Brett- APR
09-08-1999, 03:37 PM
<center><img src="http://131.107.68.28/a4org/apr_stage3mani.jpg"></center><p>In response to a thread on the A4.org comparing the advantages and disadvantages of a turbocharger versus supercharger setup, and in defense of our Stage 3 kit for the A4/Passat 1.8T, I would like to make a few comments. I know that a lot of you are wondering about the specific advantages of either setup and I would like to clear up the confusion that has been created.
When we first conceptualized the idea of a high output power package for the 1.8T motor we sat down and discussed the positive characteristics of the stock motor and how we would like to maintain these characteristics yet improve the power output considerably. Knowing the disadvantages of a supercharger system, we dismissed such a system immediately. Here are just a few reasons why we chose a turbocharger system:
1. Greatest power potential- a turbocharger system would be capable of providing the most peak horsepower yet still retain a very broad torque curve
2. Day-to-day drivability and noise output- a turbocharger setup could be designed to feel very similar to stock at low throttle positions without the annoying noise of a supercharger setup.
3. Fuel efficiency- a turbocharger system could be designed that would equal stock fuel consumption figures under normal driving. (Obviously, gas mileage would be worse if you were at full throttle.)
4. Engine electronics already in place- the stock Motronic system consists of sophisticated controls for the turbocharger system. The system is quite good and, in combination with a thorough understanding of the system, could be made to adapt quite well to a larger turbocharger setup.
5. The Engine was designed for a Turbocharger- for instance, the camshafts were designed and optimized based on the assumption that the engine was exhausting through a turbocharger. Any other form of aspiration changes this equation.
6. Stealthiness- the average person would never guess the car wasn't stock by just looking or listening to the car.
Obviously, there are downsides to everything. These are the largest obstacles we encountered:
1. Turbolag- all else being equal, a larger turbocharger will have a higher boost onset point (full boost will be released at a higher RPM). This was the most difficult (not to mention time consuming and expensive) obstacle to overcome.
2. Relative complexity- a larger turbocharger system would require a larger, more efficient intercooler, larger recirculation valve, new exhaust components, and much more. However, we did not have to relocate the stock airbox (which DOES serve a purpose, BTW) and we were able to design a system that looks stock to the average person.
OVERCOMING TURBOLAG
This required going back to the drawing board. We knew of two items that we had control of that would affect turbolag and boost onset point considerably: the turbocharger and the exhaust manifold. The turbocharger wasn't overly difficult- we chose the latest turbocharger from Garrett utilizing a twin-ball bearing design. The exhaust manifold was another story.
A turbocharger utilizes the energy present in the exhausting gases and uses this energy to pressurize the intake charge. Realize that exhaust gases are at a higher temperature than atmospheric and have a significant amount of velocity and therefore contain a great deal of potential energy. A turbocharger is designed to harness this potential energy. In contrast, a supercharger uses power that was intended to drive the wheels to compress the intake charge. This is why a turbocharger is so much more efficient overall than a supercharger. Although the effiency of a supercharger unit itself can rival that of a turbocharger, when one considers that the majority of the work required to run the turbocharger is waste energy and the supercharger uses flywheel power, the overall efficiency of the turbocharger system is much greater. This, in combination with the fact that a more dense inlet charge (in other words, forced aspiration) actually increases the overall efficieny of an engine, explains why turbochargers are used in energy critical situations such as large trucks, and power generators.
Back to manifold design, the idea was to harness as much of the potential internal energy in the exhaust charge as possible. Adding to the complexity was the fact that the exhaust gases pulsate. We also knew that we had to minimize the internal volume of the exhaust manifold while retaining excellent flow. Another important consideration was to minimize the heat loss across the exhaust manifold walls (the more heat lossed, the lower the energy potential of the exhaust gases to spin the turbo).
After months of research, development, and testing we finally came up with the final design. To reduce heat transfer across the manifold walls, we elected to have the manifold investment cast in Inconel with 5mm walls. Inconel is classified as a super-alloy and has a heat-transfer coefficient roughly one-fourth that of the stock manifold. We also found that we could reduce cylinder-to-cylinder pulse interference by intersecting cylinders one and four together and doing the same with cylinders two and three (interestingly, Audi used this same principle on the TT 225HP exhaust manifold.) Many iterations of the manifold were tested until we found the combination that provided optimal flow with minimal internal volume.
The results have proven to be worth the effort. We have decided to withhold from releasing official numbers on the kit until its final release but I think everyone will be pleased. Many of you are probably wondering when that will be. Well, everything is in the production stage, and deposits will be taken soon. Check our website frequently for release information.
Brett Augsburger
Audi Performance & Racing
1-800-680-7921
www.goapr.com
When we first conceptualized the idea of a high output power package for the 1.8T motor we sat down and discussed the positive characteristics of the stock motor and how we would like to maintain these characteristics yet improve the power output considerably. Knowing the disadvantages of a supercharger system, we dismissed such a system immediately. Here are just a few reasons why we chose a turbocharger system:
1. Greatest power potential- a turbocharger system would be capable of providing the most peak horsepower yet still retain a very broad torque curve
2. Day-to-day drivability and noise output- a turbocharger setup could be designed to feel very similar to stock at low throttle positions without the annoying noise of a supercharger setup.
3. Fuel efficiency- a turbocharger system could be designed that would equal stock fuel consumption figures under normal driving. (Obviously, gas mileage would be worse if you were at full throttle.)
4. Engine electronics already in place- the stock Motronic system consists of sophisticated controls for the turbocharger system. The system is quite good and, in combination with a thorough understanding of the system, could be made to adapt quite well to a larger turbocharger setup.
5. The Engine was designed for a Turbocharger- for instance, the camshafts were designed and optimized based on the assumption that the engine was exhausting through a turbocharger. Any other form of aspiration changes this equation.
6. Stealthiness- the average person would never guess the car wasn't stock by just looking or listening to the car.
Obviously, there are downsides to everything. These are the largest obstacles we encountered:
1. Turbolag- all else being equal, a larger turbocharger will have a higher boost onset point (full boost will be released at a higher RPM). This was the most difficult (not to mention time consuming and expensive) obstacle to overcome.
2. Relative complexity- a larger turbocharger system would require a larger, more efficient intercooler, larger recirculation valve, new exhaust components, and much more. However, we did not have to relocate the stock airbox (which DOES serve a purpose, BTW) and we were able to design a system that looks stock to the average person.
OVERCOMING TURBOLAG
This required going back to the drawing board. We knew of two items that we had control of that would affect turbolag and boost onset point considerably: the turbocharger and the exhaust manifold. The turbocharger wasn't overly difficult- we chose the latest turbocharger from Garrett utilizing a twin-ball bearing design. The exhaust manifold was another story.
A turbocharger utilizes the energy present in the exhausting gases and uses this energy to pressurize the intake charge. Realize that exhaust gases are at a higher temperature than atmospheric and have a significant amount of velocity and therefore contain a great deal of potential energy. A turbocharger is designed to harness this potential energy. In contrast, a supercharger uses power that was intended to drive the wheels to compress the intake charge. This is why a turbocharger is so much more efficient overall than a supercharger. Although the effiency of a supercharger unit itself can rival that of a turbocharger, when one considers that the majority of the work required to run the turbocharger is waste energy and the supercharger uses flywheel power, the overall efficiency of the turbocharger system is much greater. This, in combination with the fact that a more dense inlet charge (in other words, forced aspiration) actually increases the overall efficieny of an engine, explains why turbochargers are used in energy critical situations such as large trucks, and power generators.
Back to manifold design, the idea was to harness as much of the potential internal energy in the exhaust charge as possible. Adding to the complexity was the fact that the exhaust gases pulsate. We also knew that we had to minimize the internal volume of the exhaust manifold while retaining excellent flow. Another important consideration was to minimize the heat loss across the exhaust manifold walls (the more heat lossed, the lower the energy potential of the exhaust gases to spin the turbo).
After months of research, development, and testing we finally came up with the final design. To reduce heat transfer across the manifold walls, we elected to have the manifold investment cast in Inconel with 5mm walls. Inconel is classified as a super-alloy and has a heat-transfer coefficient roughly one-fourth that of the stock manifold. We also found that we could reduce cylinder-to-cylinder pulse interference by intersecting cylinders one and four together and doing the same with cylinders two and three (interestingly, Audi used this same principle on the TT 225HP exhaust manifold.) Many iterations of the manifold were tested until we found the combination that provided optimal flow with minimal internal volume.
The results have proven to be worth the effort. We have decided to withhold from releasing official numbers on the kit until its final release but I think everyone will be pleased. Many of you are probably wondering when that will be. Well, everything is in the production stage, and deposits will be taken soon. Check our website frequently for release information.
Brett Augsburger
Audi Performance & Racing
1-800-680-7921
www.goapr.com