Generally speaking tubular manifolds produce more hp per pound of boost when compared to the log style.

Given two identical engines running the same boost and same turbo, one with a log making 325hp and one with a equal length tubular making 385hp. Assume the intercooler is efficient at taking out the heat and inlet temperatures are the same.

Does the one making 385 create more stress on the engine? The boost is the same. The internal pressure is the same. It

 

The two big physical stressors are engine RPM and torque. Too much torque will cause components to fail through over stress. In addition to torque, too much engine speed results in very high stresses on all rotating components (accelerating and decelerating the pistons for instance).

Boost pressure alone isn't enough information. We need to know the pressure on BOTH sides of the engine, that is the intake manifold pressure and the turbine manifold pressure. In addition to the suction from expanding cylinder chambers, the higher the pressure differential across the engine, the more air that will be pulled through. More air packed in the cylinder = more torque and eventually more power.

With the same boost pressure (pressure UPSTREAM of the engine) the two engines make different power because they are NOT moving the same amount of air. Due to the more restrictive log turbo manifold, more pressure builds up DOWNSTREAM of the engine. This causes the overall pressure differential ACROSS the engine to decrease, reducing air flow. This is why it takes more boost pressure with log manifolds to reach the same amount of power achieved with a tubular manifold. It takes fractionally longer for the boost pressure to rise to that higher level, resulting in additional turbo lag with log manifolds. The same power can be achieved, but it just requires more spool and more boost pressure.

To answer just your stress question, the engine with more torque has more stress.

 

Thanks, never thought of it that way, but makes perfect sense.

 

Does this also mean by using a very efficient manifold vs an inefficient one could lead to needing a thick head gasket (or lower compression pistons) sooner as the cylinder fill is greater?

So the rule of above xpsi you need to lower compression is not necessarily a good rule?

 

Yes, but not because the cylinder fill is greater. If the power is the same, the "cylinder fill" should be the same. The boost pressure alone is never enough to break components. When the air is compressed and then combustion takes place, the cylinder pressure is orders of magnitude greater than the boost pressure. The headgasket reduces the compression ratio, allowing the boost pressure to be raised while avoiding pre-detonation. It does NOTHING for the engine's strength.

With the F20/F22 at stock compression with pump gas, the rule of thumb is that above 10 psi, the pre-detonation limit is too close for comfort, especially with a turbocharger. Not sure what a similar limit would be with the 3mm gasket, but it's over 19-20 psi.

 

I guess I wasn't clear on that one. I understand the reason for the lower compression, and that it does nothing for strength.

What I meant is asssuming the tubular has a greater pressure difference from less backpressure in the manifold the fill would be more, as would the power (325 vs 375), for instance. With the car making 375 , the pre-detonation window would be closer than the car making 325, even though the boost as measured at the manifold is the same.

Or put another way. Assume two identical engines making 7psi at the manifold. One tubular and one log. One making 325 and one making 375. The one making 375 is closer to the pre-detonation window correct?

And as a side question. If one was to run 110 leaded race gas, how much does that effect the pre-detonation window? (assume racecar with no cats and no o2 sensor damage issues)


Bill

 

^ you can run 110 octane that is unleaded. Once you get into 116 and above it is leaded.

It is not necessarily closer to the predetonation window, its simply more efficient. Tuning is a HUGE factor in all of this.