I want to upgrade the power in my boat but would like to stick with the small blocks. It has the merc trannies and trs drives. How much power do you think it will take to crack the 80 mph barrier? Thanks.

I'm guessing 400+hp per side.

450 hp to 475 hp, if it is a TRS boat, 27" Mirage props, about 4900 - 5000 rpm, if the drives aren't way deep; however, since you arte going small blocks, 25" Mirage 5300 rpm. My father had a 27' Magnum that ran an honest 84-85 mph (27" Mirage props spinning 5200 rpms - labbed with added cup) in the final years of ownership, but it the beginning it ran 81 mph on radar. That is how I am coming up with your numbers.

I don't think you will likely get to 80 MPH as rigged. Why? Because the HP required to offset the stock TRS drive height would be significant...maybe 500+. If you were (not sure it is possible) to raise the TRS drive height to accommodate an ideal drive height for say the Bravo 1 props, your engine height will exceed the clearance of your current engine compartment.
Even if your re-rigged the boat, you would now be on the reliability edge for the TRS drives and past the edge of the Merc transmissions. If you break a Merc tranny, finding parts may be a challenge.
Numerous high HP small block 28' Cigs and Magnums were raced in the '80's and few were able to exceed 80 MPH.
If you are still not deterred, consider switching to Bravo drives and a total re-rig including drive and engine height, new exhaust and engine hatches.
Oh...don't forget the bottom work on the hull. If you get to 80 MPH, you won't like the handling of the boat.

As usual, never a simple answer to a simple question. The first two posts did sound a little more promising though. Thanks for the replies. I'll have to mull this over.......

You're not going to do it with the Merc trannies. Listen to BenPerfected, Bravo's are the way to go. But you are limited with small blocks, and big blocks put a lot of weight in the back affecting handling and causing porpoising. Good luck with whatever you end up doing.

I will qualify my response a little further, the Magnum had the bottom blue-printed, and the X-dimension was raised. Hp was estimated, the boat had big-blocks, never porpoised at cruising speed, transmissions never failed in 19 years of ownership, broke a prop shaft once (a TRS boat). That is fact, not BS. The bottom of a 28' SS and a 27' Magnum are the same. There were three 27' Magnums in Barnegat Bay, NJ in the 70's running better than 80 mph (all had big blocks).

Benperfected's response of 500 hp per engine is probably more realistic, than 450-475 estimate, I gave you, unless you really dial it in, with props, drive height, bottom work, ect.

500+ per side at least, I run a 30 Panther scarab TRS's with 450+ a side and only see 73mph somewhat comparable. what Ben Perfercted said is probably the most honest answer you will hear. I think he has a fast 30' boat and somewhat a similar hull.

Back in the 70's my best friend and I converted a 27 Mag & a 28 Cig from Volvo to TRS. He had the cig, and also some $$. As a result he had some small bocks built that dynoed at 400 HP each (which was a lot back then !!) We put the engines on 29" centers and ground into the rub rail area to get the drives a high as possible. It wold run right at 80 anytime you wanted. My Mag would never run over 70. I hated him They were both balanced perfectly.

Today I would do it with small blocks and Bravo's. There is a 28 in Steve Lopez's shop with 6.2's that he just re-rigged, I believe it runs 80.

I also think big blocks are too much weight for that boat. The 1 big block Magnum in Chicago never did run very well.

Sounds like raising the drives is mandatory. I want to stay with the small blocks. How did the various magnums and cigs that you all are talking about handle at speed. Were they controlable or all over the place.. Would hate to go through the work and have the boat be too squirrely at top end.