Brand new 8.2L H.O. Not performing as expected

[Nick70]

I’ve got a new 8.2 HO, as well as a new Bravo III XR drive. Just hit 18hrs of break in time.
Boat is a 2006 260 Sundancer. Previously had a 8.1 Mag, Bravo III.
Old engine lost two cylinders due to clogged injectors. The cool fuel unit had been replaced by previous owner but never cleaned/inspected the fuel line, rails and injectors. It took a while of chasing to figure out what was wrong. long story short, Mercury helped me out on parts, and I went big with a complete drop in 8.2 HO, and the XR drive.
So, I went from a 375hp rated engine to a 430hp. the weird thing is, the new set up is considerably under performing. The numbers are much worse than
What they were even the last time I took it out with the original engine..and at the time it was obviously not even running properly.
Original engine cruise @ 28 mph around 3600 rpm and 14-15 gph. WOT 4600
Rpm and 40mph.
with this new engine I’m on my third set of props already.
First set was 24 pitch and max rpm was 4000 @32mph. Not good.
Second set, 20 pitch and it would go easy right up to 5000 rpm and 37 mph.
Now, I’m running 22 pitch and hit 4800 rpm @ 38 mph.
Best cruise is 25mph 17-18 gph @ 36-3700 rpm.
WOT 4800rpm 38mph.
I’m scratching my head wondering why/how my overall speed and performance is considerably worse as well as my fuel economy aprx 20% worse.
Wtf..with this new set up I was conservatively expecting a WOT top end of at least low 40’s mph..hoping for 45+, and a much better cruising range of where it’s at.
As it was with the original engine, 2mpg was just barely out of reach.
30 mph at 15 gph…so I was just expecting something positive with the new.
Nope. It’s sucking gas and not even as fast as the old clogged engine.
After all the time and money I’m almost beyond even caring anymore.
I figured it’d be a no brainer going with the new Mercruiser big block.
But I’m curious as to how the original, less powerful and clogged injector engine was doing better all around.
Same boat, same capt. Same equipment. I’ve adjusted the trim as I always do. Multiple runs. It’s unexplainable boat calculus.

 

[scoflaw]

Check the timing?

 

[Jeremygavin]

Is the new drive the same ratio as the original drive? That would change the numbers quite a bit.

 

[paulswagelock]

Something is odd for certain. You should have seen 3-4 mph increase at top end and better overall performance.
IIRC there is some set up or checking that needs done with the factory dealer software. Did that get done?

 

[Nick70]

Is the new drive the same ratio as the original drive? That would change the numbers quite a bit.

Whatever the ratio of the drive, if propped correctly you should get the performance. You know it’s propped correctly if you’re getting the desired rpm range at WOT.

 

[Nick70]

Something is odd for certain. You should have seen 3-4 mph increase at top end and better overall performance.
IIRC there is some set up or checking that needs done with the factory dealer software. Did that get done?

That’s exactly what I was expecting…3-4 more on the top end and really nice performance all around. I had an ‘09 330 with twin 8.1’s bravo’s and that thing hit 50 mph. It weighed about twice as much as the 260 as well, so my outside expectations were kind of high..these results are the exact opposite! Lol…I’ll check on the software updates. Good idea.
Btw what does IIRC mean? Thanks

 

[KevinC]

That’s exactly what I was expecting…3-4 more on the top end and really nice performance all around. I had an ‘09 330 with twin 8.1’s bravo’s and that thing hit 50 mph. It weighed about twice as much as the 260 as well, so my outside expectations were kind of high..these results are the exact opposite! Lol…I’ll check on the software updates. Good idea.
Btw what does IIRC mean? Thanks

IIRC = if I remember correctly

-Kevin

 

[Chillaxin']

No help but I feel your pain.
3 years ago, I replaced a pair of 5.7s w/260hp ea to a pair of 383s w/350 ea.
None of my performance numbers improved and I had to prop down from 22p to 20p to get close to the bottom of the recommended wot range.

Still not happy.

 

[Stray Current]

Your new numbers are similar to my current numbers on my tiring 1000 hr 6.2 in the same boat. I do 26.5-27.5 at 3900 and I’m getting 4650 +/-50 at 36 mph on 20” props wtfo.

used to hit 40 at 4850 when I bought it with 500 hours

 

[Scott215]

Could be gear ratio. Could be timing. I would go back to them and have them go over everything twice

 

[Nick70]

Check the timing?

Distributurless ignition…coil packs on this engine, crankshaft position sensor. Engine starts and runs smoothly.

 

[Nick70]

Could be gear ratio. Could be timing. I would go back to them and have them go over everything twice

The gear ratio of the new drive is 1.81:1, it was the suggested ratio and a very common one, the 24 pitch props I initially had were also suggested by the Sea Ray rep and agreed with by the Mercruiser rep as well. Oh, and the engine has distributorless ignition, it’s got coil packs and crank position sensor. It starts and runs smoothly.

 

[techmitch]

Distributurless ignition…coil packs on this engine, crankshaft position sensor. Engine starts and runs smoothly.

Seems like some good diagnostic equipment is in order. Check out @boatrboy latest post.

 

[Nick70]

No help but I feel your pain.
3 years ago, I replaced a pair of 5.7s w/260hp ea to a pair of 383s w/350 ea.
None of my performance numbers improved and I had to prop down from 22p to 20p to get close to the bottom of the recommended wot range.

Still not happy.

Yeah it’s crazy. I understand with boats there’s a lot going on. How you see vastly different numbers between different peoples boats with the same power package etc…but in situations like ours, same exact boat, same human operator, and new more powerful engines, with same or worse performance, that leaves me scratching my head.

 

[Nick70]

Seems like some good diagnostic equipment is in order. Check out @boatrboy latest post.

Will do, thanks.

 

[PlayDate]

I agree with Mitch.....diagnostics should tell the story. Not all the ponies are showing up. Off hand.....what was the torque rating between the new and the old engine?

 

[Ericinmich]

I'd look at the hp and tq curves of the engines. The new HO engine probably has intake runners and cams to spin up higher for better top end, but did it give up some mid range torque to get the extra HP on the top?

A couple sites I looked recommended the lower HP version for cruisers and your version for HP boats.

 

[Scott215]

496 vs 502. Not a huge difference depending on how the motors are built the 502 by nature should have more torque than the 496 when the 502 came out it was 502 cubic inches 502 horsepower I know Marine applications are different. I think something in the tune of the engine cuz I don't think the 496 the computer can run the 502 or did you get a new ECM with it

 

[dtfeld]

Is this a Mercury engine? I couldn’t find the torque specs, but the new 8.2 has a bigger bore but smaller stroke than the older 8.1, so maybe a difference in the torque.

Usually no replacement for displacement, but in boats, torque matters.

 

[Ericinmich]

496 vs 502. Not a huge difference depending on how the motors are built the 502 by nature should have more torque than the 496 when the 502 came out it was 502 cubic inches 502 horsepower I know Marine applications are different. I think something in the tune of the engine cuz I don't think the 496 the computer can run the 502 or did you get a new ECM with it

I just looked at the two versions of the 502 merc sells. One is 380hp (or something like that) and same block they sell the 425hp vesrion. The 425 spins a lot higher, so likely different cams to get there and maybe intake runners. It very well could be that the lower HP version would have better cams for mid range torque and better bsfc at those points.