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BMW M4 Turbo Upgrade (S55) – Which Turbo Is Best?

This article demonstrates how to use the turbo size calculator by using a BMW M4 turbo upgrade (S55 engine) as an example. The article shows how we get the numbers we need from the turbo calculator to analyse turbo compressors maps.

NOTE: This example is for illustration purposes only. We are not dealing with the practicalities of converting twins to a single. We just want to find the hypothetical ideal Garrett G-Series turbo for the M4 Competition.

Basic Numbers

To start we need to know some basic information about the S55 as it is used in the M4 Comp.

  • Boost Pressure vs RPM
  • Horsepower vs RPM
  • Engine Capacity

Step One – Finding the VE of the S55

Find the Volumetric Efficiency of the engine and from that hypothesise the Volumetric Efficiency of the engine without the turbos.

To get the numbers we need, we use data from the Bluespark website. The page gives up dyno graphs for the M4 Competition, it also gives us graphs showing the boost the engine is running throughout the rev range.

VE Calculations BMW M4 Competition S55
VE Calculations for the BMW M4 Competition S55

When we plug the Bluespark numbers into the volumetric efficiency calculator we get the numbers in the above table.

If you don’t have a dyno graph for your engine showing boost and bhp vs RPM, try searching online for images of dyno graphs from cars with the same or similar spec to your engine. If you can’t find a graph and you don’t want to go to a dyno you can try the engine horsepower calculator here (just record boost pressure as well)

These numbers tell us that the volumetric efficiency of the S55 engine in the M4 is pretty flat (graph on the right). Peak efficiency comes at around 5000rpm and it looks like it starts to tail off as it gets to 7000rpm.

From these numbers we are going to say (conservatively) that the volumetric efficiency is on average around 85% without the turbo.

Step Two – Turbo Size Calculator

Now we have a VE number we can use the turbo size calculator to see which Garrett turbo suits the engine in stock form and we can also see what turbo we can upgrade to, if we want to increase the power of the motor by increasing the boost from 15PSI to 30PSI.

BMW M4 Competition CFM, LBS/MIN and BHP numbers using the turbo size calculator
CFM & LBS/MIN numbers for the stock engine with stock boost
BMW M4 Competition CFM, LBS/MIN and BHP data if the car were running 30PSI of boost
CFM, LBS/MIN and BHP numbers if the engine were running 30 PSI of boost, 45PSI absolute.

So which turbo from Garrett suits the engine when it is running stock boost levels (around 15PSI) and which turbo would suit the engine if the boost were raised to 30PSI?

BMW M4 Turbo Upgrade – This car uses a G35-900 turbo, a size in-between the G30 and G40 turbos used in the example in this article. At 21PSI, pump, on the stock turbos it made around 546bhp at the flywheel (455rwhp). On the G35-990 it makes 520rwhp @19PSI on pump. 70whp more with 2PSI less boost. With E85 (not sure the boost) it makes 670rwhp.

Compressor Maps – BMW M4 Turbo Upgrade Analysis

We can plot the LBS/MIN and Pressure ratio numbers from the table above onto Garrett turbo compressor maps to see which turbo suits our engine.

The green dots are 30PSI of boost, the blue dots are stock boost. I took 3 turbos from the Garrett G-Series range and plotted our M4 numbers (lbs/min vs pressure ratio). First turbo is the smallest, bottom turbo is the biggest.

Garrett G30-770 turbo. Compressor map showing BMW M4 competition S55 engine running 15PSI and 30PSI of boost
G30-700. Blue dots are stock boost, green are 30PSI. For stock power this looks to be a good turbo which has a bit of room to increase the boost and still give good efficiency. However the island seems a bit narrow for the size of the engine. Probably not the turbo to go for if you are looking at running 30PSI of boost, the top right green dot is well off the island. But if you are looking for boost around 21PSI this turbo looks okay.
Garrett G30-990 turbo. Compressor map showing BMW M4 competition S55 engine running 15PSI and 30PSI of boost
G30-900 Compressor maps. Blue dots show stock boost. Green dots show 30PSI boost. The next size up from the 770. This turbo has better efficiency for higher boost at higher RPMS. At low RPMs it could lose power to the stock turbo. This turbo could be a good choice for the person looking for a turbo that can work at stock boost pressures but can also work with a big boost increase down the road.
Garrett G40-990 turbo. Compressor map showing BMW M4 competition S55 engine running 15PSI and 30PSI of boost
G40-900 Compressor maps. Blue dots show stock boost. Green dots show 30PSI boost. At stock boost, the G40 probably loses out at 4000rpm to the stock turbo, but by 5000rpm we have good efficiency and we stay in the best part of the island until redline. At 30PSI boost we are still in good efficiency territory from 4000rpm to 7000RPM. If you are looking to run 30PSI of boost in the future, this turbo could be the ticket.

Which Turbo? – BMW M4 Turbo Upgrade

For 15PSI to 21PSI, the G30-770 looks good. For 30PSI the G30-900 and the G40-900 look very close, even at lower RPMs. If I had to choose I think I’d go with the G40 just because it spends more time in the higher efficiency islands and maybe there would not be that much difference at lower RPMs, lower loads. Having said that, the G35-900 might be the optimum (as used on the car in the video above).

Wrap Up – Turbo Size Calculation

So this was a quick example of how to use the calculators on StrikeEngine to compare different sized turbos, in this case the BMW M4 but the principle is the same for any engine. The process can be used for a naturally aspirated engine which we want to put a turbo on and for engines where we want to upgrade an existing turbo.

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This page was last modified Sep 20, 2023 @ 6:05 pm

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