Reducing CR and quench/squish
Discussion
I'm looking to lengthen the stroke of a current engine using a crank with a larger rotational diameter with the appropriate rods. Providing the block can accomodate this of course, does anyone have any thoughts at this point?
As this is a turbo application the compression ratio is the primary problem. Custom pistons or a bit of OE luckery may allow me to lower it, ultimately by leaving a larger space at tdc than before e.g 1mm longer.
Mathematically this is all straighforward, but what l am worried about is the effect this will have on the squish band, affecting the mixing and combustion efficiency of the mixture. Does anyone know if this is a problem l should be worrying about?
I will be using a heron (flat) head with the combustion chamber formed in the piston crown.
As this is a turbo application the compression ratio is the primary problem. Custom pistons or a bit of OE luckery may allow me to lower it, ultimately by leaving a larger space at tdc than before e.g 1mm longer.
Mathematically this is all straighforward, but what l am worried about is the effect this will have on the squish band, affecting the mixing and combustion efficiency of the mixture. Does anyone know if this is a problem l should be worrying about?
I will be using a heron (flat) head with the combustion chamber formed in the piston crown.
Lots of people just use a lower comp height piston, or a thicker gasket. It works but there is virtually zero squish or quench so it's not the proper way to do it. The proper way to is to get the pistons right in the first place. Squish should be .040" ish to work and I'v gone to .020" on some engines. You need to see what measurement your engine already has, then check your new pistons (fitted) with the head gasket (compressed item). If your lucky you may be able to skim the block say .020" and still end up with a squish band that may work. I believe that squish is more importamt then the compression issue. So make the squish the priority and run low boost and 9.5:1 if you have no choice. That would probably be more efficient then zero squish and 8:1 c/r. The high comp motor would also be very sharp 'off the line'.
Well, to start with take the head and gasket off the car and measure where the pistons are relative to the top of the block/deck (tdc). The pistons may be .020" below or .010" above or somewhere inbetween. You need an accurate measurement and can use a depth micrometer if they are below or above. You could use feeler guages for a reasonably accurate figure. Take the measurement from above the gudgeon pin. Then you need to find out how thick your old compressed gasket is. Use a 0-1" micrometer and I measure the metal fire ring to get this thickness. When you look at your pistons in the bore they will be 'in the hole' at tdc or sticking up proud of the deck and intruding into the gasket thickness. Once you have those measurements you can determine how far your current pistons are from the head face ie, your existing squish clearance.
Then, find out what the compression height is for your stock pistons from a parts book/factory manual. Do the same for your intended pistons and see what the difference is. With all those figures you should have a fairly good idea how far your new pistons will be from the head and maybe what needs to be adjusted. If your measurements confirm the new pistons will be appx .030 from the head or less I'd suggest you should install them and check it again insitu to get an exact measurement, regards.
Then, find out what the compression height is for your stock pistons from a parts book/factory manual. Do the same for your intended pistons and see what the difference is. With all those figures you should have a fairly good idea how far your new pistons will be from the head and maybe what needs to be adjusted. If your measurements confirm the new pistons will be appx .030 from the head or less I'd suggest you should install them and check it again insitu to get an exact measurement, regards.
Well its a heron (flat) head so they would definitely be 'in the hole' at tdc. So the squish measurement is simply the tdc top of piston to head face gap?
0.03" is a very small gap. Can l assume that if the head is flat as it is with the engine l'm looking at this gap would be bigger. e.g. 90mm stoke 10:1 CR would leave a 10mm squish clearance assuming that pistons were also flat topped. (they are actually dished but for the sake of the example..)
How is this measured on dished pistons, from the edge?
0.03" is a very small gap. Can l assume that if the head is flat as it is with the engine l'm looking at this gap would be bigger. e.g. 90mm stoke 10:1 CR would leave a 10mm squish clearance assuming that pistons were also flat topped. (they are actually dished but for the sake of the example..)
How is this measured on dished pistons, from the edge?
funkihamsta said: Well its a heron (flat) head so they would definitely be 'in the hole' at tdc. So the squish measurement is simply the tdc top of piston to head face gap?
0.03" is a very small gap. Can l assume that if the head is flat as it is with the engine l'm looking at this gap would be bigger. e.g. 90mm stoke 10:1 CR would leave a 10mm squish clearance assuming that pistons were also flat topped. (they are actually dished but for the sake of the example..)
How is this measured on dished pistons, from the edge?
10mm squish is no squish at all. The piston has to get within about 30-40 thou of the head for squish to actualy achieve anything. In the case of a flat head, the piston would normaly have a "bowl" in the crown which actually forms the combustion chamber (a la crossflow). The outer edges of the piston would form the squish area.
Well said that man!
Also to measure the volume of the cylinder with the piston at tdc you would use oil from a syringe. From what you've said thus far you haven't got any squish with your current/proposed setting. Doesn't mean the engine will fail but it could be a lot better if it had squish. Could you get pistons made or find a better alternative?
Also to measure the volume of the cylinder with the piston at tdc you would use oil from a syringe. From what you've said thus far you haven't got any squish with your current/proposed setting. Doesn't mean the engine will fail but it could be a lot better if it had squish. Could you get pistons made or find a better alternative?
That's what confused me.
If the head was flat and the pistonhead was flat, then you would leave a 10mm gap under a 90mm stroke 10:1 CR.
Correct?
I guess the piston bowl, alters this and allows the piston edge to come much closer and create the squish whilst retaining the CR. Cheers guys!
Dan
If the head was flat and the pistonhead was flat, then you would leave a 10mm gap under a 90mm stroke 10:1 CR.
Correct?
I guess the piston bowl, alters this and allows the piston edge to come much closer and create the squish whilst retaining the CR. Cheers guys!
Dan
Hmm yes i see why you'd say that: but its the ratio of the cylinder volume at bdc to the volume at tdc. Given a simplistic flat suface on the cylinder crown and head, this would be 100mm * Cylinder bore area : 10mm * Cylinder bore area (if the stroke was 90mm). CR is therefore 10:1
Nahnah!
Nahnah!
funkihamsta said: That's what confused me.
If the head was flat and the pistonhead was flat, then you would leave a 10mm gap under a 90mm stroke 10:1 CR.
Correct?
I guess the piston bowl, alters this and allows the piston edge to come much closer and create the squish whilst retaining the CR. Cheers guys!
Dan
Sorry but I think it's incorrect. Suparuss, take your coat off! A measured gap of 10mm doesn't mean you have a 10:1 compression ratio. What about stroke length and cylinder diameter? The compression ratio depends on the volume of the swept area relative to the total combustion chamber volume at tdc. If you want to mail me I shall gladly try to help you or you can post some measurements, ie stroke, bore diameter, head volume, gasket thickness and it's bore diameter and don't forget the piston details ie where it is relative to the deck. Cheers.
ok.
Dumb question time. Do all engines have a squish band?
I didn't even know what a squish band was till I read this thead. I've since googled it and now have a vauge idea.
I know that its important in a 2 stroke engine but is it as important in a 4 stroke.
Also what happens if you convert a NA car into a turbo/supercharged car and simply bung in a thicker head gasket. If you lose the sqiush band won't this fundamentally affect the cooling?
Andy
Dumb question time. Do all engines have a squish band?
I didn't even know what a squish band was till I read this thead. I've since googled it and now have a vauge idea.
I know that its important in a 2 stroke engine but is it as important in a 4 stroke.
Also what happens if you convert a NA car into a turbo/supercharged car and simply bung in a thicker head gasket. If you lose the sqiush band won't this fundamentally affect the cooling?
Andy
A proper turbo conversion would use custom or lucky OE selected pistons. Thicker gaskets are a kludge. You would lose squish, NA engines of any quality probably have some squish, but how would it effect cooling? If anything inefficiently burnt fuel may serve to cool the cylinder chambers!
the squish area is also called the quench area, or quench band, as it increases the mixture around the edges of the combustion chamber, this assists in maintaining a steady (not violent) burn rate, and offsets any tendancy for high speed detonation or pre ignition to occur.
The quench area also lowers the piston crown temperature by momentarily restricting the combustion flame to just the area of the combustion chamber, this increases piston ring life, and helps prevent the piston from becoming a hot spot, able to pre ignite the fuel/air charge.
The quench area also lowers the piston crown temperature by momentarily restricting the combustion flame to just the area of the combustion chamber, this increases piston ring life, and helps prevent the piston from becoming a hot spot, able to pre ignite the fuel/air charge.
suparuss said: the squish area is also called the quench area, or quench band, as it increases the mixture around the edges of the combustion chamber, this assists in maintaining a steady (not violent) burn rate, and offsets any tendancy for high speed detonation or pre ignition to occur.
The quench area also lowers the piston crown temperature by momentarily restricting the combustion flame to just the area of the combustion chamber, this increases piston ring life, and helps prevent the piston from becoming a hot spot, able to pre ignite the fuel/air charge.
I think I agree with some of the above and what quench does but I would never regard squish and quench being the same thing. I see squish as a means to squeeze fuel from the furthest recesses of the cylinder towards an advancing flame front. This speeds up burn time as both fronts are heading towards each other. It also means more of the fuel will get burned, so it's efficient.
I look at quench slightly differently. I think the term 'quench' is used because of it's cooling applications. The D shaped quench band in the cylinder head is used to absorb heat from the expanding flame front which may be already on the verge of detonation. In effect it cools the charge temperature. Some engines have a quench area in the head with an opposite area in the piston so this would be good. Rovers are probably not so good. They have quench in the head and a bowled piston. Always thought that was strange or maybe just a poor design.
. It's good to have squish in engines including turbo's. In fact it may be of greater importance in a turbo motor. Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff