What is reversion?
12-24-2003, 03:07 PM
#2
Guest
Posts: n/a
John, you may want to read this so you know what causes Reversion too, as you're not even close...
What is Reversion? It is simply the exhaust pulse flowing backwards momentarily during the overlap phase of the camshaft at low cycling rates. During the overlap phase the piston is pushing out the last of the exhaust gases and prior to reaching top dead center (T.D.C) the intake valve and the exhaust valve is still closing. At this point in the engine cycle both valves are in the open position. At high cycling rates the inertia of the incoming intake charge and the outgoing exhaust pulses keep the exhaust flowing in the proper direction. But at low cycling rates, as the piston is pushing out the last of the exhaust gases the intake valve opens and some of the spent exhaust charge is pushed into the intake manifold. As the piston reaches T.D.C. and begins the intake stroke the exhaust valve is still not completely closed. As a result, the piston pulls from the intake and exhaust valves simultaneously causing the exhaust gases to flow in a reverse direction. This is normally not a problem until you add water into the exhaust stream. Reversion can be severe enough to stall the engine, add water to the oil, rust the exhaust seats, etc. This effect only happens at idle speeds, but remember that during shut down the engine encounters the greatest reversion.
What is Reversion? It is simply the exhaust pulse flowing backwards momentarily during the overlap phase of the camshaft at low cycling rates. During the overlap phase the piston is pushing out the last of the exhaust gases and prior to reaching top dead center (T.D.C) the intake valve and the exhaust valve is still closing. At this point in the engine cycle both valves are in the open position. At high cycling rates the inertia of the incoming intake charge and the outgoing exhaust pulses keep the exhaust flowing in the proper direction. But at low cycling rates, as the piston is pushing out the last of the exhaust gases the intake valve opens and some of the spent exhaust charge is pushed into the intake manifold. As the piston reaches T.D.C. and begins the intake stroke the exhaust valve is still not completely closed. As a result, the piston pulls from the intake and exhaust valves simultaneously causing the exhaust gases to flow in a reverse direction. This is normally not a problem until you add water into the exhaust stream. Reversion can be severe enough to stall the engine, add water to the oil, rust the exhaust seats, etc. This effect only happens at idle speeds, but remember that during shut down the engine encounters the greatest reversion.
12-25-2003, 04:31 PM
#4
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Per definition, John was correct, reversion as a word would mean water going the opposite direction intended. This could be caused by many reasons including John's. I see he unfortunately deleted his post.
As it goes with engine building in the marine industry, reversion as a slang word means what Findme so pompously described. John is correct and to those in the marine performance field, so is Findme.
Don't let one arrogant person discourage you John. I find your approach tacky Findme. There are better ways to enlighten people than the one you chose. Typically I find your fact gathering and sharing beneficial.
Dave
As it goes with engine building in the marine industry, reversion as a slang word means what Findme so pompously described. John is correct and to those in the marine performance field, so is Findme.
Don't let one arrogant person discourage you John. I find your approach tacky Findme. There are better ways to enlighten people than the one you chose. Typically I find your fact gathering and sharing beneficial.
Dave
12-26-2003, 11:37 AM
#5
VIP Member
VIP Member
I'll try to make this as "simple as possible"
This occurs only at low engine speeds, where
the flow inertia's are low.
During the overlap phase, both valves are slightly
opened ( exhaust closing, intake opening)
being both valve's are somewhat opened,
intake manifold vacuum is allowed access
to the exhaust port.
Being there's a vacuum there now,
(we're talking momentary)
the exhaust pulse will be drawn back
toward the cylinder.
Since most marine exhaust systems
expel the cooling water through the
exhaust, the discharged coolant is
exposed to the same forces the exhaust
pulse is exposed to. ( thus, you have a chance of
water being drawn back to the exhaust port)
Then we get into the fact that water
droplets are heavier that air, inertia, blah, blah, blah.
Aside from that, I think thats a pretty simple
explanation.
jt
This occurs only at low engine speeds, where
the flow inertia's are low.
During the overlap phase, both valves are slightly
opened ( exhaust closing, intake opening)
being both valve's are somewhat opened,
intake manifold vacuum is allowed access
to the exhaust port.
Being there's a vacuum there now,
(we're talking momentary)
the exhaust pulse will be drawn back
toward the cylinder.
Since most marine exhaust systems
expel the cooling water through the
exhaust, the discharged coolant is
exposed to the same forces the exhaust
pulse is exposed to. ( thus, you have a chance of
water being drawn back to the exhaust port)
Then we get into the fact that water
droplets are heavier that air, inertia, blah, blah, blah.
Aside from that, I think thats a pretty simple
explanation.
jt
12-27-2003, 12:10 PM
#6
Registered
And for those who have had this problem and resolved it, would one say that any engine with overlap of more than 110 degrees is a potential problem?
12-27-2003, 01:35 PM
#7
Originally posted by 2112
And for those who have had this problem and resolved it, would one say that any engine with overlap of more than 110 degrees is a potential problem?
And for those who have had this problem and resolved it, would one say that any engine with overlap of more than 110 degrees is a potential problem?
12-27-2003, 02:41 PM
#8
MarineKinetics
Platinum Member
I think there may be some confusion here regarding camshaft terminology. My guess is the 110 deg referred here to is LSA, not pure overlap. In order for a profile to have 110 deg overlap the cam would have to spec out (example) @ 322 deg (seat) duration intake, 322 deg (seat) duration exhaust, ground on 106 LSA. This grind is extreme and would not be suited to a marine application. It would have a very “peaky” power band and reach max HP and torque well outside the (rpm) limits of all but max competition engines.
Because overlap is a function of both duration and LSA, both must be considered in your cam choice. As duration increases so does overlap. Also as you numerically decrease (narrow) LSA overlap will increase. 110-112 deg is the generally accepted figure (depending also upon duration) in marine cam design as the narrowest LSA you want to run to avoid reversion and keep the engine in an acceptable power band.
Hope this helps,
Bob
Because overlap is a function of both duration and LSA, both must be considered in your cam choice. As duration increases so does overlap. Also as you numerically decrease (narrow) LSA overlap will increase. 110-112 deg is the generally accepted figure (depending also upon duration) in marine cam design as the narrowest LSA you want to run to avoid reversion and keep the engine in an acceptable power band.
Hope this helps,
Bob
12-29-2003, 05:58 PM
#9
Guest
Posts: n/a
rmbuilder,
It sounds like you are quite knowledgeable with cam shafts. What is the benefit of more overlap? Is it more horsepower?
I have a 502 EFI with a Procharger. I was told not to change my cam because it was already a great cam for a blown motor. Do you or anyone else have any input on this?
It sounds like you are quite knowledgeable with cam shafts. What is the benefit of more overlap? Is it more horsepower?
I have a 502 EFI with a Procharger. I was told not to change my cam because it was already a great cam for a blown motor. Do you or anyone else have any input on this?
12-30-2003, 12:34 PM
#10
MarineKinetics
Platinum Member
Overlap is the length of time, expressed in crankshaft degrees (or area expressed in inch/degrees), that defines the period between the intake valve opening point BTDC and the exhaust valve closing point ATDC. The greater the seat duration is on the intake and exhaust lobes and/or the tighter the LSA, the greater the overlap will be in degrees. It is during this overlap period, when the intake, combustion, and exhaust areas are all exposed to one another, that the pressure differentials in the respective areas interact. In some instances if velocity and inertia are low (rpm) exhaust reversion occurs when cylinder pressure is < ambient at the exhaust area. Increasing the overlap can increase peak power, but only if the exhaust system is properly designed to scavenge the cylinder and the engine is operated in the suitable rpm range.
Forced induction changes those parameters. With the introduction of a pressurized intake charge upstream the need for inertial cylinder filling and exhaust scavenging is greatly reduced. As boost levels increase (to avoid over scavenging) overlap area is generally reduced in forced induction applications by increasing the LSA.
Overlap crossover is probably one of the most important cam events, but it is the least talked about. It’s the time during overlap when the intake and exhaust valves are off their seats at exactly the same amount of opening. (Example might be .080" for each valve lift at 4 degrees BTDC.) This 4 deg value is known as the cross over centerline. Knowing cross over centerline can be useful when making performance comparisons at specified RPM values. As a rule, crossover usually occurs plus or minus 5 degrees near TDC. Examining the point of crossover is your reference point as to the advanced or retarded position of the cam. If you need to advance or retard more than 4*, you have the wrong cam. The formula for calculating overlap can be found here: http://members.uia.net/pkelley2/Overlap.html
Hope this helps,
Bob
Forced induction changes those parameters. With the introduction of a pressurized intake charge upstream the need for inertial cylinder filling and exhaust scavenging is greatly reduced. As boost levels increase (to avoid over scavenging) overlap area is generally reduced in forced induction applications by increasing the LSA.
Overlap crossover is probably one of the most important cam events, but it is the least talked about. It’s the time during overlap when the intake and exhaust valves are off their seats at exactly the same amount of opening. (Example might be .080" for each valve lift at 4 degrees BTDC.) This 4 deg value is known as the cross over centerline. Knowing cross over centerline can be useful when making performance comparisons at specified RPM values. As a rule, crossover usually occurs plus or minus 5 degrees near TDC. Examining the point of crossover is your reference point as to the advanced or retarded position of the cam. If you need to advance or retard more than 4*, you have the wrong cam. The formula for calculating overlap can be found here: http://members.uia.net/pkelley2/Overlap.html
Hope this helps,
Bob
