Raising Framerate Without Shortening Clip Length?

I have a weird video clip that is 8fps, I would like to double the frame rate but everytime I try to double the frame rate it always ends up shortening the clip. So for example I have a clip that is originally lets say 8fps and 4 minutes I double the frame rate making it 16fps but then Davinci shortens the clip to 2 minutes. Is there a way for me to double the frame rate and either keep the same clip length or double the clip length? I always assumed that if you are doubling the frame rate by adding an extra frame after every original frame then that would in effect also double the video length but that doesn't seem to be the case for some reason.

So, as you've discovered, simply changing the frame rate of the original clip will result in changing the duration of the clip,... changing frame rate doesn't create new frames, it just tells Resolve at what frame rate you want it to handle the existing frames.

But, if you edit your clip at its original frame rate (eg 8fps) into a timeline with a different frame rate (eg 24fps) then Resolve will create however many new frames are needed in order that it match the timeline frame rate... and how that happens depends on your choice in the project's / clip instance's Retime Process setting:
Nearest will simply duplicate existing frames.
Frame Blend will (as the title suggests) blend existing frames to create the new ones.
Optical Flow will use pixel interpolation to create entirely new frames based on estimating the motion of the pixels between the previous and next frames and calculating where it thinks those pixels would be in the new inbetween frames.
If you choose Optical Flow you can further enhance the accuracy of the new frames by choosing the degree of accuracy via the Motion Estimation setting... note that the more detailed the estimation process the longer each new frame will take to compute.

Does that make sense?

You can find a more complete and detailed explanation in the Reference Manual ... Frame Interpolation is described on page 106 (Chapter 5 | Project Settings) and options for working with mixed frame rates in the timeline gets a mention on page 663 (Chapter 38 | Using the Inspector in the Edit Page)

Cheers
Andy

Optical Flow will use pixel interpolation to create entirely new frames based on estimating the motion of the pixels between the previous and next frames and calculating where it thinks those pixels would be in the new inbetween frames.
If you choose Optical Flow you can further enhance the accuracy of the new frames by choosing the degree of accuracy via the Motion Estimation setting... note that the more detailed the estimation process the longer each new frame will take to compute.

Optical Flow is your best bet. Speed warp is an 'AI' type tool and can have good results. I am not sure if it is available in free or not.

These methods are very GPU intensive. They may not even run with less than 6gb of VRAM. And they will take forever even if you have a 3090.