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    If we were to design the ideal camshaft for idle speed running (600-700 RPM), what would the valve timing look like? I ask this question for two reasons. First, we normally don't run our model engines under load, or at speed much of the time where valve overlap is of benefit. Model engines will never see a dyno to demonstrate their maximum potential of speed and power. However, every model engine builder does appreciate an engine with a nice smooth idle which is where our engines spend most of their time running. A smooth idle (or running) isn't easy to attain at no load. My second reason for asking is that I am building a few vintage model stationary engines that originally ran at a slow constant speed. I would like to model that kind of operation as best I can. I understand there are other factors to slow speed operation like flywheel diameter and rim mass, carburetion, and friction but I think the valve timing is also a major factor.

    What I have observed is that some of the smoothest running (low speed) old engines used atmospheric intake valves which probably open 20 degrees ATDC and close 20 BBDC or sooner. Their exhaust valves usually close ATDC. My 1920's vintage Briggs and Stratton FH engines come to mind. They have a crude carburetor (similar to our RC carbs), atmospheric intake valve, produce reasonable power, will run over 3000 RPM, and will idle all day without missing a stroke.

    So, my question is, for slow speed operation, what type of valve timing would be "ideal" using a mechanical intake valve? Any thoughts or experience greatly appreciated.


    Great topic, besides the cam design, the tappet / follower design will play a significant role in engine performance at idle speed.
    The use of roller tappets / followers allows for both easier cam fabrication ( milling) and engine performance at low speed.
    Overlap of valves ( period during which both valves are open) tends to be smaller in slower engines ( intake opening maybe 5-6 deg before TDC and exhaust closing 20 deg after TDC).

    One engine that comes to mind is the Mastiff designed by Masson (1973), Flat four boxer type engine, runs quite smooth at low RPM's decent RPM's in the high end and overall nice performer.

    In the Mastiff, the designer uses more overlap than other similar volume engines ( Intake open 10 deg before TDC and exhaust closes 20 deg after TDC with a 110 deg Cam angle ), but I guess this was needed since the engine has pistons opposed 180 deg and is side valve design which causes some problems with scavenging of gases.

    Valve duration is another consideration, some slow speed engines use 135 deg for exhaust and 125 for intake, cam angle from 105 to 110 deg (this is dictated by the valves total open period and the overlap).

    Hope this ideas add more to the confussion.

    • CommentAuthorgbritnell
    • CommentTimeMay 27th 2010
    Hi Jeff,
    I don't think intake valve timing has as much to do with slow running as does the centrifugal weight of the flywheel. An engine is going to be more efficient, translating to running better, with an adequate intake charge. That's not to say that the timing has to be radical but with the piston not moving much between 20 degrees before TDC and 20 degrees after TDC opening it a little early won't hurt a thing. The purpose of opening it somewhat early is to allow the scavenging action of the exhaust flow to help pull in the new fuel charge. I can get my Holt engine, 1.00 bore x 1.25 stroke, down to about 500 rpm with the intake opening about 25 degrees before TDC. It then closes about 30 degrees after BDC. Getting into cam performance numbers for model engines is a tricky subject. When I get an engine to run well the way I want it I don't say to myself " I think I'll make another whole camshaft, pull the motor apart, install the new cam and reassemble the engine" just to see if it changes the way the engine runs.
    Engines that run slow have certain characteristics, low compression, long stroke and a heavy flywheel to name several. Ignition timing is based on rpm. The idea is to have the fuel charge burn up and produce the most energy at just the moment that the piston starts on it's downward stroke. The faster the engine runs the more the timing has to be advanced to maintain this accurate cycle, up to a point. How this equates to model engines I'm not exactly sure. My model 302 engine has a 1.00 bore and a .90 stroke. It will run up to just shy of 5000 rpm but still idle about 700 rpm. The timing on it is 38 degrees before TDC. It will idle fairly slow without much flywheel mass because it has 8 cylinders to pull it along. You notice the lack of flywheel weight when you rev it up, it's almost instantly.
    I know this is getting a little off of your question but what I'm trying to say is there are several factors for getting an engine to 'idle down'. I don't think valve timing plays a very critical role. OK everyone else, fire away. This is one of those personal subjects.
    I still don't understand how valve overlap at TDC benefits engine operation at, or near idle speeds. I say this because at this speed, intake manifold vacuum is relatively high due to throttle restriction. Much higher vacuum I think, than any draw created by the exhaust manifold pressure waves. So how does scavenging take place in the presence of high intake manifold vacuum? I would think exhaust gases would actually be sucked back into the cylinder during the overlap period creating combustion instability (rough idle) like a leaking intake valve. This is the main reason why I am thinking the old Briggs FH type valve timing where no scavenging is possible might might work better at low speeds (<1000 RPM).

    • CommentAuthorgbritnell
    • CommentTimeMay 28th 2010
    Hi Jeff,
    Maybe this will help explain the operation a little better.
    • CommentAuthordavid
    • CommentTimeMay 28th 2010
    I think you have similar concerns as I have and were discussed in the thread David’s Vertical.
    I understand how the overlapping of valves would work in a full size engine where there are larger flows to push or pull flows longer than expected due to inertia of the moving gas. I have doubts this happens to the same degree in a little engine.
    I hope to get back to mine in the next few weeks and make the head and rings and test the cams I made.
    I understand the link and the way it works in full size engines, especially at higher RPMs with more open throttle settings. I have no disagreement with the logic in an engine where valve timing is optimized. But, it doesn't give any insight into why smaller engines with atmospheric intake valves run so nicely at idle speeds and mechanical exhaust valve engines generally do not. When scaling engines down, air friction losses increase and dynamics will change. Scaling alone might totally negate any dynamic effect at low engine speeds. When you add the high pressure differential across the cylinder caused by high intake manifold vacuum, I think valve overlap becomes detrimental to uniform combustion and a smooth idle. Logic, and some of my antique engines lead me to that conclusion, but I am always open to other ideas.

    • CommentAuthorgbritnell
    • CommentTimeMay 28th 2010
    Hi Jeff, when I first answered your question I stated that everyone has their own opinion about valve timing. If you feel that minimizing intake valve timing would help make your engine run slower by all means go for it but comparing the operation of a hit and miss engine to a mechanically operated valve train is not apples and apples.
    As a side note, I don't think anyone in the modeling field has documented flow characteristics, spark timing, combustion chamber shape and so forth and so on. Everyone who builds an engine is just happy to have it perform well.
    "However, every model engine builder does appreciate an engine with a nice smooth idle which is where our engines spend most of their time running."

    Not exactly.

    If we are talking about multi cylinder IC engines, I like a slight rumble at idle. I want the engine to sound bigger than it really is. I have even had 2 comments to the effect that "someone finally built a model engine that sounds like a real one". I like the engine to sound like a performance version of whatever it is. For me it is as much about how it sounds, as it is about how it runs.

    As for single cylinder IC engines, I said my piece in davids vertical.
    Maybe we have gotten sidetracked a bit from my original question. What I am saying is that there are some really nice running commerical engines that have been produced that are not too much larger than some of the models we build. And some of them run exceptionally well. As model builders, maybe we can learn a few things from the early commerical engine designs. The Briggs FH, for instance, it a throtle governed 1/2 hp engine made in the 1923-1930 era. They run like watches at all speeds, including idle. They were good on power, and were one of the most successful early engines built by Briggs. The better running Briggs engines in the early F series used atmospheric intake valves. A while back a model builder named Norm Brockelsby made a dozen or so half scale models of the FH Briggs engines (1.25" bore), and they too ran exactly like the full scale FH engines. The valve timing on these engines is somewhat different to what is generally in use by most model builders. I wanted to call attention to this for those who might be interested in improving idle performance, or building models of slow running prototypes. Since I have only built three IC engines to date, I wanted to see what thoughts others had regarding the issues of trying to duplicate the atmospheric intake valve performance with a mechanically operated valve.

    Thanks to all for the comments. I will give it a try and see how it works.

    What makes for a smooth slow idle?
    1st stroke to bore ratio (the more under square the better)
    2nd connecting rod length crank radius ratio (the longer the con rod the better)
    3rd number of cylinder (the more the better)
    4th a conservative cam timing with as little overlap as possible.
    5th a flywheel proportionally on the heavy side.