Elevation change

[Scout77]

Hi All,

I have a volvo penta 3.0 along with the SX-M drive 1-97 ratio. I bought the boat in Texas and was able to easily move around the lake with a full boat and towing any tube but I now live in Utah where most of the lakes are above 4000 ft. I've changed the stock prop to one that is a 14-1/2" and 17 pitch but I'm still having trouble with getting any power. I don't know if there are different gears I could put in or maybe new jets for the carb but I haven't had any luck finding either. Any suggestions would be great.

 

[rad1026]

Re: Elevation change

I had the same experience. Bought the boat in TX and live in Albuquerque. My local lake is at 3500 ft and I could not get the boat in an acceptable RPM range even going down to a 17 pitch prop. I wound up having to re-gear my outdrive. I bought a new Merc upper unit going from 1.5 to 1.64? I think. I sold my old upper unit on ebay and it worked out well. There really isn't anyone here locally that wanted to open the unit and change out the gearing.

 

[fat fanny]

Re: Elevation change

Look in the MERC service manual for cab rebuild and you will see the specific jet sizes for altitudes. Remember an engine is a big air pump and when the air changes which it does with altitude you will need to modify it to breath better in that thinner air!

 

[wired1236]

Re: Elevation change

Yep, IMHO chaning the jets is the right thing to do. It's pretty easy and they are a lot cheaper than regearing.

 

[Don S]

Re: Elevation change

Great answers for people with Mercruisers. But he has a Volvo.

 

[SeanMcl]

Re: Elevation change

Welcome to Utah, Scout77!

That said, changing jets, and fiddling with timing will help, but not much. It will put some of the "crisp" back in your throttle which is nice for skiing and tubing.

If you boat Willard Bay or Utah Lake on a nice 90 degree day, you are losing 23, or 17%, of your rated 135 horsepower because of altitude and temp. Go to Pineview and you lose 20%, or 27 HP. Head up to Rockport or Jordanelle or Flaming Gorge and you lose 23%, or 32 HP compared to sea level.

Nothing you can do to keep your top speed, except increase the total power of the boat. You can keep your boat feeling peppy and pulling skiers up quickly by propping way down and deciding you don't care about top speed. Call a local shop like SS Marine and get a prop recommendation. Don't re-gear, it's not worth it when you can just change props.

And here is a good link that shows the local lakes. It's a good place to find new places to try.

http://www.utahhikingandlakes.com/boating.html

 

[Redrig]

Re: Elevation change

not to hijack, but while on the subject. I live in UT too and my boat has a few different props. its a 1977 OMC stringer175 hp. I have 14,15 and a 17 pitch. whats the rule with elevation? I usually just go to Powell which is about 3650 elevation and I use my 17, should I be using another?

 

[SeanMcl]

Re: Elevation change

I have been told WOT for the OMC 190 and 175 is 4200, but I don't have an official OMC manual so I'm not positive. In any case, if you are turning about 4200 at WOT wherever you boat then you are in the ballpark. After that I'd say that if you are happy with how it runs at Powell, you are using the right prop.

Can anyone who has an OMC manual confirm the WOT RPM? The one available for DL here is the Seloc, and it doesn't list it.

 

[Scout77]

Re: Elevation change

Thank you all for the advice so far. So from what I understand, other than prop size my next best option would be to change jet size. Does anyone know what size is in my Holly carburetor and what would be some of the sizes to try, Also maybe a good website that would tell me how best to change them?

 

[gryhnd]

Re: Elevation change

I have been told WOT for the OMC 190 and 175 is 4200, but I don't have an official OMC manual so I'm not positive.

According to my owner's manual (1975), the 175, 190 and 235 WOT range is 3800-4200 RPM

 

[jtmarten]

Re: Elevation change

Horsepower loss:

hp loss = (0.03) x (elevation) x (sea level (rated) horsepower) / 1000

or

hp loss= (0.00003) x (elevation) x (sea level (rated) horsepower)

 

[jtmarten]

Re: Elevation change

Thank you all for the advice so far. So from what I understand, other than prop size my next best option would be to change jet size. Does anyone know what size is in my Holly carburetor and what would be some of the sizes to try, Also maybe a good website that would tell me how best to change them?

http://www.mortec.com/carbs.htm

 

[Redrig]

Re: Elevation change

duplicate

 

[Redrig]

Re: Elevation change

dang 3800-4200, I am doing something wrong then. I have always just got the boat planing and then used the lowest RPM to keep it planing, with my 14 pitch prop that is usually around 2700 RPM,

 

[SeanMcl]

Re: Elevation change

Get the List number that is stamped into the carb (usually on the choke horn), call Holley tech support and they will tell you what Jets and Power Valve you have. Tell them your elevation and they will also tell you what jet size to change to. You can buy jets direct from Holley less than $10 a pair, or you local autoparts store might have them, nothing special about them.

Here is an exploded diagram of a Holley 2300, Holley.com probably has one for your model if that's not the right one.

http://www.holley.com/data/TechService/Technical/2300 Exploded View.pdf

The jets are #39, they screw into the Metering Block, #40, with a flat head screwdriver. You just have to unhook your fuel line, take the fuel bowl off (4 screws), and you are looking at the jets.

Take care not to damage the gaskets. Depending on the age of the boat, you might want a complete renew kit for your carb, since you are 90% disasembled at this point anyway. The renew kit will have a new power valve, float needle and seat, gaskets, accelerator pump, etc. It's all dead easy to put together and the instructions are quite good. Spray some carb cleaner around and you just rebuilt your carb!

 

[SeanMcl]

Re: Elevation change

dang 3800-4200, I am doing something wrong then. I have always just got the boat planing and then used the lowest RPM to keep it planing, with my 14 pitch prop that is usually around 2700 RPM,

That 3800 - 4200 RPM is for Wide Open Throttle. Your cruise RPM is whatever you want it to be.

 

[SeanMcl]

Re: Elevation change

Horsepower loss:

hp loss = (0.03) x (elevation) x (sea level (rated) horsepower) / 1000

or

hp loss= (0.00003) x (elevation) x (sea level (rated) horsepower)

Cool formula. I just used this website http://www.anycalculator.com/enginehorsepower.htm

It also figures in relative humidity and air temp, but close is plenty good enough for these type of things.

And nice link to the Moretec site. Lots of good info.

 

[fishrdan]

Re: Elevation change

Jetting isn't all that hard, but is a pain. Rule of thumb is - 1 jet size for every +2000', so if up at 4000' reduce jet number by 2. IE: if stock jets are 100's, put in 98's. The number should be stamped on the head of the jet, so you can remove the jet and get a smaller size. Just remember, don't run your boat with lean jets at low altitude or bad things can happen. I would rejet the carb for the lowest elevation you boat at, (4000'?) and go down in prop pitch as needed.

I have a similar setup, 140HP Mercruiser in a 2200# boat, that I run from 1000' up to 9000'. I only swap jets if over 5000' since it's a hassle. If I'm going to be at several high elevation lakes over a couple days I choose the lowest elevation and jet for elevation, so I don't have to re-jet for each elevation. If I'm just going to 9000' I'll jet for that elevation, but know that I'm stuck with that lake/elevation unless I rejet. At 1000' I run a 19P prop (could get away with a 21 at times), a 17P for 5000', and a 15P for 9000'.

Your boat/engine is going to be different, but this might give you some food for thought.