Pushrods would seem to be an endangered species these days, at
least as far as most new engine designs are concerned. The only
brand new engines built in recent years with a pushrod valvetrain
have been Chrysler’s 8.0L V10 for the Viper and Dodge Ram, and
General Motors new "LS1" 5.7L small block V8. All the
rest have either been new overhead cam engines or upgraded versions
of older existing pushrod engines.
Overhead cams really aren’t ‘new’ technology. Dual overhead cams
(DOHC) were first used back in 1912 by Peugeot in a Grand Prix
race engine. The first production DOHC engine was a 1500 cc straight
six built by Alfa Romeo in 1928.
Up until the 1950s, the "flathead" design was the most
common type of engine manufactured because placing the valves
in the block was a simple, inexpensive way to make an engine.
The heads on these engines were essentially slabs of cast iron
with no ports, no valvetrain components and no moving parts.
The flathead design was very compact and worked reasonably well
for its time. But the introduction of overhead valve (OHV) pushrod
engines in the 1950s made flatheads obsolete. The new pushrod
engines were more powerful than their flathead counterparts because
the placement of the valves over the combustion chamber allowed
the engine to breathe more efficiently.
Ironically, the same thing happened to pushrod engines when overhead
cam (OHC) engines began to appear in significant numbers in the
1970s and 1980s. The performance advantages of the new OHC engines
made their pushrod counterparts seem obsolete.
Getting rid of the lifters and pushrods reduces the reciprocating
mass of the valvetrain. This enables an OHC engine to rev higher
and produce more power with less risk of valve float. Increasing
the redline on a pushrod engine requires the use of much stiffer
springs, which increases friction, wear and the risk of breakage
in the valvetrain. Roller rockers and lightweight valve spring
retainers help but can be expensive.
An overhead cam arrangement also makes possible the use of more
than two valves per cylinder (three-, four- and five-valve heads). Multivalve heads increase airflow and allow
small displacement engines to produce more power without the added
cost and complexity of turbocharging or supercharging.
For these reasons, the Europeans and Japanese have changed most
of their engines over to overhead cam multivalve designs. But
the domestic manufacturers have been much slower to make the transition,
and will likely continue to build pushrod engines for a variety
of reasons.
One reason is cost. New pushrod engines are still being built
because overhead cam engines, especially ones with more than two
valves per cylinder and/or dual overhead cams (DOHC), are expensive
to manufacture. Double the number of valves and you also double
the number of valve springs, seats, cam followers and hydraulic
lash adjusters. The head itself also becomes much more complex,
and requires additional machining and labor to finish and assemble.
That, in turn, makes OHC heads more expensive to manufacture and
rebuild.
Increasing the complexity of the head and valvetrain also increases
the risk of something going wrong. The more parts in the engine,
the more parts there are to wear out and fail, which can create
major warranty concerns. A typical 24-valve V6 may have more than
1,100 parts in the engine compared to only about 700 in a pushrod
V6. That’s why General Motors says it has no plans to abandon
the pushrod two-valve-per-cylinder engine anytime soon. Most of
the truck engines are all pushrod designs, and most will likely
remain so.
Another reason for keeping pushrods is that a typical multivalve
OHC engine’s power curve is higher up the rpm scale. The increased
valve area of two intake valves provides improved breathing at
higher speeds but reduces air velocity at lower rpms which hurts
low end power and torque. The overall horsepower and torque numbers
are higher than a comparable two-valve-per-cylinder pushrod engine.
But to create them it generally requires more rpms, unless variable
valve timing is used to alter cam timing for improved performance
on both ends of the rpm spectrum (which adds additional cost and
complexity).
Consequently, multivalve engines tend to be geared lower and turn
more rpms at equivalent speeds to maximize their performance capabilities.
This puts more stress on engine parts (especially the valvetrain)
and increases the number of rpms the engine turns for every mile
driven. As a result, multivalve OHC heads tend to be less forgiving
than pushrod heads and require more precise machine work and assembly
to achieve reasonable durability.
Another disadvantage of the OHC multivalve design is the heads
themselves are larger. An overhead cam cylinder head is usually
several inches taller than a pushrod head. This increases overall
engine height and may limit the vehicle manufacturer’s ability
to lower the hood line for improved aerodynamics. Dual overhead
cams add width to a head and make the engine wider (particularly
a V6 or V8). This, in turn, complicates packaging as well as service
and maintenance by making a cramped engine compartment even tighter.
Extra bulk also adds weight, so OHC and especially DOHC engines
are often heavier than their pushrod counterparts. To offset the
added weight of the extra cams and valvetrain components, engineers
may try to compact the engine by reducing the size of individual
components such as valves and springs. OHC heads with bucket style
cam followers and deeply recessed valve springs that can be extremely
difficult to remove with traditional tools and techniques. So
to minimize teardown and reassembly labor, you may have to buy
special valve spring compressors and/or an OHC head disassembly
station, if you haven’t done so already.
Engineers have also tried to save weight by reducing the thickness
of head castings. Thinner castings save precious pounds but also
increase the risk of head cracking and distortion. The cooling
systems on many of these late model, high output, highly stressed
multivalve DOHC engines have little reserve capacity, so if the
radiator is low on coolant, the fan fails or the engine has a
performance problem such as detonation, the engine can overheat
more quickly. This results in a warped or cracked head, head gasket
leaks and/or cam seizure or breakage – which creates an
opportunity for rebuilders who know how to straighten and repair
these heads.
A sure sign of a warped head is a seized or broken cam. If the
cam won’t turn in the head, the head and/or cam are bent. Cam
straightness as well as the alignment of the cam bores should
be measured to identify the problem.
On some engines, cam failure and/or seizure can occur because
of lubrication problems. Upper valvetrain lubrication is extremely
important in all engines, but especially OHC engines because of
the added distance cold oil must flow when the engine is first
started. For this reason, most vehicle manufacturers recommend
a lighter viscosity 5W-30 motor oil for year round driving (especially
during cold weather). If a heavier viscosity oil is used, or the
engine is suffering from low oil pressure because of a worn oil
pump, clogged oil pickup screen, worn main bearings, etc., the
camshaft and valvetrain can suffer during cold starts.
Some Honda engines have an oil return valve located in the block
to keep oil in the head when the engine is shut off so oil pressure
will build more quickly when the engine is restarted. But the
valve can become clogged creating a blockage that prevents the
cam from being properly lubricated.
On 1986-’90 Mazda 1.6L SOHC engines, cam failures due to inadequate
cam lubrication have also been a problem. The camshaft in this
engine has only three journals (even though the cylinder head
has integral bores for five). Direct pressurized lubrication is
only provided to the first two journals. The rear journal is splash
lubricated. Consequently, the rear journal sometimes seizes and
breaks the cam following a cold start during subzero weather.
Some rebuilders prevent this problem from reoccurring by drilling
an extra oil hole to route oil to the back journal.
Head warpage is best corrected by first straightening the head
(hot or cold) to realign the OHC cam bores, then resurfacing the
face of the head as needed to restore flatness. The amount of
metal that can be safely removed on most OHC heads is limited
not only by the thickness of the casting, but also by the timing
and compression changes that occur when the head is refaced.
Resurfacing the head reduces the volume of the combustion chambers,
which may raise compression to the point where detonation becomes
a problem. Resurfacing the head also reduces its installed height,
which retards OHC cam timing. Some aftermarket head gaskets provide
added thickness to compensate for a limited amount of resurfacing.
There are also shims that can be stacked with the head gasket
to compensate for even greater changes caused by resurfacing.
Even so, there’s a limit beyond which the head has to be replaced
if it can’t be straightened or resurfaced without removing excessive
metal.
Thinner heads are also less rigid, so a growing number of engines
including many Honda and Mazda applications as well as Ford’s
4.6L V8 use a new type of "multi-layer steel" (MLS)
head gasket. This type of head gasket requires less head bolt
torque to seal which reduces cylinder bore distortion as well
as stress on the head.
But such a design also requires a much flatter and smoother surface
finish on both the cylinder head and block. In other words, you
have to more or less duplicate the mirror-like factory finish
on the head and block when these engines are rebuilt. The typical
surface finish requirement for an MLS gasket application is 20
to 30 Ra (microinches), which can best be achieved by milling
the head. This compares to a surface finish of 30 to 110 Ra for
cast iron heads (the preferred range is 60 to 100 Ra), or 30 to
60 Ra for aluminum heads (the preferred range is 50 to 60 Ra).
The flatness specifications for these MLS applications typically
allow no more than .002" (.05mm) of total distortion (that
is, block plus the head combined) in any direction.
Another source of trouble for OHC engines can be the timing belt.
The belts on older OHC engines generally have a recommended replacement
interval of 60,000 miles Ð which most motorists forget about
or neglect because of the cost to replace the belt. If the belt
breaks and the engine lacks sufficient valve-to-piston clearance,
the valves will hit the pistons and become bent.
For the above reason, the timing belts in most newer OHC engines
are now made of a more durable material that has a 100,000 mile
replacement interval. Many new OHC engines have done away with
the timing belt altogether and gone to a more durable chain drive
arrangement.
Chrysler’s new all-aluminum 2.7L DOHC V6, which goes into 1998
LH cars, follows the current design trend and employs a three-chain
drive system instead of a timing belt.
Composite cams
Another design innovation that has appeared in recent years on
some Ford OHC engines (4.6L V8 and 2.5L V6) as well as some import
engines (Mazda 2.5L V6) is "composite" camshafts. This
type of cam is made by mounting molded powder metal lobes on a
hollow steel shaft. A rod is then forced through the shaft to
expand it and lock the lobes in place.
The composite cam design offers several advantages. One is that
the design is about 30 to 40% lighter than a conventional one-piece
cast iron or steel camshaft. Another is that the cam is about
1.7 times as rigid as a cast iron camshaft, which reduces flexing
that can lead to cam damage or breakage.
The hardness of the powder metal lobes is also equal to or greater
than that of a hardened steel cam for improved durability. The
use of powder metal lobes which can be precisely molded to near
perfect shape also eliminates or reduces the amount of grinding
needed to finish the cam.
Some unique problems have been reported with composite camshafts.
The Automotive Engine Rebuilders Association (AERA) issued a technical
bulletin (TB 931) in 1992 describing "slipped lobes"
as causing a loss of compression on some Ford 4.6L OHC engines.
The lobes on some camshafts apparently loosen over time and slip
out of position, causing a loss of compression in the affected
cylinders. Since there’s no way to reposition or reattach the
lobes, the camshaft must be replaced.
A similar problem has also been described in AERA tech bulletin
TB 1329 for 1983 to 1986 Toyota Camry 2.0L 2SFE engines. This
engine also uses a composite camshaft with individual lobes mounted
on a steel shaft. A loss of compression results when a lobe slips
out of its normal position. Cam replacement is the cure here,
too.
Though Ford recommends replacing composite cams when an engine
is rebuilt, others say there’s no reason this type of cam can’t
be reused as long as the lobes are not worn, loose or cracked.
Such cams should be carefully measured for wear and inspected
for cracks along the mating surface between the lobes and shaft.
Some rebuilders who have found visible cracks in lobes have reportedly
had difficulty verifying the extent of the cracks with dry magnetic
crack detection equipment (possibly because of the nonmagnetic
ingredients used in some powder metal lobes). One suggestion would
be to use a wet crack detection technique like that used to check
a crankshaft.
According to at least one manufacturer of camshaft grinding equipment,
rebuilders should be able to regrind composite cams the same as
any other cam. If the lobes are worn, the original profile can
be restored by grinding as long as there is sufficient base circle
left in the lobe to do so. The grinding procedure is essentially
the same as that for a steel camshaft (which is harder than a
cast iron camshaft), so a different grade of stone and slower
speed is required.
Regardless of what type of camshaft an OHC engine has (cast iron,
steel or composite), replacement cams should be the same material
as the original. Steel and powder metal followers should only
be used with steel or composite cams, and vice versa.
Variable valve timing
An added feature that’s found on some OHC heads is variable valve
timing (VVT). This is a means of changing basic camshaft timing
and/or lift, duration and overlap to improve engine performance
over a broader range of speeds and operating conditions. At lower
rpm, VVT can be used to advance cam timing to increase low end
torque. At higher rpm, VVT can retard timing or change duration
for added power.
Most of the VVT applications to date have been on high-end import
luxury engines, such as Nissan’s 3.0L V6 in the 300ZX and Infinity
J30, the 4.5L V8 in the Infinity Q45, Mercedes M104 3.0L six in
the 300 SL, M119 5.0L V8 in the 500SL, and 4.2L inline turbodiesel
six in the S-Class, and Honda’s 3.0L V6 in the Accura NSX. For
1998, Lexus has added VVT to its 3.0L V6 and DOHC 32-valve 4.0L
V8. Ford offers it on its Zetec 2.0L engine in the Contour.
Most of these VVT systems use some type of hydraulic mechanism
on the end of the cam to alter cam timing. A computer controlled
solenoid routes oil pressure into the VVT cam mechanism at a predetermined
rpm to change cam timing. Mercedes, though, uses a magnetic clutch
rather than a solenoid for this purpose.
Honda’s "Variable Valve Timing and Electronic Lift Control
System" (VTEC) on the Accura NSX 3.0L DOHC V6 is unique in
that it has extra lobes on the cam and extra rocker arms that
come into play at higher rpm to increase valve lift and duration.
Oil pressure routed through a solenoid pumps up the extra rocker
arm for each pair of valves to change valve lift.
An opportunity here for a specialist might be to rebuild VVT hydraulic
units for other rebuilders. Currently, such parts are only available
from new car dealers. The units are expensive to replace, and
are fairly complex with close tolerances.
Rebuilding OHC heads
Disassembly typically takes longer with OHC heads because the
camshaft has to come out before the valves and springs can be
removed. Most pushrod heads can be taken apart with nothing more
than a C-clamp style spring compressor in about 10 minutes or
less. But a multivalve OHC head can often take up to half an hour
to disassemble, even if you have the right equipment, and require
even more time to reassemble.
Removing the cam (or cams) requires unbolting four or five cam
bearing caps (per cam), or unbolting and removing a separate cam
housing assembly. If the head has cam towers with integral cam
journals that do not come apart, the followers have to be removed
before the cam can be extracted. On some, the valves must all
be held down to pull out the cam.
Once the cam and followers or rockers are out of the way, the
valve springs can be compressed for valve removal. If the springs
are accessible, removal is usually no problem with an ordinary
C-clamp or outside style valve spring compressor. But if the springs
are recessed and have bucket style followers like those found
in various Toyota and Volkswagen engines, accessibility requires
a small footed cup style spring compressor. Reassembling the keepers
is also tricky because of limited accessibility.
Another step that’s necessary when rebuilding OHC multivalve heads
but isn’t necessary with pushrod heads is restoring the cam bores.
If the cam bores are out of alignment, worn or damaged, align
boring or honing is required. Many heads (Mitsubishi, Isuzu, Toyota,
Mazda, VW, Audi, etc.) do not have replaceable cam bearings so
if the cam bores are worn or damaged, the head can be saved by
installing an aftermarket cam with oversized journals (.015"
to .020" are popular oversizes) or machining the bores to
accept full round or half-shell aftermarket bearings.
Installed valve height is another dimension you have to pay close
attention to on OHC heads. Installed valve height affects the
geometry of the valvetrain, so the tips of the valve stems should
be ground to maintain the proper height when the valves and seats
are refaced.
If the stems are not ground to compensate for changes in height
that occur when the valves and seats are refaced, the valves may
sit too high and not allow enough clearance for proper valve adjustment.
The valves may also be subjected to side thrust forces that can
accelerate guide wear.
Concentricity is another dimension that’s important in all types
of heads, but especially multivalve designs. The smaller the valve,
the better concentricity should be to get a good seal and long
valve life. Most late model OHC engines have some type of hydraulic
valve lash adjusters either situated in the cylinder head under
one end of the cam followers, or "mini-adjusters" located
in the rocker arms. If worn or clogged, the adjusters can be expensive
to rebuild or replace. At least one aftermarket company has found
a profitable niche by specializing in rebuilt valve lifters and
cam followers.
Overhead cam multivalve engines are changing the way engines are
rebuilt, but they’ll likely never entirely replace pushrod engines
because of the tradeoffs we have described. To keep on top of
the market today, you have to have the equipment and know-how
to do both types.
