Will 1.6 rockers in place of 1.5 change my DCR?

[mopower58]

I replaced my 1.5's with 1.6's this weekend. Makes valve lift .704 instead of .660. Do you think it will change duration enough to lower DCR any?

[Eric68]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

Won't be a significant change, as a rule of thumb a one point rocker ratio change usually makes a 2-3* change in duration @ .050 at the valve.

[mopower58]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

Thanks for that info,every little bit helps!

[PackardV8]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

Dynamic compression ratio calculators, such as this one from K/B's John Erb, http://kb-silvolite.com/calc.php?action=comp take into account "intake valve closing @ .050" + 15 degrees. Anything which affects intake closing affects dynamic compression ratio.

thnx, jack vines

[MikeyT]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

What is dynamic compression ratio?

Mike

[mopower58]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

The actual compression ratio while running,which takes into account the opening and closing time of the intake valve which bleeds off compression..... I think ;D

[PackardV8]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

Static compression ratio, the one you see in the OEM spec sheet, is the total cylinder and head volume compared to the volume remaining at TDC.

Dynamic compression ratio is the real number, calculated based on the cylinder volume when the intake valve closes. Short cam timing has a high DCR. Holding the intake open longer as on full-race cams, lowers the DCR, because some compression is lost as the valve is open as the piston is rising.

thnx, jack vines

[mopower58]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

Static compression ratio, the one you see in the OEM spec sheet, is the total cylinder and head volume compared to the volume remaining at TDC.

Dynamic compression ratio is the real number, calculated based on the cylinder volume when the intake valve closes. Short cam timing has a high DCR. Holding the intake open longer as on full-race cams, lowers the DCR, because some compression is lost as the valve is open as the piston is rising.

thnx, jack vines

That is what I meant to say! ;D

[MikeyT]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

Thank you for the explanation

mike

[chad786]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

Is not the 0.050 the duration at the lifter? Additionally, changing the
rocker ratio will change the lash at the valve. IE .022 lash at the valve
equates to a (.022 / 1.5) = .0147" at the lifter. To maintain proper lash
at the lifter the lash at the valve should be: (.022 / 1.5) * 1.6 = 0.0234"
This is to keep the lifter in the "ramp" before the valve stem is contacted
by the rocker arm. - or am I opening a can of worms? Maybe I should
return to lurking.
chad786

[smallblockjay]

Re: Will 1.6 rockers in place of 1.5 change my DCR?

Bring it, bud. I sure like to hear the ideas. 8)

I'd say the dynamic compresion would be comprimised slightly by the slight increase in duration at the valve, but offset by the additional lift, provided the heads can flow a good improvement at that higher lift. If the heads can only flow well to .590 and you raise lift to .630 with the rocker, it's really a moot point. Just an over-simlified example. 8)