Fuel stand-off - intake reversion - what are the causes?
Fuel stand-off - intake reversion - what are the causes?
(OP)
A while back, I posted some questions related to a v-twin I am working on.
Here is another puzzling one. The engine is a v-twin, dual overhead cam, 4 valve configuration - designed from a clean sheet of paper - not really a copy of anything.
The motor has severe intake reversion that comes on at around 4-5k rpm. At the same time, the engine will not make peak power past low 5k rpm.
Things tried have been:
1. Cam timing and duration changes - we have been all over the map with possible changes and combinations and the engine does not really respond positively or negatively to cam changes.
2. Intake manifold configs: dual IR runners, one carb to divide to either cylinder, 48mm and 56mm throat carbs. All had reversion.
Is it safe to say that the engine will not be able to realize its power potential unless the reversion goes away? Also, what other factors could contribute to reversion besides cams and intake systems? Really look forward to some experienced opinions on this topic.
Thanks
Here is another puzzling one. The engine is a v-twin, dual overhead cam, 4 valve configuration - designed from a clean sheet of paper - not really a copy of anything.
The motor has severe intake reversion that comes on at around 4-5k rpm. At the same time, the engine will not make peak power past low 5k rpm.
Things tried have been:
1. Cam timing and duration changes - we have been all over the map with possible changes and combinations and the engine does not really respond positively or negatively to cam changes.
2. Intake manifold configs: dual IR runners, one carb to divide to either cylinder, 48mm and 56mm throat carbs. All had reversion.
Is it safe to say that the engine will not be able to realize its power potential unless the reversion goes away? Also, what other factors could contribute to reversion besides cams and intake systems? Really look forward to some experienced opinions on this topic.
Thanks





RE: Fuel stand-off - intake reversion - what are the causes?
Intake runner cross sectional area.
Intake plenum volume.
Design and shape of bell mouth to runner.
Throttle plate position and size.
Intake valve closing point.
Valve springs and cam closing ramps and their effect on valve bounce.
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
My experience, which is motorcycle engines with a separate carb / throttle body per cylinder, is that you will *always* have some degree of flow reversion, particularly at low revs. If the cam timing is long enough for the engine to make decent power at high revs then it will be long enough so that at lower revs it fills the cylinder first and then pushes some of it back out. If you run the engine without the airbox you can often see a mist of fuel being pushed back out.
If your problem develops at high revs and full load, it sounds like there is some sort of resonance developing between the intake runner and the cylinder (acting like a Helmholtz resonator). I can only see that situation happening if your intake ports or intake runners are WAY WAY too big and/or too short. You need to keep the velocity up so that the column of air stuffs the cylinder full but doesn't let any of it back out when the engine is in its powerband. I've never seen a production-design engine have that problem, but if you've gone out of bounds, maybe there's a first time.
The bike engines are just designed to deal with whatever reversion happens at lower revs. Constant-vacuum carbs have the slides close down at lower revs, which partially reflects the reversion wave; the carb is jetted to deal with whatever reversion is left. Modern fuel-injection systems use a drive-by-wire throttle that stays mostly closed at low revs, even when the rider asks for full throttle. Flat-slide carbs don't deal with the reversion; they have terrible bottom-end response ...
RE: Fuel stand-off - intake reversion - what are the causes?
I'll explain.
The design is a 4.25 bore, 4.00 stroke, 6.8" rod, 45 deg twin. We would expect to see the best power in the 6500 to 7000 rpm range. It seems that the reversion is not a problem "visually" at under 3500 rpm. Once over 4000 rpm at full throttle it comes in and get's worse and worse as the rpm climbs and never goes away.
On the dyno, the mist of fuel/air coming out the carb mouth is around a 1.5 ft column.
In terms of determining if the intake runner cross section is too large, we have gone from as low as 1.8 id to as large as 2.25 dia just after the carb body. In fact, the reversion seemed worst on the smaller runner.
Since the motor fits a Harley style frame, the intake runner length is fairly short in order to be packaged. But, for tests, we have tried adding a spacer up to 3" long behind the carb with no better results.
The carbs tried have been 48mm mikuni flatslide and S&SD butterfly style with 2.25" throat. Both had reversion at high rpm.
The intake cams have been tried as low as 226 degrees @.050 duration and 97 degree centerline (early closing point). This was not better than larger cams with later closing - makes no sense so far.
Brian, it sounds like your experience is that the reversion goes away once the revs come up. That is opposite of what we see.
Is it possible that the bore and stroke will not work well with the head flows we are working with on the 4-valve head design? We have very good low lift flow numbers as expected with the 4 valve design. Does the motor need to be more over square?
Pat, I remember you replied to my "optimum port velocity" questions a few years ago. You mentioned that the intake cam should guide the valves to meet the piston and follow it down the bore and close as soon as can be done to achieve best power - can you explain further?
I remember you also stating that the intake runner volume should be slightly larger than a comparable 2 valve. Could you give recommendations for sizing based on this 114 cuin engine?
What keeps the motor from carrying out on top end?
Thanks for the help
RE: Fuel stand-off - intake reversion - what are the causes?
speed is too slow it will continue to burn during the camshaft
overlap period causing excess pressure in the intake tract.
RE: Fuel stand-off - intake reversion - what are the causes?
This isn't something I've even considered. How do you go about finding if this is happening? What test can be done?
RE: Fuel stand-off - intake reversion - what are the causes?
What's happening on the exhaust side, and how much cam overlap do you have? If you are getting a pressure wave coming back the exhaust system during the valve overlap period, it's going to spit flow backwards even before the intake stroke starts. If the exhaust system is too restrictive to let the flow out, there could be pressure left over at the end of the exhaust stroke. This gets *worse* if the valves have high flow at low lift.
Back-of-notepad calculations suggest that your exhaust system should have primary header pipes somewhere around 40" long and 2.5" diameter. On a 4-banger you would want to join with the pipe from another cylinder at that point and carry on with about the same diameter for some distance after that.
The short intake runners are unfavorable, but with that engine configuration I agree that there's not much that can be done.
Are you drawing air from a good-sized airbox? Most bike applications just contain the reversion flow and have the engine breathe it in again.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
can be used. Excessive EGT, intake inspection, spark plug
inspection and exhaust inspection. (dry/white deposits = good
combustion heat; wet/black deposits = insufficient combustion
heat)
Here’s a good ink to reading plugs:
http:
Another thing to consider is exhaust system efficiency;
properly designed it should pull on the intake tract at RPM.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
The head/chamber configuration is modern - plug in center, shallow valve angle and squish band around where we could get it. Spend lots of time on this.
The motor seems to make best power with only 26-28 degree timing.
Brian,
I have tried cam timing all over the place including large and small overlaps - it doesn't really seems to matter. This the confusing part. I believe that there could be something in the way of being able to see the results of cam changes.
The current header pipe is 1.9 id and around 38" long through flow thru mufflers. This is the largest header pipe we used to date. The outlet of the port is 1.800.
On the intake runner, would it make sense to try to build an automotive style plenum "box" after the carb to increase volume understanding that there will be a sudden drop in velocity as air passes into the box area. There really isn't room to make the runner longer, but it could be larger in diameter before the heads.
This could contain/diffuse the reversion, but doesn't it make more sense to try to cure it in the first place?
RE: Fuel stand-off - intake reversion - what are the causes?
The other is (this may have been mentioned) that a reversion wave is timed to the RPM, if the positive wave hits the chamber during overlap it could spew out the intake....
Dont try to fix it with an air box as it will be miserable to tune with a sometimes wet reversion wave...
Cheers
I don't know anything but the people that do.
RE: Fuel stand-off - intake reversion - what are the causes?
Cheers
I don't know anything but the people that do.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
The stand off should tend to suck into the engine as revs and airspeed increases.
I agree 112 lobe centres on a 4 valve head. They scavenge a lot better than 2 valves and to much overlap increases the chance of exhaust pulses bouncing back into the inlet, although you should be able to drive through these events as they should only occur over a narrow rpm range.
It really sounds to me like cam timing is way out, like it lost a lobe type out.
I guess it's to basic to ask, have you checked the split overlap is at about TDC.
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
1. Runner lengths - intake port - 5.5 inch average between the two valves. one runner is slightly longer than another.
intake manifold - 5 inch average - one cyl in further than another because of cyl offset.
total avg runner length - 10.5 inches.
2. It is a single carb, single throat 57mm, butterfly style. The carb feeds both cylinders through a common manifold which divides to each cylinder.
3. I have not checked the split overlap, but I can offer this information:
Recent cams and settings:
1. intake 226@.050 set at 97 deg lobe center
exhaust 234@.050 set at 112 lobe center
lobe center separation = 97+112/2 = 104.5
2. intake 234@.050 set at 100 lobe center
exhaust 226@.050 set at 106 lobe center
lobe center separation = 100+106 = 103
3. intake 234@.050 set at 100 lobe center
exhaust 234@.050 set at 112 lobe center
lobe center separation = 100+112/2 = 106
All of the above do not come very close to the 112 lcs described above. Could this be an issue? Do the above figures point toward a tendancy toward reversion at higher rpm's?
If yes, then would a setting the like the following be more in line
intake 226@.050 set at 108 lobe center
exhaust 234@.050 set at 116 lobe center
lobe center separation 112.
RE: Fuel stand-off - intake reversion - what are the causes?
I would go back to basics and set it up at split overlap with 112 Deg between centres.
Fine tune from there.
Durations sound in the ball park to me.
Inlet tract is a lot to short.
Can you fabricate something up for dyno that makes both runners the same length, with a connecting plenum. Make the plenum quite large and put the carby in the middle, possibly facing away from the runner inlets so the airflow sweeps through a 120 deg or more curve. This might keep the reversion on the plenum and make a bit of room for longer stacks.
Are the valves definitely holding their seats without leaking. Is the head distorting under load and unseating the valves.
This is all grasping at straws, but your problem really does not add up.
Can you test with long, big dia open exhaust pipes.
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
Maybe it has good mixture with stand off leaning it out, but as you draw the stand off in, it is going rich and miss firing.
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
Very much appreciate your suggestions.
The a/f ratios are being read by an oxygen sensor on the front headpipe.
The next step for me is to try the 112 centerline intake and exhaust.
On the plenum idea, I think a "box" can be made up to bolt onto the existing intake manifold flange. 90 degrees to that inlet, the carb can feed through into the box. This should point the carb throat toward the front of the bike into the airstream - a good thing.
Have any ideas on what volume the box should be in relation to the engine size? Something like around 50 cu in should be possible to package. Wouldn't there be any concern about the sudden drop in velocity of the air once it first hits the plenum behind the carb?
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
I would agree that if the motor was performing as expected, then the reversion would not be much of a concern.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
optimal VE and power potential. At speed the time to fill
the cylinder is very short, when the intake valve opens
the first and only movement should be into the cylinder.
To accomplish this the pressure differential between the
intake and cylinder must be favorable. With V8 engines we
achieve this with the exhaust header, V twin... I don't know.
To me I see three options, find a way to reduce cylinder
pressure during overlap, raise the intake pressure or open
the intake valve later when conditions are more favorable.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
Going on the idea of opening up the centerlines, I'll try with the cams at 226@050 intake at 112 cl and 234@050 exh at 112.
If this shows improvement (reversion and power), then probably go even further with lowering the overlap. What would be preferable from there - later intake or earlier exh?
I have read in other places that lobe separation angles of up to 118 work for 4-valves - sound correct?
RE: Fuel stand-off - intake reversion - what are the causes?
look past the reversion and find the cause. With 2 valve heads
and 4 into one headers the results can be amazing. The
picture below is from Larry Meaux, an awesome example of
completely eliminating reversion.
v114, I can't answer your questions on ideal valve timing
events. I'm very interested in the results of widening the
separation. Please keep us informed of progress.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
One cylinder might be very lean and the other very rich due to charge robbing between the cylinders as I on a V twin, I would think the intake of one opens before the other closes. Twin carbies would be a lot nicer.
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
Pat, the o2 sensor reads only one cylinder.
Getting back to the plenum idea. In terms of geting the engine to carry higher in the rpm range, say to 7K, does the plenum have a chance to be able to help this. The idea would be that the single 57mm carb may not flow enough to supply the demand at 7k - maybe the pleunum would help?
If a plenum would be a good thing to try, how to size it? Is there a rule to go by in terms of plenum volume vs. engine displacement?
RE: Fuel stand-off - intake reversion - what are the causes?
Some fuel stand off is normal on a high performance engine.
You might have one very rich and one very lean cylinder.
The airflow might be OK, but you problem is ignition system. Are you sure it is good. Are you sure there is no valve bounce.
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
Good to hear confirmation that the plenum will make carb seem bigger.
More than one ignition system has been looked at. Also, we looked at the timing marks during a pull to 6K and it seemed like the ignition stayed fairly steady within around 2 degrees of setpoint.
Valve bounce is a possibility - We are not sure that there is no valve bounce. How can this be identified without spinning the valvetrain on a test fixture with accelerometers.
Can the plenum be made too big?
RE: Fuel stand-off - intake reversion - what are the causes?
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RE: Fuel stand-off - intake reversion - what are the causes?
Are you experiencing any detonation?
RE: Fuel stand-off - intake reversion - what are the causes?
The plenum size I have scoped out will mount in place of the existing carb on the end of the manifold and will be made to be adjustable in volume with the smallest volume starting equal to one cylinder.
The carb will mount forward facing and be about 85 degrees to the inlet of the manifold with a generous radius to the entrance of the manifold to smooth the flow.
Think this may help rpm? Nothing else has so far.
RE: Fuel stand-off - intake reversion - what are the causes?
pressure during overlap but everything you describe relates
to good combustion efficiency. Have you giving much thought
to what phase each cylinder is in during the overlap period
of opposite cylinder?
Here's a link to information you might find interesting about
software for sizing engine air flow components.
http://www.maxracesoftware.com/pipemax36xp2.htm
RE: Fuel stand-off - intake reversion - what are the causes?
That is why I suggested twin carbies at one stage. It would eliminate that as a potential problem.
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
then be like tuning a single cylinder engine.
I read this tonight on harley tech talk:
If you've ever watched a bike on a dyno with the air cleaner
off, especially one with a healthy cam, there is a nice big
cloud of fuel/air hovering over the carb inlet. This is caused
by the air pulses from reversion, and also at high rpm by
the air getting accelerated, then hitting a brick wall
(intake valve) when the intake valve closes, and bounces back.
http
RE: Fuel stand-off - intake reversion - what are the causes?
You can see the reversion effect, in this video.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
It sure seems that there is something far overshadowing the normal tuning options here. No matter what tuning options are tried, the power does not go up or down much at all.
We are not 100% that the cam drive train is keeping the timing fixed - we are in the process of installing cam position sensors and a scope to be able to see what is going on up at speed. This will hopefully rule out problems in the valvetrain.
Thanks all for the feedback - will keep you posted.
RE: Fuel stand-off - intake reversion - what are the causes?
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
1. Replaced the spring tensioned cam chain shoes with solid adjusters. These were tightened after the engine came to temperature and there is very little play or room for chain to deflect. Thying to rule out cam timing changes as rpm goes up.
2. Still have not checked valvetrain with cam position sensors.
3. Completely new ignition system module and coil.
The next plan is this:
We now have new 213 degrees @.050 cams and .380 lift. This is a big step down in duration as compared to previous. We will be installing and testing early next week.
Two questions to throw out there.
1. If reversion is really coming from cam selection, does it make sense that 213 duration should show a significant difference?
2. We have not tried the plenum idea yet as listed earlier in the post. I have been reading what I can on intake runner length sizing and we have established here that the current intake runner length is way short in this case (10.5 inches). Is it a decent possibility that the rpm limiting we are experiencing is due to the short runner in combination with no plenum?
RE: Fuel stand-off - intake reversion - what are the causes?
bellmouth shape and it's impact on cylinder filling and
spit back by Prof. G. P. Blair.
http:
Two more papers on intake sizing:
http://nt
http://nt
RE: Fuel stand-off - intake reversion - what are the causes?
it make sense that 213 duration should show a significant difference?"
I believe that something other than camshaft selection is the
primary cause of the reversion. If it were the cause I think
you would have determined it with the tests you have
performed. My current thinking is your problems are related to
tuning of the intake and exhaust runners as well as bell
month shape described by Blair.
The new camshaft design will shift the RPM range and require
different runner sizes. This should be considered if the new
design shows benefit.
RE: Fuel stand-off - intake reversion - what are the causes?
The current thinking on the smaller camshaft selection is to be able to detect whether or not we do actually have the wrong sized ports. We believe that if 213 duration works in increasing power and reduced reversion, then that may point in the direction of port sizes being too large.
The theory being that the flow velocity (and resulting momentum) is not high enough to keep the flow moving toward the port. This could actually be the whole problem.
It is puzzling though, because I thought that larger ports favored higher rpm. We see good throttle response and low end performance with this setup.
RE: Fuel stand-off - intake reversion - what are the causes?
1. Completely new style ignition system - revoved electronic module and replaced with a more conventional inductive pickup with weights for advance.
Result - no change, so ignition is not an issue.
2. Looked at A/F between front and back cylinders and saw that they were consistent
3. To test to see if higher port velocity would have an effect, we added material to the exit of the manifold where it meets the head to reduce the area by about 25%.
Result - no change
4. We still have the question of valve bounce or something similar that could explain the reversion and how it gets worse as rpm climbs.
We are using seat and nose pressures of 65 lbs and 200 lbs respectively. The spring is a Manley 22105. Total weight is 131g except the weight of the spring.
5. We still have not built a plenum.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
What does the cam manufacturer recommend for springs?
Manley says this is an Acura spring, suitable for up to 0.535 inch lift, but with a seat pressure of 82 @ 1.350".
And, if I read it right, they will tolerate up to .765" lift before binding.
http://www.manleyperf.com/sc/mk/honda/b-kits.html
These guys say Crower valves for that motor are a dainty 45 grams. Stock intake head size is 33 mm. http://
I can't imagine your valves are anywhere near that small with a 4.25 inch bore (almost 1 inch larger than the Acura).
It looks like you may be running 20 lbs less seat pressure with a much heavier valve, although max revs are obviously much lower.
I'd be looking into the possibility of shimming those valve springs to get a seat pressure nearer 90 lbs, which is what CompCams springs
provide for a FORD 4.6L 4valve motor have.
http://ww
RE: Fuel stand-off - intake reversion - what are the causes?
No hydraulic pucks - shim under style buckets
The current cam grinder has no recommendation for us on the springs - go figure. I guess it's like that for some of the bike cam grinders. This is why the question still exists on whether we are using enough spring to control the system.
We are headed toward increasing the spring to rule this theory out also.
All of our cams have had no recommendation for springs and all cams have had fairly agressive ramps which could explain some things. Maybe the valve bounce is occuring with all cams we are using and thus the reason why we are not seeing predictable changes.
RE: Fuel stand-off - intake reversion - what are the causes?
Second most likely is valve bounce having an influence on pulse in intake.
Regards
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RE: Fuel stand-off - intake reversion - what are the causes?
We have not really pursued this much more recently because it really does not package well for the rider but is seems to have been about the only thing so far to get the rpm to carry at all.
RE: Fuel stand-off - intake reversion - what are the causes?
All I know about reversions is from Jim Feuling and Jerry Branch
and if you Google them there might be some info
http://www.ssheaders.com/header.htm
Cheers
I don't know anything but the people that do.
RE: Fuel stand-off - intake reversion - what are the causes?
exhaust valves?. What kind of base circle radius are you using, do you know the lift per degree on profile, what have you set your seat pressure to and your over the nose rate, also what is your valve mass weight.
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
The more testing we do, the more the issue points toward the problem with reversion and lack of power and rpm potential being in the cylinder head itself.
It doesn't respond well to cams, pipes, type of carburetion, ignition timing, valve spring rate changes. All the things that would normally have an effect in a normal working engine.
We are looking very hard again now at the cylinder head/port design.
RE: Fuel stand-off - intake reversion - what are the causes?
The brake specific value usually falls in the range of between .42-.45 under a full throttle run up to the rpm where the hp peaks. After peak hp, the brake specific number rises substantially for the rest of the run and climbs to approx .7 or .8 by the end of the run. We are usually seeing rpm peaks in the low 5k range (not good) and we run to about 6200 rpm during a pull. By the end of the pull, the brake specific number is around 0.8.
What does this say about the engine condition? I understand this to not be a normal situation, but not quite sure on this one.
Earlier in this thread, we described how the engine will not peak power beyond the low 5k rpm range. Does this situation (low rpm peak and poor brake specific) point to anything specific to look at in correcting the problem?
Look forward to your comments here
Thanks
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
RE: Fuel stand-off - intake reversion - what are the causes?
V114 - IMO you've stacked the deck against yourself with your novel engine design.
What did you discover in the combustion chamber design?
Have you considered a dry-flow intake tract?
Si Vis Pacem Para Bellum
RE: Fuel stand-off - intake reversion - what are the causes?
Have you made any progress on the reversion or did you shelf this project?