Originally Posted by mindmachin

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!!!!

Originally Posted by prowler97

When I get my wife a 400h I plan to use 87 octane. The Toyota Highlander Hybrid reccomends using 87 octane and that has the same engine as the 400h. I think Lexus reccomends "premium" gas because it's their premium brand!

Brian Richardson
Jupiter, FL
1997 Plymouth Prowler with Trailer
2006 Mercedes Benz C230 with 6-spd
2007 Lexus RX 400h (soon)