Piston Comparison
Dec 19, 2008, 10:09 AM
#31
When I get my pistons in I am going to add all sorts of debate to this thread.
Basically one thing I learned from domestics long ago (between my grandpa, uncle, and a friend named Mardy Maltez...google him) is the smaller the ring the better. To offset the seal issues you can do lateral or vertical as ports to get it too seal back up and stay sealed. Accumulator grooves, contact reduction, etc. (starting to sound like a JE order form) can all work around some issues that are created by others.
Basically one thing I learned from domestics long ago (between my grandpa, uncle, and a friend named Mardy Maltez...google him) is the smaller the ring the better. To offset the seal issues you can do lateral or vertical as ports to get it too seal back up and stay sealed. Accumulator grooves, contact reduction, etc. (starting to sound like a JE order form) can all work around some issues that are created by others.
Dec 19, 2008, 10:54 AM
#33
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If everything else stayed the same you always make more power with more boost. More boost is more airflow, more airflow is more power. However when you reach the end of the airflow rope the next best thing is more compression...like I mentioned with the 4% rule, if you are at 500whp at 10:1, theoretically the next full point at 11:1 would net 520.
Dec 19, 2008, 10:56 AM
#34
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So what is the ideal static compression ratio?
There is no correct answer to that question, simply because there are too many variables to consider. Alcohols like Methanol and Ethanol will tolerate static cylinder pressures that make gasoline unstable. But one question is, just how much can a mixture of 85% EtOH and gasoline withstand?
Keep in mind that factors like valve timing, head gasket thickness, intake air temps, turbo/turbine housing size, etc., all factor into just how much static cylinder pressure the engine will tolerate. That is because all of these things factor into the only compression ratio the engine really sees, which is dynamic compression. A car with smaller cams and turbo will have a tougher time with high compression than long duration cams and a turbo with a big hotside.
As always, octane is the primary limiting factor. For a car that will never see anything but pump premium, the factory SCR (static compression ratio) is probably the best bet given the ample data we've seen of what is possible with that. With E85 used as a secondary fuel, I'd still stick to the factory-range SCR if strong performance on pump premium is desired.
For a dedicated E85-E98 car with larger turbo, I'd guess 10:1 should improve torque without drawbacks. Does higher SCR create faster spool? It creates a bit more torque, but it increases effciency, which reduces heat (the primary spool influencing factor). It also improves fuel economy and emissions somewhat. FWIW, WRC cars make stupid torque around 3500 rpm with a tiny turbo, 40psi boost, and 10:1 SCR using 98 RON. That doesn't make it practical for a street car however.
For an E98 car, 11:1 might be viable, but who knows? At some point, even with straight alcohol, going to a higher SCR is too much of a good thing, and the result will bring the detonation threshold down to a point whereby no more power can be realized because the ignition advance is set too far from BMT.
Just my thoughts. Now ... where's my martini?
There is no correct answer to that question, simply because there are too many variables to consider. Alcohols like Methanol and Ethanol will tolerate static cylinder pressures that make gasoline unstable. But one question is, just how much can a mixture of 85% EtOH and gasoline withstand?
Keep in mind that factors like valve timing, head gasket thickness, intake air temps, turbo/turbine housing size, etc., all factor into just how much static cylinder pressure the engine will tolerate. That is because all of these things factor into the only compression ratio the engine really sees, which is dynamic compression. A car with smaller cams and turbo will have a tougher time with high compression than long duration cams and a turbo with a big hotside.
As always, octane is the primary limiting factor. For a car that will never see anything but pump premium, the factory SCR (static compression ratio) is probably the best bet given the ample data we've seen of what is possible with that. With E85 used as a secondary fuel, I'd still stick to the factory-range SCR if strong performance on pump premium is desired.
For a dedicated E85-E98 car with larger turbo, I'd guess 10:1 should improve torque without drawbacks. Does higher SCR create faster spool? It creates a bit more torque, but it increases effciency, which reduces heat (the primary spool influencing factor). It also improves fuel economy and emissions somewhat. FWIW, WRC cars make stupid torque around 3500 rpm with a tiny turbo, 40psi boost, and 10:1 SCR using 98 RON. That doesn't make it practical for a street car however.
For an E98 car, 11:1 might be viable, but who knows? At some point, even with straight alcohol, going to a higher SCR is too much of a good thing, and the result will bring the detonation threshold down to a point whereby no more power can be realized because the ignition advance is set too far from BMT.
Just my thoughts. Now ... where's my martini?
Dec 19, 2008, 09:40 PM
#38
I have been considering going to 9.5:1 compression pistons for a while now. Unfortunately in my area the only fuel available on the pump is 93 w/ 10% ethanol. Have you done any testing with that compression ratio? I have seen a lot of stock, 9:1 and 10:1 but it seems the majority have ignored everything in between. Any reason for that?
Great write up btw. I always love pictures too 
Great write up btw
. I always love pictures too Wiseco 9.3:1, part number K597M85
Dec 23, 2008, 10:22 AM
#40
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Good topic,
When matching pistons to power targets, the shape and quench area plays an important role in the final solution. CR is something one can always work around by improving the fuel quality and type. Quence ....important in Evo's critical in Subarus.
My business does a lot of R&D at my expense and one of the things i see often in my own local environment is that most engine builders dont actualy know all the facts nor understand the science to put it all into perspective. As an example BMW has done some serious research on the piston shape to enhance power and performance of their new generation engines with no change to the CR. I.E testing piston shape only and nothing else. They will not release any technical details for obvious reasons but i have seen the shape of succesful sample that works like a dream and i must say they look a lot like your 10.1 CR pistons. Now they focus on N/A cars mostly, however in the past their high performance pistons all looked much like the evo's banges with a dish and valve recesess.
In mycase if the customer wants a high power evo (above 500Whp) then iI use JE's with 8.5 or higher CR. They are 2618, fairly quiet and i prefer the larter 1,3mm compression rings. JE builds these to my specs and everyone is happy. Customers looking for moderate power (below 500Whp) I default to the cheaper end of things but with 9:1 CR - to suit their budget, and discussions like this become interesting but trivial to them.
So many variables in an uncontroled "laboratory" means your results will mean whatever you want them to. A concrete scientific analysis would run you a few millions and serve little in terms of commercial value given the limited application.
my 2 Dollars worth.
When matching pistons to power targets, the shape and quench area plays an important role in the final solution. CR is something one can always work around by improving the fuel quality and type. Quence ....important in Evo's critical in Subarus.
My business does a lot of R&D at my expense and one of the things i see often in my own local environment is that most engine builders dont actualy know all the facts nor understand the science to put it all into perspective. As an example BMW has done some serious research on the piston shape to enhance power and performance of their new generation engines with no change to the CR. I.E testing piston shape only and nothing else. They will not release any technical details for obvious reasons but i have seen the shape of succesful sample that works like a dream and i must say they look a lot like your 10.1 CR pistons. Now they focus on N/A cars mostly, however in the past their high performance pistons all looked much like the evo's banges with a dish and valve recesess.
In mycase if the customer wants a high power evo (above 500Whp) then iI use JE's with 8.5 or higher CR. They are 2618, fairly quiet and i prefer the larter 1,3mm compression rings. JE builds these to my specs and everyone is happy. Customers looking for moderate power (below 500Whp) I default to the cheaper end of things but with 9:1 CR - to suit their budget, and discussions like this become interesting but trivial to them.
So many variables in an uncontroled "laboratory" means your results will mean whatever you want them to. A concrete scientific analysis would run you a few millions and serve little in terms of commercial value given the limited application.
my 2 Dollars worth.
Last edited by popadel; Dec 23, 2008 at 10:24 AM.
Dec 23, 2008, 11:53 AM
#41
Popadel, thank you for mentioning the piston shape. That is something that people like Endyn experimented (succesfully from all reports) with to control flamefront propogation and not allow the charge to have the chance to detonate.
Piston weight is also and important factor that I havent covered much. There are pistons on the market that are under 300g for an 85.5mm bore, and nearly that for the larger bores. Wiseco and JE both have options from research I have done and been forwarded. Lighter pistons go hand in hand with lighter rods. The lighter the rotating assembly the less parasitic losses in acceleration so it inherently makes more power an it reduces stresses on the crank at high rpm.
SilverNslow- 9.5:1 or 10:1 is the limit for that boost level on pumpgas from what I have personally seen. With your 57 trim you will have a cooler charge temp at a given boost level than a stock turbo but I dont see it really being able to run a bunch more boost.
Piston weight is also and important factor that I havent covered much. There are pistons on the market that are under 300g for an 85.5mm bore, and nearly that for the larger bores. Wiseco and JE both have options from research I have done and been forwarded. Lighter pistons go hand in hand with lighter rods. The lighter the rotating assembly the less parasitic losses in acceleration so it inherently makes more power an it reduces stresses on the crank at high rpm.
SilverNslow- 9.5:1 or 10:1 is the limit for that boost level on pumpgas from what I have personally seen. With your 57 trim you will have a cooler charge temp at a given boost level than a stock turbo but I dont see it really being able to run a bunch more boost.
Jan 5, 2009, 01:02 PM
#43
Some more stuff to look at
This is the 87mm long rod 10:1 piston that is going in my motor. It maintains some dish since its larger displacement and a IX head, has big valve reliefs and if you look close it has the accumulator groove cut in between the 1st and 2nd ring land. I wanted lateral gas ports but the wrist pin is so high it was apparently not wise. Total piston weight is 315g.
This is the 87mm long rod 10:1 piston that is going in my motor. It maintains some dish since its larger displacement and a IX head, has big valve reliefs and if you look close it has the accumulator groove cut in between the 1st and 2nd ring land. I wanted lateral gas ports but the wrist pin is so high it was apparently not wise. Total piston weight is 315g.
Jan 6, 2009, 03:18 PM
#45