Frankenstein cooling system: Temp swing up/down the mountain w/a mechanical guage?

 This is as much A story as it is a question, but, I would like a reference point from those who take old cars with mechanical gauges up the mountain.

Because I'm dumb and cheap (a really bad combo) I have a sealed cooling system from 91 and 96 (I think) Volvos in my E21.

TL;DR, how much temp swing do y'all see when driving up and down a mountain road?  With an old mechanical gauge, not like the modern ones where as long as temp is "in spec" the gauge never moves. 

Are ~15-20 degree swings from "center" air in the system or flow rates?

Short story long: Why do I ask? 

Because why get a cheap ebay radiator that fits, or spend $600 on a nice Ron Davis unit, when you can leap for joy when you think you've figured it out or frustrate yourself when you realize you haven't, all for $125 or so on boneyard parts and aluminum tubing. 

Anywho - on flat ground, the cooling gauge will run right about where you'd expect with a 170 deg thermostat (a bit past the "1/3" point).

Going up the mountain, it'll run maybe ~20 deg warmer (not quite to just about "half way"), going down the mountain, it'll run a bit cooler (at or a bit above the 1/4 mark).

Back on flat ground, runs pretty much stable, temp will maybe creep up a bit, but pretty stable. 

Still air in the system?

Flow rate not quite enough with extra load on the engine?

As I neck down the upper hose size and shorten the distance (angle from neck to upper inlet), the temp swing gets smaller - if I go too small, that swing gets wider. 

Also, in accordance with the above, the electric fan seems to come on less, and more consistent with the temp gauge the less of a "swing" I get.  The fan would sometimes come on while the gauge read cool-er, which to me says flow rate (fan temp probe close to inlet location).  That fan behavior is the same whether driving on flat ground or going up/ down the mountain. 

Could still be air in the system, I suppose, leaving a hot spot. 

Also, the coolant does have to travel "up hill" from neck to inlet.  I can lower the radiator about half inch or so, seems that would help a bit too. 

Trying to figure out when I can call it "good" or good enough with my Frankenstein system. 

I'd like to get it to the point where the temp gauge is reading a more consistent temp going up and down, but, I don't know what I should see, thus the questions, how much does your temp swing going up/down hill with a normal, well bled cooling system?

As long as it doesn't peg red and boil over don't worry about it.  The temp will go up and down, if you are worried put a new thermostat on, Mercedes does the same thing. 

Once it stops moving on consistent load, I stop getting concerned :) You're always going to see it moving around as load comes and goes - that's exactly why so many cars have gauges with a big "happy zone". I think the gauge on my Cadillac covers about 1/2 of the total range in normal driving.

I had a mechanical gauge on the cold side of the radiator in my racecar. I'd see 20-30° swing between a full course caution and racing. There's also the stock gauge on the hot side of the radiator. It would climb up to 200° and stay stead for the duration of the race.

Appreciate the inputs, less "concerned" than I was previously. 

My e36 M3 gauge stays anywhere from 190-215. 

Slippery said:

My e36 M3 gauge stays anywhere from 190-215. 

Nice, I'm in the right ballpark, then.

If I can tighten it up a little more I'll be content, if I can't, seems like I'm ok. 

as others have said...  you are ok.
 

You are likely seeing more swing because your thermostat is set lower than it probably was from the factory. Most modern cars seem to want to be about 200 degrees for emissions compliance so you have given the coolant the ability to cool under normal operation lower than the factory planned on.

I have a race car that has some inherent cooling issues - Fiat 850. Since I am in AZ I dropped by Ron Davis one day to see what they had to say about it. Even though they were aware I was not likely to buy anything from them they took a good amount of time to listen and offer information, education and suggestions I could affordably use. I came away with a less is likely to be more look toward a solution.

As far as the temp fluctuations go. The answers already provided here are what I have experienced with various vehicles too and you should be fine.

Update:  I didn't like the swing I was getting, so I monkeyed a bit more.

With the fan going on/off out of kilter with the temp gauge, I surmised that I wasn't getting coolant out of the head into the radiator, and not getting good flow through the radiator.

So, a bit more restriction on the upper half, and closer to the outlet (coming out of the head into upper radiator inlet) has the car running within ~ 5 degrees of "baseline" all the way up the hill!

At idle, it just sits there.

Fan hardly comes on now, I'll have to readjust that once it gets a bit hotter.

Hose sizes matter a lot, as does length.

Each time I monkeyed, more total distance in hose configuration meant poorer efficiency.

Anyway, food for thought.