For 7-21PSI – GT3076R looks the pick.
For 21-28PSI – G30-770 looks the pick
The Turbo Assessment Process
To assess a turbo we need to know the lbs/min and Pressure Ratio of our engine for our boost and RPM targets.
First step is to get the volumetric efficiency (VE) of the engine. When we have the VE, we choose what boost we want to make at what RPM and bolt these numbers in the turbo size calculator.
The turbo size calculator gives us the lbs/min & PR which we can plot on to turbo compressor efficiency charts to assess a turbo.
Getting the numbers
Step 1 – Finding the volumetric efficiency of the engine at various RPMs. I am going to choose 2500, 3500, 5000 and 7500RPM.
Bolting in the RPM and power numbers into the StrikeEngine volumetric efficiency calculator we get the following VE results
|K20A Tegiwa Intake||2500||65||0.9|
K20 Turbo Kit – Turbo Size Calculation
For this example I am looking for the best K20 turbo size for a stock engine and running just 0.5 BAR of boost.
I want the engine to be able to make 0.5BAR of boost from 3500RPM and maintain that boost up to 7500RPM. I’m also looking for a turbo that has some head room for more power in the future, in case we decide to up the boost at a later date/forge the motor etc. It’s preferable not to have to change turbo at a later date if at all possible!
With our boost targets and with the VE numbers we just calculated (above) we can use the StrikeEngine turbo size calculator.
Bolting the VE and boost target numbers into the turbo size calculator we get these PR, lbs/min & CFM numbers.
Now we have the lbs/min, CFM and PR (Pressure Ratio) numbers we can go to the Garrett website and use their turbo compressor graphs to find a good turbo for our requirements.
We plot the lbs/min and PR number onto the compressor charts and this tells us how efficient the turbo is at the points we chose above. Plots are in red.
Which Is Best?
The G25 looks to be the choice turbo out of the above three. Considering it is newer technology, its to be expected. However the retail price for this turbo is also considerably more than the others. For example, at the time of writing (13.10.22) Owen Developments have the G25 listed at 1201GBP and the GT2871R at 1000GBP (3071 is not listed).
Something in the middle
I have looked at the GT and G series turbos but Garrett also have their GTX series of turbos. Lets see how this turbo range stacks up versus the three turbos listed above.
The GT3071 looked to have potential, its just the efficiency was low, lets look at the GTX3071R (54mm) and see if that addresses the efficiency issue.
For higher RPM the GTX3071R looks to have the advantage over the GT3071R, however performance at low RPM looks slightly more compromised than the GT3071R so on this, I’d still choose the GT not the GTX.
Trying a smaller GTX
With the GTX2867R we have the same problem as the GT2871R, ie too small at higher RPM.
First time I see the GTX2967R is when I am writing this article. It looks to be a good fit. Plenty of headroom if we want to turn up the boost in future and also at our target boost level of 7PSI we are on the 68/70% efficiency island. But while this turbo is a good fit, its a very unusual size and the performance looks very similar to the GT3071R so I think the 3071R still has the edge.
We looked at the GT2871R, GT3017R, G25-550, GTX3071R, GTX2867R & GTX2967R
The G25-550 looks to be the superior out of all of the turbos, plenty of headroom, generally high efficiency, even at low RPMs. Again, not ideal having such a low boost target but it should do what we want.
A close second is the GT3071R, has all the same plus points, the difference being the efficiency is a bit lower all over.
Lets Bump Up The Boost
In the interests of completeness, lets increase the boost pressure to 21 PSI and see if the G25-550 & GT3071R are still suitable.
Bolting our boost targets into the turbo size calculator we get these lbs/min and CFM numbers. (You can use these numbers on any turbo compressor chart to assess it’s suitability. If you have different boost/RPM targets, use the turbo size calculator to get your numbers.)
Summary – K20 Turbo Kit Turbo Selection
Its the same story at higher RPMs, the GT3071R loses in efficiency vs the G25-550. The 3071 is being pushed close to the limit at 21PSI so if I were running this amount of boost, I’d probably go the the G25-550.
At 14PSI both turbos are in a sweet spot. If budget not a factor, again, the G25-550 looks the winner with the GT3071R close behind.
Things to think about
The VE of the engine will be reduced when we bolt a turbo manifold to it. This will move our plots slightly to the left. The numbers above also assume we are using a good intercooler, with a good exhaust with our K20 turbo kit. If the intercooler/exhaust etc the flow numbers will be less ie the plots will be moved to the left.
In short, if you think your turbo system is going to effect engine efficiency negatively, in a significant way eg you are using a log manifold, move the plots on the charts further to the left and then see if the turbo size is still suitable.
One I missed?
How does the 3076 compare to the 3071?
So we have a new winner, the GT3076R seems to do everything the G25-550 does except the 3076 seems to be in a higher efficiency island for more of the time. The only worry is the low RPM performance, it looks close to the surge line, is this a problem? Giving how manufacturers are supplying this turbo with their K20 turbo kits, it suggests not.
Brewed Motorsports supply their FullRace kits with the G30-770 as an option so lets see how the size down compares.
The G30-660 also looks good for 21PSI. Better efficiency than the 3076R at the 21PSI boost level and around the same efficiency at low RPMs. 28PSI could be on the limit of the turbo though.
If your target is running 28PSI, the 770 looks like a winner. At 28PSI we should be right in the sweet spot of this turbo.