Repower from 5.7 to 8.1 engines

That small block is pretty sick looking from the dyno sheet.

What is that engine going to run price wise?

Doug
 
douglee25 said:
That small block is pretty sick looking from the dyno sheet.

What is that engine going to run price wise?

Doug
Click to expand...

The target is about $10K~$12K. The MPI people are supplying some of these to Off Shore Power Boat Racers this season to see what breaks. Reportedly, there is a guy in Louisiana that did a pair in a 90~91 42DA in place of 7.4 carbureted motors. They cut 1500 lbs of weight out of the boat and added a total of 400 HP and 200 lb. ft of torque.
He was able to add 2" in prop pitch and the boat cruises at 30 mph at 3500 RPM and with a light load has seen 45 mph+ /GPS on the top. Cruise fuel is about 22 gph! Word is that in the last year the motors have logged close to 200 hrs with no issues. I would love to see and ride that boat to see how it was engineered.

The LSA motor is the Big Block replacement. There will not be any more produced after October this year.
 
sprink56 said:
The target is about $10K~$12K. The MPI people are supplying some of these to Off Shore Power Boat Racers this season to see what breaks. Reportedly, there is a guy in Louisiana that did a pair in a 90~91 42DA in place of 7.4 carbureted motors. They cut 1500 lbs of weight out of the boat and added a total of 400 HP and 200 lb. ft of torque.
He was able to add 2" in prop pitch and the boat cruises at 30 mph at 3500 RPM and with a light load has seen 45 mph+ /GPS on the top. Cruise fuel is about 22 gph! Word is that in the last year the motors have logged close to 200 hrs with no issues. I would love to see and ride that boat to see how it was engineered.

The LSA motor is the Big Block replacement. There will not be any more produced after October this year.
Click to expand...

Good info. If you find out anything more, please post it. I am curious to hear the reports.

Doug
 
Just curious....How old is the motor. That GM got me thinking. You could drop a procharger and a B3 on her and be out of it for a lot less.

OR.....A procharger and a Schoell "Pulse Drive" and pick up a ton of fuel economy and cut worries about flotsam.

Whatever you decide, you have a nice boat. Keep us posted.
 
Tail Chaser 661 said:
Too late,
Just wrote the check for the Merc 8.1 MAG 375 hp today.
I'm going for the lowest cruising RPM I can get.(Reliability)
That's diesel or big block power.

SB and BB both share the same motor mount location.
Replacing my 24 yr old fuel tank. Trimming two inches off the depth for more engine room. (same as small block)Then having the bottom of the tank extended 5" down into the bilge. That will put me back at the org 100 gals.
Merc 8.1 Mag comes with fresh water cooling and dry joint exhaust.
At this point, installation for SB and BB EFI engines will be the same.


I think a 383ci is a little stressed for heading straight out 100 miles offshore in a 8500lb single screw boat.
The 496ci was built as a workhorse engine.
Click to expand...
You did it! :smt038now you just have to do it. Keep us posted
 
stevea661 said:
You did it! :smt038now you just have to do it. Keep us posted
Click to expand...
Thanks steve,

I had to grab this pre 08 crate engine due to Calif and the CARB BS. (Catalytic Converter). Could not pass on the $9950.00 tag either.
Nov. would have been a better time.:smt013
Everybody might get screwed in 2010 on brand new gas engines.

Would like to do a couple of 3 Day WSB trips with my smallblock before even thinking of doing the repower.
Maybe have the Big Block in before the Yellowtail and Tuna.

Putting my 15"X 17" Stainless prop back on.
Maybe I will be happy enough to wait till the off season.

Going custom fuel tank. Should add a 30 Gal aux tank also.
I know me. Will want to check out the stringers, Beef them up, just for the hell of it. Add some glass mat in the bilge, etc.
I dont feel like dealing with any of this now.
Just want to use the dam boat :smt021
 
Are you going to do the work yourself? If so my hoist is still up from when I did my engine. Your welcome to drive up the mountain and use it to pull the old one and install the new one. The other thought is ( and this is just my opinion) that you should wait until after fish killing season to do the work. Some times, and don't ask how I know this, projects take longer to complete than we plan. Just some times.:lol:
 
Thanks Steve,
I need to get the old engine out.
Then rewire the whole boat myself. 95% of the engine wiring runs on the transom. Not very clean looking.
Mount a couple of bus bars on the transom.
Have the dash to engine wiring going thru some conduit or PVC to the engine room.
Do some glass work and bilge painting also.
I should take the boat to the fuel tank guy. Let him build a tank to get max gals

I'll let the mechanic do the install.
He has all special tools for engine alignment, bravo cable adjustment etc.

I'm hoping to get one or two..... 3 day trips in before my engine arrives. All I'm thinking about is, 50# WSB swimming around out there. And I'm still at home.:smt013
I just put my faster stainless prop back on.
Hope I can finish this year out with the small block.

Anyway, Got to get back to the boat. Frigen 95 degrees outside. Stopped for a beer break.
 
If you stay with the Alpha drive, have it rebuilt with a stronger shaft. I have seen Alpha drives, years ago, with twisted shafts from too much torque on a heavy boat and the shaft just twists.
Upgrade the motor like suggested above but upgrade the Alpha drive also. Otherwise, you will be fixing it later, I've seen it several times. JD
 
Dog clutches in an alpha can't take big block torque. If you fool around with the gear ratio and put the least amount of reduction into the drive, it can almost take the torque produced by a standard performance 454.
 
i think he's planning on going bravo, pretty sure he mentioned it somewhere. He's defanitely not the type to half ass it or do half the job.
 
Thanks berth control,

Getting Bravo 3 drive. 2.2: gear ratio with 26" pitch prop.
WOT should be 44 mph at 4500 rpm.
I will rarely take it to 3200 rpm. That's over 30 mph.
Fuel burn should be 9 GPH at 2500 rpm and 12gph at 3000rpm.
 
Last edited:
Tailchaser,

In smooth water you will be at about 12GPH at 3000. In rough to very rough, you will see 13-14GPH at 3000.

You may have to double cup those 26's or even go with 28's. I have found that double cupping gives about 100-200 rpm decrease, as compared to a solid 200 rpm decrease with standard uncupped 28's. The 26 hull likes the double cups, though, as it carries the bow better with a little less porpoise at higher trim levels.

Your WOT will be well past 44 MPH and 4500 rpm with uncupped 26's. We used to get 45-46 MPH with the old carbureted 454's at 310hp.

My 26 would consistently run 53-55 MPH fully loaded at 1200 feet above sea level with the exhaust open (and through EMI headers). It was also just barely in the rev limiter at 5050 RPM. I didn't run it that way often, but she would fly.

The 502 is different than a 496, and definitely has more oomph at the top of the RPM range (but does not have the midrange of the 496). Still, I would be surprised if you can't pass 50MPH
 
One other thing... When you put the drive on, put the "celery stick" forward to maximize negative trim. It will significantly lower the bow when climbing on plane, and allow you to stay on plane at lower speeds. Just make sure to trim up a touch as you get on plane to avoid bow steering. There is an old thread about this somewhere on this board.
 
Thanks keokie,
I was running very safe numbers on the Merc prop selector website
8K boat
500 lbs of passengers
100 gals of fuel
Basically gross weight 9100 lbs.
Came up with 14"D x 26"P and 15.25D X 26"P prop set.
That gets the 42.9 mph at 4500 rpm on the Merc site.
Add another 500 lbs, They drop you to the 24"P props

With all my fishing junk, In reality I'm probably at 8500# Gross wgt.

Merc racing has some double cup 26"P prop sets. I believe
they were 16D props.
Merc recommends going down one pitch size using them.
Not sure if I could spin them, or any double cup props with the added wgt on my boat.
I'm bottom painted also, creating even more drag.

Thanks again for the great info keokie :thumbsup:
A 50 mph top end would work for me :grin:
A good way to empty some bad gas out of the tank ,While I spank somebody. :lol:

Get 2 MPG at 3000rpm while cruising in rough water? Not bad.:grin:
And to think some CSR members think the answer is to buy a new boat :smt101 :smt018
 
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