As we continue our quest to make Project Silver Bullet even faster, and eventually dip the car into the low 10-second ET zone, we’ve found ourselves at the point where changes are necessary in terms of power and safety to get there. Horsepower is a mistress that we never tire of, so making more horsepower was always in the plans. Other changes were inevitable as well, we must take the necessary safety precautions or risk getting kicked out of the track, or worse injured because we didn’t have the right safety equipment onboard.
According to NHRA rules a full bodied car running faster than 11.49 ET needs a rollbar installed. Cars with an unaltered firewall and floorpan like Silver Bullet can run as fast as 10.00 with only a six-point rollbar installed. Since we’re already dipping into the high 10’s we’ve been pushing our luck and have had a few cross looks from officials at the drag strip.
Chris Alston Chassisworks provides detailed instructions for measuring to order a custom cage. Having a buddy handy to help hold the tape measure and these half-circles that you’ll make out of cardboard will help.
If a customer has a specific request for fitment or routing of some of the bars that our off the shelf kits doesn’t provide, then they can do a custom roll bar or cage request. -Lino Chestang, CA Chassisworks
We wanted a rollbar that we could install in the car at the Power Automedia tech center, but also that doesn’t interfere with our ability to use the car on a daily basis. This rollbar also has to meet all of the NHRA requirements to pass tech inspection. This includes its position in relation to the driver in the car, and clearances to the roof, doors, etc. We also wanted the option of changing this roll bar over to a cage later so we turned to Chris Alston Chassisworks for some help.
Chassisworks actually offers over 60 off the shelf roll bar and cage fitments, but they can also do custom bent rollbars and rollcages made to order. “If a customer has a specific request for fitment or routing of some of the bars that our off the shelf kits doesn’t provide, then they can do a custom roll bar,” says Lino Chestang of Chassisworks. By performing several measurements on the car and filling out a data sheet with that information Chassisworks is then able to construct a system to a customer’s specific needs.
The end result in our case is a custom fitted rollbar that meets all of our criteria and will satisfy the NHRA tech inspectors, as well as keep us safe at the track.
“The more exact the customer is with their measurements for a custom fitment, the better the cage will fit and they will get the final product that they want,” explained Chestang. Measuring for the custom rollbar needs to be a precise operation and Chassisworks provides the instructions on exactly how to do this. This is at least a two person operation and requires around an hour to complete. Since the car is not at the Chassisworks shop, they can only rely on the data a customer provides, and successfully recording all the measurements correctly will result in a cage that fits the car properly.
Measuring accurately will ensure the finished product is exactly what you're looking for. Each bar must be measured for in this process, and those measurements recorded on the ordering sheet. This is where time, and attention to detail will pay off.
We left performing these measurements up to shop head honcho Sean Goude. “It’s a good idea after you finish all your measurements sit down and really think about each bar and the angles. If something doesn’t make sense recheck it. Double checking your work will ensure the cage you get is exactly what you wanted,” says Goude.
Chassisworks offers their bars in 1 3/4-inch 0.134 wall mild steel, or in 1 3/4-inch .083 wall 4130 chromoly. Both meet NHRA requirements, and as long as the car has that unaltered firewall and floorpan it will satisfy requirements for safety equipment down to 10.00 seconds with just our eight bar setup. We went with the chromoly so that the added weight for our cage is minimal since we’re retaining the full interior, and all the modern conveniences of our Mustang.
A few weeks after we sent our order to Chassisworks, our chromolly roll bar arrived.
Going Behind Bars
Chassisworks does not send the cages pre-notched. “We send the bars with some extra length so customers can finalize everything for a truly custom fit,” says Chestang. If you’re installing one of these at home a tube notching tool is highly recommended, and if you don’t have one Chassisworks sells one that will help make life easier for you.
When it comes time to weld the bars in there are two choices: MIG or TIG. If using chromoly the only choice is TIG welding. There are a couple of notes to observe in welding in any roll cage, no matter what process you’re using. First is that most sanctioning bodies, including NHRA, do not allow any leniency in terms of the quality of a weld. There can’t be pores, contamination, or signs of improper metal penetration as well as grinding is not allowed on any welds. While many of us like our cars to look show quality inside and out, if you grind a weld to get it looking smooth on a roll cage install, don’t expect the tech inspectors to sign off on it.
Top Row: Left: Our chromoly cage is made up of 0.083 wall 1 3/4-inch diameter tubing. Center: 6x6-inch plates are used to secure the rollbar to the chassis. These require heating and bending to get them to form to the shape of the floorpan and wheel wells. Right: Test fitting one of the 6x6 plates. Bottom Row: Left: The plates are welded to the car's unibody. Center: Each bar is test fitted prior to being TIG welded in place. Right: Trimming, and cutting of the rear interior panels is necessary to clearance the rear rollbar struts.
Since the Mustang is a unibody car, it also requires some special considerations when welding in the rollbar. Mainly that NHRA rules require a rollbar or cage be welded to the frame of a car. In the case of unibody cars a 6×6-inch plate 1/8 inch thick is required to provide an adequate mating surface for the bars to be welded to the floor of the car. Chassisworks includes these plates when needed with their rollbars and cages.
Getting Silver Bullet ready to go faster also involves upping the horsepower ante a little more. Our ROUSH Performance supercharger package is ready for that task with a Stage 3 upgrade. The Stage 3 upgrade, part number 421597, will net Silver Bullet more boost, and push our stock Coyote engine very near the limit of the factory rods and pistons. ROUSH advertises that this kit brings the engine up to 675 hp and 585 ft-lbs of torque at the crank. The stage 3 upgrade involves replacing the supercharger pulley with a smaller 80 mm version, and adding a fuel pump voltage booster from ROUSH Performance. This upgrade will take us to 11 psi of boost. We had previously upgraded our car’s fuel system with new injectors from Deatschwerks in anticipation of doing this upgrade. You can find our injector upgrade story at the link here.
Making more power in this case will require more than just a simple pulley swap on the supercharger. The ROUSH stage 3 upgrade includes a fuel pump voltage booster, plug and play harness, and 80mm supercharger pulley. We also upgraded our fuel injectors with new 78lbs units from Deatschwerks.
Our auxiliary belt tensioner provides improved belt wrap on smaller diameter supercharger pulleys. This can really help prevent belt slip on hard drag strip launches. -Brent White, Brenspeed
That smaller diameter supercharger pulley means that there is less surface area for our supercharger belt to wrap around. To help keep us from burning through belts we chose to add a Brenspeed belt tensioner to our system. “Our auxiliary belt tensioner provides improved belt wrap on smaller diameter supercharger pulleys. This can really help prevent belt slip on hard drag strip launches, or during aggressive shifting,” says Brenspeed owner Brent White.
We previously upgraded our fuel system with new 78 lb fuel injectors from Deatschwerks. “The ROUSH stage three kit combined with our calibration typically makes enough power to exceed the limit of the factory Roush injector,” says White, thus the requirement for the change in injectors.
White also provided us with a new tune using data provided by Deatschwerks on our new injectors as well as Brenspeed’s intense R&D on ROUSH supercharger kits. Brenspeed was able to create a tuning profile that gave us the maximum amount of horsepower for the dyno and track, but still retained great drivability for the street. White was quick to point out that the injectors worked well and were a major contributor to the drivability gains.
Top Row: Left: Running a smaller diameter supercharger pulley will net us more boost. The tradeoff however is decreased surface area for our belt to grip. To maximize this we installed Brenspeed's auxiliary belt tensioner to increase belt wrap on the supercharger pulley. Center: This Y-harness is included with the ROUSH Stage 3 upgrade kit for the fuel pump voltage booster. Right: We mounted our fuel pump voltage booster in the trunk where it will easily reach the Y harness and be out of the way of heat, dirt, and debris. Bottom Row: Left: We upgraded the pressure sensor to accommodate the increase in boost pressure. Center: The Stage 2 pulley next to the Stage 3, you can see only a minor difference here but it's enough for major power gains. Right: Swapping the blower pulley is as easy as unbolting the old one, and replacing it with the new one.
When it was all said and done with our Brenspeed tune and tensioner, ROUSH Stage 3 Upgrade, and Deatschwerks injectors Silver Bullet put down 552.2 hp and 490.3 ft-lbs of torque to the rear tires. That’s a gain of 50.2 hp and 39.1 hp over our previous best pull. Studying the dyno graph below shows that torque is fat, and flat, with over 450 ft-lbs available from right around 3,500 rpm, and over 400 hp hitting the rear tires at 4,800 RPM and pulling steady all the way up to the redline, with horsepower well over the 500 mark from 5,000 rpm to the end of the run.
In order to keep our additional horsepower from being wasted in tire smoke we decided to get a set of wheels to hang on the car that would allow us to run a more drag oriented tire package. We love the looks and advantages of Weld Racing’s RT-S S71 wheels, and as such we ordered a set to fit our car. We spec’d out 18×4.5-inch for the front wheels, with the high-pad option to clear our GT500 brakes. 15×10-inch wheels were selected for the rear, allowing us to maximize sidewall height, and contact patch area.
Left: Our 18x4.5 front wheels will provide clearance for the front brakes using Weld's high-pad mounting system to space the face of the wheel away from the caliper. Right: Our 15x10 rear wheels will provide plenty of mounting surface for the 295/55/15 Mickey Thompson Drag Radials we run out back.
Weld’s high-pad option on the front wheels adds thickness to the wheel mounting pad in order to get us the caliper clearance we need. In this case the high pad option adds 2.05-inches of caliper clearance. The wheels are exceptionally lightweight, and will reduce rolling resistance on the front of the car, as well as rotating mass, which will help us get off the starting line faster, and should result in better 60-foot times. The forged design and aligned grain structure of the aluminum allows for an exceptionally strong wheel as well, that won’t delaminate and is well suited for the heavier weight of modern muscle cars.
For the 15-inch diameter rear wheels to fit our 2013 Mustang we had to relocate the rear sway bar. This also required grinding off the factory rear sway bar mounting flange as it would otherwise interfere with the smaller diameter wheel size.
Fat and Skinny
Tall, fat, and sticky, sums up our 295/55/15 Mickey Thompson ET Street Radials. These tires just work, and it’s been proven on drag strips around the world.
We developed this tire a couple of years ago as a direct replacement for the old Chevelles and such, and it spilled over into the Mustang market because it’s a true 28 x 10.5, 15-inch tire. -Tom Kundrick, Mickey Thompson Tires
To maximize traction we went to Mickey Thompson for a set of drag radials for the rear, and a set of skinny tires for the front. MT gave us the hookup with a set of 295/55/15 ET Street Radial tires, out back.
The ET Street Radial was actually born out of demand from the classic muscle car market for a true 28-inch drag radial that would offer great traction at the strip, remain street legal for the cruise to and from, and provide many of the benefits traditionally found only in a slick tire. “We developed this tire a couple of years ago as a direct replacement for the old Chevelles and such, and it spilled over into the Mustang market because it’s a true 28 x 10.5, 15-inch tire. It’s a got a taller sidewall that works very well on the drag strip. There’s just no better way to sum it up than by saying it’s just an excellent-working tire,” said Mickey Thompson’s Tom Kundrik.
The larger 28-inch sidewall should also prove more forgiving at the drag strip, allowing better traction, harder launches, and even some sidewall wrinkling. “The larger sidewall really lets the 295/55/15 tire wrinkle up. Most people don’t think that a radial tire wrinkles, but if you watch some of the really high horsepower cars in Outlaw Drag Radial and such, it will have wrinkles all the way around almost like a slick. But by the sixty-foot mark, it’s already back to round. Radials stay round and they’re very stable,” says Kundrik.
Providing the skinny up front to go with our fat rear tires in the back, are a pair of Mickey Thompson Sportsman SRs for the front of the car. These radial construction tires are designed for decreased rolling resistance and will still allow us good braking and even maneuvering through traffic on the street.
Since we don’t want to mix a set of bias-ply skinny tires with our radial rears, we also got a set of Sportsman SR tires for the front, sized 26x6x18. These tires will allow us to maintain a radial tire that can handle a variety of driving conditions on the way too and from the track. Driving in SoCal is a mixed bag of high speed interstate blasts, and sudden stops. Between rapidly evolving traffic situations on the interstate, and sometimes poorly maintained city roads, we needed a tire that will allow us to maintain control of Silver Bullet whether we’re blasting down the strip, or just trying to make it home after a day of test and tune at the track. The Sportsman SR does just that. We won’t be carving any corners with these, but if we need to avoid an obstacle in the road, or get on the brakes quick because of a phantom traffic jam -the Sportsman SR can handle it.
This tire also offers lower rolling resistance for the track, as well as light weight. All of these attributes combined with our Weld RT-S wheels up front will allow the suspension to react faster and result in better 60-foot times for Silver Bullet.
At The Track
We headed out to the track with the changes made to Silver Bullet. With more boost, grip, and reduced rotating weight, we wanted to see if our hard work has paid off. Obviously traction has greatly improved, and thanks to the addition of our Circle D Specialties converter we’re also able to get through the 60 ft faster.
The new setup really stepped up the 60-foot, but there is more room to grow. When we previously got our best 60-foot, we ran a 1.58 in mineshaft air conditions in about 45-degree weather. With the new converter and upgrades, we quickly improved to a 1.51 in 3600-feet of air in “iffy” track conditions. That .07 gain is dramatic but even more so when you consider the weather and air density conditions.
Our two best runs so far with this round of modifications.
We ran the car through the full quarter mile at Fontana and ran a 6.95, but ran out of RPM and hit the rev limiter going a 10.88 at only 119 mph. With times now securely in the 10-second zone, the roll -bar from Chassisworks isn’t just an option at this point, but mandatory as we’re no doubt marked by tech inspectors to check for it anytime we bring the car to the track. To give you some perspective, back in the mineshaft air at Irwindale in the fall we ran a 6.80 in the 1/8 mile with a 1.58 60. This car is headed to the 6.60s and 10.40s – just need the track and weather to make it happen.