Ignition Timing & Altitude
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=47988
Printed Date: April-29-2024 at 2:55am
Topic: Ignition Timing & Altitude
Posted By: Jonny Quest
Subject: Ignition Timing & Altitude
Date Posted: August-30-2019 at 4:24pm
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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 ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Replies:
Posted By: KENO
Date Posted: August-30-2019 at 7:21pm
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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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Posted By: Jonny Quest
Date Posted: August-31-2019 at 5:04pm
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 ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
KENO wrote:Posted By: gt40KS
Date 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   ------------- JCCI 1995 Ski Nautique GT40 |
Posted By: Jonny Quest
Date Posted: September-04-2019 at 12:50am
Definitely gonna have to cut a hole in the engine cover. The real question, however, is RR or standard rotation? JQ ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |