Ricardo

I do all my tuning at 5000 ft above sea level. Even the Dyno is located at this altitude.
The racetrack however is at sea level.
AT this elevation(5000 ft) The maximum reading on the MAP sensor is 80 kpa.
At the racetrack I get 98-100 kpa.

Is there a general rule of thumb I can apply to my 80 KPA (5000 ft) timing curve for the 100 KPA (sea level) race events?
Something like 1 deg retard for every 2000 ft or so?
I am assuming by looking at the stock timing curves that you are supposed to retard timing a little bit as compensation for higher map readings.


Any input greatly appreciated

RG

Hcastle97

You shouldnt have to worry about much unless you are running high boost or nitrous.. The car will pull 1 to 4 degrees out its self.. if you are running it all out with the bottle or boost humity and temp and dew point will make a difference.. You need to find out how must oxygen is in the (parts per million) at each track that is better than altitude..

Injuneer

The oxygen content of air is pretty much fixed at 21% by volume, adjusted slightly by the grains of moisture. That would put the O2 content of air at about 210,000 ppm. What does that indicate? Air density, and hence its mass, varies directly with absolute pressure. At 5,000-ft elevation, as correctly pointed out above, the normal barometric pressure is only 84kPa, compared to 102kPa at sea level. That means that the engine can only make (roughly) 83% of the power at 5,000-ft, that it could make at sea level. While the density has been reduced by 17%, the "ppm" of O2 is still the same.... 21% or 210,000 ppm. Professional drag racers (and pilots) use "density altitude", which incorporates the effects of true elevation, variations in barometric pressure, variations in ambient air temperature and moisture content of the air.

As far at the stock PCM tuning, it can in fact pull as much as 15-degrees of timing. The problem is that the PCM tends to pull out way more timing than is necessary, then very slowly puts it back, until it detects more knock. Timing will be reduced more, and power will be reduced more relying on the knock sensor to retard timing, than it would by simply setting the timing correctly for the denser air.

Ricardo

Thanks for the clarification Fred.
Your explanation is in line with my thinking on this subject.
I am actually trying to develop an optimal timing curve for my setup by using the DYNO, but this "optimal" timing will be good for 5000 ft above sea level.
I want to avoid getting knock at the racetrack.
I am looking for advice on how to extrapolate the timing curve developed for 80 KPA for the corresponding 85, 90, 95 and 100 kpa MAP values based on experience.
I can tell by looking at the stock timing curves in my original flash that GM did retard the timing 3 degs when moving from 80 to 100 KPA.

WS6T3RROR

When moving from 80 to 100kpa that is the engine manifold vacuum, not the barometer reading. The engine takes the barometer reading one time before the engine is started and works from there far as i know. The move from 80-100kpa that you're seeing in the programming is a function of engine load and by load i mean how hard you're cramming the loud pedal (80% vs WOT). If anything you can adjust the table for 80kpa to be a few more degrees than the table for 100kpa. The reason being is that with the lower air density and lower power your ve% is going to be lower so there is less particles of air and fuel in the cylinder and it takes longer to get the flame to propigate through the cylinder and get the job done, works alot like vacuum advance used to for old carbed stuff. Or for another example when the egr kicks in on a stock car the timing is advanced to make up for the basicly lower ve the engine has as a result.

In racing all you will ever get with timing is close. Engines dont really like the testing it takes to find the best timing curve. My suggestion to you is to just get the shape of the curve that the motor seems to like on the dyno, and copy it into the 100kpa column and add a degree or two through the curve or break the 2000-3000rpm or so you run in into 2 or 3 parts and vary them up and down. If your car is consistant enough you will see the difference at the track if it likes it or not.

Injuneer

Just thinking out loud... since the engine operating at 5000-ft will never see much more tha 80kPA, why not tune it for 5000-ft using the tables up to 80kPa. Then tune it for sea level in the portion of the tables from 80-100kPa? Or is that what the post above suggested in the first paragraph? Or do you think the PCM "rescales" the timing tables based on the barometer, to somehow allow the 0-100kPa values to be prorated over the range of 0-80kPa? I don't know... just asking.

Ricardo

This is the timing table for 4000 RPM and above in my original flash.



Base timing advance is referenced to MAP and RPM.
When going WOT at 5000 ft my MAP readings are 80 KPA all thru the RPM range. The PCM uses the specified timing values for this MAP reading.

When going WOT at the racetrack the MAP readings are in the range of 98 KPA all thru the powerband.

I guess I will follow the pattern applied to the original flash and retard my "optimal" timing curve developed for 80 KPA copying this decay pattern. (2-3 deg, as stated previously)

I think that both the throttle opening and barometric air pressure affect the MAP reading when the engine is operating.

WS6T3RROR

Fred, thats is what i was suggesting in my post. ie, you can run more aggressive timing at wot at 5000ft than you can at sea level (this would be your 80kpa line). Then tune your 100kpa line for sea level drag racing. Break the rpm range you run in to two or three parts and just add or subtract 1 degree or so in a certain range and see how it responds (merely to save on time and number of runs).

If you have a wideband log your air fuel ratio and see if it responds to the changes as well. Do your best to keep all things constant water temp fuel psi and conditions. Honestly, you're going to be shooting at a moving target.

Also i think you may have a tad bit much timing commanded for your 80kpa rpm range, the lt1's i have tuned all show more timing on a logger and a timing light than I have commanded maybe up to 3* or so the closer you get to 7000rpm. use freescan to verify what the pcm is actually commanding.