The frame was delivered this week. While it looked good on paper, the finished product totally blew away all expectations. The blood rushing to the head really kicked in when they put the tires on. The overall design and workmanship of the Scott’s HR&C frame left no doubt that it could handle whatever was thrown at it. Of particular note was the massive fabricated 9” Ford rear end housing. Forget bullet proof. This thing looks like it would survive a nuclear Armageddon. Here comes mega-super awesome. What about a big Whipple supercharged LS to deliver over 1,000 lb-ft of torque at low rpm with say 1,200 hp at high rpm? Yeah, let’s do it!!
The next day, you wake up with normal blood pressure, rational thinking has returned and you wonder if that’s really the way to go. Remember, this is a spec build designed to generate a profit when it’s all said and done. The turbo LS engine with a GM block which they already bought is about $8,000 less expensive than a big Whipple on a Dart LS Next block. The turbo installation however has higher fabrication costs, requires an intercooler for pump gas and will add to the project timeline. And the turbo LS is still super-awesome!
The Whipple LS is pretty much of a drop-in and includes a built-in intercooler. Time is money on a spec build and of course, money is money. The net difference of the time and money is probably in the $5,000 range. Is mega-super awesome worth it?
The question becomes will the extra $5,000 investment translate into higher value and make the car more marketable when ready for sale. Here is where we would appreciate your input. Let me lay out the case and you tell us what you think. Here are some of the things that we are thinking about:
Probable Buyer Profile: Our view of the likely buyer for this vehicle is a male over 40 years of age with economic resources to get whatever they want. They are not that price sensitive, but are very particular. Whatever they buy needs to be perfect. The new owner will probably be participating in fun, driving-oriented events such as Power Tour, Cruising the Coast, Woodward Dream Cruise, etc. Given the anticipated usage, they will want to have an automatic transmission, all the modern comforts and conveniences along with reasonable gas mileage. Gas mileage? It’s not so much the fuel cost but the range on a tank of gas.
Personal Preference: Some people are in love with turbos and others are in love with superchargers for reasons that are purely subjective. One big reason is that the sounds made the two engine types are very different, both at idle and high rpm. The ever-so-slight whistle coming from the Whipple is like a dog whistle for supercharger fans. Pull into a cruise night and they’ll all be over to see what’s under your hood before you get out of the car. Turbo fans love the high rpm whine and the sound of blow-off valves popping. Every car guy loves looking at an amazing engine under the hood. I’d call this one a tie, but whatever floats your boat is great. In general, we see a younger demographic favoring turbos and an older demographic favoring superchargers.
Throttle Response: As a gross simplification, torque is what smokes your tires while horsepower is what gets you down the track in a hurry. Turbocharged engines have an upward sloping torque curve while Whipple supercharged engines have a relatively flat torque curve. As a reminder, dynos measure torque and calculate horsepower by the formula HP = Torque x RPM ÷ 5,252. Peak horsepower for hydraulically cammed LS engines is usually in or around 7,000 rpm. At that point for two engines with the same peak horsepower, the turbo is fully spooled up and delivering near maximum torque whereas the supercharged engine’s torque is on a downward slope. What about at low rpm? With peak torque just over 4,000 rpm, it’s a no-contest win for the supercharger. Turbos need a little time to spool up their turbine/compressor rpms and move the column of air into the engine (turbo lag). A supercharger’s screws are always turning. Most twin screw superchargers have an internal bypass which slaps shut instantly at full throttle sending manifold pressure from vacuum to boost in a fraction of a second.
For street driving, the instantaneous throttle response of the twin screw supercharged engine clearly beats out the turbo because on the street you are generally going to be at low rpm. If you’re at the drag strip and staging, the turbos are already spooled when the lights go green, so turbo lag is not a factor. Smaller displacement turbo engines often bog down when punched because the unboosted engine’s torque is very low at low rpm. Our LS engines put out well over 600 hp without boost so that nasty bogging down experience is not a factor. So what do you want – explosive and instantaneous acceleration, or a more controlled lift-off?
Mechanical Considerations: Q: How do you snap the head off a bolt? A: Quickly apply a turning force (torque) to the wrench. The preceding throttle response discussion pretty much laid out that twin screw superchargers quickly apply torque which means that they can break things. While we are comfortable using a GM block for an 1,100 hp turbo LS build, we are using much stouter parts for the same horsepower on a Whipple LS build. For starters, it will be a Dart LS Next block with its intrinsically stronger design (see Dart-GM Block Comparison). This in turn allows us to use six bolt heads for greater clamping force. We would also be looking at center counter-weighted cranks for greater stiffness and reduced harmonics among other upgrades in our bag of tricks. These upgrades are what account for much of the cost differences between our turbocharged and Whipple supercharged engines.
Cruising and Gas Mileage: TGM is planning to use a BRE custom 4L80e transmission from Hughes Performance in either case and the tires are already on the frame. While not yet fixed, it is likely that the turbo LS would be paired with a 3.50:1 gear ratio differential while the Whipple supercharged LS engine rear would be 3.00:1 as it needs less gearing leverage in accelerating from low rpm. This would cut highway rpm by about 15%. The GM block has a 4.00” bore while the Dart’s is 4.125” yielding 6% more cubic inches. Turbos are intrinsically energy efficient while the twin screw superchargers get their efficiency through the power-on-demand from the internal bypass. Both will suck up a lot of fuel when you beat on them. My guess is that the Whipple supercharged engine would get about 10% better gas mileage cruising at a constant highway speed thanks to the lower gear ratio it can support.
Question for the Audience
- Should TGM stick with their turbo LS or go with a big Whipple supercharged LS?
- What are the top three reasons for your decision?
Leave your comments on our Facebook post by Friday 1/25/19. A free Borowski hoody will go to a randomly selected participant among those submitting coherent thoughts on either side of the turbo vs supercharger debate. Thanks in advance for your input!
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