View Poll Results: Which Octane do you fill up with?
Lower than 87
2
2.00%
87
33
33.00%
89
13
13.00%
91
27
27.00%
Higher than 91
25
25.00%
Voters: 100. You may not vote on this poll
What kind of gas?
03-19-08, 07:17 AM
#16
"If you use reg.and get 2mpg or so less,then you may as well stay with premium,cause your not showing any savings,by reg. fuel."
There is no logical explanation for a 2 mpg difference with 87 octane fuel. In fact, this is impossible.
There is no logical explanation for a 2 mpg difference with 87 octane fuel. In fact, this is impossible.
03-19-08, 07:22 AM
#17
Lexus Champion
I wouldn`t say it`s impossible,alot depends on your environment.Hotter climate,elevation,type & style of driving,etc.It`s probably more possible than one would think.
03-19-08, 08:04 AM
#18
Driver School Candidate
Wow, been down this road before with the people on the Acura MDX forum.
First let me say I am a retired mechanical engineer who used to work for an engine builder. Actually Cooper Bessemer. In my early days I worked in research and development as a technician while I was attending college on a co-op plan.
Anyway to the point, we tested engines running in a steady state enviroment, constant speed and load for periods of time and we adjusted the timing to get optimum performance, ie fuel effeciency for the load. The engines I worked on were very large, the type used in electrical power generation and on compressor sites to move natural gas in pipelines. Typical engine configurations were as large as 12, 16 and even 20 cylinders and up to a maximum of 16,500 HP.
These engines ran very low RPM and piston slap could be easily discerned.
We did a lot of work on cylinder/valve configurations in order to achieve total
combustion. As you might imagine fuel efficiency is very important on a 16,500 HP engine running all day long 24/7 pumping natural gas from Texas to Ohio for example. The companies like Columbia Gas Transmission want the best efficiency they can get.
A little more background here and then to my point. The 2006 RX manual did not suggest you must run 91 Octane premium, also the 2006 manual recommended 5w-30 weight oil; but the 2008 manual says 91 octane premium
is required, and they also changed the oil requirements to 5w-20 and as an alternate 0w-20. ok so what design changes to the engine have occurred, I can only guess. I could look at compression ratios ect and speculate but who knows for sure.
Anyway back to the R & D testing and results:
1. Engines run most efficiently at the proper timing setting, retarding timing for any reason not only reduces available HP, it also reduces fuel efficiency even if you are not needing the extra optimal HP available at the optimal timing.
We could always see the impact of less than optimum timing on fuel effeciency
2.In a large engine like I am talking about, piston slap really sounds terrible, when the engine knock occurs, its a loud and ugly event; but in a small car engine it isn't so loud and to boot it is deadened by other sounds, it is however just as damaging.
3.It's all in the design, how the engine is meant to be run, the compression ratio and the cylinder head design along with valve placement configurations. The gas flame front will advance differently and impact both maximum energy output and fuel efficiency, no matter what the load the design parameters of the engine cylinder, proper timing and yes optimum fuel based on the design/ implemented design goals.
Now having said all that, we need to think about how the hybrid ICE setup works, yes it runs at in a more constant RPM range with a variable load. Also like others have said driving for best fuel economy, we aren't pushing the engine into the extremes in needed HP, but the interface of the ICE and how load is applied by the hybrid system is vastly different from a conventional gas engine only setup. Ping is of course going to be more noticeable and more prevalent when power demand varies more with a regular transmission, but that does not mean that the hybrid setup with a CVT doesn't encounter load variations that would cause piston slap-ping if it weren't for the electronic
countermeasures retarding the timing as needed.
Another point as I understand it, the electronic timing anti knock system is designed to reset the timing curve over a period of time, like when you switch
gas from one tank to the next, instantaneous timing adjustments do occur, but the whole timing curve is only gradually adjusted as you burn thru a change in octane of a new tank of fuel. I am not a Lexus engineer so I am only speculating based on rumors I have heard.
Also I will state now I own a new 2008 RX 400h and I use 91 octane premium, with $4 a gallon gas $0.20 is only a 5% premium and 5% of 27 mpg is only a 1.3 MPG difference and you break even. Plus I have the added comfort of knowing I can tromp on it if I want to,I can go up steep hills without worry, I can encounter really hot summer days with the AC running and indirectly loading the engine for needed electrical power ect. However, the best part is I can go the the Lexus dealer in the case of a warranty related failure and say I followed the recommended fuel requirement and my warranty should be good
if needed.
I know way too long, to summarize though, it's your $49,000/ $50,000 car do what you want to, my 08 RX 400h calls for premium required in the manual and it's only 5% more at todays prices and I am a mechanical engineer with engine testing experience, but I still cant totally evaluate how necessary it is or the total impact of not using premium. So I will error on the side of caution following the factory recommendations and enjoy the ride when I might otherwise worry. If I want to feel the power I can and I can climb a steep hills without having to reduce speed going up it without concern.
I am older and not a heavy footed driver, I drive for economy most of the
time. I still however find times when I want to use my car close to it's design potential.
Good luck to you all!!!!
First let me say I am a retired mechanical engineer who used to work for an engine builder. Actually Cooper Bessemer. In my early days I worked in research and development as a technician while I was attending college on a co-op plan.
Anyway to the point, we tested engines running in a steady state enviroment, constant speed and load for periods of time and we adjusted the timing to get optimum performance, ie fuel effeciency for the load. The engines I worked on were very large, the type used in electrical power generation and on compressor sites to move natural gas in pipelines. Typical engine configurations were as large as 12, 16 and even 20 cylinders and up to a maximum of 16,500 HP.
These engines ran very low RPM and piston slap could be easily discerned.
We did a lot of work on cylinder/valve configurations in order to achieve total
combustion. As you might imagine fuel efficiency is very important on a 16,500 HP engine running all day long 24/7 pumping natural gas from Texas to Ohio for example. The companies like Columbia Gas Transmission want the best efficiency they can get.
A little more background here and then to my point. The 2006 RX manual did not suggest you must run 91 Octane premium, also the 2006 manual recommended 5w-30 weight oil; but the 2008 manual says 91 octane premium
is required, and they also changed the oil requirements to 5w-20 and as an alternate 0w-20. ok so what design changes to the engine have occurred, I can only guess. I could look at compression ratios ect and speculate but who knows for sure.
Anyway back to the R & D testing and results:
1. Engines run most efficiently at the proper timing setting, retarding timing for any reason not only reduces available HP, it also reduces fuel efficiency even if you are not needing the extra optimal HP available at the optimal timing.
We could always see the impact of less than optimum timing on fuel effeciency
2.In a large engine like I am talking about, piston slap really sounds terrible, when the engine knock occurs, its a loud and ugly event; but in a small car engine it isn't so loud and to boot it is deadened by other sounds, it is however just as damaging.
3.It's all in the design, how the engine is meant to be run, the compression ratio and the cylinder head design along with valve placement configurations. The gas flame front will advance differently and impact both maximum energy output and fuel efficiency, no matter what the load the design parameters of the engine cylinder, proper timing and yes optimum fuel based on the design/ implemented design goals.
Now having said all that, we need to think about how the hybrid ICE setup works, yes it runs at in a more constant RPM range with a variable load. Also like others have said driving for best fuel economy, we aren't pushing the engine into the extremes in needed HP, but the interface of the ICE and how load is applied by the hybrid system is vastly different from a conventional gas engine only setup. Ping is of course going to be more noticeable and more prevalent when power demand varies more with a regular transmission, but that does not mean that the hybrid setup with a CVT doesn't encounter load variations that would cause piston slap-ping if it weren't for the electronic
countermeasures retarding the timing as needed.
Another point as I understand it, the electronic timing anti knock system is designed to reset the timing curve over a period of time, like when you switch
gas from one tank to the next, instantaneous timing adjustments do occur, but the whole timing curve is only gradually adjusted as you burn thru a change in octane of a new tank of fuel. I am not a Lexus engineer so I am only speculating based on rumors I have heard.
Also I will state now I own a new 2008 RX 400h and I use 91 octane premium, with $4 a gallon gas $0.20 is only a 5% premium and 5% of 27 mpg is only a 1.3 MPG difference and you break even. Plus I have the added comfort of knowing I can tromp on it if I want to,I can go up steep hills without worry, I can encounter really hot summer days with the AC running and indirectly loading the engine for needed electrical power ect. However, the best part is I can go the the Lexus dealer in the case of a warranty related failure and say I followed the recommended fuel requirement and my warranty should be good
if needed.
I know way too long, to summarize though, it's your $49,000/ $50,000 car do what you want to, my 08 RX 400h calls for premium required in the manual and it's only 5% more at todays prices and I am a mechanical engineer with engine testing experience, but I still cant totally evaluate how necessary it is or the total impact of not using premium. So I will error on the side of caution following the factory recommendations and enjoy the ride when I might otherwise worry. If I want to feel the power I can and I can climb a steep hills without having to reduce speed going up it without concern.
I am older and not a heavy footed driver, I drive for economy most of the
time. I still however find times when I want to use my car close to it's design potential.
Good luck to you all!!!!
Last edited by mindmachin; 03-19-08 at 08:13 AM.
03-19-08, 08:23 AM
#19
Lexus Champion
Very well put into perspective my friend.And comming from a lad such as yourself,with your engineering backround,I will more than likely (even more so now)stay with the premium. Thank you.
05-13-08, 08:04 PM
#24
I have been using 87 since I purchased my 2008 RX 400h in December , 2007. I have been getting a consistent 22.8 MPG. Then last week I added 93 octane to about a half a tank of 87 and my display immediately jumped to 28-29 at highway speed (never saw it that high before at a steady 65 MPH) and my average has jumped about 1.5 MPG and this is over 4,000 miles. This means that my MPG really improved. I'll keep using 91+ as I think the payback is now there with $4.00 gas ($0.20 is just 5% of $4.00 and 3 MPG is about 10% so...
05-29-08, 07:28 PM
#26
I was using 91 since I bought the car in Dec. 07. I have been getting around 24 mpg (mostly highway) I recently started using 87 and am getting a steady 25 mpg. I haven't noticed any difference in performance, so I guess I will stick with the 87 octane...
05-29-08, 08:05 PM
#27
Lexus Test Driver
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I recently put in Shell V-power 93 octane in when I was in Michigan found no difference 25.5 MPG at all now today I put in Shell V-power 93 octane in Pa. I am getting 28.5 on a gallon.
05-31-08, 08:09 PM
#29
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Very interesting but I headed west from NJ to Indiana/Michigan I left with a full tank of Gulf 87 octane got 23.? MPG I blamed it on speed 75 to 85 MPH and hills next tank was 25 My friend needed gas and I filled up with them at a Shell station 87 I got 26 MPG OK so I put in next time Shell V-power not so great tried it later 26.? MPG in PA coming back I used the Shell V-power again and got 28.4 MPG IMHO some areas are getting cheaper quality fuel than others.
But hey over here they closed a station selling regular as super but the stations that sold Diesel as gas are OK.
But hey over here they closed a station selling regular as super but the stations that sold Diesel as gas are OK.
06-02-08, 07:59 AM
#30
Lexus Champion
Very interesting but I headed west from NJ to Indiana/Michigan I left with a full tank of Gulf 87 octane got 23.? MPG I blamed it on speed 75 to 85 MPH and hills next tank was 25 My friend needed gas and I filled up with them at a Shell station 87 I got 26 MPG OK so I put in next time Shell V-power not so great tried it later 26.? MPG in PA coming back I used the Shell V-power again and got 28.4 MPG IMHO some areas are getting cheaper quality fuel than others.
But hey over here they closed a station selling regular as super but the stations that sold Diesel as gas are OK.
But hey over here they closed a station selling regular as super but the stations that sold Diesel as gas are OK.