Cam timing, help !
Posted: Mon Jul 26, 2021 9:37 pm
Hi all !
while reading hours and hours on this forum I finally reached a situation where I need to ask your support
Making some great progress in restoring this lovely car which has been originally imported from Germany, it's now time to give some tlc to the original engine (1.8 120HP).
According to the many stories it would make a lot of sense to replace the original cams with the 135HP version as it will result in a bit more umpf with reasonable little effort, knowing that the engine is out of the car and in the process of new timing belts etc.
Although having quite some experience with engines in general, it's the first time of measuring the exact valve opening, duration, closing and overlap timings. I have used the following source as guidance: http://mgf.ultimatemg.com/group2/engine ... timing.htm
Just focussing on the inlet for simplicity now.
According to the source the standard 1.8i 120BHP inlet valve should open at 12 degrees before top dead center (BTDC) and close at 52 degrees after top dead center (ATDC). My actual readings are far, way far from this.
Although the engine was running fine when it was removed from the car, I first started to confirm the current timing. This seems to be fully correct to me. The notch at the front of the crankshaft pulley aligns perfectly with the mark on the plastic shroud.
At the same time, both inlet and exhaust cam are on the marks where they should be:
Then I have defined TDC by using a dial indicator on cilinder #1, the moment the needle changes from direction is the moment where the piston changes from direction too. During a small period the dial indicator stays steady while you are rotating the crankshaft, this has been averaged by using the 2 moments where the needle starts moving.
Then, TDC has been marked on the protractor fixed on the crankshaft pulley. This is the reference for all measurements:
Then I have tried to determine when the inlet valve opens and closes in relation to the absolute crankshaft position. I understand that the crankshaft rotates 2x the speed of a camshaft and therefore the readings should be divided by 2.
First measurement, tried to find the moment when the inlet valve starts to open ever to slightly. Then the same procedure was applied to find the moment when the valve was fully closed (needle of the dial indicator becomes steady while rotating the crankshaft).
This resulted in the following 2 readings:
Opening at 34 degrees BTDC
Closing at 258 degrees ATDC
Dividing these values by 2 gives:
Open @ 17 BTDC vs 12 BTDC according the source
Close @ 129 ATDC vs 52 ATDC according the source
Since it is difficult to find the moment where the valve starts or stops moving I have repeated the measurements but noted the values at 1mm valve change.
This again gives total wrong values, related to absolute position on crankshaft from TDC:
Opening at 10 degrees ATDC (1mm valve lift)
Closing at 220 degrees ATDC (1mm from fully closed)
This is not even close and can't get my head around these readings, what am I doing wrong?
while reading hours and hours on this forum I finally reached a situation where I need to ask your support
Making some great progress in restoring this lovely car which has been originally imported from Germany, it's now time to give some tlc to the original engine (1.8 120HP).
According to the many stories it would make a lot of sense to replace the original cams with the 135HP version as it will result in a bit more umpf with reasonable little effort, knowing that the engine is out of the car and in the process of new timing belts etc.
Although having quite some experience with engines in general, it's the first time of measuring the exact valve opening, duration, closing and overlap timings. I have used the following source as guidance: http://mgf.ultimatemg.com/group2/engine ... timing.htm
Just focussing on the inlet for simplicity now.
According to the source the standard 1.8i 120BHP inlet valve should open at 12 degrees before top dead center (BTDC) and close at 52 degrees after top dead center (ATDC). My actual readings are far, way far from this.
Although the engine was running fine when it was removed from the car, I first started to confirm the current timing. This seems to be fully correct to me. The notch at the front of the crankshaft pulley aligns perfectly with the mark on the plastic shroud.
At the same time, both inlet and exhaust cam are on the marks where they should be:
Then, TDC has been marked on the protractor fixed on the crankshaft pulley. This is the reference for all measurements:
First measurement, tried to find the moment when the inlet valve starts to open ever to slightly. Then the same procedure was applied to find the moment when the valve was fully closed (needle of the dial indicator becomes steady while rotating the crankshaft).
This resulted in the following 2 readings:
Opening at 34 degrees BTDC
Closing at 258 degrees ATDC
Dividing these values by 2 gives:
Open @ 17 BTDC vs 12 BTDC according the source
Close @ 129 ATDC vs 52 ATDC according the source
Since it is difficult to find the moment where the valve starts or stops moving I have repeated the measurements but noted the values at 1mm valve change.
This again gives total wrong values, related to absolute position on crankshaft from TDC:
Opening at 10 degrees ATDC (1mm valve lift)
Closing at 220 degrees ATDC (1mm from fully closed)
This is not even close and can't get my head around these readings, what am I doing wrong?
