Altitude vs Carb settings??

[Craig]

I keep thinking it would be cool to check out the Chesapeake Bay around the 9th or 10th of Aug. Had figured I couldn't and still not sure BUT just in case, I was wondering about the effect of running the boat there (Holley 600 cfm 4 bbl) given the different altitude. Maybe not a big deal, but where I normally go boating is about 2600 to 2900 feet. Is that a significant difference to run in the bay w/o a carb adjustment? Just wondering. Hate to come all that way and then cough and sputter all day. I'm definitely no carb technician. It runs good up here!

[HyperDonzi]

is that on erie? if so we will be there this saturday through next saturday.

[MOP]

I got this off a manufacturers site, if I read it right you may run a little lean. It did say prolonged use with a 1500 meter 4900ft) change in altitude would harm the engine. We need a trailering Pro here for this one!

5.2 Changes in atmospheric pressure and in air temperature

Variations in pressure or temperature cause a change in the air density and consequently a change in the fuel-air ratio and further tuning may therefore become necessary.

A decrease in atmospheric pressure with consequent decrease in air density causes a mixture enrichment and smaller jets will therefore be required.

Altitude variations also produce changes in the carburation and they too cause changes in the air density; prolonged use of a vehicle at an altitude higher than 1500 metres, the carburation of which was originally set up for operation at around sea level, would require a change of jet sizes in proportion to the pressure change.

In this case too, a decrease in pressure should be compensated by a reduction of the jet sizes. Furthermore, a lowering of air temperature produces an increase in air density and consequently a mixture weakening; therefore an increase in the jet sizes is required.

Summarising, we can say that any decrease in air pressure, any increase in altitude or in air temperature should be compensated for by a decrease in the jet sizes.

Conversely, any increase in pressure or any decrease in altitude or in temperature should be compensated by an increase in the jet sizes.

[Formula Jr]

You will be fine. If ya care, run 97 Oct. 3k isn't enough to make that much diff, and do you know the carb was tuned for 2900 feet? Denver, Tahao, Yes, but not Deep Creek. :D

[David Ochs]

Craig, My boat grew up on a lake with an elevation of about 5500 feet. I now operate it on lakes that are closer to sea level. The only change is with the prop. At sea level the motor takes a bigger wheel, but no changes made to the carb. No smoke, no sputter. You will notice a difference in performance(good thing), but with only a couple of thousand feet of elevation change, I'll bet you'll have a good couple of days. BTW, I live in CA, but used to cruise the Chesapeake a lot when younger. I think the Bay is probably some of the best boating in the USA.

[tailwind]

Hey guys,

Here is a good topic for my fellow aviation / donzi buddies. (Digger & Dalema) please chime in here! Air density and altitude are indeed related and need be considered for high altitude considerations accordingly. At Sea Level applications, our engines do require a certain ratio of fuel/air mixture. As we increase the altitude, the fuel/air mixture is changed accordingly. Increase in altitude, less density , less fuel simplified, hence leaning the mixture.

For an immediate example: Recent project (1976) on top of Mauna Kea, HI at 13,750 ft msl our piston driven equip. including cranes etc. had to be derated by a factor of 40% !!! Sucking thin air at this elevation. Would love to hear a more definitive description of the effects of altitude versus our piston driven engines. here is another one, why do fuel injected engines seem to operate better at altitude versus carbed engines?

Best regards,
Randy

[Craig]

Thanks for all the feedback guys. All interseting, but I think I like Dave's because if I take it, I don't have to change jets. Good reasoning, eh? Sounds like it is a concern but maybe not with the altitude difference between W. MD and the Ches. bay. When I was a kid, my Dad took our family on a trip out west in a '73 Ford Country Squire wagon (yuk green with wood siding -- Ooooooo!!!) . Anyway we went up Pike's Peak and he had 'er to the floor and she'd barely run. Hey now that I'm thinking about it, that old wagon may have had a 351W (same as my boat... cool!). Anyway, what's Pike's Peak, 10,000' ??? I remember he said he couldn't see the road on the turns over the big ole wagon's hood and every now and then she'd lunge ahead. Hey we made it home.
I'm running a 23" prop and RPM at WOT around 4700-4800. Think I'll just try the bay with the boat as it is this time (if I can make it -- according to Dan," time is running out to sign up for the event!"). Thanks again.

[Craig]

Oops forgot,
Hyper Baja,
No, I'm a long way from Erie. I'm in western MD, but it climbs pretty good at the far western end. I think Cumberland (where I work) is like Elev. 600 (+-) and 10 to 15 min. west (just west of Frostburg) is like Elev. 2900 (thats a high point).
Craig

[ToonaFish]

Do you think the residents of Pikes Peak tell "boat ramp" stories on the idiot tourists?

Early seventies, my family rented a passenger van in Florida for our trip out West. Not only did it stall on Pikes Peak, my father tried to do a three point turnaround on one of the switchbacks, becoming hopelessly stuck. While we blocked all traffic in both directions, my uncle went into diabetic shock. (Surely all that "only available west of the Mississippi" Coors had nothing to do with his condition.) The ambulance and the tow truck had a heck of a time reaching us due to the traffic jam we had caused. We took the tour bus up the the top the next day. And left the men at the hotel.

Bunches,

Celene 'scarred from childhood memories like these'