I see these valve timing numbers on cam cards but I dont really understand what they mean.

Lets use the Hammer cam as an example, since its pretty popular and there is a really nice pic of the cam card on John's page..



Intake opens at -1 BTDC and closes at 43 ABDC. Whats that mean? Is this a measure of crank rotation or what? Then Exhaust opens at 43 BBDC and closes at -1 ATDC. Does that mean that the exhuast begins to open 43 degrees before bottom dead center and then stay open for 179 more degrees? Wouldn't negative 1 ATDC actually be TDC?

I just now realize that if I add 180 for the distance from TDC to BDC to 43, and subtract 1 I get 222, which is the duration, but I could have the cam at 50 BBDC and -8 ATDC, how would that effect power?

J.

 

BTDC = Before top dead center
ATDC = After top dead center
BBDC = Before Bottom deat center
ABDC = After bottom dead center

This tells you at what crankshaft degrees the following events occur, in relation to TDC and BDC. if it's 0.050 numbers then it will be the rotational position with respect to TDC and BDC where each valve is at 0.050 lift.

Chris

 

Ok but what effect does moving the open and closing around BDC and TDC have? You can have the same duration, but have them opening and closing earlier and later. Is that what it means to have timing ground into the cam?

 

[quote] You can have the same duration, but have them opening and closing earlier and later. Is that what it means to have timing ground into the cam? <hr></blockquote>

Yep - if you keep the lobes the same but just make all the events earlier or sooner - well, if you change the relationship between the intake and exhaust lobe (e.g. keep the exhaust the same, but make all the intake events come xxx degrees sooner/later) then you are changing the LSA
If you move all the events (intake and exhaust) then you are changing the amount of ground in advance.

As for what they do - intake opening and exhaust closing are the primary determinants of overlap - the sooner the intake opens the more overlap you have - but also more "time" to fill the intake.

Intake closing has the largest effect on dynamic compression - the sooner it closes the higher your peak compression numbers - but you also have less time to fill the cylinder - so it's a tradeoff (and if you close it to soon your compression numbers will drop because you simply aren't filling the cylinder with anything to compress).

Exhaust opening will primarily determine how effective your exhaust evacuation is - the sooner you open it the more power you are robbing from your compression stroke - but then if you don't open it soon enough you will not be able to fully evacuate the cylinder so your next charge filling will be contaminated.

The valve events themselves are just as important, if not more, than the duration numbers - though bother are derived from the other each looks at the situation a little differently.

As you can see every event is a compromise - compression vs. charge filling, overlap vs. contamination, evxhaust evacuations vs. power stroke.

This is the big reason why cams make power at different RPM's. There will probably be a particular massflow balance that is optimal at each of these points. Unfortunately they are not referenced vs. time/flow, but crankshaft degrees. 15 degrees of overlap at 1000 rpm may be 15msec, while at 5000 rpm it's only 3 msec. Now even though the piston is moving faster you still have much less time to achieve the flow - so depending on how many crankshaft degrees you give for an event it will be "happiest" at some rpm point.


Chris Bennight