Twin scroll turbo down pipes

I now offer Dump pipes to Suit twin scroll JDM STI turbo (VF37&VF36)

Comes in short size for easy shipping and allows your old TBE to be mated up. Email for info.

$400 (inc GST) + shipping

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Over a period of 2 years I have developing an exhaust that delivers fast spool up and mid range power that hits like a bomb, torque is everything and horse power is nothing without it. What dyno graphs don't show is the gains to part throttle and the reduction in lag on the road. To get good low rpm power you need to bring boost on as fast as the turbo will allow, after all this is what makes a nice road car and a quick one too!
I have tried every variation and have found that by using an Open mouth collector to be best to get that low end power.

The splitter position and length is all very important to get the most from the system. The short one off a TDO4 used on a VF turbo will give poor results, and must be lengthened to suit the Vf turbo. The waste gate only opens to 30 odd deg, most of the air flow from this flows directly into the path of the main flow, this reduces gas flow, creates turbulence and increases the exhaust pressure between the exhaust valves and turbo. Hold them apart for longer reduces the size available for the main flow to expand, so it is important that the splitter be as short as possible once inside the collector, but it must be a neat fit in the back of the turbo giving an optimum seal.

The diameter and length of the taper on the collector also plays a role as to how and where maximum torque is delivered. Also having a huge bearing on torque band is pipe direction leading into the rear muffler, with an angled rear muffler torque is lost through the mid RPM, but with some small gains made to top end.

Using an S bend leading into the rear muffler via 2 90degs back to back offers the most torque in the mid range, reducing the angles from 2 x 90 degs to 45 degs moves peak torque up through the rpm, using almost straight pipe with angled muffler provides and very linier power band, smooth delivery of power with no "kick" as a result from noticeable losses in torque in the mid range. The S bend has a huge bearing on torque in the mid range, after consulting with Prodrive they verified this for me, a straight pipe might see more top end power, but the loss of low down out weighs the gains.

Also changing the rear tail pipe section from 3" to 2.5" helps torque, using 2.5" rear mufflers greatly help mid range with gains on WRX/STI set ups. Pressure at the turbo is not effected with this and helps to keep gas speed up for better power.

The key to good power on any turbo car is the exhaust system and its design. An exhaust will always determine the total power output of a car and how well it responds to other mod's, based on how well it works. It isn't just about how MUCH boost the exhaust lets the engine run, it is about how WELL it can run the boost it has.

So what does a good exhaust do? A good exhaust is one that allows the turbo to spin up as quickly as possible (reducing lag), but doesn't create massive turbulence in the process. Turbulence in the exhaust system reduces flow and creates back pressure at higher rpm before the turbo. This back pressure is one of the main causes of a turbo engine dropping off boost significantly as rpm rise. Minimal back pressure at high rpm allows a turbo to hold a preset boost pressure much more readily.

Large collectors allows the hot exhaust gases to expand without reducing flow, if hot gas can't expand then flow is reduced, but it is not required to hold large pipe sizes too long as the exhaust gas cools volume requirements are reduced so must the pipe size or you see loss in torque.

A poorly designed exhaust will increase back pressure as rpm rise and as boost rises. Asking a turbo to hold a high preset boost level by simply forcing gas through the turbine or restricting flow to the waste gate increases the positive pressure before the turbo and reduces scavenging on the exhaust stroke of the cycle. While some systems may offer bolt on power they can be attributed from forcing boost pressure up with restricting waste gate flow which also increases pre turbo exhaust pressure limiting any true gains from other mod's. With less of the old gases removed, there is less space for fresh charge to enter the bore meaning less potential power. This set-up will also result in an engine pinging more readily when compared to one running the same boost but with a well set-up exhaust system like one that lowers the pressure pre turbo and allows the turbo to expel gases more quickly and effectively. At this point of high pre turbo pressure no other parts you bolt on will show their true benefits unless the pre turbo pressure is reduced with either a bigger turbo OR ideally a better designed exhaust. Just fitting a bigger turbo will not solve the issue on its own - you must have a correctly designed exhaust system fitted as well. With a good system assisting optimal scavenging on the exhaust stroke, there is a greater amount of fresh charge available in the ignition stroke for more power.

So a good exhaust system does TWO main things. The first is assisting with optimal scavenging on the exhaust stroke, increasing volumetric efficiency whilst maintaining the same boost. The second is reducing back pressure at higher rpm, helping the turbo to maintain boost pressures at higher rpm. As a result of these two things, ignition timing can also be altered to take advantage of the set-up for even more power. It is common for a standard car to ping at say 18 PSI, but the same car with a good exhaust system can run 18 PSI safely (or even more!). With increased turbo efficiency and lower intake temps as a result, a combination of more ignition advance and/or more boost can be run. This in turn gives better results when compared to just having an average exhaust and forcing a higher charge of boost through.

Anyone can get gains from just bolting on a whopping big turbo, but the exhaust efficiency at any rpm will effect both boost control and peak hp in any case. Poor exhaust design means that you need a bigger turbo to achieve the same power gains (along with more lag) as a good system with a smaller turbo. This is not to mention that a good exhaust system will allow a turbo to achieve full boost up to 800~1000rpm sooner when compared to a poorly designed system, giving more torque and a more drivable car on and off boost.

 

This Pic to the right is from Prodrive with a Group A engine they were testing on an engine dyno, note it uses a open mouth with long taper dump pipe, this was back in 1996.

 

 

 

 

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