# Better Insight into Cam Timing



## Brian Rupnow (Jun 21, 2014)

I need to find some good literature on cam timing as it applies to small, single cylinder, low speed i.c. engines. Book that takes a practical, real world approach to lap, lead, etcetera. Any good suggestions?--Remember, I am in Canada, so probably an American publication would be more convenient to access.


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## Lakc (Jun 21, 2014)

Such a book would be very helpful, but I fear it does not exist. At least, not one that specifically targeted. 
What will exist, are various SAE papers on the subject, but not easy to find unless you have access to library that has them. There have been a lot of recent papers regarding variable cam timing. Some good info can be gleaned from various aftermarket camshaft grinders and performance sites. 
What I think you and I are both looking for is out there, but mostly its "reading between the lines", and not spelled out too conveniently. 
Cam timing is only part of the equation. The way I learned it long ago was air+fuel=power, and thus more air+more fuel=even more power. The fuel has to make it perfectly mixed with the air and not just running along the intake runner. That means you need to keep the velocity up and eliminate bends, dropouts, and stagnation areas in the intake side. Exhaust side is treated likewise, as not to hinder the intake and possibly to obtain some useful scavenging effect from the negative pressure pulse created as the mass of exhaust gas uses its momentum and pulls away from the closed exhaust valve. All that requires a lot of math involving the mass of air and piston speed, and calculating pressure differentials across an orifice (valve area) and thermodynamic effects of expansion from combustion.
Even more fun is calculating the clearances involved in our tiny engines and watching how they affect the actual cam timing as we can build them.


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## petertha (Jun 21, 2014)

Two posts here on HMEM if it helps. 
http://www.homemodelenginemachinist.com/f12/designing-engine-21808/
http://www.homemodelenginemachinist.com/f26/valve-timing-4-stroke-glow-17093/
I have the Malcom book & its kind of guide-lines & ratio based. I really should validate & upload my Excel tool one day, might be useful. I've also done just simple comparison tabulations across analog engines. Some parameters 'scale' to FS engines, others not, probably to be expected.
  My own personal take-way is there is actually quite a bit of variation amongst model designers even among roughly similar engines classes. Now what that actually means is another matter. Maybe the models are tolerant of variation. Or maybe there are some good runners in the mix & others less so. Hard to know unless you've seen them run. For my own purposes, I was tending towards copycat-ing parameters of commercial (RC) engines, only because that's what I'm seeking to replicate & I know firsthand they work for a living


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## KLG (Jun 25, 2014)

There is a  classic book that is really worth a read called 'Tuning for Speed' by our very famous  Ozzie Phill Irving. First Published in 1948 and now in its 6th edition  ISBN 0 908031 29 7. Do not be put off by the word speed in the title as Phill  clearly shows how different parameter changes affect the performance of the engine. He has also a book on two strokes but I do not have a copy as I was only interested in 4 strokes. If you want to approach more towards the theoretical  ''The High-Speed Internal Combustion Engine" by Sir Harry Richardo and JGG Hempson  enables some insight of engines. 
Cheers,
Kerry from Oz


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## Goldflash (Jun 25, 2014)

Try this web site 
Gives a lot of timing figures right back to Model T fords 

http://www.tildentechnologies.com/Cams/CamHistory.html

Ralph


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## Brian Rupnow (Jul 3, 2014)

I have just ordered "Miniature Internal Combustion Engines" by Malcolm Stride. I am not certain that this is the book I really want, as some of my questions regarding cam timing have risen from the engine I so recently built which was designed by Malcolm stride.--However, the man seems to "know his stuff" around small engines, and I found his overall design to be very complete and accurate and well executed. I had trouble reading some parts of his drawings, but I think that can be put down more to differences in drawing styles than any gross inaccuracies in his drawings.  My birthday is coming up very soon, and my wife asked me this morning what I wanted for a birthday gift. I thought that this book would be informative and perhaps shed some light on the aspects of small engine design that I am still rather vague about. George Britnell---You should write a book yourself if you haven't already. I found more information in your post than I have been able to find in many reference books I have looked at.---Brian


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## Swifty (Jul 3, 2014)

Hi Brian, I found Malcolm's book well worth it, but you are correct regarding his drawing style, it leaves a lot to be desired. It's very easy to be confused with his drawn parts as there seems to be no difference between hidden lines and solid lines, you really have to be very careful.

Paul.


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## Brian Rupnow (Jul 3, 2014)

I have just read a truly amazing excerpt from a book called "Model 4-stroke Petrol Engines - Designing, Running, Building" by Len Mason, which answers many of the questions I had about cam timing. I was so impressed by his clear explanation of the subject that I immediately did a Google search, and found that this book is still in print and is available from Amazon.com. The only problem is that the price of a new book is over $200 Canadian. However, (and fortunately), they also sell used copies, and have them available at much lower prices, depending of course on their condition. I have ordered a "good used" copy for about $45 plus shipping and tax, which brings it to about $61.  I now have two books on order, and have completely blown my budget for "summer technical reading".---Trust me--It is hard to explain to your wife why, when she has just ordered a book for your birthday about "them little motors", why you have suddenly spent $61 on a book about "exactly the same thing"!!!


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## gus (Jul 4, 2014)

Swifty said:


> Hi Brian, I found Malcolm's book well worth it, but you are correct regarding his drawing style, it leaves a lot to be desired. It's very easy to be confused with his drawn parts as there seems to be no difference between hidden lines and solid lines, you really have to be very careful.
> 
> Paul.




Gus easy and willing victim.:wall::hDe:
Good reason why I am taking it easy. No more goofs.


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## Lakc (Jul 4, 2014)

You cant have too many books.  
The fun thing about technical reading is it brings to mind more questions to be solved by more books!


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## mu38&Bg# (Jul 5, 2014)

I might have to look these books up, I'm curious what they have to say that's different than full scale practice.

Greg


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## Brian Rupnow (Jul 17, 2014)

I purchased the book "Miniature internal Combustion Engines" by Malcolm Stride published by Crowood Press in Wiltshire U.K.    It is an excellent book on small engine design, with easy to understand articles covering all aspects of small internal combustion engines, including a great section on cam design. The book is a hardcover, of 175 pages, with many colour pictures and technical drawings. I highly recommend it to anyone interested in model i.c. engines. I believe I paid about $40 Canadian for it, including the shipping charges. You can contact Crowood Press at [email protected]   I purchased my copy through the Canadian book store Chapters-Indigo.---Brian


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## Hopper (Jul 18, 2014)

Cam design is a black art, and few share their secrets. Not sure of the differences in scale between models and Harleys, but if you go to this below  link at Andrews Products and click on any of the "VIEW" links you can download their Harley cam specs including timing, duration, lift and what applications they suit.
It may give you some insight into what factors affect what results.
http://www.andrewsproducts.com/motorcycle-parts/2013motorcycle-catalog

Of course, Harleys have whopping great low-revving cylinders, so if you google around and find similar info for small Hondas it might suit.  (Their 250cc 6-cylinder racer of the 60s was little more than model size anyways!)


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## Brian Rupnow (Jul 19, 2014)

I have just received and read through the second book I had ordered, "Model Four Cycle Gasoline Engines" by L.C. Mason. Although it is quite a good book, it doesn't give any more information than the book by Malcolm Stride, and doesn't have as many pages, illustrations, nor technical drawings, and was written in 1976, quite a lot earlier than Malcolm Stride's book, (although very little has changed in the world of small model engines since then.)---I really don't think it is worth anywhere's near the $200 Canadian that is asked for a new copy of it on Amazon.com   So--a summary of what I have read/learned. I am quite sure I knew about 90% of the information contained in these two books. They have cleared up some of the mystery surrounding cam design, and a bit of the size relationships between various engine components. The rest of the stuff, I knew already----but then I should. I have worked as a mechanical design engineer for the past 49 years, been involved in hot-rod building and drag racing for almost as long, and have built 10 steam engines and seven i.c. engines. They will certainly be valuable reference books to add to my ever growing stack of "resource literature" about this marvelous hobby.---Brian


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## Brian Rupnow (Jul 27, 2014)

So--After multiple readings of both books, I have began to comprehend the basics of cam design.I hope you are able to read the drawing posted here, because it will explain some of the theory/mystery behind the actual design of a cam itself. The timing of cams is a whole different issue, and all I will say about it here is that a relatively mild cam for a 4 cycle i.c. engine, running at relatively low rpm (500 to 2500 RPM) would begin to open the intake valve approximately 10 degrees before the piston reached top dead center on the exhaust stroke, stay open while the piston travels from top dead center to bottom dead center on the intake stroke , and remain open until it closes at about 50 degrees after the piston has started upwards on the compression stroke. I know that sounds like a lot , 240 degrees of total cam influence, but remember that when talking about piston movement in relationship to degrees of crankshaft rotation, the piston travels relatively short distances during the 30 degrees either side of top or bottom dead center, and travels most of its total distance to be covered during the 120 remaining degrees in half a crankshaft rotation. In the drawing, I picked the 3/8" shaft diameter. That is a somewhat arbitrary number, but must remain smaller than the "base circle diameter". The .080" of valve lift is a dimension that is calculated by determining that the volume of space between the face of the valve and the seat when the valve is fully open should be equal to the volume of a cross section taken through the intake runner leading to the valve. If anyone takes exception to the information I have given, or wants to ask a question, I will be glad to discuss it. Remember--Although I have been designing mechanical devices for 49 years now, this entire cam design thing is relatively new to me too.---Brian Rupnow.


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## dsage (Jul 30, 2014)

Hi Brian:

Don't forget to figure in some valve lash. A small lash of even 5thou will make a big difference in the open / close timing and therefore duration etc of the lobe.
Evidenced by the sometimes huge difference between the "advertised" and "actual" durations quoted for full sized cams. Hot rod guys like choose and brag that they have a high duration cam but when they read the "actual" duration (after the necessary 50thou valve lash is added in for solid lifter cams) they are sometimes quite disappointed.

If you know someone with a CNC mill you might want to try our grinding program to grind your lobes or just run your numbers through it to see if they are valid.

http://gcam.lucasemail.org/

http://www.homemodelenginemachinist.com/showthread.php?t=21127

https://groups.yahoo.com/neo/groups/GCam_CNC/info

Dave Sage (Mississauga)


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## Brian Rupnow (Jul 31, 2014)

Dave--the valve lash on these little engines is very small, about .005" is all that is required. If I was going to factor that into the calculation for the valve, what, exactly, would it be added to?---Brian


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## gbritnell (Jul 31, 2014)

Brian,
The clearance will change the timing events. Lets say the timing events were calculated with no clearance and the valves opened or closed at X degrees. This would give the cam lobe a certain profile. Now if we add .003 (this is the amount I use for miniature engines) then the cam would have to rotate more to take up that clearance. When you first started talking about cam design I went back into some of my engine cam specs, one flat tappet and one roller and laid out the change in timing by eliminating the clearance and even on these small engines it changed quite a bit. Would that hurt the performance of the engine, I doubt it, but it will change it.
gbritnell


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## Swifty (Jul 31, 2014)

Brian, I reread you posting #15 where you have posted the cam drawing. After reading the following posts about valve clearance / lash, I can see that it is a real headache to design a cam to allow for the correct opening angles whilst allowing for the lash. Yes, we are only making model engines here and not formulae 1 engines, but it can see as others say, that .005" clearance will make a big difference to the valve timing / duration. I may fire up the CAD later today and play around with cam profiles to see if there is an easy way out, just for my own interest.

Paul.


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## Brian Rupnow (Jul 31, 2014)

From the two books that I have recently purchased and read, I don't believe that the allowance for clearance/valve lash is really a big deal. It may be if you are building a hi performance engine, but for little one cylinder runners that basically set on a corner of the workshop table and are started up once in a while "for fun", I just don't think it matters. It is a parameter that we should be aware of, and if we are designing an engine for a high performance application it should perhaps be taken into consideration. My question still stands though---if I did take it into consideration, then what number would it effect in the calculation and layout of the cam?---Brian


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## Swifty (Jul 31, 2014)

Just to satisfy my own curiosity, I drew up your cam profile and then allowed for .005" valve clearance, the main difference that it makes is in the duration from when the valve starts to open to when it closes, yours has 120deg, while allowing for the clearance it only gives 90.6deg duration.

As it's been stated, it's only for model engines and will work fine. I assume that full size engine makers take into consideration the valve clearance when designing cams.

Paul.


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## Lakc (Jul 31, 2014)

The amount (degrees) it changes depends on your flank radius. If your lift begins at 10 BTDC but it takes another 20 crankshaft degrees before the lobe moves to make up for the lash your opening is now at 10 ATDC. 
Now if you undercut your base circle by the amount of lash everything goes back as you designed it.
Next is factoring in how different cam follower diameters change the duration.


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## Brian Rupnow (Jul 31, 2014)

L.C. Mason states that _" Incidentally,a point on cam shapes for petrol engines is that it is normal to undercut the base circle diameter behind the nose below the nominal size by the amount of the valve clearance. In the model engineers workshop it is somewhat easier to machine the shape of the cam so that the opening and closing points correspond exactly to the timing diagram. If the base circle around the "no lift" part is then machined below size by the amount of the clearance, then the clearance will be taken up by the time the shaped cam flanks come into operation, and the valve will then lift and close at a more nearly correct time." _---This is not really clear to me. From my perspective, it's hard enough to machine the exact shape of the cam as dictated by the drawing I posted, without having to consider this additional bit of information. If the valve lash is a known factor BEFORE the cam layout is made, (which it should be) then does one just subtract .010" from the nominal base circle before the cam layout is made, or do you make the entire cam first, then go through a secondary machining sequence to put in the .005"undercut" around the base circle?--This seems like it would be terribly impractical to me.


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## gbritnell (Jul 31, 2014)

Hi Brian,
Here's a layout of the cam lobe for my 4 cylinder engine. The drawing shows the rotation angle with -0- clearance and the rotation angle with .005 clearance. 
Not enough to affect the operation of the engine but if one was building to certain opening and closing events then the .005 would make a difference. 
gbritnell 

View attachment CAM CLEARANCE DRWG ROT.pdf


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## petertha (Jul 31, 2014)

Swifty said:


> ... I drew up your cam profile and then allowed for .005" valve clearance, the main difference that it makes is in the duration from when the valve starts to open to when it closes, yours has 120deg, while allowing for the clearance it only gives 90.6deg duration.
> Paul.



That's pretty significant timing difference actually. I guess it probably makes sense though, a seemingly smallish 0.005" gap relative to a 0.070" lift is clearance 7%. I recognize that's not what translates into angular change between theoretical open/close. Just saying the same 0.005" feeler gauge gap on a 'big' engine with 0.5" lift is only 1%.

Swifty, did you do this in a Cad program by chance? I was going to suggest to Brian, he probably has the tool (Solidworks) to simulate this with layout sketch blocks. I've only dabbled with this, but I think all the features are there to set up the geometry & mates & measure the difference like what you appeared to do. If you mean pencil & paper, then I bow my head to you  please provide details! Upper pic = valve assembly. Lower pic = SW tutorial on sketch block linkage motion.


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## Swifty (Aug 1, 2014)

Petertha, yes I drew it up in CAD, it was straight forward to draw Brian's cam and then draw the .005" clearance around the base circle. I could then see when the cam actually started moving the valve and from there work out the angles.

Paul.


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## Jasonb (Aug 1, 2014)

Brian, have a look at the last paragraph here, you may find it easier to just draw it up and then put adjusted angles into the camcalc so that with the valve clearance you get the durations that you want. Far easier than undercutting.

http://modelenginenews.org/design/CamTable.php#bcr


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## mu38&Bg# (Aug 1, 2014)

.005" of lash is quite large at this scale, especially if you have small lift. Production model engines recommend, .04-.1mm(.0015-.004") and some users set it as tight as possible, while still allowing reliable starting, to get a little more power. This requires high accuracy of the cam profile. The more accurate the cam is, the less lash you can run. There is also a slight difference between lash and base circle clearance when a rocker is involved.

IMO, this only matters if you strive for maximum power output. Valve timing and valve time-area only matter at high power output.

Greg


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## Brian Rupnow (Aug 1, 2014)

The base circle diameter is a somewhat arbitrary figure, based on  roughly 6 times the valve opening, which again is a somewhat arbitrary figure. The flank radii are based on the "arbitrary" base circle diameter.The only real "absolutes" that I put into the drawing of the cam is the angle and the "cam blank diameter" which is based on the "base circle diameter plus the desired cam lift." The only "absolute" figure I can mess about with is the  angle of the cam. If Swifty's calculations are correct, and there is a real difference of 30 degrees (nominal difference between my desired 120 degrees and Swifty's arrived at figure of 91 degrees), then wouldn't it seem somewhat logical that if I wanted a REAL cam dwell of 120 degrees, I would just design the cam angle initially for 150 degrees? Then when the 30 degrees was "gobbled up" as a result of the valve lash, the cam would open and close the valve at the 120 degrees that I originally desired?


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## Swifty (Aug 1, 2014)

Brian, I think that you are on the right track, I won't be able to check on my CAD for a while, it's early morning here and my wife and I are going away on an overnight trip for our wedding anniversary. 

Paul.


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## Charles Lamont (Aug 1, 2014)

Brian Rupnow said:


> then wouldn't it seem somewhat logical that if I wanted a REAL cam dwell of 120 degrees, I would just design the cam angle initially for 150 degrees? Then when the 30 degrees was "gobbled up" as a result of the valve lash, the cam would open and close the valve at the 120 degrees that I originally desired?


This really is not the way to do it. The smooth acceleration of the valve depends on the flank arc being tangential to the base circle at the actual opening point. The geometry of the arrangement also means that, at opening, the cam flank should be in contact with the centre line of the tappet. Anything else and you are opening the valve with a impact, which will create noise and wear. This is why the clearance is best provided by reliving the base circle round the heel of the cam, with a transition to take up the clearance before opening.


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## dsage (Aug 1, 2014)

Wow !!
This discussion really took off since I posted my suggestion of allowing for the valve lash. But as usual everyone here is very knowledgeable on the topic and have given good suggestions to get around the issue. I agree that skimming a bit off the base circle at the bottom is probably the best way to solve the problem. (Something I'll consider asking John to add to the program) Right now when I grind them it's easy enough to add a bit to the duration to allow for the loss of duration when the lash is applied. It's just numbers in the Gcam program (doing so introduces the problems eluded to by Charles). Even so it has been a trial and error effort to get the cam to perform exactly the way I expect it.

In actual fact with such small lash values I'm find eccentricity of the lobe on the shaft is more of a problem. Wherein the lobe might be offset a couple of thou off the center of shaft rotation and the valve might open in the right place but not close where expected. Or sometimes even more lash has to be added to get it to close at all.

The only reason we developed the Gcam program was because it didn't make much sense to me to use cam calc to rough out a cam and then be so crude as to FILE it to shape when I had a CNC mill capable of grinding to the exact desired profile. Especially after I noticed a few thou here and there made such a large difference in the timing. Regardless of how you make the lobes, now that you are studying the problem you'll be able to do a better job at making them however you choose to do it.
   In the end it makes little or no difference. The engine is going to run just fine. It's more me trying to be a perfectionist. Having said that if I have the ability to grind a (near) perfect cam lobe then why not try and make it so.

Sage


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## Brian Rupnow (Aug 1, 2014)

dsage---I buy that line totally!!!---"In the end it makes little or no difference!!!" I have no CNC equipment, and find it a challenge to make a cam---period!!! As much as I love building these small engines, I love designing them as much or more. I think I will stick to the parameters I gave in my drawing, and forget about the undercut for valve lash. Not because the theory is flawed, (It isn't) but because there is quite enough work for me to do just designing and building an engine that runs. If someone builds an engine I design and wants to go that extra step when making a cam, then more power to them. I don't think I am going to worry about it.---Brian Rupnow


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