4.3 OMC Cobra and or...

[RVFR]

I asked this one not only for OMC guys but more for the 4.3 powerplants. I just read something that's kinda weird in that odd way, yet knowing there is definitely a difference between auto and marine engines, I have to ask. Is there more power gain when running the 4.3 on premium fuel where the timing advance is greater vs. reg fuel at 1 degree? The reason I ask is I just got a deal on a 92 Sea swirl spyder I couldn't pass on. It looks to be a fairly fun get up and go boat. When I took it out the first time I played with this and that getting a feel, then time to push my luck,, I was only able to get to 4300 RPMs, and wouldn't you know the speed O wasn't working, still though felt like low 40s. I get back looking for info find this fine place where I read a lot of 4.3 pull 4800-49 easy, now granted I have no history with this boat, yet read its empty weight is 2300lbs then add fuel and stuff it is still under 3000lbs. The second trip with the OEM speed O fixed I saw 4400 RPMS at what was 45mph, I'm thinking maybe there is something amiss here, so once going over all things that could mess this up, I find the timing at around 2-3 degrees, LOL, and I know I topped it off with regular. So this begs my question, can I expect it to pull better setting the timing to 6 and use supreme fuel? BTW it has a Michigan 14.5x19 SS prop on it. I know that would help big time on a car

 

[Scott Danforth]

premium fuel will only cost more at the pump. it wont help your motor

the only benefit you will get with the low compression 4.3 (yes, its a low compression motor compared to many motors regardless of boats or cars) is if the premium fuel is e0

your still spinning your motor too low RPM. spin that thing closer to 4800 RPM. if it wont go there, look at to why (most likely crud in the fuel system)

your timing is also a bit off.

 

[Lou C]

My boat was specified to have the 15x17 but the WOT rpm was still lower than I wanted so I went with a 15.5x15, which lowers my top speed but reduces the load on the engine as I always felt that the 4.3 was a bit on the small side for my boat (20’9”). I can get 5,000 wide open. When I repower I’m thinking of going with a new 5.7 4bbl. The drive ratio I have it 1.62:1, so it should be fine.

 

[Lou C]

PS for my year (1988) they listed the timing spec at 1* BTDC for regular gas and 6* for premium gas this is why I used at least 89 octane. I guess they felt these were prone to detonation because in fact the centrifugal advance distributor is specified for only 12* at 3200 rpm. So 6 base + 12 centrifugal is only 18* much lower than the V8s.

 

[RVFR]

I'm not so sure I would call a 4.3 low comp like a smog engine, at 9.3 that's a good mid-range for a lot of application use, like what has been stated you can run regaler at a lower advance set up, or tweak it and use supper. Think I will give the Super a try knowing it does play how well an engine runs. I have dyno charts that say it does matter, now if it's worth it cost wise is up to the end-user to decide.

 

[RVFR]

I'm seeing I see a lot of boats in my size range with a 4.3 using a 21P and still making their RPMs, so something is fishy

 

[Scott Danforth]

I'm not so sure I would call a 4.3 low comp like a smog engine, at 9.3 that's a good mid-range for a lot of application use, like what has been stated you can run regaler at a lower advance set up, or tweak it and use supper. Think I will give the Super a try knowing it does play how well an engine runs. I have dyno charts that say it does matter, now if it's worth it cost wise is up to the end-user to decide.

9.3 is same as my turbo motor in my daily. many of todays VVT motors are running 11.0:1 (including the Volvo Penta 4.3) many Mazda's are now running 13:1.

so yes, compared to todays vehicles, 9.3:1 is low

however your motor is an early gen 2 or 3 being a 92 OMG. 92 is a transition year. the Vortec heads came out in 1996

if you have a 2-barrel, you are only around 180hp.

however before you do anything. do the maintenance that was neglected by the prior owner.

you may also want to check compression. a new motor is 150 psi, a good motor will be between 135 and 150 psi. a tired motor will be 120-135 psi and below 120 is rebuild time.

 

[Lou C]

while the 9.3 or so comp ratio of the old 4.3s is low compared to modern designs we must keep in mind certain other factors
it does not use a quench design, due to the use of GM full dished pistons this limits the amount of spark advance that it could tolerate before detonation becomes an issue
it did not have a knock sensor
carbed engines do not have as good control over fuel mixture and fuel distribution as a port injected engine
so for all those reasons, these low tech engines can't compete with much newer versions of the same size displacement. However they are sure cheaper and much easier to fix when things go wrong! That's why I have not given mine up yet.

 

[RVFR]

Good points, LOL I'm an old guy from when10.0 to 1 was hot and 11-1 was way out there. 350hp 327 and the like, IIRC the ole Hemi was only 10.25. Yes with today's tech in computer control engines it's no wonder why we have what we have. That said, yea I have an inkling to do a compression test after I get caught up making sure all is good on the easy stuff. it's just that that premium timing is interesting. With that total only being 18? I am surprised that it runs as well as it does.. Interesting here was I just saw a 93 Spyder for sale with a tuned port 5.0EFI in place, Hmmm ?

 

[PITBoat]

Cobra Service manual says full throttle operating range for 4.3 is:

2bbl - 4,200-4,600rpm
4bb - 4,400-4,800rpm

and 36-44mph in a 17-20' boat with a prop of that size.

I have a 19.5' boat, 2bbl, the same prop in aluminum, slightly dinged, and see very similar performance on 89 octane. I don't think I've ever seen 4,500rpm.

 

[RVFR]

Ah-ha, there ya go, info with substance. Thank you, I never thought about looking at a manual, unfortunately, this boat didn't come with one. With that in mind, I now know what to expect. Still, though It's hard to believe there isn't a bit more, I guess if I can get to that 4600 mark I should be happy then. LOL, where do I get a 4 barrel carb set up?

 

[PITBoat]

I never thought about looking at a manual, unfortunately, this boat didn't come with one. With that in mind, I now know what to expect. Still, though It's hard to believe there isn't a bit more, I guess if I can get to that 4600 mark I should be happy then. LOL, where do I get a 4 barrel carb set up?

Mine didn't either. Outboardbooks dot com. You'll need the engine model # I believe.

I'm not an expert, but I vaguely remember something about the heads being a limiting factor on how fast these motors will spin. I guess that would depend on cam and intake too though, wouldn't it...? I don't remember the exact context of that. I have an '89 4.3 TBI in a truck, and I think it tops out around 4,500.

Other guys know about the 4bbl carbs and manifold needed.

 

[Lou C]

Normal redline is 5,000. I had mine out in the bay today and it easily pulled 5,000. This is with a 15.5x15 prop. It limits my top speed but reduces the strain on my old 4.3. Fake cat is to keep birds from turning it into their bathroom.

 

[PITBoat]

I've missed a bunch of good days - warm w/light wind, this season. Too busy with work, or just too tired to go. And it's winding down : /

 

[PITBoat]

How much attention are you giving trimming for max speed during those last coupla hundred rpm? That might make a diff if you're just getting it close and leaving it...

PO for my boat said he saw 42 with just him on a glassy day. I've seen 45 and better with people. Idk if that diff was trim mgmt, slightly different state of tune, or both.

 

[Lou C]

I know people focus on top speed but the closer your WOT rpm gets to the actual redline the less strain on the engine every time it pulls the boat up on plane. Like a trailer tow gear ratio for your truck. My boat I feel is on the big side for a 4.3, 20’9’.

 

[PITBoat]

It took me a while to learn how to get the most out of a motorcycle when I first started that years ago - I was used to the 5k redline in my V8 car, and just piddled about with the bike while working up through the gears. I finally realized the fun was to be had by running it up to the 9,500 redline (thereabouts) in every gear. That was a 450 Suzuki inline twin, and not bad. I weighed a lot less then too, lol.

I'm afraid I do the same with my 3.2Vtec Acura. It just doesn't seem right to run up in the high rpms as a normal practice, but is probably more correct. Growing up with GM V8s is too deeply ingrained.