Power Flat Spots

[emry]

Car exhaust tuning page

 

HD exhaust page, some other interesting tidbits too

 

Another car article with a good explaination of exhaust theory.

 

Interesting College page, calc's and has Thermodynamics info, may be high level for some.

 

For those of you who prefer tree based reading

 

More tree based reading.

 

And I will try and dig up my old engineering notes, I have all of those darn formulas somewhere.

 

 

I wonder if the engineers even bother with tuning length?

 

Most definately, but often designers, goverment regulations, and space requirements can dictate the real outcome.

[motoguzznix]

I doubt it, but hey, you never know.

I sure was surprised to learn from your posts that the mixture was perfectly balanced at 4000RPM.

Effects of back pressure are fairly easy to predict.

Effects of tuning length are more difficult to predict.

With the crossover all evidence is that the stock muffler does not lose low end and may in fact gain it over what is typically used, free-er flowing shorter tuning length muffers that give the bike more high RPM power.

Predicting that the same will happen without a crossover, where we are potentially getting more back pressure, does not seem to be making a risky prediction.

I suppose the back pressure with stock muffler and no crossover might produce so much back pressure that the power is robbed below 4500RPM but I doubt it, and I am positive it won't gain more power above 6500RPM and will likely lose a lot.

Going to the free flowing mufflers, back pressure is pointing us at less bottom end and more top end. I don't think that is a bold prediction.

If I said that no crossover and free flowing muffers would make more power in the low end, mid range or the high end than a typical stucchi/mistral combo, that would be a bold prediction.

I think such an arrangement will make less overall power but I have no idea where it might do better or worse.

Yah, I know, another bold prediction.

I hope Ryland proves it wrong

I try to explain things from my point of view (not knowing if I'm right):

 

The most important thing in the exhaust is the shock waves.

 

These can best occur when the exhaust sytem has no restrictions. A restricted muffler and/or crossover deminish the effect of the shock waves, but may even out the power curve. This was evident during my measurements on the dyno comparing stock muffler with my modified less restrictive mufflers.

 

Whenever the exhaust wave comes to an open end, a depression wave is reflected back to the exhaust valve thus helping to clear the cylinder. This effect takes place several times during one cycle.

 

But where is the end for the exhaust flow? Maybe the stock Xover which is an empty chamber? The 3rd chamber in the stock muffler where the exhaust tube ends? How does a crossover like the stucchi influence this system? The first chamber of the muffler in my modiefied system?

 

How can you predict any effect taking into account all these conditions? Even a correct prediction might be caused by other effets like thought at.

 

But the real problem seems to be in the intake system for me.

 

First the anabilty of the intake port to allow sufficient flow at valve lifts higher than 8 mm see here

 

Second following the discussion about gas speed I think the intake port is too big. It would be better to make a smaller port flow better to increase gas speed. The flow bench graph above shows the LM2 smaller port flow better than the bigger V11 port at higher lift. Using heads and pistons of a Cali 1100 could realise this easyly.

 

A similar effect could have a camshaft with shorter duration and fast opening rate off the seat. When the port flowed at higher lift, more liift would be benefical. Dynotec or HMB offer camshafts who follow this route.

 

The combination of the two would be very successful eliminating the flat spot.

 

High intake speed generates higher pressure when flowing against the closed intake wave so starting the flow better when the valve opens again.

 

Shorter duration of exhaust and intake allows to make use of the explosion longer and to increase the real compression stroke after the intake valve is closed. Both increase the efficiency of the engine. Shorter overlap between intake and exhaust decrease the losses of charge at lower to medium rpms.

 

Optimising the squish and the increased CR that follows doing this makes the charge burn more efficiently ending in an engine that should have come from the factory.

[Skeeve]

Thanks Emry!

Any ideas how Ryland might calculate this independent header design?

I tried some thumper sites looking for ideas, speculating that they might know how to design an exhaust without crossovers and two into ones.

But I found nothing but aftermarket slip-on options, with SuperTrapp being an apparent frequent favorite.

Thumper makers and most motorcycle makers seem to make the length of the exhaust fit the bike.

125cc-650cc, all about the same length, but diameter changes a bit.

The Buell Blast might be an exception amongst singles, although the innards of the exhaust could go back and forth a couple of times increasing the tuning length.

I wonder if the engineers even bother with tuning length?

Enzo's Cobra was allegedly great for power, but it was loud and must have been a pain during oil changes.

SORRY

 

Some ROTs ['rule of thumb'][1] I've come across over the years:

 

Exhaust pipe (header) dia.

Exhaust gases are hot = less dense = speed of sound

 

Phil Irving's[2] Tuning for Speed is in it's 6th printing & "OOP" [out of print.] Cheapest I've seen it in the last couple years is about $60 on Alibris; I don't think there was a copy available for less than $125 last time I looked. There's got to be a reason why a book that's had 6 printings [starting early '60s, last one in the early 90's] is still so popular that w/ all those copies floating around, there aren't so many that the price drops down to the realm of normal [$25-$30] prices for similar books on Amazon. I wonder why that is?

 

Ride on,

 

[1] The expression "rule of thumb" comes from an ancient English ordinance stipulating that "a man may not beat his wife with a rod thicker than the width of his thumb." This is why women today all dig musicians, w/ their long, skinny fingers. Adaptive evolution!

 

[2] Australian engineer probably most famous for working for Vincent HRD. Heard of them? He helped make that marque famous...

[Phil A]

As it would take volumes to reply to the posters on this thread, I would like to offer some things to consider in conjunction with what has been written.

This way each poster can consider/apply the thoughts to their own understaning of what is happening without me creating arguments based on subjectivity.

 

Firstly, you must appreciate that there are two completely seperate things happening in the inlet /exhaust tracts at any one time.

 

One is gas flow and one is harmonic wave activity. Each has nothing to do with each other but needs to exist in the company of the other.

 

Some of you have a handle on harmonics to some degree but no-one seems prepared to change it.

 

Harmonics travel faster thru a denser medium.

 

Consider this in conjunction with the exhaust back pressure discussions. More back pressure = more torque down low. That is, untill the resistance is a greater loss than the gain made by increasing the density via back pressure.

 

Consider this. these bikes have a "flat spot" at 2000, 4000 and 6000, and (8000(4 valvers) The 4000rpm one you are all aware of and can feel seat of the pants.

 

They make really "easy" power at 3000, 5000,and 7000 rpm. You can test this by fully loading a 4 valver with pillion and gear and head up the steepest hill you can find in say second gear. It will come on as hard at 7000 as they do in stock form at 5000.

 

This is an example of the harmonic at work. You can prove this to yourself by lenthening the inlet tract say 100 mm. It will instantly change the good power rpms and the bad power rpms to different settings.

 

The same harmonic that is discussed at length in the exhaust system is available in the inlet tract. How many of you continuously modify the inlet tract as you do the exhaust to find that elusive torque gain?

 

Dont confuse inlet speed with pressure. The higher the speed the lower the pressure. However inlet momentum, which is, weight X speed is absolutely critical.

 

BUT I hear you say, I filled in the dips in the inlet tract with resin and the flat spot improved. Yes, but I put it to you that this was due to the fact that you increased the speed of the charge therefore increasing the momentum of the charge.

This is also evidenced by the fact that on that same engine you will have lost a little top end power. This is because the smaller tract cross sectional area limits the flow at the top end.

Also if you increase the speed you decrease the density, moving the effect of the harmonic to a different spot.

The charge density is a dynamic thing it varies on every stroke depending on temperature, density, atmospheric pressure etc

 

Bigger is always better. If it wasnt you would be riding a smaller bike. If you make something bigger and the bike goes slower then something else is too small.

 

The bigger you can make the inlet tract while maintaining the momentum the more torque you will make.

 

A good example of this is my 500cc single which does 130mph on the salt (add 10% for bitumen) at 5600rpm. It has two 46mm mikunis on it. (See it on the DLRA web site)

 

Hope this gets the wheels turnin.

 

Phil A.

 

"All is not always as it seems"

[dlaing]

snip

and I think Ryland's strategy is going to fill the mid-range hole effortlessly and the battle will be all about minimizing the high end losses while keeping the sound down and not losing low end. Perhaps I wrong, but I doubt it.

Starting with a long center connector made of plain ugly automotive exhaust tubing, he could gradually cut it down to get the right tuning length, or he can do all the calculus(sorry, can't help there) and get it right the first shot .

Or it may just not make much difference.

Here is an interesting tuning experience

http://www.visi.com/~moperfserv/hamelvincent.htm

But I wish I could tell if that is two into one or two into two?????

[Tom M]

FWIW here's a simple explanation of how a tuned intake works:

http://auto.howstuffworks.com/question517.htm

[emry]

No crossover on the Vincent. Single straight pipes. Interesting that it has a flat spot at 4k too.

[dlaing]

No crossover on the Vincent. Single straight pipes. Interesting that it has a flat spot at 4k too.

hmmm, this puts a flat spot in my theory.

Lots of interesting stuff happening here.

It is interesting that when he used long/narrow pipes there is a hole at about 3950RPM and with the two shorter/wider pipes the hole moves up to about 4050.

Also, the long narrow pipes make less power everywhere

The two short/wide pipes are both a lot better, but I bet A LOT LOUDER.

But interestingly the longer of the two short pipes has a deeeeeper hole at 4000, unless that is just the dyno operator being lazy.

Amazing what a difference two inches makes.

I strongly suspect the V11 crossover is too far from the exhaust valves.

Maybe just trimming the header pipes a little, bringing the crossover towards the sump could help fill in the hole.(speculation, I know....but maybe worth trying)

[Ryland3210]

 

... Some of you have a handle on harmonics to some degree but no-one seems prepared to change it.

 

... The same harmonic that is discussed at length in the exhaust system is available in the inlet tract. How many of you continuously modify the inlet tract as you do the exhaust to find that elusive torque gain?

 

Phil A.

 

"All is not always as it seems"

 

I for one and more than glad to change exhaust harmonics, and that would certainly happen if the crossover and existing labyrinth muffler is replaced with independent straight exhaust pipes into a free flowing "silencer". If nothing else, the length of exhaust pipe will be significantly longer than the stock unit, which will decrease the fundamental frequency of that element.

 

As far as inlet tract modifications go, it is simply more difficult to make major length changes there. At one time, I think back in the '60's, Chrysler Corporation did produce an engine, probably a hemi, with tuned intake manifolds. It used twin four barrel carbs on seperate manifolds, each feeding four cylinders. The thing was gigantic, reflecting the length required to tune for the 3500 to 6000 RPM range. In my mind's eye, I recall the picture, showing a length of about 3 feet from each carb to the intake ports of the opposite bank of the V-8's cylinders. Not so easy to accomplish on a cycle.

 

Short intake lengths will keep the natural frequency high enough to stay above the max RPM range of the engine, leaving the exhaust system as a tempting, and relatively easy target for tuning.

 

 

hmmm, this puts a flat spot in my theory....

 

 

I'm not sure. As I read the fine print, he didn't use full throttle below 5K RPM.

[dlaing]

I'm not sure. As I read the fine print, he didn't use full throttle below 5K RPM.

Good point.

[Phil A]

"Short intake lengths will keep the natural frequency high enough to stay above the max RPM range of the engine, leaving the exhaust system as a tempting, and relatively easy target for tuning."

 

Even with the short inlet, The harmonic will simply complete several cycles up and down the tract between engine cycles. Eventually you reach a point where the gas flow is going into to the engine and the harmonic is going outwards away from the engine.. This stops or slows the charge causing the" flat" spot.

[Ryland3210]

Even with the short inlet, The harmonic will simply complete several cycles up and down the tract between engine cycles. Eventually you reach a point where the gas flow is going into to the engine and the harmonic is going outwards away from the engine.. This stops or slows the charge causing the" flat" spot.

 

However, with each cycle, the pressure wave amplitude is dampened, especially if the inlet end of the tract terminates into a relatively large cavity, especially if containing a filter which can dampen instead of reflecting a pressure wave.

[emry]

Dodge 300 Cross Ram. They later designed a shorter version where the carbs sat just above each valve cover. Similar intakes are often seen on dragsters and are normally called "tunnel ram" where the intakes are pointed straight up through the hood.

 

As the pressure wave travels back and forth in the intake or exhaust it is known a 1st order, 2nd order, etc. Utilizing the 1st order pressure wave often results in very long lengths. Tuning is normally done for the 3rd and up. As stated previously the pressure wave does lose some effectivness during its reflections.

[Pierre]

However, with each cycle, the pressure wave amplitude is dampened, especially if the inlet end of the tract terminates into a relatively large cavity, especially if containing a filter which can dampen instead of reflecting a pressure wave.

So Ryland, in the age old debate of "pods" vs "drilled air box" am I to understand that (assuming the intake runners remain in both set ups) the pods may be superior? IOW, is the "reflection" off the inside walls of the air box a negative that is eliminated with pods?

[dlaing]

Dodge 300 Cross Ram. They later designed a shorter version where the carbs sat just above each valve cover. Similar intakes are often seen on dragsters and are normally called "tunnel ram" where the intakes are pointed straight up through the hood.

 

As the pressure wave travels back and forth in the intake or exhaust it is known a 1st order, 2nd order, etc. Utilizing the 1st order pressure wave often results in very long lengths. Tuning is normally done for the 3rd and up. As stated previously the pressure wave does lose some effectivness during its reflections.

Does the cross ram result in noticeable throttle lag?

"For '60, the 413 gained a radical ram induction system in which each four-barrel carb fed the opposite cylinder bank via 30-inch "outrigger" tubes. The length of the runners was calculated to produce a super-charging effect in the heart of the rpm range."