Cam/valve timing...

[methuselah1]

Can anyone point me to a thread giving a 101 on cam timing? I keep on building engines, but I've never done it before! The engine in question is an aircooled Westbury Seagull, inspired by Charles Lamont. (Then there's a Kiwi, Whippet and Wallaby...)

Thanks!

Andrew UK

 

[stevehuckss396]

I don't remember a thread specifically about cam timing but I don't remember what I had for dinner yesterday. Maybe post the cams lift and duration for each lobe and we can probably talk you through it.

 

[stevehuckss396]

Not lift, lobe seperation

 

[methuselah1]

We'll this is actually a Kiwi cam, but it's exactly the same... I've made four of 'em now; the help I need is setting them against the piston TDC. I don't know how to use these diagrams.

Andrew UK

 

[Vixen]

deleted, posted by mistake

 

[Ozwes007]

Cam timing works within the Otto 4 cycle - generally it goes something like this
intake opens a small amount before Top dead centre (BTDC) ( the exhaust is open here on the close side of its cycle)on the overlap stroke, the piston goes down sucking in the fuel air mixture a little after Bottom Dead Centre(ABDC) it closes the intake valve, the piston continues upwards on the compression stroke, a little before Top Dead Centre it fires the plug. From there it goes back down with the Big Bang and a little before Bottom Dead Centre (BBDC) the exhaust starts to open, it then swings past the Bottom Dead Centre going up to the overlap stroke( called this because both the intake and exhaust are open at the same time at Top Dead Centre(TDC) as it approaches TDC we start the Intake opening (as per first part of this ) goes past TDC overlap and then closes After Top Dead Centre (ATDC).
Simple hey! The actual amount of cam timing each part has is dependant on quite a number of things including but not limited to
1. Engine size.
2. Piston Diameter
3. Rod Length
4. Stroke Length
5. Rev Range
6. Engine purpose
7. Flywheel size
8. Idle RPM
Hope this helps, been building Racing engines for nearly 50 years, of all configurations and categories and Cam timing is one of those Black art areas

 

[bluejets]

110 degrees is a good start.

 

[Charles Lamont]

deleted

 

[Charles Lamont]

We'll this is actually a Kiwi cam, but it's exactly the same.

Those dimensions are not the same as the Seal/Seagull cam.

Andrew, when you ask about cam timing are you talking about design considerations, or how to get the specfied timing in practice?

 

[methuselah1]

Hello Charles- we meet at last! Your Seagull was the inspiration for my own take on the air cooled approach- but I refer to mine as a double Seamew.

My question is how to dial in the cams (as drawn) to the specified timing.

-Andrew UK

 

[stevehuckss396]

From the diagram it seems that you have 110 degrees seperation in your lobe centers. The lobes are equal distance from TDC (55 degrees) . What I would do is..

(1) Fix a degree wheel to the crankshaft
(2) Set a pointer onto the degree wheel at zero
(3) Turn the crankshaft backward 110 degrees BTDC. Remember the cam tirns half as fast (55 degrees)
(4) Indicate the exhaust lobe to fully open
(5) Lock the camshaft using what ever means the engine was designed to use, set screw in the gear?
(6) Turn the crankshaft back to original TDC and then go the other way 110 degrees ATDC. This is where your intake lobe should be at fully open
(7) Turn back to original TDC
(8) Rotate the crankshaft 360 degrees to secondary TDC. This is where your ignition timing is measured from. Usually 6 to 36 degrees BTDC




This video will show you the process i used. The numbers are a little different but you will be able to see the mechanics involved. When it gets to the part about setting the distributor. I forgot to mention in the video that I had made the 360 degree rotation already. (step 8)



Hope this helps.

 

[Charles Lamont]

My method for assembling my built-up camshaft is explained in some detail on my website:
http://www.charleslamont.me.uk/Seagull/camshaft.html#assembly
For how I set about timing the camshaft correctly in relation to the crankshaft, the full explanation might be a bit long winded and difficult to follow, so please have a look at this diagram which I used to explain it to myself, and then ask questions. (My timing train is non-standard, in that I made a 21T idler, instead of the design 20T, to try to balance the wear with a 'hunting tooth'.)

 

[Ozwes007]

Try this video to assist. A lot of people find the terminology the hardest to understand. TDC = Top Dead Centre(piston is at the top of its stroke), BTDC = Before Top Dead Centre, ATDC = After Top Dead Centre, BDC = Bottom Dead Centre (Piston is at the bottom of its stroke), BBDC = Before Bottom Dead Centre, ABDC = After Bottom Dead Centre. On a multiple cylinder engine the number one cylinder is always used as the cylinder you set up on. Always remember the timing is based on cylinder timing (and yes some cams have cam timing and cylinder timing indicated so you can check the cam) The cam rotates at half the speed of the crank. One other thing you will need to know is the lift at the valve or cam the timing is actually performed at. Trying to dial a cam in at zero lift is impossible and extremely inaccurate. The Standards in the industry are 0.020” or 0.050” of an inch. The reason is because of the base circle of the cam through the ramp angle is the black magic area of cam design. As you regrind a cam the ramp angle will alter the opening and closing characteristics of a given cam profile.

 

[Ozwes007]

Here’s a good place to make a degree wheel to suit your cam.
https://www.blocklayer.com/degree-wheel.aspx

 

[D and D]

Excellent resource, thank you