Exhaust question

[cprracing]

I was wondering what the diffrence is bewteen taking my car to a muffler shop and having 3 inch exhaust ran from the turbo all the way to the muffler instead of buying, a down pipe then catback.

Also i was wondering if 3 inch is to big? Will i have enough back pressure?
thanks

 

[L2RTSiAWD]

Nothing just make sure it is mandrel bent. No 3inch is not too big.

Later,

 

[kpt4321]

Originally posted by cprracing

Also i was wondering if 3 inch is to big? Will i have enough back pressure?
thanks

Ahhhhhh!! I hate the way people teach newbies that engines need backpressure!! No engine, whether turbo or non turbo, wants backpressure. On a car such as a turbo DSM, the best exhaust would be no exhaust at all (obviousy Corky Bell rip off ). Anyway, 3 inch is generally the accepted standard for a "big" dsm exhaust. It'll be loud, but it's about the biggest people usually go unless they have a crazy setup.

Now, before someone says that non-turbo engines need backpressure: No, they do not. NA engines require, or benefit from, exhaust scavenging. This scavenging can be ruined by an exhuast that is too big, and therefore people often confuse it with backpressure. Backpressure bad, scavenging good.

 

[cprracing]

How much of a power diffrence am i going to see if any?? and does this help the turbo breath better.

 

[NewB2dsm]

Ahhhhhh!! I hate the way people teach newbies that engines need backpressure!! No engine, whether turbo or non turbo, wants backpressure. On a car such as a turbo DSM, the best exhaust would be no exhaust at all (obviousy Corky Bell rip off ). Anyway, 3 inch is generally the accepted standard for a "big" dsm exhaust. It'll be loud, but it's about the biggest people usually go unless they have a crazy setup.

then how come if you put 3" exhuast on a t25 your car will suck balls?? hmm this to me seams funny.. if you did not need exhuast at all why dont drag cars just run without headers?? iv seen a few do it but not common.. or is this just so they dont cautch them selves on fire? probly that. Iv heard that the 3" exhuast is bad for the t25 and 14b worse on t25 than 14b though. B/c its was too big and something like that. I raced a guy with 3" exhuast (full) and FMIC and 15psi of boost and he went no were and i have just boost and intake and 1gen bov and UICP. Hmm maybe his car was #@%#@%#@%#@% witch it could be but maybe it was b/c he had the t25 on way too big on exhuast. i dont know but... 3" exhuast to me is OVERKILL.. was not on a DSM but i remember seeing a dyno test for 2.5" and 3" to see the real difference.. I think it was like 5rwhp at LOW hp and like 20 or 25 at High HP like 500rwhp or above... and the number goes up the more HP you get. And #@%#@%#@%#@% a supra has 2 turbos that can be #@%#@%#@%#@%ing HUGE going out 1 3" exhuast. Some use bigger but most use 3" so 2.5" to me would be fine for our car. But you can probly tell a difference if you have done major mods to the car. My opion i would not pay the extra money for 3" when 2.5" will work just fine. Also alot of places around here cant do 3" madrel bend just crushed.. not good. but i have not shoped around much and im sure a local tuner knows someone who can do this. just my 2cents..

 

[turboholic]

-CPRRacing, it's depend on what you want and how much you can afford at the moment. Some might say 2.5" is enough, it might be, but down the road you want to mod your car even more, you'll need a bigger pipe, you'll have to pay for another set-up. If you have friend that have a welder, you might consider building your own, I made my 3" turbo back with >$150 in parts, it venting from the side.

 

[cprracing]

Ive got a guy down here that will do the hole thing for a $100 for 3 inch and maybe less for 2 and 1/2

 

[Kevin Jewer]

IF a car with t25 or 14b sucks balls with a 3" exhaust, the problem lies somewhere else. ITs just that its not worth spending the money on a 3" when niether of those turbos flow enough to take full advantage of it. No il side effects other than a lighter wallet.

 

[larryd]

Originally posted by kpt4321
Now, before someone says that non-turbo engines need backpressure: No, they do not. NA engines require, or benefit from, exhaust scavenging. This scavenging can be ruined by an exhuast that is too big, and therefore people often confuse it with backpressure. Backpressure bad, scavenging good.

what exactly is scavenging?

 

[Enigma_Man]

I don't know what scavenging is, but this is how it was explained to me at school by the guy who has helped build the racecar there for 20 years:

On a naturally aspirated car (it's different for a turbo, I'll explain why later) having an exhaust of a certain size helps exhaust flow at a certain rpm.

This is because the exhaust comes out the manifold/header whatever in pulses, from each cylinder.

Now these pulses travel down the exhaust pipe at a pretty good speed, Imagine having a straw for an exhaust. Each pulse would travel completely out the exhaust system basically. Now picture a big big exhaust, say a 3" exhaust on a small 4 cylinder naturally aspirated. What happens is these exhaust pulses leave the header, and enter the exhaust pipe. But because the pipe is so large, they don't travel very far, and actually sortof "bounce" back towards the engine.

Okay, for an example you have an engine running at constant rpm / load. You could theoretically pick a size for an exhaust that would set up these pulses so that they very smoothly flow down the pipe, and one pulse sort of "pulls" the next one from the next emptying cylinder out, and so on and so on. It sets up a nice smooth flow down the exhaust pipe. You don't want to go too big, because then the exhaust pulses will all sortof bounce back and forth while going down the pipe, and cause the emptying cylinder to have to push a little harder to get it out. And if it's too small, then obviously it's going to have a hard time getting the exhaust out too. More load needs a bigger exhaust to work efficiently.

So, bigger exhausts give you more torque up top and less on bottom, and smaller gives more torque down bottom. This is why most production cars come with smaller exhausts (and for fuel economy too) so that they're more "driveable" by the public.

On a turbo, this is much less important, cause the exhaust turbine screws up all the pulses anyway, and all you want to do is have as little pressure in your exhaust as possible, to spool the turbine. So, bigger is better for a turbo

-Jesse

 

[mortsayyed]

All I know is that I had a 14B running at 8 psi with a 2.5 turbo back exhaust and it was FAST. Now, I ahve a 14B with a 3" turbo back exhaust still at 8psi and it sucks major balls.

Mort

 

[mike9146]

A 14b at 8psi is going to "suck balls" no matter what exhaust you have. Why on earth would you be running *less* boost than what the car was designed to run from the factory?

Mike C.

 

[RetardedAWD]

You absolutely cannot go wrong with a biggest you can get (within reason) I have had ALOT of experience with turbo 4 cylinder motors (mostly Hondas). Even with a 1.6l dohc motor with a t3/t4 we tried a 2.5 inch exhaust and it made the car SLOW compared to 3 inch... the difference was night and day. The least restriction the better.. the more restriction that you have with FI the hotter things are going to run. It takes alot of heat stress off of internal components and the turbo. It helps spool faster too.. if the exhaust can get out of the way faster the turbine will come on boost quicker..

 

[mike9146]

"Miller you are a profit!"

Revenue-Expenses=Miller?

Mike C.

 

[cprracing]

Im confused now because some say that 3 inch is better and some say that 2.5 is better the hole restriction thing is what i thought but ive heard pros and cons so i dont know what to do now

 

[turboholic]

-3" and you won't regred it. the muffler can build the turbo back for $100? is it + parts, or they'll press bend(wrinkle) the pipe? only big shop can afford an mandrel bend machine. if it's a press bend, then at the curve after the bend the inner diameter of the pipe will be squeeze down to 2.5-2.75" from the original 3" pipe.

 

[Black95GSX]

I thought I'd throw in my .02

On an NA car, you do need back pressure... so bigger is not always better


On a Turbo car... if you go full 3" mandrel w/o a cat, yes you will gain a ton of top end. But at the same time you will lose a little bottom end torque. This is not new information, and has been proven on the dyno several times.


Also keep in mind, that people have gone 11's on a 2.5" pressbent exhaust.... it won't always make a "night and day" difference (for the record, Alan-from alaska went 11's on a 20g with a 2.5" pressbent BR exhaust on his 1g)

 

[swordfish]

On a turbo car the only back pressure needed is provided by the turbo. After the turbo the bigger the better.

jeff

 

[Kevin Jewer]

I'm going to kame a half-assed attempt to clear some things up.

Scavanging is when you use the suction created behind the "slug" of exuast from one cylinder traveling down the exhaust pipe to pull the exhaust out of the next cylinder. There isnt a steady stream of exhaust coming from the motor, it comes in pulses. So you have a "slug" of exhaust, then a gap, then another slug. Now picture this slug moving down the exhaust pipe, filling its entire cross section. As it moves it creates a low pressure area or a vacuum behind it. Exactly like a syringe (sp?) does. If you time these pulses properly, it will create maximum vacuum right when the next exhaust valve opens. The vacuum pulls the exhaust out of that cylinder completely so it can be filled entirely with fresh airfuel mixture, without exahust byproduct contamination. Now, at different rpms, the timing required is different. This is why on manifolds and headers the lenght of each runner is critical, and affects the power band. Long runners make more low end power, and short runners make more top end power. If you think about that and visualize the exhaust pulses traveling and creating vacuums you'll see why that is. This is why 4 to 1 and 4-2-1 headers make different power changes.

As this pertains to exhaust diameter, the critical factor is velocity. The air has to leave the engine and travel the entire length of the exhaust no matter what right? The next pulse is coming... If the piping is very large, it can travel slower (linear motion) and still all get out. If the piping is smaller, it has move fast to get all of it out of the way in time. So as far as scevenging goes, restriction is not the issue, exhaust gas velocity is. Now if we apply what we said above, if the air moves at greater velocity, it will create a stronger vacuum behind. So with less exhaust gasses (low rpm) it still scavenges well. With larger piping it doesnt scavenge as well, but the loss of restriction at high gas flow (rpm) allows a lot of gas to flow, and it picks up power there in the top end. Simple.

Now for turbo cars we want the biggest pipe that makes sense for our budget and flow requirements. Heres why. Low end power means two different things for NA and turbo cars. Lets say at 3000 rpm, its considered "low end" power. On a turbo car, we are most likely already in boost at that point (or at least no vacuum), so we want max flow for the sake of letting the engine breath (at 15 psi it almost doubles displacement effectively, i.e. a DSM becomes a 4 liter. Roughly ;o) Now a larger exhaust will also let the turbo spool earlier, allowing you to make boost sooner, and reduce the off-boost rpm range. Thats what we consider low end on a turbo car really.

Here is why larger exhaust is better for turbos. The turbine wheel extracts energy from the exhaust in the form of heat, and converts it to mechanical or kenetic energy to spin the shaft. The exhaust gasses dont "blow" over the wheel, they expand against it and force it to turn. Thats why the enter from the side, and then exit from the center (vise versa for the compressor side) . As opposed to blowing over it or through it. As heat is extracted from the exhaust gas to do work, temp drops (hence the difference in tapping your EGT probe in the manifold or after the turbo in the downpipe ;o) As temp drops so does pressure. Which brings us to the next point. The more pressure -differential- you have ACROSS the turbine the more work it does. That can be acomplished by increasing the pressure before it (higher rpm, leaner mixture (more heat) etc), or by dropping the pressure after the turbine. Thats the one we really have control over, by choosing a larger exhaust pipe. The larger it is, the less restriction, the lower the pressure (backpressure) just after the turbine. So the turbine can do more work. That means quicker spool. Combine that with a more efficient (less restrictive) engine system as a whole, as a result, and you get a nice power increase across the whole rpm range.

So on a NA car you need to compromise between low end and high end. On a turbo car, who cares because when boost hits 24 psi at 3400 rpm I have plenty of "low end"

Hope this wasnt too long, and helps shed some light.

 

[Kevin Jewer]

This isnt too far off topic from the original question, so here is another post that I wrote to some forum about a year ago. It makes some good points and might explain some parts a little better. But its another long one

"The idea that an engine "needs backpressure to function" is a myth. The fastest cars of them all (top fuel dragsters) just attach a pipe to the exhaust port. You can look into the head hehe. But he is right in a way. When you increase the diameter of a tube, the velocity of a given volume of a gas (in this case exhaust) will go down. At high rpm this helps because more gas can then flow before hitting the max velocity (capacity) of that particular tubing system. At low rpm however, higher gas velocity will create more torque. A very fast moving gas will help with "scavenging" or it sucks in more fresh intake air/fuel to increase volumetric efficiency. It's a bit like when you are standing on the side of the road and a semi goes by. You can feel the suction behind it, and can often see papers and sand etc following it. The same thing happens in the exhaust piping. All camshafts have some degree of "overlap" (you may have heard of the high overlap cams in the vtec motors). That is when the exhaust valve is still open when the intake valve opens. When they are both open, and the high velocity exhaust gasses are leaving with that low pressure area (suction) behind them more air/fuel is drawn into the chamber. It acts like a very small turbo setup would essentially and naturally increases torque. Now when you go and increase the pipe diameter velocity drops and this suction effect is reduced, and torque is reduced along with it. So an engine certainly doesnt need any backpressure to fuction, but higher velocity will increase low end torque, at the expense of high rpm horse power potential. Honda motors have a history of lacking low end torque so for those motors the effects of pipe diameter is significant. On the old muscle car engines that were all about torque (just by design) you could remove all back pressure and still make tons of low end torque (you would still lose a little but the top end would be substantialy improved, not a bad deal). Turbo motors can also get away with lower velocity because the turbo steps in soon enough to pack the chamber with air/fuel much more effectively than scavenging ever could anyway.

These priciples also apply to intakes by the way.... My Mazda Rx7 used what was called a "dual stage intake system." It had relatively small intake runners for low end torque (higher velocity) and then at 3400 rpm another set of intake ports opened up to alow for high rpm power (more flow capacity). This felt a bit like vtec and and similarly reduced the compromise between low end toque and high end power. Since the rx7 used a two cylinder (actually two rotor) motor with two intake ports on each they are known as "6 port motors."

As an iteresting side note.... The reason that diferent headers create different power bands has do with timing the exaust pulses (suctions) of different cylinders to increase scavenging. Even the pipe length changes scavenging characterisics because it changes the timing of the pulse relative to another cylinder on it's exhaust stroke. Typically shorter headers time the pulse in such a way that high rpm power is enhanced, while longer primary tubes tend to favor low end torque. The difference between 4-1 and 4-2-1 affects the timing and pairing of the cylinders and their respective pulses as well. Many people are too quick to say that a header "only adds 4 Hp!" or whatever the magazine tests say. But, you have to remeber that peak HP means nothing. I'm sure everyone knows that it's the "area under the curve" that matters. And a header will always out flow a manifold so when you do decide to upgrade, perhaps to forced induction, you know that you have the flow capacity to get good high rpm horse power. And you will still get the benefits of the added low end torque from the design of the header. So unless you are only going to race (high rpm hp) then a good header for a honda will be tuned for low end power, but still flow more than stock on the top end for some increase at higher rpm, and it will still have the capacity to make the most of future upgrades.

Sorry this got so long []"


Hope it helps.

 

[RetardedAWD]

Originally posted by mike9146
"Miller you are a profit!"

Revenue-Expenses=Miller?

Mike C.

Also keep in mind, that people have gone 11's on a 2.5" pressbent exhaust.... it won't always make a "night and day" difference (for the record, Alan-from alaska went 11's on a 20g with a 2.5" pressbent BR exhaust on his 1g)

Mike C:


Black95GSX: What do you think he would run with a 3 inch exhaust? =)