Alltitude jetting differences?

[C.Fisher]

Hello to all!
To start we live in columbus ohio and we just purchased a 2001 30ft checkmate with twin 454 blower
engines, holley powerchargers w/ single dbl pump carbs on them. My question is that we drove out to colorado to get the boat and the person we bought it from said that we would need to re-jet the boat to our sea level. Denver is like 5280 above and we are
around 700ft above. A site said that for every 2000
ft above to go with one jet size smaller. So with that said there is 87's in front and 88's in back
and the difference being 4540 I would install 89's front and 90's in the back. So smaller jets for higher alltitude and more jet for the lower alltitude.
Does this method ring true? Thanks in advance!

[Floored]

that method will get you close but with the blowers a hair rich to start is far better than lean. a lean backfire can be bad to very bad. but yes larger jets at lower altitude. high outside air temps will also richen the mixture as well as high humidity. try the blower section and maybe someone in you area has a similar combo. maybe Infomaniac will respond, oklahoma weather is closer to yours than Az.

[Infomaniac]

There will probably not be a big difference due to altitude since they are on a blower.
The blower makes a very good vacuum signal to the carbs. The carbs meter fuel due to the velocity of the air and not the density of the air.
Altitude will naturally have an effect on density and not velocity. The blower makes up for the lost air density.
To answer your question - I would say jet it up the 2 additional numbers and give it a try.
Put fresh plugs in it and get a good reading on them with minimum idle time.
Keep a close watch for detonation or the power falling off while deep into the throttle.
[ August 20, 2003, 10:39 AM: Message edited by: Infomaniac ]

[C.Fisher]

Thanks guys!

[Dennis Moore]

The main thing you will need, at a higher altitude, is a different prop. No matter what you do with the jets the engine will be down on power and your prop pitch will be wrong. You could detonate the engine with a prop with too much pitch. To avoid buying a new prop in Denver you better bring along one with a lower pitch.
Dennis Moore