1968 Chevy Nova Dual Exhaust System - Piping Up
How to select and build the right-sized dual exhaust system for a serious street machine
From the October, 2011 issue of Popular Hot Rodding
By Johnny Hunkins
Photography by Johnny Hunkins
Big pipes and a throaty rumble are what street machines are all about, but it’s not just about the sound; an exhaust system has to perform up to the engine’s full capability, too. Compounding the issue is the expense involved of chasing down headers, mufflers, pipes, and, in some cases, the right tools. You’ll have a lot invested in your system—even if it’s really basic, like the one on our ’68 Nova. Bottom line: It pays to get it right the first time.
For Project Nova, we selected a set of Hooker Super Comp long-tube headers (PN 2207-1HKR). These are designed to fit ’68-74 Chevy Novas, and have 17/8-inch diameter primary tubes in a 30-inch length with a 3.5-inch collector. They also feature high-temp ceramic coating, which helps retain heat inside the header for more efficient exhaust scavenging. We will get into the mechanics and sizing of our header selection in the next Nova installment, but we’ll quickly mention that these pipes were calculated to maximize the torque of our stump-pulling Dart 400 SHP-based small-block.
The exhaust aft of the pair of Super Comp collectors is our objective this month, and was handled by the exhaust professionals at Automotive Excellence in Huntington Beach, California. AE proprietor Alex Mayea has been bending up serious hot rod exhausts for seven years, and has built pipes for nearly every kind of muscle car on the road. We knew we could trust Mayea with our Nova project after seeing some of the cool projects he’s worked on in his family owned shop, so we handed him the keys to work his magic on the Chevy.
“The first thing people do is bend over and look at it from the side, so the first thing you notice is if one pipe is hanging lower than the other,” Mayea says. “We try to get that right. From a customer’s standpoint the two biggest concerns are rattling and leaks. Both of those are things we check before taking the car off the rack.” Mayea continues: “We did turndowns on this car, but most people would want tailpipes. Depending on the rearend and suspension, the tailpipes might need modifying. We can handle that, whereas a hobbyist might not be able to complete the install. Up to the mufflers, kits usually fit pretty good, but then the tailpipes can have issues.”
Note the comparative size...
Note the comparative size of Project Nova’s original 1½-inch headers installed by a previous owner (top), and the new Hooker Super Comp ceramic headers with 17/8-inch primaries. Bigger isn’t always better, although too small will hurt performance—and that was our case here. With over 500 lb-ft of torque available from our 400ci small-block at 3,700 rpm, and 523 lb-ft peaking at 4,400 rpm, a bigger tube was called for. We’ll have more on the science of primary tube sizing next month when we dive into our header story.
As a Flowmaster dealer, Mayea is well-versed in the product lineup, and agreed when we expressed an interest in keeping the Nova Flowmaster equipped. The ’68 had a basic 2-inch dual Flowmaster system on it when we bought it, as well as some undersized 1½-inch headers that were showing signs of age. Our take on it was that we liked the basic sound of this setup, but we needed something that would satisfy the flow requirements of the larger 400ci Dart small-block. With 523 hp at the crank, we were leaving a lot of sauce on the table, and a larger exhaust system would allow us to fully unleash the Dart’s fury.
After studying the Flowmaster website and getting input from Mayea, we opted for the Super 40s. Mayea told us: “The two-chamber Super 40s definitely have a more powerful exhaust tone outside, but are quieter in the interior of the car. This is for a car that’s more of a driver, but that still needs an aggressive muscle car rumble on the outside.” That’s just what we were after, so we calculated our diameter at 2¾ inches for dual pipes (see chart on p. 68). Rounding up to the next available size, we chose a Super 40 with a 3-inch inlet/outlet diameter and an offset inlet (PN 953046). This should cover us easily to almost 700 hp should we want to add a blower, turbo, or nitrous somewhere down the line.
Flowmasters are used on some...
Flowmasters are used on some of the country’s most powerful race cars, and we wanted some of that mojo for the Nova. The Super 40 (PN 953046) has a 3-inch inlet/outlet, a 5-inch-thick body, and a case that’s 13.5 inches long by 10 inches wide. The offset inlet will prevent any driveshaft interference. Alex Mayea of Automotive Excellence says: “The two-chamber Super 40s definitely have a more powerful exhaust tone outside, but are quieter in the interior of the car. This is for a car that’s more of a driver, but that still needs an aggressive muscle car rumble on the outside.”
Complementing the Hooker Super Comp headers and Flowmaster Super 40s would be Flowmaster’s universal U-Fit 3-inch dual pipe kit, which AE would adapt to our Nova chassis using the 3.5- to 3-inch collector adapters included in the Hooker header kit. We really liked the sound of the axle turndowns from the existing system, so for the new system AE replicated those by sectioning one of the mandrel bends from the U-Fit kit.
We’ve yet to make it to the chassis dyno, but we plan to do that down the line when we experiment with FAST’s new EZ EFI throttle-body fuel injection system. We’ll be doing some before/after dyno testing with back-to-back runs of carb versus EFI, plus we’ll have some online video clips to show you how the Nova sounds at wide-open throttle. Next month, however, we’ll show you some really cool pieces from Hooker that allow you to easily modify header primary tubes around underhood obstructions. Mayea and the crew at AE did such an excellent job modding our Hookers, we dare you to tell the difference! Be sure to check it out next month here in PHR.
Before building the new exhaust,...
Before building the new exhaust, AE removed the factory crossmember and replaced it with G Force’s dual-exhaust crossmember (PN RCF1-400, $219). The G Force piece went in last month with the installation of our Phoenix Transmission Products’ Turbo 400 and Switch-Pitch converter. It gave us some additional ground clearance that we needed for a larger 3-inch system, as well as greater flexibility in placing our U-Fit exhaust. If you’re converting from a stock single-hump crossmember, this is the one you want.
A non-stock steering box with...
A non-stock steering box with a non-stock header and a non-stock cylinder head port means you’re in experimental territory with headers. We had to chop out one primary tube to continue with the exhaust install. See how we made everything work next month!
Flowmaster’s U-Fit 3-inch...
Flowmaster’s U-Fit 3-inch universal kit comes with a variety of mandrel bends that can be combined and cut in almost infinite variation—the key is having the experience to tell you which moves are best. Here Mayea uses a 14-inch chopsaw to shorten a 45-degree bend for one of the back pipes. If you’re going to do custom exhaust work on your own, a chopsaw is mandatory.
A slight kick-out was needed...
A slight kick-out was needed to make some clearance between the Flowmaster 40s and the driveshaft, so Mayea put a slight kink in one of the U-Fit kit’s straight pipes. AE uses a professional swedger/bender from Bend-Pak, but for small bends like this you can easily use a low-cost hydraulic bender from someone like Harbor Freight.
While the mandrel-bent pipes...
While the mandrel-bent pipes in the Flowmaster U-Fit kit already have expanded ends for slip-fit assembly, Mayea did need to enlarge them to fit using AE’s Bend-Pak swedger. Low-cost alternative tools are available for the DIY guy without an exhaust shop.
Here’s where all the experience...
Here’s where all the experience of a shop like AE comes in. “You want to make sure it’s as symmetrical as possible,” Mayea says. “I’m looking for ground clearance, driveshaft clearance, and body clearance with the trans tunnel.” Up to this point, you have the option of routing the pipes anywhere you want, but once you start welding, you’re committed. The cool thing about the U-Fit kit is that the slip joints allow you to get a very professional mock-up going without lots of intermediate tack welding or having to erect a forest of jackstands.
Your exhaust system is only...
Your exhaust system is only going to be as good as your welder, and when it comes to MIGs, Miller has it covered. AE relies on their 10-year-old Millermatic 35 pro unit seen here, but a great choice for the homebuilder would be the dual-voltage Millermatic 211 with Auto-Set, which takes the guesswork out of setting wire feed speed and voltage.
With the exhaust still mocked...
With the exhaust still mocked up on the Nova, Mayea tacks the Flowmaster pieces together, checking periodically to see that the alignment and the clearances of the exhaust system are still where he wants them. Mayea says: “When you tack it, do it in three spots, not just one or two. If you tack it in three spots it will keep things from moving around.”
The finishing touch is a pair...
The finishing touch is a pair of turndowns just before the Wavetrac-equipped Moser 12-bolt axle. The U-Fit kit includes enough extra mandrel bends to make these by cutting it perpendicular mid-bend, and then “baloney” style on the discharge end. The hangers are a pair of Nickson Industries “Southern” pieces that Mayea sells for $2.50 each.
So How Does It Sound?
Treated with high-temp silver...
Treated with high-temp silver paint around the weld surfaces and mounted back up on the car, the fully welded Flowmaster 3-inch dual system is ready to rock. The sound with axle turndowns is simply amazing, both at cruise speed and at full throttle!
Go to www.popularhotrodding.com
and check out the video of Project Nova as we put the new Hooker/Flowmaster system through a thorough sound check!
DIY vs. A Good Shop
We opted to use the services of Automotive Excellence in Huntington Beach, California, but assembling a custom exhaust system like ours is well within the reach of the average guy, provided you’ve got the right tools. Assuming you already have a serviceable MIG welder, a compressor, jackstands, and all the normal stuff, you can expect to buy a few specialty tools for a job like this. We priced out some super affordable tools from the DIY experts at Harbor Freight that will provide similar functions to the pro tools at AE. The comparison works out like this: AE charges about $300 in labor for a job like this. You get to watch from the sidelines while the pros do it, but you won’t gain much personal experience. Buying the tools yourself, however, will give you the satisfaction of doing it yourself, while rounding out your tool collection.
Exhaust System Sizing
With the Flowmaster U-Fit...
With the Flowmaster U-Fit pieces, Super 40 mufflers, and Hooker collector adapters tack welded in place, Mayea removed the system from the Nova to make the final welds. Mayea set the Millermatic 35 to 4 amps current and the wire feed speed to between 40 and 45 inches per minute, using .035-inch wire. “You don’t want too much heat or you will blow a bunch of holes. You also need a really good ground connection,” Mayea says. Some Miller MIGs have Auto-Set, which only requires the user to set the material thickness (16 gauge in this case) and the wire thickness. Amperage and wire speed are automatically set.
It’s possible to mathematically calculate the flow requirements of an exhaust system based on the total mass flow of the system (mass of the ingested air plus the mass of the fuel), but it works out to about 2.2 cfm per engine horsepower. For a straight piece of round pipe, each square inch of cross-sectional area flows about 115 cfm, so it’s a matter of working forward from there. This handy chart takes most of the math out of sizing an exhaust system, and from this you can see that our 532hp Dart small-block breathes nicely with a dual 3-inch system.
||PIPE AREA (IN2):
||MAX HPPER PIPE:
||MAX HP(DUAL SYSTEM)
|Tools You’ll Need
|16-ton Central Hydraulics pipe bender, 3-inch capacity
|Chicago Electric heavy-duty cutoff saw, 14-inch blade
|U.S. General large tailpipe expander
Where The Money Went
|U-Fit Dual 3-inch pipe kit
|Super 40 Series Delta, 3-inch inlet/outlet
|Super Comp headers, 17/8-inch dia.
|Southern exhaust hangars