Overlap in cams is good for FI cars
02-09-2005, 05:16 AM
#32
1.0 BAR
Join Date: Jan 2005
Posts: 278
Re:Overlap in cams is good for FI cars
Originally Posted by jagojon3
Engloid is the master of stuff. And he knows ---- too. Ya crazy *** all knowing welding muthafucka
02-09-2005, 06:52 AM
#33
1.0 BAR
Join Date: Oct 2004
Posts: 277
Re:Overlap in cams is good for FI cars
Well, im not any master on this kind of thing, so i can just look at it and give my idea or perspective of things.
Im not sure how much it has to do with overlap as it has to do with the timeing of the exhaust valve.
Since its a turbo motor, and the exhuast gasses velocity are what cause the turbo to spool, than buy opening the exhuast valve eirlyer or later you change the amount of work the engine has to do to get the turbo moveing.
In a natrually asperated motor, the cam opens the valve right at the pressure drop, somewhere halfway down the combustion stroke, depending on your motors r/s ratio and other things. The valve opens slightly into the pressure front that moves the piston. This causes exhuast gasses to rush out the exhuast, and as the intake opens since the gas is flowing (and objets in motion tend to stay in motion) the piston pumps gasses out the exhuast and pulls them from the intake valve.
So this lets me belive that if you open the exhuast valve later than you have all these hot gasses that are pent up in the cylinder that dont do any work, because there not quite enough velocity to move a piston/rods/crank/drivetrane/your car/, but yet still have the heat and mass required to spool the turbo.
Im not sure how you went about tuneing the thing so its hard sayin rather is overlap that created the power, or the timeing of the exhuast or intake valves that did it. Next time you are doing this, maybe you should try to adjust the exhuast only. See what it does, than try the intake.
Thats how ive allwasy done it on my motorcycles. Anyways just my $.02
Sean
Im not sure how much it has to do with overlap as it has to do with the timeing of the exhaust valve.
Since its a turbo motor, and the exhuast gasses velocity are what cause the turbo to spool, than buy opening the exhuast valve eirlyer or later you change the amount of work the engine has to do to get the turbo moveing.
In a natrually asperated motor, the cam opens the valve right at the pressure drop, somewhere halfway down the combustion stroke, depending on your motors r/s ratio and other things. The valve opens slightly into the pressure front that moves the piston. This causes exhuast gasses to rush out the exhuast, and as the intake opens since the gas is flowing (and objets in motion tend to stay in motion) the piston pumps gasses out the exhuast and pulls them from the intake valve.
So this lets me belive that if you open the exhuast valve later than you have all these hot gasses that are pent up in the cylinder that dont do any work, because there not quite enough velocity to move a piston/rods/crank/drivetrane/your car/, but yet still have the heat and mass required to spool the turbo.
Im not sure how you went about tuneing the thing so its hard sayin rather is overlap that created the power, or the timeing of the exhuast or intake valves that did it. Next time you are doing this, maybe you should try to adjust the exhuast only. See what it does, than try the intake.
Thats how ive allwasy done it on my motorcycles. Anyways just my $.02
Sean
02-09-2005, 11:16 AM
#35
1.0 BAR
Join Date: May 2003
Posts: 326
Re:Overlap in cams is good for FI cars
This is making a generalized statement. I bet in the cars dynoed the intake pressure (boost) to turbine inlet pressure ratio was higher then 1. This would cause scavenging as in NA. If he was running a smaller more restrictive turbo like many stock FI cars, I bet the overlap would cause a reduction in power because the turbine pressure would be higher then the intake manifold pressure.
Cool observation though. I think a set of cam gears would be beneficial in either case.
Cool observation though. I think a set of cam gears would be beneficial in either case.
02-09-2005, 01:25 PM
#37
3.0 BAR
Join Date: Jan 2003
Posts: 7,082
Re:Overlap in cams is good for FI cars
sure, overlap spools the turbo better, but without knowing the rpm of your turbine and the rpm abilities of your turbo you can't tell me I'm wrong saying that it is possible to oversping your turbo and cause problems.
if we're not working with diesel engines then the air going into the combustion chamber is mixed with fuel, so if you're blowing a bunch of that air out with too much overlap you're afr readings will be much richer than they really are inside the cc, so inside the cc you're likely to lean out and blow ---- up.
I'm not trying to say overlap is shitty, since we have a dyno graph at the top proving it can be beneficial, but you can't say that you need all kinds of overlap and the more the better. the point I'm trying to say is don't go overboard with it, find the point when the benefits are highest and the draw backs are minimal, that's the point where the most power will be made.
if we're not working with diesel engines then the air going into the combustion chamber is mixed with fuel, so if you're blowing a bunch of that air out with too much overlap you're afr readings will be much richer than they really are inside the cc, so inside the cc you're likely to lean out and blow ---- up.
I'm not trying to say overlap is shitty, since we have a dyno graph at the top proving it can be beneficial, but you can't say that you need all kinds of overlap and the more the better. the point I'm trying to say is don't go overboard with it, find the point when the benefits are highest and the draw backs are minimal, that's the point where the most power will be made.
02-09-2005, 03:58 PM
#38
1.5 BAR
Join Date: Sep 2003
Posts: 708
Re:Overlap in cams is good for FI cars
Originally Posted by Kyle
if we're not working with diesel engines then the air going into the combustion chamber is mixed with fuel, so if you're blowing a bunch of that air out with too much overlap you're afr readings will be much richer than they really are inside the cc, so inside the cc you're likely to lean out and blow ---- up.
"The wideband air/fuel ratio sensor combines an oxygen-sensing "Nernst" cell from the narrow band sensor with an "oxygen pump" to create a device that gives a wide range response to various air/fuel ratios. The Nernst cell senses exhaust gas oxygen in the same as a conventional narrow band O2 sensor. If there is a difference in oxygen levels across the ZrO2 sensor element, current flows from one side to the other and produces a voltage."
and again, we're not talking horribly long durations. even at idle, and I'm going to be lazy here and not calculate at all (hopefully engloid has a formula that he can whip out), 5° of overlap out of 360° is really not going to throw an impressive amount of fuel out of the engine and into the turbo. on top of allowing the compressor side of the turbo to spin without as much backpressure, and of the added velocity hitting the exhaust wheel, think about the cooling effects the timing will have on the cylinder itself. less chance for hot spots, less chance for preignition.
02-09-2005, 04:47 PM
#39
3.0 BAR
Join Date: Jan 2003
Posts: 7,082
Re:Overlap in cams is good for FI cars
Originally Posted by warrick
Edit: found a better quote to describe how a wideband functions
"The wideband air/fuel ratio sensor combines an oxygen-sensing "Nernst" cell from the narrow band sensor with an "oxygen pump" to create a device that gives a wide range response to various air/fuel ratios. The Nernst cell senses exhaust gas oxygen in the same as a conventional narrow band O2 sensor. If there is a difference in oxygen levels across the ZrO2 sensor element, current flows from one side to the other and produces a voltage."
and again, we're not talking horribly long durations. even at idle, and I'm going to be lazy here and not calculate at all (hopefully engloid has a formula that he can whip out), 5° of overlap out of 360° is really not going to throw an impressive amount of fuel out of the engine and into the turbo. on top of allowing the compressor side of the turbo to spin without as much backpressure, and of the added velocity hitting the exhaust wheel, think about the cooling effects the timing will have on the cylinder itself. less chance for hot spots, less chance for preignition.
"The wideband air/fuel ratio sensor combines an oxygen-sensing "Nernst" cell from the narrow band sensor with an "oxygen pump" to create a device that gives a wide range response to various air/fuel ratios. The Nernst cell senses exhaust gas oxygen in the same as a conventional narrow band O2 sensor. If there is a difference in oxygen levels across the ZrO2 sensor element, current flows from one side to the other and produces a voltage."
and again, we're not talking horribly long durations. even at idle, and I'm going to be lazy here and not calculate at all (hopefully engloid has a formula that he can whip out), 5° of overlap out of 360° is really not going to throw an impressive amount of fuel out of the engine and into the turbo. on top of allowing the compressor side of the turbo to spin without as much backpressure, and of the added velocity hitting the exhaust wheel, think about the cooling effects the timing will have on the cylinder itself. less chance for hot spots, less chance for preignition.
02-09-2005, 05:03 PM
#40
1.5 BAR
Join Date: Sep 2003
Posts: 708
Re:Overlap in cams is good for FI cars
sorry, I got side tracked in thought.
the cam timing changes, dialing in the overlap as opposed to zeroing it out, will allow the cylinders to completely evacuate exhaust gas. having both intake and exhaust valves open at the same time will allow the fresh (charged) air to give a bit (not much, but still something) of cooling to the cylinder.
the cam timing changes, dialing in the overlap as opposed to zeroing it out, will allow the cylinders to completely evacuate exhaust gas. having both intake and exhaust valves open at the same time will allow the fresh (charged) air to give a bit (not much, but still something) of cooling to the cylinder.
