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dkbrooks(at)gmail.com
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 Posted: Mon Aug 27, 2007 6:59 am Post subject: Non-aviation question about fuel (octane and "knocking") |
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I suppose this could be aviation related, especially for the 912ULS owners, but my cause for posting is not.
I have a supercharged Miata that I have to feed 91+ octane gas or it will knock pretty severely when I put my foot in it, especially at high boost. I normally run around 5-7 pounds of boost around town and have no problems with knocking when I use 91. I run 13+ on the track and will only light up the knock sensor light under pretty extreme loads or on really hot days when the intercooler can't keep up.
I have been buying 91 octane gas at a local station and lately I have been getting lots of knocking at the low boost levels, and I am not driving very hard. (I live in a town with 2500 people and three of them are full time cops. Not much chance for spirited driving.) My best guess was that I was starting to lose a valve seal or maybe I had some rings going bad and had carbon building up on the piston heads and that was causing detonation. But now I am not so sure. I filled up an almost empty tank out of town over the weekend and I haven't seen the knock light blink even once. I am now wondering about the gas at my local station.
I did notice something interesting when looking at the pumps at my local station last time I was there. The pump only has two Hobbs-meter like counters at the top of the pump instead of three. I always thought those were total counters, one for each underground tank but this station sells three grades of gas. Could they be storing only two grades of gas and mixing 85 and 91 to get the 87 grade? Or worse, could they be selling 87 gas as 91? (I see one of the counters ticking away when I fill up with 91. I will put 87 in the truck next time it needs gas and I will see if both counters run at the same time.)
A petro-chemist friend once tried to explain to me about gasoline molecule chain length and detonation, "cracking" during the refining process, and who knows what else. I am not a chemist (I don't even play one on TV) so I got lost pretty quickly. What I did take away from the conversation was that his opinion was that the lower grade octane gas (the shorter molecule chains?) would pre-detonate under compression even if mixed with the high octane gas causing the whole mix to go up. (It was also his strongly expressed opinion that "octane boosters" were pretty useless as well.) I want to make it clear that I am not sure I understand what is going on in the cylinder when "knocking" occurs, so I am starting at a bit of a disadvantage on this whole issue.
So I guess my questions to this (incredibly) knowledgeable group are these:
- Does mixing different octane fuel "average" the effective octane of each grade?
- How can a regular schmoe like me find out of they are selling 89 octane gas as 91? I am pretty sure that asking isn't going to get me very far.
- Could there be additives in the gas that might encourage knocking? Would the 10% Ethanol gas cause more knocking than the undiluted fuel?
- What is really going on when the engine "knocks"? (The sensor replaces one of the engine mount bolts on the engine side. It looks like an epoxy filled hollow bolt with a wire lead coming out the end. That wire runs back to the after market ECU. Is this sensor really just a microphone listening for "knocks"? Can it be that simple?)
- What (if anything) is the difference between "knock" and "pre-detonation"? I hear both terms used and have always wondered if they meant something different.
- What do you 912ULS owners do when you can't find 91 octane gas? Is there an octane booster that you like? Do they even work at all?
- What are the minimum octane requirements for the non-912ULS engine options like the Jabiru 3300? (I am planning on building a 701 one of these fine days. Honest.)
Please forgive the posting of a not totally aviation related question to this board, but the depth of knowledge here is astounding and I figured this was about as good a place to go for help as I was likely to find. Asking technical questions on most of the auto lists usually gets responses like "Buy a real car". Besides, if I put wings on this car I sometimes think it would take to the air! Too bad it is too heavy to qualify as an E-LSA plane. The power to weight ratio is about right...
Thanks in advance, and feel free to contact me directly if you don't want to clutter up the list with this topic.
Dave
Please do not archive
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Gig Giacona
Joined: 10 Jan 2006
Posts: 1416
Location: El Dorado Arkansas USA
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| Posted: Mon Aug 27, 2007 8:04 am Post subject: Re: Non-aviation question about fuel (octane and |
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I can really on help on one of your questions.
Does mixing different octane fuel "average" the effective octane of each grade?
Yes, when you go to a station that has 3 grades look around there are only, in most cases, only two tanks. This is because to make the middle grade the pump mixes premium and unleaded.
Have you tried a full tank from another station that uses a different distributor? If the knocking goes away when you do the problem is with their gas if not it is with your car.
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_________________
W.R. "Gig" Giacona
601XL Under Construction
See my progress at www.peoamerica.net/N601WR |
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bryanmmartin
Joined: 10 Jan 2006
Posts: 1018
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| Posted: Mon Aug 27, 2007 10:28 am Post subject: Non-aviation question about fuel (octane and "knocking") |
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Nearly all fuel stations that sell multiple grades of gasoline only have two tanks, one for regular grade and one for premium grade. To get mid grade, they just mix the regular and premium in the proper proportion to get the mid grade anti knock rating.
On Aug 27, 2007, at 10:59 AM, David Brooks wrote:
| Quote: | I suppose this could be aviation related, especially for the 912ULS owners, but my cause for posting is not.
I have a supercharged Miata that I have to feed 91+ octane gas or it will knock pretty severely when I put my foot in it, ....
So I guess my questions to this (incredibly) knowledgeable group are these:
- Does mixing different octane fuel "average" the effective octane of each grade?
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Yes ,mixing one gallon of 91 octane gas with two gallons of 85 octane will give you three gallons of 87 octane.
| Quote: | - How can a regular schmoe like me find out of they are selling 89 octane gas as 91? I am pretty sure that asking isn't going to get me very far.
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It sounds like someone dumped some regular fuel in the premium tank. Unfortunately, there is no simple way to test gasoline for the proper octane level. You'll need to find out who does the inspections of that station and complain.
| Quote: | -
- Could there be additives in the gas that might encourage knocking? Would the 10% Ethanol gas cause more knocking than the undiluted fuel?
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Because of the alcohol, a small amount of water will dissolve in gasoline containing alcohol. If too much water is mixed with gasohol, the water will settle out of the mixture instead, taking all the alcohol with it. If there was a large amount of water settled out in the bottom of the tank when gasoline containing alcohol was added to the tank, it's possible that the alcohol ended up dissolved in the water at the bottom of the tank. Since the alcohol acts as an octane booster, this would result in a reduction in the octane rating of the gasoline floating on top of the water in the tank.
| Quote: | - What is really going on when the engine "knocks"? (The sensor replaces one of the engine mount bolts on the engine side. It looks like an epoxy filled hollow bolt with a wire lead coming out the end. That wire runs back to the after market ECU. Is this sensor really just a microphone listening for "knocks"? Can it be that simple?)
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That's about right.
| Quote: | - What (if anything) is the difference between "knock" and "pre-detonation"? I hear both terms used and have always wondered if they meant something different.
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Knock is caused by detonation (not pre-detonation). Pre-ignition is something entirely different.
see: http://www.hrd-performance.com/13.html
or: http://www.northernstar.ab.ca/ftp/images/pres/BrentPowers.pdf
You must live in an high altitude area if the local stations sell 85, 87 and 91 octane fuel. In the low lands, the standard grades are 87, 89 and 93 octane.
--
Bryan Martin
N61BM, CH 601 XL,
RAM Subaru, Stratus redrive.
do not archive.
[quote][b]
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_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive. |
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MaxNr(at)aol.com
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| Posted: Mon Aug 27, 2007 12:42 pm Post subject: Non-aviation question about fuel (octane and "knocking") |
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You can find more info about autofuel than you want to know at Peterson's auto fuel site (http://www.autofuelstc.com). He sells auto fuel stc's for certified A/C and has several informative links including a page for homebuilders. Regarding blending fuels: I recall one of the instructors discussing just that in the classroom in military flight school back in the last half of the 20th century. I was told that if you blended 91 and 115 octane in equal proportions, you would wind up with something around 95 octane. We were learning about the R1300 and R1340 and we were running on 115/145. 91/96 was approved. Caution. This info may have been updated since. My Lycoming O-235 has a compression ratio of 6.75:1 and its just possible that it may run OK on fuel from any third world country. Vapor lock would be the big thing to worry about as I like to fly high on cross country. Check the web site above for details. If "knocking" or detonation happens, it will make your CHT go bananas. Remember that alky rots both your brain and your carburetor.
Do not archive
Bob - Pace,FL XL/Lyc
*******************************emed/aolcom30tour [quote][b]
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dkbrooks(at)gmail.com
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| Posted: Tue Aug 28, 2007 6:17 am Post subject: Non-aviation question about fuel (octane and "knocking") |
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I never knew that the pumps blended the gas on demand. I never really needed to use anything other than super cheap stuff before I started playing around with turbos and superchargers (effectively high compression engines).
I think you are right about this station's gas being 'bad'. What started me down this path was filling up at a different station and seeing all my worries go away. I suppose the good news is that I don't have to tear down my engine to fix my problem. The bad news is that I no longer have an excuse to tear down the engine and install some more new shiny go-fast parts. I suppose I will just have to admit the fact that I want to do it because I can and get on with it.
Thanks for the info and suggestion. I love this list.
Dave
On 8/27/07, Gig Giacona < wr.giacona(at)suddenlink.net (wr.giacona(at)suddenlink.net)> wrote:[quote]--> Zenith-List message posted by: "Gig Giacona" < wr.giacona(at)suddenlink.net (wr.giacona(at)suddenlink.net)>
I can really on help on one of your questions.
Does mixing different octane fuel "average" the effective octane of each grade?
Yes, when you go to a station that has 3 grades look around there are only, in most cases, only two tanks. This is because to make the middle grade the pump mixes premium and unleaded.
Have you tried a full tank from another station that uses a different distributor? If the knocking goes away when you do the problem is with their gas if not it is with your car.
--------
W.R. "Gig" Giacona
601XL Under Construction
See my progress at www.peoamerica.net/N601WR
Read this topic online here:
[b]
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Gig Giacona
Joined: 10 Jan 2006
Posts: 1416
Location: El Dorado Arkansas USA
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dkbrooks(at)gmail.com
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| Posted: Tue Aug 28, 2007 7:17 am Post subject: Non-aviation question about fuel (octane and "knocking") |
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Hey now...
Actually, you would probably be surprised at just how fast these things will go. A couple of years ago I spent a beautiful weekend out at Reno Fernley Raceway terrorizing a bunch of Corvette owners. At 185 rwhp (not stock) and 2100 pounds (with a good track suspension setup and R-compound tires) this thing will fly past just about any street car in the corners. Sure, the long straight down the hill was like watching a Corvette freight train go by me, but I did have fun with them in the corners. And at 1/10 the price, the grin/dollar ratio is really spot on. All that and I can carry two (yes two!) whole bags of groceries in the trunk!
Dave
Do not archive (as I suspect that future 601 builders probably won't be all that interested in building a race Miata - but you never know)
On 8/28/07, Gig Giacona <wr.giacona(at)suddenlink.net (wr.giacona(at)suddenlink.net)> wrote:[quote] --> Zenith-List message posted by: "Gig Giacona" <wr.giacona(at)suddenlink.net (wr.giacona(at)suddenlink.net)>
Shiny, go fast parts in a Miata. How cute. 
dkbrooks(at)gmail.com wrote:
| Quote: | I never knew that the pumps blended the gas on demand. I never really needed to use anything other than super cheap stuff before I started playing around with turbos and superchargers (effectively high compression engines).
I think you are right about this station's gas being 'bad'. What started me down this path was filling up at a different station and seeing all my worries go away. I suppose the good news is that I don't have to tear down my engine to fix my problem. The bad news is that I no longer have an excuse to tear down the engine and install some more new shiny go-fast parts. I suppose I will just have to admit the fact that I want to do it because I can and get on with it.
Thanks for the info and suggestion. I love this list.
Dave
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--------
W.R. "Gig" Giacona
601XL Under Construction
See my progress at www.peoamerica.net/N601WR
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=131349#131349
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dkbrooks(at)gmail.com
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| Posted: Tue Aug 28, 2007 7:21 am Post subject: Non-aviation question about fuel (octane and "knocking") |
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All -
This list is fantastic. I have received so many responses with so much good information and pointers, both on-list and off, that it is going to take me a while to digest it all. I have tried to thank each person off-list but if I missed you then please accept my thanks here.
I love this list. What an amazing group of knowledgeable and helpful folks.
Dave
Do not archive
[quote][b]
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