Elevation effect on tuning? Octane?

[GetawayInMoscow]

I don't know if any of you have drive to Wyoming, for example, but you will notice that they have way lower octane there.

Apparently at higher elevations you don't need as high of octane. Does anyone have a scientific explanation for this?

Is it because the air is less dense so it's already harder for fuel to combust since there is less oxygen?

If your car is tuned for 12:1 AFR at 4,000ft and then you go down to sea level will it still be 12:1?

Hopefully someone who has taken EFI 101 can chime in, but I don't know if this is covered....

Meng? Reid?

[Steve]

When the air is less dense , the compression pressure before and after ignition is lower , therefore the temperature is lower (unless you advance the timing and increase the pressure before tdc to bring the temp back up). Octane rating is resistance to self ignition . Self ignition occurs when the temperature raises beyond the point where the fuel is stable.

[rrussell]

I don't know if any of you have drive to Wyoming, for example, but you will notice that they have way lower octane there.

Apparently at higher elevations you don't need as high of octane. Does anyone have a scientific explanation for this?

Is it because the air is less dense so it's already harder for fuel to combust since there is less oxygen?

If your car is tuned for 12:1 AFR at 4,000ft and then you go down to sea level will it still be 12:1?

Hopefully someone who has taken EFI 101 can chime in, but I don't know if this is covered....

Meng? Reid?

Its becasue of the air being less dense.

Example:

350CI engine at 5000rpm with a VE of 100% and at a elevation of 1000' will flow 36 lbs/min.

The same engine at the same rpm and VE, but at a elevation of 5000' will flow 30 lbs/min. That's equivalent to the engine at 5000' with 3lbs of boost.

Also air with a higher density has a faster flame speed during combustion then air with a lower density. Even at the same A/F ratio.

The car at the higher elevation will reach maximum cylinder pressure later in crank degrees then the car at a lower elevation.

There's more to it but this is it in a nutshell.

[sr240]

you should get the vette re-tuned here milan.. shellys probably has the soft ware to do it.. you could probably make more power here and while its cold out lol..

[GetawayInMoscow]

you should get the vette re-tuned here milan.. shellys probably has the soft ware to do it.. you could probably make more power here and while its cold out lol..

Yeah I agree...but I'm only here for 6 more weeks and then I'm moving to CO. I was just curious for my Civic

[GetawayInMoscow]

Russ thanks for the help...I had a feeling you might chime in!

[3258]

not sure i agree with the logic that the diff octane is because of elevation, but, octane at a given pump can be different depending on the time of year. Possibly it is emissions requirements of the state. the reason being is that where i am at is around 900ft elevation. the car does not care what the advertised elevation is, really the corrected altitude which changes day in day out, here from 10-2500ft taking into consideration the many variables. I do not put different fuel in my race car or street car at any different weather situation, and of course we have all drive across country with differing fuels and no problems. as mentioned before, the octane is measurement of resistance to detonation, and when the atmosphere is on any end of the spectrum, a car will run with the designed octane just fine, the change in atm pressure is small very small compared to the pressure of the compressed gas/air mix in the cylinder. It takes a big jump in combustion pressure to demand a much higer or lower octane, probably more than the atmosphere has to offer. A good example is diesel fuel....octane in the 40's, and much much higher compression, of couse that is how it fires so it must not resist detonation. A side note is that if the octane is greater than 100, the fuel has lead in it, that is why people who hang out at the track to much are brain dead.....

With tuning and elevation, the only thing the elevation affects is open loop stuff ie wide open throttle, idle etc. when it is in closed loop the ECU is shooting for a target AFR, there may be less power to be built, but the AFR will be what the program in the ECU demands based on the feedback variables (depending on the algorithm type you are running ie speed density or alpha nu) via more/less fuel injected/cycle. Some people have noticed driving to seattle that going over the pass the car can run funny. When you start your car, the MAP sensor takes a reading, and bases all calcs off that reference pressure. as the altitude changes rapidly, the ATM pressure reference is no longer valid, and thus, if you shut your car off, and start it again, it will run good again because the ECU now knows what the real ATM pressure is. A remedy that many are using is a dual MAP sensor, one always references ATM, and one Manifold pressure, and the reference is not a snapshot, but realtime updated

[GetawayInMoscow]

Okay but in states like Colorado, Wyoming, some parts of Montana, the octane ratings are totally different.

It will be like: 85.5 87 91
or sometimes: 85 87 90

[GetawayInMoscow]

Also E85 is over 100 octane with no lead.

[3258]

Also E85 is over 100 octane with no lead.

correct, it should have read "gas with over 100 octane ahd lead" not "fuel with over 100 octane has lead"

but be careful of e85, it is not a constant mix, 85% is the max, but it varies, not good for bracket racing

[3258]

Okay but in states like Colorado, Wyoming, some parts of Montana, the octane ratings are totally different.

It will be like: 85.5 87 91
or sometimes: 85 87 90

so what is that in % change from spokane?

[gregc]

but be careful of e85, it is not a constant mix, 85% is the max, but it varies, not good for bracket racing

E85 (or any variant really) test tube testers are a wonderful thing.

Have you tested the E85 around here to see the variance? Anybody?

I will be in the next week or two.

There is a very easy fix. Buy a 30 gallon drum of E100 and use it to get the right E85 mix for your racing fuel.

[Steve]

I do not put different fuel in my race car or street car at any different weather situation, and of course we have all drive across country with differing fuels and no problems. as mentioned before, the octane is measurement of resistance to detonation, and when the atmosphere is on any end of the spectrum, a car will run with the designed octane just fine, the change in atm pressure is small very small compared to the pressure of the compressed gas/air mix in the cylinder. It takes a big jump in combustion pressure to demand a much higer or lower octane, probably more than the atmosphere has to offer.

The air does affect cylinder pressure a lot . If you've ever had a car that runs on the edge of detonation and you drop elevation, it will ping. Any mechanically inclined person who drove cars from the 60's and traveled from Spokane to Seattle will tell you that they would audibly ping over there. As I tried to say before , it's really the temperature that causes detonation. If the air is very cold , and the combustion chamber surfaces are cool , it's much less likely that detonation will occur than when the air is hot and the combustion chamber temps are high. Even though the density of the air is generally higher when the temp is lower for a given altitude. If you ever check your cranking compression at high or low elevation , you will be surprised how much difference there is.

You can get leaded and unleaded race fuel too. Seems like Seattle had unleaded race fuel available at the track pump a couple years ago.

[Evosol]

Pfft... the certification I have is from GhettoTune101... sorry can't help you.
:D


Its good to know numbers and figures, but its also too easy to get too caught up in the numbers... realizing that its never just a single variable that changing... I just like to look at it in terms of the change in VE.

EDIT: no after a minute of thinking... thats not right. I don't recall, i'd have to knock off the dust fromt he old ghettotune101 manuals...

[Bad Bug Two]

E85 (or any variant really) test tube testers are a wonderful thing.

Have you tested the E85 around here to see the variance? Anybody?

I will be in the next week or two.

There is a very easy fix. Buy a 30 gallon drum of E100 and use it to get the right E85 mix for your racing fuel.

Greg,

Do you know where a person can get some straight ethanol? I think it is actually E98 (To keep people from drinking it!) I haven't had any luck sourcing any.

Thanks!

Jeff

[3258]

.......detonation and you drop elevation, it will ping. Any mechanically inclined person who drove cars from the 60's and traveled from Spokane to Seattle will tell you that they would audibly ping over there.
You can get leaded and unleaded race fuel too. Seems like Seattle had unleaded race fuel available at the track pump a couple years ago.

I am not fimilar of what happened in the 60's as that was about 20 years before my birth, but you just discussed 2 different things, detonation, and pinging, or pre ignition, (and yes they are different) they can however be cause and effect. In todays world, and EFI, there are many things that sense the load etc and also a knock sensor that is listening for the pinging you are describing, combined with better head designs and cooling systems it is not an issue like it was in old times, they are not on the edge of detonation or pinging all the time. How many times have you had that problem in the past 10 years or more? we are not taliking about 1960's 50 year old technology

as for the fuel, i guess it is possible to have bought race gas that has not lead, it is uncommon i think becase of the cost of other chemicals that give over 100 octane, but considering it is tree-huggin seattle, it would not suprise me. An additive these days is MTBE (Methyl Tertiary Butyl Ether). This chemical boosts octane ratings and it is also an oxygenate. Being an oxygenate, it reduces the amount of unburned hydrocarbons and carbon monoxide in the exhaust

[Steve]

It's true that better combustion chamber shapes , with increased turbulance and better fuel distribution, as well as running aluminum head material and better cooling systems , increased the detonation resistance of modern engines, but the effects of elevaton haven't changed. If the computer gets input from the knock sensor , it retards the timing. Which decreses the pressure btdc and slightly after tdc , as well as cutting power output, but it makes up for the lack of octane. It's a bandaid for lack of octane . The engine becomes less efficient , and increases the exhaust temp, because the burn isn't optimized . As you go up in elevation , you can increase the timing advance , to bring the pressure up to increase efficiency , (same thing as what a vacuum advance does on old cars when the throttle plates are slightly open , manifold vacuum is high and air density is low) but you'd have to change it back for low elevation. It's just easier to run lower octane fuel at high elevations ,and higher octane at low elevations.

[mark6052]

The highest octane I can find for unleaded fuel is VPs ms109 it has a r+m/2 rating of 105, thats unleaded and oxygenated. Its about 10.00 agallon. yeah its expensive, speed costs how fast you want to go.

[3258]

thats why you buy methanol at 2$ a gallon ;) (and double the volume)


or just buy the leaded stuff if you dont have the emissions stuff on your car