RexSnr

Trying to replace a water damaged, 485 hour, 310 hp 7.4 MPI Mercruiser engine and want more bottom end to get 9,000lb cruiser out of the hole faster and have a higher cruise speed at around 3,500 rpm through bigger props or gearing. I also need good economy at that cruise speed as I am likely to do big distances - hence choosing a lowish rpm. I can decide on an engine if I could get torque/HP graphs but what is the deal with these?? The Chev crate engine website has one for the HT502, but none for the 454HO. I tried to find one for the std. 7.4MPI, the Magnum 454 and the 502 Magnum - all engines combinations available to me but can only get the HT502 one and that one starts at 2400 rpm.
This website has in previous posts suggested to others that the 454HO is a good replacement for the 7.4MPI in big cruisers but I worry it won't have the low speed torque to pull it quickly out of the hole without lugging. A comparison of torque curves would sort this out quick. Anyone got any of these curves??

Smitty275

Is your existing engine scrap from having cracked when frozen or just had water where it shouldn't have? If the block is not cracked take it to a reputable engine builder and turn it into what your looking for. 496/502/540. Not knowing your block I can't tell you just how big it'll go. But there are excellent kits out there for BBC that don't break the bank. Be very specific about how you want it to perform. They'll pick the right cam, heads, and intake to make it happen.
You'll not find many dyno sheets that start under 2500rpm. It's hard on both the dyno and the engine to load them down at low rpm and expect the engine to be able to overcome the resistance.

liquidlounge

iTrader: (1)

If you don’t have Bravo 3 out drives, having those will make the biggest difference on a boat like that.

RexSnr

I have a Bravo III outdrive. The original engine on the boat has only done 485 hours but is severely corroded in the bores and the water jackets. Some of the pistons I cannot even remove. I want more torque to drive the boat comfortably out of the hole and at a decent speed but with lowish cruise speed, as I will do ling trips in it. I would like a cruise speed of 3,500- 3,800rpm and hoefully still be able to swing biggish props to give best available cruise speed. Top speed is of no interest although it would be improved under same conditions. Fully fuelled and loaded for a trip it could weigh up to 9150 pounds. I was trying to assess the torque of past and potential engines to see which made the most up to say 4,000 rpm.

outonsafari

iTrader: (4)

Start looking for a mercruiser 496 mag or mag ho complete engine, should be what your after
( my opinion & engines arent my thing )
plus all 496's have closed cooling as standard, seawater thru coolers and exhaust only, engine on coolant.

AllDodge

iTrader: (1)

My 10K lb cruiser is about where your at prior. Had a 330 hp 454 that would just barely get it out of the water. Then to a carb 502 and it did better. Was not fixed until I repowered to a 530hp (probably 500 hp at the prop) bored 502 MPI (509).

Running lower rpm doesn't mean better fuel economy, it only means you may be burning more fuel lugging the motor. Efficiency and a boat do not go together. Mine runs best at 3800 to 3900 rpm, which speed is at 38 mph. Figure highest speed at lowest rpm and lowest prop slip. I'm also have a single B3 drive using 2:0 ratio

xlint89

Slightly off topic, but still relevant to the OP if he does bore his engine.

What did you do to modify the stock 502 MPI oil cooling? Thanks

AllDodge

iTrader: (1)

Removed the fitting which attaches the remove oil filter (canister filter) and used AN-10 lines.
Installed high volume oil pump adapter
Installed a 18 by 2 inch oil cooler

xlint89

Thanks for the reply.

Do you have pics of this by chance?

Been weighing my options on building my 1998 502 MAG to around 500 HP. Haven't seen anyone address the oil cooling when increasing HP. The Mercury Racing engines went with a big oil cooler while making similar power. Thanks again

AllDodge

iTrader: (1)

Yes,
https://forums.iboats.com/threads/ch...ration.686970/