from what i gather it's a result of sequencing valve events in a certain way. i've only seen one combo that did that, and i'm wondering what exactly causes it, is it good, bad, what does it mean, etc.
thanks

 

Any and every cam grind will produce reversion at some point.

Reversion occurs as a result of a "pressure tug of war" between the intake
side of the chamber, the chamber itself, and the pressure in the exhaust port.

Think of 3 teams of bumper cars.

All three teams are driving...

Every member of each team is driving at a different speed...

Sometimes one team member will hit a valve, a catalytic, or the atmosphere and bounce back.

Sometimes that same driver will hit his own team mate and send each other
flying in the opposite direction.

Maybe the Intake team will bump the ehaust team further down the header
primary, or maybe the exhaust team will fight harder and send the Intake team
back up the intake runner ("reversion" as we know it?).

THe car with the most pressure will continue in the initial direction, but will
have lost some energy due to the collision.

At the end of the cylce (which took a blink of an eye); the team which had
the most average pressure in the direction they intended to drive, will end up
in the chamber when the valves close.

Then the big ugly piston comes up and crushes all the bumper cars.

 

That's a pretty damn good analogy.

Scroll down a bit further in this section and read the cam discussion threads. It will make your head spin, but you will learn something.

 

Reversion is when some gases are pushed back out of the intake valve... happens on cams with lots of overlap and/or late IVC events. Reversion = cam lope (one of the causes, to a certain extent)

 

good explanations.

to expand, it ultra sucks, beyond the obvoius because
1. intake charge is warmed up
2. intake charge is diluted
3. exhuast charge is still in the cylinder making the engine prone to detonation
4. exhuast scavaging loses momentium

 

yea i know, that's why i wanted a summary separate from all the other topics.

so far so good, but is reversion bad? is that just a byproduct of huge cams, or is there an actual reason why we'd want it?
all i can think of is a reason to not want it--maf's don't deal with it well.

 

you shouldn't have it going backTHAT far! its not a total filling of the intake manifold with exhuast.

basically, it happens with overlap for NA engines. the idea is that the intake charge rushes in and pushes the exhuast out, or the residuals, at the very tail end of the exhuast charge.
But at low rpm, exhuast isn't being pushed out hard and intake pressure isn't very high. So you get it pushing out the wrong end!

It can happen with boost, but its less likely. high intake manifold pressures prevent it and intake doesn't open as much durring exhuast stroke. But you're not always under boost either so it CAN happen

 

It's all bad. We don't want it, but unless we have a constant speed engine or variable lift and duration cams plus variable length intake and exhaust systems, were going to get it on a high performance engine because all those variables have been optimized for high RPM power, leaving them all wrong for low revs.

 

Yes, overlap which causes the reversion is a necessary evil. We can minimize overlap by increasing the acceleration of the valve on exhaust close and intake opening, but that forces other limits on lift or it causes excessive valvetrain loads.

Mechanically you can sometimes lessen reversion by deliberate port mismatch. IOW, if the intake manifold is a little smaller on the SSR than the head port, reverse flow or reversion hits a 'wall', but forward flow skims over it.

Similarly, 'anti-reversion' headers have a primary pipe that is bigger than the exhaust port, especially on the SSR. This may help prevent the exhaust reverting back into the port. Dave Vizard visited this in How to Build Horsepower Vol. 1 .

 

Reversion leans out the mixture because of the fuel that gets into the mainfold in the reverion,the computer than adds fuel to compensate for it and runs really rich especialy in low RPM because there is so low vacum effect in the header pipes/collectors to suck the exhaust out of the other cyllenders and allso because of backpressure,this is the cause of bad fuel milerage with cams with much overlap and you fail the emissions(This is how i understand it,correct me if i´m wrong)

 

BTW, the term also shows up in header design, referring to the fact exhaust gas can make it back into the motor.

 

RHS is on the money about MAF's not liking reversion. The problem is MAF's work well under steady state flow i.e. > 4k RPM. Below 4k the PCM "blends" VE values and MAF values to calculate air flow.

As RMP's drop, effects of reversion become greater, further disrupting the MAF values, resulting in bucking and surging.

The best tuning solution we have so far is !MAF (SD) - however, motors with larger cams will still exhibit bucking and surging within the rpm range reversion occurs most.

 

I think that in properly understanding reversion and scavenging it's important to understand the overlap region where both intake and exhaust valves are open to the combustion chamber. The main reason to have overlap, as I understand it, is that at and above a certain RPM point (varies with the cam and engine combination) the exhaust venting cycles per second become frequent enough that the pressure in the exhaust runner is less than that inside the cylinder as the exhaust valve is closing and the intake valve is beginning to open. This lowered pressure scavenges (sucks out) most of the remaining combustion byproducts that remain in the cylinder and also promote complete filling of the intake charge waiting to be transferred into the cylinder via the intake runner. All of this is good at and above the RPM transition range where scavenging begins to occur. However, it isn't a win-win situation because below this RPM range, the simultaneous opening of exhaust and intake valves cause the phenomenon already discussed called reversion where the exhaust actually backs up into the intake runners causing a diluted mixture and increase in unburned hydrocarbons and other negative things. This is one reason why factory cams have such wide lobe separation angles. If the LSA is widened, overlap (and therefore reversion and scavenging) is minimized. This improves emissions scores and can hurt peak power levels.

Steve

 

Slight tangent...

Wider LSA hurts peak numbers, but brings up the rest of the powerband, correct? Basically just makes the powerband less peaky

 

SOme things to remember:

LSA is not the primary consideration for overlap - it's EVC and IVO which define this region.

Reversion can occur above and below the tuned RPM, however once the intake
pulses become stronger, reversion is less of an issue.

Aftre the negative and positive pulses mix, the resulting pressure wave will normally
favour the intake charge. It is very difficult (impossible) to have exact phase
between exhaust and intake pressure waves over the entire engine
operating band.