Stuey.  My quoted statement above confuses even me at this point without the rest of the context.  So let’s start over.

Most solid lifter cams measure their advertised duration values at 0.020” lobe lift.  That means at 0.020” lash, the valve is also closed at the same point that the advertised duration is starting to be measured.  That makes the DCR (dynamic compression ratio) calculation easy if using 0.020” valve lash.  Any tightening of the lash will have the intake valve closing later (higher ABDC value) and this will decrease the DCR.   Any loosening of the valve lash will increase the DCR due to the intake valve closing earlier.

When degreeing in a camshaft, it’s possible to record the various intake closing events at differing lobe heights to simulate the valve lash effect.  These values can then be used to calculate the different DCR values.  Here are the DCR values when degreeing in the following Isky camshaft on a Y engine and taking into account some various lash settings.

With this chart you can see the effect that valve lash has on the DCR.

If you can post the specs on your engine, I’ll run them through the spreadsheet.  Need bore, stroke, rod length, piston dome if you have any, how far the pistons are sitting in the hole at tdc, head cc’s, brand of head gasket or style, and if you know exactly where the camshaft is installed in regards to lobe centerline.

I’ll add that if the camshaft is not degreed in, then you are running blind in knowing exactly what’s happening in that area.  A four degree change in cam timing can make a lot of difference in the tuning parameters.

Lorena, Texas (South of Waco)

Pegleg those figures at 18 thou lift were at the cam (on the lifter) and they agree with the cam data sent by John Mummert. using the multi-keyway timing chain i retarded it 2* that is to say the cam is now installed 1* advanced

re pics i used to have some on the profile page but they seem to be not available any more , also put a couple under a thread started by someone requesting pics of trucks    as for the pub i'm usually not capable of driving the camera at the end of an afternoon, the bike knows its way home.

Ted

 BORE--3.82

STROKE--3.61

ROD LGTH--6.125

PISTON DOME--flat tops with 0.4cc valve reliefs

0.009 thou in the hole

BEST GASKET 572 G-2

CAM  Y-270S

LOBE SEPERATION 108*

IL LOBE CENTRE 107*

no allowance made for the gap between piston top and top ring

using Wallace racing calculators i come up with SCR 10.77

its gonna be an eye opener to see your results

the real question is should i be using IL closes at 55* or more likely the closing time quoted on Johns web site of 64*

MANY THANKS    and i hope i haven't caused too much inconvenience

stuey 

Stuey.  Here are the results when plugging your values into my own spreadsheet.

The 0.4cc for the valve reliefs sounds low though.  Most twin valve reliefs measure closer to 4cc’s.  I’m assuming the 53° ABDC value you’re using was measured at 0.018” lobe lift?  If so, it doesn’t quite jive with what you should be seeing on a symmetrically ground lobe.  But if that 53° value measured at your valve lift value is indeed correct, then that’s the one that should be used for the DCR.  And plugging that value into the spreadsheet increases the DCR to 9.29:1.  I would verify the #1 cylinder checks by also checking the camshaft at the #6 cylinder.

Lorena, Texas (South of Waco)

i arrived at valve relief cc by pushing piston 1" down bore and filling with spirit then doing same with reliefs filled with play dough. pistons were originally pop ups which gave about 13:1 CR these were milled to 5 thou below the flat surface leaving 0.200  top thickness.  the 53* figure was on the cam follower at 18 thou both measured and and on the spec sheet.and i agree if cam is symetrical then something is odd with IL opening degrees and duration.  but my head is spinning for now and i 'll put it down to the cam grinders dark secrets. till another day when i get a second wind.

thanks its been an education

stuey