RPM drop when t-stat initially opens
 

I had a neighbors son stop over last night and wanted to test a timing
light he was doubting. Well we headed out to my garage for a beer and
hooked them both up to my Firebird and fired them her up. It was 30 degrees.
outside and car was run earlier in the week.

She went right to high idle at 1400rpms and after 30 seconds I tapped
throttle and she dropped to 1200rpms as usual. After another minute
or so the idle had climbed back up to 1350 and I again tapped throttle
and she dropped down to a 775 curb idle. Again as usual.

Both timing lights gave exact same results the whole time. for timing
and rpms. Flash was steady at high idle and curb idle.

Just as the thermostat was initially @185 opening I watched the rpms drop
down to 650 or so and the motor shuddered for a few seconds and then
recovered slowly to 800rpms. We watched it run for another 15 minutes and
it seemed it only did it once on the initial opening of stat. Just seemed odd
to me. With a 180 stat and iron heads is this normal.

I rarely run it like that, normally stat is opening as I am driving so I would never notice.
Can a drop in water temp effect a motor that fast when stat first opens?



Gerry

Haven't seen that. Only thing I can think of is with the thermostat closed, the pump is operating in shear and the water pump is basically spinning the same water around the impeller housing causing very little drag. Thermostat opens and the pump is now actually moving water and this causes more resistance. Hell, I don't know but that's the best I got...:p

x2 on that and you may be on the lean side at below 1000 rpm when the motor is still not at a full stable operating temp.
One check of that would be to repete that whole session and as the stat opened close the choke some and see if the rpm picks back up!
Also below 1000 rpm or so your ditributors advance may drop off like a rock!

Fairly fresh engine? metal could have contracted when it opened and caused more friction, just like how it idles/responds when cold. Bet it doesn't do that once the temps stabilize.

.

Only did it when stat opened first time, and once stabilized all was good.
The timing was steady at 11 initial and even remained there during low rpm and
back to normal idle of 800.
Never saw his before but with all cooling upgrades maybe water in rad is so cold
it temp robs to much heat from engine? Smaller WP pulley, HD fan clutch and 7
Blade AC fan, 2 row aluminum radiator, tightly clearanced divider plate and cast
iron impeller.

Sounds stupid to say it and never saw this when warmer out.

That's what I was guessing could be a possible cause, and yeah, less of a temp diff when it's warmer out. There's some other factors there too, intake/head temps effecting the charge, etc, but you get the idea.

.

Keep in mind that the thermostat opens very slowly in response to the rising water temperature. If you suspect that the thermostat is suddenly opening to full flow, then the thermostat is bad. Try a new one.