Okay, this is no dosbox problem...
I tested commander keen 1 and it runs a pleasant 75 Hz fullscreen using Dosbox (config as in previous post) .
The problem with Battle Isle 2 was that it uses a non-standard 360x240 videomode, and so the NVidia refresh rate fix seems to have no effect on this videomode (yet..).

Wasn't there some functionality in the dxdiag program to override
refresh rates for all video modes?

Yes I know that option in the last tab of the dxdiag screen. I seem to have filled in 85Hz there, but I doubt it ever made much difference.
Certain versions of NVidia detonator drivers under nt/2000/xp had a 'bug' that made it much harder to play fullscreen games at anything higher than 60Hz.
I haven't got it totally clear for myself how to summerize the issue. Using windows 9x with detonator drivers and 2000 with nvidia forceware drivers for years now; I always had to download certain tweak utilities to get the these refresh rates on an acceptable level.
like this one: http://www.pagehosting.co.uk/rl/
These utilities seem to patch some hidden system settings and then it seems to work properly.
Anyways, Damn the guy who ever invented 60Hz as a default. 😉

gerwin:

I played around with this once with my LCD (and nVidia drivers), and my attempts to change the fullscreen refresh rate through the display settings or lockers had no effect. Testing kept showing that I was actually at 75Hz in fullscreen no matter what I did. I think you can get some better success with your CRT.

The emulation refresh timing within dosbox is independent from the host refresh rate. I'm a little confused by what exactly you're trying to do, so it could be good to get some background on what's been discussed before. If you're interested, there was some discussion about it in this thread (which is about a year and a half old). My posts in the thread go into some detail about it:
Screen tearing in DOS games running through DOSBox

and I once started working on a patch related to it in this thread:
Vertical retrace sync patch

I'm considering revisiting the issue sometime (h-a-l-9000's recent experimental vga patch got me interested), and releasing a directx-only patch. I lost focus a bit last time..

augnober:

I have hardly any experience in using LCD screens myself. The two links you gave to the other posts; it seems they are not much about the issue I have myself IMHO.

In my current setup it it seems to be a NVidia driver configuration issue. It is about letting the driver know that it should initialize certain videomodes at 75Hz/85Hz instead of the default for most videomodes: the safe but annoying 60Hz.

The battle isle 2 issue is solved now, I used the RivaTuner utility: selected customize: the low level refresh rate settings. Selected the second tab: monitor driver wizard: vertical scan frequencies (Hz): Changed 50-160Hz range to 75-160Hz. then clicked create (creates an mydriver.inf file)
Then used this file as the driver for my monitor.
Now when I start playing BI2 in dosbox (fullscreen 360x240) it is at 84Hz according to my monitor's USD (and according to my eyes 😉).

interesting tweak ain't it?

I see. The issue was that on a CRT, 60Hz is pretty annoying and/or headache-inducing. I'm glad you got it solved 😀

Although.. on Windows at least, it is possible for the app to select from the supported refresh rates (I've seen some DirectX samples do this before, though I haven't looked at the latest samples). It could be nice if dosbox made use of this ability rather than just use the default.

Unless you personally volunteer to port DirectX to Linux and MacOSX, I don't expect DOSBox to use any DirectX specific features.

Didn't SDL1.3 try to implement refresh rate manipulation?