Hi , I found an imco 12” extension box but it’s -3 lift.
i have 26 Scarab single engine with 700hp and bravo xr.
what benefit i get if i install this box with 3inch lift ?

We had one on a 26 cat. The 3 inch lift was too much and slip percentage was high with a standard length lower. Started trialing different length spacers and found a seat spot. The boat came with the box on it so I can’t give you a before and after but I’m assuming moving the prop away and up from the transom is a good thing on most boats.

My scarab is a deep V .

How far is the propshaft below the bottom of the boat now ?

Are you aware that the engine will need to come up 3” also?

Which involve dealing with moving the front engine mounts.

You will also have to deal with alignment of the exhaust tips with a now higher
engine

That why I’m asking in here , didn’t know I need to lift motor 3” !

Your Scarab will PROBABLY like that +3 extension box. I don't know your current X dimension but MOST Scarabs came with very conservative (deep) X dimensions. Bringing the drive UP and BACK would likely be helpful to you.

It's a lot of work, because of the need to raise the engine. It also raises your center of gravity (although it would still likely be lower than your same hull with twin engine installation).

A neutral extension box and a -2 or -3 shorty lower unit would be the better option if you can run across one to try.

But - sure - if you already have it, then your main tests are whether your motor will still fit under your hatch once you raise it (I'm guessing that it will still clear but I'm not trap familiar with your particular hull).

I'm not sure if any single engine boat applications have a "notched" transom (❓) I have some experience with adding extension boxes and a bit of the trial and error that goes along with it.

I didn't have a notched transom on my 32' Thunder (side by side twin engine) I owned years ago and wanted to add some Stellings extension boxes and then one summer later I added some -2 IMCO shorty lower gear cases to my Bravo drives which really helped and improved with the handling and increase in speed, etc, but power steering ram extensions were needed along with 5- blade props for easier plane - offs, etc.

Anyway, I'm with McllnsTN for a "neutral" extension box system with the option of being able to choose or work with a -1, -2 or -3 inch lower gear case.

There are several factors that must be taken into consideration when making these kinds of mods as it can have a ripple effect depending on any of the changes needed, but in most cases moving the prop shaft back and up is usually beneficial for speed and handling.
The real trick is no matter what you do is to try and keep as many of the ripple effect mod work and expense to a minimum.
That was about 20 years ago --- it was fun and very interesting for me and I learned a lot from it. 👍🇺🇲