What's a good rule of thumb for max RPM with an inboard?
 

Coming from my parents I/o, we never wanted to go over 3800/4000 rpm with the prop on it. They traded it in for a malibu 247 with a 6.2l 383 hammerhead indmar. How many rpms would you say to not generally go over?


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5200

I believe the Hammerhead is 5600 but bftskir may be correct. I know the rotating internals for that motor are solid to 6000 and the rev limiter will keep you from getting there. I have hit the rev limiter a few times driving a Monsoon

Yeah 52-55 is max. it may go higher, but that's really hard on the motor.

That's the max but I wouldn't consistently run that...I generally try not to go over 4000 rpms in my VLX

No stock inboard engine can or will spin 5600.

You can run to the redline on any good engine and those in inboards are better than many for being tough. You can run all day at 80% of the redline without any issues on any good engine.
What you want to do is listen to the engine. You can hear when you open up the last bit of throttle as the engine starts to work really hard. On most inboards you won't hear that change until you are up to about 90% of your redline, but you can hear it. The top of the power range is all about getting air into the cylinders. The parts that take the 'g' loads in an engine are built to handle a particular speed and past that they might fail. That, with a safety margin is what sets the redline. Usually a boat is propped so that you don't go over that as the boat is under constant load, unlike a car.
The last bit of throttle is where your fuel use goes up with little effect as you are trying to burn all the air it can get in the engine and you are past its peak efficiency. As a result it is working harder with less result. However it is the parts inside which determine the RPMs you can run and that is how they set the redline. When I was young we used to run my old Merc 470 up to 5000 regularly. Then I eventually found out that the redline for that engine was supposed to be 4200. Oops. Now it has well over 3000 hours on it and still goes well except it smokes a bit. That might be because it was underwater for a day many years ago. Now I only take it to 4200 when it occasionally goes out. It will die before I bother rebuilding it.
On my boat the redline is 5200 but I don't tend go above 4500 much. I'm never worried about running at the redline for 10 minutes however.

learn something new every day, art sounds like hes got some experience in this department. I've always treated my engine more like a trucks... every now and then i'll open her up but generally cruising stay around 3000 sometimes 3500. I figured i wouldn't want to drive a truck around at 4000 all day so i probably shouldn't do that to a boat motor thats based off a 5.7 liter truck block.( or so i thought) I have the indmar ltr which is same as malibu's monsoon 5.7l 350 making 340hp

Rev limiters take out all the guesswork as far as over revving is concerned. Just check the manual for the WOT . It's going to be something like 4800 to 5200 ...the trick is getting the right prop that will get to 5000 to 5200...that's not always too easily done and in most cases won't be necessary.

Huh???

Mine will rev up to 62-6300 and run there all day long.... Rev limiter is 6400

Stock inboard engine?

Ya..... I will admit though, you are right for almost all of them.

ilmore 7.4l is not a typical GM crate engine though.

From the PCM owners manual, maximum RPM for the 6.0L motor is 5600RPM, and yes, I have seen them get there. I know Malibu uses Indmar but any recent LSx based motor will easily turn 5600. It is best to keep cruising RPM below 4500. This applies to the higher end/upgrade motors available in the last 10 years.

That ilmor will spin 5800...according to them anyway.
And a 383 hammerhead is not exactly a GM crate engine.
Let's keep it real.

I've seen a hammerhead blow a hole through the side of the block at 5k.

Our boat has a stock Indmar LS1, spins to 5800 RPM.

Anyone know what the stock indmar 5.7 350 horse does??

I think 4400 to 4800....but check engine manual for the spec

5200 is what mine will get to. My owners manual states 4800- 5200 is max rpm at wot. I've only been there a couple times to verify a new prop would reach redline.

Yup just checked. 4800-5200

Hey BFTSKIR, Lots of things have changed since 1990, get with the times. These days the roller motors in our boats spin 5200-6000RPM, my 2005 330HP PCM Excalibur 2 bolt main spins 5200RPM with the stock team prop. I also drive Malibu V drives with Monsoon motors that hit the rev-limiter at 5200-5300rpms. The LS based motors spin 400-800rpms more when propped that way and will also hit the rev-limiter. Have you been in a boat newer than 2000? Barefooter with a direct drive Sanger... no surprises here.

Ian, don't worry about it. You aren't going to hurt that motor with anything you are going to do to it.

My 90 Sanger DX has a 2011 Mercruiser 383 Stroker with 350 horsepower. Boat does 50.

exactly! of a completely different vintage, gearing, propping and hull design from the boat of question in this thread

Guys I think part of this discussion is missing. Redline and operating range are two completely different things. The old indmar/PCM/etc motors based on the 351W had a standard operating range of 4200-4600 but an actual redline capability of about 5200. That means the motor is built to run consistently for an extended period of time in that rpm range. Part of the reason we reprop when adding ballast, is to move the engine back to that range. In the boat world torque is king, so while comparing these motors to a truck motor makes sense at first, most boat motors generate peak torque dead in that RPM window, rather than the significantly lower 2k-2500rpm window of a truck.

New LSx blocks have a significantly higher operating range in part due to being roller motors, but also due to internals and a few other factors. So here are a few guidelines:
1. Figure out your factory operating range. Being propped below this will foul the motor, and above it will put a good amount of unneeded stress on the motor(properly set up, a boat will fall right in that operating range with full ballast/load at wide open throttle/WOT)
2. Don't sweat bouncing off the limited once in a while, just don't do it every day. Your boat has a limiter set below the limits of the block specifically as a safe guard, but in boats, the limit works by cutting fuel, and that's not great for any motor.
3. The OP asked for a guideline, and in my opinion, that is 4500rpms. Almost every motor that comes from Mercury, Indmar, Crusader(older boats), PCM, Ilmor, etc as a stock WOT range of 4200rpm and higher. So 4500rpms will put you right in or below that in virtually every ski/wake boat.

Maximum horsepower is rated @5800 RPM. That is a lot different than max RPM. It pulls hard right to 6300. I found the rev limiter when testing different props. Limiter was at 6400.

And the Hammerhead is based with a GM performance crate engine. The 7.4l Ilmore is a fully hand built, custom engine that uses all custom spec'd, forged, internals. Everything is specialized from the crankshaft to the valve springs....

The answer to the OP is still correct at 5200.

jesus, you make it sound like 7.4 is the rarest engine ever, everything is specialized? ya so is a 383, change the crank, rods, and bore(forge them if you like) from a standard 350 and you get a 383. put in a bigger cam you should to change the springs. (7.4L) 454s have been around since the 60s! its in corvettes, trucks, etc,. there is nothing that special about it. they pretty much just copied it from GM.

I agree, though I believe the Ilmore is built off of the GM LSx Bowtie block, so a GEN-IV SBC instead of a GEN-I like the Hammerhead. But everything is off-the-shelf, as it would be stupid and not the least bit economical to engineer valve springs, pistons, rods, etc. when there is an abundant supply of aftermarket stuff to choose from - it would be stupidly reinventing the wheel. Take those parts, spec out a cam and there you go, a "custom built motor." Your local performance engine shop can do it too.

Nothing against Ilmor and they are doing FAR more than the other inboard marinizers by a long shot, but let's put things into perspective.

You are right, the ilmore 7.4 is not the gen I big block. It may displace the same cubic inches, but is nothing like that engine.

And I didn't say that they custom engineered and manufacture every single part in the engine. They absolutely pick and choose a lot of the parts to put together the package they wanted for this engine. Some of the major components (Pistons, camshaft, and valve springs) are custom made for it, and they are used in the MC version, and the offshore racing version. My point, was that, the 7.4l is not an "off the shelf GM crate engine, like all of the others, including the Hammerhead. ilmore put together a custom package, with specifically spec'd components, and hand assembles the whole engine in-house.

And of course a local engine shop can build you a "custom built motor". But they can't tune your ECU to run with it...... And that is all besides the point anyhow. This conversation was about the engines that come in the boats....

Rip it wide open till it blows up

Why exaggerate?
Even Ilmors high performance 7.4 only spins up 5500 to 6000 wot and 6100 limit.

Also worth noting...ilmor uses GM based engines to build their 5.7,6.0,6.2 ...GM crate engines.

???

I'm not exaggerating. That is exactly what it Revs..... If you are basing that on the info for the MV8 570, that is not the same engine. Very similar, but not the same.

And yes, I know the rest of the lineup are crate engines. I never said otherwise.

What boat and where do you run 6200 rpms all day long?...I know you wouldn't exaggerate anything.

Well heck, I guess I did exaggerate. I don't run "all day long"......

When I need to run from point a to point b, in a hurry, that's what RPM it runs at WOT. Sorry for trying to use a figure of speech to get a point across.....

We have my friends 2013 Sanger DXII running an indmar 409 HP L96 propped to wot at 5350. It will tow a barefooter 51 mph. We put 150 hours on it this year. Not too many people foot over 47 mph anyway.

For 99.9% of the boats on WW max wot will be less than or equal to 5200.

Do you enjoy making completely inaccurate blanket statements?

Most people on WW wakeboard and surf. Many of those folks run a lot of ballast (especially WW members) and because of that, have to prop down to get hole shot they want or to even be able to get on plane at all. As a result, the motor runs at a higher RPM and they tend to hit the rev limiter very easily, so WOT will be where the rev limiter is set (and usually hit the rev limiter before WOT).

Really? So you're saying running at 20 or 21 mph you are hitting the rev limiter? Spinning 5000+rs...or just to get out of the hole...momentarily...every time you start out you are hitting the rev limiter?

This is exactly why I can make the blanket statement.