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Old 07-01-2014, 09:13 AM   #309
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OK James,
We don't disagree on much of anything and I don't disagree on this either. I didn't want to get to technical on the forums. In theory two identical engines in displacement in two identical vehicles, if one has a higher compression ratio it needs a higher octane fuel to achieve more HP.
If nothing else is changed just adding higher octane fuel to an engine that only requires 87 won't improve it's performance. And that's the point that was being made. Using 87 octane in an engine that requires 91 will compromise that engines ability to use the extra compression because of potential preignition.

We both know that the higher compression engine will also have other parameters changed so it can take advantage of the higher compression and potential increase in HP.

Have a great day

TeJay
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Old 07-03-2014, 06:35 AM   #310
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Taken a little further

Taking this thought a little further, maybe you technical guys can answer the same 87 vs 91 otctane based on the fact that at most stations near me the 87 octane gas is a gas/ethanol blend, generally 10%, where as the 91 octane usually is ethanol free. Now I have vague recollections of reading about the burn properties of gas vs gas/ethanol blend, or that the pure gas will have more btu's per gallon than the blend, or something along those lines. Care to comment for our benefit.
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Old 07-03-2014, 07:31 AM   #311
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Your question is multi faceted, so Ill break it down like this:

1. There is practically zero gas at a normal pump that doesnt have some level of ethanol in it now. Unless youre lucky enough to find someone selling "pure" gas, 87, 89 and 93 all has ethanol in it. Some stations have less ethanol, which is why the pumps say "up to 10%", BUT, for the most part they are pretty consistent. Having a racecar that does not prefer the corn, I have been testing my gas for ethanol content for years up until recently. I had a favorite station that consistently had less ethanol. I would wager if you actually tested what comes out of your 91 tap, its about 10% ethanol too.

2. Yes, ethanol has less BTUs per gallon than gasoline. That is one of the primary reasons why you burn more and get less with ethanol laced gas. The comparison really shows when comparing something like E85 to regular gas.

3. For the most part, I generally agree with James and TeJay, HOWEVER, I will offer one specific set of circumstances in which more octane is not a bad thing. My race motors are mild builds, 9.4:1 compression, very basic modifications. No wild cams, super duper valve jobs etc. Im running GTX260, which is 98 octane and ~$7.50 a gallon currently. Why? Because I had two meltdowns on 93 octane, which I attribute to a couple of factors. First, in each situation, I experienced a low RPM, high temperature, high load condition. In both cases, the motors detonated themselves to death in a matter of seconds. Ive got the data to back this up. Second, in both cases, I was running 93 octane with ethanol in it. Pump gas has a fair bit of "other" crap in it (detergents and additives) that I would say were contributing factors, but probably not deciding factors. I have had the same issue while running GTX260, and that motor survived. Was it better octane, less crap in the gas, or pure dumb luck that the motor didnt let go? Id like to believe a combination of the first two. Now, that being said, when would you experience these types of conditions with a Ford V10? My guess is.... never. I cant recall, even loaded to GCWR on my F53 based motorhome, ever seeing the coolant temperature above 205, even on decent climbs across the Appalachian mountains. Granted, theyre not the Rockies, but still. For reference, I generally see temps on the racecar at 205-210 and as high as 225 in the draft. Guess where the temp was when the last motor went boom?

My point is, sure, there is a time and place for higher octane than whats specified, within given circumstances of course. The Ford V10 aint it.
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Old 07-04-2014, 09:43 AM   #312
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Thanks for the post. That's some (real world), well at least the racing world, good information and another perspective on the subject.

I remember a TV show on how gas gets distributed throughout this country. There are over 150,000 miles of underground pipe lines taking fuel everywhere. It is very common for many different company tankers filling from the same location. So I guess once a Mobil tanker fills up it's now Mobil gas and when a Shell tanker fills up it now Shell gas. I also remember that some tankers put their additives in after they were loaded. They have to keep the octanes separated but from there I guess it's anybodies business or maybe nobodies business.

TeJay
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Old 07-07-2014, 09:23 PM   #313
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Quote:
Originally Posted by cucamelsmd15 View Post
3. For the most part, I generally agree with James and TeJay, HOWEVER, I will offer one specific set of circumstances in which more octane is not a bad thing. My race motors are mild builds, 9.4:1 compression, very basic modifications. No wild cams, super duper valve jobs etc. Im running GTX260, which is 98 octane and ~$7.50 a gallon currently. Why? Because I had two meltdowns on 93 octane, which I attribute to a couple of factors. First, in each situation, I experienced a low RPM, high temperature, high load condition. In both cases, the motors detonated themselves to death in a matter of seconds. Ive got the data to back this up. Second, in both cases, I was running 93 octane with ethanol in it. Pump gas has a fair bit of "other" crap in it (detergents and additives) that I would say were contributing factors, but probably not deciding factors. I have had the same issue while running GTX260, and that motor survived. Was it better octane, less crap in the gas, or pure dumb luck that the motor didnt let go? Id like to believe a combination of the first two. Now, that being said, when would you experience these types of conditions with a Ford V10? My guess is.... never. I cant recall, even loaded to GCWR on my F53 based motorhome, ever seeing the coolant temperature above 205, even on decent climbs across the Appalachian mountains. Granted, theyre not the Rockies, but still. For reference, I generally see temps on the racecar at 205-210 and as high as 225 in the draft. Guess where the temp was when the last motor went boom?

My point is, sure, there is a time and place for higher octane than whats specified, within given circumstances of course. The Ford V10 aint it.
I have to ask what type of race engine is that. 9.4:1 compression is pretty weak by todays standards with most automotive gasoline engines. In fact newer turbo cars are around 10:1 with 15 psi of boost on 87 octane fuel with a slight increase in power when 91+ is used through advanced timing.
Now base compression for a 2 stroke I would fully understand. My Jetski's are a base of 6ish:1 I believe but add the tuned pipe and the supercharging affect takes place.
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Old 07-08-2014, 06:23 AM   #314
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Originally Posted by jamesrxx951 View Post
I have to ask what type of race engine is that. 9.4:1 compression is pretty weak by todays standards with most automotive gasoline engines. In fact newer turbo cars are around 10:1 with 15 psi of boost on 87 octane fuel with a slight increase in power when 91+ is used through advanced timing.
Now base compression for a 2 stroke I would fully understand. My Jetski's are a base of 6ish:1 I believe but add the tuned pipe and the supercharging affect takes place.
A legal one.

No really, its a 20 year old 4 cylinder that was actually meant to be a turbo based motor. Mine are pretty mild builds, forged pistons/rods, medium cams and gears, backcut valves, basic port and polish. Its all I need to meet my power cap.

To TeJays point though, outside of race conditions, I doubt you would ever be able to replicate those conditions in a modern car for one reason: Automatic transmissions. Well, its probably worth clarifying that both knock sensing/control and electronic ignition/fuel control both are a key factor too, but really the transmission wont allow you to sit and lug the motor where you are high throttle input/high temperature and low RPM for an extended period of time. In my case, I probably could have saved both those motors if I had the wherewithal to pay attention to my shifting but by the time I realized what had happened, it was too late.
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Old 07-08-2014, 08:06 PM   #315
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A legal one.

No really, its a 20 year old 4 cylinder that was actually meant to be a turbo based motor. Mine are pretty mild builds, forged pistons/rods, medium cams and gears, backcut valves, basic port and polish. Its all I need to meet my power cap.

.
Any more details on these engines? What comes to mind is the Ford 2.3L engines. They were a rather popular 4 cyl build and really liked turbos. I had a turbo 2.3L when I was younger. Very fun car. I assume there is plenty of advanced spark timing or really high rpms.
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