exact cam lobe position at TDC?

[liam012]

This is just a curiosity question.

I am looking to know what the valves/rockers should be doing at TDC.

I know the TDC mark for the Crank is half the dwell between rising and falling and that is easy measurable

I am guessing at TDC the inlet valve is just after closing and the exhaust valve is closed and about to open on the down stroke to release combustion gasses

i am trying to find a way of knowing by looking at the lobes with the rocker cover off when the engine is at tdc?

Granted both lobes should be pointing 'down' as that means valves are closed but is there a more exact way of doing thi or measuring this? should a particular rocker be just about to move or is there some measurable dwell angle without the use of expensive tools?

i must repeat i want to do this by looking at the lobes NOT THE TIMING MARK. i am not trying to achieve anything in particular but it would help me understand certain things better and be useful to know.

cheers!

can anyone help?

[GeoffBob]

Since you are dealing with a 4-stroke engine, it takes two rotations of the crank for one rotation of a cam. Hence, there are two distinct cam positions that would correspond to TDC, one at the end of the exhaust stroke, and one at then end of the compression stroke.

At the end of the compression stroke both cam lobes (exhaust and inlet) will be raised upwards (more-or-less) and at the end of the exhaust stroke both will be downwards. Also, the exhuast cam will lead the inlet cam by approximately 90' of cam rotation simply because the exhaust stroke precedes the inlet stroke by 180' of crank rotation.

Of course, actual cam timing is unique to each engine, but the above applies in general to 4-stroke engines.

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Determining exact TDC from your cams is impossible without a) expert knoweldge of the cams in question and b) attaching an index wheel to the crank or cam pulley. There is no golden rule that says your inlet and exhaust cam must have the same profile, or that the exact mid point between the exhaust closing and inlet opening must correspond to TDC.

HTH
Geoff

[Turbo-Brown]

So are you trying to find TDC on the compression stroke, or exhaust?

As you say, the valves will both be shut at TDC on the compression stroke, and depending on the overlap of the cam you have, will both be open to varying degrees at TDC on the exhaust stroke.

Don't suppose that helps much really though!

[liam012]

its a 272 / 272 schrick cam
would knowing the detailed specs of it help me in any way?
thanks btw!

[liam012]

ok i know that valve lift at TDC is:

1mm inlet and 6mm outlet

so how do i measure valve lift can anyione assist!

schrick datasheets here for anyone searching in the future:
http://www.turnermotorsport.com/image/e ... W_only.pdf

[GeoffBob]

Well the fact that the cams are identical certainly makes life easier. If the cams were indeed timed (geared relative to each other) such that the mid-point between the exhaust valve closing and the inlet valve opening corresponds to TDC, then it should be quite easy to find TDC at the end of the exhaust stroke due to the fact that the tension in the valve springs will quite naturally bring the cams to rest in this position. You'll certainly feel this as you try to crank the cams manually. Any less of a turn of the cam and you'll feel more spring tension from the exhaust valve, and more and you'll feel it from the inlet valve. This position corresponds to what is called a "potential well", meaning that it is a natural position of least resistance, sort of like a ball coming to rest at the bottom of the stairs.

Unfortunately, this position does not correspond to TDC on many engines. In your case, specifically, it corresponds to 3.5mm of lift on both the inlet and exhaust (assuming the inlet and exhaust valves are equally sprung), which arrives at a moment in time slightly before TDC.

Apologies, my "up" and "down" in my first post are of course upside down since the cam on the M20 is actuating the rocker arm. The little animation I posted has the cam acting diectly on the valves, but I think you get the idea.

[liam012]

sorry to be nuiscane but how do you arrive at 3.5mm for both in and ex?

is this like a mid point between the 1mm and 6mm in and ex?

sorry but ya threw me!

how best t o measure the lift?

thanks again!

[GeoffBob]

so how do i measure valve lift can anyione assist!

With the tail end of a vernier on top of the head. You can easily see how much a valve is open by using this method.

Get your timing to the position I described above (3.5mm lift on both valves - check this!) then rotate the cam slightly backwards in order to increase the exhaust valve lift by 2.5mm (up to 6mm) and reduce the inlet lift by 2.5mm down to 1mm.

[GeoffBob]

is this like a mid point between the 1mm and 6mm in and ex?

Indeed it is. So long as the inlext and exhaust valves have equal spring tension (and so long as the cam inlet and exhaust lift profiles are identical) the 3.5mm position is the "natural" position that the cams will try to snap into as you wind the cams around to near TDC at the end of the exhaust stroke. The remainder of the setup you have to do using a vernier gauge to get the valve lift spot on (as per your Schrick data sheet).

Even if the spring tensions aren't exact you'll still land close to this position.

Oh, and of course you'll want to measure the dimension of a fully closed valve beforehand. Otherwise you won't be able to guage by how much it's open by when you get to TDC.

[liam012]

awesome

Thanks for such informative helpful answers people!

gotta love the zone

[GeoffBob]

BTW, in the absence of a timing mark I'd still rather find TDC by "feeling" the top of the piston through the plug hole .

Also, its common practice to affix (temporarily) an index wheel (like a big protractor from school trig) to the crank. You can then plot your own graph, say, of valve lift as a function of degrees of crank rotation. This is a great way to diagnose cam and timing errors, or just to convince yourself that all is as it should be.