Twin Scroll Turbo and Headers

[kenikh]

Originally Posted by WERK I

Twin scrolling will produce more efficiency at low engine speeds, but there is no advantage to twin scrolling when an engine achieves higher engine speeds (above 3000RPM, for instance).

100% correct - the Evo guys that want to make BIG power and only care about top end rarely run twin scrolls because they are superfluous.

[Speedy Squirrel]

The N55 engine produces less power than the twin sequential turbo N54 engine it replaces. It is intended to give European customers, the majority of which pefer diesels, the torque and efficiency of a diesel at a gasoline price. I think many Americans would prefer the N54 engine.

The exhaust manifold IS NOT a log manifold, it just looks like it in the cutaway. It does in fact have some very short runners of unequal length. Equal length is not critical to twin scroll operation on any engine. There is a range of variance allowable, and it depends on the mass flow, cross-section, length, and manifold pressure.

The twin scroll has little effect on residual gas after the wastegate is open. Once the wastegate is open the benefit is mostly lost. Modern engines have very low residuals in any event, due to the superior combustion chamber layout, and pistons with extremely high and thin top rings. It is just not an issue any more in a modern engine.

For our engine, the twin turbo approach is preferable. The shorter length exhaust manifold provides a benefit right off the mark, and it has all the benefits of the twin scroll in terms of pulse utilization. I don't know if it will make more power (there certainly are more twin sized turbo's to choose from than twin scroll ones, as a practical matter), but it will be more responsive.

[kenikh]

Speedy makes a very good point I neglected to make earlier, but needs more explanation.

Twin turbo is preferable on our motors IF the twin turbo application has equal length shorty headers to the sides of the motor, al la the 993 Evo turbos. This provides the benefits of equal length AND the shortest tract possible (which the rear of the motor isn't).

I will contend that a twin scroll, single turbo system is every bit as good as a twin turbo system hung off of the back of the motor.

[911st]

This is a cool graphic that displays the overlapping of the exhaust events on a merged or non- split system.

The overlap of the red bars is where two exhaust valves are open at one time.

On a split system, every other line would be on one bank of cylinders.

Originally Posted by 304065

I like to be organized so I made the following chart to help me with cam timing. Basically the operation is not more complicated than that explained in the factory manual, but I like to overcomplicate things. This chart shows what each piston is doing. But the key point, in my case, comparing the timing of Cyl1 with Cyl4 is that when Cyl1 is at the timing point (labeled OC in the chart) Cyl4 is at the firing point (labeled TC in the chart). Of course I know that the firing point is not exactly TDC but you get the idea. Green= power, Gray= exhaust, Blue= intake, Red= Compression.

[911st]

I am really not sure if one type of system is really better than the other except that equal length is a good thing to get the exhaust out in the most orderly way.

It looks like right at the time one exhaust valve is about to fully close, another has just opened and pressureizing the exhaust manfold on a common manfold system.

[Jim2]

Just received this turbo yesterday. Split vane, (ball bearing too) for my turbo 3.6 project. Building a 993 C2 turbo'd with SDS injection.

[911st]

Nice.

PM'd.

[kenikh]

Oooooh, dats niiiice...