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I believe my motor has a 160 degree thermostat, when I was out cruising for the 15 minutes my motor lasted it was bouncing between 160-180. Maybe put in a 140 t stat then?

Also, ive read on a few forums that these motors should be at 35 degrees timing at full advance. Any performance loss running 30? ( I would rather be reliable anyways )

Do you have an opinion on bumping compression to 8.5:1 since I am buying pistons anyways?

Thx

Rookie

iTrader: (4)

The 525 sc is a pretty reliable combination. You are correct with 13° base and 22° added at full timing for 35°. Problem is the minimum fuel requirements for this engine is 92 octane. You added closed cooling to the engine? If so, the 180°F water was not your friend to help with preventing detonation.

Mercury HP 525SC Specifications
Propshaft Horsepower: 490/365
Full Throttle RPM Range: 4800-5200
Displacement (CID/CC): 454/7.4
Bore (inches/mm): 4.25/107.9
Stroke (in.)/(mm): 4.00/101.5
Compression Ratio: 7.5:1
Cylinders: V-8
Fuel Requirements: 92 Posted Octane

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Yeah I added a closed cooling system, I will install a 140 degree thermostat to keep temps lower. I will always run 93+ octane from now on. Expensive mistake

badluck

35° is correct for full advance for a stock 7.5 compression engine, it comes in very late though. Is the ignition system stock?

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Yeah it's the stock thunderbolt ignition. V8-22A

Griff

Sounds like it was probably caused by too low of octane fuel in combination with 180* Tstat. Stock Tstat is 140* in a 525SC.
Next question is how do you know if you have big enough heat exchanger?? Sounds like that could be an issue as well since it over heated so fast.
Post some pics for a better evaluation of what happened.

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I had my doubts about the exchanger as well, Ive seen the same size exchanger used on stock 454 mercruisers but obviously the 525 will make alot more heat. ( It's 4" by 24" )
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I'm thinking j will go back to raw water setup because I know it works and I don't want to worry about coolant level or if the exchanger is big enough. I probably go in the salt water 10 times a year or so. I will just flush really well and use Salt Away in it.


Here's a picture of one of the 3 cylinder walls with "scuffs" on them.


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sailtexas186548

That half-moon or pie-piece area on the cylinder wall (bottom middle of pic) looks like a cylinder that sat with water in it and corroded.

i has some “rebuilds” with the same thing, ended up going .030 over although it probably would have cleaned up at .010

Griff

I'd be more curious to see what the pistons and head gaskets and spark plugs looked like.

The biggest issue with salt and the 525SC is the water ports on the intake manifold where they mate with the heads.
My engine was a 1996 with about 340 hrs on it and I'd had for a 140hrs in fresh water only HELP!!! Need Merc 525SC Intake

wayne272

iTrader: (1)

I have a cigarette bullet that was running 454mags, 8.8:1 compression with 990 heads. I installed a pair of weiand 177's, swapped to a blower grind flat tappet cam, and pro magnum roller rockers and a V6 ignition module on 93 octane. Dynoed at 575hp and boat picked up 10-15 mph.