COSPEED

Ahem....Dealer has no clue. Take these diagrams back to him and ask him politely to explain just how top tier or the cheapest is going to have ANY effect on these engines? Only octane matter now that all GM gasoline engines are GDI and fuel never touches any part of the valves or intake tract, and as the injectors now operate at 2000 PSI plus, there is little chance for a fuel injector to build any deposits. And, as the fuel is not introduced until the final 10% or so of the compression stroke, it is only present for milliseconds and the vast majority has already completed the burn before it touches the piston top or any other part of the combustion chamber. I really laugh at the new Shell commercials where they make all these false claims on how it "cleans" your engine! Only port injection engines benefit (and they do for sure) from running top tier fuel as the fuel is constantly spraying the intake valves keeping them cool and clean and deposit free:

Traditional port injection that keeps valve deposit free for 100-200k plus miles:


Today's GDI showing fuel never touches the valves any more so they run far higher temps and bake on these hard abrasive deposits:



GDI valve deposits at 20-30k miles running top tier fuel:



New GM GDI at 5k miles:


LS1 with 142,000 miles and rarely using top tier fuel, but always premium (my personal C5 for years):


New C7 with 5500 miles on it. Note if you pause the video in the beginning and look close at how the deposits are wearing the valve guides already as they cycle in these hard abrasive deposits every cycle. This cleaning method works, but I much prefer the crushed walnut shell media method:


And last, a 3 study dynoing every year at app the same time and weather each year on same dyno showing how this late model GDI car (not a GM) degraded in power and the last run right after manually cleaning the valves (do NOT use a solvent based upper induction cleaner on these hard deposits as engine damage can/will occur unlike port injection engines where they are beneficial). Note, the car had app 15k miles on it before the first dyno, and the driver puts app 15k on a year:




So, anyone, dealer, factory, oil company telling you to use top tier only is just passing on marketing BS with GDI engines unlike the port injection engines where there was substantial benefits to running it. And I love to see a service manager explain when brought in the diagrams and pictures to explain how it makes any difference to any part of the engine or fuel delivery system. It is impossible as it would have to touch the components they claim it helps to actually do anything!

If anyone has any doubts on this, ask questions and present anything you have been told.

Getting educated on these new engines is going to be difficult, but it is critical if you want to care for these properly to understand and learn the facts as the industry certainly wants this kept quiet.

Lots of advantages of GDI I can list, but this is a substantial disadvantage.

And how to properly address these issue right from the start is also critical if you want a long trouble free life out of these engines. Ask me any questions and I am happy to answer anything technical about GDI and the GM engines specifically (I am an ex GM employee and an Automotive Engineer/member of SAE, etc. that specializes in GDI, so I am more than willing to share a lifetime of experience and industry knowledge for those that wish to learn what is fact, and what is BS).




Jason

Cheers!

COSPEED

He is referring to tax exempt red dye fuel, both diesel and gasoline. Totally unrelated to top tier or ethanol in anyway.

Ask any Farmer.

Zymurgy

Jason (coSPEED2), one area that I have seen top tier make a difference is the fuel gauge sender unit in the Corvette fuel tanks. There have been reports of problems with these units when using non-top tier gas. In some cases, the owners were able to get the sender working again by running a tank with Techron added. I believe the issue is sulfur compounds that the additives in top tier fuel apparently remedy. Can you speak to this?

gtf464s

So why use 93 octane gas over 91 octane when the manual calls for 91 octane? Isn't anything in excess of 91 a waste? I know using premium (higher octane) gas in my vehicle that calls for 87 octane actually yields decreased performance.

ratman6161

Sure
https://www.revisor.mn.gov/statutes/?id=239.791

See "Subd. 12. Exemption for collector vehicle and off-road use."

Yes, there are lots of stations with 91 octane ethanol free premium in Minnesota. I use it all the time in my lawn mower and snow blower and exclusively in my two stroke snowmobile. Boaters can also use it since those all fall into the "off road use" category.

So yes, there is lots of ethanol free gas to be found in Minnesota. Its just illegal to put it in an on-road vehicle unless it meets the definition of "collector vehicle".

The quoted statute says that the pumps have to be labeled:

"NONOXYGENATED GASOLINE. FOR USE IN COLLECTOR VEHICLES OR VEHICLES ELIGIBLE TO BE LICENSED AS COLLECTOR VEHICLES, OFF-ROAD VEHICLES, MOTORCYCLES, BOATS, SNOWMOBILES, OR SMALL ENGINES ONLY."

Edit: Missed the "motorcycles" when I first replied. But you still can't legally put in a car other than as specifically listed.

COSPEED

You are correct in this point (thanks!) as that is one area in the C5 and C6 where there have been tons of examples. With all 2014 and up GM vehicles GM claims they have corrected this in the redesign of the sending unit sender and contacts so this is no longer an issue. BUT time will tell!

great post on why it is good to use. I was purely referring to the intake valve coking issues and other issues that GDI engines have that top tier does not do a thing to help.


Take HP tuners or EFI live and log the amount of KR (detonation) occurring when you accelerate with 91 VS 93. And that will be an eye opener! You can run on 87 just fine as well, but far more KR takes place, and anytime the PCM is commanding timing to be pulled and fuel trims to enrich until the detonation ceases, you are loosing both power and MPG. You want the optimum advance possible for the most efficient burn possible. These are 11.5:1 plus CR engines now and any oil ingestion as well will cause detonation. The reason these can run this high of CR is no fuel resent until that final 10% or so of the comp stroke so little chance to detonate. Add oil mist and it still occurs.

Great posts and questions/comments!!! Excellent discussion when it is civil and intelligent.


I believe that pertains to the damage that is caused by oxygenated fuels to non hardened valve seat engines, but not living in MN I could be wrong. In every State I travel through non ethanol fuel is legal to all as it is far better on the engine than ethanol. And it takes more ethanol to burn and maintain proper A/F ratio so less fuel economy as well.

Zymurgy

Because the grades around here are 87, 89 or 93. Manual calls for 91 or higher. Given my choices, 93 is what meets the requirements.

LDB

I’m neither engine designer nor oil company representative, but I did spend a career (now retired) as a chemical engineer in refining for a major oil company. During that period, I had access to our engine testing program, which admittedly was all port injection, not direct injection (retired in 2010). Top tier clearly kept port injection engines cleaner as you agree in your post. And I would agree that direct injection makes it more difficult for gasoline additives to have a cleanliness impact. But I’m not quite ready to believe it’s a cut and dried, no impact whatsoever, as you imply. Don’t view that and the following statements as saying you are wrong. I am admitting up front that I have no direct experience with direct injection. View it as my trying to understand the situation.

First, let me ask about areas other than intake valves, namely, piston crowns, spark plugs, and injectors. You imply piston crowns and spark plugs are never touched by gasoline, but that seems like a stretch. If the gasoline isn’t mixed throughout the cylinder prior to ignition by the spark plug, then it seems like you’d have a rich area where the gas is, and a lean area where it isn’t, and combustion wouldn’t proceed smoothly. For that matter, if there no gasoline around the spark plug, how does the plug ignite the mixture? So it seems to me that the combustion chamber must be thoroughly mixed, with walls wetted, and if walls are wetted, then additives should have an impact. On injectors, yes, they operate at higher pressure than port injectors, but so do diesel injectors, and they foul. Seems like eddy deposits should form regardless of injector pressure, and they will eventually interfere with spray pattern unless additives keep them from forming. So it seems to me that additives should still matter in these areas. Perhaps directionally a bit less than in port injection engines, but still important. What am I missing?

On intake valves, very clearly if your statement about all the gas being injected in the last 10% of the compression stroke is literally true, then additives can’t have an effect because the intake valve is already closed. But I have to wonder about that statement because what I’ve read in other places like GMInsiderNews from GM engineers seems to imply that injector geometry plays a role in how clean the valves stay. Since that couldn’t possibly be true if the valve is closed before injection starts, it implies that the valve is still at least partially open during at least part of the injection cycle. In fact, some posts I’ve seen imply that they used what they’ve called a pulsed injection cycle, meaning that in each intake stroke, there are 2 pulses of gas injected: a smaller one when the intake valve is still at least partially open, and the main pulse after it is fully closed. If any of these things are true, then there is at least some chance for gas to wash the valve stem, and additives could still have some impact. Comments?

I suppose the last question is, if true that additives no longer matter, why do GM and other car makers continue to recommend top tier? Yes, oil companies promote their additives, but top tier was never an oil company program. It has always been an auto company program. If none of the above factors matter any more, why do they continue to recommend it? Note that I could easily believe additives are less effective in direct injection engines. But your post implied they are totally ineffective. So I’m grappling with the question of where matters really stand in the spectrum from being just as effective as in port injection engines (which I agree seems unlikely) to being totally ineffective (which I have at least some trouble believing, but want to hear more).

CriticalmassGT

That is Tax Free diesel, it has red dye in it and is used for farm equipment. Has nothing to do with top tier fuels.

jcthorne

Also sold as home heating oil.

JoesC5

Thanks for the info. I'll have to remember that if I'm ever traveling through Minnesota.

COSPEED

Excellent post!!

Let me correct a few things I thing you misunderstood. The fuel (99% plus of it) is injected in the final 10% or so of the compression stroke at 2000 plus PSI so atomization is much improved over prior 45-55 PSI port injection. And (depending on which engine manufacturer) 90-95% has already completed the burn before any touches (if at all) the top of the piston (here is the picture of a modern GDI piston, note the drastic changes in topography over a port injection or carbureted):

GDI piston first, Port injection or carbureted second:




The entire premise behind GDI is with out fuel present during the intake and most of the compression stroke, there is little chance of any pre-ignition or detonation occurring as there is nothing to combust prematurely allowing far higher compression ratios (Audi is current experimenting with as high as 19:1!!!) . And looking at the pictures above you can see the dark spot in the center area is the only portion of the piston to see any contact with the fuel as it burns so efficiently and completely. The PR releases you are reading do not give the percentages of fuel in the alternate events, and each automaker has several patents on these events, some several quick minor squirts to one or two, and some prior to compression, etc. but in practice as if anyone takes the few minutes to remove their intake manifold to see for them selves, none of these multiple fueling events have had any significant effect on reducing or eliminating the coking issues, yet every one proclaims them as solutions. (actual seeing it for yourself goes along way...only takes a few minutes and few tools to remove and closely inspect your own valves, and all gaskets are silicone O'ring reusable). I am a member of SAE (International Society of Automotive Engineers) and participate in technical discussion quite often with engineers from all the automakers when ever I can as my specialty is GDI since 2008 with the firm I have my "real" job at, and the firm does contract work for tier one suppliers to GM/Ford and others.

What each and every automaker has found is if they introduce enough fuel early so it can contact the backside of the valve, then the benefits of GDI go away and they begin to see a recurrence of detonation and cannot run as high of CR and thus defeats the benefits. Audi began adding a small port injector as well to combat this (they referred to it in PR releases as to "improve off idle throttle response, which it also did) and found again, lost the advantage of eliminating most detonation (pre-ignition) which allows a 11.5:1 CR engine to run fine on 87 octane.

The topography is far different due to the spray patterns under such high pressures, etc. and the burn pattern, flame front, quench areas are all so uniquely designed when utilizing GDI and are all constantly evolving (at any time we have several makes/models/designs of new GDI engines here were studying).

Most all automakers also have animation that is incorrectly showing the fueling events, etc to make it appear it is different than it actually is.

Aside from the post where it was mentioned the fuel sending units and sulfur content (and ALL gasoline no matter the brand comes from the same bulk distribution tanks...visit any tank farm to see the trucks coming and going, it is the additives the different brands add that make them "top tier" or not, and ALL have to have fed mandated minimum detergent additives. That is the law, but Shell/Mobile, etc all have far better formulations than no names.

COSPEED

LDB, here is a recent paper you can read covering quite a bit on the advancements up to 2014 with GDI and there is a section dealing just on the fuel making contact with other parts of the combustion chamber and parts of the piston different from the current designs which have dealt with what you described:
http://articles.sae.org/13624/

Also addresses Audi's small port injectors and other multiple fuel timing events.

gtf464s

Interesting. Thanks for the info.

gtf464s

Gotcha. Wanted to make sure I wasn't missing out on a vette secret.

ratman6161

Yes, you are wrong

The Minnesota law has nothing to do with what fuel will run correctly in my engine. My two stroke snowmobile doesn't even have valves - but it will actually run fine on E10 even though it isn't required to. And Minnesota is not "every state" I provided the link to the exact statute.

Zymurgy

As I read the MN statutes, those apply to the SELLER, not the BUYER.

The statute says nothing about BUYING such gas.

meyerweb

Ethanol helps reduce pollution coming out of the tail pipe. In general, it is only required to be blended into gasoline in areas that have failed to meet federal air quality standards. Those are mainly populous urban and suburban areas and certain industrial areas. Outside of those areas is perfectly legal (from a federal standpoint) to sell and use E0.

BUT, as usual, things aren't quite that simple. Gasoline refiners also have a mandate to use a certain amount of ethanol each year. As gasoline consumption declines, it gets harder to meet that requirement without blending ethanol into more and more gasoline, so the big refiners tend to use E10 everywhere. It's simpler to blend all their gas as E10 than to refine, store and transport two different formulations.

LDB

Thanks for your posts. I guess I need to do some homework and go back and look at various things I’ve read before, and also try to find some confirming new stuff. Your story is internally consistent, and I have no first hand knowledge to refute it. Indeed, the soot formation which I had not read of before tends to support everything you are saying. Similarly, I can see how both pulsed injection cycles and supplemental port injectors would lose some (perhaps most) of DI benefits. But some things still trouble me.

Most of the time (the VW diesel scandal being an obvious recent exception) corporations don’t tell clear lies. Stretch a bit? Misdirect a bit? Sure. But dodge the coking issue by talking about pulsed injection and/or supplemental port injection when they know full well they do no such thing, and continue to promote top tier when they know it’s useless for DI? I’d be disappointed. Not totally astonished mind you, but disappointed. Top tier does establish auto and oil cooperation, but it’s not a marketing agreement like Mobil 1 where promotional money changes hands.

Another thing that makes me wonder is that at least by reputation, some engines seem to be much worse than others on coking. Info I’ve seen implies that it’s from pulsing, geometry, etc, but maybe the real difference is better or worse oil mist separation in the PCV system. I’m not talking catch cans. If there’s a serious mist problem, those don’t have enough guts to fix the issue. I’m talking about serious internal engine baffling and separators to drastically reduce PCV mist. Or if the engine is a turbo, proper oil seals to eliminate any oil leakage to the intake side.

Anyway, thanks again for your posts. I’ll take a renewed look at things before buying a car with a DI engine. Have had Vettes in the past, and currently a CTSV, but none so far have been DI. Meanwhile, if you have links to other serious technical papers related to DI coking, I’d be interested.

COSPEED

Wa Wa advertises it in every State they are in so I agree.