Having
recently glanced under the skin of Mopar’s pushrod Hemi V8, it
seems proper to do the same with GM’s overhead-valve (OHV) small-block engines. We’ll take a look at the LS2 6.0L and
L92 6.2L engines, both of which are relative newcomers to the line-up and are considered to be part of the GenIV family
(the better-known LS1 comes from the GenIII series).
This brings us to the first point – if there’s anything that
GM did seriously wrong with these engines, it’s the lack of a proper brand name. “Hemi” rolls off the tongue
for any of the varied characters in Chrysler’s ad campaigns, where as GM’s 3-character SPID designations come naturally
only for hardened fans of the General. Sure, there’s the Vortec name that’s been weakly marketed in the truck lineup for
about 15 years now, but does the average person comprehend that the 5.3L V8 under the hood of his Tahoe is related to
the LS7 in the Corvette Z06? Even so, GM’s modern take on the small-block V8 is one of the most widely-used engines in
the United States.
I think that GM does a great job on its cutaway displays, but I really wish they wouldn’t paint every damn part in
the same shade of silver – it makes it a bit tough to describe things. Regardless, we’ll do our best.
Let’s
start off with one of the newest additions to the GM V8 family – the L92 engine that will find its way under the hood
of the 2007 Cadillac Escalade.
I really wish I could roll this engine up to the
others that we’ve examined in this series for the purpose of showing just how compact of a package that GM has created.
There are a fair number of OHC V6 engines out there that are larger than the GenIII/GenIV engines, and
the variety of vehicles that make use of these pushrod V8s are testament to their compactness.
On the valve
covers are the coil packs. Where as most modern engines use a coil-on-plug (COP) design that completely eliminates
high-tension spark plug wires, the plug location of a wedge head doesn’t allow such a setup. If this display had plug
wires installed, you’d see them running down just ahead – to the left – of each exhaust manifold primary
tube.
Speaking of the exhaust manifold, this is definitely a decent piece of engineering. A true 4-1 long-tube
design would likely improve peak power and broaden the powerband a bit, but this is a vast improvement over older,
log-style manifolds.
We’ve spent a lot of time talking about intake
manifolds during these engine posts, and for good reason – port fuel injection has allowed the development of some
pretty radical intake geometry, and as such, intake manifolds may have overtaken cylinder heads as the most critical
contributor to an engine’s powerband.
Like we’ve seen on most other engines that are designed for something
just short of all-out top-end performance, here we find the barrel-shaped long-runner manifold. There’s plenty of
plenum volume, and a not-outrageously-large runner cross-section. In other words, this is exactly what we’d expect to
see on an engine designed to have a broad powerband that’s biased somewhat towards the lower end.
While the L92
makes use of GM’s Displacement on Demand (DoD) technology, it’s not something that lends itself well to static display.
We can see the lifters in the valley, but not much else – you’ll just have to go on faith that it’s really in there. The
toothed gear at the rear of the crank, ahead of the last crank journal, is the reluctor for the crank position sensor (a
piece of technology that keeps on showing up in these cutaways), and we can also see the spark plug at the lower left of
the photo. Note it’s aimed directly at the center of the chamber, despite the fact that it’s coming in from the side
(unlike the Hemi, which points its pair of plugs straight down into the chamber).
If there’s port geometry, I’m going
to comment on it. Compare this intake port to that of the Hemi, and it would seem obvious that the Hemi’s
straighter shot into the cylinder would offer better performance. Ah, but that’s not the case. GM has developed wedge
head port geometry to an art form, and has the intake charge turning corners and sliding over the back of the intake
valve as if it had a straight shot into the cylinder. Despite having dramatically different port geometry, the GM and
Chrysler approaches basically yield similar (and excellent) results.
Think of the L92 head as a slightly smaller
version of the LS7 part (the smaller bore of the 6.2L requires the use of smaller valves), and that gives some insight
into what kind of performance can be expected. Only three or four years ago, this level of output gave Corvette
fans reason to drool, and now it’s being dropped into an SUV. I’m not sure if that qualifies as progress, but it’s at
least something to ponder.
It should be noted that there’s a price to pay for the GenIII/IV’s head flow, and
that’s a head bolt. These engines provide four bolts around each cylinder, while the original small-block and the LT1
GenII derivative have five bolts around each cylinder. Unfortunately, that fifth bolt got in the way of the intake
runner, and so something had to go. This isn’t a problem until the engine is modified well beyond reason.
Note that the
“skirt” of the block drops down well below the centerline of the crankshaft, and supports the main bearing
caps via crossbolts. This is a very solid bottom end. That beefy oilpan adds a bit of extra structure as well.
To
the lower left of this shot, we see the valvetrain – note the beehive springs and the use of cast-aluminum rockers that
ride on needle bearings and actuate the valves with a roller tip. Ten years ago, those would have been considered
exotic features. At the middle of the right edge lies the electronically-actuated throttle body.
There’s the
actuator for the variable valve timing (VVT). While installing the VVT system on a single-cam OHV engine doesn’t offer
up any adjustability of intake/exhaust overlap timing, it should be noted that simply advancing or retarding the
overall valve timing can have significant effects on an engine’s powerband, as hot-rodders have known for decades.
Typically speaking, advancing the cam timing offers more low-end power, while upper-end performance benefits from
retarding camshaft timing. I’m sure that GM employs this system in other ways as well, such as to improve cold-start
emissions.
And now we move on to the 6.0L LS2. This is an interesting engine, for
despite the many differences between it and the L92, they both generate peak torque at the same RPM, with the LS2
having a horsepower peak about 300 RPM higher in the rev range. Ah, but there’s way more than just peak numbers, and
what these single points on the graph don’t tell us is what happens before and after the peaks on the graph, or what
happens at part throttle. Unfortunately, GM has yet to provide dyno plots for the L92, so we’ll have to revisit this
issue at a later date.
Note the different intake used on the LS2 –
it’s lower, with shorter runners. That helps not only with packaging this engine under the ultra-low hood of the
Corvette, but it also aids in moving the powerband slightly higher in the rev range.
The intake port geometry
on the LS2 is derived from that of the LS6, which means that it was basically the benchmark for pushrod V8 airflow
until the LS7 came along. A whole lot of additional peak HP awaits those brave enough to perform a camshaft swap,
but at the price of reduced low-end power and drivability.
Here’s the same beefy lower end. Note that
there’s just a simple gear hanging off the camshaft – no VVT is employed on this engine.
To no one’s surprise,
this looks a lot like the rear of the L92. The LS2 does not make use of DoD, however; it’s definitely not needed in the
light and tall-geared Corvette, but the CTS-V and GTO would likely benefit from the cylinder deactivation
scheme.
So there we go. Which is better, you ask – the Hemi, or the GenIV? Let’s put it this way – I’d be
glad to have either powerplant under the hood. Both of these engines produce an impressive combination of power, compact
packaging, and fuel economy; they just take slightly different paths.