Low compression high altitude timing?

[rangehadies]

Hello all, ive had some mixed answers, i have a 1976 caprice engine that has been sitting for a while and finally got it back up and running, it took a new carb and intake manifold as the old one was cracked and its running better but i need it to be timed.
it has very low compression from what i see online 7.8.1 and I used a static timer to get the idle timed at 10 degrees but before i do the timing to 36 or what ever it should be ive been told that because im such low compression, and also due to my altitude ( 5700 above sea level ) that i need even more timing to 42 degrees instead of the typical 36-38. what are your thoughts? is more timing from higher altitude needed? and also more timing for low compression?

[34_40]

Welcome Aboard rangehadies. It's good to see you here.

My "gut" reaction is you'll need less timing, not more. But it's early and I haven't had any coffee yet.. LOL..

I think I'd set a baseline for each timing mechanism and then slowly increase it and then road test the car. Once it starts reacting badly then back off to the previous good running settings.

[Mike P]

Welome to the board. I live at 4500 feet out here and there are a few adjustments that can make a big difference in how the car runs at altitude.


Timing AND jetting changes from the original factory setting are generally required to optimize an engine operating at higher altitude. The reason for this is the air quality at higher altitude. The higher you go the “ thinner” the air becomes and less oxygen is available to support combustion (plus less dense air results in an lower actual dynamic compression ratio on naturally aspirated engines) . Regardless of the compression ratio the ultimate goal is to get as complete and efficient fuel burn as possible.

With less oxygen, to get the most efficient burn you generally want to take a little fuel out of the mix (smaller jets or larger step rods (in the case if Quadrajets and AFBs)) and increase the timing in order to lengthen the burn time.

OK that is probably over simplified, but it is the reason why the changes are necessary. What is below applies primarily to non computer controlled engines.


You probably are aware that timing is generally measured in 3 ways; Static (where the timing is set with the engine not running), Initial (where timing is measured with the engine idling and the distributor vacuum advance is disconnected and plugged) and total timing (where the vacuum advance is connected and the engine RPM is brought up ……usually 2000-2500 RPM on stock engine…… At this speed the mechanical advance and vacuum advance have come into play and combined with the initial timing provide the timing the engine see when going down the road. This is the 36-38 degrees you are referring to in your post and IS NOT what the initial timing should be set at). To check/set the initial timing you generally need either a dial back timing light or a timing tape installed on the balancer.


As for the how, if you have buckets of money and a ton of time the ideal way to dial an engine in is to put the car on a chassis dyno, do a baseline run and then start making changes until the engine is happy. Most of us don’t have that luxury so you start with a general rules of thumb and experience as the starting point.

I live at 4500 ft elevation and generally on stock engines start out by jetting down 1 to 2 jet sizes and increasing initial timing 2 degrees. THIS IS A STARTING POINT. There is really no exact formula for exactly what changes the engine will be happiest with…….. fuel quality, air quality (which can change from day to day……..the air will be considerably denser on a 50 degree day than it is on 100 degree day…….the denser the air the more fuel and less timing is required) and engine modifications all factor in.

At your elevation I would also start at 2 degrees over the recommended factory initial timing setting and keep advancing it from there until the engine runs best (or starts kicking on the starter and back it off a couple of degrees from there). After the timing is dialed in you may want to start looking at the fuel ratio and jet changes if it’s running overly rich.







AND YOU THOUGHT I WAS DONE TYPING



A few words of caution……

When you set the timing and adjust the fuel ratio keep in mind where you might end up driving the car. If you stay at 5700 feet forever fine, but in a lot of cases people end up driving some distances and the elevation and therefore the engine timing and fuel requirement change. If you are going to make trips to lower elevations on a regular basis you probably will want to back the timing off a degree or two from what is optimum at 5700 feet and leave the carburator a jet size larger than is best at that elevation. At much lower altitudes that you may also want to step up to “better” fuel (midgrade) if you do leave the timing where it’s optimum at 5700 feet.

Finally the higher you go the more the fuel will expand as it heats up (think hot summer day) additionally fuel with ethanol added will expand even more. If you do go into the carburator or experience the engine flooding after it’s shut off on hot days you will also want to drop the float level just a bit.

20 years ago all this used to be common knowledge for the mechanics who lived and worked at higher elevations. With computers taking over more and more of the engine management functions (and doing it pretty well) it’s pretty much becoming lost knowledge. If you’re not planning on doing the adjustments yourself, ask around and find the garage with the crusty old fart who has been doing this for years and years…..you’ll be time and money ahead.



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[rangehadies]

alot of good information! one says maybe a lil less timing and other is slightly more, eek ive had this problem. so you say add 2 degrees and go from there. ive gone from 6 all the way to 15 degrees with no noticeable difference. if altitude is a factory and should add 2. should i add it to 38 at 3500 rpm and call it a day?

[34_40]

That's not really what Mike P. and I are saying, I was describing an approach to strictly the timing, but, Mike P. is correct in that the carburetor also needs adjustment when running at an higher altitude.

I don't think you need to go beyond 32 degrees timing, but after making changes and adding a few miles to the car are you pulling the plugs and reading the color, or other conditions?

[Mike P]

Any timing you add to the initial timing (idling with the vacuum line plugged) will be additive to the total timing........the 2 degrees on the initial timing will show up on the total timing end......adding 2 degrees to the initial will result in the 38 total you now have going to 40.

You are indicating that you're checking the total timing at 3500. If the total timing is not coming in until that RPM it's a little high for a street driven engine. You make that change with either heaver mechanical advance weights OR lighter advance weight springs (both of which can be purchased in the various advance weight kits. For normal street driving the engine will likely be happier with the total advance being all in at around 24-2500 RPM.


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