Exhausts and Headers

[SA E30]

So as not to continue to hijack another thread, I've made my own.

Ok, so I understand that your sayin about the restriction of the cats, but...

Here we dont have to run cats, so we dont have that restriction. I know in the early 90's we used to race Nissan Primera's here (pretty much the same the BTCC cars of the same era) , and my one friend from Nissan Motorsport was tellign me how as they went further from the motor their exhaust Dia got bigger, starting at around 50mm, and ending in a 76mm dia, and they still do this on the Grp A Nissan 350z, and I also hear that the BMW e90 330i use this same exhaust (all running a single pipe). Strange how they use a similar exhaust design on a full worked 2.0lt 4cyl and on a stock 3.5lt V6, or 3.0lt str8 six.

Which brings me to my next question, would a single pipe be much better that a dual exhaust ?

IMO, the 2 places to extract power from the M20 is on the intake and exhaust, not so much so on with stroking it as such (although it does make a difference)

[winstontj]

I'll give this a shot. Others may disagree but here's what I know as well as what I think/believe...

...Nissan Motorsport was tellign me how as they went further from the motor their exhaust Dia got bigger, starting at around 50mm, and ending in a 76mm dia, and they still do this on the Grp A Nissan 350z...

I believe that the theory behind that the increasing diameter pipe is for both noise management as well as cooling & air velocity. When people talk about tuning exhausts, headers, etc. it's only in the first primary tubes up to either the X, H, or Y sections. Once you pass the X,H or Y the goal is to get it out as quickly and easily as possible. By going to a larger bore tube you slow down the gas velocity which gives it more time in the system to cool, burn and quiet down before exiting the car. It makes it easier to be less restrictive because you now have a slower gas moving through the baffles of the muffler. This is a good idea and why many setups increase in diameter.

I also hear that the BMW e90 330i use this same exhaust (all running a single pipe). Strange how they use a similar exhaust design on a full worked 2.0lt 4cyl and on a stock 3.5lt V6, or 3.0lt str8 six.

I assume when you say same design you mean increasing in diameter. It's all about gas velocity when you talk about increasing bores and pipe dia. A 2.0L 4-cyl at 4,000rpm will move just as much air as a 4.0L V-8 at 2,000rpm. Little screamers (like the Nissan motor or an S14) If you calculate the soft rev limits of each motor as well as the displacement I'll bet you'll find that they will put out similar air flow. I'm having a brain fart moment so I'll leave the math & cubic volumes calcs up to you :-)

To add to this - this is EXACTLY WHY we don't use HUGE exhausts on street motors. To have a 4-cylinder motor that NEEDS an exhaust that big it must have some work done to it. If you just strap on a huge open exhaust you can lose power, burn out valves (it's true) and the only thing you will increase is NOISE. IMO BMW is superior in their design for appropriate performance applications. They got it pretty good the first time around so the rule of thumb is that usually increase in displacement = increase in exhaust dia. If not the stock setup can be improved with more free flowing mufflers & perhaps a header to begin with. Once you start increasing CR and adding cam (which means revs are also increased) then you start moving your power band up which means your air pump will move more air and then you have a legit reason to bolt on a larger exhaust. Bottom line is you want to tune your exhaust runners (at the manifold/header) for your power band. You will find that different diameters and bores will move your torque and power curve.

Which brings me to my next question, would a single pipe be much better that a dual exhaust ?

(all my opinion which I hope is somewhat educated) I would say that auto manufacturers take into account things like the "cool factor" of dual exhausts as well as the cost associated with producing a manifold setup for V motors. You'll notice that just about every straight 4,5 or 6-cyl motor has single exhaust which I would speculate is due to cost. BMW runs two paralell pipes but they are still joined at the X, cat & muffler. The manufacturer does what it does to sell cars and make profit which isn't always the best from a performance, track or autox point of view.

With that aside, I still say that 99% of the exhaust is in the header before the X, H or Y pipes. Once your tuning is done then you want to do whatever you can to get the exhaust out of the car quickly and easily. The only reason why most of you guys run mufflers is to get below your noise requirements (95db @ 50ft for example) but if there were no noise requriements I'd speculate that most if not all track cars would just run a header and a short one part open pipe. V motors would run two headers and two open pipes - K.I.S.S.

IMO, the 2 places to extract power from the M20 is on the intake and exhaust, not so much so on with stroking it as such (although it does make a difference)

I'm not 100% sure I follow however 'there is no replacement for displacement' isn't just a big 'Merican Musscle slogan. A stroked 3.0L m20 is going to give you more torque and HP than a stock 2.5L even both in relativly stock form. Depending on what m20 you have you could do some headwork to improve flow, intake TB can be enlarged (unless you have the big one already), exhaust, cam, etc. This is a new motor to me so I can't say with absolute certanty but you usually have ~7rwhp each in the intake & exhaust to gain from stock (imo), and more to be had with things like chip, fuel delivery, ignition control, cam, increase CR, and finally increase displacement.

Hope that answers a few things....

[PiercedE30]

very nicely put winston....do much engineering?

[SA E30]

Wow, thanks... that was really informative.

Now to ask another question, what is the importance of header diameter ? , for example, my exhaust ports are opened up quite a bit (can't remember the exact figure) , and iv'e noticed that with my headers they sit about 1mm inside the ports, which I know is affecting power, but without swapping them out for a bigger diameter and how much bigger would I go ?

Also just a little info, my car runs on a dual 57mm exhaust system, with no cats.

[winstontj]

very nicely put winston....do much engineering?

Actually I work in finance with alternative investments. Cars are just a passion and a hobby. I considered getting my ASE & opening up a shop at one point but decided that wrenching for pleasure was WAY better than wrenching to make a living.

Wow, thanks... that was really informative.

Now to ask another question, what is the importance of header diameter ? , for example, my exhaust ports are opened up quite a bit (can't remember the exact figure) , and iv'e noticed that with my headers they sit about 1mm inside the ports, which I know is affecting power, but without swapping them out for a bigger diameter and how much bigger would I go ?

Also just a little info, my car runs on a dual 57mm exhaust system, with no cats.

Time for FNG to be honest and not misrepresent himself. (FNG = effing new guy) I don't have a whole lot of real life experience with this stuff so I don't want to mislead you. I've been around it enough to know all the "general rules of thumb" but I'm not to the point where I've built a race motor and have spent the time on the dyno testing this stuff first hand. I am building a stroked M10 with an evo (87mm) crank & 92mm pistons to get ~175rwhp (all without ITB's and NA) but again, this has been done so many times before me that I just copy best practices rather than have to do any engineering of my own.

This is what I do know: First you don't ever want the turbulance (read: restriction) of a break or step change like that. You want to at a minimum make sure that your making use of the expensive head work that's been done. Take some 3M adhesive spray or some light tack (don't worry it'll burn off) and glue your exhaust manifold gasket to the head. Next take a dremel or razor and port match the gasket. Just make sure that when you put your finger in the exhaust port there isn't any overlap from the gasket. If there is that will be your point of restriction and all the money you just spent on head work will be tossed out with a $20 gasket.

Next move on to the header. The goal is exhaust gas velocity forward of the x pipe. You want to make sure that you stick with an appropriately sized header so the only thing I can say that will work with 100% certanty is buy some dyno time and try a few different combinations. Yes it's expensive but it'll be worth every penny to have someone who knows what they are doing tune your motor & exhaust on a dyno so you have accurate results. Second to that you can talk to the person who did the motor work and see what they reccomend, or finally you can CAREFULLY, GENTLY, etc. taper (if there is enough room) the inside of the header out to the bore of the head to match so there is at least a taper rather than a rough edge. IMO going too big can be worse than restricted and too small. At least with small you have the low end power and only restrict when you get up to the high revvs. With too large of a pipe you'll lose low end power and you can run the risk of burning valves.

Hope that helps.... Back to work now :(

[german g]

I run a straight 2 1/2 in pipe from IE long tube headers into only a muffler...A little better throttle response and great sound..

[JRKOUPE]

exhaust

german G....



Pm sent.

[PiercedE30]

ok.....let the truth be known now.....
Ok, you want when it comes to porting and such, you don't want to increase flow that much, you want to increase VELOCITY. In order to do that, you really need larger intake port openings than the intake ports on the intake manifold, and larger exhaust manifold ports than exhaust ports (on the head). Not by much, somewhere around 1-2mm larger. If you are just porting to port, then you end up with TOO much air/exhaust trying to enter/exit the ports which will SLOW DOWN the gas VELOCITY. Air flow means nothing, unless there is nothing to little flowing. It's all about the VELOCITY of the gases.
The faster the exhaust gases flow, the better the scavenging effect which leads to a better mixture in the engine.

[winstontj]

ok.....let the truth be known now.....

Ok, you want when it comes to porting and such, you don't want to increase flow that much, you want to increase VELOCITY. In order to do that, you really need larger intake port openings than the intake ports on the intake manifold, and larger exhaust manifold ports than exhaust ports (on the head). Not by much, somewhere around 1-2mm larger. If you are just porting to port, then you end up with TOO much air/exhaust trying to enter/exit the ports which will SLOW DOWN the gas VELOCITY. Air flow means nothing, unless there is nothing to little flowing. It's all about the VELOCITY of the gases.
The faster the exhaust gases flow, the better the scavenging effect which leads to a better mixture in the engine.

I feel like we are saying similar things but maybe are misreading each other???

There are two parts to this equation as I see it. Velocity and Volume or air moved, flow, etc. Flow can be misconstrued as an increased flow could equal increased volume of air moved OR increased velocity, which also would equal more volume. Hopefully we can agree that given the same size pipe (ex 2" dia) air/gas moving at 1 cubic foot/minute that a 1" pipe would need to have twice the velocity or air speed than the 2" to move the same amount of air/gas. If we agree on this than we have our base to build from.

Porting essentially enlarges the bore of the intake or exhaust port, similar to extrude honing an intake, using a larger TB or larger dia. exhaust. So when we have a constant RPM we move a certain amount of gas/air through the motor (X rpm * X displacement motor = X volume of air moving through the motor, (in theory the amount of air introduced into intake through the TB should directly correlate to the amount of air coming out the tailpipe but that is a whole different discussion.) So at a given RPM we move a certain volume of air. By porting or enlarging we increase the diameter of the runner/port which means the end result is a decrease in velocity while keeping the same amount of air moving. Again if we go from a 1" bore up to a 2" bore and move the same amount of medium (air/exhaust gas) through the larger diameter we will find that the medium moves in the 2" at exactly half the speed of the 1".

I think this is where I am misunderstanding your post. I understand (and agree) that a larger intake runner (manifold runner, ITB trumpet, etc.) leading into a smaller port in the head will increase velocity in the head prior to the valve because as you move a set amount of air through a progressively smaller space the velocity must increase to move the same volume of air. On the other side your post does not make sense to me though - "and larger exhaust manifold ports than exhaust ports (on the head). Not by much, somewhere around 1-2mm larger" if you move from a 50mm exhaust port in the head to a 52mm exhaust manifold runner you move from small to big. In this case 50mm to 52mm is a 4% increase. As you increase the diameter while moving a set volume of air the air speed or velocity will slow down due to the enlarged chamber size. To maintain even velocity you must increase volume as these directly correlate to each other. So what I don't understand is how can you move from a smaller port to a larger manifold AND increase flow? I believe that there is an ideal velocity or exhaust gas flow rate from the valve to the Y or X pipe which yields best scavenging effects and most efficient flow out of the engine.

I also agree that porting to port is not the best idea. Everything should be done with an intended purpose and for a reason. (I don't want to get into porting versus polishing either) To port you must have a need, just as to increase exhaust diameter or header/manifold diameters over stock you must have the ability to push a greater volume of air through the motor to achieve the same flow rate (velocity) and be at your ideal numbers.

"Air flow means nothing, unless there is nothing to little flowing. It's all about the VELOCITY of the gases." Flow and velocity are the same thing in a matter of speaking. "Velocity" and "Flow Rates" can be quantified the same however I would tend to believe that velocity (cubic volume/rate ex. minute) is a more appropriate word. Flow is a more broad word used to describe efficiency (where we could start the debate over swirl polished valves, polishing versus non polished, etc.) in terms of turbulence, restrictions and specific mediums being able to flow through a certain chamber at a set volume. When someone says "that flows well" velocity is only part of the flow equation. With intake and exhaust we factor in how gasses flow because flow patterns and rates are much more important than simple velocity. Too much velocity is a problem just as too little - there is a target to achieve that is appropriate to the motor.

With exhaust we seek a smooth efficient means to exit gases however we have also learned that torque is largely effected by exhaust flow and therefore we need to be mindful to keep the diameter down enough to maintain torque, which increases velocity however only to the point that your header can handle the volume of air you are trying to push which then leads us back to backpressure and/or the need to increase diameter lessening restrictions and turbulence (turbulence by itself is a restricting factor) which allows for more volume to flow so we can get back to hitting target velocity. It's all a balance.

Hopefully that clears up where I am coming from and some of the words that have been flying around. I know that no one likes a smart-ass new guy coming in and ruffling feathers but in an effort to share and educate I'm trying not to be abrasive.

Cheers!

[Tucker]

Speaking of smart-ass new guys, I couldn't help but make a small correction.

Hopefully we can agree that given the same size pipe (ex 2" dia) air/gas moving at 1 cubic foot/minute that a 1" pipe would need to have twice the velocity or air speed than the 2" to move the same amount of air/gas. If we agree on this than we have our base to build from.

Actually since the cross-section of pipe is pi*r^2, the 1" pipe in your example would need a gas velocity of 4x that of the 2" pipe for the same flow rate.

This really doesn't invalidate anything you posted above though. :)

-Tucker

[winstontj]

lol.... I knew I'd slip up somewhere along the way!

Sorry... chuck my whole argument out the window

[SA E30]

Ok, from what I understand here is that a small pipe will amost always give u more velocity than a bigger pipe... ?

but the trick is to find the dia that keeps the velocity up, while not creating to much back pressure.

So then it leads me to think that a bigger pipe is better, as it creates less back pressure, but at the expense u lose velocity, but with the bigger pipe you're still moving a greater volume of exhaust gasses ?

[PiercedE30]

winston, I believe we are on the same page, it sucks not being able to put thoughts into words that easily and then it's even harder trying to explain something in written form. Technical writing isn't my strong suit.

SA, even if you are putting a bigger pipe on, you are still going to be producing the same volume of gases. With a big pipe, the motor has to fill the pipe and then push the gases outwards towards the tailpipe. That will slow down the velocity of the gases, which we don't want. No matter what, your motor is always going to produce the same relative amount of gases (unless your car is turbo'd, then it throws everything out the window). (just to reiterate)

[nando]

Ok, from what I understand here is that a small pipe will amost always give u more velocity than a bigger pipe... ?

but the trick is to find the dia that keeps the velocity up, while not creating to much back pressure.

So then it leads me to think that a bigger pipe is better, as it creates less back pressure, but at the expense u lose velocity, but with the bigger pipe you're still moving a greater volume of exhaust gasses ?

the reason you don't want to sacrafice too much velocity is the scavenging effect.

[winstontj]

How much power do you have and what is the cars purpose? There is no need to reinvent the wheel. Why not just buy a header that is appropriate and put an exhaust on that will either meet your autox noise requirement or pass emissions, or both...