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    • CommentAuthorkamran
    • CommentTimeFeb 28th 2009 edited
    Hello all,
    I’m working on an inline 2 cylinder, 4 cycle engine with 1 x 1 Bore and stroke.

    Based on an article and advice of few people in some other discussion board, I decided to go with an exhaust cam that has a lift period of 140 degree and intake cam that has a lift period of 120 degree. Also I was told that these setting are used by Bob Shores Silver bullet engine. I don’t have the plan for that engine so I can’ confirm this.
    Anyway based on these setting, I have a few design questions and hoping to get some help.

    First question is what should be the correct lube separation angle? I’m assuming something like (140 + 120) / 2 which is 130 degree, but looks like that will be too much angle, so I’m thinking something like 100 degree would be a good start.

    Second question is, let say I go with what I mentioned above, then basically how many degrees BBDC the exhaust should start to open? I’m more interested in the low speed end rather high speed.
    If I open it too soon, then I could loose some of the power stroke, but I can gain in away it gives me an opportunity to get near atmospheric pressure before the exhaust cycle starts. Now if I open it too late, then I’m putting more forces to crankshaft by forcing the exhaust out and also taken away energy from the flywheel.

    Can you give me some feedback if my timing chart is correct or not, and if not what should be the correct setting?
    What do you think for setting the exhust opne at 50 BBDC?


    So if you look at the timing chart, you will the following
    Intake cam lift period = 120 deg
    Exhaust cam lift period = 140 deg
    LSA = 100 deg

    EO = 50 BBDC
    EC = 50 ATDC
    IO = 10 BTDC
    IC = 50 ABDC

    • CommentAuthorgbritnell
    • CommentTimeFeb 28th 2009
    Camshaft design involves quite a few factors for determining the shape and timing of the lobes. Along with the rpm range you would like your engine to perform at you would need to know how well your ports and valves would flow air. Realistically this isn't going to happen as no one can measure the flow in a small engine, at least not the home hobbyist, so you can only take full size engine cam specs and go from there. As far as lobe seperation this depends on the lobe shape and the opening duration. A number to start with is 110 degrees. Lobe seperation also determines valve overlap at TDC. For my 4 cylinder OHV engine I designed my own cam and these were the specs I used. IO at 20 degrees BTDC, IC at 30 degrees ABDC, EO at 20 degrees BBDC and EC at 30 degrees ATDC. My lobe seperation angle is 95 degrees. I don't know why you would want or even need all that exhaust open time. On top of that you might run into a problem with piston to valve clearance as the exhaust would still be quite open as the piston goes past TDC. Here's a link to an article that gets pretty deep into cam timing and what it does. If you're trying to design your own cam it's worth the read
    Some additional reference material:



    Make yourself crazy studying cam profile data for most engines here "Cam Lobe Profiles" near the bottom of the page;

    Web Cams has a calculator in the section titled "Degreeing in Your Cam" - at the bottom of the page;


    • CommentAuthorkamran
    • CommentTimeFeb 28th 2009 edited
    Thanks all for the info.
    Based on the calculator link that garry posted, my timing diagram is correct. Now somehow I have a feeling that 140 lift period for exhaust is little too much, but I was told that is what silver bullet used so now the question is what is a good starting point for opening the exhaust?

    Well I haven't read all links yet, but any suggestions?


    If you make your Cam Shaft with separate lobes that are loctited on, you could play around and experiment with the lobe settings with out going through the whole effort of making a new cam..... Just a thought. Could be great experimenting on what is optimal cam design for your engine.

    • CommentAuthorkamran
    • CommentTimeMar 2nd 2009
    Hi Jan,
    Yes I'm making my cam with separate lobes until I get it right, and then maybe make ome piece camshaft.

    question for gbritnell
    Based on your spect for CAM, looks like you have 50 degrees overlap and duration for both exhaust and intake are 115 degrees.
    Based on what you decided to design your CAM this way?

    Also over the weekend I did a little experiment on one of my OS 4 cycle RC engine and this is the timing for that engine is

    IO = 35 btdc
    IC = 45 abdc
    EO=65 bbdc
    EC=25 atdc

    Based on the numbers above...

    102.5 degrees seperation
    59 degrees overlap
    259 degrees intake duration (129.5)
    270 degrees exhaust duration (135)
    Lobe center intake 95.5
    Lobe center exhaust 110

    Hey Uncle Dale, is this where you got the idea for 102 degrees seperation? Seems to be what the RC world is using.

    From what i understand narrowing the seperation will increase Cylinder pressure and higher rpm potential by allowing the intake to close earlier and the exhaust to open later. Widening the seperation will improve low end and idle quality.

    Seems to make sense that the RC world would decrease the seperation because they generally operate at full throttle. The wider seperation would be better for the model engines that wont be doing any work.

    I went 110 degrees for my camshaft.
    • CommentAuthorgbritnell
    • CommentTimeMar 3rd 2009
    I know that alot of reading has been presented in the previous answers but part of what was explained on a couple of the websites is that the seperation angle doesn't describe the cam events entirely. Depending on the profile and duration of the cam this can change the seperation angle. If one has a drawing program on their computer you can layout the shape of your lobes (along with the size and shape of your lifter) and actually see what is taking place. The hardest thing for the model engine builder to do (without a cam grinding machine) is to replicate a true lobe profile with opening and closing ramps etc. Most modelers who make seperate lobes and position them on a shaft just machine a base circle with 2 flat sides and a maximum lift arc. As I tried to explain earlier, cam timing is more than just numbers. In the true performance world intake flow, exhaust flow, valve size and port postioning have as much to do with cam design as just saying what the lift, duration, and lobe seperation are. It really is informative to read some of the posted links and see exactly what is going on when these elements are changed relative to one another. When an engine is put on a dyno and they take a baseline reading with one cam then only change one element (lift, duration, lobe centerline) and make another pull it's interesting (at least to me) to see what occurs. I don't think there's many of us out there that build a miniature engine for the purpose of using it for work, we just want it to run and enjoy our machining efforts. I think from the comments that we are getting a little deeper into this than is needed. For those of you that haven't seen my 4 cylinder OHV engine runnning on Youtube I will include the link. My engine performs quite nicely. One of the elements that hasn't been discussed in helping a small engine run slower is flywheel size. I had to increase my flywheel several times to just get the engine to run where it is now.
    Gbritnell, nice running engine, nice machining..
    I do agree with your comments, been building racing engines for quite some time now, 2 / 4 stroke, and the engines we model are more for the pleasure of fabricating and running display models.
    In practice, I have built three or four different camshaft designs for 2 engines and tried them and try to tell which one is performing better is quite a challenge, I built a camshaft grinder so I can replicate the profiles easier, have changed angle duration, lift, overlap etc. They seem to have little to no impact unless you follow up with the proper design in valve geometry, port diameter, combustion chamber and compression ratio.
    Right now working in a couple of engines who will do work in boats, 56 inch hull, Mastiff and Wallaby, have made two sets of camshafts, one following ET Westbury design for the Sealion, and one design following design principles from the WALL 50 cc engine. Will have a go at both under power and determine which one works better. But my experience in boat engines says it is NOT the Camshaft that will make the difference but a good Carburetor design with nice compensation and constant fuel level via float chamber.
    So pick up a duration, degrees of overlap, match the lift to the valve opening / valve diameter ratios and give it a go.
    I prefer solid camshafts with a degree adjusting wheel so I can fine tune the timing between cam and crank.
    best regards.
    • CommentAuthorjay rich
    • CommentTimeMar 3rd 2009
    i agree with jamie so far i think the west bury designed cams are great but i could be wrong for the a.r.engine these cams work realy nice i made the mistake of makeing them from brass but later found out as they worn in the engine got better in sound and breathing meaning the way the engine takes in fuel and the exuast flow oh and by the way gb great four cylinder sounds nice and i would like to say this is a hobbie have fun with it i know we all get nuts but unless one of us invents a shrink ray gun and we need our engines for a car or bike have fun ....jay
    Hello all, this is my first time on this discussion group, although I have been following for some time now, can somebody give some feedback for the following question? A cam lobe can look/be fat or slim in shape, depending on the flank and nose radiuses, how are these radiuses arrived at? Can anybody help please?
    sunny stivala
    • CommentAuthorgbritnell
    • CommentTimeApr 21st 2009
    When you say fat or slim maybe you're refering to the shape of a flat tappet cam lobe as opposed to a roller tappet cam lobe. A cam lobe is basically a base diameter with projection for lifting the lifter which opens the valve. The shape of the radius on the high point is controlled by the duration that is calculated into the lobe design. A flat tappet lobe pushes against a flat faced lifter and therefore the specs for the lobe are controlled by its contact with that surface. A roller tappet lobe has a lifter with a roller in the end of it. The diameter of the roller is controlled by the diameter of the lifter. The roller has to be small enough to fit through the lifter bore. The shape of the roller lobe is completely different than the flat tappet lobe as the tangent point of the roller against the lobe has a completely different geometric relationship. If this is too confusing reask your question with a little more information.
    gbritnell thanks for your attention,
    Here is what I hope is a little bit more information.
    The cam lobe shape I am talking about is the one designed by 3 parts of a circle, to operate a flat tappet, by the 3 parts of a circle is meant, [1] the base circle radius [2] the flank circle radius [3] the nose circle radius,
    My question is, how are the flank radius and the nose radius sizes arrived at?
    Thanks sunny stivala
    • CommentAuthorgbritnell
    • CommentTimeApr 21st 2009
    Hi Sunny,
    Let's start with the nose radius. This would be determined by the duration specs of the cam lobe. Once the valve was fully open the lifter would ride on the nose radius for whatever the duration was. The flank radius is a ramping radius the slowly takes the lifter up and lets it down without slapping on the otherwise flat sides of the lobe. It's and extremely large radius because it's only meant to ease the lifter up until it gets to the nose radius. For model engine cams about the only way to put in a flank radius is with a cam grinder that follows a master lobe template, and then on the finished model cam it would be so small it would be almost unmeasureable. Some full sized applications just aren't needed for model work. The reason being, they don't need a 100,000 mile warranty and they aren't going to do any heavy work, in most cases. I hope this helps a little more.
    There is info in the tips and links section that tells how to make a cam with a radiused flank using a fixture to offset turn the cam in a lathe.
    gbritnell again thanks for your prompted attention,
    I still need to understand at least how the nose radius size is arrived at or calculated, I am now starting to understand that the flank radius will more or less fall into place, at least in model cam lobes, once you have the base and nose radiuses calculated. So it is the nose radius that makes the cam lobe looks what I call fat or slim.
    I fully agree with you, the best tool is a cam grinder to form a harmonic cam lobe, but although it will leave a bit of hand work it is easy to use a fixture to offset turn the cam lobes flank radius, as Steve Huchss said, might be this is the oldest method that ever was in use.
    • CommentAuthorkamran
    • CommentTimeApr 22nd 2009
    Hi Sunny,
    Are you talking about the CAM duration which how long the CAM should hold the valve open?
    If so, then depends on your design. The lower duration gives you a good idle, low rpm torque.
    As you increase the duration your idle become rougher but you improve horsepower.

    But I would say you also need to think about lobe separation or the overlap between the exhust and the intake.
    Of course for longer the duration CAM you will loose compression so then you need to worry about compression ratios and other things.
    So as you can see from my first post, I wanted to have it designed right but too many things to worry about for model engine.
    Kamran, no I am not talking about duration, I am talking about the nose radius.
    • CommentAuthorgbritnell
    • CommentTimeApr 22nd 2009
    Hi Sunny, as Kamran stated, the nose radius is controlled by the duration of the lobe. There is no one number (radius) that can be applied to a cam. Each one is different. Here is a drawing I made. Maybe this will help you understand what we are talking about.
    • CommentAuthorgbritnell
    • CommentTimeApr 22nd 2009
    I forgot about posting images here or the lack thereof. Here's a direct link to what I'm talking about.

    • CommentAuthorgbritnell
    • CommentTimeApr 22nd 2009
    Sorry, I spent a half and hour making a drawing and I can't post if for you. Maybe the administrator can help us out.
    • CommentAuthorgbritnell
    • CommentTimeApr 22nd 2009
    email me at and I'll send you the picture I drew.
    gbritnell, i e-mailed you as rquested, and thanks for the trouble.
    gbritnell and Kamran. back to the cam-lobe nose cercle.
    I draw a cercle of diameter 0.440ins/11.176mm, this is BASE CERCLE.
    I draw a cecle of diameter 0.620ins/15.748mm, this is the outside diameter of the blank.
    I draw two lines from the center of these two cercles one each side of the vertical center line, 65 degrees each side, which will give me 130 degrees duration.
    from the point were the duration line crosses the base cercle,I start draw an ark,which we call the flank radius,on a radius of 1ins/25.4mm, this will end me up with a nose radius of a certian size to be able to join the 2 flank radius line at the nose.
    If I increase the flank radius I will need a bigger nose radius to join the flanks radiuses,
    If I make the radius smaller I will need a smaller nose radius to join the flanks radiuses.
    I have/use Autocad to draw with, I wish I know how to, as other wise I would have send gbritnell a few dawings to explian things a shade better. sunny stivala
    att gbritnell, i have tried to send you a auto cad drawing in an attempt to express myself better, I do hope you get it ok.
    gbritnell I got your drawing thanks.
    The link to ->link<- has been removed when I got there.