Logo
FULL SITE MENU

454 to 350 question

8

805HD

Regular Contributor
Looking for advice on what it’s going to take to go from a 454 down to a 350, what should I be expecting on both install and performance? Can it be done or is it going to be a major headache as far as mounting? Will it line up with the old foot print of the 454


Boat is a 23 foot cobalt with a Alpha drive

My old engine was giving me nothing but problems so I pulled it with the intention of getting a new long block but current money situation has dictated otherwise.

Got a buddy with a good running 350 that I can have out of his old boat to get me on the water this summer. Which obviously, is the only reason I’m even considering this...

thanks in advance
 
Well, you'll need a prop with less pitch--for sure. From what I've seen in 28 foot Bayliners, the performance is not that far different.

Jeff
 
fastjeff said:
Well, you'll need a prop with less pitch--for sure. From what I've seen in 28 foot Bayliners, the performance is not that far different.

Jeff
Click to expand...

for sure, figured that much. Definitely not interested in speed. We really just cruise the lake/ river and spend most of our time beached or floating. The engines ready to drop in just hope it’s plug and play with the existing mounts in the hull.

im sure my alpha will appreciate the 350 more then I will lol!
 
These may help

merc 350 install dimensions


https://samerwebapp01apncus01.azure...4v_installation_drawing_865108r85-and-r88.pdf

454 install drawings for bravo but mounts in same place

https://samerwebapp01apncus01.azure...e-b11f-bc3c56b60663/74lmpibravo_1996-2000.pdf

the motor mounts look very close in dimensions but I didn’t look at it too long

probably need to go down 2-4” in pitch best to find a prop shop to try before you buy. If I recall 454/alpha combos had a1.32 rear ratio , a 350 would be about 1.5, 1.62 at steepest.

when I repoweredwent from a 4.3 to a 5.0, kept the 4.3 drive, just had to go up 2” in pitch, would think you can go the other way a step or two
 
Dieter said:
These may help

merc 350 install dimensions


https://samerwebapp01apncus01.azure...4v_installation_drawing_865108r85-and-r88.pdf

454 install drawings for bravo but mounts in same place

https://samerwebapp01apncus01.azure...e-b11f-bc3c56b60663/74lmpibravo_1996-2000.pdf

the motor mounts look very close in dimensions but I didn’t look at it too long

probably need to go down 2-4” in pitch best to find a prop shop to try before you buy. If I recall 454/alpha combos had a1.32 rear ratio , a 350 would be about 1.5, 1.62 at steepest.

when I repoweredwent from a 4.3 to a 5.0, kept the 4.3 drive, just had to go up 2” in pitch, would think you can go the other way a step or two
Click to expand...

thanks for the specs, from what I see they are identical. Which is great news for me! Really appreciate the links. Obviously I’ll double check but greatly appreciated.

Totally hear you on dropping the prop a bit!

hopefully others will chime in
 
I would suggest if the engine is not complete and you can do some simple modifications I would say,

Slightly bigger cam, Slightly higher rise intake manifold and calibrated card for combo. Try to get power up to at least 300 HP (which will increase the torque also).


A stock 350 is 260 hp.

A 350 mag is 270 -280 hp..............only difference is, carb calibration and intake manifold (slightly higher dual plane)

A 23 ft cobalt is a very heavy boat....My 21 ft cobalt is 3600lbs dry with a 350 alpha.

Note: if your 454 had the remote oil filter attached to the elbow, RE-USE this! Great opyion so changing oil filter does not require being upside down to do so.

Actually if you bump the power up you should be able to use the 454 carb as it is already calibrated for ~ 325 hp
 
Meh sounds like your use is mainly as a mobile lounge, so I would say just drop in the 350 and go. Your main issue is getting the alignment of the engine correct.
 
kghost said:
I would suggest if the engine is not complete and you can do some simple modifications I would say,

Slightly bigger cam, Slightly higher rise intake manifold and calibrated card for combo. Try to get power up to at least 300 HP (which will increase the torque also).


A stock 350 is 260 hp.

A 350 mag is 270 -280 hp..............only difference is, carb calibration and intake manifold (slightly higher dual plane)

A 23 ft cobalt is a very heavy boat....My 21 ft cobalt is 3600lbs dry with a 350 alpha.

Note: if your 454 had the remote oil filter attached to the elbow, RE-USE this! Great opyion so changing oil filter does not require being upside down to do so.

Actually if you bump the power up you should be able to use the 454 carb as it is already calibrated for ~ 325 hp
Click to expand...

What’s up jack, you really think it will be that underpowered? I don’t need a race boat but I also don’t want to be a slow boat!

next summer I plan on throwing a 454 back in there.

do you think it will be a plug and play swap as far as mounting and alignment goes or am I going to hit some issues?
 
As long as you have everything from the small block, the change should be fairly easy. That is, the exhaust manifolds, elbows, and y-pipe, alternator, power steering pump, etc. There's not much that will actually swap from one engine to the other (bracketry-wise)
You will have to remove the drive so that you can get the alignment tool thru the transom assembly and into the drive coupler. While the engine is out I would spend a few hours cleaning and painting the bilge if necessary, inspecting the engine beds for strength, and generally tidying things up back there. Don't forget to throw some love on the bilge pump, blower and anything else that's hard to get to with the engine in place.
As for Ghost's recamming suggestion....if you're gonna go to the trouble of installing a cam then just bite the bullet and buy a rebuilt big block. It's about 4 grand. Cry once and be done.
BUUUUT, and this is me....I would have the current lump checked out. What's the issue that you are so ready to kick it out of the sack so to speak? Cracked block? That would be about the only thing, right? If it's just head gaskets or valves or something all that can be repaired at fairly reasonable cost even if it means removing the engine to do so. Bottom end on these is so darn reliable that it would have to be something catastrophic that would require switching it out.
 
the 350 won't like the 1.32:1 gear.....I'd prop it at the high end of the WOT range....and don't use any more throttle than necessary and you should be ok for a season...
 
805HD said:
What’s up jack, you really think it will be that underpowered? I don’t need a race boat but I also don’t want to be a slow boat!

next summer I plan on throwing a 454 back in there.

do you think it will be a plug and play swap as far as mounting and alignment goes or am I going to hit some issues?
Click to expand...


Actually Yes. 260 hp in a 4000lb+ boat is under power.

But if you are going to rebuild the 454 then just use the 350 as is. I thought you were replacing for good.


Also if you are going to continue with the Alpha when the 454 is re-installed, DO NOT increase power of 454. The torque is about at the max a Alpha can handle...!!!!

Your fortunate it lasted this long..........

And take the advice on the prop pitch change when the 350 goes in.
 
Dieter said:
These may help

merc 350 install dimensions


https://samerwebapp01apncus01.azure...4v_installation_drawing_865108r85-and-r88.pdf

454 install drawings for bravo but mounts in same place

https://samerwebapp01apncus01.azure...e-b11f-bc3c56b60663/74lmpibravo_1996-2000.pdf

the motor mounts look very close in dimensions but I didn’t look at it too long

probably need to go down 2-4” in pitch best to find a prop shop to try before you buy. If I recall 454/alpha combos had a1.32 rear ratio , a 350 would be about 1.5, 1.62 at steepest.

when I repoweredwent from a 4.3 to a 5.0, kept the 4.3 drive, just had to go up 2” in pitch, would think you can go the other way a step or two
Click to expand...

would you happen to have a suggestion of what neighborhood I should be in as far as prop pitch goes?

with the big block I was running a 19 pitch. According to the prop wizard prop calculator I should be running something like a 17-15 pitch with the small block.. sound about right to you?
 
805HD said:
would you happen to have a suggestion of what neighborhood I should be in as far as prop pitch goes?

with the big block I was running a 19 pitch. According to the prop wizard prop calculator I should be running something like a 17-15 pitch with the small block.. sound about right to you?
Click to expand...

yes by step normally that refers to a 2” drop or increase in pitch. Would think 17-15” would be where you land. May want to consider a 4 blade for better hole shot with a smaller engine, especially if top speed isn’t your goal. Had good luck with Michigan Vortex 4 blades with under powered boat I previously had. Aluminum is relatively cheap if you need to play with pitch. Best to find a local prop shop or marina who explain what you’re doing and likely they will let you try before you buy. You want to prop it such that you can get up to 4800 rpm or so at WOT.
 
....my 23 ft Lancer displaces just about 4000# with it's MERC FWC 260 HP 5.7MIE. The hull has a 22 degree deadrise ( as much a factor as displacement) Wide open with two onboard, etc it does just over 30 mph. Cruises nicely in the mid 20s. Given Barnegat Bay's steep breaking chop most afternoons, it is plenty fast enough.
I'm running a 14.5x17 prop with a 1,61:reduction. I could probably run a 19 P prop if the engine's torque curve wasn't midrange optimized ( per MERC promo lit).
 
Last edited:
sandkicker said:
....my 23 ft Lancer displaces just about 4000# with it's MERC FWC 260 HP 5.7MIE. The hull has a 22 degree deadrise ( as much a factor as displacement) Wide open with two onboard, etc it does just over 30 mph. Cruises nicely in the mid 20s. Given Barnegat Bay's steep breaking chop most afternoons, it is plenty fast enough.
I'm running a 14.5x17 prop with a 1,61:reduction. I could probably run a 19 P prop if the engine's torque curve wasn't midrange optimized ( per MERC promo lit).
Click to expand...


this is probably a great example of what performance the OP can expect. Difference is op will has a 1.32 drive hopefully a 15” is steep enough given you are running a 17”
 
I dont know Lancer But I do know Cobalt, Cobalt is a very heavy boat......most 21 ft bow riders are what? 2600-3200 lbs?

Mine is 3600 dry weight with no one in it and no gas. Gas tank is 63 gallon tank.......you do the math
once loaded it gets much closer to 4000 lbs easily if not more, 3 batteries, 5 adults (200+ each) coolers full of beer, a bunch of other accessories,,,,,,


That is why I say a 350 for that boat is under power
 
Last edited:
Wow! That's a heavy boat.

Agreed that it will be way under-powered, and under-powered boats burn lots of gas! You might consider locating another big block--lots of them out there.

Jeff
 
Back
Top