It's the 2.4l in my new Accord. iirc, it's 10:1 and def calls for 87. I used mostly 89 so far, because it was running better then 87...fair enough. I snagged a good deal on Exxon 93, so went w/ a full tank. I know 93 burns slower, driving the piston more smoothly, but it has no greater thermal content. What's the deal? btw, the effect was instant upon changing fuel.
New car speced for 87 runs better on 93...why?
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I have a 2007 Civic LX (compression ratio 10.5:1) and use 93 octane Shell V-Power exclusively. I like the extra pep that using V-Power seems to provide. Your Accord should have a Knock sensor, which will advance/retard the timing based on gasoline quality. I believe that the higher octane gasoline will cause the timing to advance, giving the feeling of extra horsepower. If you check your Owners Manual, I believe you will find that your Accord can use 87 octane "or higher" gasoline. That is the wording Honda chose to use in my Civics Owners Manual anyway.
Contrary to popular belief, 93 octane gasoline does not burn slower. 93 octane gasoline resists detonation better than lower grades, which helps to prevent detonation before the spark plug fires. Do a search on some of my posts and you will find that this is the case.
Contrary to popular belief, 93 octane gasoline does not burn slower. 93 octane gasoline resists detonation better than lower grades, which helps to prevent detonation before the spark plug fires. Do a search on some of my posts and you will find that this is the case.
I always run Premium. I feel it's worth the extra money. My car runs better and gets better fuel mileage. One thing I always wonder, will running premium lessen or increase engine lifespan? Or have no effect at all in engine life.
Fisher is correct in saying that octane is used to control detonation. That actual quality of the gas (87 vs. 91) isn't any different if you are getting it at the same place but they use the 'Premium' label as marketing to make people think that. You can run just about anything at the pumps with the newer cars because of the sensors but I would agree that with 10:1 and over you should see some performance improvements because of the timing adjustment. I've watched some people at the track dump 110+ octane fuel in their 9-10:1 cars thinking it will give them more power and it was better fuel, well the results were them draining that gas and going back to the pump for 91.
My Soob EJ22 is factory stock at 10:1, calls for 87, and exhibits no difference in MPG when fed 87, 89, or even 93 octane, so she gets 87.....
Dave
Dave
I'm curious as to how your engines sound with the higher octane. Do you hear the slight pinging going up a small grade as you accelerate more with the lower octane? Does the engine seem to run smoother with the higher octane?
I know that the computers can compensate for a lot of variables these days, but still feel that there is a threshold level that can be crossed an have the engine sound and run better, without having to go "all out" with performance parts, racing fuel (adding a gallon here or there, or per tank is OK. I am referring to filling up with a tank of it), or octane boosters.
I know that the computers can compensate for a lot of variables these days, but still feel that there is a threshold level that can be crossed an have the engine sound and run better, without having to go "all out" with performance parts, racing fuel (adding a gallon here or there, or per tank is OK. I am referring to filling up with a tank of it), or octane boosters.
In the case of Shell Premium, the detergent package is significantly higher compared to the lower grades.
With significant technical advancements RE: engine control and managements, a lot of high compression ratio'ed gasoline engine cars can now do with just 87octane rating for the engine will dynamically "map" the ignition timing to suit the particular gasoline in-question. This is normally done in a fraction of a second and a knocksensor (usually a piezo pickup) serves as the feedback in keeping track of the gasoline octane and engine load changes.
Engines of this type does not require any exotic high test/high octane gas for the engine management control will auto-compensate it just like it should with 87octane gas.
2 things that I simply cannot stress enough:
-automobile pump gas sold in N.A. contains more or less same energy content and flame propagation speed. The difference is in the relative level of anti-knock agent added (to alter the octane rating) and perhaps the degree of gasoline-related additives such as detergents, etc.
Special race gas for engine revving on an upwards of 10,000rpms does not apply and neither should AV-gas (different composition designated for different application and needs).
-The reason why there were so many different grades of pump gas has to do with the older engine designs where due to their combustion chamber designs and a/f mixture swirls, flame front propagation behaviour,etc., as well as other factors such as combustion chamber deposits accumulation as the engine ages(decrease in combustion chamber volume/increase compression ratio as a result), the call for higher octane rating to combat pre-ignition (thus that physical "budt dyno" or "budt sinking" sensation or better overall idling and accelerating sensation.
Also is to blame for is the relative ancient ignition mapping/advance designs with yesteryear's engine management/control with distributor setup, centrifugal advance+vacuum advance motor, and static idle ignition timing setup, etc. All of these can be benefited with a bit of octane increase from minimum requirements as the mileage accumulates.
No, higher octane gas doesn't equates to slow flame speed (we are talking about automotive pump gas in N.A. and race/Av-gas doesn't apply here) at all.
My 2c's worth.
Q.
p.s. My Honda fit 1.5L comes with a 10.3:1 compression ratio and calls for 87octane rating gas. I've tried up to 94octane gas and no difference in mileage and budt-dyno sensations, except the overpaid yet unneeded anti-knock additives.
Engines of this type does not require any exotic high test/high octane gas for the engine management control will auto-compensate it just like it should with 87octane gas.
2 things that I simply cannot stress enough:
-automobile pump gas sold in N.A. contains more or less same energy content and flame propagation speed. The difference is in the relative level of anti-knock agent added (to alter the octane rating) and perhaps the degree of gasoline-related additives such as detergents, etc.
Special race gas for engine revving on an upwards of 10,000rpms does not apply and neither should AV-gas (different composition designated for different application and needs).
-The reason why there were so many different grades of pump gas has to do with the older engine designs where due to their combustion chamber designs and a/f mixture swirls, flame front propagation behaviour,etc., as well as other factors such as combustion chamber deposits accumulation as the engine ages(decrease in combustion chamber volume/increase compression ratio as a result), the call for higher octane rating to combat pre-ignition (thus that physical "budt dyno" or "budt sinking" sensation or better overall idling and accelerating sensation.
Also is to blame for is the relative ancient ignition mapping/advance designs with yesteryear's engine management/control with distributor setup, centrifugal advance+vacuum advance motor, and static idle ignition timing setup, etc. All of these can be benefited with a bit of octane increase from minimum requirements as the mileage accumulates.
No, higher octane gas doesn't equates to slow flame speed (we are talking about automotive pump gas in N.A. and race/Av-gas doesn't apply here) at all.
My 2c's worth.
Q.
p.s. My Honda fit 1.5L comes with a 10.3:1 compression ratio and calls for 87octane rating gas. I've tried up to 94octane gas and no difference in mileage and budt-dyno sensations, except the overpaid yet unneeded anti-knock additives.
You must have been tripping the knock sensor with 87.
Most knock sensors are to sensitive, and go off too easily and then pull WAY too much timing.
Honda street tuners usually unplug the sensor, and use good gas.
There was a Ford service bulletin that advocated unplugging the knock sensor to fix certain driving complaints.
So they are not a perfect system.
They can help with poor gas, but in reality in this country it just doesn't happen - it is rare.
Most knock sensors are to sensitive, and go off too easily and then pull WAY too much timing.
Honda street tuners usually unplug the sensor, and use good gas.
There was a Ford service bulletin that advocated unplugging the knock sensor to fix certain driving complaints.
So they are not a perfect system.
They can help with poor gas, but in reality in this country it just doesn't happen - it is rare.
Ok, well I don't understand what ignition timing has to do with octane. I understand how more compression = need for octane and that the amount of charge affects the ~relative~ compression. I just don't see how +/- octane = +/- timing advance.
As timing advances, detonation is more likely. When detonation starts, back off the timing and it'll go away.
Dave
Because higher octane burns at a slower rate. That is what octane is and does. Lighting the fire sooner by increasing ignition timing creates a need for the fuel to burn slower so that cylinder pressure is evenly controlled. If cylinder pressure is too high too soon, "knock" and "ping" result.
Here is the key you're all missing... dynamic versus static compression ratio.
Items that contribute to needing higher octane:
1. High Compression Ratio
2. Advanced Ignition
3. Fat lobed camshaft
4. Engine temperature (head/piston surface)
5. Coolant Temperature
6. Air inlet temperature
7. Combustion chamber design (hot spots)
8. Forced induction (turbo/super)
By having an aluminum block you get away with a higher compression due to the lower cylinder temperatures, dynamic valve timing, and dynamic computer controlled ignition timing. You might want to try 5 tanks of 93, then 5 tanks of 87, just to see if you can feel any difference. I have done a few in my subaru (10:1 CR) with no effect on mpg. However, in the hot summer I find running a slightly higher grade helps with engine power when heat soaked in the city. Otherwise 87 runs just great for me especially in the winter.
Items that contribute to needing higher octane:
1. High Compression Ratio
2. Advanced Ignition
3. Fat lobed camshaft
4. Engine temperature (head/piston surface)
5. Coolant Temperature
6. Air inlet temperature
7. Combustion chamber design (hot spots)
8. Forced induction (turbo/super)
By having an aluminum block you get away with a higher compression due to the lower cylinder temperatures, dynamic valve timing, and dynamic computer controlled ignition timing. You might want to try 5 tanks of 93, then 5 tanks of 87, just to see if you can feel any difference. I have done a few in my subaru (10:1 CR) with no effect on mpg. However, in the hot summer I find running a slightly higher grade helps with engine power when heat soaked in the city. Otherwise 87 runs just great for me especially in the winter.
Jim makes sense...but. I am sort of lost. Let's define something, detonation. We mean detonation from compression, not spark, right? So, basicly is or is it not possible to detonate low grade gas at a 10:1 ratio? If so, then we have a diesel engine, needing no SI. And we'd have engines starting up with just fuel.
Now, if the mix cannot be "detonated" by compression, how do we end up with a knock condition anyway? I understand there could be hot-spots.
To simplify, since compression (ratio) and detonation or knock are physically dependent on each other, how then does *electrical* spark ...matter? The resistance to detonation *itself* being the definition of octane.
The only way I can understand this is, like was said, the enlarging volume of the partly ignited fuel takes up a more and more space in the combustion chamber THUS changing the ACTUAL ratio of compression far beyond 10:1. The physical dimensions of the combustion chamber are just a starting point. Uh? Hmm, in this case then, the timing itself (sort) of controls the compression ratio, but starting the burn and changing the volume of physical gas in the chamber. Did I figure it out? lol.
Now, if the mix cannot be "detonated" by compression, how do we end up with a knock condition anyway? I understand there could be hot-spots.
To simplify, since compression (ratio) and detonation or knock are physically dependent on each other, how then does *electrical* spark ...matter? The resistance to detonation *itself* being the definition of octane.
The only way I can understand this is, like was said, the enlarging volume of the partly ignited fuel takes up a more and more space in the combustion chamber THUS changing the ACTUAL ratio of compression far beyond 10:1. The physical dimensions of the combustion chamber are just a starting point. Uh? Hmm, in this case then, the timing itself (sort) of controls the compression ratio, but starting the burn and changing the volume of physical gas in the chamber. Did I figure it out? lol.
I see this conversation went deeper into detail than I originally would have thought. Yes, we can talk about static/dynamic compression, flame propegation, cam profiles, etc.. you're right they all play a role. But for a simple explanation for the original question, detonation occurs when the pressure applied on the fuel charge, combined with combustion tempertures cause the charge to ignite before proper piston position. Octane helps stabilize the charge from this pre-detonation and allows the spark to control when the mixture is burned. When you have to much octane for the temps/pressures of your engine, flame propogation is difficult to achieve. Spark timing can help some with inadequate fuels, cam timing can 'move' dynamic comp. pressures up or down the rpm range which will help with pre-detonation more than spark timing. Cam timing that is good for starting and low rpm isn't always true for higher rpm, that's why you are seeing more engines with variable cam timing to make the engine more efficient throughout the rpm range. I hope this made some sense. It's such a 'complicated and involved' topic to cover over a forum.
Not enough octane or pre-detonation sounds like some nuts in bolts in a coffee can being shaken by a 10 year old with ADHD. The other direction the engine sounds like you pulled a few spark plug wires. The motor has a hard time generating power and everyone around has tears in their eyes.
We can't have compressioin detonation before the valves are closed. So again, more confusion. How could cams matter? I'm going to wait for a while and think before I post again.
Scenario: You're running 87 octane on a hot day in a hot engine (Think poorly maintained Tercel in Death Valley)
Detonation will occur because during the compression stroke (valves close) the gasoline ignites from pressure/heat alone before the spark actually ignites the fuel. This is what is avoided by running a higher octane gas. Ignition temperature/pressure is simply higher.
Gasoline is a mix of heptanes (7-carbons) and octanes (8-carbons) - Im not going to get into the chemistry beyond this but know that more carbons = harder to explode.
Detonation will occur because during the compression stroke (valves close) the gasoline ignites from pressure/heat alone before the spark actually ignites the fuel. This is what is avoided by running a higher octane gas. Ignition temperature/pressure is simply higher.
Gasoline is a mix of heptanes (7-carbons) and octanes (8-carbons) - Im not going to get into the chemistry beyond this but know that more carbons = harder to explode.
simply put... running 93 gives your engine a higher limit for spark ignition and combustion temperatures. If your ignition timing table can adjust that far to take advantage of it, you will get more power.
You will encounter detonation most often during low-rpm, high torque, full-throttle (3rd gear, 2000RPM, throttle on floor) if you have any.
You will encounter detonation most often during low-rpm, high torque, full-throttle (3rd gear, 2000RPM, throttle on floor) if you have any.
Try to think in terms of a new term. "Cylinder Pressure". Cylinder pressure is effected by all those things posted. The higher the cylinder pressure, the harder the piston is forced down, and the more the power. Cylinder pressure is raised with hotter cam profiles, smoother ports of the right size, less restrictive exhaust, proper fuel mixture, optimally timed ignition, and of course, forced induction.
Also think i terms of inertia. The column of air entering the cylinder has a lot of inertia. That is why the valves can be held open longer and for that matter why the valves can even overlap.
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