Fuel stand-off - intake reversion - what are the causes? 2

[v114]

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

 

[FoMoCoMoFo]

I would play with the lobe centers,and or exhaust or ? you could build a big plenum and reduce the standoff but if the reversion and standoff are not causing A/F mixture problems then its not a problem its a symptom. and you would not be too concerned with it if power and revs were meeting expectations. would you?

 

[v114]

In my mind, the reversion is a problem because at the rpm it is noticed (visually) coincides with where the power seems to drop off. As the rpm's come up, the reversion gets worse and never goes away. It makes me think that the rpm potential of the motor will not be achieved until the reversion is under control.

I would agree that if the motor was performing as expected, then the reversion would not be much of a concern.

 

[FoMoCoMoFo]

I guess what I'm suggesting is that you have a problem, that's causing power to drop fast and early and causing reversion/standoff, but that the reversion is secondary (a symptom and a clue) and that if reversion is not causing fuel metering problems (you say it is not) then i don't think its a problem in itself. as always i could be wrong.

 

[automotivebreath]

IMO reversion at RPM is a major restriction on archiving
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.

 

[FoMoCoMoFo]

all true, no argument with any of the above. just trying to get causes and effects in order.

 

[v114]

I should be able to try the following tommorrow.

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?

 

[automotivebreath]

FoMoCoMoFo, no offense intended, we agree the solution is to
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.

 

[FoMoCoMoFo]

none taken.

 

[patprimmer]

Does the oxygen sensor read both cylinders or just one.

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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[v114]

Did the 112 and 112 intake /exhaust cl test yesterday. As usual, this made almost no difference in the peak power or rpm where it made it. The reversion did seem to be a bit less as compared to other cam settings in the past. Still, though, during a pull on dyno, your hand will get wet with gas a few inches in front of the carb mouth. Can't believe this is optimized.

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?

 

[patprimmer]

The plenum will make the carby seem bigger.

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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[v114]

With the currrent set up, we will swap the o2 sensor between cylinders to check for lean and rich conditions.

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?

 

[patprimmer]

You won't have room to make it to big. Most are to small due to packaging restrictions. From memory, from quite a few years ago, at least one cylinder volume is minimum, engine size is probably getting well into diminishing return area.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[automotivebreath]

v114,
Are you experiencing any detonation?

 

[v114]

No detonation except when we radically change the ignition curve (very high advance rate). Normally, we run around around 26-28 degrees total advance with no problems.

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.

 

[automotivebreath]

I keep coming back to combustion problems creating high cylinder
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

 

[patprimmer]

automotivebreath

That is why I suggested twin carbies at one stage. It would eliminate that as a potential problem.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[automotivebreath]

Yes Pat two carbs would eliminate that possibility, it would
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://groups.msn.com/harleytechtal...sage=1152025&LastModified=4675628912205815110

 

[dicer]

http://video.google.com/videoplay?docid=-7393939630149644203&q=V10+Engine

You can see the reversion effect, in this video.

 

[Zapster]

V114, is there any chance that you have lost control of your valve timing? Valve float or cam follower pump up?