Twin Turbo vs. Single Turbo V6: A Dissertation

Mike@GTM

Now, on a street car, the things that matter most, is how much fun the car is to drive. Usually, good boost response, low boost threshold (boost comes on at lower engine rpms) and decent top-end power are what make a car fun to drive on the street or occasional track days. A twin turbo system does these things extremely well and cannot be beat…even by a well engineered single turbo system. The single turbo system has too many compromises to make as effective of a street setup.

On a full on drag race car (tube frame, gutted, no A/C, no power steering, no ABS, modified firewall, etc.) it makes sense to use a big single turbo to make 1300+ horsepower (even though that much power can still be made on twins). The reason it makes more sense is efficiency. A bigger turbocharger is simply more efficient at compressing air than a small turbocharger (or two). Besides, at the point of building a full on drag car, it’s OK if you have a 5 – 6” diameter downpipe running where your passenger’s feet would be in a street car.

So, wait, a bigger turbo is more efficient? Why yes, but it’s also a bigger diameter compressor and turbine wheel. That means that it has a higher polar moment of inertia and therefore has very poor boost response.

Ultimately, for a 370Z on a stock engine, a single turbo is a very poor choice for a street driven car. It will have a boost threshold 1500 – 2500rpm later than twins, have poorer boost response and/or top end horsepower. If you are spending $5-7k+ on a turbo system, why throw away so much performance by using a single turbo system?
I mean, I understand the desire to be different from everyone else, but when you consider that most 370Z’s aren’t FI in the first place, just being FI at all is being different from everyone else all by itself.

One other tidbit I’d like to add. There was a discussion about having cats before a turbo. It is a terrible idea for not only the reasons already laid out here, but also from a reliability standpoint. Coming from the Subaru world where the 2002 and 2003 WRX had a cat mounted pre-turbo (along with two more in the downpipe), that pre-cat was notorious for coming apart and taking out the turbo. Subaru had so many warranty issues that they eliminated that pre-cat in all later WRX’s and STi’s. Also, the cat eats up enthalpy like a sponge and leaves less for the turbo to use. One of the first mods a 2002 – 2003 WRX owner does is change the up-pipe.
Also, I know someone will eventually mention heat wrapping the long exhaust runners in a single turbo system. As a Subaru guy, at one time, I thought it would be fun to install a nice set of stainless steel headers on my car and heat wrap it to retain some enthalpy. Unfortunately, thin stainless piping and heat wrap still have a significantly lower R value than a cast iron manifold with heat shields and I eventually went back to the stock exhaust manifold for more bottom end torque, better boost response, lower boost threshold and overall better drivability around town. Not to mention the top end gains with the headers were marginal at best…they were Tomei headers in case you’re wondering.

All in all, this is just information for people to know. This is a culmination of mechanical engineering knowledge coupled with years of experience in working both in the high performance building industry and high performance car industry and being an enthusiast. I just wanted to share this information with people so they can make informed decisions. This is not to knock other companies’ products as I can appreciate how much hard work it is to bring a product to market.