Your should be running faster than that. Those boats with the 5.7 EFI (260-270hp) ran 50-52.

50hp willget you in 60's.

Alpha will live behind 350-400 hp 5.7. Idle (neutral) speed needs to be lower than 800rpm.

GPS or speedo?

Most agree that's a reasonable max. Some have torn em up with less and some have run forever with more. A lot depends on how you treat it and how you maintain it. Take it easy on the holeshots, change the oil regularly, make sure it never runs low, and run a drive shower if you regularly make long runs on plane.

Our 18 Barons ran 58 on radar back in 1978 with a stock 260 with pre Alpha and cleaver . We were 1 mph faster than the 18 Donzis

Reason we radiated them was Merc came down with a gun and a trailer full of props after we installed a 280 with a TRS that we complained was a dog.

Best we got out of the identical boat with the TRS /280 hp was 53.

I can see a Donzi z22 only doing 52 with 300 horse .that is twice the freeboard and about a foot more beam than the boats I am referring.

This subject has been discussed may times on OSO. Just do a little searching. I've built quite a few Vortec 350/383's to 400 hp and more. The biggest issue is the stock efi intake manifold. Replace that with a good 4bbl intake and carb, correct cam and good exhaust will get you to 400hp. If you want to stay with efi you need to go to an aftermarket efi intake, throttle body and program the ecu accordingly.

Bob

Put an Alpha SS on it and pick up 5 mph or so

no such thing as gen 2 alpha ss. you would be looking at a transom assy swap complete to run an extinct drive. now, it is a Jasper engine. factory specs. WHICH factory? for all you know it is an emissions cam in there. may not even be roller lifters. and they are DEFINATELY cast pistons, crank. Jasper engines are mass produced for durability. they also machine the valve seats in WAY too deep. they cut everything to the max so they don't have to measure anything., or change machine set-ups. all bores .040. valve stems may now be 3/8ths. you need to pull the intake, get a digital caliper and see what you have for lifters and valve lift at the cam.

I have a 350 mag with a whipple charger on it pushing 550hp with a cam piston and the vorteck heads.i am also running a Blackhawk drive running in the 90s

Yup.

A classic small block "top end cam and exhaust kit" Ive seen add anywhere from 50-100HP in a small block with high quality components (AFR/Brodix)

Tune it on a dyno with marine headers/manifold, and drop it in the boat and rock. Tons of guys have a combo in their drawer for this.

I been to at lest 2 dozen dyno session and never yet seen even an expert builder nail on the first tune, but a few jet swap in and outs and kaboom you get the best sth combo can give.

Doing that in the boat is a nightmare, and if something like an intake manifold gasket has a seal problem its a lot harder to deal with in a bilge than on a stand with a guy opposite you with the mill exposed and at chest height you can fix that stuff in minutes.

Whatever it is you do, please show it to us so we can enjoy the fun with you!

UD

Which stock EFI intake do you refer too? There are stock EFI SB intakes and throttle bodies that will suffice for the HP required.

I'm referring to the late model intake. The early 350 mags and 6.2's had a 4 bbl type intake with an adapter for the throttle body which was not too bad, but still about 20-3 0hp below a good aftermarket intake and and the larger 4 bbl type throttle body.

Bob