[Travis Q]

More on backpressure (its time for a coffee break this morning, I'm dragging).....

To boil it down why backpressure is important in a drag racing application, it all has to do with max engine speed. In nearly any drag racing scenario, engine speed allows for a greater window with converter or clutch tuning, and equates to more power strokes per second. If you have two engines that make the SAME peak/average power, but one does so at 1000 higher rpm, and the race car is geared to take advantage of that additional rpm, that will be the faster car, even though the power number is the SAME.

A turbocharged engine with a high backpressure number can make the exact same power number that one with very low backpressure can. It can make the same average power over its operating range that a low backpressure engine can over its operating range. The difference is what that operating range is! High backpressure forces you to run a camshaft with very little overlap. Small overlap numbers means that the engine isn't going to rev. Also, high backpressure applications (like street cars) are not overly sensitive to valve timing events, since the cams are already so small. You can put nearly anything in it, and so long as the overlap isn't too crazy, it will run just fine.

Low overlap engines act a lot more like a NA engine, because there starts to be a small amount of scavenging on the overlap cycle. So these engines can sometimes be a lot more picky about camshafts.

The bottom line is that for the typical drag race engine, it's not worth losing sleep over backpressure, because most guys don't really want to spin their engines over 8000 rpm anyway, or don't have the components to take advantage of it if they did. The gains from reducing backpressure aren't going to come from a huge gain in horsepower; they are going to come from the additional engine speed afforded by a low backpressure combination. And for a typical T/S application, a high backpressure combination may not be all bad....a high backpressure combination is usually one that has a very responsive turbo combination, which means that it will be really easy to get the turbos to follow the boost controller.

[Tom Vaught]

I will post a couple of comments from the Garrett People and the Borg Warner people about pressure across the inlet and outlet of the Turbine.

Garrett:

"http://www.turbobygarrett.com/turbobygarrett/turbine_housing_AR_and_housing_sizing
Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to breathe"" effectively at high RPM, adversely affecting peak engine power."
Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds.
When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband characteristic.
Here's a simplistic look at comparing turbine housing geometry with different applications. By comparing different turbine housing A/R, it is often possible to determine the intended use of the system.
Imagine two 3.5L engines both using GT30R turbochargers. The only difference between the two engines is a different turbine housing A/R; otherwise the two engines are identical:
1. Engine #1 has turbine housing with an A/R of 0.63
2. Engine #2 has a turbine housing with an A/R of 1.06.
What can we infer about the intended use and the turbocharger matching for each engine?
Engine#1: This engine is using a smaller A/R turbine housing (0.63) thus biased more towards low-end torque and optimal boost response. Many would describe this as being more "fun" to drive on the street, as normal daily driving habits tend to favor transient response. However, at higher engine speeds, this smaller A/R housing will result in high back pressure, which can result in a loss of top end power. This type of engine performance is desirable for street applications where the low speed boost response and transient conditions are more important than top end power.
Engine #2: This engine is using a larger A/R turbine housing (1.06) and is biased towards peak horsepower, while sacrificing transient response and torque at very low engine speeds. The larger A/R turbine housing will continue to minimize backpressure at high rpm, to the benefit of engine peak power. On the other hand, this will also raise the engine speed at which the turbo can provide boost, increasing time to boost. The performance of Engine #2 is more desirable for racing applications than Engine #1 since Engine #2 will be operating at high engine speeds most of the time."

So basically Garrett is singing the same old song: "higher engine speeds, smaller A/R housing will result in high back pressure, which can result in a loss of top end power"


Borg Warner:

http://www.turbodriven.com/en/turbof...gnTurbine.aspx

Borg Warner says that Higher Pressure across the Turbine is not necessarily a bad thing:

The turbocharger turbine, which consists of a turbine wheel and a turbine housing, converts the engine exhaust gas into mechanical energy to drive the compressor. The gas, which is restricted by the turbine's flow cross-sectional area, results in a pressure and temperature drop between the inlet and outlet. This pressure drop is converted by the turbine into kinetic energy to drive the turbine wheel. Energy transfer then happens (from kinetic energy into shaft power in the turbine wheel, which is designed so that nearly all the kinetic energy is converted by the time the gas reaches the wheel outlet.

The turbine performance increases as the pressure drop between the inlet and outlet increases, i.e. when more exhaust gas is dammed upstream of the turbine as a result of a higher engine speed, or in the case of an exhaust gas temperature rise due to higher exhaust gas energy. The turbine's characteristic behaviour is determined by the specific flow cross-section, the throat cross-section, in the transition area of the inlet channel to the volute. By reducing this throat cross-section, more exhaust gas is dammed upstream of the turbine and the turbine performance increases as a result of the higher pressure ratio.

A smaller flow cross-section therefore results in higher boost pressures.

The turbine's flow cross-sectional area can be easily varied by changing the turbine housing. Besides the turbine housing flow cross-sectional area, the exit area at the wheel inlet also influences the turbine's mass flow capacity. The machining of a turbine wheel cast contour allows the cross-sectional area and, therefore, the boost pressure, to be adjusted. A contour enlargement results in a larger flow cross-sectional area of the turbine.

For a high overall turbocharger efficiency, the co-ordination of compressor and turbine wheel diameters is of vital importance. The position of the operating point on the compressor map determines the turbocharger speed. The turbine wheel diameter has to be such that the turbine efficiency is maximised in this operating range.

The turbine is rarely subjected to constant exhaust pressure. In pulse turbocharged commercial diesel engines, twin-entry turbines allow exhaust gas pulsations to be optimised, because a higher turbine pressure ratio is reached in a shorter time. Thus, through the increasing pressure ratio, the efficiency rises, improving the all-important time interval when a high, more efficient mass flow is passing through the turbine. As a result of this improved exhaust gas energy utilisation, the engine's boost pressure characteristics and, hence, torque behaviour is improved, particularly at low engine speeds.

I personally tend to agree with Travis Q and the Borg Warner people in that High Turbine Pressure Numbers do not necessarily always result in Top End HP losses. Basically you have to look at what you are trying to do in each application and then go from there. "Rules of Thumb" are ok but actual data that says "if i do this then I actually got this" is always better. "One piece of data is worth 1000 Opinions" (James Clarke)


Tom Vaught

[ponjohn]

If superchargers need so much HP to produce then why are they a viable option vs. turbos?

Relative simplicity?

[ventura7211]

Originally Posted by ponjohn

If superchargers need so much HP to produce then why are they a viable option vs. turbos?

Relative simplicity?

Sort of. They are considered "easier to race" like a nitrous car vs a turbo car. It is an easier transition from nitrous to boost with a procharger than to turbo due to the methods of power management used.

Over-simply put, you slow the crank down, you control the power, you can do that with a graphs in a digital 7 just like you can with a nitrous car. With turbos you have boost controllers to deal with to program the ramps. Basically, pro-chargers have been viewed as less of a learning curve and something you could eventually do on your own, while turbos have had a stigma of always needing to have a "tuner" on hand helping with your program to go fast and win races.

Then when it comes to driving the car you stage a procharger car just like a nitrous/NA car, while a turbo car you had use to have to learn how to build boost, ride the breaks, bump in etc. Of course now, you put the right converter in the car and it comes up on boost quicker, stages easier, etc. Or you can use a "bump box" like a friend of mine makes where you bump the trans break button:

http://www.leashelectronics.com/Puls...tem-PL2000.htm

If you already had a nitrous car, it was easier to convert to pro charger as you could often re-use many components that were on the car vs "having to start fresh" with a turbo swap.

Obviously things have been changing at a rapid rate for turbo cars in the past couple of years, the stigma about the tuners is going away as more guys are "doing it on their own", procharger can not keep up with the technology advances in turbos in class racing leading more and more guys going to turbos which probably has something to do with that, I know of one car and another in progress that can be swapped from turbo to procharger in a few hours (power adder, converter, cam, gear, EFI program) in anticipation of class rule changes if one power adder has the advantage over the other in the class, the turbo and procharger uses the same ramair inlet in the lower front fascia, if you have the extra 10-15K to spend for the parts to sit in the trailer, it is what you can do when you want to win a championship...

[ponjohn]

Interesting for sure.