[ SAE International Tech Newsletter ]
Viper V10 introduces cam-in-cam variable valve timing
Although it looks like one camshaft with a phaser at the left end, it's
really a hollow tube-type shaft with a second, solid shaft through it, with
the phaser connected to both. The intake cam lobe assembly (A) has a pin that
goes through a slot in the hollow camshaft and is pressed into the solid cam.
The exhaust cam lobe assembly (B) is pressed into position on the hollow
The 2007 Dodge Viper SRT-10 is attracting headlines for its new V10 engine,
which is rated at 600 hp (450 kW) at 6100 rpm, a huge increase from the
previous 510 hp (380 kW) at 5600 rpm. Yes, the engine now has twin throttle
bodies, a new deep-breathing intake system with larger intake valves, and more
aggressive cam lobe profiles.
More significant, however, is that it is also the first production
pushrod-valve engine with truly variable valve timing (VVT) -- on the exhaust
valves. Using a phaser on a camshaft-within-a-camshaft design (Mahle's
CamInCam), the V10 can change exhaust valve timing up to 45* vs intake valve
timing, although only 36* actually are used.
This compares with General Motors' simpler VCT (variable cam timing) on 3.5-
and 3.9-L V6 engines, in which a phaser on a single solid shaft changes timing
equally on both intake and exhaust lobes.
The Viper design, developed with Mechadyne International, provides the
advantage of VVT while retaining the compact size of a pushrod-type,
cam-in-block engine. Chrysler Group did look at a dual overhead-cam
arrangement to advance the Viper V10 performance, but it simply would not fit.
Exhaust-valve control was chosen over intake to provide the valve timing
variability needed to improve the idle and part-throttle combustion stability,
normally an issue with engines whose camshafts are tailored for high-end
power. The performance camshafts produce high exhaust-gas dilution of the
intake charge at low rpm/light loads. This results in unstable combustion,
which can affect idle quality, emissions, and the ability of the engine
computer to detect misfire, which is a federal emissions requirement.
Intake valve control was not chosen because its prime benefit would be
improved low-end torque, something the 8.4-L V10 has plenty of, although the
560-lb/ft (760-N/m) peak is at 5000 rpm. Intake control would simply
"translate to more tire smoke on a Viper," explained Chrysler SRT engineer
Although technology exists for both intake and exhaust VVT with the pushrod
cam-in-block, it would require new designs both for the camshaft and phaser,
deemed an unnecessary complexity and cost at this time. The exhaust-only VVT
design enables use of a modified Mahle hollow camshaft and a reworked
off-the-shelf cam phaser by INA.
The intake valve cam lobe assemblies are slid onto the hollow exhaust camshaft
and positioned above the slots, in a build sequence that permits exhaust lobes
to be pressed into position on the hollow shaft. Locking pins are driven
through the intake cam lobe assemblies. They pass through the slots in the
hollow shaft and are pressed into the pin holes in the solid shaft, locking
the intake lobes to the solid shaft. When the solid shaft is turned, the
phaser can move the hollow exhaust camshaft between advance and retard
positions vs. the solid camshaft, within the range allowed by the slots and
the solid pins that go through them.
The 500-rpm increase for peak power was enabled by a number of changes that
reduce valvetrain weight, including new valvetrain geometry. Although the
intake valves are larger in diameter for improved breathing, the stems are
hollow and shorter. The exhaust valve stems also are shorter.
Although the Viper engine displacement was increased from 8.3 to 8.4 L and the
bore was increased from 102.3 to 103 mm (4.03 to 4.06 in), the objective was
not the tiny increase in swept volume. The higher engine performance called
for stronger reciprocating parts, particularly the connecting rod. With just
the small bore change, the SRT V10 could take the already-available rod and
piston assembly (with a floating wrist pin) from the 6.1-L SRT Hemi V8.
The V10 cooling system was upgraded with reshaped chambers for better flow,
and two new two-speed electric fans that provide more airflow replaced the
Although the Viper SRT V10 VVT valvetrain now is the technology leader for
cam-in-block engines, it's known that GM has a small-block V8 in development
with variable intake- and exhaust-valve timing in a three-valve configuration.
We may not see a four-valve pushrod gasoline engine, despite use of that
valvetrain in diesel V8s. However, cam-in-block pushrod gasoline engines have
a high-tech, high-performance future, and both Chrysler Group and GM keep
engineering teams on the job of developing it.