High altitude - carb vs. fuel injection

[crqflier]

Posted this in the general discussion forum, but thought there may be more insight in the engine tech forum. Planning to buy my first boat, 19-22 foot bowrider. Engines under consideration are 4.3l carb 190 hp, 4.3l FI 220 hp, and 5.0l carb 225 hp. The lake elevation is a bit over 7000 feet.

My question is, how big of a consideration is carb vs. fuel injection? Is it best to look at FI instead of carb? Will I need to have the carb tuned for altitude? If so, I'm guessing going back to sea level would also require some type of tune? Should I not worry about it? There are tons more carb boats available used than FI.

Thanks.

 

[bruceb58]

Re: High altitude - carb vs. fuel injection

I wouldn't worry about it. Just make sure you look for a larger motor than you normally would for a boat used at sea level. I have a 24' boat with a carbed 7.4L. I have a house at Tahoe(6300 ft) and it runs fine although it has a lot less power than at sea level. The 7.4L makes up for it though.

If the only place you boat is the 7000 feet, you can get the carb rejetted but you would have to change the jets back if you went down in elevation or you will harm your engine. It would be too lean.

 

[Aloysius]

Re: High altitude - carb vs. fuel injection

The nice thing about a FI engine with a MAP (manifold absolute pressure) sensor is that it automatically compensates for altitude.

 

[fishrdan]

Re: High altitude - carb vs. fuel injection

For high altitude,,, there is no replacement for displacement, go with the bigger engine. You will be down on power at least 25% at 7000' so that 190HP V6 will run like 135HP and the 220HP V8 will run like 165HP. The down side to the carbed engine is it will need carb tuning, both boats will most likely need prop changes made (unless they are running at that elevation right now).

The only reason I might opt for the FI V6 is if you are going up>down>up in elevation often. The carbed engine will need work every time you make major elevation changes. I run from 1000' to 9000' and elevation is a killer on boat performance, royal PITA making all the engine changes too...

 

[zbnutcase]

Re: High altitude - carb vs. fuel injection

Fuel injection is the way to go...

 

[HT32BSX115]

Re: High altitude - carb vs. fuel injection

If I had a choice, I would always choose fuel injection.

You will be getting a newer boat. Eventually they'll all be EFI.

Ditto on the MORE cubes.......

a V-6 is going to be somewhat anemic at 7000' MSL. PLUS you may have to put a lower pitch prop on the boat too.....(consider a 4 blade prop)


regards,


Rick

 

[Aloysius]

Re: High altitude - carb vs. fuel injection

Just like aircraft engines, power drops substantially with altitude. While EFI compensates for the altitude change, it doesn't provide the same power at all altitudes, just optimizes the engine automatically for the altitude/temperature variations.

 

[crqflier]

Re: High altitude - carb vs. fuel injection

Thanks for all the comments. Especially like the flying references . There's definitely a huge performance hit flying into the local airport.

Hadn't really considered going with the 5.7L. Though it would be overkill. Haven't seen too many on the market until you get to 22'. Might be the way to go.

The speed limit on the lake is 35 mph - and practically, more than that is dodgy because there's generally a ~15 knot wind generated chop. So I'm not too worried about top end. More concerned about getting out of the hole and getting skier's up, etc.

 

[crqflier]

Re: High altitude - carb vs. fuel injection

One more quick related question. Is there a signifcantly different torque curve (specifically low end) for the 4.3l 220hp MPI vs. the 5.0l 225hp carb?

I would guess the v8 has a better/higher low end torque profile, but i've looked around this site and the mercury marine site and haven't been able to find any HP/TQ/RPM graphs (or even stated max torque @ rmp specs).

With the discussion of performance loss, i'm starting to put more emphasis on power/weight ratio. There's a nice four winns 190 with the volvo 5.0GL - dry weight spec is under 2800 lbs. The best I've seen on boats in my price range.

 

[extreme76]

Re: High altitude - carb vs. fuel injection

I would guess the v8 has a better/higher low end torque profile, but i've looked around this site and the mercury marine site and haven't been able to find any HP/TQ/RPM graphs (or even stated max torque @ rmp specs).

Some info here on the fuel injected versions.
http://www.gm.com/vehicles/innovati...engines/specialized/marine/marine_engines.jsp

 

[HT32BSX115]

Re: High altitude - carb vs. fuel injection

Just like aircraft engines, power drops substantially with altitude.

That's true.... A fuel injected Lycoming N/A 200 Hp engine will only develop about 150hp (75%) at 5000 MSL at full throttle.

At 7000' it would be substantially less. (and that's with the mixture manually adjusted for best power using fuel flow and exhaust gas temp)

So the 190 HP carbed engine (running a little too rich) at 7000ft would be pretty wimpy.....

I would go for an EFI V-8

 

[Aloysius]

Re: High altitude - carb vs. fuel injection

Frankly, the only reason GM makes/sells a 5.0 is for marketing. The ONLY significant difference between a 5.0 and 5.7 is the cylinder bore. Same rods, same stroke crank.

So, if you're considering a 5.0, a 5.7 weighs exactly the same, and is physically identical.

Often, if you go to GM industrial engine site, you can see torque/hp curves for your particular engine.

Maybe a "normalizer" on the engine to maintain power at altitude??

 

[crqflier]

Re: High altitude - carb vs. fuel injection

It funny. When I first started looking for a boat a month or so ago, I was considering a 17' with 3.0l. Now I'm at 19+ feet and the 5.0 vs. 5.7 argument is making sense - that's almost double the size engine and HP than when I started! Thanks for all the great input.

 

[HT32BSX115]

Re: High altitude - carb vs. fuel injection

If you were going to stay at sea level or so, the 3.0L engine would probably be adequate.

7000' is another story....

 

[Gromulin]

Re: High altitude - carb vs. fuel injection

For high altitude,,, there is no replacement for displacement, go with the bigger engine. You will be down on power at least 25% at 7000'

Interesting. I understand the concept, but did not think there was that much % lost at altitude. I boat between 0' (Sac River), 466' (Folsom Lake), 4,000' (Union Valley) and up to 6300' (Tahoe...eventually). I have a 5.7 GSi...and am now really glad I have injected.

And like the man said...no replacement for displacement.

Ask yourself this question: Have you EVER thought "Gee, I wish this vehicle had less power" ?

Are there tables / equations to spitball HP loss at differing altitudes?

 

[HT32BSX115]

Re: High altitude - carb vs. fuel injection

Be glad you're "Duo-Propped" too


Last year I did a little running at Tahoe in my 87 FourWinns 211 Liberator.

There was Me, My brother, my wife, his wife, my mother, her mother, my 2 daughters, my brother's daughter and 3 100lb dogs.

9 people, 3 (big)dogs and the boat STILL planed!!! (went about 40-45 GPS MPH too @WOT!!)

I attribute that to the Bravo III because it was a real DOG with the previous 460 King Cobra. (I don't think it would have planed at all with the previous engine/drive or if I had an SBC, )


A good approximation is the charts you can find online for aircraft engines. (Continental/Lycoming) All the HP/ALT charts are for best power fuel mixture at various altitudes.

 

[crqflier]

Re: High altitude - carb vs. fuel injection

Are there tables / equations to spitball HP loss at differing altitudes?

Airplane ops manuals all have detailed charts for HP at given altitude and temperature (another critical factor - impacts density). One example: 2400 rpm at 4k feet = 78% max bhp, at 8k feet drops to 65% max bhp - this is on a normally aspirated, fuel injected, 6 cyl.

Suppose if boats had mixture control (don't believe they do, do they?), then carb vs. FI would be a simple twist of the mixture.

Guess i'm starting to answer my own original question...

 

[HT32BSX115]

Re: High altitude - carb vs. fuel injection

Suppose if boats had mixture control (don't believe they do, do they?), then carb vs. FI would be a simple twist of the mixture.

Only the EFI engines .....

You could do it with a carbed engine but you'd have to readjust it every time you change the throttle setting. You'd end up running over someone twiddling with it!!!

 

[Gromulin]

Re: High altitude - carb vs. fuel injection

A good approximation is the charts you can find online for aircraft engines. (Continental/Lycoming) All the HP/ALT charts are for best power fuel mixture at various altitudes.

Thanks. Now I have something to look up during my conference calls today