This is a long one Gents please bear with it,
I have a Big Valve engine with the correct (and standard cams). The specs are:-
Inlet opens 22 deg BTDC
Inlet closes 62 deg ABDC
Exhaust opens 62 deg BBDC
Exhaust closes 22 deg ATDC
This gives a MOP of 110 degrees.
Ok so far, I think…
Anyway I have had the block decked and hence need to retime the cams and have used the following method
Set engine to “true” TDC (use DTI)
Place DTI on number 4 exhaust tappet (always under tension due to rotation of the engine)
Rotate engine to remove slack and stop at TDC again
Now slowly rotate engine 'till tappet moves (opens) to 50 thou, read angle from timing disc.
Rotate engine 'till tappet starts to close and gets back to 50 thou point on DTI, read angle from disc.
The results at 50 thou were:-
Angle when opening 64.5 deg BBDC
Angle when closing 16 deg BTDC
therefore MOP = (64.5+180+16)/2 -16 = 114.25 therefore advanced by 4.25
Now here is the problem,
When I repeated the exercise I used 60 thou of lift and got the following results:-
The results at 60 thou were
Angle when opening 62.5 deg BBDC
Angle when closing 18 deg BTDC
therefore MOP = (62.5+180+18)/2 -18 = 112.25 therefore advanced by 2.25
When I repeated the exercise at 70 thou of lift I got the following results:-
The results at 70 thou were
Angle when opening 60.5 deg BBDC
Angle when closing 20 deg BTDC
therefore MOP = (60.5+180+20)/2 -20 = 110.25 therefore advanced by 0.25
I have not checked the inlet cam yet but I cannot understand the readings for the exhaust and I dont want to go any further. I have also tried to do the same at max lift of the cam (ie either side of the dwell) by the following method:-
Set engine to “true” TDC (use DTI)
Place DTI on number 4 exhaust tappet (always under tension due to rotation of the engine)
Rotate engine to remove slack and stop at TDC again
Now slowly rotate engine 'till tappet moves to max lift, read angle from timing disc.
Rotate engine 'till tappet just starts to close, read angle from disc.
The results at max lift were
Stop at max lift (on start of dwell) 132.5 deg BTDC
Start when closing (coming off dwell) 125.5 deg BTDC
therefore MOP = (132.5-125.5)/2 +125.5 = 129 therefore advanced by 19
Would appreciate some help
Thanks
MArk