Can anyone tell me why a GDI needs 98 fuel to avoid the carbon problem! I can't get my head around the excess carbon problem being caused just by low octane fuel.I can understand octane making a difference to power and smoothness etc but why does it cause problems in the intake system!
franc123,
Aug 29, 4:21am
Apparantly because it burns cleaner which causes less deposits from the EGR system. The increased octane has little if any affect on GDI's.
thejazzpianoma,
Aug 29, 4:23am
In simple terms, it dosn't combust properly because the engine is not designed for it.
I can give you the long winded (even by Jazz standards, pun intended) speel but really that's what it boils down to.
Aside from that running 91 in a direct injection car is completely pointless because you are going to lose significantly more in economy than the saving in fuel cost.
thejazzpianoma,
Aug 29, 4:24am
Nope, the increased octane has a huge impact on any direct injection engine, it actually needs it to function as designed. The high octane fuel only burns cleaner because the engine is operating as designed.
thejazzpianoma,
Aug 29, 4:29am
I should point something out though to make it clear. In general with direct injection engines running 91 is what causes the real carbon issues.
95 won't generally soot them up too much but your economy is usually poo compared to 98. This is because the car will struggle to get into its fuel stratification mode which is really the point of direct injection and where you get the economy from.
The early GDI's from memory try and force themselves to run in FSI mode pretty much all of the time.I suspect is why they are so much worse for carboning up than other manufacturers designs. I am not 100% certain on this aspect though.
monaro17,
Aug 29, 4:32am
Genuine question. What about the new SIDI direct injection commodores! The recommended fuel is 91 octane
thejazzpianoma,
Aug 29, 4:36am
I have heard of low octane direct injection before in other vehicles too. Clearly its possible, you could certainly design it to function on 91 but I can't see how you could do it without losing most of the advantage.
Sorry I don't know how often the Commodore actually gets into a proper fuel stratification mode if at all. It may have some strategy to help it that I am unaware of I know pretty much nothing about that engine.
thejazzpianoma,
Aug 29, 4:45am
Genuine question back at you monaro. Where did you get the info about them running on low octane from!
I am wondering whether they are saying they can be run on 91 (as a selling point) but quietly in the background the recommended octane is actually 95 or 98.
I am not saying that is the case but if it is it would make a lot of sense. You see the newer direct injection engines as explained above will only turn on the FSI mode when the conditions allow for it. So it would be easy and fair to produce such an engine, say its fine to run on 91 (which technically it would be as it would do no harm) while actually specifying 98 as the recommended fuel. Of course it would only have the stated economy advantages when running the 98.
Just a thought as I am seeing lots of similar sneaky marketing from lots of manufacturers at the moment.
monaro17,
Aug 29, 4:50am
Manufacturer says recommended fuel is 91. Then again it later says peak power and torque achieved with premium unleaded. So yes perhaps you are right. Interesting none the less
v8_eta,
Aug 29, 4:53am
Injectors can play up using 91. Fuel pump problems. Dont let car run on empty. Lowest i would go is 95. Car runs and sounds better on 98. idles better as well
thejazzpianoma,
Aug 29, 4:55am
Hmmm, the clincher would be whether the stated economy is also achieved with premium unleaded.
FSI mode won't do anything for power/torque, that will be measured while giving it the boot and FSI will be disengaged I would think.
It highlights that they are not adverse to using this sort of marketing so makes you think.
At the end of the day, unless they have some technology I have not seen or considered as a concept the physics of the situation says you need 98 to get an FSI capable engine to perform near what its capable of.
thejazzpianoma,
Aug 29, 4:57am
This would tend to support my theory that the early GDI's are trying to run in FSI mode all the time. It all fits when you think about it. That being the case even 95 may still be causing some avoidable carbon buildup.
v8_eta,
Aug 29, 5:05am
I dont doubt that. just saying that some weeks ya maybe a little strapped for cash. so wouldnt go lower than 95.
franc123,
Aug 29, 5:07am
As I said earlier, its because of running large amounts of EGR, this was partially solved on later engines buy using a smooth bakelite manifold instead of a rough cast alloy one. If the manufacturer recommends 91 there is no real benefit to be had from going higher, some slight fuel economy gains may be noted but thats all.
thejazzpianoma,
Aug 29, 5:12am
No offense intended here franc123 but I really don't think you understand what fsi does/is. There is some merit in varying the EGR but that is more secondary or ambulance at the bottom of the cliff. The rest of your post regarding the economy makes no real sense at all given that running 91 proper FSI isn't even possible.
m16d,
Aug 29, 5:13am
To answer the OP.our GDI pings on 91 so thats why we run it on 95
phillip.weston,
Aug 29, 5:24am
The Mitsubishi GDI manual itself states running on 91 octane is OK but not preferred. It's not so much the lower octane which is the issue, it's that the lower octane fuel is generally lower quality fuel.
thejazzpianoma,
Aug 29, 5:27am
What they say when they print the manual and what they find out down the track can be different things though. Not having a go at Mitsubishi on that one, I think pretty much all manufacturers have been caught out with that sort of thing.
I disagree on the quality of the fuel issue, but am open to suggestion. I know direct injection technology pretty well. but you know a LOT more about Mitsubishi's than me.
rovercitroen,
Aug 29, 5:52am
I am often amazed when buying a car that lots of people are fixated on whether it will run on 91 or not. I believe that if Holden in NZ said the Commodore had to use high octane fuel it would cost them lots of sales, especially to fleets. Regardless of the fact that the SIDI engine is probably more economical and better performing on 95 octane or higher.
thejazzpianoma,
Aug 29, 5:54am
+1 You are exactly right. To put it bluntly, people are stupid and everyone bends over backwards to cater to the stupidity instead of what works best.
I suspect you are quite correct that Holden have compromised the real running costs significantly for this very reason.
edangus,
Aug 29, 2:33pm
Ok. Can I please have an opinion or two! I am looking at buying an rvr and am not sure if it is just a bit sluggish 2.0 or suffering from carbon build up. Any way to tell! If so whats the fix! New head! Cheers
splinter67,
Aug 29, 2:40pm
My mum has an rvr nice to drive her one has plenty grunt for a four cylinder but you will need a small fortune to run it quite expensive on gas
gadgit3,
Aug 29, 2:50pm
From what I have learnt about GDI and Toyota D4 engines it's not so much about the fuel used but more about the quality of oil used. Low ash full syn oils will leave alot less carbon deposits while using heavy EGR % and lean burn when compaired to a cheap mineral oil. And the direct injected diesels are worse.
sred69,
Aug 29, 3:02pm
The engine oil used will have more of a effect on the carbon build up on the back of the valves and inlet manifold than 91 octane. Always use a synthetic oil and the best fuel you can and not that you won't get a build up but it prolongs the time between any work needed on the intake system.
smac,
Aug 29, 3:09pm
I've just proved the opposite of this with a Falcon - Ford say recommended fuel is "91 octane or higher". Yet on 95 it is actually cheaper to run. I think as mentioned above the marketing department have more swap than the engineers. The ad guy would have asked the enginner guy if it runs on 91 ok. The engineer would have said "yeah, but not as well.". The ad guy would have got all excited and run out of the office before heard the rest.
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