350mag v20

fixit · #1 11-25-2014, 01:29 AM

im mid swap on my I/O 1991 v20

taking a dead 4.3l out and swapping in a 350mag mpi motor (300hp) with a matching bravo 1 leg

im wondering what kind of top speed ill get and what pitch prop would be appropriate?

Im going to add trim tabs too, any other thoughts?

spareparts · #2 11-25-2014, 06:54 AM

sounds like a good swap, I'd suggest starting with a 23P Mirage, that should put you in the low 60's. That engine is rated to turn 4800-5000, you may need to step back to a 21P depending on how its loaded. Trim tabs are a must

THEFERMANATOR · #3 11-25-2014, 10:15 AM

That engine is sensitive to propping, just a bit to much pitch and it will bog down. Put one in the SEARAY I did years back, and it was a DOG, only topped out at 4200 RPM's and about 30 MPH. Had the prop repitched from a 17 to a 16 and it jumped up to 4800 RPM's, and topped out at almost 40MPH. It should move right along in the water though.

RidgeRunner · #4 11-25-2014, 01:31 PM

GarageNC has one with a similar powerplant that is peppy so I've heard..
It should go fast enough, I will guess mid-50's. Welcome aboard BTW..

fixit · #5 11-25-2014, 03:01 PM

Thanks for the welcoming replies
Been a long time lurker didn't realize that was my first post!

I've been looking at Bennett hydraulic tabs ( I was told they are one of the best)
Any idea on what size?

I'll have to get some Picts ups
Can't seem to do it on my phone

fixit · #6 02-16-2015, 02:01 AM

got the boat wet today,
after a few runs her best speed was 42 knots (gps) with a 19p turning at 4800/bravo leg 1.5;1

wondering if an 18p or a smalled diameter 19p would get it up to 5000rpm

RidgeRunner · #7 02-16-2015, 07:57 AM

Nice top end. You should have a nice cruise speed. I am not sure going up in RPM by going down in pitch is going to do anything for your top end. It might, but I think you are already on the top end of what the camshaft is prepared to do. I remember the old 260 Mercruiser (350 chevy with a carb) had such a mild profile cam it was not helping anything to prop it so it would turn over 4600. Others are far more knowledgeable on that topic. 42 knts is scootin'.

Congratulations, that was an undertaking.
It would be better for me if you

and posted it here. Just saying. LOL!

spareparts · #8 02-16-2015, 08:53 AM

if it has vortec heads( 8 5/16 intake bolts), you are supposed to prop it out to 4600-5000 rpm, if it has non vortec heads(12 3/8 bolts) 4400-4800 RPM. Check your tach to make sure its accurate. With the bravo you are loosing a little hp in the drive compared to an Alpha, but its a much beefier drive. For top end, you may want to play with a prop with a little more rake than the Mirage like a tempest, but its going to hard to beat the all around performance of a Mirage

THEFERMANATOR · #9 02-16-2015, 10:29 AM

Being an MPI engine, it should be a VORTEC based engine. Also, keep it UNDER 5000RPM's. The rev limiter in some of the MEFI controllers is rather violant to say the least once it hits 5000RPM's. The one I did the swap with was a MEFI 2 ECM, and the second it hit 5000 RPM's, it would shut off until it dropped below roughly 4600 RPM's. It was not a smooth limiter at all, bout slid out of the seat when it shut off so quick. And like spare said, the bravo leg is taking away a good bit from your top end.