RV.Net Open Roads Forum: Class A Motorhomes: Fuel economy and additives

RV Blog

  |  

RV Sales

  |  

Campgrounds

  |  

RV Parks

  |  

RV Club

  |  

RV Buyers Guide

  |  

Roadside Assistance

  |  

Extended Service Plan

  |  

RV Travel Assistance

  |  

RV Credit Card

  |  

RV Loans

Open Roads Forum Already a member? Login here.   If not, Register Today!  |  Help

Newest  |  Active  |  Popular  |  RVing FAQ Forum Rules  |  Forum Posting Help and Support  |  Contact  

Search:   Advanced Search

Search only in Class A Motorhomes

Open Roads Forum  >  Class A Motorhomes  >  Maintenance Issues & Tips

 > Fuel economy and additives

Reply to Topic  |  Subscribe  |  Print Topic  |  Post New Topic  | 
Page of 4  
Prev  |  Next
Sponsored By:
ItsyRV

Desert SW

Senior Member

Joined: 10/02/2018

View Profile



Good Sam RV Club Member


Posted: 06/12/19 10:21am Link  |  Quote  |  Print  |  Notify Moderator

Tom in Tulsa wrote:

WOW. an RV full of info. would moving up the octane to 89 help or would that just add more cost for 50 gal. I am using 87.

Your engine was designed to perform best at a certain octane. Using a higher octane doesn't improve anything. Read the owner's manual and use the octane rating they state. Using a lower octane could cause engine damage.


1994 Itasca SunDancer 21RB - Chevy G-30 chassis.


rgatijnet1

Florida

Senior Member

Joined: 06/22/2009

View Profile



Good Sam RV Club Member


Posted: 06/12/19 10:28am Link  |  Quote  |  Print  |  Notify Moderator

Your engine has a sophisticated computer management system. If you use the MINIMUM fuel octane as recommended by the owner's manual, which is probably 87 octane, the computer will make the necessary adjustments to give you the best performance/mileage. Using a higher octane fuel will not improve the mileage on an engine that has the computer calibrated for 87 octane.

msturtz

Washington

Senior Member

Joined: 06/28/2013

View Profile





Offline
Posted: 06/12/19 10:41am Link  |  Quote  |  Print  |  Notify Moderator

rgatijnet1 wrote:

Your engine has a sophisticated computer management system. If you use the MINIMUM fuel octane as recommended by the owner's manual, which is probably 87 octane, the computer will make the necessary adjustments to give you the best performance/mileage. Using a higher octane fuel will not improve the mileage on an engine that has the computer calibrated for 87 octane.


This is absolutely correct, what is worse is if the ECU (Engine computer) is calibrated for a certain minimum octane and a higher octane fuel is used then fuel economy can actually suffer i.e. get worse. Octane is resistance to pre-ignition or knock. This occurs when the air-fuel mixture ignites before the spark plug is fired by the ECU. Gasoline engines are homogeneous ignition combustion engines where the ignition is controlled by the ECU sending a spark to the spark plug vs. stratified charge compression ignition in diesel engines that rely on injecting fuel at precisely the time the ECU wants ignition to occur. It should be noted that with gasoline engines that do rely on a higher octane fuel such as boosted engines e.g. Ford EcoBoost or higher performance engines such as the GM 6.2L V8 can and will use 92 octane fuel and will have better fuel economy on higher octane fuel. The reason for this is the higher compression ratios and or high boost pressure. In these engines, the ECU will back off the timing and or actually inject more fuel if engine pre-ignition (knock) is detected. This would necessarily cause poorer fuel economy than using the recommended octane fuel.


FMCA member

Rick Jay

Greater Springfield area, MA

Senior Member

Joined: 02/02/2003

View Profile


Offline
Posted: 06/12/19 11:48am Link  |  Quote  |  Print  |  Notify Moderator

In my experience, the use of 10% ethanol fuel reduces my MPG by about 10%. I know the "experts" say it should only be reduced 2-3%, but in 4 different vehicles, INCLUDING my 8.1L W-22 Motorhome, the measured reduction in MPG over time hovers right around 10%. Pretty much all we have here in the NorthEast and I believe most of the East Coast, is 10% ethanol blends. It's darn near impossible to find pure gas. But when I'm on the road and DO find it, I use it!

So, IF you can find REAL gas for under a 10% price premium, I'd say go for it!

~Rick


2005 Georgie Boy Cruise Master 3625 DS on a Workhorse W-22
Rick, Gail, 1 girl (22-Angel, Lexi96.org), 1 girl (17), 2 boys (19 & 16).
2001 Honda Odyssey, Demco Aluminator tow bar & tow plate, SMI Silent Partner brake controller.


msturtz

Washington

Senior Member

Joined: 06/28/2013

View Profile





Offline
Posted: 06/12/19 12:00pm Link  |  Quote  |  Print  |  Notify Moderator

Rick Jay wrote:

In my experience, the use of 10% ethanol fuel reduces my MPG by about 10%. I know the "experts" say it should only be reduced 2-3%, but in 4 different vehicles, INCLUDING my 8.1L W-22 Motorhome, the measured reduction in MPG over time hovers right around 10%. Pretty much all we have here in the NorthEast and I believe most of the East Coast, is 10% ethanol blends. It's **** near impossible to find pure gas. But when I'm on the road and DO find it, I use it!

So, IF you can find REAL gas for under a 10% price premium, I'd say go for it!

~Rick


This is because that engine isn’t optimized for ethanol. Ethanol is challenging because the percentage isn’t consistent. The ECU must be able to dynamically determine the ethanol percentage on the fly and adjust the timing and amount of fuel injected. If isn’t optimized then the fuel economy suffers.

Sam Spade

North Central Florida

Senior Member

Joined: 11/05/2015

View Profile


Offline
Posted: 06/12/19 12:21pm Link  |  Quote  |  Print  |  Notify Moderator

Rick Jay wrote:



So, IF you can find REAL gas for under a 10% price premium, I'd say go for it!

~Rick


And then actually check your mileage to see if you got any improvement at all in your given situation.


'07 Damon Outlaw 3611
CanAm Spyder in the "trunk"

Sam Spade

North Central Florida

Senior Member

Joined: 11/05/2015

View Profile


Offline
Posted: 06/12/19 12:25pm Link  |  Quote  |  Print  |  Notify Moderator

msturtz wrote:

The ECU must be able to dynamically determine the ethanol percentage on the fly and adjust the timing and amount of fuel injected. If isn’t optimized then the fuel economy suffers.


And the "magic" computer shows up again.

Even the most modern vehicles are not able to "determine the ethanol percentage".....on the fly or otherwise.
It can't measure the octane either.

The oxygen sensors in the exhaust can make some tiny adjustments to the mixture but that doesn't imply that it is measuring either the ethanol OR the octane.

The fuel management ECU is not nearly as "smart" as some of you give it credit for.

rgatijnet1

Florida

Senior Member

Joined: 06/22/2009

View Profile



Good Sam RV Club Member


Posted: 06/12/19 12:52pm Link  |  Quote  |  Print  |  Notify Moderator

That is correct concerning ethanol. If you look at the placard on the pump it usually says something to the effect that the fuel may contain UP TO 10% ethanol so your ECM has to adjust accordingly. When at high altitudes out West, you will sometimes see stations advertising that they have 85 octane. I have used it in my GM 8.1L with no issues at all. When I was leaving the high altitudes, I would top it off with 87 octane. I'm not sure if the Ford can use 85 or not.

Chum lee

Albuquerque, NM

Senior Member

Joined: 08/03/2015

View Profile


Offline
Posted: 06/12/19 01:38pm Link  |  Quote  |  Print  |  Notify Moderator

rgatijnet1 wrote:

I'm not sure if the Ford can use 85 or not.


I can use less than 87 octane fuel but it lights the Check Engine Light on mine (stock 1999 V10 2 valve) every time I hit a hill of any magnitude. I notice no drivability issues, but clearly, the ECU doesn't like it. If I clear the fault code, it returns promptly on the next hill or in a strong headwind. Switching to 87 octane cures the CEL issue every time. The Owners Manual clearly warns against using less than 87 octane fuel. Sometimes I forget to check when buying fuel at higher elevations. Ooooops. That **** light is on . . . . . . again.

Chum lee

midnightsadie

ohio

Senior Member

Joined: 01/07/2008

View Profile


Offline
Posted: 06/12/19 04:12pm Link  |  Quote  |  Print  |  Notify Moderator

so in the end of this fuel mpg post we arn,t driving great gandpa,s truck. the computer knows whats best.

Reply to Topic  |  Subscribe  |  Print Topic  |  Post New Topic  | 
Page of 4  
Prev  |  Next

Open Roads Forum  >  Class A Motorhomes  >  Maintenance Issues & Tips

 > Fuel economy and additives
Search:   Advanced Search

Search only in Class A Motorhomes


New posts No new posts
Closed, new posts Closed, no new posts
Moved, new posts Moved, no new posts

Adjust text size:

© 2019 CWI, Inc. © 2019 Good Sam Enterprises, LLC. All Rights Reserved. | Terms of Use | PRIVACY POLICY | YOUR PRIVACY RIGHTS