Is it okay to add more HP?

[Dreamweaver]

I am going through the twin 351s and would like to swap out the heads with cast iron factory GT40 heads ( automotive use)
They will add an additional 20 to 40 horsepower per motor bolt on,they are a much more efficient then the stock currant heads.

The question being is everything else ie transmissions,props and shafts factory rated at the currant horsepower ?
Or is there some lea way ? Thanks.

78 F28 twin 351 windsors.

[32pacemaker]

the velvet drive c71 is rated at 340 HP, and what a great improvement the stock 351 Heads are very restrictive especially on the exhaust side should gain some economy

[prowlersfish]

Not sure if you have the 71 or 72 velvet drives ( if you have velvet drives) , The true HP rating of the gears vary with rpm as they are limited really by torque . There is a chart in some velvet drive books and on there site . However I don't think you with have any issues as I really do not think you will see much gain if any with just bolting on the heads .

[Dreamweaver]

V Drive ratio is stamped E-1.0 71c after looking them up.

Currant heads are a mismatch and valves were repaired and replaced some larger then the others,push in studs are pulling out,I have not run the engines yet just bent the push rods from stuck valves.

GT40 heads have screw in studs all ready and the spark plugs come straight out making it much easier to service.

From the late 1960s through the early to mid-1990s the 351 Windsor had a long history of being marinized by Holman Moody Marine, Redline of Lewiston, ID (now defunct), Pleasure Craft Marine (PCM), and Indmar for use in about every make of recreational boat, including; Correct Craft, Ski Supreme and MasterCraft inboard competition ski boats. The early marinized engines were rated at 220 hp (164 kW). Most PCM and Indmar marinized 351’s were rated at 240 hp (179 kW). In the early 1990s, a 260 hp (194 kW) version as well as a "HO" (High Output) version that used GT-40 heads and the Holley 4160 marine carburetor was rated at 285 hp (213 kW). A few 351 GT-40/HO engines were marinized equipped with Throttle Body fuel Injection (TBI) and were rated at 310 hp (231 kW). The marine industry's relationship with the 351W platform ended when Ford was unable or unwilling to compete with GM's mass production of TBI and MPI equipped engines in mass quantity. It was during that time that the recreational marine community's small-block V8 platform of choice shifted to the 350 cu in (5.7L) GM L31 (vortec 5700) engine series.

So I should be okay at the 285 hp rate,not going to mess with the fuel injection aspect and maybe add electronic ingition kit.Thanks for the input. I wonder how twin turbos would work?

[prowlersfish]

I don't see you having and issues with the GT40 heads . The 351w is a strong marine engine . I have owned my share Merc / OMC/ marine power and some cross breeds I built .


Twin turbos ? I hope your kidding . If not you got the wrong boat to hot rod .

[Paul]

I'm think that you'll probably have to go up in prop pitch due to the added HP. Keep in mind that these boats really don't handle that well at higher speeds.

[prowlersfish]

I'm think that you'll probably have to go up in prop pitch due to the added HP. Keep in mind that these boats really don't handle that well at higher speeds.

That would depend on what rpm the hp gain is at . If the max hp was at 4200 rpm before and then its at 4600 rpm you may not want to add pitch . I would stay with the current pitch and see what she turns and adjust as needed . Much better to be under pitched then over .

[Paul]

Agreed. I was assuming that the boat would already be operating in the correct RPM range.