In a spring day in 1966 a brand new Chevrolet Impala sedan showed up in our neighbor’s driveway. I went to check it out, and the first thing I noticed as I approached it from the passenger side rear was an exhaust outlet on that side, which meant dual exhausts. Wow; did he get a 427? This mild-mannered retired guy? But when I got to the front fender, there was no distinctive “427 Turbo-Jet” badge. What gives? A 283 with dual exhaust? That makes no sense?
The owner saw me coming, and happily obliged my request to look under the hood. Sure enough, it was a 283, but with 220 horsepower stickers on the air cleaner and valve covers instead of the usual 195. But why did it have dual exhausts when the much more popular 327 and even the 396 came with a single exhaust?
I can’t really answer that, but a little digging brought home just how effective dual exhausts were in increasing actual (net) horsepower, on this and some other engines too.
I had long committed to memory all the available engines on new cars ever since I discovered brochures at dealers when I was about nine or so. Especially when it came to Chevys; I had them down pat, even the bore, stroke and compression rations. So it’s not that I hadn’t noticed that Chevrolet had added this 220 hp version of the 283 for 1966, although I had already wondered at the time why. Seemed like the gap between the standard 195 hp 283 and the 327 wasn’t exactly all that wide.
Well, that had been the case in previous years since the 327 arrived in 1962, when there had been both 250 and 300 hp versions. But for 1966, only a 275 hp version was now available, undoubtedly to make a more even gap between the 283 and the 325 hp 396. So it makes some sense in that regard, but the 283/220 was gone again in 1967, never to return. I suspect not too many folks ordered it, although it cost only $37; the 275 hp 327 cost $93.
I had failed to read or memorize the fine print on these engines. If I had, I would have noticed that the 283/220 came with dual exhausts, and the 327/275 as well as the 396/325 didn’t. Curious, especially to a thirteen year-old kid. It just didn’t make sense.
Before we go any further, I should note that this four-barrel, dual exhaust L77 220 hp version of the 283 had also been available in 1964 in both the Chevy II and the Chevelle. In fact it was the top option on both that year, although three versions of the 327 (250, 300, 365 hp) were added to the ’64 Chevelle mid-year, to counter the popular GTO and other new mid-sized muscle cars. Also curiously, the 283/220 was dropped in both those lines for 1965 but then returned for 1966, also just for one year.
I was utterly unable to find an image on the web of an original 283/220 installed. But you all know what a 283 looks like, right? Here’s a two barrel version.
Supposedly this is an L77 283/220 in a ’66 Chevelle. Note that the exhaust manifolds on the Chevelle are different, not the “ram horns” on the full size application, presumably due to packaging constraints. The stock carb was a Carter four barrel, essentially the same as used on the 250 and 275 hp 327s.
Regarding my headline, about the power of dual exhausts. Chevrolet used to list both gross and net (as installed) hp ratings for their cars in the AMA spec sheets, except for the higher output versions. A closer look at that somewhat blurry chart shows that the net hp increase for the 283/220 over the 283/195 was 35 hp (150 to 185), ten more than the 25 hp increase in gross hp (195 to 220). How does that work?
And notice that the net hp of the 283/220 (185) is an unusually high 84% of the gross hp, whereas the same ratio on the 283/195, 327 and 396 are all between 75% and 77%.
Gross hp ratings were derived by removing the exhaust system and running with open pipes, as well as removing other power-robbing ancillaries, air cleaner, any smog devices and advancing ignition advance as much as possible. It measured the theoretical potential of the engine, essentially set up as in a race car. Net hp ratings are “as installed” with all the ancillaries, air cleaner, stock ignition advance and a full stock exhaust system.
So the unusually high net hp percentage of the 283/220 in comparison to the others is then solely due to the installation of the dual exhaust system. No wonder putting on duals was such a popular modification back in the day. Old Hot Rod magazines in the ’50s were chock-full of ads for dual exhaust systems.
It seems safe to say that duals gave a bigger kick for the buck than just adding a four barrel carb and intake; in fact, as the 327 shows with its 210 net hp, only 25 more than the 283/220, it didn’t help all that much, despite the additional 44 cubic inches.
Here’s another graphic example, from the 1971 Plymouth Fury brochure. Look at the two versions of the 383: although the advertised gross hp increases 25 hp—from 275 to 300—with the addition of a four barrel carb and dual exhausts, the net hp jumps a whopping 60 hp, from 190 to 250! That’s a jump from 69% of gross to 83%, thanks to the dual exhausts. Part of that may be due to these gross numbers not being 100% reliable, but there’s absolutely no doubt that these jumps when dual exhausts are also tested in the “as installed” SAE net hp tests, a significant improvement results.
This also explains why Cadillac’s big V8s suffered a disproportionately big hit when they switched to net hp ratings. In 1971 they still listed both, with the 472 having 345 gross/220 net, and the 500 inch Eldorado V8 having 365 gross/235 net hp. In both cases, the net is only 64% of the gross. Cadillac had switched to single exhausts some years back, which of course was not reflected in their previous gross hp ratings. There may have been other factors too, but undoubtedly the single exhaust reduced net output disproportionately.
What does all of this mean in the real world? Dual exhausts made a more significant impact on actual power and performance (and fuel economy) than might be expected, or just the switch to a four barrel carb. With a single exhaust, the potential of a four barrel carb to deliver more air and fuel was inherently restricted, in some cases considerably so.
Related reading:
How Many “Real” Horsepower Did The 1971 426 Hemi Really Make? A Look At The Gross vs. Net HP Games
Ate Up With Motor: Understanding Gross Versus Net Horsepower Ratings
Automotive History: 1957 Chevrolet Fuel-Injected 283 V8 – Ahead Of Its Time And The Competition
Very interesting information. Having grown up in the “dual exhaust era” of the fifties, there is much to be said for this modification. Thanks for giving us technical information/enlightenment. (VWOOM, VWOOM!)
This all makes me wonder about modern dual exhausts. My ’88 T-Bird, the ’97 Grand Prix GTP, the 2016 Civic EX-T, and even my 2019 Mazda CX-5 have (‘had’ in the case of the older cars) what appears to be “Dual Exhaust”.
But how much better is it than a single pipe out the back like on my 2007 V6 Mustang?
In the case of the 5.0 T-Bird and 3.8L Supercharged engines, the headers/manifolds come off of the engine into two small cats. Then they join under the car into one larger cat. Then behind the cat, they split into “duals”. Wouldn’t the merge point also be the choke point?
And what of the 4-cylinder engines in the two Japanese examples I cite? What advantage is dual exhaust here? You have one manifold/header, which at that point only to splits to duals after the catalytic converter?
Perhaps it’s still helpful in reducing back pressure.
Great Piece Paul! – I love the ’66 Chevy example. I am quite sure my Dad’s ’66 Impala back in the day only had the 195hp single exhaust 2-barrel version. He only started buying the faster trim level cars when he got older. 😂
Seems that many of today’s “duals” are simply a rear muffler with two outlets, not a system from the manifold on down. Does this setup really offer any performance improvements or are they mostly cosmetic?
Modern systems are carefully designed to perform well, since the net hp ratings and EPA numbers are generated with “as installed” exhaust. Of course they can be improved for maximum power but with significantly greater noise, as we hear all-too often on the streets thanks to “fart cans”. But yes, it’s very typical to split the exhaust after the cat for appearance.
This was interesting. I always knew that adding a 4 bbl and duals would wake up an engine, but had never thought about their respective contributions.
A family friend had a 1965 Ford Galaxie with a 352 and factory dual exhaust. I recall him complaining loudly about having to replace the system at double the cost every two years or so. He claimed that a dual system wore out faster than a single because it didn’t warm up as quickly and thoroughly as a single, leaving moisture to rust out the system. Back then everyone was regularly replacing muffler and pipes, with shops like Rayco and Meineke everywhere. Things have really improved. I haven’t replaced muffler since 1982 and the muffler shops have all but vanished. Another example of the canard that “they don’t build them like they used too”.
I can attest to the change in performance (for the better) when I had duals installed on my 69 Caprice with the 396/4bbl. The original single exhaust had such a small diameter, it looked sad! Basically exhaling through a drinking straw.
I also replaced the snorkel air cleaner (itself quite restrictive) with an open-element one. I don’t drive it in the winter so I don’t need the heating aspects for cold weather performance. Relax – I kept the original air cleaner so it can be returned to stock.
I have to say that the 396 is a great engine. It drives so effortlessly and is well-suited to the full-size Chevrolets.
A decent exhaust flow works great dual pipes can achieve that, but so can a bigger single system with the factory restriction like cats removed though these Chevrolet engines predate cats anyway,
More carby will give a bump in power but that extra air has to get out too.
Now my 68 Cougar, with J code 302-4V, came with dual exhaust. After that exhaust didn’t matter what with company cars and then Japanese imports. However, when I bought the 67 Parklane in 2003 one of the first things for the car was to put duals on for the 410. At 145,000 miles hard to say if I noticed anything. In 2010, when I got the Polara, I put dual exhaust on the 360. I have yet to do anything on the 360 in the Ambassador. Maybe yes, maybe no. As for the 410 it will be interesting to see how the 410 runs as a newly rebuilt engine if I can ever get a hydraulic camshaft.
Regarding the note about the big-block 396 with single exhaust…
It is my understanding/recollection that if you wanted a fast big-block car, you’d order a 427. The 396 was your choice if you wanted to tow your boat or camper. The folks who wanted to tow weren’t interested in the raucous note of dual exhausts, they just wanted the power.
It’s not quite as simple as that. Prior to 1966, the 300hp 327, a common choice, came with dual exhausts. As did quite a few other big cars with 4-barrel medium-sized engines. And they weren’t “raucous” unless they used low-restriction mufflers, which were only used on the true hi-po engines.
And if you really did want the full power of a 396 or such, you weren’t going to get it with a single exhaust. That’s the whole point of this post.
And that 325hp 396 did come with dual exhausts standard in the Chevelle, Camaro and Nova.
It also explains why the 396 didn’t do all that well in some full-size comparisons with the Ford 390 and Chrysler 383 4-barrels, as those did come with duals.
I had a 2003 Honda Civic Hybrid with a very strange breathing setup. Instead of having 16 valves and a spark plug in the center of the valves, it had only 8 valves and 2 spark plugs per cylinder where the other 2 valves would have been. The intake valve would induce swirl to better mix the fuel/air mixture as it entered the cylinder. The 2 spark plugs would fire sequentially or simultaneously depending on engine speed to make sure to completely burn the entire charge.
It had a comparatively low red line from other high revving Honda engines and you could feel it choke on its own exhaust up near the red line because it couldn’t breathe. Unfortunately, the size and shape of the combustion chamber limited the size of the intake and exhaust valves, so it was always a bit compromised on breathing, higher in the rev range. Dual exhaust would have done nothing because it couldn’t physically eject the exhaust fast enough from the combustion chambers.
(When it was time to change the spark plugs, got into an argument with the mechanic about only replacing half of them. “Nope, I want all 8 replaced at the same time!” He was having a hard time understanding why a 4 cylinder engine would have 8 spark plugs and 8 individual coil packs.)
Engines used in hybrids typically are designed for a quite different range of operating parameters. And yes, changing the exhaust would have been useless. Modern exhaust systems are very carefully optimized.