Mercury 90 4stroke on tempest prop upgrade

[Ricelake]

Hey guys I've just bought a 2015 tempest 175 with a mercury 90 four stroke. The boat weighs 1010 lbs. It's got a 13x19 prop and on the gauges I'm getting 49/50mph at 5200 RPM.

I've read the motors are capable of 6000 rpm. I'm happy with the low end and hell I'm impressed with the top end. But who doesn't want more speed haha.

I'm new to props. I'm thinking I'd like to have a prop to hang on the wall thats for top speed. I realize I'll loose some bottom end but I'll keep the 13x19 for that.

Any suggestions? I'm not sure if I need a 14x17 or a 13x21 or a 4 blade or if I even know what I'm talking about. Haha

Thanks in advance!

 

[QBhoy]

Hi

first off I think you need to get accurate speed and rpm numbers. I’d suspect you might be on or over the verge of impossible numbers there ?

 

[jimmbo]

Well a 4 Blade is the way to go, if you want a lower top speed. What is your Engine's WOT range? "I've read the motors are capable of 6000 rpm" doesn't cut it. Some 90s in that era, have an rpm range of 5000 - 5500, others 5000 - 6000. You need to know what it is

Now if the engine is running at a RPM lower than the in Upper Half of the WOT range, you will need to decrease Pitch.

49/50 mph on a 17 ft with a 90 is actually pretty good, and to go faster will likely take a bigger engine

 

[achris]

Hello Ricelake and :welcome: aboard.

As engines may not be the same year as the boat they are bolted to (have a look at my boat and engine to see a great example of how and why that happens ) we ask that you post the serial number of the engine so we all know exactly what you have. I have just been through this with another post, the engine is actually 4 years older than the boat... so, the year of the boat only tells us the year of the boat, nothing about the engine.

Props: Complete can of worms. different props work differently on different hulls. What works on one boat may not work so well on a different boat, even of the same brand!

In general...
More blades are LESS efficient than less blades. So a 4 blade will be smoother, but for the same top end RPM give you less speed.

Prop pitch is measured in inches, and usually the sizes vary at 2" intervals. Each 2" change in pitch will change the WOT revs by about 400. So, a 19" topping out at 5200rpm will have a 17" prop at about 5600rpm.

Try to prop towards the upper end of the recommended rpm range. So with your 90, the rev range (based on a quick look-up, not your serial number) is 5000-6000. Personally, I'd change to the 17" (don't worry about the diameter, the prop engineers have done the math on that so you get the best option. Usually you don't get to spec diameter and pitch, just the pitch).

The other thing, accurate readings (tacho and speed). Dash tachos aren't nearly accurate enough to use for checking props. Use a digital tacho that reads directly from the engine. Same for the speedo, use a GPS, and give it time to 'settle'. GPS only update every second or so, so changes to speed and direction take time to be displayed. If you don't have a GPS on the boat, download a free GPS app to your phone and use that.... (I use GPSTest)

Chris.....

 

[WesNewell]

The 75, 90, and 115 hp 4s mercs are all based on the same 2.1L block. The 75 rpm range is limited by the ecm and an air restricter in the intake manifold. The 90 is not rpm limited but does have an air restricter in the intake manifold and iirc does have a different ecm than the 115. The hp of all these engines can be increased by removing the restricter, and by more by having the ecm reprogrammed. Removing the air restricter on your 90 should increase your hp enough to go to a 21P prop and possibly gain a little rpm in the process. If you want to get up to ~120 hp on that engine, you can also have the ecm reprogrammed at http://www.simonmotorsports.com , but that will cost you. Removing the air restricter alone cost nothing if you diy and should increase power considerably, especially if you are at altitude.Contact Simon Motor sports to verify this. I'm not sure about the 90 hp, but positive about the 75.

 

[ahicks]

My take-
The 19" prop turning 5200 will make a good all around prop - unless you are pulling somebody. If the priority is there (pulling), you can increase the pulling performance compared to what you have now (decrease the time it takes to plane the boat out) and maybe pick up a mile an hour or 2, by going to a 17" pitch prop. The down side of the 17p is that these big 4 cylinder engines start sounding pretty busy when they get up into the high 5000's.

Bottom line, the 19p will make the better cruise prop, while the 17p will be the better performance/ski prop. If you're looking for a prop to hang on the wall, try a 17p.

 

[QBhoy]

The 75, 90, and 115 hp 4s mercs are all based on the same 2.1L block. The 75 rpm range is limited by the ecm and an air restricter in the intake manifold. The 90 is not rpm limited but does have an air restricter in the intake manifold and iirc does have a different ecm than the 115. The hp of all these engines can be increased by removing the restricter, and by more by having the ecm reprogrammed. Removing the air restricter on your 90 should increase your hp enough to go to a 21P prop and possibly gain a little rpm in the process. If you want to get up to ~120 hp on that engine, you can also have the ecm reprogrammed at http://www.simonmotorsports.com , but that will cost you. Removing the air restricter alone cost nothing if you diy and should increase power considerably, especially if you are at altitude.Contact Simon Motor sports to verify this. I'm not sure about the 90 hp, but positive about the 75.

Read this with great interest Wes. Great info. So I have a 115 CT 2.1l engine. It’s an incredible thing for power and I’m certain is putting out more than the claimed figure on the badge. So does mine have a removable restrictor too ? I know it limits at 6150ish rpm for sure. I also know that there is a pro Xs version of mine that has a slightly higher rpm limit. Is this the only difference between mine and the pro Xs ? Can I remove some sort of restriction on mine ? I know run a larger pitch prop which will see a little over 6000 rpm. Can’t complain as it is now at all. Manages a staggering 44 mph gps on a 19ft cuddy. But more power is always welcome.
thanks in advance.
Apologies for hijack !

edit*** just read that the 115 has no restriction and if going from a 75/90hp to get 115/120...it needs ecu flashed as mentioned but maybe the cam and heads are different too ? Not sure. I’d also like to know if my 115CT could be turned into a pro Xs easily.

 

[WesNewell]

I don't know the answer to that. If you check with Simon Motor sports I'm sure they could answer your questions. I'd assume you could get a little more power out of it with a custom flashed ecm. How much more? I don't know. But I don't think the cost would be worth it. IIRC it cost ~$600 for the ecm flash.

 

[Ricelake]

Hi

first off I think you need to get accurate speed and rpm numbers. I’d suspect you might be on or over the verge of impossible numbers there ?

Yeah that's what I thought too lol. My other boat is a checkmate with a 150 tower of power so I've felt true 50 before and it felt close. I'm going to gps it today. Stay tuned. Thanks

 

[Ricelake]

Well a 4 Blade is the way to go, if you want a lower top speed. What is your Engine's WOT range? "I've read the motors are capable of 6000 rpm" doesn't cut it. Some 90s in that era, have an rpm range of 5000 - 5500, others 5000 - 6000. You need to know what it is

Now if the engine is running at a RPM lower than the in Upper Half of the WOT range, you will need to decrease Pitch.

49/50 mph on a 17 ft with a 90 is actually pretty good, and to go faster will likely take a bigger engine

Hey man in my manual the specs for WOT rpm are 5000-6000. So I'll have to drop a pitch to get to the upper half. Thanks

 

[Ricelake]

Hey guys thanks very much for all the replies I was going to reply to each but there is too many and its may two four. I'll update you guys with more information when I've got more time. Thanks again fellas enjoy the water!!!