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# Exhaust backpressure the myth

Backpressure: The myth and why it's wrong.

I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?

I often wonder how the myth "Engines need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

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## ThumperTalk

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• Ok, I idolize, worship the new 2018 YZ450F (new for me because I bought it late in the year).
Haven't had any starting issues or had to use the new starter plunger until the winter temperatures crept in.
I keep it in an off and on heated garage, but in the morning it's cold, and would not start so easily anymore as it did in the summer, where I never before had to use the starter circuit.
The F.S.M says to push the plunger all the way in, then hit the starter, and let the engine run/rev at 3000-5000rpm!!! for 1-2 minutes.
Then, your supposed to operate the throttle, as the chop throttle direction is supposed to kick the plunger back out to its N.O.T. position.
I did exactly this, but, when I operate the throttle, the throttle close direction DOESN'T kick the plunger back out.
The engine was revving at about 3100rpm for only about 40 seconds, when, to my dismay, I looked down at the engine & exhaust pipe; the exhaust pipe exiting the cylinder port was glowing so hot I swear it was almost molten!
Screaming inside my head, I quickly grabbed a pair of long slip joint pliers to pull the plunger back out (or else I'd no doubt brand myself on the glowing exhaust), and the motor rpm dropped to a more conventional warm/idle up speed.
I then had to accelerate the water pump briefly, then cut the engine. I waited a few seconds, and started it up again, and varied the throttle slightly. I repeated this a few times in order to cycle the motor, and safely remove the heat conduction thru the cylinder head.
My YZ450F runs great, post incident, and has more power than ever now. My only issue is now I have to always have a pair of pliers when cold starting the powerhouse, because the throttle and starter plunger still don't communicate. I DON'T let it run for more than like 15 seconds on fast idle anymore.
My question is, WHY did the pipe heat up so much if that is how long it's recommended to fast idle, AND does anyone else have this issue with their starter plunger sticking, or had their pipe heat up orange hot?
I went to the dealership to correspond with the mechanics there and no one was able to give me any information.
Please, ONLY 2018+ YZ450F owners need reply on this thread, as it is a BRAND NEW starting device on a fuel injected engine for Yamaha, so other instances or experiences will not apply.

• Cone pipe and SX CDI I removed off my 16 Te300 before I sold it. Pipe has couple real small dings other wise like new. If you race or just want endless rev with out sacrificing low end then this is the combo you want.
• By wcoats
The first header pictured it the one on the TTR125 with what looks like a supertrapp muffler.  the second one is the header on my other TTR125 with FMF.

• With a little bit of work on your part, Wiseco Garage Buddy Steel Valve Kits can help your dirt toys deliver years of service. Read on for full details on these reliable and affordable valve replacement kits.

One of the basic truths of the imperfect world we live in is that the people who design machines are not the same people who have to maintain those machines. This often leads to situations where something that seemed like the way to go on the CAD screen turns out to be more difficult or more expensive to fix in the real world than it otherwise would be. Exotic materials and painstaking processes that are economical to implement when you’re mass-producing something turn out to be expensive to service in the field.

Today's 4-strokes are engineered to be high-tech, but the parts come with a big price tag.
In this single-serving, throw-it-away-when-it-breaks world, there are some noble souls who take a stand and say that we should be able to service and maintain things ourselves instead of discarding them, bringing new life to machines that need a bit of a refresh. Such is the case with Wiseco’s Garage Buddy Steel Valve Kits for a variety of popular dirt bike and ATV applications.

Wiseco Garage Buddy Steel Valve Kits were engineered to be a more reliable and affordable option for riders who need to replace valves in their modern four-stroke machines. Read on for complete details!
When faced with the price tag on factory replacement parts for bikes that came with trick valvetrain components, many owners cringe at the price of refurbishing a tired engine. However, with the right components at the right price, turning your dirt bike’s mid-life crisis around and letting it catch its second wind can be easy.
Win on Sunday, Sell on Monday
With the incredibly impressive machines under race tents worldwide, nobody wants to buy a new bike that has a whiff of “outdated” technology surrounding it, so a lot of the high-end features that really only make a difference to the top one percent of professional racers become must-haves for weekend warriors who just want to trail ride with their kids. When those parts wear out, the exotic bragging rights come with a cost, though.
“Titanium is a great valve material due to the strength-to-weight ratio, and also the material’s ability to deal with the high temperature of combustion,” Wiseco Product Manager Dave Sulecki explains. “The light weight is important for engine acceleration; imagine how a heavy component takes more energy to move, and you can see where titanium is ideal when the camshaft needs to accelerate the valve quickly with less energy, and you can see that a lightweight component would be critical for a high-end racing engine.”

Titanium is popular for valves for its light weight properties, but they are expensive to manufacture and can wear out faster than steel.
While those race-spec valves come standard because they’re a positive selling point on the dealership floor, they’re mostly there for bragging rights instead of making a difference you’ll feel when twisting the throttle yourself, and it’s cheaper for the manufacturer to make everything to one specification than it is to have separate designs. “This light weight and performance comes at a greater cost,” Sulecki adds. “The material is more expensive, and costs more to machine or form into a valve. Additionally, the titanium requires a special coating to deal with the heat and wear, which also adds cost. This expense is needed for the highest performing engines, like the type you find in nearly all levels of racing from motocross up to Formula 1.”
Sticker Shock
Even expensive, exotic materials wear out, though, and when it’s time to freshen up the valvetrain of your bike, you might be surprised to see just how much it will cost to replace like-for-like with factory components. Per Sulecki, “Steel valves are a great low cost alternative to titanium, and offer longevity, reliability, and improved wear over titanium. Some customers are not always racing their vehicles, and just want longer service intervals and the peace of mind that comes with this material.”

"Steel valves are a great low cost alternative to titanium, and offer longevity, reliability, and improved wear over titanium." - Dave Sulecki, Wiseco Powersports Product Manager
That’s where Wiseco’s Garage Buddy Steel Valve Kits enter the picture. They’re designed to be an affordable way to refresh your high-tech dirt bike’s valvetrain. Although they may not be made from titanium, that doesn’t mean they aren’t precision-engineered. “Because steel valves are a small percentage heavier than titanium valves, heavier-rate valve springs are required to control the valve and protect the engine from valve float (the condition where the heavier valve will stay open under high RPM engine speeds),” Sulecki explains. “These springs are included with the Garage Buddy Steel Valve Kits.”
Garage Buddy Steel Valve Kits are available separately for both intake and exhaust valves. They come complete with the valves, springs, and even a free packet of cam lube to make sure every box is checked during your reassembly.

Converting to steel valves requires using valve springs designed for the specific weight of the valve. Springs are included with Garage Buddy Steel Valve Kits.
Wiseco’s extensive experience with powersports valvetrain components provides confidence that their conversion kits are engineered to restore showroom-floor performance, and they utilize stock retainers, seals, shims, and other components for affordability and drop-in compatibility. The springs are crafted from premium chrome vanadium steel, and the nitrided steel valves can actually outlast an OEM titanium valve by a factor of three or more.

Wiseco's nitrided steel valves are designed to utilize stock retainers, keepers, and seals. The steel conversion valve springs are manufactured from chrome vanadium steel.
Time For A Change
So, how do you know when it’s time to replace the stock components, short of a dropped valve or broken spring? Per Sulecki, “Valves and valve springs wear over time, like any highly-stressed engine component. When you are checking the valve clearance, or making shim adjustments, this is always a good indicator how quickly the valves are wearing or receding into the seat.” Keeping an eye on these telltales during your regular maintenance will allow you to judge when your factory valves and springs are reaching the end of their service life.

Entire engine in need of a refresh? Garage Buddy also offers Complete Engine Rebuild Kits, check them out here.
“When you are inspecting your top end for general overall health, such as the piston and ring condition, this is the best time to take a closer look at the valves and valve springs,” he continues. “Valves and springs need to be removed from the cylinder head for full inspection. Once these are removed, you can look closely at the condition of the valve face where it seals to the valve seat, and also the condition of the valve head overall and the stem condition. Any cupping or damage to the valve face means it is time to replace the valve, and any similar wear to the valve seat means replacement or re-cutting will be needed.”

Inspecting your valves for wear while doing a top end is a good idea. Closely inspect the sealing surface of the valve for cupping, and inspect the rest of the valve for wear or damage. It's a good idea to also check the groove at the top of the stem for signs of wear to avoid breakage.
Over time, springs become less elastic and may no longer be able to control valve motion at high speeds, but it’s not the sort of wear that is immediately obvious to the naked eye. Sulecki suggests, “Valve springs should be inspected for free length, and also overall condition, looking for any wear marks or defects that can lead to spring failure.” Any nicks or cracks are a sure sign of impending doom, and your cue to replace the entire set.

Valve spring free length can be measured and compared to the recommended spec to get an idea of wear on the spring.
Doing the Job Right
Depending on your level of mechanical aptitude and how well-equipped your garage is, valve replacement might be a job you want to subcontract to a professional. “For most all valve replacements, it is a good idea to work with a qualified builder if you are not sure about the condition of any of these components,” Sulecki suggests. “The work can be done in your own workshop, but there are some special tools required to remove the valves from the head, and having an experienced eye on these items is always the best approach if you are not sure what to look for. An OEM service manual is always the best place to start, they will provide information about any special tools, and guidelines of what to look for regarding valves, valve seats, and even valve guides, and their condition.”

When replacing your valves, be sure to use proper tools and follow all procedures and specifications outlined in your owner's manual. If you're unsure about performing your own valve maintenance, we recommend taking your machine to a trustworthy and certified shop.
Whether tackling the job yourself or letting a pro handle your top-end maintenance, you’ll save time and money by seeing to all the wear-prone components at the same time. Sulecki adds, “When replacing valves, it is a good idea to inspect the top end for any concerning issues or conditions. Inspect the valve seals, valve keepers and seats, shim buckets, the condition of the cylinder head (flatness and sealing condition), and cam chain condition.” Needless to say, the time to service or replace these components is while everything is apart in the first place, and by using quality components like Wiseco’s Garage Buddy Steel Valve Kits, you’ll protect your investment for many off-road seasons to come.

Wiseco Garage Buddy Steel Valve Kits are available separately for both intake and exhaust valves.

• I have some questions to clear up, concerning uncorking a 2012 TTR250, that I need to keep road legal to Australian NSW laws.
Is it the pipe coming from the Bike or the Exhaust (The Header) that deals with noise I get the impression that it's the Exhaust.
I know the Exhaust system is restrictive but is the problem in the Exhaust or in the pipe.
If it's the Exhaust, is there a aftermarket one that will help uncork the Bike but not exceed noise limit of 94 decibels? It seems the powercore is loud at 96 without a insert.
Also when buying a Exhaust you don't need a full new system do you? As in new pipe coming from the bike aswell, they should mount straight on yeah?
Also is it possible to rejet the carb, and do airbox mods, with out an exhaust? Or would it be better to just leave it stock?
I know lots of dumb questions but I would appreciate any help.

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