The idea of remote mounting turbochargers isn’t anything new. In fact, companies like STS Turbo exist with remote-mounted kits as their specialty. Whether you’re trying to be sneaky about things, or just have absolutely no room for a turbocharger or two under the hood, remote mount turbochargers have their place in the automotive spectrum.
However, many people look at the kits, and all of the associated charge piping and exhaust tubing, and think, “How on earth does the turbo spool with all that volume?!” While there is no denying that the volume12 feet of tubing adds to both the intake and exhaust tract will affect the system, exactly how much is often the subject of guesswork.
To answer the question, Richard Holdener fabricated a test setup at Westech Performance to be able to test a remote mounted twin-turbo system, as well as the length of the exhaust independent from the length of the discharge tube tract. For the test, Holdener set up a 4.8-liter LS engine with a pair of twin GT35 61mm turbochargers, running through an air-to-water intercooler.
Holdener’s baseline was a peak of 613 horsepower at 6,600 rpm, 560.4 lb-ft at 4,800 rpm, and full boost (7 psi) being reached by 3,500 rpm. He also ran the combo at three other boost levels, resulting in the expected increases, with a peak of 13 psi yielding 719.6 horsepower and 659.6 lb-ft of torque and full boost not arriving until after 4,000 rpm.
His first test was to run the engine at 7 psi, with the exhaust tract lengthened by 12 feet, and the intake tract in its previous (short) configuration. In the video, you can hear that the engine seems to be laboring more throughout the RPM range, and the initial peak numbers don’t appear to have changed much, with 609.1 horsepower and 565 lb-ft of torque.
The real story being told in that dyno pull isn’t the peak number at all, but rather what’s happening at and below 4,000 rpm. Looking at the two 7-psi runs laid on top of one another, you can see that the extended exhaust tubing results in a much “laggier” response, with full boost being reached approximately 500 rpm later. Surprisingly, once full boost is reached, both curves mirrored one another.
On the 13 psi run, the results were the same, only with the additional lag further exacerbated as the remote exhaust struggled to get the turbocharger up to full boost. Once there, though, like the lower-boost run, the two curves matched each other.
The next test was to extend the turbocharger discharge tubing by a total of eight feet. Running the same 7 psi as previously tested, the engine made the exact same peak horsepower and torque, and while there was some minor deviation below 4,000 rpm, Holdener attributes that to dyno run variances than actual performance changes.
Holdener confirms this with a 9 psi comparison, where the curves match identically. Following with the 11-psi test, the longer air intake appears to make slightly more power on the top end than the shorter, “standard” length setup. “I really don’t think there is much of a change in response rate from the length of the turbocharger discharge tube. But there was a big change when extending the exhaust length,” Holdener says. “So there you go: the air intake, not so much. The exhaust, definitely yes.”
However, we have to wonder if turbochargers more appropriately sized to the engine and its intended powerband would have mitigated some of the disparity in response time between the two setups. But, that’s not what this test was aimed at, and the results of the test are clear, that the exhaust system volume has a far greater effect on the power output than the intake tract volume.
It also raised more questions as well: while the exhaust loses a significant amount of thermal energy in a remote-mounted setup (which is probably the biggest reason for the reduction in responsiveness), how much do intake air temps differ from right at the outlet of the turbocharger(s) to the throttle-body inlet?
What would happen with all that intake tubing but retaining the stock exhaust manifold setup? We’ve seen racers in no-intercooler forced-induction classes run unnecessarily long intake tubing in the hopes of it acting as a cooling chamber… but does that really work? Hopefully, we can answer those questions down the road.