Max RPM WOT 1989 Merc 150 xr4

[Turkey]

I've been looking for max rpm on this 150 xr4 but haven't had much luck in finding it. I know I need to switch props from the one it came with because the rpm's will get really high ~7000. I don't know what prop it currently has on it other than it is 3 bladed stainless.

 

[j_martin]

Re: Max RPM WOT 1989 Merc 150 xr4

Mine came with a little "advance module" that made it back off real fast at 5500 rpm or so. I threw it out.

I run mine at 5800 rpm turning a 24 in high five on a 18' fiberglass tracker bass boat. It's pretty snappy at that.

I think book WOT rpm is 5000 to 5500. Most folks I talk to that know these motors say they would rather spin high than low.


hope it helps
John

 

[Turkey]

Re: Max RPM WOT 1989 Merc 150 xr4

Well at around 7000-7100 is where it stops we hit around 60-65 mph with 2 people, in a 17' bassboat. The prop on the motor needs to be refurbished or replaced anyway so might as well take the rpms down at the same time.

The motor runs great and still pulls great that high; but, from reading here I see that melted pistons are in my future if I don't slow the thing down a bit. Is it possible to increase the speed while lowering the rpms with just a prop change? Would it be better to switch to a 4 bladed prop instead of 3? Or simply switch to a higher pitch?

I'm quite new at this and need to get some advice. I've read the prop sticky but that really doesn't help me as much as getting custom answers. So far this site has been great and I truly appreciate it. I'm sure I'll be here often. Hopefully not to oten though

 

[j_martin]

Re: Max RPM WOT 1989 Merc 150 xr4

I have a feeling that you own a fairly highly modded motor. It probably wants to run that high, but I sure wouldn't run any kind of a damaged prop like that. The XR4 lower unit is geared kinda high, like 1.78:1, and it's light to boot, can't take too much of a beating. I would run Merc High-Perf lube in it.

If it is modded, and someone who doesn't understand that tries to correct it, like put a steep prop on it without backing off the timing for instance, you could grenade it in a hurry.

I'd check in over at screamandfly.com and see what the guys there think.

hope it helps
John

 

[Turkey]

Re: Max RPM WOT 1989 Merc 150 xr4

I posted this in the prop section and got a lil more info...the prop I have is a 21p ballistic. I'm not sure about a moded motor...under tha cowl looks like stock paint and wiring.

I know this motor has a bad rep(over spin = burnned pistons) but mine is impressive if I have low compression on a single cylinder I would be amazed... I know a mechanic and he is allready kind of thinking of making a pro stock drag boat out of it. I know the dealer reworked the carbs and it is running a lil rich...but I won't cry over a rich mixture. I am more worried about the prop...I took it off an had it put on a gauge today and it is only a 21"p and this motor will cavitate it. It has a hell of a hole shot..

as I learn more about it I'll keep you posted...I love speed but I wanna go fishin, and that's what this boat is for. But as anyone knows, speed = good!!!

 

[Turkey]

Re: Max RPM WOT 1989 Merc 150 xr4

I have a feeling that you own a fairly highly modded motor. It probably wants to run that high, but I sure wouldn't run any kind of a damaged prop like that. The XR4 lower unit is geared kinda high, like 1.78:1, and it's light to boot, can't take too much of a beating. I would run Merc High-Perf lube in it.

If it is modded, and someone who doesn't understand that tries to correct it, like put a steep prop on it without backing off the timing for instance, you could grenade it in a hurry.

I'd check in over at screamandfly.com and see what the guys there think.

hope it helps
John

I appreciate this post, but it looks like stock paint under ther cowl...the things runs like crazy though...I'm thinking about changing props to a 23 or 25 and testing it. If I get more speed and and a slower hole shot I think that will tell me the tach is right or wrong. I woldn't mind anything up to 100mph as long as it doesn't hurt the motor.

 

[Texasmark]

Re: Max RPM WOT 1989 Merc 150 xr4

The book says that you can get two lower units with that enigine: 1.78 or 1.87. The latter is the 4 1/2 bullet dia (gearbox housing) and is what's usually used in that hp range. You can also get the former with the 4 1/4 bullet which is what is used with the midrange engines, like my 90. The smaller gearbox with lower ratio provides improved performance for light fast boats.

Measure the outer diameter of the gearbox housing right where the prop mounts and you will know your gear ratio.

The book says RPM range is 5000-5600. I have run Mercs (different engine types) at 6000 for years and never had a problem.

BAM is a high performance site on the www and has a prop calculator. You might stroll over there and plug in some numbers for your desired new prop pitch to get you down to a more respectable rpm.

You probably will be surprised because with what I am hearing from you, and know what happened to me, as you increase pitch and load the engine more, putting it back into or nearer to it's recommended operating range, it will get quieter and produce more usable power, hence your speed will increase disproportionaly faster.

Don't overlook Ballistics while you are shopping props; they are a division of Michigan Wheel (MWC). I bought mine (have 3 now) on here.

HTH,

Mark

 

[j_martin]

Re: Max RPM WOT 1989 Merc 150 xr4

The smaller lower unit on that motor is a 4 1/4 inch, 1.78:1 gear ratio. It's hard to find a steep enough prop because very few V6's had that lower. If you're looking for a prop, you have to go to the V4 charts for them.

I'm running a 24" high five on a relatively heavy bass boat, turning 5800 rpm or better.

That motor, a year earlier, was the 175 (at the powerhead). Right out of the crate its about 162. Shave the heads a bit, and you can gain 10 easily. Throw away or break the advance module, and get a little more.

That lower unit is quick, because it's small. If you take good care of it, and run only high performance lube in it, and don't try to run it surfacing the prop, it'll last a long time. Neglect it and it'll put a pinion out the side in a heartbeat.

If your compression is over 125, it's been shaved. (I bet 145) I would run 92 octane gas in it.

That engine's a jewel, enjoy it.

John

ps here's mine pushing the bass bote, full gas tank, a little water in the swimmin' pools, tackle on board, seats removed for testing and service.

 

[j_martin]

Re: Max RPM WOT 1989 Merc 150 xr4

I know this motor has a bad rep(over spin = burnned pistons) but mine is impressive if I have low compression on a single cylinder I would be amazed... I know a mechanic and he is allready kind of thinking of making a pro stock drag boat out of it. I know the dealer reworked the carbs and it is running a lil rich...but I won't cry over a rich mixture. I am more worried about the prop...I took it off an had it put on a gauge today and it is only a 21"p and this motor will cavitate it. It has a hell of a hole shot..

These motors like to spin, but they require a ton of fuel to do it. If they lean out at WOT, they'll grenade in a few seconds.

Be sure your fuel is good, and free flowing. I put in a racor filter/water seperator, threw away the bayonet connector and went directly to the oil mixer with the fuel line, threw away the mercury thumb sized fuel filter in the engine. I improved the gas tank venting with a racor vent guard. I rebuild the diaphragm fuel pump once a year. It's fat and happy.

The prop calculator shows you're slipping a little at 65, about 15%. I run a 24" high five and slip about 10%, so I go that fast with a heavier boat at 5800 rpm. I have a 22 I think I'll try. It was on the boat before i chit canned the advance module, and let it spin over 6000, but went flat rapidly. With the extreme torque and smaller prop diameter, I'd seriously consider a 4-5 blade screw for it.

hope it helps
John

 

[Turkey]

Re: Max RPM WOT 1989 Merc 150 xr4

Well I still don't have the prop on it and I measured it and it is kind of hard to tell just eyeballing it, the output shaft being in the way and all, the number I was seeing was 4 3/8ths od...but not really sure if that means anything. It is really hard to eyeball a 1/4 difference when you can't get the tape flush to the surface...so good info but I can't use that...grrr.

The guy at the prop place said it was the smaller one though...I guess he would know better than me and I'll go with that. He was nice enough to say that he would sell me a prop but allow me to test it to see if we get the desired result, and if it didn't drop the rpm I could bring it back. So I guess that's what I will be doing. I'll probably switch to an aluminum prop right now for cost concerns. Think I should just go straight to a 25p?

Yeah the friend that works on boats said I should go to a 4 blade and about the same pitch as you guys. He doesn't work on boats now and has a rep for story telling but so far he seems to be spot on with everything, so I guess I'll trust him. I'll allways double and triple check though...last thing I need it to pop the cork on this thing...it is too much fun...haha

I just wanted to say thanks to everyone, you guys have been super helpful.

 

[j_martin]

Re: Max RPM WOT 1989 Merc 150 xr4

It's the 4 1/4 inch 1.78:1 gear ratio XR4 LU.

John

 

[Turkey]

Re: Max RPM WOT 1989 Merc 150 xr4

The mechanic friend said I should switch to a 4 bladed 24p prop on this boat, we measured it from the back of the transom to the bow and it comes up 16'6" So it is a lil smaller than I thought. He also thinks that this motor is way to big for this particular hull length, but it has a good solid transom and thinks it is more than ok to handle it. I'm still worried about taking it out untill I get a new prop though. I'm a speed junkie and will take it as fast as I can and that's what brought me here. The engine sounded like it was really wound up...kinda like the old 2 stroke motor cycles if they had exhaust pipes under water basically. He would like to put a Raker 4 bladed racing prop and jackplate on it, or at least pull the motor out of the water a lil more. Pad to shaft centerline with the motor all the way down is 5+ inches. It has a whale tail on it and I'm hapy with it. It is a fishing boat afterall...

 

[j_martin]

Re: Max RPM WOT 1989 Merc 150 xr4

I run a 24 pitch high five (5 blade) on a relatively heavy 18' bass boat. I think 24 would be minimum for that, maybe even higher.

When it comes to raising the motor, it should help handling at high speeds. Watch out for lost water pressure, and resist the temptation to run the prop piercing the surface with this lower unit. It won't take the beating very long.
As I said before, I run a high five comfortably below the surface, about an inch or 2 below the pad 6 inches back on a jack plate.


It does sound like you might be overpowered on that boat, especially if you get it propped right. Be careful.

hope it helps
John

 

[Claurvick]

Re: Max RPM WOT 1989 Merc 150 xr4

I run a 24 pitch high five (5 blade) on a relatively heavy 18' bass boat. I think 24 would be minimum for that, maybe even higher.

When it comes to raising the motor, it should help handling at high speeds. Watch out for lost water pressure, and resist the temptation to run the prop piercing the surface with this lower unit. It won't take the beating very long.
As I said before, I run a high five comfortably below the surface, about an inch or 2 below the pad 6 inches back on a jack plate.


It does sound like you might be overpowered on that boat, especially if you get it propped right. Be careful.

hope it helps
John

Hi John was looking on here to try and find the right prop for my 89 tracker with a 150 mariner. And I finally found someone that has the same boat. What size in diameter was your high five? 13? Or 12 3/4

 

[jbcurt00]

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