Exhaust Cam Timing
Posted: Sun Oct 30, 2022 14:22
It's been a few years since I started on the Excel resto (too many other projects). But she finally hit the road a month ago with a shiny new MOT for the first time since 2007. She was glorious until the engine trashed itself again. It started as a sudden change in engine note and slight loss of power. I slowed looking for somewhere to stop when it lost power completely with a tinkling sound. I banged the clutch in and rolled to a standstill. After a tow home I gave it a thorough check over. She would turn over freely but a compression check showed zero on all cylinders.
The exhaust cam could be seen to have slipped three teeth. Intake was still aligned perfectly. I whipped the head off and sure enough has eight S bent exhaust valves. Fortunately as I was only doing about 20mph when it finally let go there was only light marking to the piston crowns and no signs on the valve seats. I checked the piston heights and all appeared on spec. This was apparently the second time the exhaust valves had had an interference problem as I discovered when I stripped the engine initially.
The engine is not exactly standard. The car had been used for track days and has a gas flowed head, an equal length 421 stainless steel exhaust, L14 inlet cam and a re-profiled exhaust cam and vernier cam pulleys. I had clocked the exhaust cam and the curved almost exactly matched the L14 profile. After rebuilding the engine I took it for tuning to an 'expert' to set up the cam and ignition timing as I didn't have a dial gauge that would read into the cylinders. I said that I wanted to make best use of the set up, but to retain some driveability. The cam timing they gave it was 2 degrees advance on inlet and 2 1/2 degrees advance on exhaust.
The timing belt was slightly over tensioned which initially gave a whine but which eventually went away. I couldn't check the ignition set up without a running engine but the engine had felt strong, although I did think it sounded a bit tinny above 4,000 revs. So I am concerned about what has caused this belt slip and what to do different to avoid a third failure.
It does occur to me that the L14 profile has a steep ramp up and long duration. Combined with 2 1/2 degrees of advance the cam is trying to punch the valves open whilst the cylinder pressures are still very high way before BDC. Is it just asking too much for the relatively poor engagement between the belt and pulley. Close examination of the belt (Powergrip) showed a row of teeth that were abraded on the driving side of the teeth with raised fibres. I am tempted to revert to a 107 cam with zero timing, but presumably the valves interfered on the and of the exhaust stroke so effectively retarding exhaust timing make interference more likely if the problem actually lies elsewhere.
Richard
The exhaust cam could be seen to have slipped three teeth. Intake was still aligned perfectly. I whipped the head off and sure enough has eight S bent exhaust valves. Fortunately as I was only doing about 20mph when it finally let go there was only light marking to the piston crowns and no signs on the valve seats. I checked the piston heights and all appeared on spec. This was apparently the second time the exhaust valves had had an interference problem as I discovered when I stripped the engine initially.
The engine is not exactly standard. The car had been used for track days and has a gas flowed head, an equal length 421 stainless steel exhaust, L14 inlet cam and a re-profiled exhaust cam and vernier cam pulleys. I had clocked the exhaust cam and the curved almost exactly matched the L14 profile. After rebuilding the engine I took it for tuning to an 'expert' to set up the cam and ignition timing as I didn't have a dial gauge that would read into the cylinders. I said that I wanted to make best use of the set up, but to retain some driveability. The cam timing they gave it was 2 degrees advance on inlet and 2 1/2 degrees advance on exhaust.
The timing belt was slightly over tensioned which initially gave a whine but which eventually went away. I couldn't check the ignition set up without a running engine but the engine had felt strong, although I did think it sounded a bit tinny above 4,000 revs. So I am concerned about what has caused this belt slip and what to do different to avoid a third failure.
It does occur to me that the L14 profile has a steep ramp up and long duration. Combined with 2 1/2 degrees of advance the cam is trying to punch the valves open whilst the cylinder pressures are still very high way before BDC. Is it just asking too much for the relatively poor engagement between the belt and pulley. Close examination of the belt (Powergrip) showed a row of teeth that were abraded on the driving side of the teeth with raised fibres. I am tempted to revert to a 107 cam with zero timing, but presumably the valves interfered on the and of the exhaust stroke so effectively retarding exhaust timing make interference more likely if the problem actually lies elsewhere.
Richard