Twin Turbo Tech
Advantages Over Superchargers, Plus a Few Pointers on Tire Width and Dragstrip Dynamics
By Bill Watson, Jeff Koch
Photography: Jeff Koch
Online Editor’s note: The November ’01 issue of Hot Rod featured an extraordinary twin-turbo ’66 Mustang owned by Bill Watson, a jet engineer with Garrett/Honeywell. He generously supplied us with way more detail on his project car that we could fit in that issue, so we decided to include this question-and-answer interview that explains why he considers turbocharging superior to either supercharging or stroking an engine for more power. You may not agree with all of his points, but he knows his stuff. (After all, his Fastback runs in the 11s on—get this—SlimFast 215-section tires!) Blower buffs are invited to email us their alternative views on the best way to obtain boost.
Q: Other than the fact that the company you work for makes turbos, why go the turbo route? Why not use a supercharger or build a stroker? Are turbos simply the most efficient way to go?
A:There are four main reasons why I think turbos are superior to a blower, but of course I can’t argue that the supercharger isn’t easier. It is. I can appreciate why supercharger kits are more popular for the bolt-on crowd. But here are the reasons that I prefer turbos:
1) Turbos always will outperform supercharged cars when it comes to power production. It’s simple—you don’t have to spend (for instance) 70 crankshaft horsepower to drive compressors by taking engine horsepower right back off the crank. True, you back-pressure the engine on a turbo car, but much of the compressor horsepower comes from heat—and that is "free." The bottom line is that you don’t sap as much power out of the engine to get your pressurized air.
2) Easily adjustable boost! It’s a breeze to set up a knob inside the passenger compartment to adjust the wastegate setting up or down. When I drive in the mountains, I can raise boost to get back the power lost at higher altitudes. On the other hand, if I buy cheap gas I can turn it down, or change boost to compensate for summer or winter air temperatures (or for that matter, morning or evening temps too!). With a blower car you have to change pulleys—not that hard, mind you, but very few people are going to change pulleys as they’re climbing mountains, or at a gas station after buying a tank of cheap gas.
3) Midrange torque!! Boost vs. rpm is far superior on a turbo car. With a belt-driven centrifugal blower, you have to choose a pulley size so you won’t overboost at redline. But that means that at every rpm below that, you aren’t running as much boost as you can. The nature of centrifugal blowers is parabolic, meaning at half the redline rpm, you make less than half of the boost. So on a typical 5.0L, you might only make 3 psi of boost at 3,000 rpm, where a turbo car can easily be on the wastegate for 11 psi. That’s why I make 550 lb-ft at 3,000 rpm. Hey, if you cut my boost to 3 psi at 3,000, my torque would be down to 340—that’s 200 lb-ft less! To me, the best way to optimize a belt-driven centrifugal blower car would be to loosen the torque converter. Then you can hit the gas, immediately be at 4,000 rpm for example, make boost in an instant, and have a bolt-on car that just hauls. The only downside there is the economy will take a hit unless you can design in a lockup converter when you want to just cruise down the freeway. The turbo doesn’t have any of these compromises.
4) Finally, turbos run proportional to demand. What I’m getting at is that compressor speed is dependent on airflow, which comes from two main variables: engine rpm and throttle position, if you will. Now, a belt-driven blower car’s compressor speed is dependent on only one variable: rpm. So why care? Because when you’re just maintaining speed on flat and level ground, trying to make some mileage, a turbo car’s compressors are going very slowly and hence don’t increase pressure before the throttle body. But a belt-driven blower car has no idea what the throttle position is; the compressor is simply geared to the crankshaft, so it’s spinning much faster and making, say 2 psi in our example here. (And if it’s a Roots-type blower, it’s making 6, 8, or even 10 psi!)
So what do you do? You have to back out of the throttle even more. That means throttling losses are up (bad for mileage), and you’re pumping hot compressed air through your intercooler all day if you have one, and you’re using crankshaft horsepower to compress the air—then throttling it back anyway! At least put the throttle body before a blower; you can see why positive displacement (Roots or screw compressors) blowers have the throttle plate before them to avoid this problem. Then it’s much more of a non-issue.
You’ve also asked why I didn’t go to more cubic inches. Of course that works, but as you note the mileage wouldn’t be as good, plus I’d still have to build a 7000rpm motor to make this kind of power on something under 400 cubes. Then that requires all the money for rpm-related hardware to accommodate moving the operating range up, which I have spent zero dollars on. The turbo approach was different, in the sense that you spend the same money but on different hardware. I’ve done the cam/carb/intake/heads/headers/loose converter thing before. It was fun, but this time I wanted something that was a real sleeper until you lean on it.
Q: Surely you could have in theory used a stock intake, or not? Why not? What was the practical reasoning behind the intake design? Longer tubes equals greater torque?
A: Yes, I could have used a stock intake, but there were two big reasons I didn’t. One, the upper intake is extremely visible and typically is where most people’s eyes go when a hood is raised. This is where it’s easy to set the car apart from the others and do something different.
Two, when a stock intake is used on early Mustangs, the throttle body and the chassis bracing (the "export brace") can’t co-exist without cutting things up. I valued the export brace’s contribution to chassis stiffness enough to leave it intact. (I still need to work in the other option, a "Monte Carlo bar" that is common on early Mustangs which goes straight across from shock tower to shock tower. I used to run one but the latest mods required me to yank it for a while.
In addition, I was trying to maximize tube length for good bottom end torque, hence the runner shape going all the way to the valve covers. By most hot rod standards, 302 cubic inches isn’t a ton of displacement, so I wanted to maximize torque down where it would be driven. Plus, as you’ll note below, it allows you to run larger turbine nozzles so you can make more top end, too.
Q: Any idea how long intake runners are currently versus stock intake?
A: They’re about the same length, maybe an inch longer. Even a stock car with stock cam has a torque peak around 3,250 rpm, so I figured the headwork and larger diameter intake tubes would allow torque to hold on longer before starting to drop off, and I expected it to have peak torque around 3,600 to 3,800 or so. Hey, torque at 3,000 is 265 lb-ft normally aspirated (550 with 11 psi) and it was a pleasant surprise to find the torque just kept climbing between 3,000 and 4,100, for normally aspirated and blown peaks of 314 and 620 lb-ft, respectively. What great mid-range! I’m not Edelbrock, so I only had one attempt on this intake. I got lucky—it works better than I expected.
Linear Mode
