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Things to consider when picking and deciding on a turbo size for your build.

Does a large turbo car make a terrible streetcar?

Contributed by: Enginebasics.com

So you have decided to turbocharge your car or upgrade your existing turbo, but are stuck on what size of turbo would be right for you.  It would be nice if turbocharging was like many other decisions where bigger is better, but not so.  While 90% of the internet would tell you to pick the smallest turbocharger that meets your horsepower goals I would like to offer a different point of view after we go over the pro’s and con’s of each.

Small turbochargers

Pro’s

1. Spool faster providing more torque lower in the power band.
2. Easier install due to smaller dimensions.
3. Cheaper

Con’s

1. Torque falls in the upper RPM of the power band.
2. Smaller turbine wheels cause more exhaust manifold pressure (EMAP) causing more cylinder reversion of exhaust gasses, raising cylinder temperatures causing the motor to become more prone to detonation and pre-ignition.
3. Don’t make as much power as larger turbochargers.
4. Torque usually comes on rapidly over a small rpm band causing traction problems.

Large Turbochargers

Pro’s

1. Usually maintain torque all the way to redline.
2. Large turbine wheels allow good flow for better volumetric efficiency and less exhaust gas reversion, which helps in avoiding cylinder detonation and pre-ignition.
3. More progressive torque curve that comes online in a more progressive manor.

Con’s.

1. Laggy.  Meaning the time till the second boosted torque curve over the N/A torque curve takes time to obtain.
2. Physically larger, therefor can be harder to install in a given space.
3.  Cost more than a smaller turbo.

Now that we have talked a little about the pro’s and con’s of each let’s talk about why picking the smallest turbo for our power goals isn’t necessarily the end all advice to turbocharger sizing.  Since we need a specific example to talk about let’s go with a mid size turbo and a mid size motor like a 3.0 Liter Toyota Supra motor (2jz).  If we say our goal is say 650 wheel horsepower, then we would be looking for a turbo that can flow upwards of 60 lbs/min of air.  Strong choices would be a Garrett GT35R or GTX35R, or a Precision 6262, or a Borg Warner S362.  We go to the dyno and continue turning up the boost pressure till we hit our 650 HP goal.  We eventually do meet our goal of 650whp but we had to run such high boost pressure to reach that goal, that race gas was required.  Now this power can only be had if the owner is willing to grab his wallet for $9 dollar a gallon race gas in the tank.  A bonus though of running such high boost and hitting high boost low in the RPM band was we made tons of torque too.  We made 650 HP and 650 Torque.  How exciting, but then our engine builder told us that making that much torque will most likely bend our stock rods and beat on our bearings a lot more.  To fix this we need to be grabbing for our wallet once again and have the motor fully built with forged parts.  Well the owner empties his wallet and hits the street.  1st gear the owner stabs the throttle and the turbo lights right up and so does the tires, so they shift to 2nd but the torque comes on so rapidly with the little turbo and so low in the rpm band that again the tires light right up again.  They now back out of the peddle so the torque drops and traction can be found right around 5000 rpm where they are able to get back into the go peddle but are then unsatisfied as they can feel the engine torque falling off as the small turbo’s turbine wheel is too much of a restriction at high engine RPM.  Also they think to themselves that if they wanted to wait till 5000 RPM to get into the go peddle they would have just bought a larger turbo.

Now let’s do the same scenario with a large turbo.  The owner has the same goal of 650hp, but chooses to go with a larger turbo like a Garrett GTX40/88r or 6766 or Borg Warner S366.  These turbo’s are all rated at 80+ lbs/min of airflow and will flow enough to make 800+ wheel HP.  With the same goal of 650 whp the owner hits the dyno and is able to make 650 whp on pump gas do to the fact that not much boost pressure is required to get the large compressor and turbine wheel to flow enough air to make 650 HP.  The owner is excited that he can make this power with cheap pump gas and enjoy that power daily.  The dyno operator than says that power is 650 whp, but because they didn’t turn up the boost pressure very high and because the turbo didn’t spool up till later in the RPM where the motors volumetric efficiency was decreasing, the engine only made 575 ft/lbs of torque, but it was able to hold this torque almost all the way out to redline.  The owner would have liked to make more torque, since more is better, but then the engine builder lets the owner know that since the motor didn’t make more than 600 ft/lbs or torque he would likely be fine running the stock rods and pistons, and that a built motor is not required.  With this news the owner celebrates and leaves his wallet in his back pocket.  Next the car takes to the street. In 1st gear the turbo is lazy so not much boost pressure is made, but the car is still very quick since traction is maintained and a 3.0L naturally aspirated motor is still very peppy.  2nd gear comes and the larger turbo spools a lot more linear and comes up much later in the RPM with less torque so that again, traction is maintained pretty well.   The owner shifts to 3rd and traction is all there and so is the turbo.  What is nice is the larger turbo pulls hard all the way to redline.

So which set-up is better?  Well if money was infinite and traction was infinite than the smallest turbo to reach your HP goal wins hands down.   Better response, more torque, and with infinite traction it would put car lengths on the larger turbo car till the larger turbo cars boost came in, and also would be much more responsive and fun to drive.  BUT, in the real world where money matters, and traction is a big problem, which one is better??……there is no right answer. People always want more power it seems, and buying a new $1,500 dollar turbocharger in the never ending quest for more more more sounds expensive.  So, this is some food for thought to those that say to buy the smallest turbo that satisfies your HP goals.  An owner should think about a lot more than just reaching their HP goal.