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#6: 12-02-2010, 08:56 AM
 
 m sprank
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Learn stuff - Turbo lag, how to beat it. Part II
The following is part 2 of an extract from a discussion paper put together by our engineers called "Turbo lag and the TMS solution". Part 1 precedes the post.
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TMS principle

By designing and adjusting a TMS valve to specifically suit the application, GFB has made it possible to reduce turbo lag during gearshifts or when modulating the throttle by holding some pressure in the intercooler and piping. By venting only enough air to prevent compressor surge, pressure in the intercooler can be maintained for as long as the turbo’s inertia will allow.

During a high RPM/full throttle gearshift for example, boost pressure in the intercooler with a factory valve will typically drop to zero (atmospheric) before the throttle is re-opened. With a GFB TMS however, the rate at which the boost pressure drops can be reduced, so that it is possible to have positive pressure in the intercooler when the throttle re-opens. This gives more power immediately as pressure is higher, and also reduces the time taken to reach maximum boost.

Example

To demonstrate this, our test bed was a 1.8L turbo engine, with a large front-mount intercooler running 12psi peak boost. A data logger measured the throttle position and the manifold pressure.

Since this engine had no factory valve fitted, we used one from a Mitsubishi EVO IX to begin with for a base run.

A full boost 2nd-3rd gearshift was performed at 6000RPM and the results logged. From the data we can measure the actual lag time from when the throttle first starts to re-open, to when full boost is reached.

The factory valve was then replaced with a GFB TMS valve, and the spring was adjusted as firm as possible without incurring compressor surge and the test repeated.


Results

The graph opposite (refer TMS_lag.jpg) shows manifold pressure and throttle position, starting from part-way through a gearshift, since it’s what happens when the throttle re-opens that we’re interested in.

Importantly, the manifold pressure trace shows two distinct zones: Initial pressure rise and turbo lag. The initial pressure rise is similar to the behaviour of a normally aspirated engine in that as the throttle opens, the manifold pressure rapidly rises from vacuum to the supply pressure (i.e. the pressure upstream of the throttle – on an NA engine this will usually be atmospheric pressure).

The pressure increase then abruptly reduces in rate as the manifold equalizes with the supply pressure, and the turbo is still spooling up. The rate of pressure increase during spool up is determined by the engine and turbo system itself, and the time taken doing so is defined as turbo lag.

This graph (refer TMS_boost.jpg) shows the two test runs overlaid, with the red line representing the manifold pressure with the GFB TMS fitted, and the black line is with the factory valve fitted.

It can be seen that with the GFB TMS, the initial manifold pressure rise doesn’t stop at zero (atmospheric) as it does with the factory valve, but continues to 2.5psi. This means that the supply pressure in the intercooler when the throttle is re-opened is 2.5psi rather than zero, and as a consequence, peak boost is achieved 0.21 seconds (30%) sooner than with the factory valve.

Peak numbers aside, it can be seen on the graph that higher boost pressure (and therefore engine power) is available during the entire spool up process.

Conclusion

A GFB TMS valve can help maintain boost pressure in the intercooler and pipes whenever the throttle is closed with the following benefits:
• Higher boost available upon re-opening the throttle for instant response
• More boost throughout the spool-up process (up to 40%)
• Shorter lag time (reduced by up to 30%)
• No compressor surge

When driving, a factory diverter valve will typically create a slight hesitation immediately upon re-opening the throttle, followed by a noticeable turbo lag. Fitting a GFB TMS will sharpen the response by eliminating the hesitation and then reducing time to peak boost. During a quarter mile race (where 3-4 gearshifts can occur) the time saving is quite significant, and on the circuit the power delivery upon corner exit is sharper and more responsive to the throttle opening.