I'm reading my manual for the port timing specs and it talks in terms of BBDC and ABDC.

Exhaust

open-86.5 BBDC
close-86.5 ABDC
duration 173 degrees


Scavenging (which assume means transfer ports??)

open 59.5 BBDC
close 59.5 ABDC
duration 119 degrees

That's fine. Problem is I'm reading a tuning article that talks in ATDC and BTDC. How does a timing spec in bottom dead center "translate" to top dead center?

also, does duration refer to the number of degrees of the crank cycle that the port is open?

Thanks.

 

TDC (top dead center) will be 180 degrees from BDC (bottom dead center) the A and B will stand for Before and After. To translate your specs from BDC to TDC, take the number from the specs and subtract that from 180 and reverse the A(or B) prefix, ie exhaust open is listed as open-86.5 BBDC which would be 93.5 ATDC. You are correct in that duration is the amount of rotation of the crank.

######### TDC
####### 180 (0)
## Before / ..... \ After
###### 90 ..... 90
#### After \ ..... / Before
####### 0(180)
######### BDC

This chart should provide a good visual (crank rotation is shown as clockwise)