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Ignition Timing & Altitude

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Jonny Quest View Drop Down
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    Posted: August-30-2019 at 4:24pm
We've all heard various "rules of thumb" when it comes to tweaking engine performance. I live and ski between 4,400 and 6,500 feet of elevation. So far, my only altitude tweak is on my QF M-650 carby where the primary and secondary jets are leaned-out 2 sizes. Now I'm considering a tweak to the ignition timing on my DUI dizzy set-up.

I've heard one old rule-of-thumb for timing and elevation that says "1/2 degre timing advance for every 1,000 feet of elevation". So, my Pro Boss (PLP) engine specs show ignition timing at idle of 10 degrees BTDC. PCM manual shows 8.3:1 compression ratio.

I run 89 octane fuel. What do you gents think of increasing ignition timing to 12 degrees at idle? Maybe a bit more?

JQ
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KENO View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-30-2019 at 7:21pm
It's pretty easy to find thumb rules that say 1 degree per 1000 ft also.

If you go to 12 you should have 36 total   14 would be 38 total assuming the DUI has 24 degrees of mechanical advance as it was sent to you

I guess it all depends on how close to the ragged edge you want to be at higher rpm's

There must be some reason why you want to jack it up

I say ............go for it 2 degrees at a time, remembering that too much initial makes for a looong slooow crank that'll make you think your battery is dead.

12 initial 36 total should be no problem at all especially since you need less octane at higher altitudes. Lots of people run numbers like that at sea level with 89 octane

Do some reading on detonation symptoms first

Don't they sell 85 octane moose whiz out there in the mountains?
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Jonny Quest View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonny Quest Quote  Post ReplyReply Direct Link To This Post Posted: August-31-2019 at 5:04pm
Originally posted by KENO KENO wrote:

There must be some reason why you want to jack it up


Just trying to squeeze a bit more performance out of the engine. 4,400 feet of elevation is a 15% reduction in barometric pressure. At 6,000 feet, that loss increases to a 20% reduction. Loss of ambient air pressure = a reduction in the engine's volumetric efficiency. Tweaking the ignition timing to properly compensate for the elevation is a relatively easy fix. The next step is to cut a hole in the top of the engine cover and mount a blower.

OK, maybe not that...

JQ
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2003 Ski Nautique 206 Limited

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Post Options Post Options   Thanks (0) Thanks(0)   Quote gt40KS Quote  Post ReplyReply Direct Link To This Post Posted: September-03-2019 at 11:44pm
I believe I've got the answer to your loss of air pressure JQ ....



That oughta do it       
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Jonny Quest View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonny Quest Quote  Post ReplyReply Direct Link To This Post Posted: September-04-2019 at 12:50am
Originally posted by gt40KS gt40KS wrote:

I believe I've got the answer to your loss of air pressure JQ ....



That oughta do it       


Definitely gonna have to cut a hole in the engine cover. The real question, however, is RR or standard rotation?

JQ
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2003 Ski Nautique 206 Limited

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