Driven5 wrote:
sen2two wrote:
So, now that I have purchased aftermarket cams, I want to degree them. To do this, I want adjustable cams gears.
As far as I'm aware, at least most engines should not typically "need" adjustable cam gears with standard aftermarket cams. It's more a matter of how finely tuned of an engine you "want" to build, and how much getting there is worth to you. The cams should typically be designed from the manufacturer to work as intended by the manufacturer with an OEM style installation, especially those not requiring aftermarket pistons. Beyond that adjustable cam gears basically just allow you to move the torque curve around a little bit. But without spending a bunch of time (and/or money) getting them tuned, then there is minimal benefit. Also not all cam gears are designed the same, so some may be easier to make adjustable than others...And some are even somewhat adjustable from the factory if you know what you're doing.
This post is full of misinformation.
There is easily 8% hp or torque (often mutually exclusive) to be gained in the majority of engines.
You don't always end up with a cam of the duration you actually need.
Your cam grinder is limited by the cam core, as to where he can place the lobe, and its size.
No engine wants symmetrical valve events, and no engine wants equal lobe angles.
Anyway, here is what the original poster asked for. I no longer see it hosted anywhere online but copied here:
Quote:
Sprocket Power
by David Vizard
"Your motor has 4 valves per cylinder, dual overhead cams, and the potential for a lot of power -- if the cams are dialed in right."
Heard this before? Buy a set of adjustable sprockets and dial in your 4 valver's cams for maximum performance. Sounds good - but with infinite adjustability where do you start? It's common enough knowledge that the cam timing must be correctly set for maximum output and adjustable cam sprockets allow us to do just that. The ability to adjust the cam timing is, as we shall see, a very important factor for more reasons than just that machining head and block faces alters the production factory settings. The question is once this adjustability is acquired how can you most quickly arrive at optimal cam timing.
There are traditionally four routes toward achieving optimal cam timing. The first is to time the cams as suggested by someone who claims they have had good luck with particular numbers. The second is to follow cam grinders instructions. Thirdly you can start with what ever you believe might work and road test until best results are seen. Last and obviously most effective is to get on a dyno, chassis or engine, and test and adjust as necessary.
Considerations.
So far this all sounds like there is a simple deal leading to the desired end result. And there would be but for one or two major factors. Our first option, namely using the timing recommended by someone lucky enough to get good results. Going this route is seriously flawed. Ask yourself just how much of an expert this person may be if, after the availability of over 100 years of automotive technology, their results still boil down to nothing but luck. In this day and age where competition is so fierce that winning is counted in thousandths of a second, understanding what's going on not luck, is your only real hope of success.
OK then lets go with the cam grinders recommendation after all they did design the cam. Without knowing different this looks to be a sure fire route to success - but it's not. Here's the problem. The cam grinder cannot test every combination that their cams are likely to be used in. So their recommendations are in fact also their best guess at what will normally be close for a motor similar to yours. Well just how far off can these cam experts be? Putting some numbers to it for a typical 2 liter 4 valve motor we can say from experience that 25% of the time the grinder recommended timing figures can be off to the tune of a 8-10 hp loss. Why? Not because they don't necessarily know their job but because they don't intimately know your motor. Let's put it another way. The correctness of the cam timing is not in the cams themselves but in the motor. It is not want the cams want but what the motor wants that counts. This, as we shall see later, can change dramatically with seemingly minor changes in the motor spec. Unless your are prepared to accept that your cams may not be delivering their best you have no option but to use adjustable sprockets to get the timing right with one of the last two options i.e. road/track testing or dyno.
Fundamentals.
Without having some idea of how changes in cam timing affect the power you will be a little like the amateur/beginner photographer who in the position of having a camera with adjustable speed, aperture, focus etc. In this instance the adjustability does not actually help take better pictures but gives an infinite opportunity to screw up what is taken unless luck intervenes. The chances are better pictures would have resulted from the use of a cheap and simple camera.
Just as with the camera we have a situation where each change in cam event timing can influence some other factor and after 50 adjustments the timing may still not be right even though a dyno was used. Ultimately success can be had but dyno time costs money so it's best if you can arrive at functional valve events in the shortest possible time. This is what we will now consider.
Timing Variables.
When two separately adjustable cams are involved we can change the Lobe Centerline Angle (abbreviated as LCA and shown in Fig 1) and the advance/retard position of the cams as a pair (Fig 2).
Obviously there will have to be a certain amount of adjustment of each of these variables to be able to measure a change in output. Of all cam specs the LCA, not the duration, as we are so often told, is the most important variable. In the situation we have here this is under our control. Adjustment of the LCA alters all intake and exhaust opening and closing points. Of these the overlap area and the closing point of the intake valve are the most influential. These two important aspects have a combined effect on output making the final adjustment of the LCA in 1degree increments worthwhile.
For most normally aspirated multi valve motors the LCA will usually fall between 116 and 108. However there can be cases of both wider and tighter requirements than this although they are not so common. The position of both cams can be in the advanced, retarded or straight up location. Until close to optimum this can be adjusted in 2 degree increments and almost always falls between 6 degrees retarded to 4 degrees advanced. This means there could, in most instances be 54 combinations available within this span of adjustment. If you are paying for dyno time you don't want to go through 54 adjustments to find the best. Read on to find out how certain factors effect the valve event timing and how to short cut this down to typically 6 adjustments or less. This feature deals with the valve event timing required of normally aspirated (non super or turbo charged) motors. A MotorTec feature on valve events for nitrous, super and turbo charged motors will be appearing in about 3 months time.
If all the why's and wherefore's of optimal timing that follows are too heavy you can skip to the last part of this feature and learn how to time cams the fastest way but you won't be quite the expert.
Motor Characteristics Affecting Optimal Valve Event Timing.
If you don't feel up to absorbing the theory then skip this part and go to the 'Setting Up Your Cams' section.
Influences on the LCA.
After you have read this section it will be clear why the cam grinder cannot specify the cam timing for your motor any closer than an educated guess. It will also reveal why there is no such thing as a secret cam spec that will work for your motor. If someone is advertising a confidential or secret cam spec then, rest assured, it is most likely a marketing ploy. Sure they may have an effective cam spec for their motors but that does not mean it's the same for yours unless your motor is identical.
For any high performance normally aspirated motor with an effective exhaust what happens in the first third of the induction stroke has the greatest influence on the success of the cylinder fill. Let us now go through a list of variables that affect the optimum LCA and advance/retard. However before we do that here is a point of reference to help visualize why things work the way they do. For any high performance normally aspirated motor with an effective exhaust what happens in the first third of the induction stroke has the greatest influence on the success of the cylinder fill. If the air has not built optimum velocity and delivered a near optimally filled to that point in the stroke it will not make up for it at the other end of the stroke. Understanding this will be a key to understanding why it is relatively critical that, for a given cam duration, the optimum overlap (and therefore the LCA) for the motors head/displacement ratio must be used.
Displacement influence.
For a given cylinder head the larger the motor under it the tighter the optimal LCA becomes and, as might be expected, the smaller, the wider the optimal angle becomes. This means if you had a set of cams that worked dynamite on a 1600 cc motor when set at 114 degree LCA a stretch to 1800 cc would need the LCA tightened up to probably 112. So how much difference can 2 degrees of error make? In terms of a pair of 284 degree cams timed straight up, the 114 LCA results in 28-76-76-28 (intake opens 28 BTDC closes 76 ABDC - exhaust opens 76 BBDC and closes 28 ATDC) timing with 56 degrees of overlap. For the 112 LCA setup this becomes 30-74-74-30 with 60 degrees of overlap. This seems like a small change and not much of a deal. The biggest change is in the overlap. This went from 56 degrees to 60. In terms of degrees that's an increase of 7%. Even this does not sound like a really big deal but in practice the motor is sensitive not to overlap degrees but overlap area and tightening the LCA here increased the overlap area by no less than 14.7%. Now that is a big change and this relates to a relatively large change in output as is seen from the graph Fig 4.
Incidentally the cam timing spec called for by the cam producer was 114 LCA! If we had stuck, as is so often recommended, to the cam grinders timing figures our motor would have been shy almost 6 hp and 6 lbs-ft.
This test serves to emphasize the greater importance of the early part of the intake stroke on the success of the remaining part.
Cylinder Head Influence.
Since, as has just been shown, the ratio of cylinder head flow to displacement is a prime factor it is not unreasonable to assume that changes in flow, while holding the capacity of the motor constant, has a similar effect on optimal LCA. However it is not the peak flow number that is the greatest influence but the low lift numbers. As I have said before any porter that tells you low lift numbers are not an influence needs to get up to speed. In reality the main advantage of a four valve motor is in it's ability to deliver a huge increase in low lift flow. This is where most of the extra power from a multi valve motor, over it's two valve counterpart, comes from.
Much bigger output improvements were made as the LCA was tighten up to optimum than were lost by going too tight... If flow figures increase dramatically in the range where the piston is in the middle 70% of it's stroke an increase in flow numbers has virtually no effect on the optimal LCA. If the low lift flow is increased dramatically then the LCA will need to be spread. This can easily be appreciated if you consider that an increase in low lift flow appears, to the filling cylinder, exactly like a tightening of the LCA. Remember a cylinder is almost insensitive to opening but almost totally sensitive to flow. Extra flow early on appears to the cylinder as extra lift or extra overlap. If your head porter does a really trick job on your 4 V heads there could be as much as a 20% increase in low lift flow. To the cylinder this could appear as 20% more overlap and that may simply be too much. At this point let's consider what happens as the overlap is increased.
From the curves in chart Fig 5 it can be seen that as the LCA is tightened up several things happen. Starting at 124 degrees we see that the motor will pull right down to very low rpm. It will also idle nicely. As the LCA is tightened up so the torque comes up but the cam looses some of it's low speed manners. At first the torque comes up quickly as we approach the optimum of 112 degree LCA. Adjusting from 112 to 108 only sacrificed a marginal amount of top end power and virtually nothing on peak torque. However the cam was far more aggressive in terms of how the motor comes up on the cam. When the LCA was too wide the motor behaved faultlessly but failed to make a lot of power and more importantly torque. Much bigger output improvements were made as the LCA was tighten up to optimum than were lost by going too tight. One or even two degrees too tight on the LCA for a race motor will not cost much in the way of discernable power loss but the motor will be noticeably less well mannered and it's rpm band in the downward direction severely curtailed. On the other hand too wide on the LCA costs quite a bit more power and delivers only a more mildly mannered motor. For a road motor to retain it's best manners consistent with maximum output it is important to use the right LCA, not too much or too little.
There are two implications from the forgoing. First if performance is the #1 consideration it is better to have the LCA slightly (that really does mean slightly) too tight than too wide. Secondly if the cams need to be set on a wider LCA than required for best output solely to get the idle quality required then only one conclusion can be drawn. Namely that we have the classic case of too much cam duration for the application. Shorter cams on the right LCA will perform better.
Another cylinder head related aspect that has a considerable influence on the optimal LCA is the CR. The higher the CR the wider the LCA needs to be. Typically raising the CR from 11/1 to 13/1 will call for the LCA to be about 1 to maybe 1½ degrees wider. Understanding this has many little assets one being that the wider the LCA the smaller the piston valve cutouts need to be. Another is that for a street motor this higher CR allows the use of a cam of greater duration before an unacceptable idle is encountered.
Exhaust Influences.
The influence the exhaust brings to bear is varied and depends on two aspects. First the more restrictive the exhaust the wider the optimal LCA needs to be. The reason here is simple. A more restrictive exhaust means the scavenging function during overlap is less effective. Too much overlap from a LCA that is too tight means pollution of the incoming charge. Result, loss of both flexibility and torque/power. On occasion a motor with a really effective header/collector may produce slightly more power with a little backpressure from the muffler. This is not an argument for the manufactures of inefficient mufflers to use as a means to sell their mufflers but an indication that the LCA, without the small restriction the mufflers may be delivering, is too tight. What's happening here is that the exhaust along with the overlap produced by a tighter LCA (which increases overlap) is over scavenging the cylinder and a certain amount of fresh charge is being pulled right through the combustion chamber into the exhaust port. A small amount of backpressure cancels this out and the motor makes a little more power. However the fix is not to increase backpressure via a muffler but to retime the cams to a wider LCA. The results will be better.
The other aspect of the exhaust system is it's ability to scavenge via inertial and pressure wave action. Up to a point the better the scavenging the tighter the LCA needs to be but if the extraction is increased past a certain point of effectiveness the LCA required will start to spread wider.
Induction Influence.
In practice the influence of the induction system's flow capability can be small unless the system is restrictive. In a situation where race rules call for all the air to pass through a given size orifice designed with the sole intent of limiting power we find that the point at which the intake closes becomes much more important. Other than possibly needing a shorter intake duration than a totally unrestricted motor the LCA usually needs to be tighter but that assumes the exhaust scavenges well. For any plenum type intake such as used on Honda's and most other modern motors, we find that there is normally some high rpm restriction unless steps are taken to the contrary.
Port size can also have a significant influence. Bigger ports require the LCA to spread where as smaller ports tend to like tighter LCA's. As for pressure wave effects on the intake we have to allow that they occur over a much smaller rpm range than the exhaust and are generally less intense. The best effects here are gained by having the intake optimal for a point between the lowest race rpm used and peak power. Generally speaking the effects of the intake pressure wave are greatest at the end of the induction stroke rather than the beginning (which is the case for the exhaust). As a result replacing an indifferently tuned induction system with a significantly better one will call for a slightly wider LCA.
Cam Profiles.
Last on the list is the profiles used. If these are aggressive they will require a wider LCA and if relatively slow opening then a tighter LCA will be best.
Influences on the Optimum Advance/Retard.
Now we have an idea how major motor characteristics effect the LCA angle we can go through the influence on the advance and retard requirements much quicker as they are linked.
Displacement influence.
The bigger the motor for a given cylinder head the more advanced the valve events need to be. If the LCA is about right, most 4 valve motors produce maximum output with around 0 -2 degrees of advance especially if the intake and exhaust duration is about the same. If the cams are in at too wide a LCA, the best power will be with more advance. If for some reason the displacement is reduced all these rules are reversed.
Cylinder Head Influence.
Assuming something representing normal exists we find that improving high lift flow on it's own has little effect on the advance/retard needed. However if the low lift flow is good or improved since the last rebuild/dyno test then the cams will require retarding. If low lift flow is poor then more advance is required. Because a 4 valves greatest asset is large amounts of low lift flow we find, in general, that they do not require the amount of advance we see in 2 valve motors. A five valve motor usually is best with a retarded cam position due mainly to the high low lift flow ratio between intake and exhaust.
Exhaust Influences.
If the scavenging is improved the cam will need to be advanced slightly and the reverse if it is made worse for what ever reason. If backpressure goes up the cam works best with more retard and vise versa.
Induction Influence.
If induction is restricted and or port velocity low then the cams will need to be more advanced with the opposite being true if the ports flow well and or have higher velocity.
Cam Profiles.
The severity of the cam profiles has little influence on the advance/retard setting.
OK we have now looked at the major influences that affect the cam timing. You should now be in a position to better understand a) why you need adjustable sprockets and b) accepting any recommendation (from a pro or otherwise) for your motors valve events as final is at least 90% sure to be costing you power. Now lets get down to how to get the best timing with the least amount of adjustments.
Setting up Your Cams - Six Steps to Optimal Cam Timing.
OK lets pick a starting point and this will apply whether you are track testing or dyno testing. I will assume at this point that your motor is near stock capacity with good exhaust and intake and a mildly ported head. This makes it middle of the road. If it's not you will swing your starting position for the cams LCA appropriately i.e. tighter for a bigger motor and wider for a smaller motor etc.
Most motors will want a LCA between 112 -114. Check the intake valves don't foul the pistons when the intake centerline is at 104. Then set the LCA to 116 degrees timed in straight up (no advance or retard).
Testing can now be started. Make enough dyno runs or timed tests to get a solid baseline. Next retime the cams to 114 degree LCA and retest. At this point one of three things cam happen. Over the rpm range of interest the power can a) go up, b) stay the same and c) go down.
If a) the power goes up tighten the LCA by 2 degrees.
If b) the power is virtually the same widen the LCA by 1 degree.
If c) the power is down widen the LCA by 3 degrees.
Retest the motor.
Again the situation will be either a), b) or c).
If a) power increases adjust the in the same direction (wider or tighter) as per the previous test by 2 degrees.
If b) the power is virtually the same leave the setting as is or widen the LCA by 1 degree if you have not tested at such a setting before.
If c) the power is down reverse the direction of adjustment from the last adjustment by 1 degree.
Retest the motor.
Again the situation will be either a), b) or c).
If a) the power increases adjust the LCA in the same direction (wider or tighter) as per the previous test by 2 degrees.
If b) the power is virtually the same leave the setting as is or widen the LCA by 1 degree if you have not tested at such a setting before.
If c) the power is down widen the LCA by 1 degrees.
Retest the motor.
About now it will be evident that the optimum LCA will have been established.
Now is time to adjust the advance/retard to optimum. Unless the head has really good low lift flow try advancing the cams by 2 degrees. If the power picks up try another two degrees. Unless your motor has some unusual characteristics it won't need 4 degrees and will probably make less power so go back to 2 degrees of advance.
If the heads have the flow characteristics just mentioned try retarding the cam by two degrees and 4 if indications are that's what it wants. However 4 degrees will be less common.
If you have followed all this your cams will, for all practical purposes, be optimum. In most cases this will have been reached after only six adjustments.
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