5.0 cobra H.O. conversion to a CUMMINS 4BT????? any suggestions?

jj50cal

Seaman Apprentice
Joined
Jun 13, 2012
Messages
49
I am considering changing out my 94 four wins 5.0 H.O. to a cummins 4BT. does anyone have suggestions? or should I set it on fire and buy something else. this will be the 3rd 5.0 H.O. that would be put into it and just tired of werking on them to keep them running.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,468
or should I set it on fire and buy something else.

Ayuh,..... Considerin' OMC has been outa business for decades now,.....

I'd think tryin' to put a diesel in front of a Cobra is gonna be more problems, than any kinda joy,......

Even if the drive can take the torque, yer gonna have gear ratio issues, tryin' to match the available rpms,.....
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,306
the harmonics on the 4B at 420 and 780 RPM will destroy the drive

if you want a diesel boat, buy one with a diesel in it.
 

jj50cal

Seaman Apprentice
Joined
Jun 13, 2012
Messages
49
tired of trying to get the horsepower needed to get off plane. or I should say plane off.... :facepalm:, see I quit werking on motors when I joined the fire dept, but my son is now into them, but it seems to lean more to rattle bangers, than gas. if he could produce a 5.0 ford engine into a 2 stroke, believe me he would. but he has made the swap from gas to diesel in generators, welders, and small cars, so his train of thought is (why not) a boat.
 

jj50cal

Seaman Apprentice
Joined
Jun 13, 2012
Messages
49
no it just seem to be overlapping, I ended up talking to different people and getting better advice, then as usual life happens, and stuff gets put on a back burner, specially since I do not make my living from a boat, boat is a privilege, not a right.
 

jj50cal

Seaman Apprentice
Joined
Jun 13, 2012
Messages
49
after all.... LIFE overlaps, when it doesn't, your dead...…...
 

hvymtl939

Chief Petty Officer
Joined
Feb 6, 2017
Messages
495
I remember seeing a video a few years back of a Baja Islander that was 4bt swapped. IIRC they were running an alpha drive.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,306
the 4BTM was never paired with anything other than a Bravo from Mercruiser directly, and they were problematic. even the 2.8 liter marine motor came with a bravo as the diesels will shred an alpha drive with the low RPM harmonics and the torque.

the 4BTM has a really bad reputation with the inboard transmissions as well

the QSB6.7M was paired with the NXT drive for some spec race boats, however was not competitive.
 

jj50cal

Seaman Apprentice
Joined
Jun 13, 2012
Messages
49
thanks guys, think i'm just gonna stick with the 5.0 fomoco, and the omc, just gonna add a bigger camshaft, and if that don't do it, well, it's just gonna be a tow boat.... :)
 

jj50cal

Seaman Apprentice
Joined
Jun 13, 2012
Messages
49
will the 5.8 not destroy the lower unit? if not i have one already waiting to go...
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,468
Ayuh,..... The 5.8l used that same drive, originally,....
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Gear ratio and rpm difference would be your issue. Not worth it.
 

jj50cal

Seaman Apprentice
Joined
Jun 13, 2012
Messages
49
would the rpm's not be the same,? look, i'm just doing my best at makin' momma happy and get her back on the water, specially since now she is gonna be a granma. 5.0 just seems to suck, but i have a marine 5.8 ready to go, but have been reluctant to use it. pleeeeease help...
 
Top