AAA study concludes "top tier" gasoline is worth the extra cost

[namesrejected]

So, its been said many times in this thread, but keeps coming up. Octane, and Top Tier are two different things.

Octane is basically as many have said, " The fuels ability to burn slower ". Not hotter, slower. Top Tier fuel is basically brands of fuel that meet requirements set by some committee or something. Wikipedia has a good read on it, although I didn't read the entire thing https://en.wikipedia.org/wiki/Top_Tier_Detergent_Gasoline

As far as the contaminates on intake valves, here is my theory.

Back in the good ole days when vehicles used carburetors, all the exhaust was dumped out of the tailpipe, and all positive crankcase pressure was dumped into the atmosphere. Vehicle engines stayed cleaner, and ran longer, and were a lot easier to work on.

Over time, we got these new things called injectors, PCV valves, and EGR valves. They all aim to make the world a better place, but cause engines "mainly intake manifolds, and intake valves " to foul up. See, with the carburetor fuel went by the intake valves, effectively keeping them clean. When injectors ( direct port injection, throttle body injection didn't have this problem ) were introduced the intake valves lost there constant cleaning from the gasoline because now the gasoline was sprayed directly into the combustion chamber, and never went by the intake valves to keep them clean.

To make this matter worst, The EGR valve dumps exhaust gas into the intake, and the PCV valve dumps crankcase pressure into the intake. These are the two main reasons why intake valves foul up so quickly. The exhaust gas, and gasses out of the crankcase contain high levels of carbon that get cooked onto the valves. The valves are never cleaned because the fuel on modern vehicles is injected directly into the combustion chamber, and never goes through the intake itself to clean the valves.

( If you want to keep your valves from fouling up quickly you block the EGR, and dump crankcase pressure outside ) Hey! Who wrote that! Not me! No, don't do it, you will get a check engine light, and your car will not run well at all. You also might add a speck of carbon or something like that. * Yes, that last comment was a tolling comment *

I have taken the intake manifold off of my 2010 Traverse with 150000 miles to do a sparkplug change, and the valves were horribly fouled, as were the pistons. I filled each combustion chamber with seafoam additive, and filled the intake side of the head with the same, leaving it for 2 days. I then removed as much of the seafoam additive as I could, then I cranked the engine with the plugs still removed to get the rest out. I will say, it made a huge difference in performance.

On a final note, there are two types of products on the market for cleaning your intake valves. One is an additive you put in your fuel, and those don't work because the fuel never comes into contact with that part of the intake valve anyways. The other type is a spray you inject into the intake while running the engine. Those work, but it will take several treatments for a lot of carbon, and they make a lot of smoke.

 

[commanderasus]

This is nothing more than marketing hype. While I was learning to become a mechanic we could use the school shop to work on our cars. We only had so many hours to do this. My engine lost oil pressure and I had to fix it before I could drive 1 hour to get home. After pulling the oil pan off we discovered a turned rod bearing and the crank was damaged from this. I got my friend to take me home so I could get another crankshaft from my spare engine. To make a long story short, this crankshaft was a shorter stroke than the original by 0.230 inch. I discovered this the next day when I did a compression test. The old engine had 300 psi of compression. The repaired one had only 150 psi. This engine ran good but had no power. I asked the instructor how I could figure out what was wrong without taking the engine apart. He told me to take one spark plug out and bring the piston to the top and feel for the top of the piston. I said this should be no problem as they were high compression pistons. I could not touch the piston at all. So I began to experiment with the tune up. This was a 1969 Chevy Camaro so no electronic ignition or EFI. I advanced the timing and the car ran better. When I got home I went to the engine I pulled the crank from. It was a 327, not a 350. Now I know why the pistons don't come to the top. Since I was a student I didn't have much money to get another crank so I just kept experimenting with the tune up. I ended up welding the advance mechanism in the distributor so there was no advance, then just kept advancing the timing until it would not crank over. I ended up with 55 degrees of timing. This was locked in, so the timing was always at 55 degrees. This engine ran great, just run the key and it was instant starting. When we studied fuel mileage I told the instructor what I had done to make the car run right. He said that I had a very low compression engine and it probably would not get good mileage so he said this would be a good example for the test. We ran the test where the gas tank is shut off and you got your fuel from a gas container that was mounted under the hood. We did three tests and at 65 mph this engine got 45 mpg. The instructor was shocked. WE put the car on the chassis dyno as he wanted to see how much power this combination made. The result was 350 hp at the wheels. To finish the story we did the mileage test with all the two grades of gasoline that were available at the time. No difference was detected. Fast forward 10 years. I am now a crew chief on a racing team. I learned to blend our own fuel. We gave our fuel to the local mechanics institute for EPA style testing and all the tests came back the same - a 50% reduction for all numbers. They also tested this in junk yard engines that had over 90,000 miles on them, still a 50% reduction. We did this in our little shop without millions of $ in research money. The whole automotive industry is in bed with each other, it is nothing more than profit, profit, profit. If we can get the public to buy our "new technology" with clever marketing tall the better. Fuel injection was in widespread commercial use in diesel engines by the mid-1920s.
An early use of indirect gasoline injection dates back to 1902, when French aviation engineer Leon Levavasseur installed it on his pioneering Antoinette 8V aircraft powerplant, the first V8 engine of any type ever produced in any quantity.

 

[Robby-san]

"Developed in 2004, top tier gasoline is a term used to refer to gasoline that has been treated with detergent additives designed to protect engines from carbon buildup and deposits that collect on the intake valves. "
They are talking about the additives. The different grades (87,89,93) are a lot more than a 3 cents difference.

 

[Axiarus]

Well for a start I haven't seen 92 anywhere yet so yes, Australia, Japan, Europe DO have readily available 98 RON and California seems to either not have it or very sparsely so my point still stands. Jap and Europe have higher as well.

Also for the record, I believe you are incorrect and that 93-94 AKI tends to equal 98 RON for the markets I have cited.

87, 89, 91 AKI seems to be everywhere.


In the higher grade fuels, it burns hotter and they also tend to put cleansing additives as well, both of which help prevent carbon build-up.

They put cleansing additives in their lower grades as well, it all depends on where you go. Again, putting higher octane gas in a car designed for lower octane will degrade performance.

 

[Axiarus]

At least after I switched to 98 my car has been running much better. Also they add ethanol made from third world peoples food in 95 here and it's been reported to break engines and is not recommended by manufacturers so there's really no choice anymore as 95 and 98 are only octanes on offer. Maybe bad running on high octane gas is american car thing as you have octanes below 90 and guess at that point you need different type of fuel filters and such.

It depends on compression ratio. Higher octane for higher compression ratios, it does not matter where you are in the world.

 

[bea108]

I do wish we'd pick one scale to compare octane. You're RON numbers are fairly absurd when compared with the US measuring standard. Our regular is 87 octane. We couldn't even buy a "98 octane fuel" from a pump. You'd have to buy octane concentrate to get those numbers, and pour a crap load of it in to boot.

Before tetra-ethyl lead was outlawed, you could buy 101 octane gas at the pump. (Which was, IIRC about the same octane as standard aviation gas). With 100 or better, you could run compression ration ratios in excess of 11:1, which after all, is what the octane rating is about.

Everybody's "gasoline" has methanol in it these days. Get used to it.

And still, none of this discussion of octane or methanol has one blessed thing to do with the actual topic.

(And yes, I'm off topic quite frequently, thanks for noticing, and you're welcome).

That's ethanol and there's different octane numbers in the US due to altitude, that is, at higher altitudes you need less octane than at sea level and if you just use top tier gas anyway you'll be doing your car, and you a favor.

 

[bea108]

It depends on compression ratio. Higher octane for higher compression ratios, it does not matter where you are in the world.

Yeah it does. At higher altitudes you need less octane for the same car than at sea level due to air density.

 

[captaincranky]

Yeah it does. At higher altitudes you need less octane for the same car than at sea level due to air density.

I don't doubt that, since you can pick up any box of cake mix and find instructions for "high altitude baking". I do question if a sea level, or any refinery,might chance sending lower octane fuel to Denver CO because of its "mile high status", as opposed to the gas it might send to San Francisco.

It doesn't seem to me they could chance the fact the car would stay in Denver for a whole tankful.. But hey, with today's questionable business practices, I wouldn't make book on it..

OTOH, if you are going to stick around Denver, you could stuff a set of extra high domed pistons in your ride, while getting away with using the same octane gas..(y)

 

[bea108]

I don't doubt that, since you can pick up any box of cake mix and find instructions for "high altitude baking". I do question if a sea level, or any refinery,might chance sending lower octane fuel to Denver CO because of its "mile high status", as opposed to the gas it might send to San Francisco.

It doesn't seem to me they could chance the fact the car would stay in Denver for a whole tankful.. But hey, with today's questionable business practices, I wouldn't make book on it..

OTOH, if you are going to stick around Denver, you could stuff a set of extra high domed pistons in your ride, while getting away with using the same octane gas..(y)

Even if the car goes from Denver to San Fran, you'll need gas by the time you get there anyway, usually. Also refineries change blending during the year as winter gas needs to be more volatile than summer blend. In fact if you filled up in January in Denver, parked the car and didn't drive it until 85 degree day in July, you might experience vapor lock because the fuel is too volatile, although today's electric fuel pumps deliver higher pressure so you might get away with no problems. But you'd need to go to a higher octane with high domed pistons because of the compression increase.

 

[captaincranky]

But you'd need to go to a higher octane with high domed pistons because of the compression increase.

Well, not so much if the thinner air theory is true, and it is. Hence, you wouldn't get the same effective compression ratio with the same bore, stroke, and combustion chamber volume with thinner air. Now, how much extra compression ratio thinner air would allow, is a job for a physics major to determine. Whether there's enough differential to matter, would just be a guess on my part. You might actually be able to get away with a quarter point, say 11:1 bumped up to 11.25:1. But again, that's just a guess.

I can tell you this about much pressure altitude, nobody's jet fighter is cracking mach 2+ at sea level, while they'll pretty much all do it at 40,000 feet.

 

[mailpup]

I do question if a sea level, or any refinery,might chance sending lower octane fuel to Denver CO because of its "mile high status", as opposed to the gas it might send to San Francisco.

As a matter of fact the gas available in Denver is about two octane points lower than that at sea level. Instead of 87 octane it was 85. I'm from Los Angeles but during driving trips to Denver and the Rockies that's what I noticed.

 

[captaincranky]

As a matter of fact the gas available in Denver is about two octane points lower than that at sea level. Instead of 87 octane it was 85. I'm from Los Angeles but during driving trips to Denver and the Rockies that's what I noticed.

Although, (to me at least), it's rather counter-intuitive, you might actually have to spring for "plus+", if you're heading out of town?

 

[mailpup]

Maybe but in my experience I never noticed a problem. I suppose it's possible but if it was a big enough problem, I'm sure we would have heard of it by now.

 

[noc81]

. . .On a final note, there are two types of products on the market for cleaning your intake valves. One is an additive you put in your fuel, and those don't work because the fuel never comes into contact with that part of the intake valve anyways. The other type is a spray you inject into the intake while running the engine. Those work, but it will take several treatments for a lot of carbon, and they make a lot of smoke.

Great points, maybe these guys will start to listen.. xD

I find seafoam to be great at clearing out intake/valve buildup when 'injected' into the manifold vacuum via the brake booster line. It should be done with extraordinary care though, as it has the potential to hydro-lock your engine if it takes in too much too quick. It's also something that should be done regularly (in small doses) after 60k miles or so.. I've done it on older cars that never received much love, and it almost seemed traumatic for them -- just too much all at once, and it just pushes all that buildup back down the line and can foul up your EGR circuit.

FWIW, I have an '82 Trans Am with a '70 z28 small block, with a Carter AFB chokeless carburetor and very basic emissions controls -- no catalytic converter, no EGR, no carbon canister, just a PCV valve (the fumes in the PCV circuit are indeed harmful, but venting positive crankcase pressure into the atmosphere will force all your oil to blow out through your dipstick -- why breathers only work on real race engines with dry sumps). I blew a lifter and pushrod a few years back, prompting me to do a top-end rebuild. I was amazed to find that my perfectly dialed in ignition timings (hard as hell to get right with no ECM) and properly calibrated carb allowed for the engine to run cleanly enough that there wasn't a spec of carbon buildup to be found. Over 400hp, and I can even pass an emissions test without any of the original equipment. x]

 

[brentano]

well loosening of the tolerances in an engine is more to do to with the driving habits of the owner, but nice jab there. My whole point is that carbon build up isn't really an issue unless you want to reach 300k.

It's not a jab.

Australia couldn't import BMW and VW diesels until around 2005 due to diesel grade for example.

At least after I switched to 98 my car has been running much better. Also they add ethanol made from third world peoples food in 95 here and it's been reported to break engines and is not recommended by manufacturers so there's really no choice anymore as 95 and 98 are only octanes on offer. Maybe bad running on high octane gas is american car thing as you have octanes below 90 and guess at that point you need different type of fuel filters and such.

It depends on compression ratio. Higher octane for higher compression ratios, it does not matter where you are in the world.

I live in Australia and have been driving for 50 years.
Octane ratings came in after we changed from leaded to unleaded petrol, yes we have 91 Regular 95 and 98 Octane Premium, then we have 10% Ethanol, as most manufacturers only recommend 10% Ethanol.
Ethanol is also rated at 95 Octane it burns at a higher temp and older cars from the leaded petrol era are not advised to use Ethanol due to accelerated damage to valve seals.
When I used to run a car not required to use Premium fuel I would use 1 Tank full of 98 Octane once a month to help clean the engine.
VW Golfs in Australia have had serious problems with drivers that would ignore the manufacturers requirements for Premium Fuel and continued to use 91 Octane and in many cases caused serious engine breakdowns which VW refused to repair under warranty.
If you don't believe that Premium Fuel changes the nature of your engine try this. My lawn mower is a 1984 model yes 1984 Briggs & Stratton 148cc without an engine overhaul, the first model out for unleaded petrol and I use only 98 Octane with it, but I have tested it with 91 and the engine at the exact same throttle level will rev much slower.
I drive now on special diesel meaning low phosphor.

 

[captaincranky]

I live in Australia and have been driving for 50 years.
Octane ratings came in after we changed from leaded to unleaded petrol, yes we have 91 Regular 95 and 98 Octane Premium, then we have 10% Ethanol, as most manufacturers only recommend 10% Ethanol.

Well, I simply can't picture gasoline with NO octane rating leaded or otherwise. In fact, when the US switched from leaded to unleaded gas, 1974 to 1975, engines lost at minimum 35% of their horsepower rating, primarily due to a substantial reduction of compression. Leaded fuels could be easily made to 100 octane with our method of testing, which produces lower numbers than European methods, and it would seem, your fuels as well.

IEthanol is also rated at 95 Octane it burns at a higher temp and older cars from the leaded petrol era are not advised to use Ethanol due to accelerated damage to valve seals.

Alcohols have a very low surface tension, and high solvency. One might expect trouble from it washing oil from engine surfaces. If the valve seals aren't in contact with crankcase oils, that certainly would accelerate their wear.

IVW Golfs in Australia have had serious problems with drivers that would ignore the manufacturers requirements for Premium Fuel and continued to use 91 Octane and in many cases caused serious engine breakdowns which VW refused to repair under warranty.

I'm thinking Volkswagen is having tough times all over, after the diesel emissions scandal. I honest to god have no idea why people keep harping about octane. This thread has nothing to do with octane, it's about the quality of gasoline, with respect, to refining, and fuel additives.

IIf you don't believe that Premium Fuel changes the nature of your engine try this. My lawn mower is a 1984 model yes 1984 Briggs & Stratton 148cc without an engine overhaul, the first model out for unleaded petrol and I use only 98 Octane with it, but I have tested it with 91 and the engine at the exact same throttle level will rev much slower.
I drive now on special diesel meaning low phosphor.

Well I suppose things are different in the land down under, but here in the US, sulfur is the pollutant of most concern.

And FWIW, more "octane" makes gasoline burn slower. Any horsepower gains exist in the ability to run higher compression engines.

Our "Oldsmobiles" used to say it right on the air cleaners "ultra high compression".

And IIRC, on the gas caps as well.

 

[PreachJohn]

US provides very low grade fuel in general - it is completely unsurprising about these results. To be honest, I thought grade of fuel and carbon buildup was common knowledge?

87/89/91 in California... their "top tier" is considered regular in Australia and I would imagine Japan and Europe where 98+ octane is common.

Octane has nothing to do with carbon build up, it is all about compression. Putting higher octane in a car rated for lower octane will do nothing special. It will actually, more than likely, hurt performance.

Every motor can at any state of wear/condition only use a certain amount of octane to advantage. More octane than that, you are only wasting your money. More than that again, you may start to burn your valves. And 'likely, hurt your performance' too.
Another case of if some is good, more is not necessarily better.

 

[PreachJohn]

US provides very low grade fuel in general - it is completely unsurprising about these results. To be honest, I thought grade of fuel and carbon buildup was common knowledge?

87/89/91 in California... their "top tier" is considered regular in Australia and I would imagine Japan and Europe where 98+ octane is common.

There is the fact that the US measures octane differently from other countries so our numbers are comparable to theirs just our formula ends up with different numbers. Most of Europe and many other countries use RON to measure octane while the US, Canada and a few others use AKI. Since they measure gas differently the AKI octane numbers come out lower than RON numbers for the same quality gas, because of that AKI 92 octane is about the same as RON 98 octane. You don't have "better" gas, you have the same quality gas just measured in a different way. Don't be so quick to jump to conclusions, you might trip and fall.

In Canada octane level is measured at the pumps in the Gas Station. In the USA octane level is measured at the Refinery.
The volatile butane, which contributes to the octane level reading dissipates in transit and storage. By the time it is in the tank of your vehicle it is somewhat lower in octane rating than posted.
If I buy 87 octane here in Canada, for that reason alone I buy at least 88 rated octane fuel when in the States. Notwithstanding the different calculation formulas. Which changes things again.

 

[brentano]

I live in Australia and have been driving for 50 years.
Octane ratings came in after we changed from leaded to unleaded petrol, yes we have 91 Regular 95 and 98 Octane Premium, then we have 10% Ethanol, as most manufacturers only recommend 10% Ethanol.

Well, I simply can't picture gasoline with NO octane rating leaded or otherwise. In fact, when the US switched from leaded to unleaded gas, 1974 to 1975, engines lost at minimum 35% of their horsepower rating, primarily due to a substantial reduction of compression. Leaded fuels could be easily made to 100 octane with our method of testing, which produces lower numbers than European methods, and it would seem, your fuels as well.

IEthanol is also rated at 95 Octane it burns at a higher temp and older cars from the leaded petrol era are not advised to use Ethanol due to accelerated damage to valve seals.

Alcohols have a very low surface tension, and high solvency. One might expect trouble from it washing oil from engine surfaces. If the valve seals aren't in contact with crankcase oils, that certainly would accelerate their wear.

IVW Golfs in Australia have had serious problems with drivers that would ignore the manufacturers requirements for Premium Fuel and continued to use 91 Octane and in many cases caused serious engine breakdowns which VW refused to repair under warranty.

I'm thinking Volkswagen is having tough times all over, after the diesel emissions scandal. I honest to god have no idea why people keep harping about octane. This thread has nothing to do with octane, it's about the quality of gasoline, with respect, to refining, and fuel additives.

IIf you don't believe that Premium Fuel changes the nature of your engine try this. My lawn mower is a 1984 model yes 1984 Briggs & Stratton 148cc without an engine overhaul, the first model out for unleaded petrol and I use only 98 Octane with it, but I have tested it with 91 and the engine at the exact same throttle level will rev much slower.
I drive now on special diesel meaning low phosphor.

Well I suppose things are different in the land down under, but here in the US, sulfur is the pollutant of most concern.

And FWIW, more "octane" makes gasoline burn slower. Any horsepower gains exist in the ability to run higher compression engines.

Our "Oldsmobiles" used to say it right on the air cleaners "ultra high compression".

And IIRC, on the gas caps as well.

You say Octane has nothing to do with this thread, well here in Australia it has, simply because our higher Octane Fuels have a finer refining process and added additives to keep modern engines cleaner and give you increased miles per gallon plus slightly better performance. Providing suitable compression ratings.
Higher Octane = more milage, cleaner engine, more adatieves to prevent corrosion and carbon build up, more $$$$$$ per litre. We pay on average 10 cents more per liter for 95 and 14 cents for 98 Octane.

 

[captaincranky]

You say Octane has nothing to do with this thread, well here in Australia it has, simply because our higher Octane Fuels have a finer refining process and added additives to keep modern engines cleaner and give you increased miles per gallon plus slightly better performance. Providing suitable compression ratings.
Higher Octane = more milage, cleaner engine, more adatieves to prevent corrosion and carbon build up, more $$$$$$ per litre. We pay on average 10 cents more per liter for 95 and 14 cents for 98 Octane.

Well, the premise of this thread is there are good refineries which make good gas, and no so good refineries which make not so good gas.

I don't think any of the people who are going on about "octane", have much of an inkling what it's for, or why it's necessary for some engines, but not for others.

If you like, I'll give you a few "buzz words" to bandy about. ""top dead center", "spark lead", "detonation","compression ratio".

So, in any event, all I've basically gathered is the every country in the world has better gas with higher octane than the US. You can't have detergents in gas if you don't pay the price and buy higher octane fuel than the motor needs to run on. Oh well, who didn't know the oil companies were ripping us off.

At least up here we (sometimes), can mention the price per gallon without too much sticker shock. (At least for the time being, anyway).

But then I forgot, the metric system doesn't have an equivalent measurement to our, "gallon", so that's pretty much a moot point.

 

[noc81]

Can say it a thousand times, and nobody seems to listen. Come on guys, being a nerd (or at least a geek) should certainly afford you enough technical understanding to understand how octane works..

ALL that higher octane means is the resistance of fuel to combust under pressure. As we aren't talking diesel here, if your gas combusts prematurely under pressure (referred to as detonation) it will cause undue stress on your valve train, as it will be combusting while your piston is on its combustion stroke still, and not on it's power stroke, which exerts pressure backwards on the crankshaft actually robbing a great amount of power while causing untold damage. If the gas can make it to the maximum compression without combusting, then your spark circuit combusts it near the top of the arc leading into the power stroke.

Problems ONLY occur in cases where the compression of the engine is high enough to pre-combust the gas/air mixture before the spark, and otherwise there is ABSOLUTELY no functional difference of different octane blends otherwise. Tout whatever snake oil voodoo claims you like, this is basic science. If you run a standard compression engine, there is no need to run a higher octane fuel -- it's just money spent for your peace of mind.

 

[noc81]

If you want to get more into the technical aspects of the different petroleum distillates used to stabilize gas into a certain octane, most are non polar solvents. Think of them as being more fat soluble than water soluble, which gives them their more slippery lubricating nature.

Added alcohol/ethanol, on the other hand, is a much more polar solvent, and has properties of being mostly water soluble and slightly fat soluble. It has a higher octane (though E85 obviously is blended to have a lower octane) so it is obviously more likely to be added into gas nearer to the 10% maximum in higher octane fuels. It's polar nature makes it a naturally more 'dry' feeling fuel. As it is more slippery than the other compounds in gas, it does slightly lessen the overall natural lubricating properties. Which means it is ever so slightly harsher on valves and seals (not as much as most would claim), which may reduce the overall appearance of gas varnishing, but will have absolutely no affect on performance or carbon buildup.

 

[Axiarus]

Can say it a thousand times, and nobody seems to listen. Come on guys, being a nerd (or at least a geek) should certainly afford you enough technical understanding to understand how octane works..

ALL that higher octane means is the resistance of fuel to combust under pressure. As we aren't talking diesel here, if your gas combusts prematurely under pressure (referred to as detonation) it will cause undue stress on your valve train, as it will be combusting while your piston is on its combustion stroke still, and not on it's power stroke, which exerts pressure backwards on the crankshaft actually robbing a great amount of power while causing untold damage. If the gas can make it to the maximum compression without combusting, then your spark circuit combusts it near the top of the arc leading into the power stroke.

Problems ONLY occur in cases where the compression of the engine is high enough to pre-combust the gas/air mixture before the spark, and otherwise there is ABSOLUTELY no functional difference of different octane blends otherwise. Tout whatever snake oil voodoo claims you like, this is basic science. If you run a standard compression engine, there is no need to run a higher octane fuel -- it's just money spent for your peace of mind.

Thank you. Someone gets it. Octane is all about compression ratios, nothing else.

 

[captaincranky]

Thank you. Someone gets it. Octane is all about compression ratios, nothing else.

I've gotten that, from the very beginning.

In fact, look at all the hints I dropped in post #70!

...[ ]...If you like, I'll give you a few "buzz words" to bandy about. ""top dead center", "spark lead", "detonation","compression ratio"..

In fact, given an engine set up to run on regular gasoline, too much octane could conceivably cause you to lose a tiny bit of power, as opposed to gaining power. That's because you could have the maximum fuel explosion pressure after TDC, when the piston is already traveling downward. <Which would reduce the effective compression ration dramatically!

Once upon a time, you could prove that dynamic in a much more dramatic fashion. All you had to do, is retard the ignition timing from factory optimum. The spark then would occur after TDC, and engine power goes to s***.

The same effect is possible with high octane fuel in a low compression engine, although I do doubt the effect would be as noticeable as say, pulling the vacuum line off the distributor.

Another hint, you would have to plug the vacuum line you pulled off the distributor, or the vacuum leak would invalidate the test.

Just out of curiosity, does anybody remember having to set engine timing with a strobe light, instead of the ECM doing it for you via a crank position sensor?

 

[noc81]

Actually, I have to break out my trusty old timing light this very afternoon.. The sbc in my trans am hasn't been started in years; so I need to pull the distributor, pull the plugs and squirt some oil in each chamber, turn the engine by hand a few cycles, put the engine at absolute TDC, prime the oil pump/system, and then put the distributor back in and hopefully time it close enough to get it started so that I can rotate it to the proper ignition advance with the vacuum advance plugged. I should state that I blueprinted my distributor, working out the perfect advance curve (both vacuum and centrifugal) to give me a good balance of performance with plenty of advance while cruising at low rpm in overdrive.

So yeah, I remember how to time an engine.. x]

Oh, and I was mostly referring to yours and others' posts when saying 'how many times..'.