It’s fairly common automotive knowledge that pushrod engines each typically have one camshaft. That camshaft sits in the block, and — via hydraulic lifters — it pushes pushrods up against rocker arms that actuate intake and exhaust valves at the top of the motor. A solid, one-piece camshaft basically means your exhaust lobes and intake lobes are going to be at the same position relative to each other, which means the intake and exhaust valves will open at the same time relative to one another; this isn’t always ideal. And solving that usually means switching to overhead camshafts, which makes a motor taller and can require serious changes to a vehicle’s body. So, starting in 2008, the fully mental Dodge Viper supercar’s pushrod V10 engine featured a fancy camshaft called the “CamInCam,” a technology from German supplier MAHLE. Here’s how it works.
While General Motors was first to market with variable valve timing in a pushrod application, it used a one-piece camshaft with a phaser (a phaser just rotates the whole camshaft relative to the crankshaft (and thus it changes when the valve opens relative to the piston position)) to vary both intake valve and exhaust valve timing simultaneously, and — because it was one solid piece of metal — featured a fixed lobe separation angle.
So, what is lobe separation angle, and why does it matter? The answer to the first part is short and sweet — to open the valves in an engine, each pushrod is pushed by an eccentric camshaft lobe (OK there’s a lifter between the lobe and the pushrod), and lobe separation angle is the distance between the peaks on the intake cam lobe and exhaust cam lobe. Check out the chart below for a visual illustration.
Generally, a narrower lobe separation angle benefits power, but it comes with real-world drawbacks, both when building a production engine and when building a hopped-up engine for your own car. As Hot Rod magazine reports, “Tighter lobe separation results in a rougher idle and loss of idle manifold vacuum, two characteristics that should be minimized (relative to cam duration) to enhance driveability of a street engine.” Oh, and engine drivability isn’t the only concern with a small lobe separation angle — vacuum-boosted power brake performance can also suffer. Renowned performance outfitter Summit Racing notes that “For vehicles with power brakes, low vacuum also makes for a very hard brake pedal.”
Of course, there are also complexities to more lobe separation angle in the real world. Adding a little bit of overlap during certain conditions at low engine speeds essentially leads to a passive form of exhaust gas recirculation, which is a process that uses a small quantity of exhaust gases to dilute the fuel-air mixture in entering the cylinders. Exhaust gas recirculation primarily reduces nitrogen oxide emissions and can also slightly bump gas mileage. The problem is that too much valve overlap at low engine speeds can push some of the unburned air-fuel mixture coming in through the intake valves out of the exhaust valves, and neither catalytic converters nor tailpipe sniffer tests love unburned air-fuel mixture coming down the exhaust manifold. As with any part of engine tuning, overlap is a tricky balancing act.
However, modern technology generally exists to solve such problems, and independently varying intake and/or exhaust valve timing brings the best of both worlds to the party — little overlap at low engine speeds to benefit torque, idle stability, and vacuum, and more overlap at high engine speeds to boost power up top. This is not possible with a solid camshaft, as the lobes are in the same spots relative to one another. This is where the fourth-generation Viper’s CamInCam variable valve timing, a MAHLE product not unlike one offered by British engineering firm Mechadyne, comes in clutch.
First, a little disclosure: The animation in the videos below (one of which is the MAHLE CamInCam and the other of which is DuoCam by Mechadyne) depict overhead cam setups and not cam-in-block pushrod setups like that in the 2008 Dodge Viper, although the principles are the same. With that out of the way, let’s dive into how the Viper’s variable valve timing works.
Instead of a one-piece camshaft design, the fourth-generation Dodge Viper employs a hollow exhaust camshaft riding atop the solid intake camshaft, with total rotation limited by pins on the intake camshaft and slots in the exhaust camshaft. Think of it as two camshafts arranged concentrically, rather than in the parallel configuration typically found in overhead cam engines. Together, they make up one cam assembly that can be slotted right where a one-piece camshaft would normally go. The final piece of the puzzle is an oil-driven phaser on the end of the camshaft assembly then varies the angle of the hollow exhaust camshaft relative to the solid intake camshaft, up to 36 degrees. In theory, this concentric design can move its exhaust camshaft through 45 degrees of rotation, but Chrysler deemed using the full sweep of rotation unnecessary to meet powertrain goals. The result? Variable exhaust camshaft timing in a pushrod engine, and innovation good enough to win an Automotive News PACE Award.
Automotive News sets the stage for why the PACE Award-winning tech came to be in the first place:
In 2004, Chrysler’s Street and Racing Technology (SRT) team faced a challenge to the Dodge Viper. Like many big, high performance engines, this one displayed combustion instability at light loads due to aggressive valve timing for high-speed power, and did not meet EPA standards.
Variable valve timing (VVT) would mean a new, taller, overhead cam engine, hence investment, delay, and styling changes. However, a never-implemented 1908 patent described an intake camshaft mounted inside the exhaust camshaft, so intake and exhaust timing could be advanced or retarded independently. The UK engineering firm, Mechadyne, had introduced the concept to Chrysler and built a prototype, which was too complex.
The story goes on to say that MAHLE, already a Chrysler partner, could use its existing know-how to offer 40 degrees of intake-exhaust variation using “an inner shaft that allowed the CamInCam® VVT camshaft to fit into the space of a single standard one.” Per the story, the results of the changes to Dodge’s V10 engine were increases in combustion stability, better fuel economy, better exhaust gas recirculation performance, and more power. “Working together, MAHLE, INA, and Chrysler accomplished what had not been done in the century since the first Cam-In-Cam patent in 1908: independent timing of exhaust and intake valves in one camshaft.”
So yeah, thanks to the novel camshaft design combined with a slight displacement bump from 8.3 to 8.4 liters, the fourth-generation Viper V10 picked up 90 horsepower over the third-generation motor and made peak power 500 RPM higher in the rev range. Peak torque saw a smaller gain of 25 lb.-ft., but the bigger story is that drivability characteristics were maintained, even with the wild bump in peak power. The idle in a fourth-generation Viper is remarkably smooth, with none of the rowdy chop you’d normally expect in a peaky pushrod engine.
It’s worth noting for the sake of clarity that the Viper’s variable valve timing doesn’t effect valve lift like Honda’s VTEC or BMW’s Valvetronic, meaning the Viper engine’s valves open the same amount regardless of engine speed. However, it was the first application of independently variable exhaust valve timing on a pushrod engine, an impressive feat that’s wonderfully anachronistic considering the Viper’s reputation of being fast but somewhat crude.
Weirdly, the overhead valve cam-in-cam system found in the Viper seems to largely be a technological dead-end for now. Ford’s Godzilla V8, GM’s small-block engine family, and Stellantis’ Hemi V8 all use simple cam phasing with fixed overlap, just like the pushrod 3.5-liter V6 found in mid-aughts Chevrolet Malibus. However, Mechadyne still has a concentric cam-in-cam setup, now called DuoCam, waiting in the wings. Perhaps some ultimate cam-in-block V8 can take this tech down from the shelf and dust it off?
(Photo credits: Dodge, Chevrolet, Mahle — also, you can read more about this technology in this SAE paper)
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