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valve overlap and you, an enthusiasts perspective

Discussion in 'The Hokey Ass Message Board' started by NintendoKD, Oct 4, 2012.

  1. NintendoKD
    Joined: Sep 6, 2008
    Posts: 139

    NintendoKD
    Member

    Can someone, please break this down for me Barney style? I am having the greatest trouble understanding the role of valve overlap and performance. I have done the usual google search, and used the search feature here, however, I am not really sure on a number of things.

    0 degrees being TDC
    180 being BDC
    this I understand, as well as the ratio for four stroke engines. Overlap at low engine speeds causes the "lope" but I don't understand why. I think that this has to do with vacuum, but am not sure. Won't this make the engine less efficient? "I have timed an engine by using a vacuum gauge" as the vacuum produced at idle will be insufficient to produce enough suction through the venturis to atomize fuel in an efficient way? I am a bit confused and would like some guidance on this. As I understand things, overlap was introduced around the 60's to a greater extent, but was in use before that to a much lesser extent. Most of the diagrams only show a "perfect" combustion cycle, where the exhaust valve is closed before the intake opens. I understand that scavenging hot exhaust will help to vaporize fuel and aid in better combustion, but there is unburnt fuel in the exhaust resulting in an imbalance in the air fuel ratio, "stoichiometry" in simpler terms supersaturation of fuel to air due to unburnt hydrocarbons in the exhaust stream. Correct me if I'm dead wrong here. diesels and forced induction engines are different, however, there cannot be any overlap due to the very principle of forced induction and the diesel cycle, as losses in pressure through the exhaust would result in a large decrease in overall volumetric efficiency. Really I'm looking for cold, hard facts concerning valve overlap and the performance/economy side of the house "I realize you cannot always have both" this may also answer some burning questions in others minds as well.

    Thanks,

    Nintendo:cool:
     
  2. Torana68
    Joined: Jan 28, 2008
    Posts: 1,415

    Torana68
    ALLIANCE MEMBER
    from Australia

     
    Last edited: Oct 4, 2012
  3. NintendoKD
    Joined: Sep 6, 2008
    Posts: 139

    NintendoKD
    Member

    that helped some and was very simple. Thanks, can you expound more on the tuning aspect? is this like using a combination of back pressure and burning gasses to accelerate exhaust gasses similar to a scramjet or jet type engine "not exactly" basically put unburt fuel will continue to burn in the exhaust, and changing things like the amount of back pressure, can effect performance, similar in effect to changing the pipe on a 2- stroke?:confused:
     
  4. NintendoKD
    Joined: Sep 6, 2008
    Posts: 139

    NintendoKD
    Member

    As I understand things, overlap was introduced around the 60's to a greater extent, but was in use before that to a much lesser extent.
    smart people have probably doing that to cams since the '20's

    got it, more or less I was referring to literature I found on production camshafts, "big three" thanks
     

  5. NintendoKD
    Joined: Sep 6, 2008
    Posts: 139

    NintendoKD
    Member

    no you cant, you can have a high HP or max economy engine but you can build a good compromise engine if you follow the rules, meaning no huge lumpy cam , cause it sounds good, no huge carb/s cause they look good etc
    Roger

    an old fellah I know from maine, "deals in stanley steam cars" told me this
    performance
    economy
    reliability
    now pick two, but remember, your exponent is money, the more of one you want the more the other will cost to achieve I kind of took that one with me.
     
  6. NintendoKD
    Joined: Sep 6, 2008
    Posts: 139

    NintendoKD
    Member

    yes not at all efficient with a lumpy cam at idle, its designed to work with high speed air/fuel at high RPM, what it does at low RPM isnt something to care about if your after high RPM HP.

    can you expound more on this please? is this a similar to the effect of why variable valvetrains are so successful today? in all various types and colors? I find this interesting, the dizzy had a mechanical advance, it seems only logical to apply the same principle to a rotating camshaft. I need some more reading material;)
     
  7. Johnunit
    Joined: Dec 31, 2010
    Posts: 93

    Johnunit
    Member
    from Toronto

    I wouldn't worry too much about the effect of what unburnt fuel does in the exhaust as far as the forces it exerts on the combustion chamber intake charge.


    In layman's terms, here is the purpose of overlap:

    At a certain RPM, the amount that the inertia of the spent air/fuel mix going through the closing exhaust valve pulls the fresh/unburnt intake charge through and into the cylinder will be of benefit. This is, in an engine with a camshaft suitable for the rest of the engine and the car it'll be put in, right in the 'power band'.

    The idea is that the suction from all that air now shooting through the exhaust and wanting to take everything behind it with it will help pull ALL the spent air/fuel from the cylinder and will give a head start for the intake charge. The head start means that instead of just the force of the vacuum/suction from the piston going down, you have the force of the exhaust helping to pull in the intake charge. Ideally, the exhaust valve fully closes right as the last of the spent air/fuel goes through it and right before the first of the new stuff from the intake comes with it and is wasted out the exhaust. Then you have an intake charge with extra inertia/speed from being pulled along by the exiting spent air/fuel mix, and it crams itself in with that intertia or momentum. That means when the intake valve closes there's more of it (a denser charge with more energy) to ignite, and thus more power.

    Where the lumpy idle comes in is that you can only design a camshaft to do this at a certain RPM range. On a typical american v8, maybe 2500rpm or so. This means that below that (with increasing severity as the rpms drop) the exhaust valve hangs open long enough for the intake to be spat out the exhaust, or even for it to sort of yo-yo back and send exhaust into the engine again. As you might imagine it's a very unstable process, with different things happening each time the engine turns round because of a chain reaction from the last cycle's issues, so you get an engine that hunts and pops and farts and won't run properly at low RPMs.


    At RPM's above the speed the cam is designed for, you don't get a missfire condition usually like you do at idle, but you will see decreasing benefits from the overlap because there won't be enough time with both valve open for this sucking/scavenging to take place. The engine is turning faster, so the valve isn't open as long.


    Yes a variable valve timing system is attempting to, essentially, recreate two camshafts (or an infinite number, depending on how smart the system is). Sometimes it's about overlap, but other times it's about retarding and advancing the cam (so that the intake and exhaust 'events' happen early or later in relation to the piston's movement). Generally though, the main idea is to have a cam that is designed for, say, a 2500-4500rpm powerband at low rpms, and then change the valve events to be later, or longer, or with more overlap, or all three as the engine reaches higher rpm's, so that you can end up with valve events at 8000RPM that wouldn't allow the car to idle below 1500RPM. Since the cam is moved about in a fashion to be more economical and idle more smoothly when you go back to idle, you now have the best of both worlds. Often incredibly complex, as you might imagine, but when it works it's a huge advantage as far as creating a wide useable RPM range. They've even figured out how to move the valves around to simulate EGR function, but that's a whole other topic and one I don't 100% understand.

    Hope this helps.
     
    Last edited: Oct 4, 2012
  8. NintendoKD
    Joined: Sep 6, 2008
    Posts: 139

    NintendoKD
    Member

    What is the potential difference, in torque, or horsepower at a given rpm, "say 5000rpm" for the same engine running lets say, a serious radical cam, a mild cam, and a cam with no valve overlap, numbers would be helpful? how does this effect engine vacuum? what is the difference off the line?
     
  9. NintendoKD
    Joined: Sep 6, 2008
    Posts: 139

    NintendoKD
    Member

    horsepower is not really my goal here, so occams razor, all other things being equal, the cam shaft will do what at idle and do what at 5krpm? and why? the idea of burning an exhaust valve or intake valve is not appealing to me, so I guess I stand more on the side of reliability and economy? Everything has been extremely helpful so far. thank you all, I hope this helps someone else.:cool:
     
  10. Johnunit
    Joined: Dec 31, 2010
    Posts: 93

    Johnunit
    Member
    from Toronto

    What an engine "wants" depends on all sorts of things, when it comes to cam design. I'm a small block ford guy mostly, so I'll use what I know. At 5000rpm on a stock head 302ci/5.0 mustang, with either a mild performance carburetor and intake (600cfm holley 4 barrel and an edelbrock performer RPM let's say), or the stock fuel injection system on 86-95 mustangs, you might go from 220hp to 270 or so by going from the stock cam for those later mustangs to about as rough a cam as you could reasonably run on the street, if you do nothing else to the engine. Where it gets complex is that a different cam will affect how much other changes increase power. For instance, generally speaking as cams get more aggressive the exhaust system gets more important.

    The vacuum effect in my example would be enough to potentially require re-tuning the carb. Definitely re-tuning the computer on a fuel injected car, though that's beyond the scope of this site. The main issue that people run into with a lack of vacuum on a stripped down car like most HAMBmobiles is with the brake booster. The power brakes rely on engine vacuum to work. Most systems are already designed to handle short-term no-vacuum situations at full throttle, but if you're pulling relatively little (say less than 10" as opposed to the 18-23 on most stock engines) at idle, that can take the 'power' out of power brakes. Some hamb-era cars also used vacuum for wipers, meaning a vacuum leak or opening the throttle for too long left you blinded in the rain. Not exactly safe. As you get a "choppier" or rougher idle, the reduction in vacuum is often not steady. Whereas a production engine making 20" of vacuum should have basically no movement of the needle on a vacuum gauge at idle, a car only making 6-10" will often vary significantly. This is part of the 'yoyo' thing I mentioned earlier. More vacuum dumps more fuel in, causes a weird misfire, kills vacuum, reduces the amount of fuel being pulled in, you get the idea.


    "power off the line" is sort of hard to quantify because you can change what RPM range the engine is in "off the line" and for how long by changing the transmission and rear end or even your driving habit. For instance, the car I'm building will need to make power starting at 2400RPM or so because I'm keeping the overdrive transmission, running a 'highway' rear end ratio meaning less mechanical advantage and lower RPM's compared to wheel speed in every gear, and I'll be using a torque converter that "stalls" at 2400rpm when you floor it. For that last one, picture being at a stop light and holding the brakes down hard, you floor it, and the tires don't slip and you don't go forward. What speed is the engine turning? That's your automatic transmission torque converter's stall speed. On a production car it's usually below 2000RPM, meaning that's the speed the engine has to make full throttle power starting at. On a race car it can be north of 5000RPM, while a street/strip can can be anywhere in between. Of course as you get to higher RPM's, the increases your cruising RPMs, reduces how much the car will roll forward in drive at idle, and just generally makes the car suck more gas and behave less nicely at low speeds.


    What does all that mean for the camshaft, and engine in general? Let's say I build two identical cars. Same tire size, same transmission, same weight, etc. The only difference is the torque converter stall speed and the camshaft:

    I put a cam in one engine so it makes 300 lb/ft of torque at 2500rpm, and 350 lb ft of torque at 4000rpm. A mild performance 'streetable' cam based on those numbers with no other knowledge of the engine. I have that engine mated to a converter that 'stalls' at 2500rpm. This means when you floor it, before the wheels are spinning fast enough for gearing to allow the engine to speed up beyond it, the engine is at 2500rpm. So this engine makes 300lb/ft of torque off the line. Not bad.

    I put a cam in that car's twin that is much more aggressive. let's say it can only must 200 lb/ft of torque at 2500rpm, but 450 lb ft at 4000rpm. Obviously I've made a BIG tradeoff for more high-rpm power at the expense of low-rpm torque, right? Where things get tricky is that if I put a torque converter in this car that stalls at 4000rpm, I'm making 450lb ft off the line, instead of 300 lb ft. This despite the engine making FAR less 'torque down low' than the other. If I had just slapped this cam (note the power difference I gave is extreme) in the first car, without changing the converter, I'd have a car that was a slug off the line relatively speaking. You'd probably already be speeding before the engine spun fast enough to feel the benefits of that new cam. But I've hidden that trade-off in power by running a torque converter that never allows the engine to be under load at the rpm's where it makes poor power. Of course in a manual transmission car this is mostly done by changing when you shift and having to keep RPM's higher when you first take off from the line. That's not without it's annoyances and tradeoffs (clutch wear, possibly not being able to cruise in the city in top gear, etc.)


    I realize there's a lot of vagueries involved in my answer, but it's a pretty complex subject that's difficult to give definitive answers to without having a specific cam and engine in mind.



    a non-hambfriendly aside:

    A cam with no overlap, on modern cars, is somewhat rare. The cam in a non-performance early small block is laughably "small", as in short duration (how long the valves are open), low lift (how far the valves open), and minimal overlap compared to most modern cars. This is partly because as electronic fuel injection and the like have gotten better the rough idle and fuel consumption issues have been lessened. The engine can compensate for it more readily. Critically, in a fuel injected car there isn't as direct a reliance on vacuum and intake air speed for fuel delivery and atomization. Instead of being sucked out of little holes in the carb at whatever rate the sucking motion causes, the computer can actually read the air/fuel mixture in the exhaust, how much throttle you are using, and a half dozen or more other variables to decide how much fuel is needed. On top of that, instead of the fuel coming out in whatever form and pattern the air around it causes, fuel injection of course squirts the fuel at high pressure in a nicely atomized spray. While that spray can still puddle on the bottom of the intake runner or do other silly things, it only has the 5 inches or so in the actual head to do it, instead of having to navigate the entire intake manifold.
     
  11. A-Wall
    Joined: Aug 6, 2008
    Posts: 488

    A-Wall
    Member

    so this is an awesome thread.
     
  12. davo461
    Joined: May 13, 2007
    Posts: 345

    davo461
    Member

    Lots of overlap can lead to a 'lean-burn' = holes in pistons. I managed to do that a long time ago; nasty.
     
  13. Camshafts can be very challenging to figure out.
    My first lesson with overlap, also known as the scavenge cycle, was installing a cam in a 283 that had 120# compression before and lowering it to 80# after the cam installation.
    The rpm range of the engine also changed with the cam install, as to kill the bottom end and add all of the power to the top end !!! >>>>.
     
  14. In fact no one sat down and decided a cam needed overlap. They discovered that in making a cam with sufficient duration to run at higher RPM that the intake and exhaust wound up being opened slightly at the end of ex beginning of int. Ed Isky for one explored using this deal to produce a bit more power. Headers play a big part in this in an unblown engine. The header produces a neg pressure due to the refracted wave which changes from pos pessure to neg pessure. This neg pressure pulls slightly at the slightly open (now) intake valve and starts ever so slightly the flow of intake air /fuel into the port BEFORE the piston actually starts down the bore. Might seem like nothing or not much but if one tunes for it, it can make a significant difference since the cylinder is starting to fill at this point , be it ever so slightly. Otherwise it would have to wait till the piston was moving downward to recieve a neg pressure signal to induce or begin air /fuel flow through the intake valve. Ideally we would like max flow and lift to occur at max piston speed but this is not practical (although not impossible) To get that to happen or encourage anything in that direction we need to start flow ASAP since max piston speed occurs BEFORE the 90 degree point and max lift is often only achievable at 108 to 110 degree approx on a straight up cam install. (MAX piston speed occurs at the point where the con rod and the crank throw form a 90 degree angle in relation to the mains)
    Overlap is often bantered around a lot but it is a consequence of large duration not the intended goal. Hot rodders just figured out how to use it to an advantage. In a blown engine it can be used to even further advantage. During this period the burnt fuel(smokyness) is completely eliminated in the cylinder and fresh air /fuel replaces it with a wee bit sneaking out into the exhaust port just before the door (ex valve) is slammed shut.. By now the piston is moving downward and the intake valveis opening wider and wider and the ALREADY established air /fuel flow grows quickly.
    Just remember it is what occurs when the intake and exhaust lobe both have large duration.(ie more than 180 degrees or greater than the number of degrees in one stroke of a 4 stroke cycle) Drawing the old cam card diagrams. Inside /outsde C helps undersand it immensely.
    The MOST important valve event is intake closing but that is a whole other discussion.
    Don
     
    Last edited: Oct 4, 2012
  15. 73RR
    Joined: Jan 29, 2007
    Posts: 7,174

    73RR
    Member

    Is this just a case of "how does this work?" or is there a particular engine project that you are trying to sort out?
    A lopey idle can be caused by different events in different engines: sbc's like lobe centers down around 106° but an EarlyHemi will barely run (at idle) at that number. This tight lobe center deal is the focus of CompCams 'Thumper' series. They make plenty of noise but are not a great all around performance cam since they are making too many comprises.
    GM used some big overlap numbers back in the 60's in addition to large, for the time, duration numbers during the all-out muscle car advertising wars. None of the cars ran appreciably harder than the next guy, but they 'sounded' like they should.
    In a blown engine, big overlap numbers are used to help keep the combustion chamber and valves cool as the fresh fuel charge washes through. Fuel efficiency is not a concern here.

    .
     
  16. S_Mazza
    Joined: Apr 27, 2011
    Posts: 363

    S_Mazza
    Member

    I think this post gets most to the heart of the matter. If air worked digitally, cams wouldn't really have overlap. But since it takes a while to get the air moving, you have to make some trade-offs.
     
  17. V4F
    Joined: Aug 8, 2008
    Posts: 4,382

    V4F
    Member
    from middle ca.

    it causes the "ka-chunk ka-chunk" that peeps like . my lil banger has a small ka-junk with the cam i have ........ raw fuel at an idle ...........
     
  18. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
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    Overlap is an unintended consequence of having the valves open when they need to be open, to get the air in, and the exhaust out, at high rpm.

    In other words, you can usually ignore overlap, and concentrate on having the valve timing set up so the engine makes the most torque at the RPM range you'll be operating the engine.
     
  19. original reason for all of this techno stuff was discovered by a cat named Otto! Seems he thought the exhaust valve and the intake valve should both be open at tdc as to draw a cool intake charge by the exhaust valve to help cool the exhaust valve.
    Term commonly known as the Otto-Cycle !! And the phrase as the 5th cycle engine !!
    Confused yet ??? >>>>.
     
  20. I didnt think Otto had a clue about overlap. I am willing to learn if he did. His contibution is suck squeeze bang and blow from my understanding. What have you read that would add overlap to that from herr Otto?
    Don
     
  21. in one of Isky's early publications Ed gives credit to Otto for the discovery. In later publications Isky claims fame to the Development of the 5th Cycle Camshaft >>>>.
     
  22. greg32
    Joined: Jun 21, 2007
    Posts: 2,230

    greg32
    Member
    from Indiana

    Great thread, one other comment, with a roller lifter you can open the valve much faster than a flat tappet. Therefore, you can have more valve opening for a given degree of rotation. So, you can get the opening and closing events and timing you want without as much overlap as a flat tappet cam.
     
  23. rustednutz
    Joined: Nov 20, 2010
    Posts: 1,580

    rustednutz
    Member
    from tulsa, ok

    No one yet has mentioned the efficiency of the heads combustion chamber. How efficient the chamber is determines basically what the cam specs should be. Look at the modern LS1 engines, they have an extremely efficient combustion chamber and port design that allow them to make big horsepower from small cubic inch. The more efficient the head the less overlap is needed. Again using a modern engine as an example the lobe separation angles are much wider than earlier engines. We used to run 106 to 108 degrees LSA and now modern engines use 112 to 114 degrees and make 500 HP with good fuel mileage. My son and I just built a 496ci BBC for his OT street strip car. We used aluminum heads with a modern design combustion chamber, 10.5:1 compression and a relatively small solid roller cam that we ordered with a 111 degree LSA. The results: 718.8HP @ 6300rpm and 637.5 lb/ft of torque @ 5400rpm. The power band is unreal with 578.3 lb/ft of torque @ 3600rpm and it still has 599.2 lb/ft @ 6300rpm. The point is, you must look at all your components and what you want your vehicle to do and then chose your cam last. Everything must match to get the most performance.
     
  24. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,856

    Mike51Merc
    Member

    Great thread.
    In 2 cycle engines, the ports (which act as valves) will always have some overlap for the scavenging effect. The expansion chamber exhaust pipes are designed to set up harmonic pressure scavenging and the residual unburnt fuel gets burned inside the pipe, not adding HP per se, but adding to the scavenging effect which in turn increases horsepower. This is why 2 cycle engines smoke a lot on startup until the pipe gets hot, then they smoke just a little.

    I've wondered why expansion exhaust hasn't been tried on 4 cycle engines.


    It also bears mentioning that a factory stock non-racing naturally aspirated 600cc 2 cyl 2 stroke engine can make over 120HP (but not alot of torque), but it does get only about 10MPG. They've also created variable exhaust valves (which are simply pressure activated and spring loaded) that help restore the low end torque that's lost by the above mentioned porting.
     
  25. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    Some great info here. Maybe I can contribute some amusement.

    How did they figure out to use overlap? An old time English motorcycle racer talked of testing his bike on the Brooklands race track in the early 20s. One day as he was going around he suddenly picked some extra power and 2 or 3 MPH in top speed. On investigating, he discovered one of his valve springs was broken and the broken ends happend to interlock so that he still had some spring action but weak. He deduced the weak spring allowed the valve to "float" off the cam and stay open longer.

    So he went home and designed a longer duration cam. He then drew the cam profile on bits of cardboard, glued the cardboard to the cam lobes, and filed them to shape with a fine tooth file. The bike went faster and this led to some further thinking and developing of cam designs.

    Whether this is how the importance of cam overlap was discovered I don't know but it happened pretty early in the history of engines.

    Another story. The first Buick OHV V8s had an unusual restrictive head design with small valves. When they first designed the head they had no idea the "horsepower race" was going to take place the way it did. To keep up, they had to design some radical cams (for a stocker). By 1957 they had so much overlap it was hard to get them to idle down smoothly unless everything was just right. Their stock cams were next thing to a racing cam.
     
  26. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    Has anyone ever tried building a 2 stroke with sealed, lubricated bearings in the bottom end, and fuel injection in the transfer passage?

    The bottom end would see nothing but fresh air, the fuel would be injected directly into the cylinder, if it was timed right none would go out into the exhaust. The piston and rings could be lubed by synthetic oil pumped thru a tiny hole in the cylinder wall, just a whisper would be enough.

    This should clean up the pollution and improve the fuel efficiency enough to make the 2 stroke competitive on emissions and gas mileage.

    I thought of this when 2 stroke street bikes were disappearing off the market in the late seventies. It seemed so obvious I kept expecting some big company to do it, but I don't think they ever did.
     
  27. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,856

    Mike51Merc
    Member

    I know that snowmobiles and outboard motors both have direct injection systems available.
     
  28. Everything must match to get the most performance.

    Rustedntz.
    Well put. Right there you have spoken the biggest and most important speed secret ever.
     
  29. porsche930dude
    Joined: Jan 5, 2008
    Posts: 274

    porsche930dude
    Member

    thought id post about my experience with this. I play around with small enegines. 3-5hp briggs and the like. they come with a good deal of overlap. It seems alot of that is to make it easier to pull start and run smoother due to less compression. A good trick is to shave the valve stems to increase valve lash and effectively bump the compression by reducing the overlap and lenthening the time the valves are closed. Even though the valves arnt open as long or as high. it still bumps the power up significantly and makes it easier to start because there is more vacume in the intake. Though alot harder to pull the chord and you often get kickback if your not carefull. I also notice on the older engines pre 1955 dont have much overlap at all so you cant do this
     
  30. Neat thread here guys.
     

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