4" Dump Pipe. Pro's And Cons. Should I Get One?

[nelsonian101]

OK, at this stage definitely don't want the expense of a full system and happy with stock exhaust noise.

Air : With a GTX3576R, 5" 100cpi cat, 4" intake, Plaz 1000hp cooler.

Fuel : Eflex E70, ID1000's, Surge tank, dual rail, regulator, twin 044's, Walbro intank.

(Have always had stock dump pipe with the high flow cat.)

Will I lose low end grunt with slower spool time with a bigger dump pipe?

Not interested in massive power over 400rwkw. Never have been.

What I do need is quick spool, low-midrange grunt with a peak of maybe 360rwkw with maybe a high boost 380-390rwkw for the occasional drags.

Should I get one?

Please educate me.

[MattyP]

You'll gain spool if anything. I know I did.

The less restriction the turbo has the easier it spins up.

But I could be wrong.

["Captain Retard"]

From what I've learned from researching is turbos become more efficient when the pressure drop from turbo intake to out let is bigger.

So correct me if I'm wrong but you want high velocity and pressure on the turbo intake and very low pressure on the outlet.

This will increase spool ramp rate.

The further you can move the restriction along the exhaust the better for turbo street cars.

I think, still getting my head around turbocharged engines.

Edited 18/11/14 06:53 AM by "Captain Retard"

[BRK]

No, you will not. However, how much overall power you will gain with the factory mufflers still in place is a big question. The mufflers will now be a "cork" imo.

[Ralph Wiggum]

4" dumps and known to reduce low end torque compared to a 3.5" At you power level I would stick with a good quality 3.5" dump and cat combo.

[nelsonian101]

Does my 2008 FG XR6T have a 3" dump pipe or 3.5"?

So going to just a larger 4" dump pipe and retaining the 5" 100cpi cat with actually reduce low end torque??

This where is gets confusing as this advice contradicts.

4" dumps and known to reduce low end torque compared to a 3.5" At you power level I would stick with a good quality 3.5" dump and cat combo.

Edited 18/11/14 08:01 AM by nelsonian101

[bjc]

I had the xforce split dump pipe and 3.5inch stainless system with no cat on my BA turbo. The standard turbo was happy to spool up to a tuned in but uncontrolled 21psi before getting off it and detuning to 18psi. Unfortunately over the years I never kept the dyno sheets.

From this forums point of view though a 4in dump pipe will give more top end but the split dump from xforce gave more mid range power, if this is true or not I don't know and only those that have tested the theory can comment actual information on these cars and not science unless your IQ is 195...

["Captain Retard"]

Hmmm this is interesting.

I'll have to do more reading to find out why this is so.

["Captain Retard"]

It took me awhile but I found some info that might be useful.

Exhaust design is one of the best areas to get great gains if done properly. There are many factors to consider when designing an exhaust system. There are also different requirements for turbo and non-turbo engines. But, firstly we need to talk about backpressure. "backpressure makes torque" is often quoted but not always true. The reality is that backpressure is the enemy. You want to keep it as low as possible but keeping the velocity up. However doing the things that keep velocity high involves slightly more backpressure under some conditions. You want to keep the gasses moving as quickly as possible to make both good torque and top end power. The perfect exhaust system would keep the gasses moving as fast as they did coming out of the cylinder and have zero backpressure. However this is impossible to achieve in the real world.

Turbo System Exhaust

Is bigger better all throughout the powerband or does big diameters loose bottom end torque? Should I use 2.5", 3.0", 3.5", 4.0" for best power? Does reducing the pipe diameter towards the end of the exhaust help power? What is the best turbo outlet design? How does a cat-less exhaust perform compared to a high flow cat?

These are the questions that everyone asks but again there is no simple answer. Post turbo pipe diameter is only one small parameter in many that determine how well the exhaust performs.

Pre-turbo Exhaust

The exhaust manifold / system before the turbo and the turbo itself have a greater effect on backpressure than the exhaust behind it. You want the least restriction after the turbo as possible for both top end power and quick spool-up. Careful attention has to be paid to keep velocity high before the turbo and in the exhaust housing of the turbo to spool the turbo up as quickly as possible while not choking off the exhaust gasses on the top end.

The exhaust manifold can be simpler in some ways than a non-turbo header. Bigger dividends can be had by getting the exhaust gasses to the turbo with the least amount of restriction, highest velocity, and the most heat rather than worrying about a tuned equal length design. Like non-turbo systems, it is optimal to make an equal length header, but often the space requirements do not allow for a tuned equal length manifold. This helps explain why we usually get near identical results from a factory manifold when compared to the aftermarket ones. The factory manifold gets the gasses to the turbo as quickly as possible and goes a good job of keeping the heat in. Aftermarket manifolds tend to take a longer path and loose quite a bit of heat in the process (reduction in exhaust gas temperature = smaller gas volume = loss in velocity). In this case a good manifold collector does play a more important role than the length of the pipes. If all of the gasses ram together at a steep angle it causes a lot of turbulence, creates backpressure, slows velocity, and tends to make a mess of things before the turbo, which is the worst spot for inefficiency on a turbo charged car. A good manifold design would be something like a 4 into 1 design that uses castings or good thermal coatings as much as possible to keep heat in and would also get the gasses to the turbo as quickly as possible. The manifold collector would have to be longer and transitions nice and smooth.

Turbo Exhaust Housing

The size and design of the exhaust housing plays a major roll in the spool-up characteristics of the turbo and its ultimate power potential. There has to be a balance met if you want to have the quickest spooling turbo for your power goals. If you go with too large of an exhaust housing you greatly increase lag. Too small of exhaust housing and you severely limit the amount of boost and top end power you can make. You can only push so much gas volume through a small housing without having negative side effects. Adding to the complication is that each psi / bar of boost created makes a ratio of backpressure before the turbo. It is different for each turbo, the amount of boost you are running, the size of the motor, RPM, and load on the motor. Once you start trying to push too much through the exhaust section of a turbo (running too much boost for the turbo) you start making a huge ratio of backpressure, and it only gets higher the more boost you run. This not only limits the amount of power you can make, but makes EGT go up, hinders the motor's ability to get the burnt gasses out of the motor, and makes the car more prone to detonation. This is also a big cause for failed pre-cats in the downpipe. Choose too large of an exhaust housing for the application and it takes the turbo too long to spool, effecting torque production. The best way to make good torque on a turbo motor is to spool up the turbo as quickly as possible. Also, who cares how big your turbo is, or what power it can theoretically produce if you can never spool it up or if it falls out of the powerband every time you shift.

Adding another factor is the design of the exhaust wheel. It has to have good aerodynamic properties or it is inefficient. A more efficient wheel design means that you will make more power and/or less lag.

Post Turbo Exhaust

The least amount of backpressure possible is the main performance goal of a post turbo exhaust. There are a lot of factors that affect this. Turbulence is one main factor. If the gasses are all stagnating and/or running into protrusions or running into each other it creates more backpressure than a well designed system. The more laminar (smooth and straight) the gas flow, the more the system can flow for a given pipe diameter. Steep angles and abrupt pipe diameter chances should be avoided. The methods of collecting the outlet gasses and the wastegate gasses add another part of the equation to change. It would be optimal not to join the outlet from the turbo and the wastegate together, but the real world messes with our fun. Just dumping the wastegate to atmosphere is great for a racecar, but not a street car. So a street exhaust should combine them to get all of the gasses through the same cat and exhaust box system. Some of the turbo outlet designs include: flanges with a simple pipe, bell mouths, divorced wastegate, and split bell mouths You also have castings and formed piping to choose from. Which one works best is also determined by quite a few different factors and how well they are designed and manufactured.

Flange w/Simple Pipe - The only advantages to this design are cost and simplicity. The pipe does not have to be formed and the flange is simple therefore reducing cost. The labour to weld the pipe to the flange is easy and therefore less costly as well. That is the main factor that make it desirable to the factory and why it is used on the stock exhaust. The wastegate gasses joining the turbo gasses right at the turbo outlet does create turbulence in the worst spot post turbo and reduces flow, thus not making it as desirable for performance as other designs.

Bell Mouth - This method is much closer to optimal for joining the gasses from the outlets. There is more room for them to join and if the transition is done properly it can flow very well into the main piping. It packages very well and does not have a lot of complexity, making for less to break. We have gotten the best results from this type of downpipe so far. Boost response has been the best out of the outlet designs we have tuned on, it is easy to put a wideband oxygen sensor bung into. We have also had the fewest problems with this design.

Split Bell Mouth - This design separates the gasses in the beginning of the turbo outlet and joins them at the rear of the bell mouth section. It works well and has some of the advantages of the bell mouth and some of the advantages of the divorced wastegate designs. The main deterrent for this is the cost and complexity of adding the splitter. I am a fan of keeping things as simple as possible while still making the product work well.

Divorced Wastegate - Keeping the gasses from the turbo outlet and wastegate separate until farther back in the system is an attempt to combine the advantages of not collecting the gasses and the real world. Combining them far back is closer to optimal than collecting them closer to the outlets. It is also critical to power production and spool-up to join the pipes smoothly and avoid turbulence. The disadvantages are that you add a lot of cost and complexity. You have big temperature differences on each pipe and that makes for a system that can crack. Putting in flex or expansion joints helps, but adds even further complexity and yet another part to fail. Between 4" and 6" is the best compromise before blending in the wastegate back into the system.

Cast Outlets - Castings have the advantage of keeping a lot of heat in the exhaust as well as freedom with design. You can basically make it almost any shape you want. The disadvantages are more weight and cost. Cast iron pieces can weigh a ton and that is a valid concern for many people. The casting form that the piece is made in is also very expensive and depending on complexity can range from a couple of thousand dollars to well up in the tens of thousands.

Formed Piping -Forming pipe has almost as much design freedom as a casting with less expense and less weight. The only disadvantage lies in if it is not done properly. Poor forming can look bad and effect flow by having creases and crimped spots. You can also get the piping too thin if you try to stretch the metal too far. If done improperly you can also make the metal brittle and it will usually happen where the metal is the thinnest. This is why we use mandrel formed bends which have no ovality and no creases etc due to the forming of the bend. In addition, where possible, no bend angles over 60 deg are used as over this angle, backpressure starts to increase.

Remember, you will only flow as well as the greatest restriction. If you have a poor cat or exhaust silencer box design, then it will choke the flow no matter how good the rest of the system is designed. Fortunately straight through exhaust silencers and newer high flow cats flow very well. Having a cat is not only good for the environment, but we have seen very little power difference in levels in excess of over 350 h.p. Also, a cat tends to quiet things down a little.

Pipe diameter does have an effect on flow rates as well, but again it is not the major factor in most cases. 2.5" may flow enough for 300-350 h.p. without being a restriction. 3" is usually capable of flowing 500-600 h.p. before becoming a restriction. This is assuming that you have designed the rest of the system up to par. There are also full 3.5" systems and those that start out at 4" and taper down. Unless you are making over 500-600 h.p. anything over 3" is a case of diminishing returns and in most cases has no advantage. There is more to gain going from 2.5" up to 3" than there is going from 3" to 3.5". A 3" system will not loose torque compared to a 2.5" system if designed properly. In fact if designed properly 3" may be capable of making better low end torque than 2.5". Again, since the way to make the most torque with a turbo exhaust is to get the turbo to spool-up as quickly as possible, it should be the main goal of the entire exhaust system and good flow after the turbo is one way to achieve it. We use 3" for 3.0T Supra, 2.0T Impreza etc as we want our system to flow enough to be capable of coping with a customer's changing goals ie there will always be more modifications to follow after a good flowing exhaust system. Properly designed we can offer it to the big power crowd while still appeasing the low end torque club.

The only reason to reduce the size towards the end of the pipe is for packaging, cost, and noise reasons. Tapering the diameter does not make more power, torque, or bring on boost faster. However having smaller pipe towards the end has less effect that having smaller piping at the beginning. In other words a system that has 3" pipe for the majority, and necks down to 2.5" at the end will flow enough for more power than a complete 2.5" system. The further downstream you neck down the exhaust the better……..if you decide to neck it down. Attracting unwanted attention and not hearing your stereo or you passenger would make for an exhaust system great for a racecar, but poor for the average Joe Public. Law enforcement and your neighbours do not appreciate loud exhausts either, even if you do.

Edited 18/11/14 01:50 PM by "Captain Retard"

[BRK]

Interesting text above. Even though it is a very well made piece, I will stay with the opinion that the factory mufflers will be the limiter in this situation.

The now very old air power systems phase three kit which claimed 390KW at the engine and reportedly dynoed 330 at the tyres still used the ba/bf factory dump with 76.2 outlet, a "hi flow" cat then went to an 89mm single into a muffler then split into twin 63mm into the final muffler.

I have not measure the back pressure on the FG systems - sadly no access to a vehicle. I did test the rear muffler on the twin Manta system and could not really get a reading on the restriction. The test gear was not too charming but the reading showed almost no change. I was sceptical so I will say there was some back opressure - but it was very, very low.

If you wanted too, and maybe you alreay know, but you can drill/tap the the rear muffler outlet and use the second 02 sensor port to measure the back pressure in the overall exhaust. IMO, that might give you a better idea of where to head. There is some testing of bell mouth versus split type dump pipes on one of the muffler sites with results that do favour the 4 inch pipe. However, there is insufficient data supplied to really prove the overall superiority of the larger set-up other than in top-end power.