Typically when we spec out the right exhaust system for one of our project cars our only concerns are fitment quality and proper airflow for maximum horsepower and torque production. And don’t forget aggressive sound, ’cause we’re not the strong silent types.
This time around, however, we have an additional consideration with our GM Performance Parts LS3 E-Rod project known as the EcoNova: We got cats. As part of the smog legality that comes with proper installation of the E-Rod system, four new production catalytic converters (just like those on a fifth-gen Camaro) must be installed into the exhaust system to the factory manifolds. So rather than header clearance issues, we’ve got converter clearance issues to contend with. That’s a first for us.
It’s also a first for the aftermarket, since cat-back kits don’t exist for ’70s-era cars. Actually, kits for our later-generation X-body cars like our ’77 Nova don’t exist. Or do they? We’ve long insisted that ’75-79 Novas used the same unibody architecture as ’68-74 cars, just wrapped with different sheetmetal—and kits do exist for those. We could have just started from scratch with a stack of tubing and bends, but we had a point to prove, and we figured that we could help create a new part number for a company and throw some love to the underserved fourth-gen X-body crowd.
So, we called Flowmaster Mufflers in Santa Rosa, California, to get one of their header-back American Thunder kits for third-gen Novas. They thought we were a bit crazy and recommended we take the EcoNova over to Big John’s performance in Santa Clarita, California, for a little help from master fabricator Stephen Munson in case our bet didn’t pay off.
Flowmaster’s third-gen Nova...
Flowmaster’s third-gen Nova exhaust kit (PN 817158, $483 street price) offers 2-1/2-inch mandrel-bent 16-gauge 409 stainless steel tubing instead of the typical pinched, constricted bends formed out of mild steel. These systems are intended for the DIY guys, but a lot of professional shops like Big John’s use them because the pre-bent tubes are massive time-savers that are perfectly laid out for most muscle car applications.
GM Performance Parts did an...
GM Performance Parts did an excellent job with the basic shape of the universal fitment catalytic pipes included in the kit, but they’re likely going to need minor tweaking. They dropped a bit too low on our Nova and needed a slight re-clocking to ensure sensor clearance, so we cut the flanges off with a bandsaw to adjust the angle slightly.
It’s critical to emissions...
It’s critical to emissions compliance that our oxygen sensors remain in the stock location, so Stephen Munson was careful to ensure the distance from the flange to the sensor bung remained the same when he re-clocked and welded them back on.
With that minor adjustment,...
With that minor adjustment, the catalytic pipe slid right up between the lower control arm mount and the 4L70E’s bellhousing. This was the tightest side; the driver side had oodles of room. It’s worth noting that this fitment was achieved with Hooker’s “stock location” LS adapter plates on stock GM clamshell mounts with Energy Suspension inserts. Opting for a different engine placement may create interference issues with the lower control arm.
Interestingly, the passenger-side...
Interestingly, the passenger-side rear catalytic converter has a slight clock to it that works nicely for transmission pan clearance, which we can only assume was designed for the Camaro fitment, but it really worked nicely on our Nova as well. GM Performance Parts did their homework on these pipes.
Now that we know exactly where...
Now that we know exactly where the catalytic pipes are going to route, it’s time to get the pre-assembled Flowmaster H-pipe assembly into place. Through dyno testing Flowmaster has confirmed what we’ve seen in most of our own projects: H-pipes and X-pipes make the same power in most applications. H-pipes give that classic muscle car thump we love though.
Using a chop saw, Munson cut...
Using a chop saw, Munson cut a few inches off of the pipes that would typically connect to a set of long-tube headers and mated them up to the cat pipes. These are universal fitment for different full-length headers, but we were blown away by how naturally the Flowmaster system fit to the cat pipes with only minor trimming. Note how perfectly the H-pipe follows the dual-hump crossmember, and check out the extremely nice band clamps Flowmaster includes to squeeze the pipes and mufflers together.
Munson, our metal burner for...
Munson, our metal burner for the day, specializes in custom suspension work as well as forced induction system plumbing and, of course, exhaust. Using a pre-bent Flowmaster kit like this makes life easy for him on simple projects like this so he can focus on tailoring the kit for an individual car and driveline package.
Our E-Rod cat pipes came with...
Our E-Rod cat pipes came with these killer GM exhaust couplers that are a true 2-1/2 inches, making them a great option for creating a midsection that can be unbolted out of the exhaust for easier access to the trans if it should need to be pulled. We don’t have the PN, but they stamped 63.50 and are stock on the fifth-gen Camaro SS.
Now it’s time to jump to the...
Now it’s time to jump to the rear of the kit. It’s much easier to fish the tailpipes in prior to adding the mufflers, so they go in first. Ours slid in with the EcoNova at ride height, but some cars may need to droop the rearend a bit to get them in. After that Munson slid the muffler into place and held it with a stand to find the proper orientation.
Here’s the kit fully installed....
Here’s the kit fully installed. Despite being designed for a third-generation ’68-74 Nova with headers, the kit fits on ’75-79 fourth-gen Novas with catalytic converters like it had been custom crafted specifically for this project. Amazing. And just like that, Flowmaster now has a new set of part numbers for bolt-in kits for fourth-gen X-body cars.
Speaking of mufflers, since...
Speaking of mufflers, since we have four cats quieting us down, we opted for Flowmaster’s iconic, aggressive sounding original 40-Series mufflers to make sure the LS3 had a noticeable exhaust note when under throttle. If you want a little less growl, Flowmaster has several options that will work.
To hang the mufflers there...
To hang the mufflers there are four factory holes located in the rear unibody on a stiffening crossmember that the kit uses to mount the hanger. If your holes are usable, the supplied 1/4-20 x 1-1/4 Allen bolts and jack nuts will attach the hanger. Ours were a bit mangled from a previous install, so Munson welded ours directly to the crossmember. From there, the rubber isolator attaches to the hanger welded to the muffler.
One thing that exhaust shops...
One thing that exhaust shops use to judge the quality of an install is how symmetrical the pipes are. Check out the Flowmaster kit from the rear: every component is dead-on equal height.
Before tightening the band...
Before tightening the band clamps for the tailpipe, Munson rotated it to fit for optimum clearance over the axle and without touching the inner wheelwell or leaf spring. Interestingly, the driver-side framerail is a little different than the passenger side and the fitment is a little tighter. The length on the Flowmaster kit was perfect; we didn’t even need to trim the tips.
Just as important is how nicely...
Just as important is how nicely it tucks up to the chassis for ground clearance. Plus, nobody likes to see pipes hanging down. The Flowmaster kit gets top marks on this; it’s barely visible even with the car elevated above eye level.