OMP and boost. A potential problem.
05-22-2007, 04:57 PM
#1
OMP and boost. A potential problem.
I think I may have just figured out something about the oil metering system that may be of some importance - especially to those running FI.
The metering jets have an air line attached to them that allows outside air pressure in to the back of the nozzles. The nozzles themselves feed into the combustion chamber around the same time as the fuel injectors, which means under normal operation (which is to say on an N/A motor), the pressure differential is always positive toward the combustion chamber. The air pressure is always higher on the outside than the inside since the pressure in the chamber at that point is always at atmospheric pressure or below (really, it is almost always in a vacuum to some extent).
The MOP is calibrated for this situation and delivers oil at a pressure relative to this difference.
Now, what happens when the chamber is pressurized by a turbo or a supercharger?
Well, since the air lines to the back of the nozzles are plumbed to the intake, the pressure differential is now reversed.
Two things can arise from this:
This is bad.
A check valve on this air hose will probably eliminate the return of oil to the intake, but it will not completely address the pressure differential.
I think simply plumbing this line to the manifold would neutralize the negative differential, but I have yet to try that and to see the effect.
The metering jets have an air line attached to them that allows outside air pressure in to the back of the nozzles. The nozzles themselves feed into the combustion chamber around the same time as the fuel injectors, which means under normal operation (which is to say on an N/A motor), the pressure differential is always positive toward the combustion chamber. The air pressure is always higher on the outside than the inside since the pressure in the chamber at that point is always at atmospheric pressure or below (really, it is almost always in a vacuum to some extent).
The MOP is calibrated for this situation and delivers oil at a pressure relative to this difference.
Now, what happens when the chamber is pressurized by a turbo or a supercharger?
Well, since the air lines to the back of the nozzles are plumbed to the intake, the pressure differential is now reversed.
Two things can arise from this:
1) oil can be forced back into the intake, and/or;
2) the rate of oil delivery is reduced or, potentially, completely stopped!
I don't know what the actual spray pressure is at the nozzles, but you can be sure that you can subtract the chamber pressure from whatever the actual number is. Since the oil volume is really very low to begin with (something on the order of 250:1), lowering the delivery pressure even a tiny bit would have (I think) an enormous impact on the quantity of oil that actually makes it to the combustion chamber.2) the rate of oil delivery is reduced or, potentially, completely stopped!
This is bad.
A check valve on this air hose will probably eliminate the return of oil to the intake, but it will not completely address the pressure differential.
I think simply plumbing this line to the manifold would neutralize the negative differential, but I have yet to try that and to see the effect.
05-22-2007, 05:49 PM
#5
Mazsport.net
Join Date: Mar 2005
Location: In the shop
Posts: 753
Likes: 0
Received 0 Likes
on
0 Posts
Dont worry Jeff there are check valves built into the "bolts,connector" that attatch the OMP lines to the rotor housings. They do fail, at least the older ones did, I use one of the check valves we sell in that line on turbo installs and motor jobs as a backup. On some of the RX-7s Mazda allowed that tube to be routed to intake duct as if their intention was to have the oil stop and not push the oil back down the tubes but not allow the pressures to equalize and the OMP's VERY slight pressure to continue to inject oil. I have some of my own thoughts as too why, but nothing from Mazda.
05-22-2007, 05:59 PM
#6
Where is the check valve?
I have the entire assembly in my hand (well, its on the bench as I type) and applying positive pressure to any of the nozzles produces air escaping from the tube that goes to the intake.
Is there supposed to be a check valve integrated into the air distribution block or the nozzles themselves?
I have the entire assembly in my hand (well, its on the bench as I type) and applying positive pressure to any of the nozzles produces air escaping from the tube that goes to the intake.
Is there supposed to be a check valve integrated into the air distribution block or the nozzles themselves?
05-22-2007, 06:03 PM
#8
Mazsport.net
Join Date: Mar 2005
Location: In the shop
Posts: 753
Likes: 0
Received 0 Likes
on
0 Posts
Where is the check valve?
I have the entire assembly in my hand (well, its on the bench as I type) and applying positive pressure to any of the nozzles produces air escaping from the tube that goes to the intake.
Is there supposed to be a check valve integrated into the air distribution block or the nozzles themselves?
I have the entire assembly in my hand (well, its on the bench as I type) and applying positive pressure to any of the nozzles produces air escaping from the tube that goes to the intake.
Is there supposed to be a check valve integrated into the air distribution block or the nozzles themselves?
05-22-2007, 07:02 PM
#10
Registered
Unless something has changed the one way valves are built into the oil injectors themselves. You can blow one way through them but not the other.
EDIT: I see Scott answered that one already.
EDIT: I see Scott answered that one already.
05-22-2007, 07:35 PM
#12
Registered User
The application of barometric pressure to the top of the oil injector prevents negative pressure to the oil inlet(ie stops oil being sucke in by engine vacuum,and the check valve is to prevent oil from flowing into the air hose.
Oil pressure to the oil injectors is basic engine oil pressure less losses in the metering process.I don't have a clue whatthe losses are
Oil pressure to the oil injectors is basic engine oil pressure less losses in the metering process.I don't have a clue whatthe losses are
05-22-2007, 09:08 PM
#18
Registered User
I was confused!
I was confused(it happens sometimes when you get to 63)but it is not a ball check valve.I have attached a picture.Hopefully this is clear enough to see.
05-23-2007, 09:03 AM
#19
Ok, this has made me very weary. I have a turbo on my car and I dont have and will not have premix anytime soon.
I though about this all night yesterday, analyzed everyone's posts on the matter, especially that last picture and I came up with a suggestion:
Wouldn't plumbing this line to the intake pipe, just before it plumbs to the throttle body(like the injector air hose which is plumbed there), solve the problem? The combustion chamber would always have an equal or lower pressure(mostly lower) than that pipe making sure that the checkvalves arent overworked(more like barely used). When you let off the throttle, the BOV oppens letting boost out of ths pipe but the throttle is already closed meaning that there is already a vacuum inside the chamber which means the flow would be toward the chamber and not the oposite. When you are under boost the pressure just before the throttle would always be higher than that of the chamber simply because the flow is not perfert through the intake manifold and the chamber wouldnt ever reach the same pressure as before the throttle body because the port would close before this happened.
I can understand that if you plumb it to the intake pipe/filter pipe before the turbo, like the greddy instructions suggest, that when under boost the pressure in the chamber would be way higher than that of that pipe therefore relying on the checkvalves to keep you oil from getting sucked out to the intake hose and possibly(likely) starving the engine of oil if they fail. But if you plumb it to where I'm suggesting, even if your checkvalves went bad the pressure differential would be mantained and your engine wouldnt starve of oil.
I sat down and did a couple of diagrams trying to encompass every possible scenario of boost and vacuum vs chamber pressure(not real pressures, just vage estimates) with the hose plummed in different places and this is the only place where the pressure differential would alwways be mantained. I will try this on my car and see what happens but I'm pretty sure it should solve the problem.
Please comment on my logic, I'm all ears....MM?RG?Mazsport? anyone?
Thanks in advance....
Chris....Esmeril
I though about this all night yesterday, analyzed everyone's posts on the matter, especially that last picture and I came up with a suggestion:
Wouldn't plumbing this line to the intake pipe, just before it plumbs to the throttle body(like the injector air hose which is plumbed there), solve the problem? The combustion chamber would always have an equal or lower pressure(mostly lower) than that pipe making sure that the checkvalves arent overworked(more like barely used). When you let off the throttle, the BOV oppens letting boost out of ths pipe but the throttle is already closed meaning that there is already a vacuum inside the chamber which means the flow would be toward the chamber and not the oposite. When you are under boost the pressure just before the throttle would always be higher than that of the chamber simply because the flow is not perfert through the intake manifold and the chamber wouldnt ever reach the same pressure as before the throttle body because the port would close before this happened.
I can understand that if you plumb it to the intake pipe/filter pipe before the turbo, like the greddy instructions suggest, that when under boost the pressure in the chamber would be way higher than that of that pipe therefore relying on the checkvalves to keep you oil from getting sucked out to the intake hose and possibly(likely) starving the engine of oil if they fail. But if you plumb it to where I'm suggesting, even if your checkvalves went bad the pressure differential would be mantained and your engine wouldnt starve of oil.
I sat down and did a couple of diagrams trying to encompass every possible scenario of boost and vacuum vs chamber pressure(not real pressures, just vage estimates) with the hose plummed in different places and this is the only place where the pressure differential would alwways be mantained. I will try this on my car and see what happens but I'm pretty sure it should solve the problem.
Please comment on my logic, I'm all ears....MM?RG?Mazsport? anyone?
Thanks in advance....
Chris....Esmeril
Last edited by ChrisRX8PR; 05-23-2007 at 09:06 AM.
05-23-2007, 09:48 AM
#20
Or, possibly better, plumb it to the compressor outlet pipe? The compressor outlet, even under boost, should be a couple PSI higher than the manifold pressure.
Question: Is the oil actually under pressure? It sounds kind of like the OMP prevents the engine oil pressure from creating flow and merely allows X oil to pass under very little pressure. Then, the velocity of the air going into the engine draws oil out ala an oil carburetor.
Question: Is the oil actually under pressure? It sounds kind of like the OMP prevents the engine oil pressure from creating flow and merely allows X oil to pass under very little pressure. Then, the velocity of the air going into the engine draws oil out ala an oil carburetor.
Last edited by maxxdamigz; 05-23-2007 at 09:51 AM.
05-23-2007, 11:57 AM
#21
I think that is where tuning comes in. If you're running 80 psi fuel and 10 psi boost, you probably get 70 psi effective spray but you just up your pulse duration a bit and tune around it. If you tune for A/F, the fuel pressure is already taken care of. Do highly boosted engines (20+ psi) running higher fuel pressures to compensate? Or is increased pulse duration, injector flow adequate? A base pressure differential is probably required for the proper spray pattern.
05-23-2007, 01:12 PM
#22
Scott gets around it by using a return fuel system with a 1:1 regulator.
That is the right way to do it.
I just tune for final A/F. So far, pulse durations have never been far off of calculated values, even at elevated boost levels.
At some point, just for peace of mind, I am going to go to a conventional return fuel system with a beefier in-tank pump and a 1:1 regulator.
Plumbing the oil nozzles to the nipples on the lower intake manifold would be the best place for it.
However, as noted by Scott and RG, there is supposed to be a check valve in the oil nozzles, eliminating the need for that.
The question is, why aren't my nozzles "checked"
Actually, I went back and played with all 8 nozzles from my first two motors.
One or two of them would "check" after I sprayed carb cleaner in them.
I suspect that they are gummed. That isn't good, either.
An easy check for anyone that is worried about this is to simply use a vacuum pump and hook it up to the hose that goes to the entire nozzle assembly. If it holds a vacuum, your nozzles are OK.
If it doesn't, you then need to get to the individual nozzles and find which ones are leaking. They are pretty well buried under the intake manifold.
That is the right way to do it.
I just tune for final A/F. So far, pulse durations have never been far off of calculated values, even at elevated boost levels.
At some point, just for peace of mind, I am going to go to a conventional return fuel system with a beefier in-tank pump and a 1:1 regulator.
Plumbing the oil nozzles to the nipples on the lower intake manifold would be the best place for it.
However, as noted by Scott and RG, there is supposed to be a check valve in the oil nozzles, eliminating the need for that.
The question is, why aren't my nozzles "checked"
Actually, I went back and played with all 8 nozzles from my first two motors.
One or two of them would "check" after I sprayed carb cleaner in them.
I suspect that they are gummed. That isn't good, either.
An easy check for anyone that is worried about this is to simply use a vacuum pump and hook it up to the hose that goes to the entire nozzle assembly. If it holds a vacuum, your nozzles are OK.
If it doesn't, you then need to get to the individual nozzles and find which ones are leaking. They are pretty well buried under the intake manifold.
05-23-2007, 02:23 PM
#23
MM,
I know you probably have a logical reason for saying that plumbing after the throttle body is better than on the pipe before it(from the Intercooler) which was my susgestion. I would like to understand the reason you would choose to plumb the hose to the intake manifold instead of where I was suggesting. Thanks in advance.
Chris....Esmeril
I know you probably have a logical reason for saying that plumbing after the throttle body is better than on the pipe before it(from the Intercooler) which was my susgestion. I would like to understand the reason you would choose to plumb the hose to the intake manifold instead of where I was suggesting. Thanks in advance.
Chris....Esmeril
05-23-2007, 02:37 PM
#24
Because the pressure at the IM is exactly the same as the pressure in the chamber.
Before the TB (anywhere in the charge pipe after the turbo), the pressure is higher and is often in boost while the manifold is still in vacuum.
The idea is to eliminate any pressure differential.
Before the TB (anywhere in the charge pipe after the turbo), the pressure is higher and is often in boost while the manifold is still in vacuum.
The idea is to eliminate any pressure differential.
05-23-2007, 02:59 PM
#25
I see. That makes sense. I was mistaken in thinking that a positive pressure differential going towards the combustion chamber was desired but now I get it. Alright, I guess I'll be installing a large nipple into my manifold after the throttle body and see how it works. Thanks for the help.
Chris
Chris