Mercruiser MCM 5.7L Alpha vs MCM 5.7L Bravo

[AShipShow]

Thinking this is probably a simple answer, but what is the difference between these two motors that gives the difference in horsepower? (My assumption is the Bravo motor has the vortec heads and 4bbl carb, but just my guess).

Otherwise, they to appear to have the same motor specs.

 

[Scott Danforth]

fueling and timing along with cam and pistons for the performance and interupt switch for the drive

Note the 5.7 on the alpha has a 9.1:1 compression and the 350 Mag has 9.3. both are using vortec heads

keep in mind the specs you are looking at are old specs that used GM motors

 

[AShipShow]

Scott,

Just trying to get some more clarification. This is from manual #17 (93-97 GM Small blocks). My bayliner has a 5.7 with a 2 barrel mercarb. I'm assuming mine is the MCM 5.7L Alpha (without looking at serial number, i know, this is the only way to tell for sure). My assumption this whole time was the MCM was prevortec and the MIE was vortec...

The alpha and bravo versions of the MCM 5.7 both have 9.1:1 according to the table above...

Just curious where Mercruiser came up with an extra 15 hp.

So sounds like carb and cam?

 

[Scott Danforth]

to get higher compression is also head combustion chamber or pistons.

1996 was the year that vortec headed motors came from GM.

look at your intake manifold. if you have 8 bolts, you have vortec heads. if you have 12 bolts, you have a non-vortec.

is your desire for more power? going from a 2-barrel to 4-barrel will help

 

[AShipShow]

Mines definitely a non-vortec. Its funny, later in the same manual, it says all 5.7L models are 9.3:1 compression lol, so it contradicts itself so I guess the specs should be taken with a grain of salt.

I don't necessarily have a desire for more horsepower... and honestly, I think I'd rather have less (just enough to get the boat on plane quickly). I'd rather put less stress on my motor than try to squeeze every last hp out of it.

 

[QBhoy]

I must admit, I don’t know of any 5.7 Merc with an alpha that are only 235. I know plenty of them with an alpha that are 250hp with only 2 barrel carbs though and some that are 260 with an alpha and 4 barrel. Must be a rare engine with only 235hp. I’d think most are 250 or 260.

 

[Scott Danforth]

Mines definitely a non-vortec. Its funny, later in the same manual, it says all 5.7L models are 9.3:1 compression lol, so it contradicts itself so I guess the specs should be taken with a grain of salt.

I don't necessarily have a desire for more horsepower... and honestly, I think I'd rather have less (just enough to get the boat on plane quickly). I'd rather put less stress on my motor than try to squeeze every last hp out of it.

your existing motor is not stressed........ its just breathing thru a straw and cant catch its breath

here is what I would do with your existing motor

pull heads...... swap to vortec + 30hp (Improved fuel/air swirling - basically better efficiency)
get new intake (4 barrel and appropriate carb) + ~20hp (improved breathing)

if you want more than that, verify if your motor is flat tappet or roller when you have the intake off, then order a XM-264H or XM-264HR cam as needed. while it wont give you a lot at the upper end, it will fatten up the low to mid-range (Improved breathing)

 

[Scott Danforth]

I must admit, I don’t know of any 5.7 Merc with an alpha that are only 235. I know plenty of them with an alpha that are 250hp with only 2 barrel carbs though and some that are 260 with an alpha and 4 barrel. Must be a rare engine with only 235hp. I’d think most are 250 or 260.

some were listed as 210hp (5.7 with 2-V carb) in the early 00's. they derated them to push the MPI 5.0's

 

[AShipShow]

here is what I would do with your existing motor

pull heads...... swap to vortec + 30hp (Improved fuel/air swirling - basically better efficiency)
get new intake (4 barrel and appropriate carb) + ~20hp (improved breathing)

This is exactly my long term plan. I had basically the same plans on my last boat with a non-vortec 4.3. Even had the intake manifold and heads, then i sold the boat...

Just not sure if I'd go down the road of pulling truck heads from a junk yard for 50 bucks and refurb them myself, or just buy reman heads... Hopefully i have a while until I have to make that decision. Thinking I'll save that for next time I need to pull the motor for a exhaust manifold replacement... I'm thinking I have another 3 or 4 years on them.

 

[Scott Danforth]

new heads are about $900 a pair direct from GMPP. a bit less if you watch the sales
reconditioned heads are about $400 a pair (most engine shops near dirt tracks will have them on the shelf with ARP studs, seals cut, and a few other mods already done)
used heads are about $100 a pair from LKQ

 

[achris]

MCM = MerCruiser Marine (stern drives). MIE = Mercruiser Inboard Engine.

Read the specs! It says PROPSHAFT HORSEPOWER... The Alpha drive loses about 15hp more through it than a Bravo.

Mystery solved.

Chris.......

 

[achris]

I must admit, I don’t know of any 5.7 Merc with an alpha that are only 235. I know plenty of them with an alpha that are 250hp with only 2 barrel carbs though and some that are 260 with an alpha and 4 barrel. Must be a rare engine with only 235hp. I’d think most are 250 or 260.

235hp 5.7 Alpha - That is a propshaft rating. That's a non-vortec, 4 bbl. Was the old '260'.
250hp 5.7 Alpha 2bbl - Again propshaft rating, vortec heads.
260hp Alpha. Crankshaft hp, non-vortec heads, 4bbl.

Hope that clarifies.

Chris.........

 

[QBhoy]

235hp 5.7 Alpha - That is a propshaft rating. That's a non-vortec, 4 bbl. Was the old '260'.
250hp 5.7 Alpha 2bbl - Again propshaft rating, vortec heads.
260hp Alpha. Crankshaft hp, non-vortec heads, 4bbl.

Hope that clarifies.

Chris.........

Good info there. Left wondering about the 350 mag 4 barrel carb perhaps. The one that would be around at same time and likely after the 250 5.7 vortec 2 barrel. Would that surely be a vortec too and what HP ? I thought that was 260hp and rated from the same place the 250 2 barrel was ?

 

[achris]

350Mag 4bbl. 250 PROPSHAFT horsepower. Through an Alpha, that's about 275 crank HP. They get that from higher compression, etc. But still NOT vortec. The vortec 350Mag is 300 (crankshaft) hp.

 

[AShipShow]

The Alpha drive loses about 15hp more through it than a Bravo.

Chris.......

I have a hard time believing that... That just doesn't make sense.
a larger drive with larger gears, larger bearings, more surface area has LESS efficiency losses??

Is the bravo drive rated to higher rpm? that would make sense to me, not that an alpha is less efficient...

 

[achris]

I have a hard time believing that... That just doesn't make sense.
a larger drive with larger gears, larger bearings, more surface area has LESS efficiency losses??

Is the bravo drive rated to higher rpm? that would make sense to me, not that an alpha is less efficient...

It doesn't matter what you believe or don't believe. That is the figure quoted to me by the Merc service training school instructor when I did my certification course. And all documentation bears that out.

And yes, Bravo rated to 600hp (370 with a diesel), Alpha to 300hp.

Chris......

 

[Rick Stephens]

350Mag 4bbl. 250 PROPSHAFT horsepower. Through an Alpha, that's about 275 crank HP. They get that from higher compression, etc. But still NOT vortec. The vortec 350Mag is 300 (crankshaft) hp.

Higher compression? I thought it was the 'Magnum' sticker.

 

[Davetowz]

MCM = MerCruiser Marine (stern drives). MIE = Mercruiser Inboard Engine.

Read the specs! It says PROPSHAFT HORSEPOWER... The Alpha drive loses about 15hp more through it than a Bravo.

Mystery solved.

Chris.......

Thanks, I was always curious about the MCM vs MIE nomenclature.

 

[Davetowz]

Higher compression? I thought it was the 'Magnum' sticker.

Stickers always increase power output.

 

[AShipShow]

Good to know! the previous owner of my boat put about 10 stickers on it... Might have to upgrade to a bravo drive with all these stickers.