Is my Turbocharger too big? Turbo Sizing Tech Talk

Protomotive Other March 24, 2023

Is your turbo too big? Let’s talk about that in our Turbo Sizing Tech talk

Part 1 is up on Youtube! Is my Turbo too big?.

Are my turbos too big? Turbo sizing Tech talk.

Welcome to Protomotive, where we’re passionate about all things Porsche performance. We specialize in tuning, engine building, and performance products, and we’re dedicated to helping you get the most out of your vehicle. In this Blog, we’re going to talk about the size of turbos and how they affect the performance of a Porsche 911. You might think that a larger turbo means more power, but as we’ll explain, that’s not always the case. So, sit back, relax, and let us guide you through the world of Porsche performance. Let’s get started!

Q_1: What would be some signs that my turbo is too big?

Severe lag would be one. When the little turbo honda civic puts 3 car lengths on you while you’re waiting for your boost to come up could be a little embarrassing…
If it doesn’t fit under your hood and you end up cutting a hole to make it fit, lol…
If you have constant surge out of the compressor when you’re trying to build boost, the compressor is too large to build boost without enough airflow to support that boost.

Q_2: Does anyone really NEED a bigger turbo?

Lol, not really? Maybe in specific instances like a racing class where you’re competing and need to make a specific amount of power, sure. For the street, it’s more bragging rights than anything, and in many cases smaller turbo’d cars will outrun the big turbo cars before they even get spooled up. We used to run the Porsches vs. Supras back in the early 2000’s when the Supra’s were just capping the 1000hp mark. We were running around 700hp then on the Porsches and would walk all over them with our superior power under the curve. This was mostly street racing stuff. Sure a properly prepared Supra at the ¼ mile with huge stickies and racing injection could turn a quicker ¼. But that’s in a special situation as opposed to meeting on the Mexico Hwy and just launching after 3 horn blasts…

Q_3: Do big turbos ALWAYS make more hp than a small turbo?

Believe it or not, they don’t…

Q_4: So, then what are the disadvantages of a big vs. small turbo?

In a specific instance where you’re running 91 octane fuel in a knock sensitive engine, the small turbo actually has an advantage over the larger turbo. This is due to the small turbine producing higher backpressure leaving residual exhaust gasses in the combustion chamber. Those residual exhaust gasses will raise the octane or dilute the mixture giving the smaller turbo the advantage of better timing and lower knock sensitivity… Whereas the larger turbo with higher flow, lowers the backpressures and evacuates all the exhaust gasses from the chamber giving you a stronger mixture. However, this isn’t to its advantage with low octane high compression engines. With the undiluted mixture, the timing needs to be retarded much further, and the engine becomes more knock sensitive so you’re unable to take advantage of the higher flow the larger turbo gives you.

Q_5: And then, what are the major advantages of a big vs. small turbo?

There was one specific engine that went nuts with a larger turbo. It just got stupid happy with these turbos on it. A 997 gt2 with the gt3794 turbo on it made 1134 whp on the first pull at only 18psi. Vs. a smaller gt3586r or even a xona 7864 would max out just over that taking almost 2 bar to get there. Then the gt2 went on to 1569whp at about 2 bar, where the other smaller ones couldn’t get over 1280… However, that was running e85 on a 4.2L fully built engine that the design intent was for huge power to begin with. Not just a bolt-on application.
However, that’s not always the case and many times a turbo upgrade simply doesn’t make much difference. Boost for boost, turbos in a tight range will produce very similar power. You may get slightly better response or efficiency out of one or the other, but unless the one you’re comparing is so far out of range, they’ll still be close.
On a 993 and 996 turbo, the oem turbos are pretty small. They have 25lb compressors on them, so are just maxed out at 500hp crank, or about 430 at the tires, depending on the dyno you’re on. This makes them able to produce a lot of torque low, but just running out of steam on top and not even being able to hold the boost till redline.. You might hit 1.2 bar on the low end, while falling to 0.9 bar by redline, no matter how hard you try, you’re just out of turbo. So even a compressor upgrade like a K1616G, or now a K1665G2 can improve your boost response, improve efficiency and by holding the boost to redline now also produce up to 700hp crank / 630whp from what we’ve seen. You’ll see a similar effect on the k24’s even though they have larger turbines and compressor wheels, they also just run out of steam. They have 30lb compressors and can make about 530whp / 600hp crank and we’ve seen multiple times that they just fall back to about 1.2 bar by redline no matter how hard you try to turn up the boost. Like the k16’s a simple compressor upgrade to a k2418g with 40lb compressors gives each turbo a 100hp gain for a total of 200hp more capacity. That’s where the fun begins! We’ve even done k2471’s that have produced right at 900hp to the crank! With the same turbine!

Q_6: What turbo could I replace my oem turbo with and not have to change anything else?

What car are we talking about here? There’s quite a few differences we could speak about depending on the car we start with. For instance, are we talking a vgt turbo (997t)? Or standard turbo (996tt / 993tt / 911tt). Are we talking older stuff like 930 with a big single turbo or twin turbo from 993tt up? Etc. etc. Also the fuel injection makes a huge difference as to whether or not we’ll need to adjust anything or not. On a 930 or 964 turbo that uses CIS injection which doesn’t have computer control you can update the turbo with no other changes and get a small but nice improvement. On a 993tt you could run into issues by installing larger turbos since you’ll get a check engine light when the airflow falls out of the range of the diagnostics for throttle vs. rpm vs. boost. And more so on the newer cars since the diagnostics have gotten tighter and tighter. For the most part you can get away with it on older vehicles prior to obd2.

Q_7: When I start my mod path, should I start with a turbocharger or something else?

If your turbocharger is worn out or damaged, you could upgrade your turbocharger when replacing or rebuilding it. However, if you’re starting an upgrade path the turbo is usually a bit further down on the list. We’ll typically start with tuning since no matter what hardware you install it usually requires tuning. And with tuning only you can already experience huge power increases, especially on turbo cars 🙂

Q_8: Can I get whatever power I want out of my turbo with a compressor upgrade?

A: Now wouldn’t that be awesome! Unfortunately, it’s not true though. Turbos have compressors and turbine wheels. They BOTH need to flow enough to support the power level of the engine. A case in point is vgt turbos on the 997.1TT’s. We’ve seen all kinds of crazy compressor upgrades on them where a very mild compressor update is about all they can handle. The big 68mm compressor wheels on those tiny turbos end up producing a lot of backpressure. Think of it like you’re riding up a hill on your 21 speed bicycle and you’re in top gear. I don’t know about you but I can’t do it. Instead, I’d switch to a lower gear. Well, think of the turbine wheel to compressor wheel diameter like the gears in your bicycle. If you have a tiny little turbine wheel and a giant compressor, the pressure required by the turbine to spin up that compressor is huge! On the vgt’s you can compensate for that by closing the vgt vanes down to get the boost up. But what you’re really doing is just increasing the backpressure even higher, which decreases the flow through the engine and the net power increase is going the wrong way. By opening the vanes back up, reducing the boost and tuning it properly they’ll make better power than running the higher boost with the vanes closed down. Whereas a smaller compressor wheel on the same turbine wheel can produce the same power with lower backpressures and better boost response. So, you need to consider the turbine wheel when you’re looking at turbo sizing.

Q_9: Well, then what if I put a bigger turbine housing on. Will that help with the backpressure?

Changing the a/r ratio of the turbocharger can have an effect on the boost response and boost to back pressure ratios. However, all that exhaust still needs to get through the turbine wheel. What the big turbine housing is doing is slowing the exhaust gasses down before they hit the turbine wheel. This typically just adds lag, and may pick up a small amount of power. We’ve more experienced the a/r of the turbos should fall in the 0.7 range plus or minus a little bit. Too far either way just makes the turbo unhappy. If you need to go out of that range too much to get the response or power you’re trying to hit, you should have a different size turbine wheel.

Q_10: Are there any upgrades that would have a similar effect to installing a big turbo? Or can we optimize the rest of the turbosystem to take full advantage of the turbo we already have?

Absolutely!!! We’ve built some amazing cars with the stock turbos. The first one that comes to mind is a 78 930 turbo we built for a very fun client in California. We removed the recirc bypass valve, made a long neck intercooler, upgraded the air filter, tweaked the distributor, installed a 1 bar spring and a euro exhaust and the car was super responsive and much more powerful than stock. Too many people just assume the oem 3ldz turbo was terrible on those cars, but it actually had near 400hp capacity. So on a car that made only 286hp stock, that’s a huge upgrade without changing the turbo, but instead, all the other supporting mods so the turbo could actually do the job.
Another car we had some fun with, without changing the turbo, was a great client in Michigan. He had a 996tt, and his wife wanted to buy him some fun bits for his car for a Christmas present. We worked on a fun kit together and boy did he love it! Leaving the stock turbos on the car, we removed the oem intake system which is horribly restrictive and installed our fenderwell intakes instead. We installed our ypipe and intercoolers along with a 997.1TT throttle body and our 74mm plenum upgrade. That combined with our xpipe exhaust made this 996t a little bullet! By pulling the oem intake and improving the entire airflow path, including the exhaust we were able to take full advantage of those little k16’s while improving boost response and overall efficiency of the system.

Thanks for checking out our Tech talk blog on Turbo Sizing! We hope you found it informative and educational. If you’re interested in maximizing the performance of your Porsche 911, be sure to check out our website for more information on our services and products. And don’t forget to subscribe on our youtube channel and enable notifiactions to stay up-to-date on all the latest news, tips, and tricks from the world of high-performance Porsche 911s.

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