I just stole that, thanksOriginally Posted by Buckets4Me
Forum whore
I just stole that, thanks
Muddler/Dabbler.
They sound great, where can I get one? - great article, might almost convince some people! eg:-
A long time ago, I bought a Suzuki RM80 aircooled mx bike for one of my kids, and the cow cocky I was buying it from insisted that it was really worth more than I was paying, because it had a powerband!
We had that bike for over two years and I think we were screwed because I never did find any powerband on it!
Forum whore
Who knows how ""modern"" this thinking is but I found the description informative, especially the bit about expansion ratios in the diffuser.
http://karting.archive.netcopy.co.uk...tuned-exhausts
PART TWO
PRIMARY SYSTEM: The plumbing from the engine to the end of the megaphone helps the engine to pump air/fuel mixture. By breathing better, the engine has a good chance of producing more power. The low pressure created by the primary system is accomplished without the aid of the rest of the exhaust system. There are motorcycles and karts using just the lead-in pipe and the megaphone to good advantage. Unfortunately when used alone the bare megaphone is terribly noisy and also does not produce the broad torque range so essential to karting.
These primary systems can be tuned, however, to out-perform a machine using no exhaust system.
An understanding of how to tune a primary system for any two-stroke style of exhaust can best be developed by first examining an engine fitted with an unadorned straight pipe shown below. This lengthy explanation is worthwhile if you have not waded through it before.
When the exhaust gases flash into the pipe a high pressure disturbance is created which travels downstream towards the open end of the pipe at the speed of sound. This sonic pressure wave travels through the contents of the exhaust pipe. The important change that takes place occurs just as this positive pressure pulse leaves the pipe. A suction is created in the end of the tube which initiates the propogation of a negative wave back up the exhaust pipe toward the port. This wave, which is below atmospheric pressure, travels upstream through the particle flow with a relative velocity equal to the speed of sound. Here we have a strong low pressure wave returning back to the engine. This low pressure pulse could greatly aid the flow of exhaust from the cylinder if it were timed to arrive at the engine at just the proper instant.
Cut-and-dry methods used with an engine on a dyno indicate that this length is quite fussy for any given tuned rpm. The engine runs well at the tuned rpm, but does poorly 1,000 rpm either side of that speed. It turns out that a straight pipe without a megaphone returns a shortduration, high energy negative pressure pulse. A lowenergy, wide-duration wave would be better suited to the engine's needs. A straight pipe is worthless on a kart track, because of the rpm range it demands.
We can help the engine somewhat by adding a step to the end of our original straight section as shown below. It should help to extend the tuned rpm.
This pipe has two sudden changes of cross section at its open end and returns two negative pulses from the original 50 psi charge. Note that the timing of the first negative pulse is unchanged as it travels over the same round trip distance.
The second pulse which is reflected from the large diameter step must arrive up-stream at the port later. Its round trip distance is 2L. This has the favourable effect of extending the low pressure way duration, giving a better tuning effect.
If the steps were made very close together, a patient person would construct something resembling a diverging megaphone. Acoustically, the built-up pipe and the smooth megaphone produce the same effect—they return a series of overlapping negative pulses. The sum of these pulses produce a smooth wave which extends the useful tuned rpm range of the engine. Any high output exhaust system for the 2-stroke will have this megaphone.
The megaphone is a very essential part of the volume tuned exhaust system. A constant cross section primary system (no megaphone) was tried during the development programme. It resulted in a narrow tuned speed range and was also low on power—even though it exhausted into the volume chamber. The "S" shaped lead-in pipe of Figure No. 1 has a nottoo-constant diameter of 11 inches. I have no data concerning a different diameter. However, the length of this pipe (our length 1) is easily varied at the flex tube. A shorter length takes less time for our positive-wavecum-negative—wave to make a round trip. This change to a reduced length would be better therefore at a higher engine speed.
The diverging megaphone is eleven inches long and measures 70 surface to surface. By comparing the areas (not diameters) of each end of the megaphone we find a ratio of about four. Let me speculate for a minute on this number and its alleged influence on engine performance.
The energy contained in the reflected negative wave is taken from the original positive pulse as it moves downstream in the exhaust pipe. A slow tapering, long megaphone removes less energy from the positive pulse than a wide angled unit of equal length. The slow tapering megaphone with a total angle around returns a relatively weak negative wave, leaving considerable energy remaining in the positive wave as it leaves the megaphone. This should result in a strong back-stuffing wave when the positive pulse reflects off the closed end of the secondary chamber.
A wide angled megaphone in the 8° range uses more of the initial energy in producing a strong series of overlapping negative pulses. The energy left over for the positive back stuffing wave is therefore less than we had in the 4° diffuser.
The expansion ratio of the megaphone is a convenient means to compare this characteristic. Data taken from many two-stroke exhaust systems show that they use an expansion ratio which falls in the range of 3 to 7. In general, the observation is that engines with a narrow tuned range (and high peak power) use a low cone angle with an expansion ratio of 4.
A wide angle diffuser seems to be less sensitive to engine speed change, and retains its tuned range longer. This megaphone has a vigorous scavenging influence because of the strong negative wave it produces. Backstuffing, on the other hand, will be weaker as less energy is available to reflect off the closed end of the chamber. Timing of the long duration scavenging is also less critical than the shorter duration back-stuffing. It is therefore less sensitive to rpm change.
The Komet, PeriIla and Saetta expansion chambers have an expansion ratio of six. The McCulloch Volume tuned exhaust uses a lower ratio of four. An expansion ratio of six is quite large compared to the majority of racing exhaust systems being used today. For example:— 73cc Yamaha single — 1.90 inlet, 3.40 body; area ratio 3.20 250cc Suzuki twin — 1.50 inlet, 3.00 body; area ratio 4.00 344cc Greeves single — 1.75 inlet, 3.00 body; area ratio 2.94 250cc Yamaha twin — 1.45 inlet, 2.95 body; area ratio 4.10 250cc BuItaco single — 1.80 inlet, 3.31 body; area ratio 3.38 125cc Alpha twin — 1.25 inlet, 3,33 body; area ratio 7.32 100cc Komet single — 1.44 inlet, 3.57 body; area ratio 6.15 100cc PeriIla single — 1.44 inlet, 3.57 body; area ratio 6.15 Judging from just the information above, it appears that the "rpm-sensitive" motorcycles produce the best power with an expansion ratio of about four. Karting, with its broad torque band requirements, looks to favour a ratio of about six when the exhaust system is of the expansion chamber variety.
Judging from the broad torque band shown in Figure II, any change to the McC system would yield only a small improvement. Unless the reader is truly enthused about experimenting with the exhaust system, it is best to simply leave this parameter fixed.
Dyno testing shows that changes to the megaphone length affect performance only slightly. It is not as fussy as the critical inlet pipe length. The secret of tuning the exhaust's primary system lies in the length of the lead-in pipe. It controls the timing of the reflected wave, hence the tuned rpm. Long pipes require more round trip time and look the best at low engine speed. Figure II shows the power change caused by shortening the the lead-in-pipe on a McCulloch system. The data was obtained by a dynamometer test—the horsepower values have been purposely removed to indicate only the trend of the modification.
This curve points out the need for proper gearing. Engine speed must be kept above 6,000 rpm and below 9,500 rpm when using the stock 3 inch flex length. Removing the flex althogether requires a minimum speed of 7,000 rpm with a maximum above 10.000 rpm if you want to use the exhaust system to best advantage, Note that there is a good power band of 3,000 rpm. Many motorcycle systems are quite peaky with only 1,500 rpm leeway. The only variable in the test of Figure II was the effective internal length of the stainless flex tube. The plotted values are typical of those found in all tuned exhausts— the peak power at the maximum rpm must be sacrificed to obtain a broad torque range. Fast circuits such as Willow Springs Raceway, for example, keep the average rpm high, allowing a shorter lead-in than the 3-inch production length. Most manufacturers tune their systems for the "average track condition" leaving latitude for the fellow who tunes for the specific condition.
Figure III was included in this article just to show that the average exhaust temperature rises with engine speed. This increases the wave velocity in the exhaust system which tends to keep an accelerating engine in tune. The temperature rise, unfortunately, does not keep pace with the requirements imposed by the increased rpm. Hence, we must tune the system for the desired speed range. The temperature rises because the engine traps more of the mixture delivered because there is simply less time available for the air-fuel mixture to escape. This gives more efficient operation. Power drops because less mixture is delivered with increased speed above the tuned range—yet the temperature continues to climb because of the increased trapping efficiency.
Figure IV shows that the wave speed through exhaust gas is a function of the temperature (it is independent of the pressure). Note that the average speed will be in the 1,600' to 1,900 per second range.
The exhaust products move through the exhaust system at a much slower speed than the sonic waves— this velocity being around 300 fps. Hence there are two velocities in the pipe—we need to be concerned with only the high speed sonic waves which accomplish the tuning we are looking for. A sonic wave in the exhaust stream is analogous to a ripple moving upstream or downstream in a slowly moving channel of water. The wave can travel much faster than the average particle speed in the water channel.
Before we go on to the secondary systems let me restate the objective of the primary system. It must convert part of the positive energy, leaving the engine, into a delayed negative pressure wave. This wave, which can be thought of as a series of overlapping pulses, should arrive at the cylinder just before the transfer ports open. This low pressure area aids the scavenging of exhaust gas. It also should over-scavenge the cylinder, pulling some fresh charge into the exhaust pipe. The secondary system will complete this scavenging sequence.
SECONDARY SYSTEM:
A disassembled Volume Tuned Exhaust System without the cylindrical can is worthless. Dyno data shows that when the stock primary system is used alone, less power is produced than when the engine is exhausting directly to the atmosphere. This is contrary to what one expects from a lead-in-pipe and megaphone. Obviously, the secondary system changes the characteristics of the primary system. Any change, therefore, to the secondary system also influences the operation of the primary system. The secondary system of the Volume Tuned exhaust accomplishes the same thing as its counterpart on the expansion-chamber exhaust. As already mentioned, this portion of the exhaust system must send a positive pulse upstream toward the exhaust port to back-stuff the over-scavenged mixture into the cylinder.
The common expansion chamber does this by reflecting the original positive pulse off the semi-closed end of the chamber. Note that a closed end pipe reflects a positive pressure wave back to the source with the same positive pressure. A sign change does not occur. There is a well defined path for the wave because of the continuous nature of the chamber walls.
On the other hand, the volume tuned exhaust system has no organized path for this wave to follow until it gets back into the primary system. At first it was thought that the exhaust system relied only on the pressure buildup in the entire volume chamber to produce the back stuffing wave. This was the theory during the first months of exhaust system development which resulted in the volume tuned name for the system. However, if you measure the distance from the flat end of the can to the exhaust port, you will find the 28-inch length to be about the same as that found on an expansion-chamber tuned for the same rpm. This is an ideal surface to reflect a pressure wave. The positive wave could also reflect from the other end of the inlet end of the can or from the cylindrical chamber wall. The randomness in the volume chamber probably accounts for the silencing action.
What actually happens to produce the back-stuffing wave is somewhat of a mystery to me. We do know, however, that the factory set-up uses a can length of 11 inches. Decent power was produced during the development programme with shorter lengths of 10 and 8 inches, while shifting the peaking speed to a higher rpm. One fellow at a Willow Springs IKF enduro was using a homemade volume tuned exhaust system. The exhaust system looked similar to the McC layout except for the dimensions of the volume chamber which were altered —even though the volume remained unchanged. The modification was made because of a space problem in his "B-Bomb" powered F.K.E.
His dyno testing indicated a sizeable gain at the lowend, and also at the top-end. Unfortunately it had a flat spot in the mid-range that fell below the power produced by an exhaust stub, which has not tuning effect, This mid-range problem adds weight to the argument of maintaining a length in the 28 inch range from the exhaust port to the can end.
Just for the purpose of illustration for he back stuffing wave to arrive at the exhaust port just as the transfer ports close. Use a 9,000 rpm speed for the McC-91. The exhaust ports opens 82 before BDC, the transfer closes 61° after BDC; for a total required wave duration of 143 degrees of crankshaft rotation. A little arithmetic shows that it takes 0.00265 seconds for the engine to turn through this crank angle. In that short time, the wave must traverse up and back in a 28 inch system for a total earth length of 56 inches. The required wave velocity comes out to be 1760fps, indicating a reasonable average exhaust temperature of 875')F.
Getting on to the last item, number 5, we intuitively see that the pressure rise and average pressure in the volume chamber is also influenced by the area of the 5 inch long outlet pipe. A whole range of these diameters were experimentally tried on the dyno one at a time. From this data, power curves were plotted that slowly climb to a peak value and then fall off again. The quiet sounding small diameter pipes give a slight ( a very slight) increase in bottom end torque, while sacrificing mid-range and top-end power.
Excessively large diameters, in addition to poor power, result in a lot of noise. The 0.780 ID stock outlet is on the small side of the optimum power peak for a 100cc engine. Increasing this outlet would boost power a few percent in the 6,000 to 10,000 rpm range—but, as you might expect, this results in a discouraging increase in the noise level.
If the same experiment were tried on the larger B-class engines, the same trend would be observed. The specific change noticed would be that it takes a larger diameter pipe to reach the peak of the power curve. The 7.5 cubic inch McC-101, for instance, needs an outlet pipe diameter of just over an inch for best power and rpm.
If you want to experiment with this effect, weld on a 1-1/8 inch diameter outlet pipe in place of the present stub. Be sure to increase the hole in the exhaust system can to match the larger ID tube. The length of the pipe incidentally does not adversely influence power unless you get it excessively long (12in.). Some muffling is provided in the outlet pipe—so keep the length in the Sin. range. After the exhilarating effect of all that noise has worn you down, the exhaust system can be quietened by pinching the end of the tube down to reduce the area toward the stock dimension. Flatten the pipe until the area reduction begins to eat into your lap time. You will end up with an outlet area about the same as the stock set up. Make sure you leave the exhaust stub on the side wall. Less noise is created here when compared to a similar pipe on the flat end of the cylindrical can. The reason for this noise increase can be surmised by examining the pressure pulses in the volume chamber. When the initial positive wave reflects from the closed end of the can, the wall feels the effect from both pulses. The original wave and the reflected wave add, giving a greater pressure, hence a louder noise. The side of the can, on the other hand, does not lie directly in the path of the pressure pulse, resulting in a lower peak pressure.
If this hypothesis is correct, similar results can be expected by moving the tail pipe on the expansion-chamber type exhaust system to a a central location on the chamber wall. If you can not meet the silencing rules with your stock expansion chamber, this experiment is worth a try.
CONCLUSIONS: The purpose of the volume tuned exhaust system is to recover the energy remaining in the cylinder when the exhaust ports open—it must do this while maintaining a silencing effect. Both objectives are accomplished by controlling the resonant pressure waves in their primary and secondary systems. The right exhaust system re presents a good compromise between peak power, torque range, and noise. It can easily be re-tuned for a specific track requirement.
If you bother to try the tuning possibilities listed here, you would find that the biggest and the easiest gain in re-tuning can be made by altering only one dimension. This, the very important inlet pipe length. Most systems can be adjusted at the flex tube for varying track conditions. A change in the flex tube length alters the wave distance travelled in the primary system and also changes the total distance travelled by the back-stuffing wave. The flex tube length therefore has the ability to re-tune both the primary and the secondary system. If done with care, the results of your tuning will be a reduction in lap time and you will still have an exhaust system that is quieter than any production expansion chamber.
Exhaust System Design for the Karter In an effort to enable you to modify your present Expansion Chamber type exhaust system, or assist you in designing a more efficient exhaust system utilizing an expansion chamber, the following information is being printed to serve as a guideline.
IMPORTANT —Keep in mind that the following is not intended to be a suitable system for any particular engine, since that is dependent on several variables such as gearing used, fuel used, engine porting layout, type of track being used and entails considerable advance calculation. It offers a starting point only to design an effective system.
A properly designed exhaust system increases an engine's volumetric efficiency by complementing and raising the mean effective pressure, due to increased "breathing". The exhaust system offers what may achieve, the largest single increase of usable horsepower of any modification applicable to a two-stroke engine, however it remains a relatively dormant area of knowledge. Those willing to spend the time, study and effort to truly understand it are in a distinct minority.
The increase in power is achieved due to the action of what is commonly referred to as "pressure waves" in the expelled exhaust gases. By controlling the wave action, through exhaust system design, to correlate with and compliment port timings. A considerable power increase is possible. Since the wave action in a resonant chamber or expansion chamber is extremely complex. It is practical here, only to supply simple guidelines toward this end by offering what has been learned from experimental work, practical application and studies conducted on the subject.
The point in the RPM scale where maximum torque will be developed, depends on the distance from the exhaust ports' "inner edge" to the large end of the convergent cone, as well as the length and diameter of the system's outlet tip. The overall volume of the entire system also has an effect. A rough rule concerning this matter is that, "as the point where maximum torque is developed, rises on the RPM scale, the length and the volume of the expansion chamber decreases. But the outlet tip increases in length and reduces in diameter." This would mean then, that compared to an exhaust pipe designed primarily for use on a tight sprint track, an enduro tuned pipe would be relatively smaller, with a longer outlet tip of smaller diameter.
The megaphone (diverging cone) angle will lie between 6 degrees and 12 degrees, for a 100 c.c. engine. This cone should increase in diameter 1.4 to 1.8 times the engine bore diameter. It's length should be in the area of 8 to 10 times the bore diameter.
The second cone should reduce the diameter back down to .5 to .6 times the bore size in a length of 3 to 4 times bore diameter.
The outlet tip length will be 1 to 3 times bore diameter, dependent on RPM desires, and .2 to .3 times bore in diameter.
Diameter of the header pipe on engine will be determined by the exhaust port size and shape, but should fall within area of .6 to .7 of bore diameter.
Please remember that these figures are only a guide and final design will also be influenced by individual engine characteristics, as well as the type of track you will use it on.
Once you have designed a truly superior exhaust system, (within the current sound level requirements), and its efficiency increases, some engine modifications (in open" classes) which may be needed and will help even further, are "increasing size of high speed carburettor jet size and reduced exhaust port "lead time". These are both due directly to increased efficiency of your properly designed exhaust system.
VOL UME TUNED EXHAUSTS, written by DALE HERBRANDSON and reproduced by courtesy of XARTER NEWS
Anorak
I am guessing you went with the "white background forum option" Rob LOL
I can read it only by Highlighting the text with my cursor......
Kinky is using a feather. Perverted is using the whole chicken
Was worth a crack
'May result in tearing the arse out of the space time continuum' hahaha (sorry no emotive icons on app)
Forum whore
all the colours of the rainbow
"Instructions are just the manufacturers opinion on how to install it" Tim Taylor of "Tool Time"
“Saying what we think gives us a wider conversational range than saying what we know.” - Cullen Hightower
Forum whore
Ok now that work is settling down to easy 12 hour days and the 50 is on track, I can get back to the EFI project again in my spare time.
Started making a new injector manifold that points the small Ecotrons injectors across the transfer port windows, previously the outside pair had just fired straight into the B transfer ports and impinged on the back of the cylinder liner.
I want some of those "Power Bands" and I am looking forward to completing the trial EFI project so then I can get onto the Suzuki GP125/RGV250 EFI air cooled cylinder conversion with TPS and Digital ignition. I am looking for mid 30's rwhp and a wide power spread from the RGV power valve, ATAC header, 50mm Trombone and variable stinger nozzle venture.
The rotary valve will be increased from 112mm to 126 and the crankcase volume increased as much as I dare. The EFI is so I can re visit the plenum as previously with a carb there was a lot of raw fuel sloshing around in the bottom of the plenum and this upset the fueling.
Also the Ball Valve inlet is 34mm diameter and with the Ball Valve inside the plenum the plenums 24mm inlet is less of a restriction than having the 24mm carburetor equivalent nozzle in the inlet tract itself. And as a plan B in case I can't get the plenum to run as well as I would like is a large boost bottle arrangement that will sit between the 24mm restricted Ball valve and the large rotary valve window.
I see the biggest issue with the air cooled RGV250 cylinder will be getting the heat away from the underside of the exhaust port tract area. This is what we do with the GP125 cylinders and I have new and bigger ideas about what we can do to extract heat from the RGV cylinder in this area.
Sure, it will not be as good as having water cooling but then there are ideas about an H2O CVT unit based on the RG400 cylinder that could very well become the follow on project.
Anorak
Thought i had posted this (but looking back maybe not)............
Kinky is using a feather. Perverted is using the whole chicken
Pristine Knee Sliders
I'm planning to make a 2T LC cylinder head using inserts. Any ideas on what alloy spec would be best, and where I might get billets thereof?
I found a place (Znoelli) selling 6061 T6 & T651 billets, would those be good composition/hardness/etc?
All suggestions & guidance welcomed...
Ta
So I'm putting the MVX bucket together again. It's getting a GT125 up it again for now. Mr Nabbs prolly gonna get his arse roped into the building thereof.
But I've got a wee gem of a question that some of you knowledgeable types might enlighten me on.
The MVX motor is a V3 with a terrible issue with fucking the rear slug. The things fire both front pots at the same time, then the rear (middle) 90 or 270 degrees later.
Anyhoo, can I get the crank pressed apart, replace the middle webbs and crank pin with a straight bar from the inner webbs of the front slugs...at the same time as having the front two timed to 180 degrees?
From there I chuck a pair of Honda scooter slugs and barrels. Boom, 100cc water cooled two stroke twin, with a great six speed box?
Forum whore
6061 T6 is a good all round alloy for machining engine ( and autogyro ) parts.
I use 6061 T6 351 to make autogyro rotorheads amongst other things, good strength, good corrosion resistance, machines very well, accepts anodising well.
Forum whore
Doubt anyone has done this with a MVX but definitely doable I imagine. A 100cc H2O motor with a sweet gearbox sounds like it would be worth the effort.
Bob Haldane had a TR250 development motor with one pot, the left side flywheels were replaced with a straight shaft, worked a treat. I used it to develop a methanol setup for a TQ midget he was sponsoring at the time. The TQ midget had to be air cooled and later they used TZ cylinders with large holes cut in the water jackets, with methanol they worked a treat like that too.
Anorak
From memory the MVX250 has the same gearbox and ratios as a VT250F and i think a CBR250r
they are good close ratios
edit i think the VTR250 has the same box as a CBR250r (the four)
bit still pretty sure the MVX is the same box as the VT250F
Kinky is using a feather. Perverted is using the whole chicken
Ta
I was just concearned that the pin replacing a pair of webbs might not stay located, and twist. But it shouldn't if I think about it, since the left side was always supplying drive from a long way away from the primary drive cog.
Mmmmmkay, looks like I need to go and save a buggered MVX lump from a trip to the scrappy.
First is very tall, then nice short steps right through the box. I dunno why this didn't occur to me before now. P'raps the GT will remain on the shelf...or move to someone elses.
Anorak
I see a fly in your ointment drew the MVX has a 48mm stroke...
Maybe a Aircooled 125.........
Kinky is using a feather. Perverted is using the whole chicken
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