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Discussion Starter · #1 ·
I HATE how a lot of people on here have the misconception that losing backpressure on an exhaust system is the cause of loss in torque.

Don't waste your time trying to figure it out. its a MYTH that contradicts the most basic concepts of internal combustion engines.

1) Backpressure is the parasitic load added to the upstroke of the piston, trying to push the spent gasses out of the cylinder. The more power wasted pushing gasses out of the cylinder, the less power there is available to turn the crank.

2) Backpressure is the residual gasses that remain in the cylinder after the exhaust stroke has finished, and interferes with the induction of the fresh charge. Obviously, the less fresh charge the engine gets, the less power it can produce.

The optimum backpressure is zero. How much back pressure do you think is designed into NASCAR racers, Indy cars, dragsters, racing boats,...etc.

People confuse backpressure with scavenging. Scavenging is GOOD (it lowers backpressure), but backpressure is BAD.

And it makes no difference whether you are talking forced induction, N/A, 2 cycle, 4 cycle, diesel, planes, boats, cars, cycle, or lawn mowers.

and about scavenging:

Velocity is a component of the scavenging effect of a tuned exhaust. But scavenging ONLY takes place it the interface of differnt pulse discharges. As the exhaust from one cylinder passes across the exhaust tube of another cylinder, it will lower the pressure in that tube, and in another poster's words, "help suck it out".

So the objective of exhaust tuning is to configure the individual tubes so the exhaust flow pass the tubes from other cylinders at exactly the right time. And the velocity at that point does indeed influence the amount of scavenging assist.

But please note that this scavenging only occurs at the interface of different cylinders. It only takes place in the tuned header section of the exhaust system. Once all of tubing and all of the discharges have come together in a common collector or pipe, there is no scavenging, and velocity is no longer a factor.

The principles involved in designing "tuned exhaust system" (actually headers or extractors) do NOT apply to exhaust system in general.

The more quickly and efficiently exhaust gasses can leave the combustion chamber, the more power you can make. And the quickest and most efficient path is one with zero back pressure (or even a slight vacuum, which may be achieved with a tuned header/extractor).

I know there are a bunch of people on here that know exactly what I am talking about, I just want to put the backpressure stories to rest. can this be a sticky?
 

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Discussion Starter · #2 ·
Also coming from a forced induction car background this makes me more sensitive on the subject, heres some more good reading for those that are interested:

"If you think of your vehicle's engine as basically a big pump, the need for free-flowing exhausts becomes obvious. Air in, air out—that's what it's all about.

The goal is to get exhaust out of the system as quickly and thoroughly as possible—greater heat dissipation allows the engine to run cooler, and less exhaust in the cylinders allows more fresh air in for a better burn."

http://www.advanceautoparts.com/eng...20021201eh.html

"We have never seen a case where backpressure improved a properly tuned engine's performance. As far as we are concerned, it is an urban myth that increased backpressure can improve performance. "

http://autospeed.drive.com.au/cms/A_1502/article.html

"There is a myth that engines need lots of backpressure to run properly. A myth indeed! High performance engine builders have known for years one of the quickest ways to greater power and enhanced engine breathing is through exhaust system modifications"

http://www.engineperformancecenter....ust_systems.asp

"The fact is ANY engine - especially turbos - will benefit from the highest flowing exhaust possible. This generally means the largest possible pipe diameter, with the least restrictive mufflers to boot. So long as correct air/fuel mixtures are maintained and there is no detonation, we are yet to see a genuine dyno graph showing the contrary - regardless of the theoretical reasons some people give. (In a naturally aspirated system, the large pipe should start after the tuned length part of the exhaust.)"

http://autospeed.drive.com.au/A_0145/cms/article.html
 

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Jerz -

Great info, but I have to ask... what got you so wound up about backpressure? Are you always this passionate about your exhaust? LOL.
 

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So how come most modern 600cc sportbikes have EXUP valves? I agree with your discussion that reducing backpressure aids in making PEAK horsepower, but doesn't controlling exhaust velocity help make a bike more drivable in midrange rpms? I wouldn't think any manufacturers would put exhaust valves on their modern bikes unless there was some benefit.
 

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I think the benefit of exhaust valves is for low RPM's when the exhaust over-scavenges the cyl. Causing reduced mileage and probably effectively less compression.

I seem to recall the exhaust valves were effecively modifying the speed of the gases, dynamically retuning the pipe resonance.
 

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Over-scavenging?

At lower engine speeds (and therefore lower airflows), the exhaust valve probably helps to increase the exhaust velocity. You cannot tune an exhaust for the whole range of engine speeds, to the exhaust valve helps make up for this.
 

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Discussion Starter · #8 ·
shenion and kpt4321 probably have it right. a 637 cc motor doesnt have much torque at low RPMs at all and the valve helps with that. or so I think it does.
 

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Discussion Starter · #12 ·
Originally posted by stryder420
soooo.....does removing the cat help the air flow? or does it just help the high end performance with the sacarfice of the low end?
removing the cat will leave a big hollow bulge, any ill effects wont be noticed from the butt dyno though.
 

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I guess that I have had exhaust velocity confused with back pressure, thanks for pointing it out. Jerzefigga-what type of forced induction vehicle do you have and how much power are you making? Just curious as i have a built syclone and am big into turbo cars.
 

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Discussion Starter · #15 ·
look at your piping where the cat is, it is a bigger transition in the pipe compared to the rest.

nick041881 I had a 91 MR2 turbo. I made 243 HP to the rear wheels on the stock fuel system. Of course with supporting mods (HUGE IC with proper sized pipes, always 93 or 94 octane, bigger OEM turbo pushing 18psi, intake, DP, MBC and a whole bunch of other stuff)

It was a fun little car that whupped ass. I LOVE the GMC turbo cars. I was looking for a Syclone but they are hard as hell to find. Theres a bunch of Typhoons by me though but I would much rather have the pick up.
 

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great info jerzefigga

nick041881 you are a lucky man. I would love to have a Syclone but as jerzefigga said they are really hard to come by.
 

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ya i love my truck! i took a low mile syclone that was bone stock and started cutting it all up! i put about $17k into it in modifications and never got it on the dyno but we estimate that it was running somewhere around 650hp and torque a little higher than that on full boost. cracked a cylinder wall not that long ago and now i have an expensive paper weight!!

While i was building my truck i bought an 89 mr2 as a daily driver and man it was fun. my buddy is big into mr2 turbos and owns two of them. not the funnest cars to work on though!!!! i remember that we had to replace some hose called the "hose from hell" that runs underneath the turbo on the back of the motor....that was fun!
 

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sounds like one of the coolant pipes on my r32 skyline thats up the back of the engine leading to under the intake manifolds. in the end we had the car jacked up, numerous socket extensions and two felxi joint thingies to undo the bolts of the starter motor while the whole engine intack was still in the car (went from the top near the battery tray, under the intake manifold runners, and around the starter motor slightly), so i could cram my left hand (and im right handed, but my right hand couldnt bend the right way, so i felt unco indeed) up from underneither to get this six inch long hose off without being able to see what im doing. that is my new definition of fun. silly jap cars. :)

sorry to slighty go off topic..
 

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Discussion Starter · #19 ·
Originally posted by nick041881
ya i love my truck! i took a low mile syclone that was bone stock and started cutting it all up! i put about $17k into it in modifications and never got it on the dyno but we estimate that it was running somewhere around 650hp and torque a little higher than that on full boost. cracked a cylinder wall not that long ago and now i have an expensive paper weight!!

While i was building my truck i bought an 89 mr2 as a daily driver and man it was fun. my buddy is big into mr2 turbos and owns two of them. not the funnest cars to work on though!!!! i remember that we had to replace some hose called the "hose from hell" that runs underneath the turbo on the back of the motor....that was fun!
ahh, the hose from hell. theres 2 of those actually, that main one under the manifold and one connected to the oil cooler on the right side of the engine. such a BITCH to remove with the manifold/turbo in place but do-able.
 

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Back on topic.... why don't we just install a high speed sucking fan inside the pipe somewhere that will withstand the temperature? That will increase the exaust flow/create a vacuum and increase power.

(I know I'm wrong somehow, but I want to hear the explanation)
 
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