I can only tell you the spacer lets me get on plane easier and the boat feels better when your driving it

I have boosted the motors so I don't think I am any slower
And my engine builder thought we needed to bring the rpms back down to 5400 to 5600 for several reasons I can't really explain

The boat still excelerates great

Also I slung a blade and did a ton of damage to the boat because it hit the drive and the other prop
My Insurance company said it was metal fatigue that broke the prop
Dropping the x should help with that a little

Over all I like the spacer my boat is an 2007 the 08 and up have different props

I guess it just depends which boat and props you have

Gnorthga: You used a one-half inch spacer, and went from 25% slip to 17% slip? I wanted to be clear about the size of the spacer you are using, before considering the move myself.

How has your experience and happiness with the spacers been since your letter above in January, how is top speed now vs before, etc?

We also did a Stage 1 computer upgrade to our 2007 42' with 700sci. We got 96 mph at 5900rpm or so, about 29% slip. We had to let off the gas because we were in breakwater area, the lake was NASTY, but our slip is still quite high.
We had gone 98mph at 5400 before the reflash, last year; so basically the rpm/hp bump got us more slip but no more speed, but I had the drives up a bit higher this time than I had last year, to follow the Fountain Performance Report specs.

With some tweaking down of drives should be somewhat better than 29% slip,but not any 17%, or the 16% slip required to hit the Fountain Performance Report's" 105mph at 5350rpm".
Next time will try tucking the drives in more than 5.5 (where Fountain report said was optimal), though that is already a bit tucked in from level.

I graphed out our drives' trim "indicated" settings vs bottom of boat with a digital level...but where to MEASURE the "bottom of boat neutral" is a question. The step in the hull was what I considered "0 degrees/neutral" for the hull in toto, it's a bit different from the keel pad/s.

How hard is it to add a spacer? Doesn't it need two longer shafts transmitting the power down to the lower unit, or is it just splines that slide to adjust to the longer distance?

I love the boat with the spacer it stays hooked up better in rough water

You need longer studs and oiler tubs

check your grounds on the back of the block that can make it back fire if they are lose or corroded

I hear that Gerald

you learned that one the hard way