Mercruiser MPi fuelling self adjustment

[QBhoy]

Hi all...one for the guys that know more than I on the matter...

Although I had strongly suspected that the mpi small blocks were able to adjust timing and fuelling to adapt to different octane ratings in fuel, only today I read that they actually do.
I had wondered this because occasionally I go between regular 95 and 97/98 and even sometimes 99 fuel. Just depending on where I get it and time of year.
although I’m well aware there is no need for it and these things are designed to run on even lower grades than I have at the pumps here, I tend to run some higher fuel through it just before winter storage and sometimes through the season. Without doubt it runs smoother with the good stuff, but anyway that’s another topic.

what I would like to know is how long it takes for the ecu to adjust to a higher or lower rating of fuel (all things being considered and for talking sake if they were never mixed, which most times they are). So if I had 50l of 95 in the tank then completely ran it dry...then put 50l of 99 in it. Would the ecu adjust instantly to the fuel ? Or would it take a while ? If so, what determines such a thing ?

As mentioned, I’m aware it’s not required and probably not something that we need to discuss in those terms, but would just like to know the workings of it.

Thanks

 

[Bondo]

what I would like to know is how long it takes for the ecu to adjust to a higher or lower rating of fuel

Ayuh,..... I believe you'll find it happens several times a second,......

 

[alldodge]

While high octane will not hurt the motor, it can reduce amount of available power. The lower octane fuel burns faster and thereby being able to release all the power contained within. Higher fuel burns slower and may not get a complete fuel burn in enough time to get it all

 

[GA_Boater]

All ECUs make adjustments based on time. X number of occurrences over y amount of time. Without a lab full of gear to measure octane, most likely the knock sensor is the prime input for X.

Look at it like this - If you get a engine check light and fix the problem, it takes a predetermined number of engine start/stop cycles to turn the light off unless a scan tool resets the ECU. Knock sensing would be a shorter time, but still not instantaneous.

 

[achris]

...what I would like to know is how long it takes for the ecu to adjust to a higher or lower rating of fuel..

For any given engine speed (rpm) and load (MAP sensor value) the timing will be set at a particular advance, then (within a few milliseconds) the ECU will start advancing the timing until it detects a knock, it then retards the timing very slightly. This cycle is continuous so any change in fuel quality, engine speed or engine load will be adjusted for immediately (or within a few revolutions of the engine).

Chris......

 

[QBhoy]

Brilliant guys. So to go on from there...if I have a mix of regular and super gas...and occasionally dozed the fuel with a treatment that is known for apparently boosting octane, could the timing be a little confused from one minute to the next ?

 

[racerone]

The computer adjusts things going by a number of inputs.-------Engine temperature / air temperature / air flow / throttle position / crank position sensor / oxygen content in exhaust etc, etc.-----Adjust injectors nearly instantly, faster than you can think.----Injector opening time is measured in micro seconds.----Constantly adjusted all done in the blink of an eye.

 

[GA_Boater]

Brilliant guys. So to go on from there...if I have a mix of regular and super gas...and occasionally dozed the fuel with a treatment that is known for apparently boosting octane, could the timing be a little confused from one minute to the next ?

If the octane booster isn't mixing with the gas, you better find another product. :smile:

 

[achris]

Brilliant guys. So to go on from there...if I have a mix of regular and super gas...and occasionally dozed the fuel with a treatment that is known for apparently boosting octane, could the timing be a little confused from one minute to the next ?

Read my post above! (and everyone else's)...

Machines don't get 'confused', that's a human quality. Machines respond to inputs and programming. If the fuel quality/resistance to knocking changes from one second to the next, the ECU will deal with it.

And why are you mixing fuels? Your owner's handbook will state the required RON number of the fuel. Put that in and stick with it. Putting a higher octane in will just cost you more money. It won't make your engine run better, or cleaner or faster.... And 'additives' are just snake oil design specifically to extract $$$ from your wallet and deposit into someone else's... Just put the petrol in and drive the thing. No boosters, and cleaners or magic fairy dust required. All I would add to straight petrol is, maybe, a bottle of meths if I suspected water in the tank.

....----Constantly adjusted all done in the blink of an eye.

And this is why MPI engines beat points and carbs all day, every day. And MPI engines (all of them, not just marine) are tuning themselves hundreds of times a SECOND! An MPI engine never gets out of tune, EVER! And if you do get a belly full of bad fuel, the MPI engine will compensate by retarding the timing. A points and carb engine will just blow a nice hole in a piston or 3.

Chris..........

EDIT: And this is not a repair topic, I'm moving it out of the repair forum.

 

[QBhoy]

If the octane booster isn't mixing with the gas, you better find another product. :smile:

I’m sure it is...but with the amount I use my boat..it’s not a case of filling up then running it down then filling up again. It’s done by what I’ll need each time in Jerry cans. So 20/30/40 litres at a time.

 

[QBhoy]

FFS! :facepalm:

Read my post above! (and everyone else's)...

Machines don't get 'confused', that's a human quality. Machines respond to inputs and programming. If the fuel quality/resistance to knocking changes from one second to the next, the ECU will deal with it.

And why are you mixing fuels? Your owner's handbook will state the required RON number of the fuel. Put that in and stick with it. Putting a higher octane in will just cost you more money. It won't make your engine run better, or cleaner or faster.... And 'additives' are just snake oil design specifically to extract $$$ from your wallet and deposit into someone else's... Just put the petrol in and drive the thing. No boosters, and cleaners or magic fairy dust required. All I would add to straight petrol is, maybe, a bottle of meths if I suspected water in the tank.



And this is why MPI engines beat points and carbs all day, every day. And MPI engines (all of them, not just marine) are tuning themselves hundreds of times a SECOND! An MPI engine never gets out of tune, EVER! And if you do get a belly full of bad fuel, the MPI engine will compensate by retarding the timing. A points and carb engine will just blow a nice hole in a piston or 3.

Chris..........

EDIT: And this is not a repair topic, I'm moving it out of the repair forum.

Jeez. Sorry I asked then.

I run different fuels because of the reason stated above.
the fuels here in the U.K. are differently rated than over there. Totally different thing really. The maim reason is usually at end of season before storage.
Im not to well versed on the ecu workings of these. Can pull the rest of it to bits no bother, just not an ecu guy really.
forget moving it. Just delete it. If you can’t be bothered with such trivial things.

 

[achris]

... If you can’t be bothered with such trivial things.

It's not about 'not being bothered'. It's about having to explain it 10 different ways. You got told at least twice that the ECU adjusts within millisecs, then asked if the timing would get 'confused'...

Chris.......

 

[Scott Danforth]

No boosters, and cleaners or magic fairy dust required.

I like magic fairy dust ......

however as Chris and everyone else mentioned..... no need to go over the minimum octane rating. the motor is not tuned to take advantage of it more than a minor timing adjustment.

 

[alldodge]

:baby:op2::grouphug:

 

[achris]

.....
the fuels here in the U.K. are differently rated than over there. Totally different thing really. ...

Just had a chance to check. Fuel octane reporting in the UK is exactly the same as here in Australia, which is the same as the US. RON, research octane number, is the same standard worldwide. Use 91 (as per the owner's handbook) and you'll be fine... No need for any boosters... Waste of time and money, even when winterizing.

 

[QBhoy]

Just had a chance to check. Fuel octane reporting in the UK is exactly the same as here in Australia, which is the same as the US. RON, research octane number, is the same standard worldwide. Use 91 (as per the owner's handbook) and you'll be fine... No need for any boosters... Waste of time and money, even when winterizing.

It’s in Ron here. But if a different rating

 

[QBhoy]

Just had a chance to check. Fuel octane reporting in the UK is exactly the same as here in Australia, which is the same as the US. RON, research octane number, is the same standard worldwide. Use 91 (as per the owner's handbook) and you'll be fine... No need for any boosters... Waste of time and money, even when winterizing.

Chris it is categorically not the same rated system at the pumps. There is no such fuel as 91 here.
95 then 97-99 super.

 

[Scott Danforth]

US uses (R+M)/2 method

however 87 octane on the (R+M)/2 method is the same fuel as 91R in the UK...... its the low-grade pump swill in both countries........ also the minimum swill the boat is designed to run on.

stole this from here http://www.pencilgeek.org/2009/05/oc...nversions.html

Actual octane rating table

EURO RON	MON	US: (R+M)/2
90	83	86.5
92	85	88.5
95	87	91
96	88	92
98	90	94
100	91.5	95.75
105	95	100
110	99	104.5

Formula for converting EURO to US rating
I've also come up with a pretty good formula that approximates US octane rating based on EURO RON rating, and visa versa. The formula is US=RON*(21/22):

EURO RON	US (R+M)/2
90	85.9
92	87.8
95	90.7
96	91.6
98	93.5
100	95.5
105	100.2
110	105

Formula for converting US to EURO octane
And reversing that forumula, wet get: RON = US*(22/21)

US (R+M)/2	EURO RON
87	91.1
88	92.2
89	93.2
90	94.3
91	95.3
92	96.4
93	97.4
94	98.5
95	99.5
96	100.6
97	101.6
98	102.7
99	103.7
100	104.8

 

[achris]

Chris it is categorically not the same rated system at the pumps. There is no such fuel as 91 here. 95 then 97-99 super.

Yep, same rating, RON, as here. The US post on the pumps the average of RON and MON (motor octane number). It can't be different, because the RON and MON are STANDARDIZED ratings, standardized worldwide.

This is a clip from a document going into a lot more detail if you are interested, but this is the important line....

[FONT=Arial,Helvetica,Geneva,Swiss,SunSans-Regular]Octane rating number is defined as a value used to indicate the resistance of a motor fuel to knock. Octane numbers are based on a scale on which isooctane is 100 (minimal knock) and heptane is 0 (bad knock). A gasoline with an octane number of 92 has the same knock as a mixture of 92% isooctane and 8% heptane.

From that you can understand that it doesn't matter where you are in the world, if the pump says 'RON' it's the same RON as anywhere else in the world. (BTW, I have seen 76RON in some parts of the world. )

Read the full document -> [/FONT]http://www.csgnetwork.com/octaneratecalc.html

Also, read this article from the UK on exactly what fuels you have and what numbers you see on the pumps.... (You don't have 91 RON, so use 95...)

https://www.simplemotoring.co.uk/car-fuel-types/ (and your Mercruiser engine isn't much more than a car engine at the end of the day. A few safety features and a milder cam and that's pretty much it....)

In Aust, we have 91, 95 and 98. Standard, Super and Premium. It appears in the UK you only have Super and Premium. But the 95 you have is the same as the 95 I have.

Here's a clip from the Owner's handbook for your Merc engine....



Chris......

 

[QBhoy]

Yep, same rating, RON, as here. The US post on the pumps the average of RON and MON (motor octane number). It can't be different, because the RON and MON are STANDARDIZED ratings, standardized worldwide.

This is a clip from a document going into a lot more detail if you are interested, but this is the important line....



From that you can understand that it doesn't matter where you are in the world, if the pump says 'RON' it's the same RON as anywhere else in the world. (BTW, I have seen 76RON in some parts of the world. )

Read the full document -> [/FONT]http://www.csgnetwork.com/octaneratecalc.html

Also, read this article from the UK on exactly what fuels you have and what numbers you see on the pumps.... (You don't have 91 RON, so use 95...)

https://www.simplemotoring.co.uk/car-fuel-types/ (and your Mercruiser engine isn't much more than a car engine at the end of the day. A few safety features and a milder cam and that's pretty much it....)

In Aust, we have 91, 95 and 98. Standard, Super and Premium. It appears in the UK you only have Super and Premium. But the 95 you have is the same as the 95 I have.

Here's a clip from the Owner's handbook for your Merc engine....



Chris......

Chris. I’m well aware what RON is. Then some...I’m just telling you we don’t have 91 at the pumps. End of story.
Read above. I only said it’s different rated at our pumps. Pretty sure I didn’t say the Ron was. Think I’m about done with this...