Chapter 6
Performance Camshaft Considerations
A camshaft upgrade is a long-established
method of increasing the horsepower and/or torque output of an engine.
A camshaft with higher lift and longer duration than a stock camshaft
can enable more air to be pushed through the engine, because it forces
the valves to stay open longer, which allows more air flow through the
heads. When matched with complementing air flow and spark-timing adjustments,
a camshaft can provide a significant performance improvement for a comparatively
modest investment.
On late-model vehicles, however, changing
a camshaft requires a tuning adjustment to compensate for the engine’s
new operating parameters, for which the engine computer wasn’t programmed.
The primary issue is the difference in vacuum created by a replacement
camshaft; this typically means a significant reduction in vacuum at
idle—up to 5 in—compared with the stock configuration of 19-20 in
at idle on a 5.7L Hemi.
As mentioned throughout this book,
the Hemi’s computer and speed-density air-metering system work against
such modifications. And, as this book was written, there was no software
available to the public or to most professional tuners to reprogram
the factory computer.
Still, enterprising enthusiasts are
making do, although compromises in driveability and/or maximum performance
are the reality. Reluctance on DaimlerChrysler’s part to release the
programming tables on the Hemi’s computer has caused consternation
on the part of aftermarket manufacturers and tuners, but several companies
have worked to overcome these issues. In a nutshell, a camshaft swap
should be accompanied by a controller upgrade that changes the fuel
curve to match the lower-vacuum properties of the new camshaft; this
enables good driveability and a smooth idle.
Several manufacturers already offer
higher-lift performance camshafts for the Hemi engine, including COMP
Cams (www.compcamshafts.com), which offers three distinct part numbers
for the 5.7L Hemi. (Installation and testing of a COMP XE 268 is outlined
in this chapter.) Another outfit offering camshafts for the Hemi is
KRC Performance (www.krcperformance.com), which offers several custom
grinds for a variety of performance niches.
Swap Tips
Fortunately, replacing the Hemi’s
hydraulic camshaft can be accomplished without removing the cylinder
heads. By removing the rockers and pushrods, holding the valves in place,
and rotating the engine, the camshaft can be pulled out without interference.
(The lifters, however, are located beneath the cylinder heads and would
require the heads’ removal for their replacement.)
Replacing the camshaft—at least with
one of COMP’s units—will also require an upgrade to stiffer valve-springs
that can support the higher lift of the aftermarket camshaft—the stock
units would simply bind up and fail otherwise. The stock beehive-type
5.7L springs have spring rates of only 300 lbs and 100 lbs at 1.83-in
installed height. Like the camshaft, replacing the springs can be accomplished
without removing the heads; compressed air and an overhead-valve compressor
enable the procedure by holding the valves in place while the retainers
and keepers are removed.
On the subject of valve clearance,
COMP Cams noted a possible interference issue early on with the 5.7L’s
slightly domed piston, which didn’t have any valve-relief provisions.
COMP responded by changing the advance and using a wider lobe separation.
In their words: “Be very careful
with tighter lobe separations, more advance or larger durations if making
a custom grind. Both the 112-501-11 [XE268] and 112-501-11 [XE273] run
as tight of piston-to-valve clearance as should be [used] with stock
pistons on [the] 5.7L.”
But while the physical swap of the
camshaft and springs is comparatively simple, it’s the in-vehicle
operation that becomes the sticking point. COMP Cams advises computer
upgrades for optimal performance and driveability, as does every other
installer interviewed for this book. The problem, of course, is that
such tuning upgrades aren’t widely available.
To a certain degree, the factory computer
will “learn” the new camshaft, but as the experience outlined in
the camshaft installation section reveals, this is by no means a perfect
scenario. Companies such as KRC Performance and Michigan-based B&G
Performance (www.bgperformance.com) offer control-module upgrades, but
they are performed in-house and require the vehicle owner to remove
the powertrain control module and send it to the vendor. This, unfortunately,
means down time for one’s vehicle. Also, such controller adjustments
can cost $400—$500.
KRC Performance has worked with COMP
Cams and Superchips to develop appropriate fuel-curve programming for
Comp's trio of Hemi camshafts and KRC’s custom grinds. Superchips
holds these programs and offers them for download on a Superchips Flashpaq
handheld programmer (see Chapter 9), allowing the user to upload the
appropriate camshaft calibration without removing the factory controller
and sending it out for modification. By far, this is the easiest method
of getting the best “tune” for a new camshaft—the caveat is that
the customer has to use one of Comps’ or KRC’s camshafts (see sidebar
“KRC’s Camshafts”).
Captions
06-01
Companies like COMPCams have developed
higher-lift, replacement camshafts for the 5.7L Hemi, with camshafts
for the 6.1L engine in development as this book went to press. In general,
aftermarket camshafts offer noticeable performance improvements, but
the lack of tuning for the Hemi ultimately means compromised driveability
in many cases.
06-24
KRC Performance offers several custom-grind
camshafts for Hemi engines, including mild-lift versions that can squeak
by without computer upgrades to “monster” camshafts that require
stand-alone controllers.
Camshaft Terms
Advance/retard: The difference in crankshaft
degrees between the intake-lobe centerline and the exhaust-lobe centerline.
Base circle: The base circle of the
camshaft lobe, where no lift occurs.
Centerline (or lobe centerline): The
maximum point of lift of the intake or the exhaust lobe, measured in
crankshaft degrees.
Duration: The number of crankshaft
degrees that a valve will be off the seat.
Duration at 0.050 in: The number of
crankshaft degrees a valve will be open at 0.050-in, or more, lift.
Lift (gross valve lift): The total
lift of the valve, determined by multiplying the camshaft lobe lift
rate by the rocker-arm ratio.
Lobe separation angle: The difference
between the intake-lobe centerline and the exhaust-lobe centerline.
Rocker-arm ratio: The output ratio
of the rocker arm measured against the input of the camshaft lobe lift
rate.
KRC Performance’s Marty Fletcher
on Hemi Camshafts and Tuning
As evidenced by the real-world experience
of Jared Martin and his modified Ram, aftermarket camshafts such as
those from COMP Cams, or even Fletcher’s own KRC Performance, can
significantly increase performance, but will compromise driveability
without proper tuning.
“Vacuum at idle goes way down with
one of these higher-lift camshafts,” said Fletcher. “Because of
this, the computer assumes the throttle is open, so it adds a ton of
fuel to compensate and the result is poor driveability and erratic idle.”
According to Fletcher, a stock Hemi
pulls 19-20 in of vacuum at idle, but running an aftermarket camshaft
can result in as little as 13 in of vacuum at idle. This not only causes
idle and driveability problems, but also can affect braking performance.
Fletcher overcomes the driveability issue by flashing the factory computer
with new programming that re-maps the fuel curve.
“Because the factory computer doesn’t
recognize the new camshaft, it keeps trying to adjust fuel for what
it thinks is the original camshaft,” he said. “We re-map the fuel
so that it matches the camshaft; this cures the idle problem.”
But not immediately; Fletcher warns
that even with the re-mapped fuel curve, the computer still needs to
learn the performance of the engine with the new camshaft. This requires,
perhaps, up to a dozen or more operating cycles.
“The biggest mistake people make
is reinstalling the re-flashed computer and expecting everything to
be perfect from the get-go,” he said. “It takes a couple of days
of repeated running and stopping cycles for the computer to adapt—after
that, driveability is excellent.”
Fletcher has worked with COMP Cams
and Superchips on appropriate tuning programs for the camshafts offered
by COMP and KRC. Currently, customers with Superchips Flashpaq handheld
programmers can—for a fee—download from Superchips’ web site (www.superchips.com)
the appropriate program for Comp’s or KRC’s camshafts and upload
it to their vehicle. Or, KRC can flash the computer at its Florida shop.
KRC’S CAMSHAFTS
KRC Performance offers five performance
camshafts for Hemi engines, each based on the specific grind specs determined
by KRC’s Marty Fletcher. Three of them are designed to work—with
proper tuning—with the stock computer and provide uncompromised driveability.
Here’s a look at them (specifications based on 0.050-lift duration
and 1.6-ratio rocker arms):
KRC 210—A relatively mild “RV”
camshaft aimed at trucks and SUVs, with 0.210/0.214 lift and 523/526
duration specs on a 112-degree centerline. This camshaft works with
the stock valvesprings and makes 18 in of vacuum, meaning a computer
upgrade is probably not necessary. KRC claims this camshaft is good
for 35 hp and 25 ft-lbs of torque.
KRC 218—A performance camshaft designed
for the street, with 0.218/0.222 lift and 530/533 duration on a 112-degree
centerline. Fletcher says this camshaft has a noticeable lope to the
idle, works best with headers, and is rated at 50 additional hp. Programming
is suggested.
KRC 220—Designed for street and strip,
Fletcher says this camshaft comes on strong between 2,800 and 5,800
rpm and can help a Ram put down more than 300 rear-wheel hp. Lift is
0.220/0.222 and duration is 547/557, all on a 114-degree centerline.
Stronger beehive valvesprings are required and tuning is a must. Headers
are recommended.
KRC 236—Designed for racing and compatible
with stand-alone control systems, this 0.236/0.240 lift and 566/573
camshaft (114-degree centerline) works best with high-compression, naturally
aspirated engine combinations.
KRC Turbo—Developed specifically
for the boost dynamics of single-turbo combinations, this camshaft is
compatible with the factory computer or a FAST stand-alone system. It
requires stronger beehive springs and has -4 degrees of overlap. Lift
is 0.228/0.224 and duration is 554/547, on a 115 +4 degree centerline.
KRC Performance has developed fuel
maps for all of its camshafts and they can be uploaded via a Superchips
Flashpaq programmer by downloading the program from Superchips’ web
site.