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Focus ST Big Turbo – Which Size Is Best?

In this article we are going to find the most the best Focus ST big turbo size.

Step 1 Finding the VE – Focus ST Big Turbo Selection

We need to VE of the engine at different RPMs. In this case we are going to choose 3000, 4500 and 6000rpm.

To find the VE we need dyno graphs of an engine with a similar spec to ours and a dyno graph which shows power, RPM and boost pressure.

We will use this dynograph. Stock power is quoted as 252bhp for the Focus ST, this graphs gives peak power as 270bhp.

Virtual dynograph from here https://www.stratifiedauto.com/blog/page/11/

Getting the volumetric efficiency

RPMPowerBoostVE without turbo (Calculated)
3000200181.04
4500270200.88
6000270180.7

Bolting in the power numbers, the boost and the RPM into the engine volumetric efficiency calculator we get the volumetric efficiency (VE) numbers in the right column of the table above.

Low down VE is excellent but the engine starts to lose efficiency at 6000rpm. I am going to assume that part of this drop in volumetric efficiency is down to the small turbo fitted from the factory so I am going to say the actual VE, without the turbo at 6000rpm is 0.78**.

Step 2 – Focus ST Big Turbo Selection

No we have the VE of the engine, we can use the turbo size calculator to find our pressure ratios and airflow numbers.

My targets for the Focus ST big turbo is 4psi of boost by 3000rpm, 18PSI by 4500rpm and 20PSI by 6000rpm

Bolting the VE and the boost targets into the turbo size calculator, it gives us the following Pressure Ratio (PR) numbers and the air flow numbers in lbs/ft and CFM. (Right side columns in the table below)

RPMBoostVEPRlbs/minCFM
150000.85*1345
300041.041.210140
4500180.882.224311
6000200.78**2.330390

*Guesstimated VE at 1500rpm.

Step 3 – Analysing the turbos

We are going to look at Garrrett turbos.

We are going to plot the airflow (lbs/min) and pressure ratio numbers (from the table above) onto turbo compressor maps to find the most suitable turbo size.

G25-550

The G25-550 looks pretty good but we are left of the efficiency islands with stock power levels.

GTX3076-Gen2

This looks worse than the G25-550. More to the left of the efficiency islands and again, even with breathing mods and boost increases I’d say it would still be bigger than necessary.

GT3076R

This is the oldest design of the three turbos on this page. It is not as efficient at higher boost levels and the efficiency islands in general are smaller than the other turbos. Having said all that, breathing mods will move the plots to the right which is good. There is room for more boost but not much.

Which One?

The GT3076 and the G25 look the most suitable. I said above that the G25-550 looked a bit big but I should take into consideration that the efficiency islands are higher for this turbo ie peak efficiency is 80%.

So even though the plots are to the left of the map, the efficiency at these points is equivalent to the GT3076 and better than the GTX3076.

The benefit the G25 has over the GT3076 is the extra room it has to run more boost and at higher RPM. But the G25 is a more expensive turbo. I think either turbo will be good for stock boost pressures and will be able to make the most out of breathing mods like tubular manifolds and big bore downpipes.


Is there a better turbo than these?

Lets look at the next size down from the GT3076R

GT3071R

Everything is a little bit closer to peak efficiency

The GT3071R has all the good things of the GT3076R but with more good. It is a better choice on almost every level.

It has around the same headroom for running more boost.

Breathing mods will move us to an even higher efficiency island

We are at peak efficiency with the stock boost pressure

Focus ST Big Turbo – The Winner

For me, the GT3071R is the clear winner from this group, given the price (for the boost targets given). Good on stock engine and allows us to run up to 25-30PSI of boost

I don’t see any reason to choose the GT3076R or the GTX3076R.

If the budget is bigger the G25-550 has all the headroom that the 3071R gives us but more of it. Performance on stock engine probably as good at the 3071R but it has more room for even bigger boost pressures (35PSI ish), breathing mods and stroker kits.


Examples of Focus STs with Big Turbos

GTX2867R

Running 23.6PSI makes around 350bhp (presumably at the wheels)

ATP

ATP have dynographs comparing the GTX3071R, GTX2867R, GTX3067R


3071R


Further Reading

Edge Autosport has done a great article detailing the Ford Focus ST Big Turbo kits available and highlights the plus and minuses of each turbo kit.

This page was last modified Apr 27, 2022 @ 9:49 pm

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