The Confusion Factor - A Collection of Misunderstood Ideas and Terms   By B. Rawls

 

 

Camshaft Overlap and Compression- A very common idea, although for the most part incorrect, is that overlap bleeds off compression. Overlap, by itself, does not bleed off compression. Overlap is the angle between the exhaust closing and intake opening and is used to tune the exhaust's ability draw in additional intake charge as well as tuning idle vacuum and controlling power band width. Cylinder pressure is generated after the intake valve has closed, through the ignition process, and before the exhaust opens; in other words the compression cycle and ignition cycle of the 4 cycle or 4 stroke engine.  Within practical limits, an early intake closing and late exhaust opening will maintain the highest cylinder pressure. Overlap can be increased by narrowing the Lobe Separation Angle while holding the intake and exhaust lobe duration constant. In doing so, the cylinder pressure or dynamic compression can actually increase as the earlier intake closing pairs up with the delayed exhaust opening. Overlap can also be increasing by widening intake and exhaust lobe duration while the Lobe Separation Angle is held constant. This decreases cylinder pressure or bleeds off compression.  In both scenarios the overlap was increased, but the outcome differs as the intake closing and exhaust opening relationships change.  The true culprit that bleeds off compression is the whole collection valve events.

 

Exhaust System Diameter and Engine Horsepower- A popular idea is to select/size the exhaust system components to the engine's horsepower output. This idea typically attributes a header diameter or an exhaust system diameter to a particular horsepower level. To resolve this, look at how an engine operates and consider one cylinder. The cylinder will move a volume of air based on its crankshaft geometry, rpm, and sealing capability. The amount of air that can enter the cylinder is dependent on the intake flow capability, crank geometry, rpm, and valve timing as a minimum consideration. Likewise, the amount of air that exits the cylinder is dependent on the same characteristics.

An engine's output is usually thought of in terms of horsepower. Actually, an engine produces torque, and the horsepower is calculated through a units conversion. The amount of torque an engine can produce is directly related to the amount of cylinder pressure generated. This is all affected by the same previous characteristics (intake and exhaust capability, crank geometry, rpm, valve timing, etc). So basically an engine's power output is about air exchange capability. Using this line of thinking, look at the exhaust path again. The exhaust system is more reflective of the engine's ability to move air, as opposed to horsepower numbers. Engine output does not address the breathing aspects of the engine and is probably not a good rule to use for exhaust sizing.

There is a very good reason that tuners/engineers/specialist have attempted to assign exhaust to intake relationships around 70-80% for a typical natural aspirated set-up. In non-detailed terms, it is a range that offers a good balance for power capability. Other relationships, such as 1:1, are used and they work very well, but these methods have to be applied and tuned for very specific circumstances. This relationship does not stop on the flow bench; it goes all the way from the intake path opening to the exhaust system termination. In short, try to maintain exhaust sizes that are in line with the intake capability. Also, do not stop your analysis at the intake and exhaust paths. If the engine already has the camshaft, look at the valve events. If the specs favor a restricted exhaust (indicated by early and wider exhaust openings with wider lobe separation angles), then size it accordingly by using exhaust components with smaller cross-sections. If the valve timing specs favor the intake, then the engine needs some serious exhaust flow capability, which is only possible with larger cross-sections.

This section was written with natural aspirated combinations in mind. However, by using the 'air exchange' rationale, it becomes apparent why forced induction engines typically benefit from increased exhaust flow capability. Also, look at the nitrous combinations. The intake system remains virtually unchanged, yet with the major increases in cylinder pressure it acts like a substantially larger engine on the exhaust side, requiring earlier exhaust openings and/or higher exhaust flow capability.

 

Lobe Separation Angle and Engine Usage- There are many terms associated with camshafts that get tossed around often.  Lobe Separation Angle (LSA) is a term that receives a lot of attention, but mostly incorrectly.  For some reason, when cam application and selection is discussed, LSA seems to come first and gets linked to engine usage.  Categorically, narrow LSAs are associated with racing applications with narrow/peaky operating ranges.  Wide LSA’s are associated with streetability, broad powerband response, and exhaust emissions.  A very effective argument to this approach is to inquire about camshafts used in Pro-Stock and Competition Eliminator drag sanctioned classes that utilize Lobe Separation Angles around 114 degrees, and occasionally in the 116 to 118 degree range.  These are racing engines that can achieve 3 hp/cid and have powerbands that are often within a narrow 2000 rpm envelope; clearly violating the LSA selection guidelines.  Another approach might be to compare a 283 cid Chevrolet Super Stock to a 280 cid Chevrolet Competition Eliminator.  The camshaft on the Super Stock application might have a 104 LSA, while the Comp Eliminator has a 114 LSA. 

These examples reveal a key piece of info regarding LSA.  If you really look at the engine combinations, the more breathing capability a motor has (relative to its displacement), the wider the LSA can end up.  This observation indicates that LSA and engine usage comparisons are not globally valid.  If it is so easy to point out very well established examples that violate the criteria, maybe the premise of LSA versus engine usage (void of specific engine parameters) is not a valid cam selection criterion at all.    

Another key piece of info is that different engine combinations require completely different valve events.  Once those valve events are determined, and lobe requirements are established, the LSA is calculated, and the camshaft can be manufactured.  Maybe, LSA should be thought of as a camshaft manufacturing term as opposed to a camshaft design criteria.

 

Custom Ground Camshafts- When optimized performance of an engine combination is desired, the camshaft design parameters are calculated from the engine and vehicle specifications to perform within specific operating conditions. Let me emphasize that last statement, 'within specific operating conditions'. In no way was total maximum power for the engine implied. The intent is to maximize performance within the intended design parameters. If that means taking a pro-stock motor and wanting to run it from 2000-5000 rpm, then so be it.

The camshaft's seat timing events, ramp rate, and lift are directly related to the intake and exhaust flow capabilities, crankshaft geometry, static compression, rpm range, as well as other criteria. A camshaft selected in this manner, becomes personalized to that particular engine combination. Usually a custom grind is selected as an intake lobe and exhaust lobe with a particular phasing to each other (lobe separation angle, LSA) and sometimes a specified amount of advance or retard is built in. Although, it could easily end up having completely reengineered lobe characteristics, requiring new lobe masters with specialized ramp requirements. It is possible for an off-the-shelf camshaft to be a classified as a 'custom'. If the cam design is calculated for a particular combination and an off-the-shelf part number fits the bill, then for all practical purposes that part number is a 'custom' cam (but only for that particular set-up).

Typically, cam catalogs do not specifically list custom ground camshafts, because the possibilities are endless. They stick to particular series or families of camshafts. The super stock grinds come closest to an off-the-shelf grind that is truly optimized for a combination. There will be small differences due to header sizes and engine builders’ secrets, but usually the catalogs are pretty close to a good baseline. Likewise, brand to brand, the grinds will be very similar because of the 'class' dictated combinations and the flow characteristics are so well documented.

 

Old Camshafts Lacked  Sound Design Principles- To quote Chevy High Performance’s Cam-Tastic! Issue from March 2000, Camshaft Basics, “In the old days of camshaft design most cams were designed with exactly the same duration on the intake and exhaust lobes”. 

I have seen other articles and books make similar claims.  It is time to cut to the chase on this and clear the air of misinformation.  Real camshaft design has always addressed the needs of the engine. It’s the high performance marketplace that, for some reason, skirted the whole idea of what the camshaft does, adopting this same intake and exhaust lobe subject line. Therefore, they are the ones staking claim to noticing the change over the past 20 or so years. In short, real engineering design in valve events and camshaft technology has always been around.  Here a few examples:

In the 1930s, the Chevrolet Brothers’ Frontenac Stagger-Valve cylinder head conversion utilized 7 degrees more intake duration than exhaust.

In the late 1940s, Offenhauser was using cams with different intake and exhaust lobes on their speedway motors.

In 1959, Almquest Engineering (pioneers in the hot rod mail order business, beginning in the 1940s) offered different camshaft grinds for the flathead Ford V8’s, and some of those utilized more exhaust lobe duration.

In 1966, Ford designed the camshaft for its 289 Trans-Am Program that utilized 14 degrees less exhaust seat duration, to match the 90% exhaust to intake ratio of the seriously hogged out exhaust ports with 1.625 valves.

The Z-28 “Special Off-Road” Camshaft utilized more than 10 degrees of additional exhaust duration.

For an article entitled “Camshaft Basics”, it missed some relevant history.  The article claims the design process changed over the years.  Technology has certainly advanced, but the design process of matching the camshaft to the motor hasn’t.

 

Degreeing Camshafts- There is no special magic involved for degreeing a camshaft during installation, but this is not the same thing as random advancing, retarding, or installing the gears 'lined up'. Degreeing a camshaft involves definite known values for valve events. Typically this is specified as an Intake Centerline or as opening/closing events at specific lobe lifts. This is done to insure the cam is installed per specific requirements, such as a recommendation from an engine builder or the vendor's data sheet for that camshaft grind. Manufacturing tolerances and shop practices do not guarantee that the cam matches the data sheet, when installed at crank gear 'zero'. The cam will usually need to be advanced or retarded to the correct location. If it is correct, at crank gear 'zero', then the cam has still been degreed. It just did not require any additional tweaking to meet the requirements. Verifying the installation is what degreeing a cam is all about. A common misused term is the 'straight-up' installation. Typically this is described as installing the cam at crank gear 'zero'. This is 100% wrong. ‘Straight-up’ refers to the intake and exhaust centerlines being the same. In other words the cam will have no advance or retard during installation, regardless of the amount of advance/retard ground in by the vendor. In reality, the cam may have to be advanced or retarded (from crank gear 'zero') significantly to arrive at a ‘straight-up’ position. 

 

Piston To Valve Clearance- Piston clearance is a function of lobe geometry and phasing to the piston. Cam lift should not be a deciding a factor in clearance issues. Valves will hit the piston in the overlap period, while exhaust is closing and intake is opening. Exhaust clearance problems will typically occur just before TDC and intake just after TDC, not at max lift. Some cylinder head venders and other component manufacturers advertise a max duration or lift before clearance issues arise. This is very misleading. Maximum safe duration is a totally bogus value, and is completely worthless without knowing anything about the ramp rates or actual timing/phasing events of the installation. At least with maximum safe lift, the vendor can apply a ridiculously fast ramp at a very early opening/closing and arrive at a somewhat meaningful measurement, but without knowing the design specifics the information is still next to useless.

 

Adjusting Lash on Mechanical/Solid Cams- If valve lash changes significantly over time, then something is wrong. Cam wear is very slight, along the order of .002 or less. Lash settings should be taken/adjusted at the same temperature and same order as the previous or original setting. This is the only way to rule out expansion/contraction of the components from temperature changes. This temperature delta is usually the culprit of most valve lash dilemmas. At initial start-up and break-in of a new set-up: cam, lifters, rockers, pushrods, valve job, etc., the lash may move around during the break-in procedure and for a short time after. This is because all the parts are seating into their new wear patterns. Once this occurs, the lash setting should stay steady.  If lash settings change more than .005” then there has been a component failure (loosened hardware or actual mechanical failure)

 

Hydraulic Lifter Preload and Pump-Up- Hydraulic lifters are intended to make up for valvetrain dimensional differences as well as providing a self-adjusting method of maintaining valve lash, or rather the lack of. By setting the valvetrain so the lifter plunger is depressed slightly, the lifter is able to compensate for these differences, making a convenient hassle-free valvetrain set-up. For performance applications, lifter preload is not needed or wanted. As rpm's increase, the lifter has a tendency to bounce over the back of the lobe as it comes back down from the maximum lift point. The pressurized oil fills the lifter body to account for this bouncing. Eventually, after several engine revolutions (fractions of a second), the oil can completely fill the lifter body and the plunger will be pushed up to its full travel (pump-up). Higher oil pressures can amplify this problem. With the lifter pre-loaded, this can cause a valve to run off its seat and can cause piston clearance issues if and when pump-up occurs. By setting the valvetrain preload at “zero lash”, or just beyond, as felt by the hands and fingers during the adjustment process, lifter pump up is prevented and in most cases, the cam will rev higher. This adjustment process will typically end up with about .003” to .007” of lifter preload.  Ford tech and tuning articles in the late 60's actually urged 'stock' class racers to run a positive lash of .001”-.003” on hydraulic cams.

 

Pushrod Length and Valve Stem Centerline- Incorrect pushrod length can be detrimental to valve guide wear. Most sources say that centering the rocker contact patch on the valve stem centerline at mid valve lift is the correct method for determining the optimum pushrod length. This method is wrong and can actually cause more harm than good. The method only applies when the valvetrain geometry is correct. This means that the rocker arm lengths and stud placement and valve tip heights are all perfect. This is rarely the case. To illustrate this, think of the valve angle and the rocker stud angle. They are usually not the same. If a longer or shorter valve is installed, then the relationship of the valve tip to the rocker stud centerline has changed. Heads that have had multiple valve jobs can also see this relationship change. Notice, the rocker length (pivot to tip) remains unchanged, so the rocker contact patch will have to move off the valve centerline some particular distance for optimum geometry to be maintained.

The optimum length, for component longevity, is the length that will give the least rocker arm contact area on the valve stem. In other words the narrowest wear pattern. This assures that the relationship is optimized and the valve stem centerline is tangent to the rocker arm’s circular swept path. The optimum rocker tip contact point probably will not coincide with the valve stem centerline. What is the acceptable limit for being offset from the valve stem centerline? That will depend on the set-up. A safe margin to strive for is about +/-.080" of the centerline of an 11/32 diameter valve stem. No part of the wear pattern should be outside of this .160" wide envelope. As the pushrod length is changed, the pattern will change noticeably. As the geometry becomes closer to optimum, the pattern will get narrowest. If the narrowest pattern is too far from the valve stem centerline, then the valve to rocker relationship has to be changed. In this case, the valve stem length or the rocker arm will need to be changed.  This does not imply a change of rocker ratio, but rather the sweep radius.

 

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