I had (mistakenly) been under the impression that a "bigger" turbo meant more lag and was only useful above certain rpms compared to the stock vtg turbo. After speaking with another forum member who has the upgraded vtg turbos I learned that the OPPOSITE is true - ie. it spools up quicker with more power down low.

Apart from the extra visit from the wallet inspector is there any downside to the upgraded vtg? I assume that it by itself doesn't negatively affect driveability etc...

Anyone have mods done to their car and then add JUST the upgraded vtg? for example, anyone go from the AWE650 kit to the 700? I'd be interested in any opinions on the difference (from the butt dyno as well as formal measurements if anyone has them).

What would be the advantage of billet turbos vs non-billet? I see that Champion has introduced billet ones and was wondering what the benefit is with them (apart from the different sized inlet/outlets)?

I think Bob posted somewhere that the cost difference between the 997tt turbos and the GT2 turbos is 9k. Wow, obviously there is more performance from the GT2 turbos but is it just a matter of bigger?

 

I am still under the impression (based on previous experience with other turbo cars I've upgraded) that bigger turbos mean more lag. I would love to have some real proof to the contrary.

 

This is a large help in eliminating the feared "Turbo Lag" when referring to upgraded VTG turbos.

The Problem With Turbochargers: Turbochargers tend not to facilitate effective aspect ratio (known as A/R Ratio) when required to. This is due to the fact optimal aspect ratio at high speeds is very different from that at low speeds. For example, if the aspect ratio is too small, the turbo will eventually choke the engine at high speeds; conversely, if the aspect ratio is larger than usual, the turbo will fail to the necessary boost at low speed. All of this malfunction will lead to high exhaust manifold pressure, high pumping losses and eventually lower power output for the engine.

The solution to the problem is Variable Turbine Geometry (VTG) also known as Variable Geometry Turbocharger (VGT), or a Variable Nozzle Turbine (VNT). A turbocharger engine that has VTG has small variable (movable) vanes which can direct exhaust flow onto the turbine blades. The vanes angles are regulated by an actuator. To optimize the turbine behavior, the vanes angles vary throughout the engine RPM range.

http://www.youtube.com/watch?v=m644r...layer_embedded

 

+1 Rep for u ..
Thanx for clarifying this. I'm pretty sure every1 could underistand what you said up there ..

 

When properly matched with the correct ECU tune, upgraded vgts are more efficient than stock. This is nicely demonstrated in the dyno provided by Evoms. If you go to a bigger turbo such as gt3076, then there will be more lag at the lower end.

 

I think while the the lag of the modified VTG is less than stock, you have a feeling of more lag because the modded VTG produce much more torque than stock on boost.
From Evoms graph in their catalog, EVT 750 vs stock 997TT torque: at 2500rpm stock 345lb-ft mod vs. 365lb-ft, at 3000rpm 428 vs. 480, at 3500rpm 510 vs. 690.
Even at lower RPM, modded vtg produced higher torque than stock, but the difference torque at 2500rpm compared to 3500rpm on stock vtg is 165lb-ft, on modded vtg is 325lb-ft.
So the stock seem to be more "linear" than modded vtg.
Between 2500 and 3500rpm modded vtq torque felt like to explode due to higher increasing rate than stock.

 

I am confused by your post, how does more torque give a feeling of more lag? Not possible.
If anything I would like to see the torque not fall off at 5500rpm, but believe this is reaching the limits of efficiency of modified VTG's and they just cannot flow any more cfm. Anyone ever see a compressor map for stock or modded VTG's?

 

This sounds like what I'm looking for (explosive torque starting at 2500 to 3500)!!!! The stock system is no slouch but it could kick more in this range.

 

Thanks for the informative post.

Can you explain the billet thing and why it would be desirable to have a turbo made from a billet (if that's the correct way to word it)?

Is the GT2 turbo just a higher flowing version of the one on the 997tt?

 

A chassis dyno does not do an adequate job demonstrating lag, especially because the dyno load will affect the lower RPM range where it is not possible to distinguish between lag and boost threshold.

The boost threshold of a turbo system describes the lower range which the compressor will operate. Below a certain rate of flow at any given pressure multiplier, a given compressor will not produce significant boost. This limits boost at particular RPM regardless of exhaust gas pressure.

Turbochargers start producing boost only above a certain exhaust flow rate (depending on the size of the turbo) which is determined by the engine displacement, RPM, and throttle opening. Without an appropriate exhaust gas flow, they logically cannot force air into the engine. The point at WOT in which the mass flow through the turbo is strong enough to force air into the engine is known as the boost threshold rpm.

If you drive along at constant RPM above the boost threshold, (about 2300 for the stock VTG) you are not loading the engine, and not producing boost. Once you push down on the throttle, a data trace you get from a performance box or Durametric data log will show how long it takes for the boost to start being delivered.

The boost coming in is represented by the longitudinal G line (i.e. how many "G's" you are pulling). You can easily see how much lag you are getting by looking at how this line ramps up.

 

You can not demonstrate lag or boost threshold using a dyno, without simultaneously plotting boost pressure. Everything that is stated here is conjecture based on a plot of torque versus RPM. The delay in onset of torque is not necessarily due to lag...it could be due to artifact, dyno load, dyno setup, ECU tuning, timing, or a number of other factors.

If you want to demonstrate LAG, you must plot RPM versus rise in boost.

 

totally agree .. and in some cases maybe toque spread along RPM range ..

 

Thanks for the clarification and I totally agree with you that a chassis dyno is not the best way to reflect lag. My post was meant to address the OP's question below. Also, I posted a 30-100 3rd gear run on my upgraded vgts in a different thread if anyone wants to compare lag.

Anyone have mods done to their car and then add JUST the upgraded vtg? for example, anyone go from the AWE650 kit to the 700? I'd be interested in any opinions on the difference (from the butt dyno as well as formal measurements if anyone has them).


I don't think there is any proven performance enhancement of billet over cast turbines. Perhaps longevity.

That's a good question. I've tried to get a compressor map for my upgraded vgts but they seem to be a highly guarded secret.

 

The beauty of the Upgraded VTGs is the Turbine remains stock.. Larger compressors dont normally created more lag but the POWER is made by larger Compressor sizing.. The great thing about the VTG is you can increase compressor wheel without having to increase the turbine to keep up, so in return the lag doesnt change much at all..

With a standard turbo that is 55mm compressor / 55mm Turbine you cannot upgrade that properly to a 67mm compressor without increasing the Turbine side to keep up with it.. If you do increase the turbine well then comes the added lag..

With the VTG able to spool so quick and still flow the big HP they are a great option up to a certain HP level.. What that level is im not 100% just yet but ill find out soon as i get my 67mm VTGs added to my setup..

Mike

 

There is some controversy about the advantages of billet versus cast turbocharger upgrades. Recently billet turbo upgrades have received significant advertising from tuners, however, the technology is not new. It’s been around since the 80s.

The material does not necessarily make a billet wheel produce more power, rather it is the ability to use CAD/CAM to design a billet wheel. From a technical perspective, a billet compressor wheel is CNC’ed out of a solid piece of billet aluminum where as a traditional wheel is cast out of molten aluminum. The theory is a CNC'ed billet compressor wheel can be created with “less bulk” and less actual material, and CAD designs can take advantage of the new found “space” within the turbocharger inlet by redesigning the compressor aerodynamics to produce greater CFM with the same size inducer.

You are dead-on...If the design of the compressor wheel is the same (billet versus cast) there should be no significant performance difference (aside from actual weight of the wheel and the physics of weight reduction.)

As far as strength of the material is concerned, here is a quote from Garrett:

"in OE applications, fully-machined wheels can withstand higher centrifugal stresses due to differences in the base material. Forgings are inherently stronger than the typical casting in this respect. However, when Honeywell engineers choose a fully-machined wheel over a cast wheel, they are only doing so to prolong the wheel's life in extreme duty-cycle OE applications where the turbo speed is constantly cycled. A typical example is a city bus, in which the turbo is frequently subjected to rapid transitions between high-load (full throttle) and low-load (idle). Compressor wheels can fail in low-cycle fatigue (LCF) in these applications, which is where fully-machined wheels offer an advantage in strength and lifetime. However, the typical aftermarket turbocharger will not be subjected to such extreme cycling."