dkbrooks(at)gmail.com
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 Posted: Tue Aug 28, 2007 6:37 am Post subject: Non-aviation question about fuel (octane and "knockin g") |
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The compression test is a good idea. This car has not lived an easy life - probably 500+ miles a weekend several times a year on the track under racing (time trial) conditions at high boost. It is an amazing engine but every mechanical system has its limits.
As for the fuel system and rebuilding the carbs I am in luck. This is a fuel injected car and the race computer is fully mappable. I have total control over spark timing as well as mixture all across the whole range of throttle position, boost/vacuum, ambient air temp/pressure and RPM. Then the computer will do real time fine adjustments for EGT and O2 as well as knocks. (The knock sensor in this system is purely a defensive device - it automatically enriches the air/fuel ratio and retards the timing by as much as 5 degrees when it detects knocking. Kind of takes the wind out of the sails but it saves the hard parts.)
Which makes me wonder why fuel injection systems like this aren't more common on small piston planes. Wouldn't this setup also remove the possibility of carb icing? There isn't a venturi so there shouldn't be any rapid cooling in the intake path, ergo no ice. Or am I missing something?
Dave
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On 8/28/07, ashontz <ashontz(at)nbme.org (ashontz(at)nbme.org)> wrote:[quote] --> Zenith-List message posted by: "ashontz" <ashontz(at)nbme.org (ashontz(at)nbme.org)>
Just to rule out any other major problems, I'd do a compression test on all the cylinders. A month ago I had to change the head gasket in my old '85 Nissan pickup. It was knocking like crazy. Turned out the head gasket broke right between the #3 and #4 cylinders. No coolant leak (no white smoke out the tailpipe), but the compressed intake from one cylinder was making it into the adjacent cylinder during the powerstroke of the other cylinder during the 1-3-4-2 firing order, making for a HARD knock. Seeing as how it's an boosted engine, that's a lot of compression in there which is hard on the engine and seals.
If the compression is good, check the timing and make sure it's not too far advanced. Depending on the year of the car, it may still have a vacuum controlled spark advance. Setting the spark advance for the correct idle speed could do a world of good. You'll have to move the spark advance, then the idle screw, then the spark advance, then the idle screw etc... to get it right because one affects the other. Once you have it close, then if it still doesn't sound right per ear you can move the spark advance a degree or two one way or the other to your liking, usually no more than a degree, and usually it'll need to be retarded, not advanced.
If it's carburated, it may be time to replace some parts in there too if it's over ten years old or so which I'm guessing it is. Specifically, replace the needle valve, set the float, and replace the accellerator pump. A rebuild kit just to do those parts is only $30.
The most I've ever heard out of that truck of mine when tuned and under load with 87 octane was pinging. You shouldn't hear hard knocking.
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[quote="n801bh(at)netzero.com"]You can probably bet the gas station is selling low octane in the high octane pump. They know it takes some pretty complex equiptment to test the true octane in gas and are gambling no one is going to complain. Even if someone did, the testers sent out by the state to comfirm the given octane rating are probably not going to get an accurate number either because of various factors beyond their control. Good luck....
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Ben Haas
N801BH
www.haaspowerair.com
-- "David Brooks" ?wrote:
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
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--------
Andy Shontz
CH601XL - Corvair
www.mykitlog.com/ashontz
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