B5 S4 Turbo Upgrade – Which Garrett is OPTIMAL?
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I’ve been perusing myaudis4.com and following his labour of love and the results he has gotten from various stock location S4 turbo upgrades on his B5. The results have been less than impressive IMO. Forced Performance plus many other turbos were tested an from what I can see, for the cost, the BW RS4 turbo upgrade looks ideal, for the price and if you want to maintain low down spool.
The question I want to answer in this article is this.
Is it worth making custom exhaust manifolds and downpipes in order to fit a Garrett turbo to a B5 S4?
Can we keep similar response and power low down while at the same time liberating top end power and all the while keeping the engine as reliable as it would be with the RS4 K04 turbos. Can we get a decent increase in power without putting additional stress on the motor.
The Process
I’ve done many turbo selection videos and the process for the S4 B5 turbo upgrade selection will follow the same formula.
First we’ll find the volumetric efficiency of the engine at the boost and RPM points we are interested in.
Then we’ll bolt our VE numbers and the corresponding boost targets into the StrikeEngine turbo calculator to get our compressor plot points.
And after that, we’ll finally bolt our plots on to some Garrett turbo compressor maps to see which is most suited to the Audi B5 S4 2.7T.
Volumetric Efficiency – B5 S4
myaudis4 has an absolute ton of datalog info including a dyno run with boost levels so I’ll use that.
I’ll tailor the turbo selection to the author of this site. He wants big power top end, conservative boost and close to RS4 turbo response.
With that in mind I’m going to choose 2500RPM, 4500RPM and 6000RPM and 15PSI, 23PSI and 23PSI as our boost targets at these RPMs.
| RPM | BOOST | Power (Flywheel) | VE |
| 2500 | 15 | 180 | 0.92 |
| 4500 | 23 | 420 | 0.93 |
| 6000 | 23 | 480 | 0.8 |
Turbo Numbers
Now we’ll bolt the VE numbers into the turbo calculator
| RPM | Boost | VE | lbs/min | Pressure Ratio (PR) | Predicted Power |
| 2500 | 15 | 0.92 | 17/8.5 | 2 | 177 |
| 4500 | 23 | 0.93 (0.95 used) | 40/20 | 2.6 | 419 |
| 6000 | 23 | 0.8 (0.9 used) | 51/25.5 | 2.6 | 530 |
The VE numbers have been upped for the calculation because logically speaking our Garrett turbo will be less restrictive, so will our manifold and downpipes.
As is normally the case, hypothetically we can get big gains at the top with a properly sized turbo. Question is, does Garrett have a turbo that can outperform the Borg Warner RS4 turbos?
Garrett Turbos for the S4
Plotting our lbs/min numbers and PR numbers from above into some compressor maps we get these results.
The reality is, even the smallest of Garret turbos, GTX series and after, are a bit big for our S4 turbo upgrade. The best suited is the GTX2860R Gen 2. This is what it looks like.
Even with the smallest sized turbo upgrade from Garrett, we are going to have to give up some low end power. Maybe @ 3000RPM we can get to 15 PSI of boost. We are in surge before that. The good thing about this turbo is the headroom. We can up the boost by maybe another 5 PSI and still be in a good place. The blue dot is 28PSI at 6000RPM.
That’s the best of the usual turbo suspects. Less look at something that’s not so popular.
The Club Line is a smaller turbo but also with less technology. And it shows. Near as makes no difference low RPM response vs the GTX2860, and its maxxed out at 23 PSI boost.
B5 S4 Turbo Upgrade Selection
As I said, tricky to beat the RS4 turbos. But if we are willing to give up some boost pressure below 3000RPM, the GTX2860R Gen 2 should pay us back at the top end. And it has potential for upgrades in the future. If we run it at 28PSI, we should be getting around 600bhp. And even then, it’s still got a bit more to give.
What about torque?
How much torque are we making to get to 600bhp and 6000RPM?
Around 520lbs/ft. This is around 100lbs/ft more than myaudis4 was running with the RS4 turbo setup.
MRC offer a 480bhp package that has 480lbs/ft.
500lbs/ft might be close to the limit of the stock engine, consult your tuner.
And the MRC numbers are dynoed up to 7000RPM.
If we take 500lbs/ft up to 7000RPM we get 666BHP.
