I have a 88 si Ls/vtec, b16 head W/ gsr cams, B17 gsr tranny. Tell me what sizes we thinkin?

 

2.5" is fine or around 60 mm.

are you going to go FI or not? if you do, look at 3" pipes.

 

i have 2 1/2 setup header back and its sufficient enough clearance is also an issue when you start getting bigger than 2, 2 1/2 mine seems fine only a couple time have i dragged bottom going in or out parking lot it wat you want its your car hop this helped

 

For most N/A 4 cylinders, 2 1/4" is just fine. On bored & Stroked ones and small turbo's 2 1/2". After that you can figure it gets larger.
What most people that put 2 1/2"+ on a stocker seem to forget is that you want some back pressure. If not your truly effecting your engines flow and power numbers.
You want just large enough to allow the engine to breathe efficiently. If the engine has to fill an area larger than needed your doing nothing but creating flow pressure walls inside the exhaust that the engine is constantly fighting.

 

why does everyone here think back pressure a good thing? i hear this term at least 3 times evertime i get on. its not a good thing. if the system has pressure in it it makes the engine work harded to expell more exhaust. engines do not need back pressure. think if you take off the exhaust completely (cat back) you gain horse power. its not rocket science it is simple physics

 

Well lets see...
I have been working on cars for pretty near 20 years.
I have seen people do like you and remove the cats and everything, and sure enough they gain a few HP but they also loose horrible amounts of Torque. You know the part of the equation that determines how quickly you go from a stop.
And believe it or not since the 80's automobile engines have been designed to work with a limited amount of back pressure. *Sarcastically* I think it has something to do with the scavenging effect on the exhaust and having it work properly and actually complete the desired effect.
*Straight* Having worked on turbine engines I know a little bit about flow, how pressures work, and what they effect.

Here's a broken down barney type experiment that makes the point entirely...
Get several different diameter sizes of tubing... you know... straw's, heater hose remnants, radiator hose remnants. Get a good selection. Just try to get them all the same length.
Now put each of them up to your mouth and blow through them.
The straws will have lots of back pressure, but lots of velocity.
The smaller lines 1/4"-1/2" will be about equal, little back pressure, but you can still feel the air coming out.
The larger ones, have -0- back pressure but next to no flow rate out the end (unless you cheat and partially cover the end)
This is exactly what I am talking about. If your exhaust pipe is too large, your exhaust flow is crap, meaning it is choking the engine. Why? Because the exhaust is NOT FLOWING it is sitting in the pipe stagnate and barely moving. This is where the engine really has to work harder to create flow so the system can operate correctly.

If you want to properly size your exhaust figure your intake CFM Flow and subtract 15%, this will give your rough exhaust CFM Flow. You then match this to the closest tubing size.


Modified by boostedtsi at 7:53 PM 9/2/2008

 

Back pressure, Exhaust velocity and scavenging.

The myth: “engines need some backpressure.”


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 its consequences are. I'm sure many of you have heard or read the phrase "engines need some backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.


How the myth came about:


It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realizes that he has lost power right through the power band. The connection is made in his throbbing brain….

Put on 3" pipe = loss of backpressure = loss of power.

Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.


The other myth: “engines can get burned valves from not enough backpressure”

How this myth came about:

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust for his.

Once these vehicles received performance mods that reduced backpressure, they tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. 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 pulses 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.

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 its 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 power band is located 2-3000 RPM you'd want a narrower pipe than if your power band 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.

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).

Conclusion.


SO it turns out that engines don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.


YOU DO NOT NEED BACKPRESSURE

 

just what I edited in to my post but in much shorter version.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by boostedtsi &raquo;</TD></TR><TR><TD CLASS="quote">just what I edited in to my post but in much shorter version.
</TD></TR></TABLE>

You seem to still be saying that some backpressure is good though, which is wrong.

 

well dude he has been working on cars for 20+ years so obviously he knows everything. when in actuality he does not. backpressure is bad. if you get a 2.5" exhaust on a crx by a reputable company that has done its r&d you will gain power. but the power gained is factored by other modifications.
in summation a honda crx with an ls/vtec will gain power from a 2.5" exhaust and even more so one without a cat. the cat is just a restriction used for only emissions nothing else.

 

where did I say that I knew it all? (and to correct you I said nearly 20 years. Not 20+ so try to keep the information correct) Nowhere in my reply. I said that I know about flow and pressures and the effects they have, which I do.

I stated that most people go throwing much too big of an exhaust on their car. That will hurt performance no matter what you say.

And if I had the choice of slight back pressure or too large of an exhaust, I would take the back pressure. The engine would simply run better.

 

As per my last reply, very rarely does it work out that the tubing is sized perfect for your required flow rates. And I reiterate, you only need the size your engine can flow, nothing bigger. We seem to agree on that point.

While given my experience with turbine engines, I see it all the time where most of them tend to like the slightest back pressure to be the most efficient.

And yes I have seen many an engine burn up valves with open exhaust. You seemed to point that out in your long reply as well.

I agree (and seemed to state that before in my long reply) to say that you want as little as possible, but as in my last comment, I would prefer minimal back pressure to too large a tubing.