Max RPM with WOT 8.1 Mercruisers

[ipasgas1]

We are looking at a 2005 Sea Ray 390 Motor Yacht with the 8.1 Mercruiser engines. During the sea trial, it only made 3900 rpm WOT and the starboard engine was backfiring. The broker had a mechanic look at the engines and he changed the sparkplugs and fuel filters as well as did a compression check which was normal. The subsequent sea trial produced 4100 rpm WOT. The book says rpm at WOT should be 4400-4800. The broker says it is propped tall as the previous owner used it as a trawler. Not sure what that means but does that sound reasonable? During the out of water, bottom paint looked fine, props looked good. Surveyor said port cutlass would need replaced soon but starboard was fine. Any thoughts? We are getting her for a great price as long as the engines are good (395 hours).
Thanks,
Dan

[JKP]

Just curious, at WOT what was the indicated speed? A good friend has a similar boat, but it is powered with diesels.

[ranger58sb]

Originally Posted by ipasgas1

We are looking at a 2005 Sea Ray 390 Motor Yacht with the 8.1 Mercruiser engines. ...

The subsequent sea trial produced 4100 rpm WOT. The book says rpm at WOT should be 4400-4800. The broker says it is propped tall as the previous owner used it as a trawler. Not sure what that means but does that sound reasonable?

Could be plausible. You might be able to ask a prop shop for informal evaluation, given diameter and pitch info....

That's a heavy boat, very likely a better candidate for diesels... unless you intend to run it slow as maybe did the previous owner. (If that turns out to be true...)

-Chris

[a64pilot]

I believe it's exhaust risers that kill many big block Merc's.
Check on that and if I'm right expect to replace them if they have not been recently.
They go bad from corrosion and leak water into the cylinders seizing the engine

[ranger58sb]

Originally Posted by a64pilot

I believe it's exhaust risers that kill many big block Merc's.
Check on that and if I'm right expect to replace them if they have not been recently.
They go bad from corrosion and leak water into the cylinders seizing the engine

Good point. Our owners club members who run gas in saltwater say they replace cast iron elbows and sometimes risers (apparently depending on engine) every 5 years. Most invasive corrosion isn't detectable from the outside...

-Chris

[Fiveslide]

Years ago, I lost some RPM at wot after I rebuilt and cleaned my carb with a kit from napa. It went from about 4600 down to 3900. Rebuilt it again with a new kit with the same results. I went to a boat dealership and spent triple the price on a kit and got my RPM back. Does it have carbs or fuel injected?

My current inboard v8 boat only shows about 3800 at wot, but the speed measured by GPS and the sound of the engine have me convinced the tach just isn't functioning properly.

If the compression tests are truly normal, I think it's props, exhaust or fuel delivery problem.

[seabreez]

Originally Posted by ipasgas1

We are looking at a 2005 Sea Ray 390 Motor Yacht with the 8.1 Mercruiser engines. During the sea trial, it only made 3900 rpm WOT and the starboard engine was backfiring. The broker had a mechanic look at the engines and he changed the sparkplugs and fuel filters as well as did a compression check which was normal. The subsequent sea trial produced 4100 rpm WOT. The book says rpm at WOT should be 4400-4800. The broker says it is propped tall as the previous owner used it as a trawler. Not sure what that means but does that sound reasonable? During the out of water, bottom paint looked fine, props looked good. Surveyor said port cutlass would need replaced soon but starboard was fine. Any thoughts? We are getting her for a great price as long as the engines are good (395 hours).
Thanks,
Dan

To verify your RPMs I would install a portable RPM from a mechanic and verify my readings on the dash tachometers. With that check if the tachometer readings are accurate, the props maybe have been tuned to turn less with more inches per revolution to make it more fuel efficient at lower speeds. That would sound right if the previous owner was using the vessel as a thrawler ... The normal revolutions are 4450 for those engines. Also get a good survey on those engines as they are known for head gaskets leaking. I just did another one last fall for a friend. Good luck on your purchase...

[ipasgas1]

At WOT, he achieved 18.9 knots which seems reasonable...

[jimbunyard]

The rated RPM is 4400-4800 for the 370 HP 8.1.


According to the 'Yachting' boat test here,
Yacht, Yachting, Trawler, Luxury Yachts | Yachting Magazine


"Our test boat was equipped with twin 370 hp MerCruiser 8.1 gas engines turning a set of four-blade 23-by-22-inch props. If you're looking to do a little more extended cruising, Sea Ray offers twin Cummins 480CEs. With the gas package, expect a peppy performance. We achieved a top speed of 25 knots and a cruising speed of around 20 knots."


So 19 sounds a little slow. The 'trawler' logic escapes me. If someone wasn't interested in planing speed, why overprop the engine? Better (at least in my book) to prop the boat and engine appropriately to reach designed RPM at planing speed, and for 'trawler operation' just putt around...

Regarding the exhaust risers, some of the risers on the 496 come from Mercruiser in stainless steel, p/ns 865331A01 and A02 and 862354A2 come to mind. A lot of the problems seem to stem from lack of maintenance; before summarily replacing a 650.00 stainless riser, get someone who knows to ensure that it needs replacing, and not the gasket...

[Three Sisters]

Originally Posted by ipasgas1

We are looking at a 2005 Sea Ray 390 Motor Yacht with the 8.1 Mercruiser engines. During the sea trial, it only made 3900 rpm WOT and the starboard engine was backfiring. The broker had a mechanic look at the engines and he changed the sparkplugs and fuel filters as well as did a compression check which was normal. The subsequent sea trial produced 4100 rpm WOT. The book says rpm at WOT should be 4400-4800. The broker says it is propped tall as the previous owner used it as a trawler. Not sure what that means but does that sound reasonable? During the out of water, bottom paint looked fine, props looked good. Surveyor said port cutlass would need replaced soon but starboard was fine. Any thoughts? We are getting her for a great price as long as the engines are good (395 hours).
Thanks,
Dan

Hi.

My only thoughts are what prop size is indicated on the surveyor's report. And where is the paperwork for the prop change as this is a several thousand dollar changeover. Where the spare set of props would be. And if this prop change affected the engine warranty from Mercruiser if done when the boat was new because overproped changes loading on the engines through the rpm range. .... ?????? Good Luck

[Shrew]

I spent 8 years with a Mercruiser 8.1 (496). The Max RPM Range is 4400 -4800 rpms. The closer to 4800 the better. The broker indicating the props are 'tall' is commonly used in gearing. It is a PC way of saying "It's overpropped".

On a boat that large, you would be placing a tremendous amount of load on the engines if overpropped.

By overpropping, we are not talking about prop diameter. We are talking about the pitch. The Pitch is too high. It will take a decrease of 1" of pitch to increase rpm's by 200.

[Intrepid-FL]

Normal Full Throttle Range is 4200 to 4600 RPM per Mercruiser specs for the 8.1S Horizon Engine. The HO (High Output) Mag version has a WOT range of 4400 to 4800 RPM.