Which octane for my situation? A carbon conundrum

[TurboDieselPoint]

First off, hello Bob is the oil guy forum! I'm very excited to be a new member of this knowledgeable community! I've browsed here from time to time, reading topics topics discussing motor oil or small lawn equipment engines. Now I am here to seek your advice. I am about to get my hands on a brand new 2017 VW Passat with the 1.8 TSI which has a turbocharged, direct-injection 4-cylinder (compression ratio 9.6:1). Having been in the diesel game for years without a gasoline car, I want to make sure I'm not going to screw up and use the wrong grade of gasoline in my new car. VW states the car should be used with regular gas having a minimum octane of 87. No matter what, never ever go lower than 87, they say! Additionally, VW gives no recommendation or implies no gain for using higher than 87 except for in one discreet location: their website's spec sheet for the engine states that the maximum 170 horsepower output was "achieved using premium fuel," whatever that means. Can the 170 HP be achieved with regular,too...who knows? I don't really care either. What does bother me is that where I live all regular grade gas sold at the pumps is 86 octane, not 87, due to the altitude (3300 feet above sea level). Where I want to fill (Costco--clean pumps, cheap, fresh fuel, and Top Tier!) the only grades offered are 86 octane and 91 octane. Since I wouldn't want to violate the octane requirements for my new turbo VW, I see three different options: 1. Fill with 91 octane 2. Fill 20% 91 octane followed by 80% 86 octane with each fill-up 3. Abandon Costco and buy mid-grade 88 octane elsewhere I would prefer option #1 since I trust Costco and mixing grades of gas sounds like a pain. Plus, I'm not sure I could achieve a homogeneous mixture of 86 and 91 in the tank every time anyways. This (maybe?) could cause troubles for the car if it briefly ingests small, unmixed portions of 86 when it wasn't expecting it. I know the knock sensor would react and pull timing...but that is a reaction to knock or detonation, not absolute prevention of it! The only concern I have with burning 91 all the time is that I've heard it might cause carbon buildup inside engines not intended for premium, which is something I do NOT want to have happen. If I burned 91 for the life of this car, would the engine likely develop abnormal in-cylinder carbon deposits? Thanks, TurboDieselPoint

 

[Colt45ws]

Just use 91. THe turbo makes up for the altitude so you are wise for avoiding 86. Your engine is really best suited to premium fuel, they just program it with the assumption you will use 87 all the time. You'll get better power and maybe economy on it.

 

[earlyre]

i would think with the turbo, having the higher octane would not be a problem... i've never heard of premium causing carbon build up before, and a quick google just shows a lot of folks sayinggoing back and forth with basically yeah-huh! nu-uhh! it's weird they don't offer a mid-grade. stations that do have it, the dispenser just mixes reg&premium for you.. around here it's 87, 89, and 91-93 I would just try to find another top tier station in your area and get the mid grade. or just get the premium. 50/50

 

[B320i]

Run a few tanks of the 91 and see what the economy and performance is like compared to your "withches brew" proposal. I'll wager the engine is more responsive, and gets better economy with higher octance.

 

[Cujet]

As mentioned above, the turbocharger negates the affect of altitude. So using the lower octane regular of your location is not going to result in full output. Most likely, the engine will compensate via the knock sensor and you will have lower efficiency and lower power. Because those engines make plenty of boost at low to mid range RPM's, it's likely the higher octane fuel will provide a more pleasant experience in everyday driving. Many people don't care about peak power, but fail to understand that part throttle, low and mid range output is affected by octane in these highly tuned, direct injected, turbocharged engines. In fact, it's not unusual, in normal acceleration, to see the ECU drive the throttle plate near fully open rapidly, with just a small input on the pedal, then boost increases, the throttle plate is then driven back towards closed to manage boost and acceleration. The engine generates high cylinder pressures at lower RPM and results in acceptable acceleration without drama or high RPM. Every one of the new crop of turbocharged engines does better with premium, and VW is no different. Often so much so, it offsets the increased price.

 

[gregk24]

If 87 is not available just bump up to the next grade which sounds like 91 in your case. Make sure you are using top tier fuel and a good 502.00 spec oil. Castrol 0w40 would be my recommendation. Also, when you have opportunity from time to time and are on the highway drop it down into 4th gear and let the engine wind out at 4K rpm for a while. At one point this was VW recommendation to help ward off carbon build up. I do this with the VW in my sig every once in a while when the engine is at operating temperature. Enjoy the new VW! The 1.8 TSI is a gem.

 

[kschachn]

Originally Posted By: TurboDieselPoint

The only concern I have with burning 91 all the time is that I've heard it might cause carbon buildup inside engines not intended for premium, which is something I do NOT want to have happen. If I burned 91 for the life of this car, would the engine likely develop abnormal in-cylinder carbon deposits?

As others have stated that will not happen.

 

[mk378]

The only negative of using premium gas when it's not necessary is that it costs more. But when the manufacturer says the car will achieve full performance only with premium, it is usually worth using it because mpg improves.

 

[wemay]

Congrats on the new purchase. The Passat is the most handsome car in the segment IMO. Just use one grade higher than the 86 octane available. Costco is an excellent choice since its high octane price often rivals other's regular.

 

[deanm11]

You could also rotate tanks of 86 and 91. Just don't let the tank get down below about 1/4.

 

[dbias]

I just did the tdi buyback and got a new Audi A4. It has the same EA888 gen3 but in the 2.0 flavor. Audi of course recommends 91 octane but in my area the grades are 87,89 and 93. I am running the Shell 93 octane V Power as I think going 2 over on octane is better than 2 under. Having looked at my engine I clearly see 8 fuel injectors, so two for each cylinder. I cannot find very good up to date information on this engine but a couple articles from 2015 talked about their plan to use both direct injection and port injection to help with the particulate matter problems of direct injection engines. So hopefully if this is true I think the combination of V Power fuel and port injection will keep carbon problems at bay.

 

[slacktide_bitog]

Costco's price on 91 could be cheaper than another station's 88 Your best option may very well be to just use Costco's 91

 

[raytseng]

to be precise on your concern. If I burned 91 for the life of this car, would the engine likely develop abnormal in-cylinder carbon deposits ... as compared to running 87. there still maybe a likelihood, given this is a vw. but it is not going to be because of the higher octane rated fuel I vote use 91.

 

[Rhymingmechanic]

Originally Posted By: slacktide_bitog

Costco's price on 91 could be cheaper than another station's 88 Your best option may very well be to just use Costco's 91

Welcome. Slacktide makes a good point. If it was mine, I might try several tanks of 88 and then several of 91, tracking mileage manually and paying attention to the butt dyno. My naturally aspirated Chevy 5.3 has a lot better low and midrange response on 93 compared to 87, but the extra cost doesn't justify premium for my driving style. Your situation with a turbo may be a lot different.

 

[NJ_Car_Owner]

Just an observation from a non mechanical, not car savvy person ... My owner's manual (Ford Edge) says not to use any gas below 87. It also says for their eco boost (turbo) engines that to get better performance use higher octane gas; I would imagine that would be true for their 3.5 liter V6. I think this was already mentioned. From what I've read - top tier gasoline may clean the engine as it burns; at least that's what BP advertises. Modern engines analyze the fuel burn and adjust things in the engine to make it run correctly. If your car can use 87 but you put in higher you may see a performance/gas mileage increase but it may be minimal. This is assuming what I've read is true. As a side note, my son tried higher octane gas in his Sonata turbo and really didn't see the difference to justify the price difference. My take on your situation - it's sounds easy for you to get Costco gas, I would use the 91 and don't worry about it. If you happen to be somewhere and gassing up and 87 is available and it's cheaper (usually is) buy that; a tank of non top tier gas will probably not hurt anything as long as it isn't total garbage gas. If it's in the owners manual it shouldn't hurt anything; if it specifically state not to use it I would be concerned. I wouldn't go out of my way to get gas and I have bought higher octane gas when a station was out of regular. My view isn't at all technical, it is just how I would go about it.

 

[TurboDieselPoint]

Thanks for the responses and the warm welcome, everyone! I appreciate it! I forgot to say earlier: I have read on VW Vortex that some members think the engine (1.8 TSI Gen 3) in my car will not advance timing to take advantage of premium. It simply retards timing if it needs to. I'm not sure if this is precisely true or if it changes things regarding my concern, but I thought I'd at least mention it. Nevertheless, it sounds like I don't have to worry about additional in-cylinder carbon buildup from an incomplete burn of 91 octane gasoline instead of the unavailable-to-me 87 octane fuel. When the car arrives, I'll experiment and try a few tanks of pure 88 and pure 91 octane to see which I like better. However, I know I will like the Costco station better and their 91 actually tends to be the same price or cheaper than 88 elsewhere. TurboDieselPoint

 

[y_p_w]

I don't know if a turbo necessarily negates the effects of altitude. It would definitely improve the situation. Some of the first uses of what used to be called a turbosupercharger was for aircraft engines that would gasp for air at high altitude. General Electric tested their turbocharged aircraft engine by hauling the engine up to Pikes Peak for testing. Isn't the boost from a turbo relative to atmospheric pressure? So even with a turbo the octane demand might still be lower than at lower altitude.

 

[slacktide_bitog]

Originally Posted By: TurboDieselPoint

I know I will like the Costco station better and their 91 actually tends to be the same price or cheaper than 88 elsewhere.

In this case, definitely use Costco premium. Especially since their 91 is cheaper than other station' 88.

 

[Cujet]

Originally Posted By: y_p_w

I don't know if a turbo necessarily negates the effects of altitude. It would definitely improve the situation. Some of the first uses of what used to be called a turbosupercharger was for aircraft engines that would gasp for air at high altitude. General Electric tested their turbocharged aircraft engine by hauling the engine up to Pikes Peak for testing. Isn't the boost from a turbo relative to atmospheric pressure? So even with a turbo the octane demand might still be lower than at lower altitude.

For our discussion, a modern direct injected, turbocharged car will perform almost exactly as well at the top of Pikes Peak, as it does at sea level. The boost level is an absolute measurement, not a differential with the outside air pressure. Yes, at extreme elevations, the turbocharger has to spin faster to achieve a given manifold pressure. However, from a drivers point of view, this is not particularly problematic, other than the driver may notice slightly increased turbo lag time. During operations at normal "high" elevations, say 5000-7000 feet, Ecoboost, TSI and other DI-Turbo engines give up nothing significant as far as HP or response. And the owners manuals are specific about octane requirements. Because manifold pressure, and therefore cylinder pressure is managed to absolutes (and not referenced to ambient pressure) in modern turbocharged engines, octane requirements are hard and fast. Furthermore, all modern DI turbo engines will manage timing according to knock. Premium will result in a bit more output.

 

[y_p_w]

Originally Posted By: Cujet

Originally Posted By: y_p_w

I don't know if a turbo necessarily negates the effects of altitude. It would definitely improve the situation. Some of the first uses of what used to be called a turbosupercharger was for aircraft engines that would gasp for air at high altitude. General Electric tested their turbocharged aircraft engine by hauling the engine up to Pikes Peak for testing. Isn't the boost from a turbo relative to atmospheric pressure? So even with a turbo the octane demand might still be lower than at lower altitude.

For our discussion, a modern direct injected, turbocharged car will perform almost exactly as well at the top of Pikes Peak, as it does at sea level. The boost level is an absolute measurement, not a differential with the outside air pressure. Yes, at extreme elevations, the turbocharger has to spin faster to achieve a given manifold pressure. However, from a drivers point of view, this is not particularly problematic, other than the driver may notice slightly increased turbo lag time. During operations at normal "high" elevations, say 5000-7000 feet, Ecoboost, TSI and other DI-Turbo engines give up nothing significant as far as HP or response. And the owners manuals are specific about octane requirements. Because manifold pressure, and therefore cylinder pressure is managed to absolutes (and not referenced to ambient pressure) in modern turbocharged engines, octane requirements are hard and fast. Furthermore, all modern DI turbo engines will manage timing according to knock. Premium will result in a bit more output.

But isn't the boost on top of atmospheric pressure? There's still a performance hit since there might be 3.5 psi lower pressure at a mile up.