one question please  why ,with the aluminium heads, haven't you raised the target DCR to 9:1 from 8:1 with the iron heads?

many thanks

stuey 

i'm mulling over a similar worry hence the question.  i have not fired her up yet due to financial restraints but soon.

i originally had a scr of 13:1 but after skimming the pistons/grinding the combustion chambers i'm down to 10.75:1.

Mr Mummerts aly heads are generally reported to measure at 60cc but mine came in between 56.0  - 57.3.

so i gouged them out to 57.3. i'm also using the 270 cam which according to Mr Mummerts "cam card" has IC at 64* ATDC

but even after retarding cam 2* my IC at 55* giving a DCR of 9.1:1 . i'm pretty well convinced our petrol is the same as yours just using diferent RON/PON numbers.  as i'm not a racer and will probably not use WOT (much) i'm hoping to get away with the pinking issue.

i stand to be shot down on all this but thats how i learn. i'm not an experienced petrol head i mostly ride a bicycle, to and from the pub!

thanks every body

stuey

UK

Mark.  DCR is just another tool to help in building an engine combination but it’s not absolutely definitive or the last word.  It does however do a better job than the SCR in predicting what a particular engine combination will do on pump gasoline.  Here are the calculations I get for your combination.  I used 110° for the camshaft lobe centerline angle and installed 2° advanced or at 108° intake lobe centerline.  Zero deck was also used in the calculation.

As long as good premium fuel is used, then you should be able to optimize the ignition timing assuming the carbs are jetted correctly.  It’s when you get that mislabeled or bad load of fuel that you may notice some clatter at which point backing up the timing should clear that up.  If you hear some clatter, then make that timing adjustment.  Just keep in mind that any backed up or retarded timing will affect overall performance and drivability.  Also play around with different brands of fuel if you get some clatter at different times.  A 160° thermostat versus a 180° one will also make a difference if you're on the edge. 

thanks for reply it is much appreciated.   my mistake re ATDC should be ABDC

my big problem is in understanding cam specs.my cam is one of Johns 270 cams.

John sent the following cam card info

                                                  LIFT      DUR.        OPEN             CLOSE        AREA

IL-- lobe centerline 105.00 ATDC     0.018    261.51    28.50 BTDC     53.01 ABDC    22.74

after retarding 2*     107                                         26                 55  

the following was lifted from Johns website

Y-270-S   272°   224°@.050"     108° Lobe Separation     Valve lift  .440"        Valve Lash .018"

IO   28   BTDC                  IC   64   ABDC                  SPRING PRESSURE 95# SEAT   230# OPEN

EO  64   BBDC                  EC  28  ATDC . 

     note both sets  of info open at 28* but closure times are 9* adrift.  am i not understanding the effects of lash which is .018????

if the closure time is 64* i'm happy if it is 55* i must do further work.???

comments please

thanks

stuey

sorry about print size don't understand that either 

i think what i don't understand is in the above......the figure given in the cam spec is IL closes 53* ABDC at 18 thou lift.

that would be true at the valve given a 1:1 ratio rocker but i'm using Dove 1.6:1 ratio so that gives 28.8 thou at the valve minus 18 thou lash valve is open 10.8 thou. is that where my missing 11* degrees are hiding(the 64* quoted on Johns web site)   if so that then begs the question does the IL open at 39* BTDC???

please beer with me i'm more than a bit slow

stu

  by the way, how about a picture of the truck.........and maybe the PUB!!                               

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.

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.

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