Weird TDC Question

[jon9116]

Weird scenario and need some help. My 86 c10 has a 355 with aluminum heads, mutha thumpr cam (came with the truck unfortunately) and what feels like domed pistons (felt them through cylinder 1.

Anyways, the truck has the wrong balancer. Measure teeth are in the 2 o clock position and the balancer mark is at the 12. I attempted to find TDC with a sparkplug stop in number 1. Did the drill. Hand turned the crank clockwise until it stopped then counter clockwise till it stopped. Made my mark in the middle between the two marks. Now what's puzzling me is I feel the most out blown pressure about 40-60 degrees before my mark of TDC. Is that common? I thought I would feel the absolute most pressure at my mark between the two stops (my new TDC).

If I stick a screw driver into cylinder 1, I can feel the piston top (not at my mark of TDC but at the point I felt the most pressure.

I ended up going with the pressure point for my TDC instead of the stopper mark. Timed it at about 16 initial timing with an overall 36 degree totally timing (vacuum advance plugged) according to my new timing tape placement. It fired up and runs.

Am I missing something?

 

[rich weyand]

Look at the top line in this graph:
https://en.wikipedia.org/wiki/Piston_motion_equations#Example_graph_of_piston_motion

For a pretty wide range of angles, the piston will seem like it is at the top. Top "dead center" means just that, the dead center of the piston curve in this graph. That value should be from your piston-stop marks if you did it correctly.

Your maximum blow out of the spark plug hole will be at the maximum velocity point for the piston, which is earlier than top dead center. See the velocity curve in the lower part of the same graph. When the piston is moving fastest, it will blow the most air, which is not at top dead center.

I would try it at 16* BTDC relative to the piston stop marks, then hook up the vacuum advance and take it for a drive and see what you have. With the timing set as you have it, you are way advanced, but the vacuum advance being disconnected is keeping you from having a problem at the moment. Hook up the advance and try to drive it, and you will have big troubles, and lots of knock, I predict.

 

[jon9116]

This makes sense and is what I suspected. Thank you for clarifying. I'll remark and restab the distributor and see what happens