New world camshafts the heart of performance
By BILL GUDE
Society of Automotive Engineers International
It all started with a big Bang! Back about 1680 when Christian Huygens a Dutch scientist, made the first recorded attempt to create a function internal combustion engine using gunpowder as fuel. History does not give us the results, but most likely it was a no survivor one-time experiment.
From its primitive beginning to the fire breathing blown fuel rocket science engines of today, The 4 stroke engine has intrigued the most brilliant of minds and sprit, striving to unleash every last screaming ounce of power.
At the heart of the 4 stroke engine is the camshaft, and by most opinions is the most critical part of the engine in regards to usable power output. This should provide a sound separation of facts and myths. It should also provide an in-depth overview of cam profiles, camshaft billets, advantage of reground cams, properties of materials, heat-treating, dealing with rocker ratios, selecting camshafts for race or street.
New world camshaft thinking
The fuel injected engines 1988 and up, of the new import crowd has required dynamic changes in camshaft thinking. The bullet proof valve spring and retainer designs that come stock on most import engines opens the door wide open to new and innovative concepts, to deliver high powered bolt in camshafts. If the cam engineer is creative a stable but aggressive cam profile may be successfully used on most factory spring and retainers. This makes caming great for the average budget, as the buyer will not have to remove the head to install a performance camshaft. However the camshaft engineer must first shed his old way of thanking of fit and function on the out dated overhead valve engines.
Factory O.E.M. cams are cast from a special Iron alloy. The molting raw material is poured into steel molds in batches of 100 or more at a time, the steel mold cools the molting material quite quickly, this quick cooling process has a self heat treating effect on the camshafts, and the heat treat process is no longer needed. The camshaft is hard all the way thru and thru, not just on the surface, and may be reground many times and not affect the harness of the material... This has made regrinding of the stock cams the preferred choice by many cam grinders, who want a bulletproof reliable material.
Beware of so called cast billets, usually produced in third world country’s they sometime have wear or breakage problems, as the metallurgy and blend of alloys are not as refined as the O.M.E. Factory process resulting in a degraded product
Camshaft materials cont.
Steel billet cams are machined from one solid piece of alloy steel, then pre ground to the basic configuration and heat treated, (42 Rockwell on the c scale), and then finish ground
to produce the end product. Gude Performance may be the only company to offer this type billet for the Imports at this time. (See fig.1)
Sand- cast camshafts are cast in a sand molds, and cool slowly rendering the base material very soft, and may be heat treated but the material will have less strength than parts cast in steel molds.
The camshaft is possibly the most misunderstood component of the engine. This is most likely due to the time element involved in the actual physics of the 4-stroke engine. To comprehend the camshaft, we must first understand the basic 4 strokes and function of each stroke, in relation to the 4-stroke engine.
Basic Terminology of the 4 Stroke Engine. NOTE: (This is a must)
TOP DEAD CENTER (T.D.C.) the point at which the piston is at the absolute top of its travel or stroke. 0 degrees. AFTER TOP DEAD CENTER (A.T.D.C.) the piston is past T.D.C. but not more than 90 degrees. BEFORE BOTTOM CENTER (B.B.C.) piston has passed 90 degrees of crank rotation but has not reached 180 degrees at which would be BOTTOM CENTER (B.C.). AFTER BOTTOM CENTER (A.B.C.) piston has passed bottom center 180 degrees but has not passed 270 degrees of crank rotation. BEFORE TOP DEAD CENTER (B.T.D.C.) piston has passed 270 degrees of crank rotation.
Basic Camshaft Terminology.
Cam lobe- The part of the camshaft that performs the work of moving the rocker arm or bucket which intern moves the valve.
Rocker arm- The mechanical part that transfers the cam movement to the valve.
Cam followers- or Buckets, Transfer cam movement to the valve.
Toe of cam- The peak end of the lobe.
Heel- The low end of the lobe opposite the toe.
Base circle- The diameter of the lobe when measured 90 degrees to the toe or heel.
Cam lift-. The distance the cam lobe measures form center line to peak of toe.
Valve lift- The distance the valve is lifted of its seat, by the camshaft lobe, measured in thousands of an inch. (Sometimes in mm).
Duration- The amount of time the valve is lifted of its seat measured in degrees.
Overlap- The period of time both the intake and exhaust vale or open at the same time.
Lobe center-, sometimes called displacement angle, and is expressed in degrees.
Effective flow-, The amount of time in degrees that the valve is of its seat, at an effective or significant flow rate
Basic Concept of Camshaft Timing
Advanced cam timing will usually improve low R.P.M. performance but may hurt high R.P.M .output. Retarding cam timing will usually favor high R.P.M. at a possible loss to low R.M.P. performance. Duration longer duration will make more power at high R...P M. but requires a higher idle speed, and will sometimes even be erratic and reduce power at lower R.P.M. Overlap, high overlap cams will also make idle erratic but do have the benefit, of breathing better at high R.M.P. Dual cam engines, can be tuned more effectively because, you can change the intake cam and exhaust cam timing separately which sets the overlap. A quick note here, in most turbo street applications like short overlap camshafts, as it will spool the turbine quicker at a lower R.P.M.
The Dream Cam
Choosing a camshaft is sometime very confusing, as most want maximum power, stock idle, fantastic low end torque, and never ending high R.M.P. pull, plus great gas mileage.
They get all focused on the number game, lift, duration, lobe centers, and an R.P.M. range that’s so far in Never Never Land that the cam engineer just kind of grins and nods, when told they want to twist 10,000 R.P.M. This type of R.P.M. is the very rare exception; even built bottom ends won’t be around long at this type of R. P.M. Keep your game plan in the real world and be the winner. All the horsepower in the world, won’t win one race if you don’t finish. So pick a camshaft profile the engine can live with, and when you are in the winners circle smile.
Choosing The Right Cam for The Right Job
Questions that need answers for correct cam selection are.
Type of racing,
Current shift points in R.P.M.
Shift points desired,
R.P.M. drop when shifting into each gear.
Top R.P.M. expected or needed. (Will the engine live at this R.P.M?)
Engine compression ratio,
Combustion chamber mods.
Setup used injection, carburetors, turbo charged, supercharged
Fuel to be used,
Variable cam timing.
Street use if any, will the engine need to pass emissions, or utilize air conditioning or power steering, all this is an absolute must to make sure your camshaft will perform in the environment you ask of it. Pick a camshaft that will fit you guidelines. A camshaft is selected by the specific R.P.M. need of the engine, not by popular lift and duration numbers. or stages and never never by Internet jabber which is usually just that.
Camshafts and Gearing relationship
As a reference when the engine and gearing are well matched the power drop when shifting from one gear to the next will never drop to less than 94% of maximum power
An example of how camshaft selection affects this. Let assume we build 2 engines. Engine one Has a R.P.M. shift point of 8000 R.P.M. and is build to run this R.P.M.
Engine number two R.P.M. shift point of 7400 R.P.M a similar engine but setup as a daily driver.
We want to get the maximum performance out of both engines. When shifting engine number one at 8500 R.P.M. into the next gear the maximum R.P.M. drop is 1500 R.M.P. or drops to 7500...A good camshaft for engine number one will have good pull at 7000 R.P.M. approximately 86 % of max power and make maximum power around 7800 R.P.M. holding maximum power to the 8800 R.P.M.
Engine number two is shifted at 7800 R.P.M. Into the next gear, putting the R.P.M. at 5800. If we use the same cam that we selected for engine number one, engine number two would fall into a unfavorable R.P.M. or to low for this cam to start pulling as the Engine speed drops to 5800 R.P.M. or 70% of max power and only reached max power just at the shift occurred, and not taking advantage of max power for any usable amount of time, plus the 70 % number is just to low, and will take to long to recover and start to pull again. The usable horsepower will look like this. Engine number one was able to use an average of 135 hp.and with the same cam, engine number two was only able to use an average of 122 hp. so it is obvious that engine number two needs a cam that would reach max power approximately 7000 R.P.M. and hold 96% at 7800 R.P.M. and will hold 92% of its power at 5800 R.P.M. with this type of cam the number two engine now would using an average of 127.4 hp The moral of story what’s right for one is not always right for the other. This brings us to adjustable cam timing sprockets.
Timing with adjustable sprockets
Camshaft sprockets are usually used to correct belt stretch or cam tuning for the optimum. An example of using adjustable cam sprockets. Let’s say we have the cam that was selected for engine number one, in engine number two, by advancing the camshaft the horsepower curve can be rearranged to make the cam more usable. As advancing the cam timing will bring the power curve down to a lower usable R.P.M. for engine number two. By advancing the cam timing lets say 5 degrees, we might be able to make the
power curve look something like this. Maximum power at 7200 R.P.M. now the cam will
pull better at the 5400 R.P.M. as it will now be at 78% of max power, instead of 70% at stock timing. Now let’s say we have engine number two’s camshaft in engine number one. It obvious we will need more high R.P.M. power. By retarding this cam 5 degrees the altered curve may look something like this. Maximum power will occur at 7600
R.P.M. resulting in a more useable high R.P.M. which engine one likes. The same cam grind could be helped on the top end, by grinding more overlap into the same profile.
A good point here for the dual cam engines, as each cam may be advance or retarded separately so the overlap can be adjusted with in reasonable limits.
(See fig 4)
Putting this stuff to work.
For those who want to indulge their gray matter and advance to a whole new level read on, your are winners, as the fallowing is usually only discussed behind closed doors and secret meetings of the select few.
A little advice after 30 plus years of experience, into designing camshaft profiles that are stable at extreme high rpm with stock or light valve spring loading, this increasing horse power and engine reliability, as high valve spring loads can case excessive wear and result in total valve stem failure destroying a expensive engine
The general theory is Intake stroke-1- the piston travels down the bore, the intake valve opens, fuel and air are inducted into the cylinder. / Compression stroke -2- the air and mixture are compressed. / Power stokes- 3- ignition occurs igniting the compressed
mixture the hot expanding gases force the piston down the bore rotating the crank./ Exhaust stroke – 4 - the piston comes up forcing the spent gases out the exhaust valve. And the whole process starts over.
What really happens on overlap?
The problem with the general theory, their is no time element install, because of time the intake valve dose not open on the intake stroke, but at a precise time on the exhaust stroke. And the exhaust valve opens at a precise time on the power stroke, because of the time it takes to start the mechanics, of the flow into the head and combustion chamber, or to waiting exhaust system, the valve events are started in advance of the time the event is required to happen. This timing is critical, and just a few degrees will make a big difference in the overall performance of the camshaft.
The object is to, hold the valves open for a longer period of time during the cycle, and get more air and fuel mixture into the waiting chamber. The trick is to hold the exhaust valve open as long as possible, to let the header and exhaust scavenges out the remaining spent gases. This also starts a ram effect in the intake port, which will actually help push out the spent gases with the fresh incoming charge. Yet at the same time let enough cool mixture pass over the heated exhaust valve, as to cool the valve preventing warping or burning. This is referred to as the overlap period, as both intake and exhaust valve are open at the same time. Overlap is most helpful at high and mid R.P.M., as the scavenging effect lets the engine breath better. However, a large amount of overlap will cause poor idle and sluggish throttle response when used on low compression engines. It takes very little overlap to create big idle problems on o.e.m fuel injected engines. (Not always the hot ticket for street strip setups), The next timing demand is to open the intake valve as early as practical, and hold it open as long as possible into the compression stroke, closing it at the right moment, as not to let the oncoming piston force to much of the incoming charge back up the intake runner, causing reverse flow, and rising heck with flow in the intake manifold plenum, and disrupting flow to the other cylinders. This condition will absolutely kill low R.P.M. torque. The runner length of the intake manifold will to some extent effect the selection of valve closing time, as a long runner manifold is more tolerant of reverse flow. For this reason, the cam engineer spends countless hours testing different cam profiles, for different applications. Keep in mind you pay for the testing when you buy a cam, not so much the actual work. And you usually get what you pay for
A word on rockers and followers
The rocker arm for the most part, is a mechanical device, and is usually made in one pieces with a brazed on hardened wear pad. One end of the rocker is stationary, or a pivot. The cam rides on the rocker wear pad, and when rotated to its exocentric position will activate the rocker, and push open the valve. Some rockers have a 1 to 1 ratio, and some have as much as 1.8 ratios or more, putting this into actual terms. If the camshaft has a lift of .250, and the rocker ratio is 1.5, the actual valve lift is .375. The formula is as shown .250 x 1.5 = 375. The rocker ratio can be changed, by changing the camshaft base circle, changing the rocker position in relation to the camshaft. This will affect net lift, either in a positive or a negative way. In other words in some situations the rocker ratio will be reduced, or sometimes it will be increased depending on the rocker pivot point location. Example take the same .250 lift camshaft, by changing the base circle we end up with a reduced rocker ratio of 1.4 now working the math .250 x 1.4 = .350 net valve lift, and if the rocker ratio increases lets say to 1.6, the math will work out to .250 x .1.6 =.400, net valve lift. So it’s pretty obvious, to get the lift you want, the engineer will have to calculate the rocker ratio in relation to the camshaft base circle. This is at the point some cam grinders throw in the towel when regrinding camshafts, and go for the billet so they won’t have to engineer out the related rocker ratio and base circle changes.
However, sometimes the billet option back fires as adding lift to camshafts will cause the nose of the cam to run off the rocker or follower, and ruin the camshaft, so to get the added lift, the base circle will have to be kept down. And at this point the billets really loose some of their attraction. (See Fig.2) Adding to the billet problem, is the fact that if the billet camshaft is kept at a large base circle, (if that is possible) the lobe will have more surface, area which increases the surface speed causing more wear friction robbing power. Just one more reason the reground cams are a good if not better choice than billets. However on the bucket type follower, the reground camshaft will sometimes need a custom made shim on the valve tip to accommodate the smaller base circle. Also there is no rocker ratio, so the cam lift is net valve lift, with this type of follower billets will also be limited in lift, if the stock base circle is retained, as the nose of the camshaft will not ride on the follower causing damage to cam and follower. (See fig.3)
REMEMBER NOTHING IS SICKER!!!! THAN A OVER CAMSHAFTED - OVER FUEL INJECTED,AND OVER THROTTLE BODIED ENGINES
YA SEE IT’S SO SIMPLE !!!