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

 

[v114]

Did the run at intake and exhaust cams set to 112 centerline and basically nothing changed form other settings. Hp, torque, and rpm peak we all within about 5% of other cam settings.

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.

 

[patprimmer]

An adjustable strobe light is a very useful tool for veiwing valve train motion in slow motion

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]

Couple of things tried, again with no different power results:

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?

 

[automotivebreath]

I recently read a very good Paper on the subject of intake
bellmouth shape and it's impact on cylinder filling and
spit back by Prof. G. P. Blair.

http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

Two more papers on intake sizing:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090861_1993090861.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930084998_1993084998.pdf

 

[automotivebreath]

"1. If reversion is really coming from cam selection, does
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.

 

[v114]

Automotivebreath,

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.

 

[v114]

Some more tests have been done.

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.

 

[bigpaul]

what does the engine sound like when runing

 

[EfiOz]

Does this engine have a hydraulic lash adjuster system? Sorry but I can't find it mentioned anywhere. Lifter pump up could explain it, especially with it getting worse with more rpm.

 

[Tmoose]

How do the valve seats look?
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://www.tunerschoice.com/xq/aspx/dept_id.196/display_id.108/qx/Product.htm

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://www.compperformancegroupstor...e=CC&Product_Code=26123-32&Category_Code=Ford

 

[v114]

Engine sounds fairly responsive, not the best it's ever been, but not terrible. On the dyno, you can tell that the engine is not pulling hard on top where is should and seems to labor

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.

 

[patprimmer]

I still think it is most likely runner length and single carby and consequential interaction between pressure pulses from each cylinder. Twin carbs on IR manifolds will prove that point.

Second most likely is valve bounce having an influence on pulse in intake.

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]

In the past, with an earlier version of this motor design, we did rig that one with dual carbs and ir manifold. We did that test to understand whether the reversion was due to interaction between front and back cylinders or just confined to each cylinder. The reversion and lack of high rpm power was still there with the ir setup. We then machined out the area between the runners to make the front and back communicate and then the rpm came up and the reversion looked much better. The peak went from 5200 to about 5800 rpm - better, but still not what we wanted. That motor also had 65 lbs on the valve seats. Could that have been bouncing valves? Don't know
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.

 

[thundair]

I know a lot of reversion can be controlled by stepped headers...Have you looked into that setup?

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.

 

[bigpaul]

may be stating the obvious but do you have clearance on both
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.

 

[PEW]

Did v114 finally cure his problem or did he jump off the balcony?

 

[v114]

The balcony looked very good, but we didn't jump yet.

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.

 

[v114]

As we continue to test, we have observed a common situation:

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

 

[FoMoCoMoFo]

Is a portion of the fuel you see leaving the carb in the wrong direction not coming back ?

 

[v114]

I'm sure some is lost