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Cams: advance/retard basics

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  • Cams: advance/retard basics

    Hi yawl. Just need someone to confirm my logic here.

    Lets say you have a safe window of adjustment during cam timing to advance or retard both intake and exhaust cams, but its one or the other as a pair, no option to mix it up.

    Generally, advancing both cams (both events occurring earlier) gives power higher in the rev band verses retarding both cams, assuming overlap is unchanged, correct?

    Jake Larsen

  • #2
    I'm always in Retard mode when it comes to cams....

    Ohhhh you want to know about the actual cam timing!

    Yes, I think you have your logic correct.
    Chris L.

    Spray paint and tire shine doesn't equate to a "restoration!"


    • #3
      if you have adjustable gears on both cams you advance/retard them separate from each other, this would be the same as changing the lobe separation angle (overlap) on a single cam engine like the typical american V8

      tons of info on the web what the combination of changes would typically create
      Last edited by TeamM3; 03-20-2005, 11:42 AM.
      A friend will come bail you out of jail, but a TRUE friend will be sitting next to you in the jail cell saying, "Dude, that was focking awesome!"


      • #4
        Retarding the cam timing and making the intake valves close later actually raises HP in the upper RPM range.

        88 M3 - LACHSSILBER/M TECH
        85 323I S52 - ALPINEWEISS/SCHWARZ


        • #5
          Re: Cams: advance/retard basics

          Originally posted by Jake
          Generally, advancing both cams (both events occurring earlier) gives power higher in the rev band verses retarding both cams, assuming overlap is unchanged, correct?
          I bet you get a variety of responses to this one Jake.

          There is no simple answer to your question, based on what I have learned about cam timing.

          But there are two fundamental ways of looking at it:

          1. You can think in terms of duration and overlap

          2. You can think in terms of the specific intake/exhaust valve open/close events

          I prefer using method #2 lately, it makes it easier for me to visualize things. Using this frame of reference the intake closing point is generally considered the most important, as it directly determines how much you will compress the intake charge (your dynamic compression ratio), and whether or not you get flow reversion.

          Good luck!



          • #6
            Thanks guys. The question is a bit general I know, but that's because I'm not exactly fussing with the S14 at the moment. But the logic applies to all engines so I thought I'd throw it out there.

            Here's the situation. I can lock the cams down in position, 100% on the mark. But crank position has 1 degree of movement when I lock it in place. Aiming for the middle is boring, so I thought I'd set it up on the slight advanced side of the house as torque is not an issue. That is assuming Iím thinking along the right lines.

            Jake Larsen


            • #7
              "2. You can think in terms of the specific intake/exhaust valve open/close events"

              As far as to what I have picked up, it's down to exhaust opening point and inlet closing point which are the most important timing events. The more you hold the 'power' stroke (with a later opening exhaust) = more torque in the lower half of the rev band. The more you hold the 'compression' stroke (with a later opening inlet) = more torque in the lower half of the rev abnd but with less loss at the top. Also a narrower the LSA and a longer duration is a better basis for a 'street' set up. I think .....!

              Have a look at these, I've got quite a few of these with different combos, it makes things a bit easier to look at a diagram


              Inlet events in blue - exhaust events in red
              Last edited by UK215M3; 03-20-2005, 11:14 PM.


              • #8

                if you could keep the intake closing point
                and the exhaust open point the same, but increase
                the exhaust closing point while increasing the intake
                opening point == more overlap, you will generate
                more torque earlier, fatter torque range without
                appreciably changing the point where the engine
                falls off. Basically, you will increase power across
                the usable rpm band and the usable rpm band will NOT

                Duration or overlap are terms often used in contradictory ways.
                The closer you look, you will find the statements based
                on some durations numbers make some kind of assumption,
                which may not be compatible with your goals.

                As Gustave says, its all about actual valve timing events.
                Everything else is just a derivative where someone makes
                some statesments based on a specific app. but in another
                it doesnt work out quite that way. Looking at valve timing
                is more failsafe in that regard.

                cam peak timing location or what some people call
                "lobe seperation angle" only indirectly give you
                an idea of what is happening as well. That is an old concept
                because in old days, they were not independently
                adjustable. our cams ARE independently adjustable,
                but still dont do what I said above in first paragraph justice
                you need new cams to do this

                remember one thing: you can run quite LONG DURATION
                cams and still make a bucket load of torque down low!
                That is normally contrary to many articles on various internet
                sites (mainly because the application is different).

                also remember, in the old days, lots of overlap or more
                specifically high TDC valve lifts made idle very lumpy and
                difficult. but they had shitty ignition/injection systems.
                enter more modern electronic fuel injection and now
                you can use timing and injection MAPS which are also
                specific for idle or part throttle or full throttle. In the old
                days you could run too much advance at idle otherwise
                the engine would be difficult to start (kick back), be
                we CAN run LOTS of idle ignition advance because the
                start is handled different. So instead of running a lazy
                0 degree idle (stock S14 e.g.) we can run 12+ degrees
                advance. We can also bump up the timing in all the
                off cam areas to increase torque and driveability. Take back
                the timing where the cam comes on, and so on. All of this
                was not possible to this degree in the old days, hence
                the poor running -- which leads to the myth that big cams
                (long durations) do not run well. The main issue with big
                cams is emmisions. However, today, VANOS is used,
                so TDC valve lift is controlled for idle/part throttle/full
                throttle. It sort of gives you the best of both worlds.
                Variable Valve lift (e.g. valvetronic) takes the idea further
                although its evolution is not finished of course.

                dont know how much of this applies to push rod engines.




                • #9
                  Very thorough post as usual John. Thank you! Wish I had an engine dyno to fool around with.

                  Jake Larsen


                  • #10
                    Cams: advance/retard basics

                    I can't add much to the previous posts but I want to clarify that overlap and lobe separation angle (LSA) are not the same thing as implied in one of the posts. LSA is the angular distance between the peak inlet and exhaust timing events on the cam(s) and is one of the few camshaft angles that is referenced in crankshaft degrees. Overlap is the angular measure of the period when both the inlet and exhaust valves are open and is in camshaft degrees. Taking a single cam engine as an example, it is possible to change the overlap by changing the cam duration (profile) and yet have the LSA remain the same. HTH

                    One day I will finish my camshaft write up including measured profiles of common S14 cams and have Gustave host it on his site. - Jim


                    • #11
                      LSA is usually a term used for a 2V engine where a
                      single cam drives both intake and exhaust.
                      On 4V engines with 2 overhead cams, we only
                      speak of peak cam timing, e.g. on intake the amount of
                      degrees after TDC at which the cam achieves maximum
                      lift. Changing this moves all the valve timing events in
                      either direction, which can be too much. Likewise just
                      increasing duration often incurs later peak timing which
                      shifts the power band up. the really interesting
                      part are the actual timing events. overlap, duration etc.
                      are just secondary derived figures and not the primary
                      concern when designing a new cam.



                      • #12
                        Actually, I think one needs to be conversant with all the different terms. Peak timing is certainly useful when designing a new cam but few of on this list will be doing that. LSA however comes in to play whenever anyone here wants to use the EVO cam gear which can either increase or decrease the LSA depending on installation. This of course also impacts the peak timing event.

                        However, attempted point of my post was that many of these terms are used interchangably even when it is not correct. This may lead to misunderstandings, for example,: "Likewise just
                        increasing duration often incurs later peak timing " is true in some circumstances but it is not always true and therefore it would be wise to be as fluent as possible with the terminology. Carefully read, the quoted statement is 100% accurate, casually read, it could lead to the wrong conclusion.


                        • #13
                          >"Likewise just
                          >increasing duration often incurs later peak timing " is true in >some circumstances

                          in practice this is true in *most* circumstances and that
                          is why I wrote it that way. In almost all situations but
                          race cams this is true.

                          I should have expanded on that sentence. The reason
                          it is true is that most cam manufacturers make a compromise
                          due to the pistons. whereas my race cams e.g.
                          have increased duration and EARLIER peak timing with
                          NO compromise due to pistons. IOW, you INSTALL
                          pistons to work with the cams, not just the valve
                          to piston clearance, but also the compression ratio.

                          BTW, LSA on a dual overhead cam setup IS the same
                          idea as "peak timing". Peak timing is defined as per my
                          earlier post. but changing it is a band-aid IMO.
                          2 posts earlier I wrote what will really make power
                          given a certain rev limit, and I am quite confident in that.

                          John <-- "conversant" with the terms


                          • #14
                            well I was trying not to engage with you on the same argument over and over again, but it is not a "band-aid", it's a vital pece of overall cam timing. It may not come into play for the particular group of cams and other engine parts you've chosen to use (Motorsport), but that is not necessarily the case in any other situation. You can only ever really know for sure by getting it on the dyno because as you have stated, it's the actual application that counts.
                            A friend will come bail you out of jail, but a TRUE friend will be sitting next to you in the jail cell saying, "Dude, that was focking awesome!"