Direct Correlation Between Fuel Type And Transmission Stress

[LexKid630]

Whoever says that there's no difference between higher octane and low octane gasolines is WRONG. Even though the rx300 calls for 87, it runs MUCH better with premium. I have owned rx300's since 99 and have always used either the mid-grade or premium/ultra gas. My daughter filled it up with the 87 last week and didn't tell me. I drove it and noticed a HUGE difference in power, engine revs and transmission feel. I came home thinking there was something wrong with the car! Since i filled it up with premium yesterday, everything's back to normal.

This made me think! With the lower octane gasoline in the engine, it seemed like the transmission didn't catch into each gear as quicly and smoothly as it did on premium. Not only does the engine have to work much harder, but the transmission must as well, right?

Just trying to think of something else to help prevent these failures... Anyone have any input? And be brutal, i have no gas octane sensitivity :P

[wwest]

Whoever says that there's no difference between higher octane and low octane gasolines is WRONG. Even though the rx300 calls for 87, it runs MUCH better with premium. I have owned rx300's since 99 and have always used either the mid-grade or premium/ultra gas. My daughter filled it up with the 87 last week and didn't tell me. I drove it and noticed a HUGE difference in power, engine revs and transmission feel. I came home thinking there was something wrong with the car! Since i filled it up with premium yesterday, everything's back to normal.

This made me think! With the lower octane gasoline in the engine, it seemed like the transmission didn't catch into each gear as quicly and smoothly as it did on premium. Not only does the engine have to work much harder, but the transmission must as well, right?

Just trying to think of something else to help prevent these failures... Anyone have any input? And be brutal, i have no gas octane sensitivity :P

If an engine is designed from the get-go to take advantage of the higher octane of premium fuels it almost always has a higher compression ratio as the primary difference between it and engines that are designed to use regular grade fuels. If you fuel it with a low octane fuel the engine ECU will simply enrich the mixture in order to keep the high compression engine operating optimatelycorrectly.

On the other hand if the engine is designed for low grade fuel to begin with what can be done to take advantage of preminum fuels?

Reduce, lean out, the A/F mixture ratio..??

It seems to me that absent that higher compression ratio using premium fuels can have no advantage.

[blk_on_blk]

Unless you're going to run a turbo or something to bump up the pressure in the combustion chamber, you're not going to be able to take advantage of premium fuels. If you saw a negative difference with 87, then you might have gotten some bad gas at that station. The most benefit using higher octane on a lower compression engine will be is if you have a problem with detonation; switching to a higher octane will reduce the predetonation/knocking/pinging... that's about it. So, if you have a noticeable difference in running premium verses 87, then you either got some bad/contaminated gas, or you've got something up in the engine that needs addressing. You might want to pull a front spark plug and check for carbon build up. You can always run Seafoam or other combustion chamber cleaner and see if you notice a difference.

[TunedRX300]

So you are saying the transformer outside of your house blew because coal was burned to generate electricity instead of nuclear?

[wwest]

So you are saying the transformer outside of your house blew because coal was burned to generate electricity instead of nuclear?

I wonder how much the compression ratio changes if I use a "cold" running spark plug vs a "HOT" one....??

[Ericok]

When using regular gas in an engine designed for premium, the ECU doesn't enrich the mixture - rather the knock sensor tells the ECU to !Removed! the timing. If your gas is crappy enough, no amount of retarded timing will help and the engine will ping, knock, or detonate because the ECU has a limit on the amount of retarded ignition that can be applied.

[tmastres]

I'll vote for the bad gas theory on this one. At the very most I'd say that 91 gas might improve top-end performance, but that 87 would not make the car run crappy. I run 87 in mine all the time and it runs fine at 104k mi.. Its all in yer head.....

[wwest]

When using regular gas in an engine designed for premium, the ECU doesn't enrich the mixture - rather the knock sensor tells the ECU to !Removed! the timing. If your gas is crappy enough, no amount of retarded timing will help and the engine will ping, knock, or detonate because the ECU has a limit on the amount of retarded ignition that can be applied.

"no amount of retarded timing will help..."

Yes, and that's exactly why most most modern day EFI system's will and do enrich the mixture.

You are correct, retarding the timing will help... provided the knock/ping is due to timing being too far advanced.

But putting a too low grade fuel in a high compression engine is more likely to result in dieseling, is it not?

Chicken or egg...?

Knock/ping, !Removed! the timing....Oops, that didn't help, knock/ping must have been due to dieseling.....

I suppose the systems are now smart enough that if they detect knock/ping co-incident with, or just after ignition spark....

[Ericok]

When using regular gas in an engine designed for premium, the ECU doesn't enrich the mixture - rather the knock sensor tells the ECU to !Removed! the timing. If your gas is crappy enough, no amount of retarded timing will help and the engine will ping, knock, or detonate because the ECU has a limit on the amount of retarded ignition that can be applied.

Yes, and that's exactly why most most modern day EFI system's will and do enrich the mixture.

You are correct, retarding the timing will help... provided the knock/ping is due to timing being too far advanced.

But putting a too low grade fuel in a high compression engine is more likely to result in dieseling, is it not?

Knock/ping, !Removed! the timing....Oops, that didn't help, knock/ping must have been due to dieseling.....

I suppose the systems are now smart enough that if they detect knock/ping co-incident with, or just after ignition spark....

Shouldn't have dieseling with a fuel injected engine (unless one of the injectors is leaking) since the ECU is able, via the injectors, to shut off fuel flow completely - so in a properly running engine there won't be any extra fuel to sustain dieseling.

Even if the ECU enriched the mixture, it's ability is severely limited because of emissions control - particularly since this engine was designed to meet California Low Emission Vehicle standards.

This from the Lexus.com FAQ:

"Use of lower grade fuel will typically cause some degree of improper combustion, which results in engine knock. When the engine management system detects knock, such as with the use of lower grade fuel, the ignition timing will be retarded. While retarding ignition timing helps to minimize knock, it also reduces available performance and fuel economy....Regular grade fuel may be your best option, if, based on your driving conditions and driving style, you don't notice a difference in performance or economy between regular and premium."

[wwest]

When using regular gas in an engine designed for premium, the ECU doesn't enrich the mixture - rather the knock sensor tells the ECU to !Removed! the timing. If your gas is crappy enough, no amount of retarded timing will help and the engine will ping, knock, or detonate because the ECU has a limit on the amount of retarded ignition that can be applied.

Yes, and that's exactly why most most modern day EFI system's will and do enrich the mixture.

You are correct, retarding the timing will help... provided the knock/ping is due to timing being too far advanced.

But putting a too low grade fuel in a high compression engine is more likely to result in dieseling, is it not?

Knock/ping, !Removed! the timing....Oops, that didn't help, knock/ping must have been due to dieseling.....

I suppose the systems are now smart enough that if they detect knock/ping co-incident with, or just after ignition spark....

Shouldn't have dieseling with a fuel injected engine (unless one of the injectors is leaking) since the ECU is able, via the injectors, to shut off fuel flow completely - so in a properly running engine there won't be any extra fuel to sustain dieseling.

Even if the ECU enriched the mixture, it's ability is severely limited because of emissions control - particularly since this engine was designed to meet California Low Emission Vehicle standards.

This from the Lexus.com FAQ:

"Use of lower grade fuel will typically cause some degree of improper combustion, which results in engine knock. When the engine management system detects knock, such as with the use of lower grade fuel, the ignition timing will be retarded. While retarding ignition timing helps to minimize knock, it also reduces available performance and fuel economy....Regular grade fuel may be your best option, if, based on your driving conditions and driving style, you don't notice a difference in performance or economy between regular and premium."

I wasn't discussing throttle closed "run-after" dieseling, that went away way back in the stone ages (carburation) with the use of idle solenoids to completely close the throttle. Nor at idle or normal cruise which is when the mixture is controlled for EXACTING emissions level by the oxygen sensor signal. At the same time the MAF sensor signal is being calibrated using the oxygen sensor as reference. When you put the engine under load, acceleration, uphill, the A/F mixture is enriched, >12:1, beyond the sensing range of the oxygen sensor.

And then's when you're most likely to get dieseling due to too low fuel octane for the HIGH compression ratio which is correctable via EFI fuel enrichment.