Go Back   Jeep Garage - Jeep Forum > Jeep Platform Discussion > Grand Cherokee - WK2 -

Join Jeep Garage Today
Reply
 
Thread Tools Display Modes
 
  #13  
Old 04-26-2013, 11:02 AM
The Evil Twin's Avatar
Member
My Jeep: 2011 5.7L WK2
 
Join Date: Jan 2013
Location: Gainesville, VA
Posts: 363
Thanks: 4
Thanked 13 Times in 11 Posts
Rep Power: 361212
The Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond reputeThe Evil Twin has a reputation beyond repute
Garage
Re: What fuel for 14 5.7

Quote:
Originally Posted by ColoradoDriver View Post
Since I live at 8500FT elevation, there is certainly less pressure and thus slower burning fuel mixture. My concern is that modern vehicles already adjust their timing (and fuel amounts) for the higher altitude. So the engine management system is still looking for 89 octane. I know it will "back-off" timing if it detects pre-ignition, but I would rather just feed it the 89 it expects. (Years ago vehicle manufactures sold cars with milled "Denver Heads" to increase internal pressures. Also the carbs came with leaner "Denver Jets." That would not work today because we all drive back down to sea level, and the EPA would not allow those mods.)

Also, several previous vehicles owned were turbo charged. So the altitude excuse to sell lower octane fuel really did not apply for turbo/supercharged engines. For those engines, 93 octane would be nice "up here," but they only sell 91.
A car doesn't know what octane fuel is in the tank. The recommended rating (89 for example) is what the vehicle was designed for during use at or near sea level. As altitude increases, the need for a fuel to resist pre detonation decreases by about 1 ON per 1000 feet. Roughly. With that in mind, 85-86 (R+M/2) is equivalent to 89 (R+M/2) at 6000 feet.
On turbo engines, this still applies. Boost ratio remains the same regardless of air density.
( atmosphere pressure + required boost) / atmosphere = p2/p1 ratio
Lower air density at the intake would result in less pressure on the exit of the spool. Of course, we are talking absolute pressures here.
Lower octane fuels generally deliver more energy per gallon. Ethanol is a common way to cheaply increase the octane rating of a fuel. Although ethanol contains less energy per unit of measure than does petrol.
I usually use 87 unless towing or it is really hot around here. FWIW, I am damn near at sea level.
__________________

__________________
#132-CCS Racing

2014 Sponsors: Amsoil, www.DC-MOTO.net, Manassas Honda Kawasaki Suzuki, www.MARRC.org
Amsoil 5w-20 synthetic meets Chrysler MS-6395 for my Jeep!
Reply With Quote
The Following User Says Thank You to The Evil Twin For This Useful Post:
  #14  
Old 04-26-2013, 01:12 PM
Premium Member
 
Join Date: Mar 2013
Location: Woodland Park, CO
Posts: 101
Thanks: 13
Thanked 8 Times in 8 Posts
Rep Power: 718
ColoradoDriver is on a distinguished road
Re: What fuel for 14 5.7

My point is that modern vehicles measure atmospheric pressure (including temperature and humidity) and take that into consideration when determining ignition and fuel mapping. The mapping also includes assumptions as to fuel being used. For the 5.7L Hemi, Jeep probably assumes 89 octane. So the initial mappings are appropriate for that fuel. Once the engine fires, sensors provide feedback to the engine management system so that the initial mappings can be refined to the actual properties of combustion.

Turbocharged engines, with variable compressor vanes, compensate for most of the lost of atmospheric pressures at altitude.
__________________

__________________
Reply With Quote
  #15  
Old 04-26-2013, 02:56 PM
bill_de's Avatar
Premium Member
My Jeep: 2014 5.7L WK2
 
Join Date: Sep 2012
Posts: 6,372
Thanks: 336
Thanked 362 Times in 315 Posts
Rep Power: 1801927
bill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond repute
Re: What fuel for 14 5.7

Quote:
Originally Posted by ColoradoDriver View Post
My point is that modern vehicles measure atmospheric pressure (including temperature and humidity) and take that into consideration when determining ignition and fuel mapping. The mapping also includes assumptions as to fuel being used. For the 5.7L Hemi, Jeep probably assumes 89 octane. So the initial mappings are appropriate for that fuel. Once the engine fires, sensors provide feedback to the engine management system so that the initial mappings can be refined to the actual properties of combustion.

Turbocharged engines, with variable compressor vanes, compensate for most of the lost of atmospheric pressures at altitude.
You might be making a lot of assumptions. I think it's a lot simpler than that. When the system senses pre-detonation it retards the timing. It doesn't assume what gas you chose.



.
__________________
If you need a shoulder to cry on ...

... pull over to the side of the road!
Reply With Quote
  #16  
Old 04-26-2013, 04:10 PM
Premium Member
 
Join Date: Mar 2013
Location: Woodland Park, CO
Posts: 101
Thanks: 13
Thanked 8 Times in 8 Posts
Rep Power: 718
ColoradoDriver is on a distinguished road
Re: What fuel for 14 5.7

Quote:
Originally Posted by bill_de View Post
You might be making a lot of assumptions. I think it's a lot simpler than that. When the system senses pre-detonation it retards the timing. It doesn't assume what gas you chose.


The system starts with an assumption of what it will take to start the engine, otherwise it could not start the engine. Once running, it looks for pre-detonation, unburned fuel, etc. I understand that it will back off the timing if octane is lower than expected. Not really much of an issue for non-boosted engines like the Hemi. But for turbos and superchargers, the lower octane degrades potential power and economy.

Just one of my pet peeves. Was a real issue on my previous twin-turbo Audi.
__________________
Reply With Quote
  #17  
Old 04-27-2013, 08:56 AM
Member
My Jeep: 2014 3.6L WK2
 
Join Date: Jan 2013
Location: Michigan
Posts: 841
Thanks: 8
Thanked 19 Times in 14 Posts
Rep Power: 127311
loveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond reputeloveracing1988 has a reputation beyond repute
Quote:
Originally Posted by bill_de View Post
You might be making a lot of assumptions. I think it's a lot simpler than that. When the system senses pre-detonation it retards the timing. It doesn't assume what gas you chose.



.
It will assume you put 89 in it and then scale back timing from there. It is looking for pre detonation to see if it needs to pull timing or if it is just fine. Assumes is kind of a misnomer. It sets itself up to run the higher octane, otherwise it won't be running maximum timing that can be run. I don't really think any of that made sense but it does in my head... I'm still half asleep at work.
__________________
Reply With Quote
  #18  
Old 04-27-2013, 01:30 PM
bill_de's Avatar
Premium Member
My Jeep: 2014 5.7L WK2
 
Join Date: Sep 2012
Posts: 6,372
Thanks: 336
Thanked 362 Times in 315 Posts
Rep Power: 1801927
bill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond reputebill_de has a reputation beyond repute
Re: What fuel for 14 5.7

At the higher altitude it won't detect pre-detonation with lower octane gas, so it will not adjust the timing to correct for it. It's the pressure in the combustion chamber that counts. That is lower at high altitude because the air is 'thinner'.
__________________

__________________
If you need a shoulder to cry on ...

... pull over to the side of the road!
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Better fuel economy..? JeremyJamesDec15 Engine Performance/Intakes/Exhausts 28 04-29-2013 08:10 AM
Something with fuel Tuzmaster Grand Cherokee - ZJ 4 01-02-2013 04:25 PM
fuel pump help warthogdriver Troubleshooting/Problems 10 07-02-2012 11:52 PM
Need better fuel economy qbny94 Engine Performance/Intake/Exhaust 21 09-05-2011 09:32 AM
fuel pump help Vinnysrt8 Troubleshooting/Problems Discussion 1 11-21-2010 05:51 AM

Powered by vBadvanced CMPS v3.2.3

All times are GMT -5. The time now is 04:47 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Copyright 2012 - JeepGarage.Org
The Jeep Grand Cherokee Owners Community

JeepGarage.org is in no way associated with or endorsed by Chrysler Group LLC.