Exhaust Performance and Scavenging

A very clear and concise explanation of the process of exhaust gas scavenging and how it's affected by exhaust size, length and cam timing.

The Basics

As are most things automotive, the exhaust system is a compromise. It must balance noise and performance. In addition, the exhaust system can be designed to increase performance in the low RPM range or at the top end. All of these aspects must be taken into account when designing the best possible setup for a given application. A larger pipe diameter typically will reduce backpressure allowing the engine to expend less energy pushing out exaust gas. If the system was being restricted, this will result in more power at a certain range of RPMs. The downside to larger pipes is a loss of power in other areas of the power band and increased noise.

Scavenging

At first glance one might think that a maximum pipe size would result in the best performance. It should provide the most flow. This is true for a steady stream of gas. An engine ejects exhaust gas out of the ports in pulses though. This results in a stop-go-stop-go situation as the exhaust valve opens and closes. A properly sized pipe will help to suck the gases out of the cylinder at certain RPMs. Since air has mass and velocity, it has momentum. When in the scavenging RPM, the exhaust charge exiting actually creates a vaccum for part of the exhaust stroke. This provides for a nice boost in power and torque. For ideal scavenging an exhaust speed of 240-260 feet per second is desired. Thus as engine speed increases for a given displacement so must the pipe diameter.

Below is a typical graph of exhaust port pressure vs. piston position:


NOTE: The exhaust pressure during scavenging nets out to be positive. Thus there is not an all around vaccum. Energy is still being wasted pushing exhaust gas out. The scavenging is not "free". Work is still done. Scavenging merely helps to aliviate what would be even more work being done by the engine. Also note that the psi values will vary from engine to engine, but the curve remains approximately the same. Values over 6 psi are generally considered excessive backpressure.

The graph above is a typical representation of what exhaust port pressure looks like as the engine rotates through almost a full revolution during the scavenging RPMs. Exhaust pressure builds as soon as the exhaust valve opens. This occurs before bottom dead center. Most of the torque the engine generates is produced by 90 degrees, thus it is not necessary to keep the exhaust valve closed much past 90. By opening the exhaust valve early, the high cylinder pressure helps to accelerate the charge in the exhaust manifolds. Pressure peaks before bottom dead center and then rapidly falls as the accelerated charge pulls the exhaust out of the engine. As the piston approaches top dead center, the intake valve opens. Pressure drops again and the exhaust gas actually helps to suck the intake charge into the cylinders.

These ideal conditions can only be met at certain RPMs. To do this one can vary the size of their piping to chose what will provide them the best performance. Smaller pipes will move an equal volume of gas at a higher speed. This typically comes at the expense of increased backpressure (resistance to flow) which inturn leads to poorer performance. A smaller pipe will help to maintain the ideal exhaust gas velocity when the engine is running in the lower RPMs. This provides for a torque boost off the line with improved streetability and driveability. For this reason many manufacturers size their pipes on the more conservative side. As the RPMs increase though, the backpressure builds due to the excessively small pipe size. This pushes the velocity of the exhaust gas out of the scavenging range and saps power on the top end, reducing performance in that spectrum.

A larger pipe will maintain ideal exhaust gas velocity on the top end. This will increase peak power. The downside comes with a slow exhaust charge on the low end. Their will not be enough suction to aid in the expulsion of exhaust gas. Torque will suffer in the low ranges for this reason. Too large or too small of a pipe will push the scavenging bonuses out of the useable RPM range and will result in diminished overall performance. It is important to size the piping correctly for the desired application: low end or top end. One should also consider how this will effect the rest of the car. For instance, if the car has an automatic with a stall speed of 1,500 RPMs it won't do any good to have all the power on the top end when off the line performance will suffer terribly. Likewise if the car is mainly driven in the city or through heavy traffic, a lack of low end grunt might make driving more of a chore.

On valve overlap scavenging also helps to draw in the fresh intake charge. The pressure in the exhaust port drops again and the exhaust flows more easily. It should be noted though that too much exhaust velocity on overlap can cause "over scavenging". This occurs when the exhaust vaccum becomes so great, due to overly small tubing, that it actually sucks the intake charge right through. This leaves spent charge in the cylinders and causes a loss in performance.

Common Views

A couple of commonly recommended modifications are increasing backpressure for lowend performance, and the use of equal length headers. Backpressure is almost always a bad thing. The higher velocity of a smaller pipe is what one wants on the low end. Unfortunately this ultimately chokes off the top end to an extent. Since these two go hand in hand (small pipes and top end back pressure), people often consider them the same. They are not. Putting a sharp bend in a pipe will cause high backpressure, but it will have little benefit for performance and scavenging. Backpressure is very useful for quieting an exhaust though and muffling the sound waves. As for equal length headers, companies often claim that they help with scavenging. They are used to line the exhaust pulses up. This can help during scavenging as the exiting charges will not try to occupy the same collector at the same time. Instead, the exhaust pulses will alternate and "zipper" there way into the collector. This results in smooth flow and optimal performance. Equal length headers, or headers designed to take advantage of scavenging in general are a plus. It is important to buy from a reputable company though as equal lengths will not guarentee optimal performance. Dynometer testing is the best way. Most reputable exhaust companies do make use of a dyno which gives some assurance as to what one is getting.

Summation

1. Choose a smaller pipe for better low RPM performance, a larger one for high RPM performance.

2. Remeber that peaks are not everything, as seen in Understanding Performance, thus a proper sized pipe will deliver better results than one that may give the highest peak low end or top end performance.

3. Back pressure is almost always a bad thing.

Article from: http://autolounge.net/tech/exhaust.html

Beyond Superbikes: The Yoshimura Hayabusa Fujio Yoshimura talks about the Japanese built GSX-1300R Racer

196 horsepower, 408 pounds, 200mph

"Racing Superbikes is okay, but there is no excitement," deadpanned Fujio Yoshimura as he began his explanation of why the roadracing planet's best known tuning firm is not competing in the Japanese All Japan Superbike championship in 1999.

"The (Suzuki) factory does their own thing, and there's no chance for a (satellite team). I decided to let (the factory) do the all-out war, and we'll do the X Formula class. I think that's more fun."

The Japanese X Formula class bears some resemblance to the AMA's Formula Extreme in that both classes are aimed at the bigger displacement bikes such as the Honda CBR900RR and the Yamaha R-1. There are three significant differences in Japan, and they are that the factories are barred from having in-house racing teams in X Formula, X Formula bikes can be claimed for 2,500,000 yen (~$20,000), and the X Formula bikes compete with the Superbikes on the track. These three significant differences combine to level the playing field between the factory teams and the privateers.

Well, level is a generous statement, as the factory supported superbikes still out-qualify and out-run the X Formula bikes. At least the difference in lap times between the shiny teams and the duct tape teams doesn't approach three to five seconds, as is starting to happen in AMA Superbike races.

"The factory system is so precise now, so efficient," the son of legendary Pops Yoshimura lamented. "Everything must be tested and done well. For a privateer team like us, we just don't have the resources." Yoshimura's claim of being a privateer team lacking in resources defies what is seen at the race track in America, but the purposes of Yoshimura USA and Yoshimura Japan are much different.

The factory wants to do their own development and things, you know," continued Yoshimura. "The U.S. side of Suzuki, they don't have any factory to do such an operation, so we're doing it for them. They have their own team for motocross, but they don't have anything for roadracing. (Suzuki U.S.) have their own team for motocross, but they don't have anything for roadracing. (The U.S. factory bikes) come directly from Suzuki-Japan to U.S. Yoshimura. I own the company, but I let the guys in California do the business."

Hence in Japan Yoshimura is completely on his own for turning the Hayabusa from a street bike to a race bike.

As for the modifications to the Hayabusa, the answer was classic Yoshimura:
"You can't do anything."

Uhhh .... Nothing? Fujio, come on.

Yoshimura then admitted to his Hayabusa having special pistons.

And a special camshaft.

And specially made close ratio gearbox.

And the Superbike kit GSX-R750 fuel injection system, with higher pressure and in-tank pump.

And the GSX-R750 Superbike kit swingarm.


"We're researching this bike for the people in America, Europe, and also in Japan. I know they love those things," Yoshimura explained. "These special parts will be available in another two months. I'm sure the people in the States will want a lot of tuning parts."

"Right now we're putting out about 196 hp. We began at 165. So we added another 30 hp without spending too much money, just changing the camshaft and the pistons."

Besides the swingarm change, Yoshimura has worked hard at reducing the weight of the Hayabusa. "It's down to 185 kg (408 lb.) now, about 100 lb. lighter. But it's still 50 lb. heavier than a Superbike. It's quite a handicap."

Even with the extra weight, the Yoshimura Hayabusa can accelerate to an impressive top speed figure. "I think we're the fastest on the track, 284 kph (176 mph). We're getting 10 kph on the Superbikes."

Suzuki race kit swingarm =$15,000Further weight reduction and further horsepower improvements can be realized, but at this stage in the Hayabusa development the concern has been transmission durability. "It comes down to the size of the motorcycle. The actual transmission gears are so heavy duty for the size of the bike. It will not shift as well as the racing one, because the gears are so heavy. I don't think there's so much we can do about that, we have to live with that. That's my concern though, how the transmission would hold up with that much horsepower and torque."

If Yoshimura had stayed with Superbikes, this kind the question of transmission capability would probably never come up. There lies the appeal of the X Formula class: the lack of technology creates opportunity for new technology more so than a situation where technology is plentiful.

"A lot of people ask me, 'Why you start racing the Hayabusa?'," Yoshimura admitted. "I keep telling them that I can't go back 25 years when the original Superbike racing started in the States with the Kawasaki Z-1. Those days are gone. Superbike is not like that anymore. The fans are a lot more enthusiastic about the out come of that bike (the Hayabusa) than Superbikes. With me, it's the same."

-- Images and text by Tracy Hagen. Copyright 1999, All Rights Reserved

SPECIFICATIONS	POWER：142kw(193ps) / 10,000rpm TORQUE：142.2Nm (14.5kgm) / 8,000rpm WEIGHT:198kg (DRY) STD : 215kg
MOTORCYCLE	GSX1300R 2000Model
ENGINE PARTS	Tri-Oval Titanium Cyclone Exhaust System Pipe : Titanium / Silencer : Carbon φ81 High Compression PistonKit (12.0:1 Forged) ST-1 Camshaft Set
TUNING UP (ENGINE)	Cylinder Head Porting Valve Polished & Lightened Dyno Tested Re-assemblled by YOSHIMURA FACTORY BMC Air Filter
TUNING UP (CHASSIS)	Original Oil Lock Piece,Piston Valve Shim & Oil Height
CHASSIS PARTS	Aluminium Fuel Tank	Capacity : 24Litter
	Fairing Kit	Upper,Lower, Carbon Front Fender, Seat, Screen, Tool Box, Seat Rail, Mirror, Head Light (High : Halogen / Low : HID),Turn Signal etc.
	Fairing: Coloring	YOSHIMURA Original Colour(Red & Gun Metal)
	Magnesium Step Kit	Adjustable Back: 19 or 31mm/UP: 49 or 61mm
	Hydraulic Houses	Front Brake, Rear Brake and Clutch Hoses
	Digital Dual Temp Meter Set	Water & Oil Temperatures,Voltage,time & Stopwatch
	Chassis Protector
	Brake Pad Set
OTHERS	Racing Stand Serial Number : Cylinder Head Serial Number : Original Name Plate Yoshimura Hayabusa X-1 Book

Crower's 6-stroke engine

Briefly this is how a 6-stroke engine works:

The Crower six-stroke engine is a high-efficiency variant of an internal combustion engine under development by Bruce Crower.

Two extra strokes are added to the customary internal combustion engine four stroke Otto cycle, which makes a six stroke engine. A third down-stroke is a "steam stroke" and a third up-stroke exhausts the expanded steam while venting heat from the engine.

The engine cold starts on the Otto cycle, coasting through the fifth and sixth strokes for a short period. After the combustion chamber temperature reaches approximately 400 degrees Fahrenheit (200 °C), a mechanical operation phases in the fifth and sixth strokes. Just before the fifth stroke, water is injected directly into the hot combustion chamber via the engine's fuel injector pump, creating steam and another power stroke. The phase change from liquid to steam removes the excess heat of the combustion stroke forcing the piston down (a second power stroke). As a substantial portion of engine heat now leaves the cylinder in the form of steam, no cooling system radiator is required. Energy that is dissipated in conventional arrangements by the radiation cooling system has been converted into additional power strokes.

In Crower's prototype, the water for the steam portion of the cycle is consumed at a rate approximately equal to that of the fuel, but in production models, the steam will be recaptured in a condenser for re-use. Heat will be available from the condenser to provide interior heating of the vehicle, much as a conventional heater core works in cars and trucks today.

- article taken from wikipedia

The following is an article taken from Autoweek of an interview with Bruce Crower:

Bruce Crower has lived, breathed and built hot engines his whole life. Now he’s working on a cool one—one that harnesses normally-wasted heat energy by creating steam inside the combustion chamber, and using it to boost the engine’s power output and also to control its temperature.

“I’ve been trying to think how to capture radiator losses for over 30 years,” explains the veteran camshaft grinder and race engine builder. “One morning about 18 months ago I woke up, like from a dream, and I knew immediately that I had the answer.”

Hurrying to his comprehensively-equipped home workshop in the rural hills outside San Diego, he began drawing and machining parts, and installing them in a highly modified, single-cylinder industrial powerplant, a 12-hp diesel he converted to use gasoline. He bolted that to a test frame, poured equal amounts of fuel and water into twin tanks, and pulled the starter-rope.

“My first reaction was, ‘Gulp! It runs!’” the 75-year-old inventor remembers. “And then this ‘snow’ started falling on me. I thought, ‘What hath God wrought…’”

The “snow” was flakes of white paint blasted from the ceiling by the powerful pulses of exhaust gas and steam emitted from the open exhaust stack, which pointed straight up.

Over the following year Crower undertook a methodical development program, in particular trying out numerous variations in camshaft profiles and timing as he narrowed the operating parameters of his patented six-stroke cycle.

Recently he’s been trying variations of the double-lobe exhaust cams to delay and even eliminate the opening of the exhaust valve after the first power stroke, to “recompress” the combustion gasses and thus increase the force of the steam-stroke.

The engine has yet to operate against a load on a dyno, but his testing to date encourages Crower to expect that once he gets hard numbers, the engine will show normal levels of power on substantially less fuel, and without overheating.

“It’ll run for an hour and you can literally put your hand on it. It’s warm, yeah, but it’s not scorching hot. Any conventional engine running without a water jacket or fins, you couldn’t do that.”

Indeed, the test unit has no external cooling system—no water jacket, no water pump, no radiator; nothing. It does retain fins because it came with them, but Crower indicates the engine would be more efficient if he took the trouble to grind them off. He has discarded the original cooling fan.

So far he has used only gasoline, but Bruce believes a diesel-fueled test engine he is now constructing—with a hand-made billet head incorporating the one-third-speed camshaft—will realize the true potential of his concept.

Potential…and Questions

Crower invites us to imagine a car or truck (he speaks of a Bonneville streamliner, too) free of a radiator and its associated air ducting, fan, plumbing, coolant weight, etc.

“Especially an 18-wheeler, they’ve got that massive radiator that weighs 800, 1000 pounds. Not necessary,” he asserts. “In those big trucks, they look at payload as their bread and butter. If you get 1000 lb. or more off the truck…”

Offsetting that, of course, would be the need to carry large quantities of water, and water is heavier than gasoline or diesel oil. Preliminary estimates suggest a Crower cycle engine will use roughly as many gallons of water as fuel.

And Crower feels the water should be distilled, to prevent deposits inside the system, so a supply infrastructure will have to be created. (He uses rainwater in his testing.) Keeping the water from freezing will be another challenge.

But the inventor sees overriding benefits. “Can you imagine how much fuel goes into radiator losses every day in America? A good spark-ignition engine is about 24 percent efficient; ie., about 24 cents of your gasoline dollar ends up in power. The rest goes out in heat loss through the exhaust or radiator, and in driving the water pump and the fan and other friction losses.

“A good diesel is about 30 percent efficient, a good turbo diesel about 33 percent. But you still have radiators and heavy components, and fan losses are extremely high on a big diesel truck.”

Bottom-line, Bruce estimates his new operating cycle could improve a typical engine’s fuel consumption by 40 percent. He also anticipates that exhaust emissions may be greatly reduced. It’s all thanks to the steam.

“A lot of people don’t know that water expands 1600 times when it goes from liquid into steam. Sixteen hundred! This is why steam power is so good. But it’s dangerous…”

The danger of a boiler explosion has long been a factor in engineering—and in operating—steam powerplants of all kinds, and Crower is properly wary of the miniature boiler he has conjured up inside his test engine. That’s one reason he chose to use one originally manufactured as a diesel, for its inherent strength, though he installed a carburetor and ignition system so it could burn gasoline at first.

The original diesel fuel injector system now supplies the water spray to generate the steam-stroke.

In addition to producing extra power, the injected water cools the piston and exhaust valve, which suggests to Crower that he could raise the compression ratio. “I’ve done this many times on regular engines: 15-to-1 on gasoline for the first five seconds works pretty good until you get some chamber heat and then suddenly it gets into pinging. But with the chamber being chilled, I bet 12-, 13-to-1 will be no problem on cheap fuel.

“So what we can maybe do is have fuels that aren’t quite as good…It’ll save a nickel a gallon not having to keep three grades going.”

As for his hope of lowering emissions, Bruce speculates the steam might purge “cling-on hydrocarbons” out of the combustion chamber. “This thing may turn out to be so clean that you won’t have to have a catalytic converter.

But he admits that’s unknown, saying “there’s a lot of experimenting still to be done.” Which prospect makes him smile. He thrives on this kind of challenge.

Bruce’s Background

“You’ve kinda got to be in the cam business and know the dynamics of engines,” Bruce Crower says about how the idea occurred to him. And he certainly has that background.

He was building and racing hot rods (and hot bikes), manufacturing speed equipment and operating his own speed shop in his home town of Phoenix when he was still a teen.

After moving to San Diego in the 1950s, among other exploits he dropped a Hemi into a Hudson and drove it to a 157-mph speed record at Bonneville.

Inevitably, the inventive and inexhaustible Crower built up a major equipment business in superchargers, intake manifolds, clutches and, especially, camshafts. He’s also credited with first suggesting a rear wing to Don Garlits—in 1963, three years before Jim Hall’s winged Chaparral. Bruce Crower is now in Florida’s Drag Racing Hall of Fame.

Crower actually had introduced a wing two years earlier, during practice on Jim Rathmann's 1961 Indianapolis car—five years before Jim Hall’s winged Chaparral. Bruce had been crewing at the Speedway since 1954 (Jimmy Bryan, second place), and had been part of Rathmann's 1960 victory effort. He was likewise on the winning teams in 1966 (Graham Hill) and 1967 (AJ Foyt). Three decades later, in 1998, Eddie Cheever won with Crower cams.

Bruce even produced his own complete Indy engine, a flat-8 that didn’t quite make the field in 1977 and then was rendered obsolete (due to its width) by the advent of ground-effect tunnels. But the Crower 8 and its automatic clutch did win an SAE award for innovation.

Today, Crower Cams and Equipment Company employs about 160 people in five facilities, and manufactures not only cams but crankshafts and connecting rods—including titanium rods for (unnamed) Formula One customers.

Bruce Crower can’t be called retired now, but he’s happy to let the company he founded “roll along” while he “plays with cars.” That’s how he looks at the intensive R&D work he carries out in the privacy of his 13-acre horse property near the rural community of Jamul.

One of several projects is building up Honda S2000 engines for the Midget raced by his granddaughter, Ashley Swanson. (“I think she’s on par with Danica Patrick,” says the proud grampa.)

But his prime focus is proving his six-stroke engine is as revolutionary as he believes it is. “I’ve been trying to find something wrong with the whole basic idea for almost a year,” he says, “but I think we’re going to have a very marketable item.”

Then he adds philosophically, “If it turns out to be great, fine. If it doesn’t, it’s just another year out of my life that I’ve had a lot of fun doing something.”

If this engine makes it out onto the market, it'll be really exciting to see it running in front of you.

It's coming...

Coming next month, looking forward to the improved accuracy of tune.

Exhausts

New look with stainless steel midpipes

Exhaust systems according to Wikipedia:

conveys burnt gases from an internal combustion engine and typically includes a collection of pipes. In the most basic sense, the exhaust system simply vents waste gases from the engine. Depending on the overall system design, the exhaust gas may flow through a turbocharger to increase engine power, a catalytic converter to reduce air pollution, and a muffler to reduce noise.

It amazing how much difference this collection of pipe can add character to the bike. It's the vocal box of the bike giving it the manly growl every time the engine starts and revs. I shall just skip the technicalities and let the video do the talking.

The HMFs are brutally loud but they make very good power... Mid range and top range difference is really significant. But you wouldn't be seeing that as often as I would I them on. Yoshis are the legal way to go.. shall ride around in those when in sg.