite of the fact the compression is lowered.
This is actually the most confusing statement in your whole response. Is it that performance will improve in spite of the lowered compression or is it that it will improve in spite of the decreased quench effect? And that is also my whole reason for the question… is the .070” recommendation “just a safe number that doesn’t hurt the effect,” (there are engines out there that you have to run close to that number to prevent piston to head contact, though most people looking at this information don’t own those) or is there some magic that happens with a .070” quench distance and the groove that doesn’t at .040” or even worse, something about the .040” + groove that doesn’t work together?
[b]I recommend milling the heads to obtain the desired compression and set the quench distance with the proper thickness gasket.
If I understood what the advantage/reason for the .070” number was then I’m more then capable of accommodating it (either mill the heads or more likely, swap the stock 64cc heads for a set of 58cc heads that I have sitting in the garage, and again, use a standard head gasket that should put the quench distance somewhere >.066”). But it’s just as easy for me to lower the quench distance to a more traditionally advantageous .040” range “while I’m at it.”
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Re:Grooves and Quench Distance
Date: 2005/08/27 03:00
By: Silverback
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I guess that I can sum up my whole question with the middle section of what I just wrote:
is the .070” recommendation “just a safe number that doesn’t hurt the effect,” or is there some magic that happens with a .070” quench distance and the groove that doesn’t at .040” or even worse, something about the .040” + groove that doesn’t work together?
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Re:Grooves and Quench Distance
Date: 2005/08/27 14:00
By: Automotivebreath
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automotivebreath said: I find that with the grooves the quench clearance can be opened to .070" and performance will improve in spite of the fact the compression is lowered.
Silverback said: This is actually the most confusing statement in your whole response. Is it that performance will improve in spite of the lowered compression or is it that it will improve in spite of the decreased quench effect? And that is also my whole reason for the question… is the .070” recommendation “just a safe number that doesn’t hurt the effect,” (there are engines out there that you have to run close to that number to prevent piston to head contact, though most people looking at this information don’t own those) or is there some magic that happens with a .070” quench distance and the groove that doesn’t at .040” or even worse, something about the .040” + groove that doesn’t work together?
For years I would work with the block deck height and head gasket thickness to get my piston to head clearance down to .035" to .040" for maximum performance. This became the norm never straying above .045" because it was the right thing to do.
When I started testing with Singh's guidance, I stuck with my normal piston to head clearance. The groves had a big impact running a piston to head clearance of .040". He questioned the tight clearance and hinted that I should try to open it up. I followed his lead and changed the head gasket to a thicker one. I was surprised to see that with the thick gasket and lower compression that goes with it, performance improved and the engine was more resistant to detonation.
My conclusion is before Singh introduced the groove concept we would run the tight clearance to push as much of the air fuel mixture out of the squish area into the combustion chamber cavity that we could, knowing that the flame would not travel in the tight quench area. With this some unburned fuel was left behind. With grooved heads we are now able to penetrate flame travel into the quench area; a little extra clearance aids this penetration.
It's important to note there is nothing magical about .070", what I'm saying is if you set the clearance tight with the grooves you will begin to have flame penetration into the quench area. If you decide to open the clearance the flame penetration will be enhanced. I believe if you go wider than say .080" to .090" performance will drop, I haven’t tried this.
All of my testing is done with production connecting rods. Someone using aluminum rods should alter the recommended clearances accordingly.
Post edited by: automotivebreath, at: 2005/08/27 18:22
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