Reversion ?

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Lasher

Reversion ?

Post by Lasher »

There seems to be somewhat of a controversy in what causes " Reversion ".
There are those that say that the earlier one opens the intake valve , then any exhaust will find it's way into the intake track.... Either by pressure within the cylinder or by a higher than atmospheric pressure within the exhaust port or haveing a vacuum within the intake tract....

Others say that there is little pressure left within the cylinder and opening the valve early will cause no problems and it's a late intake closing after the crank has dwelled @ BDC and started to push the rod/piston up on the compression stroke while the intake is still open that is the main cause of Reversion.

Advancing the cam will cause the intake to open sooner thus exposing the intake to more reversion....but this generally seems to increase low to mid range torque....so does the intake closing earlier crate more torque than the reversion causes a hinderence or is this reversion a higher RPM problem only.....if it is actually caused by an early opening intake ?

If the reversion is indeed exhaust air being introduced into the intake tract it would seem that some deposits would be left in the intake tract. I have seen only two examples of this light brown deposit being left on and intake manifolds runners......both oval track engines......Yet my street car has a mild mechanical cam with cast iron ex manifolds and a full exhaust system and it also show no evidence of any ex deposits anywhere in the intake tract....

Any ideas or comments ?

Thank You...
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Stock eliminator drag cars

Post by speedtalk »

Lasher wrote:If the reversion is indeed exhaust air being introduced into the intake tract it would seem that some deposits would be left in the intake tract. I have seen only two examples of this light brown deposit being left on and intake manifolds runners......both oval track engines......Yet my street car has a mild mechanical cam with cast iron ex manifolds and a full exhaust system and it also show no evidence of any ex deposits anywhere in the intake tract....
I see it a lot on Stock Eliminator drag car engines - stock induction system, low lift, large duration, normal lobe sep and headers.
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Lasher...

Post by Lasher... »

Thanks Don....

Ya those NHRA stockers are not exactly your " Normal " cams .... Especiallly the dwell type nearly square lobe designs........

It's kinda of weird thing...... A buddy ran several classes of Comp Eliminator with cams from 311 to 332 deg @ .020 ...... When he ran injectors the smaller 311 would create a large amount of standoff at idle but showed no eveidence of deposits ..... when he switched back to carbs with several diffferent cam and intakes.... ( 1X750 , 1 X 830 , 2 /660s , 2 / 750's / 2 X 1050 's , 1x1150 ) none had any deposits.....

The only two that I have ever really seen were both Ford Clevelands 4v heads in oval track racing...... Large heads , single plane and 280/290 @ .020 type oval flat tappet cams..... with the need to be into and somewhat out of the throttle..... perhaps raising manifold vac to pull any leftover ex back into the intake ?............. Yet I've seen other OT engine with no deposits in the bowls ,ports , runners or plenum....

I'm afraid I have never really been around many NHRA Stocker guys tho....

I used to run a small Holley 390 CFM on a 331 with a small flat tappet single pattern cam..... After 10 solid years ther wasnt even any fuel stain in the manifold.....it was spotless.....

This reversion concept is an odd one to me...... The theories say different things and the evidence on the parts show something else..... Still confused..... Do appreciate the info.....
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Post by speedtalk »

I remember the first time I saw fuel standoff - I couldn't believe the engine would even run like that.

I just finished an interview with Jim McFarland on intake manifolds and he talked about what goes on inside. He also showed some interest in coming on again and talking about combustion. It would be great to get his thoughts on the subject. The first part of his manifold interview is this Friday. Check it out.
https://www.speed-talk.com/broadcast/index.html

The interview I did with Jere Stahl had an interesting conversation about scavenging.
https://www.speed-talk.com/shows/012_jere_stahl.html

There doesn't seem to be one thing to point to say this is the problem.

I think we can both agree though, it can't be good for performance.
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Lasher

Post by Lasher »

Thanks again Don....

Just got off work.... 15 hour day today....

The interview with Jere Stahl is one I would like to get from you ....I'll EMail you later....

I dont know what he says but I am thus far unconvinced that the scavaging that everyone talks about is as abundant as they make it seem....

I think that there could be this effect within a somewhat narrow RPM/Load range....But certinally not at all RPM ranges..... I would " Guess " that the intake and exhaust would both have to be down to atmospheric pressure in order just to reach equalibrium... and then the exhaust stream to pull a vacuum it'self to help purge the chambers..... But to actually pull the intake charge through...... Especially if the chamber is ...well some chambers are better than others.....

I've read alot of the theory of refelcted negative waves timed to reach the cylinder during overlap..... And again seems like it would have to be load sensitive ..... Can this scavaging work at an idle ? .... If the intake is pulling a vacuum and the exhaust stream is low in volume due to the idle condition...IE throttles closed.... Seems hard to belive that in this situation the exh would have enough momentum to overcome the intake manifold vac and actually pull air through from the intake ports ,through the chamber and out the exh ?..... Perhaps in a peak power situation under heavy load.... in maybe a limited RPM range with the engine accelerating.....But I still have dificulty beliving it is doing this at all RPM / load ranges.... others belive that it does do that.... I dont honestly know for sure....But I'm sceptical..... I hope I'm not coming across as rude.....I dont mean to be.

I cant say as I understand how it all works but then again thats one of the reasons I'm here.... to try and learn...

I certinally dont mean to be ignorant as to the hows and whys of this subject but sometimes it doesnt completly make sence to me....So when given the chance to bounce these ideas around I useually will do so.... All in the attempt to learn.... :D
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Post by speedtalk »

Lasher wrote:I dont know what he says but I am thus far unconvinced that the scavaging that everyone talks about is as abundant as they make it seem....
That's exactly what Stahl thinks
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Post by Guest »

speedtalk wrote:
Lasher wrote:I dont know what he says but I am thus far unconvinced that the scavaging that everyone talks about is as abundant as they make it seem....
That's exactly what Stahl thinks

Really !!?? .... And from a header manufacturer.... Hmmmm thats intresting...... Well he certinally has alot of people who disagree with him !

Gonna have to get that interview ! This should be informative .
Lasher

Post by Lasher »

whoops...forgot to put my screen name in the above post...it's mine. Sorry my bad.
Shaun Tiede

Reversion

Post by Shaun Tiede »

Reversion comes from more than just opening and closing points. It also comes from bounce. Consider the strength of a reversion during the compression stroke after the valve has closed and the cylinder is begining to pressurize. Springs can help, obviously, but the bounce created by a high negative nose decelleration and/or a high seating velocity will also put a reversion in the port that will delay cylinder filling on the next intake cycle. In my opinion, there is no use for a cam that requires 375+lbs seat pressures to control this in a N/A engine. The closing point of the intake valve has a lot to do with reversion too. We all know the later it closes, the more cylinder pressure/cranking compression will be kissed goodbye. The later intake closing hurts pressure more so than the earlier opening. Don't forget the rod/stroke ratio and its scenario. If you create enough velocity by minimizing reversion, you build more inertia ram. That inertia ram helps to reduce the reversion you get from a late closing. The UltraDyne cam takes this issue into consideration. Incidently, I can get all of the UltraDyne's anyone wants.
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Post by Lasher... »

Thanks Shaun,

Shaun may I ask you a question slightly off topic here ? Harold had mentioned that prior to deciding to go to work for Lunati that there was talk of selling UltraDyne to one or more of the employees . Do you know anything about that or what happened ? I dont mean to be nosey or anything but was just curious if that ever happened ?
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Post by Guest »

Here is one of the views someone had posted about reversion on another board. This was in discussing many of the late model EFI grinds where folks feel get wrapped up in wide LSAs rather than looking at valve events, then advance the cam, and wonder why it surges when they have a bigger cam in the car.

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Simply put, on an N/A motor the intake aircharge is not assisted. (leaving wave dynamics of the aircharge out for a moment). After the combustion stroke there is tremendous pressure in the cylinder. As soon as the exhaust valve cracks open it flows a LOT of air. It's basically boosted out of the cylinder if you want to look at it like this. Having the exhaust valve open too early not only costs heat (power) and velocity through the exhaust runners, it also empties the cylinder before the intake valve is open enough to take advantage of the pressure differential. (in a limited overlap/smogable camshaft this is especially true) This causes exhaust reversion and is one of the key factors in surging problems. By the airflow reversing course it is loosing a lot of it's inertia. Typically this is overcome before peak torque however. So only low-speed issues are present. At the track these motors are always above 4500rpm so this does not affect track times too much. Stilll....there is significant power lost by allowing reversion. So it makes sense to open the exhaust valve a little later and increase the overlap a bit. By adding advance into the camshaft this makes the problem even worse as now you're opening the exhaust a few more degrees earlier..... and shortening the effectiveness of the intake unless you have significant overlap flow to over come this.
Simply put, advancing a cam makes it more exhaust bias relative to TDC. Retarding a cam makes it more intake bias relative to TDC.
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Post by Guest »

Anti-reversion headers were found to help engines with high overlap that needed a wide power band. As most race engines gear around this it isn't much of an issue with a typical racing engine, but for those who want to street a very high output engine, there may be some assistance by going to this header design.

They seemed to have a short time in the limelight - presumably because they were complicated to make and customers did not want to pay the extra cost. I've seen one or two header experts "claim" they didn't work - of course they didn't make them either - but I've seen reliable evidence by others who tested them properly that was conclusive they DID reduce reversion.

Although they do not add any power on the top end, the improvement in breathing on the bottom end and mid-range may result in a slight increase in torque in the lower end of the race power band - and that could be helpful.

for a street engine - I still think it is worthwhile for big cams as it allows the engine to pull full load a full 1000rpm earlier than without A/R coned headers, run better vacuum etc - and all this presumably due to less reversion. It's worth a try, and one application I'm keen to try one day is AR headers in a turbo application as reducing reversion will theorectically help turbine speeds... yet to experiment on this
Denzil

Post by Denzil »

Anonymous wrote:
speedtalk wrote:
Lasher wrote:I dont know what he says but I am thus far unconvinced that the scavaging that everyone talks about is as abundant as they make it seem....
That's exactly what Stahl thinks

Really !!?? .... And from a header manufacturer.... Hmmmm thats intresting...... Well he certinally has alot of people who disagree with him !

Gonna have to get that interview ! This should be informative .
Yup. Mr. Stahl will talk your ear off about the subject.
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