More boost at lower RPM's with smaller exhaust? part 2

[Fiorelli]

After that last post I don't know if I should even post this. but I ran into some more information and have more questions.

In the book "Turbochargers" by Hugh Maclnnes page 39.

ADJUSTING THE POWER CURVE I'm not going to type the hole page and post the graphs (scanner dosen't work) hopefully you have the book if you need more information. but he says

An engine with the largest-possible carbureator and lowest-possible exhaust restriction has a wide-open-throttle acceleration line like the solid line on Figure 4-11 This leaves something to be desired at the mid range.

Where I have my problem.

If a smaller turbine-housing A/R or nozzle area is used, the engine will be overboosted at maximum rpm. Overboosting can be avoided by using a control device such as a wastegate. Controls are covered in Chapter 10.

At full RPM's I'm developing all the boost I want and need!!!

It is possible to have a more desirable wide-open-throttle operating line, as shown by a dotted line in Figure 4-11. It is possible to achieve this condition without using a movable control. Any one or a combination of the methods described here will improve the mid-range torque without overboosting at maximum engine speed.

Exhaust Restriction A similar result may be obtained by restricting the turbine exhaust, causing a back pressure on the turbine of about 4in Hg at maximum speed. Although a turbcharged engine is relatively quiet without the use of a muffler, this will result in an even quieter-running engine.

This makes me thing that maybe as crazy as it sounds to me a small exhaust pipe after the turbo would accually help this case.

While I agree with the person that thought I needed a smaller waste gated turbo this turbo was free. We have not turbo rule and this turbo is very old, big and shot! so when they try to control my tractor they will limit turbo size then I'll get the right turbo. After that dosen't control my tractor and the right tractors still aren't winning they will outlawing the hole tractor. It would be nice to make this process take as long as possible.

For the person that thought this was a good example of a turbo that was to big. If the turbo was in good shape I'd probably have to much boost. Yes the turbo is off an 855 CID cummins. But I'm 809 CID running the same RPM range as the 855 getting simular result. Boost a few hundred rpms faster would solve my problem. Or a slipper clutch so I could take off at a higher than the 1000 to 1200 I'm running now.

To explane this situation a little more. My final drive ratio is 15 engine revolution to 1 axle 2145 RPM limit with a tire with a 190" rolling circum. My ground speed is close to 20MPH the wheel speed is around 25MPH. I'm sure you have seen the super stock diesel tractors that shoot lots of black smoke 50 ft in the air and better they run about 50 MPH wheel speed at 6000 to 7000 RPM. At 6500 my wheel speed would be 77.9MPH. My tires are very close to the same dia but not as wide.

 

[XPC66]

The passages you have quoted relates to over boosting. You can either use a wastegate or restrict the flow of exhaust through the turbine.

The wastegate portal is sized to match the restriction of the down pipe. This is why making the down pipe less restrictive tends to lead to boost spike and boost creep..... and the drilling out the portal becomes necessary.

Restricting the exducer side of the turbine does not improve mass flow into the engine.

Having the wrong sized backend only mismatches the compressor to the required flow effectively changing the range of engine rpm to move from surge to choke.