348 Checking Cam Timing & my engine brief knock

Yes, I intend to use my TDC dial gauge to take readings at TDC-s of a left and a right piston with the gauge screwed-in until it contacts the spark plug seat. This may give me an indication of (expected) higher reading at TDC of a right piston (being closer to the spark plug seat, more push-up of the dial gauge rod) if the right head has been skimmed and roughly by how much.

Here's my home made TDC dial gauge, now with correct M12 thread.

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I think you had a special vision (at a long distance). I took measurements, left & right, of the heights from spark plug seat to piston crown with a dial gauge and they are practically identical. A couple of thou difference is, I'm sure, caused by the fact that the gauge plunger is sitting on the rough carbon layer on the pistons.

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I also managed to find a picture of a right 348 head (same year of casting as my head) which shows that the step is actually very small thus I could not see it on my head in situ.

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So, it appears that the compression difference I have left-right is due to unequal cam timing. I have just read that a few degrees difference in the intake cam timing can cause 10 or more psi difference in the compression readings. I might have a bit "late" left intake cam and a bit "early" right intake cam which, in aggregate, cause my 30 psi difference but there could be other contributing factors like more carbon build-up in my right head for some reason. It seems that, during the last belt replacement, the cams were not properly degreed as I found matching paint marks on the sprockets and on the cam seal housings.

I will be driving the car for some time and, in the meantime, get things ready for my engine out service (already have a degree wheel and some other parts for it).

 

Re-checked the cam timing today with the belt covers on (the quick check described in the service manual) and found all sprocket marks aligning with the slots well.

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Of course, the well timed sprockets does not mean that the camshafts are also correctly timed. Will check that during the engine out service.

Put some fresh petrol in the tank + Techron and started the engine for the first time since the knocks. It ran fine (nice and smooth idle, as before) with no signs of any knocking. So, this confirmed that I just had detonation when I leaned-out the mixture in the left bank (CO screw at 400 instead of 383 Ohm). This should not normally cause detonation but, in my case, it did due to too much carbon in the combustion chambers and stale petrol.

Will drive the s***t (I mean...carbon) out of it over the next few months and then check the compression again.

 

Some new findings regarding the earlier reported compression readings on my engine showing 30 psi difference between the banks - each cylinder on left showed 150 psi, each cylinder on right showed 180 psi (cold engine).

All speculations I made earlier are out. I recently started the timing belt change etc. service and, after I had a good look at the camshaft markings, I found out that the Left Intake camshaft was out (late) as shown on the picture below, i.e. by exactly one tooth. Looking at the paint marks made on the cam sprockets during the previous service (by the PO), I could establish that the cam was out by one tooth because that was how the last belt was installed, not because it skipped.

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In my previous thread above, I showed pictures of the slots at the belt covers and reported that all cams showed as correctly timed. Well, I was wrong. What I saw in the slot of the left intake cam (2nd pic) was not the timing mark as I thought but something else that just looked like the timing mark. The actual timing mark was ~ 10 mm (1 tooth) away.

So, the intake cam being late by one tooth explains the lower compression on the left bank. The later closing of the intake valves after the bottom dead centre, i.e. during the initial upward travel of the pistons, caused some of the sucked-in air to be pushed back out pass the intake valves leaving less air to be compressed.

I am still working on the engine service so I will do another compression test (after correction of the intake cam timing) later. I am confident that it will show good results.

As for the brief metallic "knock" I had, thinking that it was caused by detonation appearing after I changed the position of the CO screw on the left MAF to 400 and disappearing after I put the CO screw back to 383 ohm, it is quite likely that it was actually a rattle coming from, as I discovered during the service, the rather loose outer fence of the main cam belt sprocket (now replaced with a new improved sprocket) which just coincided with my changing of the CO screw position. Perhaps changing the CO screw value caused some resonant vibration that rattled the loose sprocket fence, who knows.

 

I eliminated the head height in my post #28 above.

 

You were absolutely right here. 150 psi was low due to the intake cam being one tooth late and 180 psi was correct/good.

Did a compression test today, after correction of the left intake cam timing, cold engine (sat for ~2 months now), 13,900 total mileage (pressures in Psi):

8 - 183 .............. 1 - 183

7 - 182 .............. 2 - 182

6 - 182 .............. 3 - 183

5 - 182 .............. 4 - 183

Cylinders 5 to 8 (Left Bank) were 150 psi before (cam 1 tooth late), now practically exactly the same as the Right Bank. So, 1 tooth late intake cam results in a lot lower static compression. Still, the engine ran quite smooth. This, I believe, is because one tooth late intake camshaft may not affect the dynamic compression as much as it affects the static compression.

Anyhow, I guess I will soon see that my engine can run even smoother than what I thought was "quite smooth". Thanks everyone for good comments and suggestions.