19psi

See this topic raised all the time and I thought maybe I could explain it so everyone will understand what is going on. Maybe a moderator could sticky it?

PLEASE take your time and read this thoroughly, even twice if you have to. I realize it's quite a bit to take in for some people without a performance background.


WOT= Wide Open Throttle
TDC= Top Dead Center (Where the piston is at it's maximum height during the compression stroke)

I'm going to use 87 as regular, and 93 as premium. I realize that in different parts of the country there are different ranges, so you'll have to apply it to what you have.
First of all, octane is the measure of the fuel's resistance to burning during the combustion process in the cylinder.
So 87 burns more easily than 89 which burns more easily than 93.

The only reason for high octane is to make maximum horsepower and torque because of it's resistance to pre ignition, otherwise known as knock, or pinging.

Our engines make the most power at high rpms, close to redline, and obvoiusly at Wide Open Throttle (WOT). Under those conditions, pre ignition will occur with low octane fuel, which will be detected by the knock sensor. The ecu(computer) will then retard timing, preventing pre ignition, but also holding back Horse Power and Torque.

The main reason we see timing pulled on a high octane rated engine is because of compression. Most Lexus engines are high compression. The high compression requires that the fuel not "blow up" before it is scheduled to, because the compression itself will cause the fuel to want to burn, even without a spark. Low octane fuel will start to burn too far before the piston reaches TDC. This produces knock or more properly pre ignitin. If allowed to burn uncontrollably, this will stress the piston and rod to the point of failure, as the rod is being phyically forced UP by the crank , while the combustion reaction is trying to force it DOWN. This is how we see rods sticking out of the side of the block.
As a safeguard, the knock sensor sees this pre ignition and informs the ecu. The ecu responds by retarding ignition timing; making the spark occur later in the compression stroke. This will prevent the low octane fuel from burning ahead of time, saving the engine from damage while restricting power.
Over the course of a few drive cycles, the ecu see the continued pre ignition, and places the timing map on a permanent retarded state so that it won't be continually pulling timing from the high performance map.
If you then start using premium, the ecu will see the lack of knock, and slowly start giving timing back. Or if you know you will be using 93 exclusively again, you could reset the ecu which will start out again on the high performance map. (Resetting of the ECU is accomplished by pulling the EFI Fuse in the engine bay)

For maximum power, we want the spark and combustion process to occur at a precise moment before the piston reaches TDC. This is what is known as ignition timing. Too far, and we pre ignite. Too late, and we lose power and torque.

To is it ok to use 87? Yes, if you just cruise along all the time and granny drive, then you can use 87 all the time with no ill effects. If you don't go full throttle to redline regularly, you'll be fine. Just don't expect maximum power if you one day need to floor the throttle to get out of the way of a semi truck.

ewenpg

Nice to know, thanks!

MoJoMan

It is my understanding that NOT USING premium will cause a predetination problem which may lead to long term damage to the engine. I choose to spend the extra $15 per month and give the car what it deserves. Besides, my 94 only has 225 horses so it needs all the help it can get.

dfkd

Wll as the original post explains, the ecu will advance the timing to prevent that predetination and thus there should be no long term damage.

Also the 94 should clock in at 250hp / 260lbft (though as I remember only about 180hp gets to the wheels).

GKLCPA

An excellent and clear explanation. I ran 87 octane over the past couple of weeks and switched back to 93 last week. There was a noticeable difference in power between the two. I'm not necessarily a speed demon, but the car runs a lot better on premium IMO.

19psi

this is true, but read my explanation again and you'll understand the rest of the story.

007LS400

I have no technical reasoning, but I use what is recommended on the gauge cluster; Premium fuel only, not Premium fuel maybe. Why drive around on eggshells waiting for something to go wrong?

I think the Lexus engineers know what they are doing, but who knows.

Saving a few dollars at the pump will cost you hundreds, if not thousands down the road in unnecessary repairs.

jcrome04

It's about time you get this thing written up! haha jk!

Great info!

Now I hope all the noobs stop getting so excited over 87 lol

XareeLex

IMHO it comes down to this.
My LS400 fills up from empty for about 18 gallons. I have found a gas station very close to my house that retails premium for about 8cents more than regular(87).
18(gallons)X8cents= $1.44
The difference in price for me, per gas tank is about $1.50
Therefore, for a $1.50, I will definitely drive my car per manufacturers requirement and fill it up with premium fuel. Like someone said, I am sure those Lexus technicians did not just sit one day and decide to "punish" us by recommending PREMIUM FUEL ONLY. There is a reasoning behind it and I will stand by their word.

That ladies and gentlemen, is my humble opinion and ideed it is not intended to underscore the research done above, its just my POV.

heyhowyado

good stuff....thanks for posting!

Celsior400

iTrader: (1)

You drive a lexus... quit being such a penny pincher...


I don't understand the reason behind driving a luxury and being cheap?

Neofate

iTrader: (1)

Good answer, and yes you are right the ECU will self correct to prevent damage, via timing control.

However, just to be technical -- The engine decreases the timing in such a situation, not increases

I agree, with your last statement, using premium as is recommended, is just , generally, the best route to go. We need to suck it up and pay the extra 3-5$ a tank vs 87 octane.

If you do use 87 octane, it wouldn't be a bad idea to buy a bottle of octane booster in the gas station (usually about a dollar and change) to add to the low octane gas. (This would somewhat offset the difference to a degree , for a cheap price).

Neofate

iTrader: (1)

Very well put! My sentiments exactly.

The reason is in the original post. Lexus engineers built this engine to utilize high compression, as such it needs a high octane fuel to operate AS designed. It isn't a 'pre-caution' .. Premium fuel IS, absolutely, needed for the engine to operate as designed.

I too agree that go ahead and pay the little extra for premium on fill up, as it has many benefits.

A) Your car will have the power it was built to have.

B) Your fuel system might stay a little cleaner than if you used 87.

C) It makes you feel better inside

D) Cars DO have personalities, when you treat them well, they start behaving better to reward you. (Call me crazy, but I have always noticed this with all my vehicles) hehe..

The list goes on and on.. but for the price difference, it is really a no brainer.

Also : I second the notion that this should be stickied!

Dutch

ecu RETARDS timing, please dont go around explaining to your friends why they should use xx octane b/c if they dont their engine will advance timing...

Neofate

iTrader: (1)

Heh, I already mentioned that mistake. I'm sure it was just a slip.. I was trying to be nice about it though. -- The basic principle and point was correct in his/her response.

If I came across as a **** in my correction I apologize dfkd.