I've never been able to find a reasonable explanation of back pressure and why it may or may not be beneficial. However I always felt there was something to it, I just didn't have the facts. The reason I thought there must be something to it? Take a look at a Yamaha sportbike (which originated this I believe....called the EXUP valve) and the more recent Hondas and Suzukis and you'll see a power valve. It looks more or less like a throttlebody but is located in the exhaust. This valve then opens or closes a certain amount (controlled by the ECU) depending on speed, rpms, throttle opening etc.. the idea is to make the maximum HP at different rpm ranges by controlling the pressure waves.

If you can, take a look at the new February issue of T.W.O magazine, it's a British motorcycle magazine, there's an article talking about electronics on motorcycles. They have a very informative little section on exhausts and pressures where they describe the use of the above mentioned valve, and helps to explain the mysterious "backpressure".

Here's the gist of it:
1. Exhaust valve opens, high pressure wave travels down exhaust pipe, when it reaches the end of the pipe it is reflected back up the pipe as a low pressure wave.

2. If you time the low pressure waves right they'll come back up the pipe to the exhaust valve area just as it's reopening, and will reflect off of the valve area and return back down the pipe again (towards the end of the pipe,) but as they reflect off the valve area the pressure pulses help to scavenge exhaust gas out of the cylinder. It stays a low pressure wave when it bounces off of the valve area and heads back out the exhaust.

3. There's is where it gets a little weird but I think I have it right: The low pressure waves mentioned above go to the end of the exhaust pipe, but are reflected this time as a high pressure wave. This high pressure wave travels back up the exhaust pipe towards the valves (and once again if timed right,) just as the exhaust valve is closing it will hit the valve area and actually force the fresh air charge (currently filling the cylinder) that is trying to escape the closing valve back into the cylinder, thus giving better filling and greater efficiency.

Now the way to control these waves would typically be exhaust pipe length and diameter, BUT there is a different pipe length that would be optimal for each rpm range. So what the exhaust power valve does in the motorcycles is adjust these pressure waves as best as they can to correlate with the appropriate rpm range, and thus provide the best power and torque. Each manufacturer has a different way of doing it. Honda goes a step further than the other's single valves and actually controls all 4 exhaust openings to get the best results.

It's an interesting little article and was the first time exhaust pressure waves were explained so that they made any sense to me. I'm pretty sure I got the above info right, but check out the article. Maybe I'll grab a copy and write the couple paragraphs in here fully.

So now the big question, why haven't they bothered with this on cars? And especially this car which needs all the help it can get to increase power through the lowmiddle rpm range.

Also the above is similar for intake pressure waves.

Also on the intake side is another area where a valve would help the S2k with cylinder filling at lowmedium rpms. Valve closes partly at lower rpms to allow the air to accelerate and fill the cylinders better. Then at higher rpms the valve opens all the way to not choke off the cylinders as they need more air for higher rpm breathing. Same thing, if it works on a bike, why not on a car?

Hope this helps and sheds a little info on this often confused subject.

 

good thread....i'm interested to know if that would work on a car, as well....it sounds like a good idea.

 

First off, note that what is being discussed is not backpressure - IOW, a flow restriction usually measured as a pressure level.

In terms of what you folks are talking about - variable exhausts and intakes, they are done on some cars. A number of cars use variable chamber mufflers, although they are more for noise management than low end torque gains (accomplishing both though). Variable intakes are very common too. Every Honda V6 sold in the US now uses a variable plenum system and the K24 engine in the CRV and K20A3 in the base RSX use a variable runner system as well. And that's just Honda.

However, you'll find that very few race bikes use the variable exhaust system. They add weight, complexity, and are often a restriction at high rpms. Just check the gains available from a good header/exhaust on a Suzuki GSXR1000 with the exhaust valve removed.

On the variable intake side, they also tend to be a bit restrictive at high rpms. That's why you don't see them on ITRs, RSX-Rs, S2000s, etc. In F1, variable length runners are used, but they are not cheap, nor practical, nor space efficient for production cars - yet. They also depend on the usage of individual throttle bodies, which are present on only a few production cars these days - the cheapest being the E46 M3.

UL

 

When selecting proper exhaust, there is a term we performance guy's like to call sonic tunning. The variable exhaust is cool and all but you will never find that on a race car. Adds too much weight...hahaha...well anyway back to what I was saying. You hear the term "tuned exhaust" There is a reason that racers scramble to find the right headers that work for there particular type of racing. And it is not just exhaust, racers change the height of their velosity stacks on their racing throttle boddies. There is horsepower to be found on both ends of our motors.

Much like targa46 read, when sonic tuning the goal is to get just the right amount of scavenging... I think that's how you spell it, well my spelling is not important. The exhaust is used in-conjunction with the piston to remove the exhaust from the cylinder... ok we know that from targa...(cool right up by the way). It also helps the induction system, like a sort of natural supercharging that gets more mixure into the cylinder then the engine vaccum alone can pull in.

Here is how it works, and I learned this awhile ago so there may be a different way. As the exhaust valve opens and the gases move to the positive pressure, a pressure pulse is created. This pressure pulse runs throught the header pipe at the speed of sound, which is about 1,700 feet per second at exhaust temp. if i remember right. the speed of the pulse dose not depend on the speed of gasses. In fact, the pulse reaches the end of the exhaust much faster then the gases. when it reaches the end, it inverts, becoming a negative pressure pulse, or vaccum and rushes up the pipe to the engine. This negative pressure pulse is what pushes the freash A/F mixure in the combustion chamber for a more complete burn.

Obviously as most of us know the way to optimize this to our advantage is to adjust the header length so that the negative pulse arrives at just the right time. But note that a fully tuned exhaust will only perform best under a narrow range of rpm's. around 1000 give or take. So it is best that the rpm used is at the peak torque range for best results. Making pipes short lets the pressure reach the exhaust port faster good for high rpms. Making it longer is better for low ends, every one knows this. I have seen differences when the lenght of my high-tech headers are moved a few inches.

I could get into the intake sonic therory but i don't want to bore you guy's...... in closing though sonic tuning is a track thing. Big power gains are only possible over short rpm ranges and speeds useful at the track not in traffic. The best approach is to plan a system concentrating on low restriction and high velocity in the intake and exhaust.

Dan
Akari Motorsports

 

Thanks guys for your replies, it's hard to find good info about some of this stuff. Regarding the intakes, that was a mistake on my part, you're right, there are a lot of variable length intakes out there and if I do remember correctly Honda abandoned this on the S2k due to it restricting airflow required for it to rev to 9k. Will this change now that the 04's have a lower redline? 8k is nothing to sneeze at but it's considerably lower than 9k. There are a few motors out there that rev to ~8k and use variable length runners (Porsche, Ferrari...)so maybe this might be feasible for the newer model.

You guys mention racing and how gains are made removing these devices, but racing is just that. High revs all the time, and you're mostly looking for top end stuff. What I'd want to know is how these would help in real world driving to prop up the midrange. I'd have to dig around to find some motorcycle dynos to show how getting rid of the valve affects the motor. I'd be willing to bet top end would increase, but lowmidrange would suffer. Hence it's introduction on street bikes where a broader powerband is preferred.

On the flip side I also understand that the gains using such devices might not really be worth their complexity. It'd be interesting to see what would happen if it were properly engineered though. I'd think there'd need to be a ton of R&D though to get the valve to move correctly with respect to throttle opening, rpms, and all that so that it would actually help and not hinder. Oh well, maybe we'll see something someday!

 

Flow dynamics is much more complicated than what they have described. And they are also partly wrong about when the low pressure pulse is sent back upstream in the exhaust.

When the internal diameter of the exhaust changes from small to large a low pressure pulse is sent upstream. This happens when the exhaust pulse hits the resonator, the catalytic converter, the muffler, and then the atmosphere. This is why it is important to have mandrel bent piping instead of crush bent. Crush bent has tons of internal diameter changes and these pulses get so overlapped they you can't use them at all. When you use mandrel bent, you can more easily control these pulses. But it is still very hard to do.

It is so hard to do that I will guarantee you that any exhaust that is sold to people as an aftermarket exhaust for a production car, will not utilize this at all. This flow has to be tested and modeled using 3d flow modeling software. This software costs a minimum of $15,000 to start. The company I work for has like 2 liscenses and that's all. I can see racing teams in F1 and Nascar, etc. using this. But for an aftermarket company, I SERIOUSLY doubt it.

If an aftermarket claims they have a "tuned" exhaust to maximize scavenging, they are lying.