B58 Big Turbo Selection – Which Size Is BEST?
Another installment of the turbo selection series. In this article we look at the BMW B58 engine and we look at what aftermarket turbo is best suited for the engine. We will not be looking at turbos that use the stock manifold. Custom tubular manifold with a Garrett turbo only, but which one?
Step 1 – B58 Big Turbo Selection
As always, before we can dig in to compressors maps we need to find the volumetric efficiency of the engine and to do that we’ll use the StrikeEngine volumetric efficiency calculator.
To get our base data we’ll use this dyno graph from a Toyota Supra.
For the purpose of this article we’ll look at 3000 RPM, 4500RPM and 6500RPM.
From the above graph (green line), at 3000RPM the engine is making 240bhp with 16PSI boost, at 4500RPM 340bhp at 14PSI, at 6500RPM 460bhp at 20PSI boost.
Unfortunately I don’t know the mods on this car so we’ll have to make assumptions namely that it is stock with a program only.
B58 Volumetric Efficiency
Given that, bolting these power, RPM and boost numbers into the VE calculator, we get these efficiency numbers (minus turbo) for the BMW B58.
3000RPM = 0.89
4500RPM = 0.9
6500RPM = 0.7
Turbo Plots
Bolting the VE numbers into the StrikeEngine turbo calculator we get these numbers
| RPM | VE | lbs/min | PR | Boost Target PSI | Predicted Power |
| 3000 | 0.89 | 22.7 | 2.08 | 16PSI | 237 |
| 4500 | 0.9 | 44.55 | 2.7 | 25PSI | 465 |
| 6500 | 0.7 (0.8 used) | 57.2 | 2.7 | 25PSI | 597 |
With the lbs/min and PR (Pressure Ratio) numbers we can now plot these points on turbo compressor maps to find the optimum Garrett turbo.
Garret Turbo Analysis
Gen 1 G-Series turbos first
Gen 2 G-Series
Thoughts
The G30-770 looks ideal for our boost targets but we are close to the limit of the turbo. There is not a lot of room to increase airflow later with bigger boost of more efficient exhaust, manifold, intercooler, intake etc.
The Gen 2 G30-825 58mm looks as good as the G30-770 at low RPM but looks much better at higher RPM. There is more room to increase the boost and power. The G30-825 is more future proofed. For example if we go to 31 PSI and 7000RPM we get the orange point on the graph below. Not a horrendous point to be at.
But this is probably the limit for this turbo with a hypothetical power of 724bhp (603whp) .
Summary – B58 Big Turbo Upgrade
All this sounds great. Until you realise the stock turbos can make 600bhp (500whp)…..
To maintain stock driveability ie decent boost at 3000RPM while have power at the top the stock turbos are hard to beat. If you want to keep stock driveability and gain an extra hundred bhp at the top the G30-825 is the choice. But is it worth spending all this money for an extra 100bhp at the top over the stock turbos?
1000BHP
To get up to 1000bhp, boost at 3000rpm is going to drop versus stock. The G42-1450 Gen 1 can do 1100bhp (fly) at 45PSI, 7000RPM, but at 3000rpm its going to struggle to make 10psi of boost without going into surge and below this boost pressure the turbo is not going to be doing anything. In short, looking at Garrett turbos at least. If you’re going for 1000bhp some compromises will have to be made in day-to-day driving.
G35-990 Gen 2 62mm will do 833bhp at 31PSI and 7000RPM. It will probably do 10PSI at 3000RPM but its also at the limit, slightly better than the G42-1450 but not much. And its no match for the G42 at the top, so probably may as well just go for the G42.
BMW Got It Right
It looks like BMW did a pretty good job with the stock B58 turbo sizing. Good low end with decent high end power. Sure incremental gains can be made with turbo upgrades but is the cost worth it?
Try It Yourself
Bolt your own boost and RPM targets into the turbo calculator to get your airflow and pressure ratio numbers and use these to analyse any turbo (that is supplied with a turbo compressor map) you wish.
