Reversion

[SBC]

When I took my carb off my intake had a dark brown color to it. The spacer also had the same discoloration to it. I was told that it was caused by reversion. What is reversion and how do I fix it if this is the problem? Thanks in advance for any help.

[Awesome Bill]

This can be several different things, down low with and engine with heads, cam and carb to big, there will be a cloud of gas fumes that hang out because the air speed is almost down to nothing. I assume your saying the carb and intake manifold are like black colored or have what looks like carbon sticking to everything? Most the time your bitten by the same old same old, everything too big. It is possible your carb is acting up also. Power valve open, too rich, too big, etc. Give me a break down of what size engine, heads, cam, duration @ .050, intake spacers etc. I can pretty my get you on the right track.

[want-a-be]

Yes,...we need to know what kind of parts you're running in the engine. It's use would also be nice to know. Was the cam degreed in when it was installed? Lining the dots up on the 2 gears isn't degreeing in the cam. This and every thing Bill mentioned could be your problem.

Another thing that could be a possibility is a poor valve job. I'm betting that the closer you get to the intake valve the more it resembles your exhaust runners. How old are these heads?

Don

[SBC]

The motor is a 434 small block chevy. intake lobe was degreed to 102 which is what the cam card said to degree it to. lobe sep is 106, 276 283 @ .050, lobe lift is .4567 .4267. It has apprx. 14:1 compression, 950 carb with a 2" HVH super sucker spacer. It is used for drag racing only. The heads are brand new with 8 runs on them, they are the pro 1 platimun series 230 runner. The valve package came from dart. The color I am talking about does not look like the charcoal/ black look that you sometimes see with a rich condition. Thanks again for your help.

[want-a-be]

Are the heads off or just the carb? If the heads are off...I'd touch the seats. New heads season,...which means the seats move a bit. All new heads do this. I usually plan on freshening the heads when the engine comes back for a season freshen up.

Don

[lun40119]

Don, I had that problem with the turd if you remember, and I am not sure if it was the valve job, but it sure got better with the new heads, everything else the same.

[Awesome Bill]

There is your problem, 106 lobe center is way too tight for that much stroke. Get that out of there and put something in with the same specs just 112-114 Lobe Center. It will take care of it. Also, do you run powervalves? I don't even think about it with a race engine? Carb could be rich, did you dyno?

[lun40119]

Bill, I was talking to Steve they other day, he told me with this new engine the 106 will leave harder and make more tq. Could you go into alittle detail why you say the wider LSA will run better. I understand why the tighter LSA will make more reversion with overlap, just not how it will run better throughout the entire powerband with the wider LSA. Thanks

[Awesome Bill]

Steve is a very good tech and I know him well. I do not not why he would of put you in that cam. Without being rude, I will leave it @ that. It will not leave harder, it does not make more torque with that tight lobe sep. It may make the same in one small area that fall off like a lead sinker going to the bottom with that much crank in the engine. The wider lobe sep will flatten your torgue curve out and take away the bottome end issues your having. Lobe seps like that do not make a good torque curve with this engine. I have dyno tested the 434 more than any one in the county and have hundreds of pulls and they all like the 110-114 lobe seps and the power output proves it. You don't have to take my word of 30 years of engine building, then dyno testing them, and actually taking these cars to the track and see it run knowledge if you want to. But I am telling you for the reason I have listed, they don't like that tight lobe sep with the duration you listed. None of my engines have a cloud of fuel over the carb while they are running? Hope this helps and if you want a camshaft that works, let me know.

[lun40119]

Its not that I didn't believe you.......I was just wondering why a longer stroke engine would like a wider lsa, versus, and shorter stroke engine with a tighter lsa. Skin is getting thicker, go ahead, let me have it.....:D

[Trmnatr]

Its not that I didn't believe you.......I was just wondering why a longer stroke engine would like a wider lsa, versus, and shorter stroke engine with a tighter lsa. Skin is getting thicker, go ahead, let me have it.....:D

A tighter lobe seperation opens the valve earlier (we all can agree on that),,

and i say reversion doesnt come from this Because when you advance your cam your intake valve opens earlier so how does a early intake valve opening create more reversion ?

Reversion kills torque down low, you make more torque down low when you advance the cam so where does reversion come from ?

In My Goofy Opinion Reversion comes from the intake valve closing later because you hit BDC and as the piston starts going up your working with Airflow That Has Already Been Put Into Motion On The Intake Stroke so if the Airflow doesnt remain into motion as the piston starts traveling up the cylinder on compression stroke (this is why dynamic compression is lower than static) it pushes the air/gas mixture back into the intake port as it has lost its "interia ram" and now because the intake valve is still open with no airflow left in motion we have a power loss now and REVERSION

Anybody who says the early intake valve opening is what creates reversion needs to put a degree wheel on a camshaft and advance it 6 degrees and see what happens,, The intake valve opens 6 degrees sooner and closes 6 degrees sooner

Now being a advanced my cam 6 degrees and gained low speed torque do you think i gained reversion from the early intake valve opening or lost reversion ?

Think about it and let me know :D

[Trmnatr]

The motor is a 434 small block chevy. intake lobe was degreed to 102 which is what the cam card said to degree it to. lobe sep is 106, 276 283 @ .050, lobe lift is .4567 .4267. It has apprx. 14:1 compression, 950 carb with a 2" HVH super sucker spacer. It is used for drag racing only. The heads are brand new with 8 runs on them, they are the pro 1 platimun series 230 runner. The valve package came from dart. The color I am talking about does not look like the charcoal/ black look that you sometimes see with a rich condition. Thanks again for your help.

Is that a Holley 950HP or a custom 950 ?

First thing i would do is try a 1000hp or a 1050 Dominator

I have seen where a carburetor is too small it looks as if we have a problem of Reversion because with the smaller venturii, when you can use a larger venturii, the smaller venturii creates a restriction on the compression stroke when the intake valve is still hung open working off Airflow Already In Motion and the carburetor is choking it off as there is no signal to keep it pulling the (interia ram) Airflow Already In Motion in the engine so you have to not choke it

Your options are
1) close the intake valve sooner by advancing cam a few degrees
2) drop duration 8 degrees
3) larger venturii carburetor that is properly set up for your engine combo

[Awesome Bill]

these #'s on this cam would work very well with a 3.5-3.75 crank maybe, would most likely not be my pick but I would run it. As far as why this and that in particular, do some reading and then do some dyno testing with a real dyno not a calculator and see what happens.

We just dyno test a 434 with a set of BMF SBC CNC ported 225cc runners with 2.10 intakes and it idles @ 800 rpm with no prblems and I have never seen the fuel discoloration like his in anything I have done. The cam on this engine is 273/280@.050 with a 112 and .700 lift and is @ a 106. First pull with mufflers and no vacuum pump after getting the bugs out was 691 @ 6800 and 602lbs @ 5400. This engine will push 715-720 maybe and like I said has a big 1050 dominator which makes less power than a 950 would down low and does not have any reversion down low. I have seen engines spitting fuel right back up the carb or stand back and watch the cloud of fuel over the carb, and especially seen it with injection. Seems not to hurt them above 4000 rpm, it all goes away. But, does not mean it is correct.

That camshaft I have just posted is an Erson, get one and try it, you'll love it and most likely pick up 2 10ths if matched with the right converter and gear for tire size and rpm needed to make 650/700 hp run to its maximum in your car.

[Trmnatr]

these #'s on this cam would work very well with a 3.5-3.75 crank maybe, would most likely not be my pick but I would run it. As far as why this and that in particular, do some reading and then do some dyno testing with a real dyno not a calculator and see what happens.

We just dyno test a 434 with a set of BMF SBC CNC ported 225cc runners with 2.10 intakes and it idles @ 800 rpm with no prblems and I have never seen the fuel discoloration like his in anything I have done. The cam on this engine is 273/280@.050 with a 112 and .700 lift and is @ a 106. First pull with mufflers and no vacuum pump after getting the bugs out was 691 @ 6800 and 602lbs @ 5400. This engine will push 715-720 maybe and like I said has a big 1050 dominator which makes less power than a 950 would down low and does not have any reversion down low. I have seen engines spitting fuel right back up the carb or stand back and watch the cloud of fuel over the carb, and especially seen it with injection. Seems not to hurt them above 4000 rpm, it all goes away. But, does not mean it is correct.

That camshaft I have just posted is an Erson, get one and try it, you'll love it and most likely pick up 2 10ths if matched with the right converter and gear for tire size and rpm needed to make 650/700 hp run to its maximum in your car.

I have used camshafts 7 degrees larger than his 276-284 with the same head and less cubic inches (100ci's) and i dont have any problems with reversion

Reversion comes from the intake valve closing too late or/and too small of a venturii, a wide lobe center with the same duration is gonna make the intake valve close even later

[lun40119]

Ok......I have always been under the impression that a tighter lobe sep is going to et quicker if the car is set up right. So now I am confused........

[SBC]

Carb is a quick fuel 950.

[Trmnatr]

Ok......I have always been under the impression that a tighter lobe sep is going to et quicker if the car is set up right. So now I am confused........


Every small block i have seen, even some big blocks, will ET quicker with a tighter lobe seperation.

There are cases where the induction package doesnt restrict the airflow that already in motion on the very start of the compression stroke,, so they use wide lobe centers, they are:
- LS vette engines
- Pro Stock
- Forced induction

I would want a 1.562" venturii on that engine or a Dominator

I use a 1000hp built by myself with annular boosters on a pretty much stock bottom end and 327 -291 heads that runs 10.40's, i will list that combo later

[SBC]

How many jet sizes should I go up when switching from power valves to block offs? Thanks again.

[want-a-be]

these #'s on this cam would work very well with a 3.5-3.75 crank maybe, would most likely not be my pick but I would run it. As far as why this and that in particular, do some reading and then do some dyno testing with a real dyno not a calculator and see what happens.

We just dyno test a 434 with a set of BMF SBC CNC ported 225cc runners with 2.10 intakes and it idles @ 800 rpm with no prblems and I have never seen the fuel discoloration like his in anything I have done. The cam on this engine is 273/280@.050 with a 112 and .700 lift and is @ a 106. First pull with mufflers and no vacuum pump after getting the bugs out was 691 @ 6800 and 602lbs @ 5400. This engine will push 715-720 maybe and like I said has a big 1050 dominator which makes less power than a 950 would down low and does not have any reversion down low. I have seen engines spitting fuel right back up the carb or stand back and watch the cloud of fuel over the carb, and especially seen it with injection. Seems not to hurt them above 4000 rpm, it all goes away. But, does not mean it is correct.

That camshaft I have just posted is an Erson, get one and try it, you'll love it and most likely pick up 2 10ths if matched with the right converter and gear for tire size and rpm needed to make 650/700 hp run to its maximum in your car.

LOL Bill,...don't take this wrong but the panets must be aligned or something. Cause we agreeing to much today...lol

I totally agree with the wider lobe sepperations. 110-114. My reasoning may or may not be the same as Bill. But like Bill says,... Depending on the engines and use. I rarely use less then a 110 LSA. I think we have talked about that some Jake.

I'd much rather trim a few peak HP off the top in order to widen the torque curve by another 500 RPMs or so. Just my opinion there, but,...I'm not a huge fan of narrow torque curves with high peak numbers.

JMO

Don

[Trmnatr]

LOL Bill,...don't take this wrong but the panets must be aligned or something. Cause we agreeing to much today...lol

I totally agree with the wider lobe sepperations. 110-114. My reasoning may or may not be the same as Bill. But like Bill says,... Depending on the engines and use. I rarely use less then a 110 LSA. I think we have talked about that some Jake.

I'd much rather trim a few peak HP off the top in order to widen the torque curve by another 500 RPMs or so. Just my opinion there, but,...I'm not a huge fan of narrow torque curves with high peak numbers.

JMO

Don


A car has a 5,000rpm stall and a 6,500rpm shift point,, Whats gonna run faster down the track ? A wide powerband from a wide lobe center or a narrow powerband from a narrow lobe center in the 5,000-7,000rpm band this example will be in which by the way is most bracket engines ?

Aint this stuff fun :D

[want-a-be]

imo...1500 rpms isn't a wide power band. 2000 rpms is and up is. Which power band are you saying is in most Bracket engines? The 2k wide one is usually what I'm getting. Usually in the neighbor hood of 5300 pk TQ and 7200-7400 pk HP.

Don

[Awesome Bill]

Planets must be working. A broader torque curve with heavy cars or tighter coverters always run faster. Peak #'s mean little to nothing to me, I would much rather maximize the combination used rather than say I have 1 spot that makes peak power and that must mean I will run faster.

[Awesome Bill]

ofcourse it is in drag racing. If your shift point is 6500, it will not like the fall back under peak torque and you will always pull back up way slower until peak torque occurs in the shift changes making a very slow e.t. That is why it is important to DYNO your engine! If your shift point falls back any more than 1200 rpm, I get concerned. That is just racing.

[lun40119]

What are the "for instances" where a tight lsa would be ideal. What you two are saying goes against everything that I have ever seen. The biggest lsa I have ever run in one of my engines was a 108°. My new turd JR, will have a 106° lsa. Already ordered the cam, should be here today.

[Trmnatr]

What are the "for instances" where a tight lsa would be ideal. What you two are saying goes against everything that I have ever seen. The biggest lsa I have ever run in one of my engines was a 108°. My new turd JR, will have a 106° lsa. Already ordered the cam, should be here today.


For me, Tighter lobe seperations have always went quicker

104-106 degree for me unless its a stick with no hook when using 104-106, then 108 degree is what i use

[Trmnatr]

What are the "for instances" where a tight lsa would be ideal. What you two are saying goes against everything that I have ever seen. The biggest lsa I have ever run in one of my engines was a 108°. My new turd JR, will have a 106° lsa. Already ordered the cam, should be here today.

Read This Jake
http://www.dartheads.com/dartboard/showthread.php?t=647

[Awesome Bill]

tigher lobe seps are one thing and putting the intake centerline somewhere is something else. That is old school thinking and about 15 years back.

[Trmnatr]

I was home out sick today so i did some searching on articles the proove where reversion comes from,, I have thought this way since the first time i advanced a cam for more low end torque and got it and noticed what the IVO did

Here is the article i found in the link below along with a copy/paste of it below,,,,,,,
http://www.iskycams.com/techtips.php#2002

What Causes Intake Reversion? Once and for all, let us have the TRUTH!

With the proliferation of the Motorsports Industry over the years, many new faces have come on the scene. In the cam grinding business today, there are many younger, less experienced companies struggling for recognition of their talents and a few have turned to postulating new theories in order to attract attention. However, they are I believe unfortunately, too often guilty of shooting from the hip.

Two in particular are responsible for perpetuating the "myth" that an earlier opening of the intake valve (even by a mere 2 or 3 degrees) causes the phenomenon known as "reversion". Nothing could be further from the truth! This misconception not only defies common sense, it also establishes a false premise from which other, incorrect conclusions can be drawn. Simply put, those who focus on overlap are on the wrong end of the cam-timing diagram!

Reversion, carburetor/Injector "stand-off" or the general effect of the backing up of the intake Fuel/Air charge normally associated with longer duration high-performance camshafts is actually caused by a Later Intake Closing! How do we know this to be true? The answer lies in the basic principles of physics. For as with geometry and trigonometry, these sacred truths do not change simply because someone chooses to ignore them in an attempt to garner a reputation.

Specifically, when the intake valve opens some 40 or more degrees before T.D.C. at the end of the exhaust stroke, very little (virtually no) exhaust gases remain in the cylinder. The piston is in the vicinity of T.D.C. (only .425" down the hole @40o BTDC - on a typical 350" Chevy with 5.700" rods) and no appreciable threat is posed to the forthcoming intake charge. The "False Reversion Hypothesis" taken to an extreme would lead one to the equally false conclusion that any overlapping of the intake and exhaust valves is totally undesirable. Automotive engineers of the late 1800's and early 1900's used to think this way. They were deathly afraid of overlap, so much so they actually employed "Negative" overlap (minus 5 or 10 degrees) to be absolutely sure none would occur. What was the result? These engines were severely "throttled back" or limited to low speeds and mediocre output. [ Reference: Iskenderian's Tech Article "Cam Degreeing is Simple"] But, more progressive engineers of the early 1920's who performed "brazen experiments" with longer duration cams proved these overlap fears to be only so much "stuff and nonsense", as both power, rpm and performance were actually improved. These engineers demonstrated that overlap did not cause engines to quiver, backfire or lock-up on the spot! Although, the ignorance displayed by their predecessors is easily explained by their lack of experience, (internal combustion engine design being in it's infancy) it was none the less the result of an incorrect hypothesis.

Should you need further persuasion that reversion is not caused by earlier intake opening and the resulting extension of valve overlap, consider this: What happens when you advance any camshaft? The intake as well as the exhaust valves open earlier. Does this advancing of the cam cause more reversion? Of course not. Throttle response and torque are enhanced. Yet, if these theories were correct wouldn't the engine run more poorly, especially at lower RPM? The answer is obviously yes, and because so, these theories are invalid. A brief look at what's happening on the other end of the valve-timing diagram will tell you why.

For when a camshaft is advanced, not only do both valves open earlier but they of course also close earlier - and here in lies the key to reducing Intake Reversion. Close your intake valves earlier and any tendency for the occurrence of Reversion or the backing up of the intake charge as the piston rises on the compression stroke will be reduced. It's not complex, nor is it a mystery. And the circumstances surrounding it's occurrence have not changed. In fact any experienced mechanic could tell you as much, for, as Ed's good friend the legendary Smokey Yunick might say, "Only country smarts are required to solve the problem."

[Awesome Bill]

Yeah, even if you advance your camshaft to the moon, you will not change lobe sep. The lobe sep is set and thats it. Advancing or retarding cam timing only moves peak rpm up or down and having both valves open @ the same time and low RPM cause the pulsations threw the intake runner. This also has more to do with the exhaust than intake? I will let you think about that one a while. Not going to give everything to you rookies.

You do have to remember when you push air back up, its helping the cylinders that are pulling in? Or is it? So some over lap and revulsion could be needed. It also gets the intake air moving when the exhaust is leaving the head, hopefully fast. But to wrap it up, to much overlap with duration and slow air speed, causes revulsion.

Put the same camshaft in a small headed engine with a big crank and it goes away, shorter the stroke and ofcourse lobe profile, it tends to like more rpm. A camshaft is not just a camshaft, its it the key to the engines performance.

Every engine is different when you have now got larger heads, intake, shorter or longer stroke and rods, piston speed etc. You can not just say its one thing or another. When you have a cloud of fuel hanging around the top of the carb and it turns it black, you have problems idling around, but may perform great. Been there done that and when it is needed, I use it.

I ususllay use revulsion to my advantage when I have a too small a head and to larger of an engine. I can give it time to move as much air in and out and scavenge as well as possible being the head is small. We do it with the Oval port Steel Gm Heads on 454 and larger engines. I can see the fuel backing up when looking into the carb and have seen flames shoot out the carb when quick carb action is applied. Get the rpm up over 2500 or so and the air then starts getting in and out really good. If you don't know what your doing, your going to always have things that you have happen because usually it is applied wrong!

[Trmnatr]

Yeah, even if you advance your camshaft to the moon, you will not change lobe sep. The lobe sep is set and thats it. Advancing or retarding cam timing only moves peak rpm up or down and having both valves open @ the same time and low RPM cause the pulsations threw the intake runner. This also has more to do with the exhaust than intake? I will let you think about that one a while. Not going to give everything to you rookies.

You do have to remember when you push air back up, its helping the cylinders that are pulling in? Or is it? So some over lap and revulsion could be needed. It also gets the intake air moving when the exhaust is leaving the head, hopefully fast. But to wrap it up, to much overlap with duration and slow air speed, causes revulsion.

Put the same camshaft in a small headed engine with a big crank and it goes away, shorter the stroke and ofcourse lobe profile, it tends to like more rpm. A camshaft is not just a camshaft, its it the key to the engines performance.

Every engine is different when you have now got larger heads, intake, shorter or longer stroke and rods, piston speed etc. You can not just say its one thing or another. When you have a cloud of fuel hanging around the top of the carb and it turns it black, you have problems idling around, but may perform great. Been there done that and when it is needed, I use it.

I ususllay use revulsion to my advantage when I have a too small a head and to larger of an engine. I can give it time to move as much air in and out and scavenge as well as possible being the head is small. We do it with the Oval port Steel Gm Heads on 454 and larger engines. I can see the fuel backing up when looking into the carb and have seen flames shoot out the carb when quick carb action is applied. Get the rpm up over 2500 or so and the air then starts getting in and out really good. If you don't know what your doing, your going to always have things that you have happen because usually it is applied wrong!

Bill,

nobody said that advancing the cam changes lobe seperation, rookie. lol. Please go back and highlight where i said advacing the cam changes the lobe center. It changes the intake center line or Max Lift point

Why do you think you gain low rpm torque when you advance the cam. more or less reversion ? But when we advance the cam we open the valves sooner - thiink about it and study a degree wheel and opening/closing events

I have ran camshafts larger than most people would want, they have always went quicker down the track. I dont care what it does on the dyno, we dont race a dyno down the track, we race a car

So i guess im wrong and a CAMSHAFT grinder that has been doing this 50+ years because you say so ?

[want-a-be]

Hey SBC,...have you gotten the answer you're looking for yet, or have you found the cause of the problem?

Don

[Trmnatr]

Hey SBC,...have you gotten the answer you're looking for yet, or have you found the cause of the problem?

Don

Im sure there is every point of view ever made on reversion in this thread :D

[Awesome Bill]

never said it changes the lobe sep, you indicated by advancing the camshaft would stop the revulsion. Some cases it has nothing to do with it. And if you advance every camshaft just to advance a camshaft, what happens is you have to retard the camshaft to get it to come in @ the proper timing events, such as fastest piston speed, fastest air speed and actual cam timing events.

If you use different cam grinds, they all have to be proper sized and installed in the correct position for maximu power. Advancing a camshaft in most cases will hurt top end power and with drag cars where rpm leaving the line exceed 6000 rpm, advancing some cams will kill et. I don't race a dyno either, but it is a tool when we change something and it increases or decreases hp in our power band we are looking for we know and don't have to shoot in the dark as people like you do with the try it first and see if it works.

I have seen people like you wear your engines out as well as customers and never get the power band or af ratio correct in a years time if ever. So when you can afford to or choose to purchase a dyno or even a flow bench, which we all concede that we don't race them, but they are valuable tools for making good power.

Put your cams where you like and you think they run the best. Means nothing because you never change the camshaft @ the track to see if it goes faster. You can't tell and don't tell us you pull the timing cover to change it. That don't happen. Maybe a belt drive cam timing change is possible but I don't even see guys do that. Dynos find the power and eliminates guess work. Try one, you'll like it.