M52 Turbo Kit – Which Turbo Size Is Best?
In this article I’m going to take you through the process of choosing the correct size turbo for your M52 turbo kit.
You can change the boost and RPM targets I use to whatever suits you. Lets get started.
Key Points – M52 Turbo Kit Size
The G30-770 looks to be well suited as a turbo in an M52 turbo kit for a 328i engine. If we can sort the VE issue at high RPM.
If not, the GT3076R looks to be suitable also.
Finding The Best Turbo – The Process
- Find the VE (volumetric efficiency) of the engine – Using the StrikeEngine volumetric efficiency calculator
- Find the lbs/min & PR for our boost and RPM targets – Using the StrikeEngine turbo size calculator
- Plot the lbs/min and PR onto various turbo compressor maps to find the best sized turbo.
Finding the numbers
VE
To get the VE of our M52 engine, we need a to see a dyno graph showing power of the (stock) engine against RPM. I found a dyno graph here.
I’m interested in the VE at 3000, 5000 and 6500RPM (Feel free to calculate the VE for whatever RPM you want ).
Putting the power and RPM numbers into the VE calculator it gives us the VE numbers of the engine at these RPMs (right column below). I have multiplied the dyno graph numbers by 1.15 to get power at the engine.
| Engine | RPM | Power | VE |
| Stock M52 328i | 3000 | 109 | 0.9 |
| 5000 | 201 | 1 | |
| 6500 | 196 | 0.75 |
lbs/min & Pressure Ratio (PR) for the compressor maps
Now we have the VE numbers, we need to decide what boost we want to make at these same RPM points. For this article I’m going to say I want to get 7PSI at 3000RPM and 14PSI at 5000 and 6500RPM. You can choose whatever boost targets you need/want.
| RPM | Boost Target (PSI) | VE | PR | lbs/min | CFM |
| 3000 | 7 | 0.9 | 1.5 | 15 | 197 |
| 5000 | 14 | 1 | 2 | 38 | 494 |
| 6500 | 14 | 0.75 | 2 | 37 | 487 |
Having put the boost targets with the corresponding RPM and VE into the turbo size calculator we get out the PR and lbs/min numbers we need to analyse turbo compressor maps from Garrett.
Note. The M52 328i looks to be very restricted at the top end which leads to the 5000 and 6500rpm points have the same airflow. This means we will only have one plot on the compressor graphs for these two RPM points.
Garrett Turbos
The GT3076R is usually a good place to start so lets see how it suits our M52 328i engine
As usual, for an NA engine making this sort of power the GT3076R looks to be about perfect. Relatively good efficiency at 3000RPM and very good efficiency at 5000 & 6500RPM.
Lets assume for a moment we can sort out the efficiency drop off at 6500RPM (perhaps a good turbo manifold without a cat will make a big difference at 6500RPM and get us back to say 85% efficiency) will the GT3076R still be a good choice?
Going back to the turbo size calculator and bolting in 0.85 VE for 6500RPM instead of 0.75, we get this plot (below, green). And while we’re at it, lets add 0.9 VE as well for the sake of completeness (purple plot)
For 0.85 VE, we still look to be okay but 0.9 VE is cutting things a bit close IE we are seeing a big drop off in efficiency at 6500RPM.
In short, at 0.9 VE at 6500RPM the GT3076R does not look great.
Lets see if we can find a better turbo for the top end.
G30-770
Putting the same plot points onto the G30-770 compressor map we get this
At low 3000RPM we are at 60% efficiency which is not great, we could be losing some performance here vs the GT3076R but at higher RPMs we are bang in the sweet spot at the current 0.75 VE and if we can get the VE up to 0.90.
What about less than 3000RPM?
I’m concerned with the G30-770 below 3000RPM. Lets see if my concern is justified.
I found this dyno graph, not a stock M52 (it has an M50 intake manifold and S52 cams) but hopefully it gives up a ballpark figure of what the engine is doing at 2000RPM, where, lets say, for this example, we don’t need any boost, IE our boost target at 2000RPM is zero PSI.
https://www.bimmerforums.com/forum/showthread.php?2176885-MMW-Dyno-Repository-Lots-of-interesting-info!
Bolting 2000RPM, and 69BHP into the VE calculator we get a VE of 0.85
Putting the 0.85 VE into the turbo calculator with a zero PSI boost target at 2000RPM we get the plot below (blue)
And…. I don’t see any issue at 2000RPM.
M52 Turbo Kit – Wrap Up
In summary, the G30-770 looks to have the edge over the GT3076R and especially if we could improve the engine breathing at 6500RPM. And the added bonus of the G30-770 is we have more room to increase the boost if we wanted to in the future.
If you have a 320i/520i or a 325i/525i and not a 328, just bolt your own dyno numbers in the VE calculator and follow the process above.
This article is not law, it’s a hypothetical optimum, it doesn’t take into consideration quality of the tune, compression ratio of the engine, intercooler piping size, intercooler size etc etc. Consider it as a very rough guide.
