Starting in the ’50s and continuing into the 1970s, many Detroit automakers offered optional dual exhaust systems, often for surprising little money. Though long favored by hot-rodders, dual exhausts never got the same attention or the same marketing push as multiple carburetors, higher compression, and other performance features. Here’s why — and why dual exhausts were a highly worthwhile performance option despite their relative obscurity.
If you have even a vague idea of how internal combustion engines work, you’re probably familiar with the old aphorism that an engine is basically an air pump — the amount of power an engine can produce depends heavily on how well it can breathe. However, there’s also a flip side to that: To produce more usable power, an engine needs to be able to effectively “exhale” exhaust gases as well. Any exhaust system imposes some restriction on the exhaust gases flowing through it, and in a four-stroke engine, the pistons must work against that restriction on each exhaust stroke.
Even straight pipes create some back pressure; mufflers and catalytic converters add more, as do turbocharger turbines. As the above graph illustrates, the cost in horsepower and fuel consumption for a given back pressure increases considerably with speed, so keeping exhaust restriction to a minimum is very important for good overall performance.
Developing low-restriction exhaust plumbing becomes more complicated with vee or horizontally opposed engines because they have more than one exhaust manifold. For cost and packaging reasons, automakers often connect both manifolds to a common exhaust pipe, which is lighter and cheaper, but imposes more restriction on the outflow of exhaust gases, much like a funnel. Moreover, many engines use an asymmetrical crossover pipe to connect the manifold on one side to an exhaust pipe on the other side, meaning that back pressure is uneven between the cylinder banks.
In general, you get significantly less back pressure if each exhaust manifold has its own exhaust pipe and own muffler(s), which can also have the advantage of allowing both sides of the exhaust system to be as close to symmetrical as possible. Dual exhaust systems are still a compromise (the pipes may still have to snake around or under other underbody components), but overall, they create less restriction than a single exhaust.
Cheap Power, Little Publicity
Dual exhaust systems are more expensive than single exhausts, for obvious reasons, but automakers have not been averse to offering dual exhaust at extra cost for buyers who wanted them, either as part of a “power-pack” package or as a standalone option.
If you ordered duals on a new car, they were often surprisingly inexpensive. For instance, while dual exhausts were standard on the pricey 1962 Oldsmobile Starfire pictured above …
… you could also order the same dual exhaust system on most full-size 1962 Oldsmobiles for a very reasonable $26.47. A decade later, in 1972, Oldsmobile was still charging only $29.70 for the dual exhaust option, while Pontiac charged $30. Not every automaker offered dual exhausts, and they weren’t available on every car or every engine, but where they were, they were often a bargain.
So, why didn’t dual exhausts get more attention? Even fairly casual enthusiasts have heard of special carburetion setups like the old Pontiac Tri-Power system, but I think a lot of people assumed — and still assume — that dual exhausts were basically a cosmetic option, like chrome exhaust tips. There was one big reason for that: Until the early 1970s, the power and torque increases provided by dual exhausts were not reflected in advertised horsepower ratings, at all.
Before 1972, most automakers advertised “gross” horsepower and torque ratings for their U.S. models rather than the as-installed “net” ratings cited for modern engines. Over the years, there have been a number of different engineering procedures for determining an engine’s gross output, but all of them measure power and torque WITHOUT the stock exhaust system and muffler(s). The hottest 1957 Chevrolet fuel injected 283-cid V-8, which Chevrolet famously advertised at 283 gross horsepower (1 hp per cubic inch), came with dual exhausts, but its gross output would have been the same if the “fuelie” engine had included a single exhaust with a crossover pipe, since gross ratings are determined with the stock exhaust system removed. However, single exhaust would definitely have reduced the 283 hp engine’s as-installed net power (which 1957 Chevrolet engineering specifications reveal was 240 hp).
Unfortunately, domestic automakers only rarely released net ratings before a now-obscure California law effectively forced them to, starting in 1972. A few manufacturers did occasionally publish net ratings prior to that — until 1971 (when some automakers published gross and net ratings side by side), the net figures generally weren’t advertised, but you could sometimes find them in the fine print of the detailed specifications. Unfortunately, where earlier net ratings are available, they’re usually for “power-pack” engines that also included different carburetors, higher compression, and/or different camshafts, making it hard to determine exactly how much of their extra net power was due to their dual exhausts.
For example, in 1961, Chevrolet offered an optional Super Turbo-Fire 283 V-8 (RPO 410), which included a four-barrel carburetor, a half-point higher compression, and a slightly hotter camshaft. You could order this with dual exhausts (RPO 220) for an extra $19. Either way, Chevrolet advertised the Super Turbo-Fire at 230 gross horsepower, compared to 170 gross hp for the standard two-barrel Turbo-Fire 283. The Chevrolet specifications credit the Super Turbo-Fire engine with 175 net horsepower, compared to just 135 net hp for the standard 283.
It’s very likely that the net ratings Chevrolet published for the Super Turbo-Fire engine were taken with dual exhausts (I don’t think the cam, carburetor, and higher compression would add up to 40 net hp). However, Chevrolet didn’t specify one way or the other, and estimating how much of the RPO 410 engine’s 40 hp edge over the standard two-barrel 283 could be attributed to each of the Super Turbo-Fire engine’s other features is a matter of guesswork.
As Paul has previously discussed, the one-year-only 1966 L77 version of the Chevrolet 283-cid V-8 at least reduced this ambiguity. The L77 had the same compression ratio and valve timing as the 1966 two-barrel Turbo-Fire 283 — as far as I can see from the specifications, it really only differed in its four-barrel carburetor and standard dual exhaust system.
Chevrolet advertised the L77 at 220 hp gross, compared to 195 hp for the two-barrel 283, a difference of 25 hp. However, the difference in their net ratings was actually even bigger than that: According to Chevrolet factory data, the L77 had 185 net hp, compared to just 150 net hp for the standard two-barrel engine, a difference of 35 hp. Some of the L77 engine’s advantage was due to the four-barrel carburetor, but in as-installed net terms, the L77 engine was even more powerful than the standard 283 than the advertised ratings suggested, thanks to its dual exhausts.
You can see the dilemma for manufacturers: Dual exhausts provided a significant edge in real-world power, but as long as the industry remained committed to advertising gross ratings, the advantages were basically invisible. Because net ratings are always lower than gross ratings for a given engine, no automaker wanted to be the first or only one to adopt them for advertising purposes.
It wasn’t until the industry-wide adoption of net ratings for 1972 that the benefits of dual exhausts became a little clearer. Here are some comparative figures showing how much dual exhausts were worth on some 1973-model GM V-8 engines:
- Buick 350-4V: 175 hp/270 lb-ft single exhaust; 190 hp/285 lb-ft dual exhaust — GAIN: 15 net hp
- Buick 455-4V: 225 hp/360 lb-ft single exhaust; 250 hp/375 lb-ft dual exhaust — GAIN: 25 net hp
- Oldsmobile 350-4V: 180 hp/275 hp single exhaust; 200 hp/300 lb-ft dual exhaust — GAIN: 20 net hp
- Pontiac 350-2V: 150 hp/270 lb-ft single exhaust; 175 hp/280 lb-ft dual exhaust — GAIN: 25 net hp
- Pontiac 455-4V: 215 hp/350 lb-ft single exhaust; 250 hp/370 lb-ft dual exhaust — GAIN: 25 net hp
Unlike many previous power-pack engines, the power and torque gains listed above were due entirely to the dual exhausts. For an option price of only about $30, you’d be VERY hard-pressed to find a greater performance value than that! While comparable figures for older models are hard to come by for the reasons I explained above, there’s no reason to assume dual exhausts weren’t just as beneficial for earlier engines.
On top of that, dual exhausts were really more useful for normal street use than other common hop-up techniques. For street driving, it’s very easy to end up with way too much carburetion; increasing spark advance or compression can cause ruinous detonation; and without variable valve timing, a hotter camshaft tends to lose at the low end what it gains at higher RPM. Dual exhausts, in contrast, give even mildly tuned engines perceptibly more pep, with few real drawbacks other than a couple of extra pounds, a little more exhaust noise, and the added cost of eventually replacing two rotted-out mufflers rather than one.
Dual Exhausts in the Age of Catalytic Converters
The age of cheap dual exhausts hit a snag in the mid-’70s with the adoption of catalytic converters for emissions control. A catalytic converter, especially an early one, adds some back pressure, but it’s not an insurmountable problem; there are low-restriction catalysts that pose no great performance penalty. The bigger issue is routing the exhaust flow to and from the cat, and deciding whether the market will bear the cost of multiple catalysts. If not, the catalytic converter can become a bottleneck.
One of the most notorious examples was the 1975 Chevrolet Corvette. Previously, even the mildest Corvette engines had come with dual exhausts, which had given them excellent street performance and enabled them to consistently punch above their weight (and rated gross outputs). For 1975, the ‘Vette got a new single-catalyst exhaust system with a crossover pipe and single exhaust pipe ahead of the catalytic converter, and a single pipe branching out into separate mufflers and outlets behind the cat. Chevrolet still called this dual exhausts, but it wasn’t, and it cut the output of the four-barrel 350-cid L48 engine to a mere 165 net hp for 1975. For subsequent years, Chevrolet was able to make up some of the lost power elsewhere, but the restrictive exhaust system remained through the rest of the C3 generation.
By the mid-1980s, the costs had come down enough that true dual exhaust systems were again available — typically on sportier models, but sometimes in some unexpected places. For instance, the Ford LTD Crown Victoria and Mercury Grand Marquis could have dual exhausts as part of the trailer-towing package. Ford didn’t consistently quote a separate power output for the 302 with duals in these Panther-platform full-size cars, but in the years when they did, the dual exhaust system was worth an extra 15 hp and 15 lb-ft of torque. Dual exhausts were also added to the hotter versions of the 302 offered in the Ford Mustang and Mercury Capri, contributing to that engine’s march past the 200 hp mark.
Starting in the late ’80s, some Camaro and Firebird engines also gained a dual converter exhaust system.
The N10 dual converter exhaust system on the F-bodies was theoretically a separate option (costing $155 in 1989, later rolled into option groups that also included features like an engine oil cooler), but it was required with certain of the hottest V-8 engines, frustrating direct comparisons. However, broadly speaking, the duals were worth between 10 and 20 horsepower compared to the earlier single-exhaust versions.
From 1991, the new Ford 4.6-liter modular V-8 in the Town Car could be ordered with either single exhaust, with 190 hp and 260 lb-ft of torque, or dual exhausts, with 210 hp and 270 lb-ft. The dual exhaust system cost a mere $83 on a 1991 or 1992 Town Car. (You could also get dual exhausts on the same engine in the redesigned Ford Crown Victoria and Mercury Grand Marquis starting in 1992, though not as a standalone option.)
The 1992 GM LT1 5.7-liter V-8 got a new low-restriction exhaust system with symmetrical cast iron exhaust manifolds and dual catalytic converters. (Although the above factory illustration suggests a single central cat, there were actually two close-coupled ones with an integral heat shield.) This and other changes brought the LT1 engine to an even 300 net hp, 50 hp more than the L98 5.7-liter V-8 it replaced.
Automakers still didn’t necessarily make it easy to see at a glance how much power a dual exhaust system was worth. Some engines were still available only with singles or only with duals, and if dual exhausts were buried in the fine print of some heavy-duty towing package, only the most committed buyers might realize the option was available, much less order it. However, even with ’90s engine technology, a good dual exhaust system still typically provided about a 10 percent increase in net power, often for a very reasonable price on a new car.
If you’re wondering, inline-four and inline-six engines can and do sometimes use dual exhausts (although they arguably need them less than vee or horizontally engines do).
One last important thing to keep in mind: Getting the performance benefits of dual exhaust really requires true duals: separate pipes all the way from the manifold(s) to the tailpipes. Don’t assume that a car has dual exhausts just because it has separate outlets, or even separate mufflers — some cars have had four or more exhaust outlets even though there was still only one exhaust pipe further upstream! Multiple pipes mean more power; multiple outlets from the same pipe don’t enhance much other than noise and ego.
Related Reading
1966 220 HP 283 Chevrolet V8 – A One Year Oddity That Proves The Power Of Dual Exhausts (by Paul N)
1973 Pontiac LeMans Sport Coupe: The Poor Man’s GTO, With A 3-Speed Manual Transmission (by me)
I’m looking forward to reading the comments on this post, as there’s a lot of opinions about the best exhaust configurations.
The advantage of an extra pipe isn’t going to be solely in the maximum flow, it will also be in how the pulses of exhaust pressure can be used to improve scavenging. As I understand (and I look forward to being corrected!) a well-tuned exhaust makes a big difference as the valves overlap, and the piston is at the top of its non-power stroke – the inertia of the gases going through the exhaust sucks exhaust gases out and helps get inertia into the air/fuel mix. I assume that these pulses (and the reverse pulses) are easier to manage when you have less cylinders exiting into one pipe.
The dark arts of having crossovers between dual exhausts is far beyond me, but I’m sure people will have opinions there as well.
You are correct that a crossover in a dual exhaust can further improve performance over duals w/o the crossover. For a while the hot aftermarket set up for a Foxstang was an X-pipe for its dual exhaust. I don’t remember the claims from back in the day but actual dyno tests did show an improvement vs separate duals. The H-pipe is also an improvement vs separate duals and that is what was used in those Panthers that had dual exhaust.
Found a video showing the difference in power and torque curves between no cross over vs an H or X pipe.
The amount of exhaust flow that an engine produces is a function of displacement and RPM, not engine configuration (vee or inline). And single/dual/quad/whatever is irrelevant. What matters is flow rate and resulting backpressure. A single 3″ pipe has roughly the same cross section area as a pair of 2.25″ pipes. Arguably the single pipe with the same area has less boundary layer drag (and thus higher flow) since it has a smaller surface area for the same cross section. And finally, even the 1972 net HP ratings are suspicious, as the SAE Net HP rating standard did not include the full as-installed exhaust system in the dyno tests. The number and severity of bends has as much impact on flow and backpressure as does pipe size.
I agree a single larger pipe can flow as much or more as two smaller pipes. The other part of that equation is the muffler, and possibly Cat, if not combined with a correspondingly higher flow muffler and Cat those items will be the bottle neck.
The problem with most of the 70’s and 80’s dual outlet exhausts was that they combine 2 pipes into one of the same diameter, and feed through the same Cat as the single exhaust cars. Of course packaging plays a huge role in all of that. The picture of the Trans Am’s set up really shows that a true dual exhaust was a pain for those engineers that had to try and fit it later.
I didn’t say that vee engines have more exhaust flow, I said, “Developing low-restriction exhaust plumbing becomes more complicated” and that having multiple manifolds dumping into a single pipe is generally more restrictive. You’re arguing points I did not make.
No idea where you’re getting this from. J245 defines net power as the brake output of a “fully-equipped engine,” which 1.6.2 defines as including but not restricted to “the intake air system, exhaust system, cooling system, generator, starter, and emission control equipment.” The procedures in 3.6.2 further stipulate “Muffler, exhaust pipe, and tail pipe — on or system providing equivalent restriction.” The “or system providing equivalent restriction” stipulation is repeated verbatim in J1349.
Chrysler Corporation proved your one pipe claim, with the first generation Plymouth Barracuda’s low-restriction single exhaust system for their 273 V-8s, thanks to having misplaced these engine’s oil filter locations, not making a separate pipe possible, without substituting upper wishbones for struts
Great topic, what goes in must come out.
Glad you mentioned the Panthers. They had to be one of if not the last vehicle where single and dual exhaust was available on the same engine. Sure for most of them it was wrapped up in a package but those packages still gave quite a lot of bang for the buck.
The most common civilian package was the HPP, Handing and Performance Package, that retailed for around $600 in the early years if I remember right. Not only did you get duals, with a H-pipe, you got different springs, shocks, marginally lower ride height, sway bars, steering programing and/or ratio, higher numerical rear axle ratio and depending on the year larger tires and/or wheels.
You also got duals with the short lived towing package or of course the police package, each with their own specific combination of springs, shocks, sway bars, steering but all sharing duals and a higher axle ratio than the base cars.
Great point about the Panther cars. We bought an 04 Grand Marquis several years back and I noticed an exhaust rattle so I took it to our local shop. Apparently the car traveled extensively on gravel roads and the muffler was dented up. But this exhaust guy, who I appreciate his honesty, told me that if I didn’t mind a small amount of noise, because that car has four catverts he could add the left side pipe and an H pipe and it would make the car run better. It’s been 4 or 5 years now and that car runs just great.
Also, we live up on a ridge with lots of deer. There is just enough noise that they scatter when we are climbing.
Yup it is an easy add, one of my 92’s was a single pipe car and back then I was able to purchase the H-pipe and tail pipes from my local auto parts store. Pretty quick job since the H-pipe bolts to the Cats.
Of the six cars I have only the 68 Cougar came with dual exhaust behind the 302-4V. I kept the 68 Mustang 289-2V with a single when I restored it back in 1985-90. The F100 kept it’s single exhaust while I haven’t given thought to the Ambassador wagon yet. Both the Parklane and Polara were intentionally changed out to dual exhaust after obtaining them. Time was spent on diameter of each pipe relative to the particular engine and then to the mufflers. The shop commented on my selections as to why I didn’t overall exhaust like people over carburate. Both cars had an H-pipe installed where I wanted them so they didn’t interfere. Will be interested in how a fresh 410 FE will now move the Parklane vs the tired engine that was near the end of effective life. Below the Polara.
I have always kept this on my computer.
In the northeast, in the pre-cat days, you were lucky to get 3 winters out of an exhaust system. Replacement costs would be double to replace a dual exhaust. I think that alone might have been a good enough reason to stick with a single exhaust for anyone who lives in rust land.
Ditto. Leaded gas and salty roads spelled early death for many an exhaust. Larger mid-price cars like Mercurys, Oldsmobiles and their ilk were more likely to duals. In a small town with severe winters, it wasn’t uncommon for cars to be driven for short trips of under 5 miles. Dual exhausts were slower to warm up than a single and often needed early replacement. The Ford dealer where I worked had frequent customer requests to replace dual exhausts with single exhausts. It wasn’t unusual to see this by the 3rd winter of a car’s life.
As Evan mentioned above, exhaust systems back in the day rotted out in only a few years. From what I have gleaned, dual exhausts rotted even faster as there was less gas flow through each pipe when compared with a single exhaust. We tend to forget how disposable exhaust system were. Midas, for example, used to be Midas Muffler.
In my family garage, we did a good business making stock American cars go faster. The first step was the ditch the cat and install a real dual exhaust. That really perked the cars up. Adding an intake and cam worked even better. The actual cost of doing this was surprisingly modest.
Adding dual pipes was well known among budding hot rodders going way back. On 50’s and ’60’s cars it was often the first modification done to a stock car. Not only did it add a bit of power, it also provided the basis for further engine mods. It also established the street cred of the owner. Nothing looks quite as serious as a couple of pencil tip, or slash cut chrome tipped pipes sticking out from under the rear bumper. A pair of glass packs provided the melody, and most of the time that was as far as speed modifications went.
Although both of my Mustangs have aftermarket Cat Back exhaust systems, one is Magna Flow and the other was FlowMaster, I realize that current OEM exhaust systems have a lot of engineering behind their development. They usually provide the best combination of low emissions, quiet sound, and the best performance. Modifying them usually just results in more noise and often poorer performance.
I always wanted a car with duals, but even the big, expensive cars I owned with 4 bbl carbs (63 Cadillac, 64 Imperial) came from the factory with single systems.
Nothing much more to add but I’m built a fresh true dual exhaust for my Cougar out of stainless steel last summer and am a true believer in its benefits considering the car came with a 2” dual into 2” single back to 2” dual from the factory. Kind of like the F bodies it’s a tortured path in my car, only around the fuel tank rather than the frame.
Some people replace the well-designed twin-pipe system on the Honda S2000 with a big single-bore.
A bit anti-social, if one isn’t careful…
A couple of observations.
During the malaise era, Chrysler Canada was the least shy about publishing their net HP figures in brochures. Ford was positively phobic about it. One had to look deeper into other publications to find them. GM was in the middle.
Ford didn’t publish any HP figures in brochures again until about the mid-80s
The 1971 GM and Chrysler sales literature published both, and to this day it’s a useful tool to extrapolate net and gross figures.
In fact, if you do a dot plot around these figures, net is about 70 percent of gross most of the time. However, there are a couple of outliers that are interesting.
Example 1-1971 340 Mopar goes from 275 gross to 235 net, 85 percent of gross.
Example 2, 1971 Ford 351 CJ Q-code goes from 280 gross to 264 net in 1973. 94 percent of gross.
Example 3 1971 Ford Boss 351 330 gross 1972 351 HO (basically same thing except with much lower compression)
275 net. 83 percent of gross. Now in this case, the lowered compression could account for much ot it,
but the 1971 330 figure is arguably under rated to begin with.
My conclusion is that these figures were caused by a combination of a) under rating the gross figure to begin with
(most Mopar guys will tell you a pre-smog 340 made more than 275)
and b) less parasitic intake and exhaust components, such as dual-snorkel or unsilenced air cleaners, and low-restriction exhausts.
I have mentioned before that my parents had a 1976 Ford Elite with a 460. It was a Canada emissions package with dual exhaust and no cats. The only figure I can find is a 202 HP rating, which I presume is the US single exhaust with cat figure.
Ford never took into account the differences of various exhaust configurations (and there were many) , a sharp contrast to the aforementioned early 1970s GM brochures. Whether it was out of laziness or just being tight lipped, I don’t know.
This car felt much stronger than that. Not only would it burn rubber very well, it had amazing kickdown response , especially down into 1st at city speeds. It was able to pin the 120 mph speedo (no telling how fast it really was, most speedos have error) . Stopwatch 0-60 figures consistently came well under 9 seconds , Not only that, it could produce MPG figures (admittedly Imperial gallons) in the 20-21 range at legal speeds (16.6 to 17.4 adjusted for US gallons). I would love to know what the actual HP figure was. It’s obvious it was stronger than what Ford published.
One more curious item re this topic. Looking at the left and middle the attached option sheet for the 1977 MK-V, it states that dual exhaust was required in “noise legislated areas”. This seem counter intuitive. I wonder what the reasoning was.
I don’t have any good sources for later Canadian market specs if they’re not in the brochure — the U.S. trades compiled data from the MVMA specifications, but they don’t account for differences due to different emissions equipment and so forth.
One thing to be cautious with about the 1971 net ratings is that they don’t always directly line up with earlier versions of the same engines, due to changes for emissions. (For example, even setting aside the question of deliberate underrating, the 1970–1971 Chrysler 340 lost a little bit of compression and the slightly warmer manual transmission camshaft.)
There is really not a single formula for converting older (pre-1971) gross ratings to as-installed net output because the older standards for gross power allowed so much variation. For example, it was common to use manual spark advance, allowing the advance to be set for maximum output at any given speed range. So, on one hand, you had some engines that were rated in mostly standard form with the exhaust system and maybe the air cleaner removed, and others that were extensively tweaked for the highest possible rating, regardless of how relevant that was to the as-installed production engine.
On that era big block vehicles the “whoosh” from a full throttle engine pushing all that exhaust through one tail pipe could be fairly loud. So it wouldn’t surprise me if duals actually quieted things down just from that. The other thing is the only thought that 2 must be better than 1 and it does make sense that two mufflers of the same size and construction will have greater sound deadening capability. Note at the time it was not uncommon for the muffler for one of the mufflers (and tail pipe) to be the same ones used on the single exhaust version.
My 2000 GMT400 K2500 has the last of the cast iron Chevy small block in it, 5.7 Vortec. The factory setup is dual cats into one single muffler and exhaust.