Last time we checked in with our project C6, it had just received all-new motivation courtesy of Chevrolet Performance and ProCharger, a combination that proved to make more than 730 rear-wheel horsepower even with a fairly conservative tune and subsisting on a diet of garden-variety pump gas. It’s an encouraging start to be sure, but we also know there’s more power to be had with just a few small tweaks.
Still, this isn’t a dyno queen – we’re building this car to be driven on the street, at the strip. And to that end, we don’t just want big numbers – we want that twist to be usable, and that means we need to dial the powerband in to be a bit more agreeable to the Corvette’s drivetrain, suspension, and tire setup. We all love a good smoke show, but incinerating the tires isn’t a good look when you’re trying to put down fast times at the track.
With that in mind, we got with the folks at Fuel Air Spark Technology and snagged their short, mid-length, and long intake runner sets for FAST’s LSXR intake to see how the different runner lengths affect the power delivery. We also scored a set of their new ignition coils to see if there’s a few ponies that can be unlocked there, as well, because, hey…who doesn’t like the prospect of easily bolting on low-cost horsepower?
Sparking Up More Grunt
Before we dug into the intake runner swaps, we decided to hook up FAST’s new XR-Series Gen IV ignition coils to see if they’d have an effect on power without any other changes.
“It’s nice because they’re designed to bolt right in place just like a factory coil,” says Petty. “So installation couldn’t be simpler.”
You might be tempted to assume that all coils are more or less the same, but as we soon discovered, that’s not the case. We were very pleasantly surprised when the car picked up around 10 horsepower with this simple swap. When we compared the overlays of the OEM coil and the FAST piece, we saw that not only did it make more peak power, it averaged more horsepower over the entire curve.
FAST’s Chris Potter says that part of it has to do with the engine combination we’re working with here.
“A boosted engine will most certainly benefit from the hotter spark provided by the FAST ignition coils,” he says. “The added volume of air and fuel trying to be compressed by the piston tries to extinguish the spark, so a coil with more energy to provide a hotter spark is much more resilient to cylinder pressure, ensuring that the flame kernel is strong enough to propagate throughout the rest of the combustion process. It’s safe to say that a coil upgrade is absolutely required anytime an LS engine climbs above 900 lb-ft of torque at the flywheel. We’ve seen that time and time again in our dyno testing.”
That led us to wonder if changing the coil dwell time could deliver even more power. Dwell time as the amount of time the coil is energized but not firing. A longer dwell time can allow the coil to build up more energy to be released. But Potter says that’s a tricky proposition, and more likely to result in losses rather than gains. “Most modern coils are pretty sensitive to dwell time. Too little dwell time will result in a weak spark, and too much dwell time can saturate the coil and cause it to overheat. It is best to consult your coil manufacturer for recommended dwell times and stick with those. If your coil is sized for your engine package, there should really be no need to play around with it.”
Fair enough – time to move on to the intake.
A Runner For Every Occasion
Installing these runners was a lot easier than I expected. It’s just a matter of pulling the intake off, setting it on the bench, removing the top, unscrewing the torx screws and making the swap. We’re fortunate that the LS design uses an O-ring style seal, so you don’t have to worry about replacing gaskets or dealing with RTV. It’s a really straight-forward process, and it took less time than we had allotted for.
The first swap probably took about half an hour, and once we were familiar with the process, each one afterward probably took about ten minutes. Initially I wasn’t sure where the manifold halves split, so the first time around I took the throttle body off and all of that. But FAST’s manifold design is really clever: Although the halves separate, the throttle body flange stays together in one piece, so you don’t even need to take the throttle body off – you can just pull it all off as one assembly.
In terms of which runner length makes the most sense for a given application, Potter offers some basic guidelines.
“It is generally understood that a longer intake runner lends itself to creating the most peak torque,” he says. “Similarly, a short runner is best suited for an engine capable of very high RPM. It really depends where you want to run the engine – do you need the torque to pull you out of the corner, or do you want the horsepower to run through the lights?”
For builds like this one, which will see plenty of track time but is ultimately a street-driven car, Potter typically expects the long runner to be the most effective option because of the way it delivers low-end torque. He said, “The long runner is probably best for the majority of our customers. Anytime you’re driving the car on the street, the extra torque of the long runner will be easy to feel. But if you have an engine that is capable of 8500 rpm or more, then there are definitely benefits of the short runner.”
And unsurprisingly, the mid-length runner ostensibly splits the difference, and that can prove to be a happy medium when low-end torque isn’t the main goal.
“We have some road race customers who like the mid-length runner best, as they have found that it offers a great amount of peak horsepower with a little sacrifice in low end torque,” Potter adds. “It acts almost like a natural traction as they apply throttle coming out of a corner.”
It is generally understood that a longer intake runner lends itself to creating the most peak torque. Similarly, a short runner is best suited for an engine capable of very high RPM. It really depends where you want to run the engine: Do you need the torque to pull you out of the corner, or do you want the horsepower to run through the lights? – Chris Potter, Fuel Air Spark Technology
While the results delivered from each test fell more or less in line with what Petty expected, he says that there’s a bit more work to do on the tuning end than some might assume going in. “When we swapped from the long runners to the mid-lengths, we did a pull and lost all over the place,” he recalls. “And that was just because the fuel and ignition maps needed some adjustment – once we got those dialed in, then we started seeing the gains we were looking for. It took about three pulls and tuning sessions with each swap to really get everything where we wanted it.”
Potter points out that the aspiration can play a big role on how the runners affect power, and that tuners aren’t really limited to these three runner length options as a hard and fast rule.
“The change between runners is much more noticeable on a naturally-aspirated engine, as the pressure pulses react much stronger in atmospheric pressure. Once an engine enters boost, there is always positive pressure at the backside of the intake valve, so the pressure pulse behaves very differently. And different length runners can most certainly be mixed – it’s another way to fine-tune the engine and get every last drop of power out of a combination. They can be used to tune the airflow into each cylinder and move the peak power up or down the RPM range, among other things. Generally speaking, what you’ll find is that if you run four long runners and four short runners, the power and torque curve will lie dead in the center between running all longs or all shorts. The same holds true for any combination of lengths.”
Despite the long runners being the most street-friendly of the bunch, we may opt to stick with the short ones to make the power more manageable out of the hole. While we want as much peak power as possible, and even with the M/T ET Street R rubber on the car, it’s really tough to get it to hook with the way the car is set up. We would rather lose some bottom end and gain some top end in this situation. There are also some planned mods which will play to the short runners’ strengths. With a camshaft and valvetrain change, the short runners will shine even more. In the future, we’re going to go in a direction with the engine combination that really speaks to what those runners are all about.