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3.9 and premium

Discussion in '2002 - 2005 Ford Thunderbird' started by whvt01, Jun 1, 2003.

  1. This has been discussed quite a bit. With most saying they
    use what the manual says. But has anyone read just
    why the motor needs premium. Its not a large displacement or
    producing that much horsepower (250). There are
    numerous makes producing that kind of hp out of V6's
    and still running on regular. Even the town car motor is about
    the same size and HP with a regular rating. I can see
    the higher quality Jaguar motors with high horsepower
    but not the base 3.9 in the Thunderbird.
  2. Premimum fuel is required because of the compression ratio. I believe the Bird has a 10.5:1 compression ratio. It'll run just fine on regular while the milage is low but later on it will develop ignition knock. On my 03, I get a lot lower gas milage with regular. It doesn't knock, but it's cheaper for me to buy high test because of the difference in gas milage.
  3. Compression ratio, had no idea what the rating was on
    the 3.9.. Also I didn't realize just how small the motor
    was till yesterday saw a post referring to it in cubic inches.
    I'm still use to the old C.I.D theory motor references.

    Something like 240 cid. I had a jeep and a turbo Buick
    with similar sized motors, which felt under powered to
    me. When think of V8's still think of larger displacements
    which aren't there anymore. So what little is there guess
    the bird is doing pretty good.

    The gas mileage on the car is probably around 13 combined
    C/H so wouldn't want to lower it any more by using regular.
    That is one thristy little motor.
  4. My new 2003 has more than enough power for me, even though I would like a supercharger. I use super premium and my mileage is about 16/23 C/H. That is real good compared to my 2002 V8 mountaineer. I do not want to try reg in this vehicle to save a few $$ after what the car cost.
  5. tbird

    tbird Forum Moderator Staff Member Lifetime Donor

    In case you are curious:

    Convert Liters to Cubic Inches = Liters / .016386

    3.9L engine = 238 Cubic Inches

    4.6L engine = 280 Cubic Inches

    5.4L engine = 329.5 Cubic Inches
    ChuckH262 likes this.
  6. You can also take the liters times 61 which will give you the cubic inch displacement. It is a slightly less accurate way but it works for estimating. 5.0 liter 61 x 5 = 305 or 3.9 liter 3.9 x 61 = 237.9 . It will get you by in a pintch.
    ChuckH262 likes this.
  7. I work for an oil company major.

    Premium VS Regular gas makes NO difference in gas milage!!!
    The gasoline suppliers will attest to that!
    Premium is for better performance ONLY. Better power and drivability.
    As reported our cars need 91 octane since the compression ratio is over 10/1, use of a lower octane eg 87 for regular, will cause pinning, premature detonation which results in run on.

    The use of a mid grade at 90 octane would be OK, if the car is not used in severe service, pulling a trailor, racing, in hot stop and go traffic etc.

    If you let the motor ping and or run on and Ford can prove you used poor fuel ' which is easy to do, you can expect big bills!
  8. In regards to engine displacement. Ford rounds up or down depending on what the promotions or media department wants. The 302 (5.0) Mustang has a 4.00" Bore and 3.00" stroke. Therefore the displacement is actually 301.592894746 cubic inches. This translates to a 4.94222 liter engine. However, since chevorlet had a 305 or 5.0 advertised engine; Ford decided to go with rounding up to a 5.0 instead of a 4.9 as most elementry school children learn to do in rounding numbers.

    For the Thunderbird, its displacement was chosen by media departments, as well as, Lincoln, jaguar, and toyota and its legal threats to Ford. It seems Jaguar uses the same or mostly the same engine and calls it the 4.0 (at least in 2000). Lincoln wanted to call the Lincoln LS the Lincoln LS4 and LS3. Toyota (Lexus) threatened to file a lawsuit if this was done because it too closely matched the Lexus LS400 and LS300 at the time (2000). Since the pending lawsuit required Ford to drop the LS4 in favor of LS; Ford also decided to not compete with its own company (Jaguar, Ford owns this company) and lowered the displacement to 3.9. For the most part that is the story.

    The Thunderbird displacement is set by the bore and stroke and no amount of advertising, legal battles, etc can change that. The bore is 3.39" and the stoke is 3.35" (if my ford supplied information is correct). Therefore the car has a 241.893429460 cubic inch engine (Motor Trend Car of the Year reported 241.9 which more than close enough, I call is 242 cubic inches) All that said the liter displacement is 3.96392310974 or using rounding 4.0 Liters per the lession taught to all of us in grade school.

    I know everyone will be suprise to know that my license plate says 4.0 V8. It sounds better and is more correct than 3.9.

    Anyway, in response to gas mileage. Yes you will get slightly less miles per gallon on regular versus super unleaded, because the engine retards the timing under harse conditions due to knock sensors and this lowers the performance of the cars. You will not notice this for the most part. The decrease in mileage per gallon is made up for in the cost per gallon. Think of it this way, miles per dollar. The car may lose 1 mile per gallon on regular (no data to back the exact loss, but I have tried both and this works for my car). If you get 23 miles per gallon on super and 22 miles per gallon on regular and the cost difference is 20 cents gallon (1.30 regular and 1.50 super) then the car goes 16.9 miles per dollar on regular and 15.3 miles per dollar on super. For purely economic reasons, the better value is in the regular gas.

    I use super because I am one of those fools who claim I need all the performance I can get regardless of cost. However, in my wifes Acura TL 3.2 which also calls for super, I use regular. My wife only wants to get places and does not care to get every last horsepower from the engine. She choses to use the money elsewhere.

    Well, I hope this clears up any questions you may have had.

    Paul Hackleman, P.E.
    ChuckH262 likes this.
  9. 4.2 Liter Jaguar

    I did a little research on the Jaguar engine. It has a longer stroke. Bore is the same. It must have a different crankshaft. Also the supercharged version has 9:1 comression and the 300HP engine has 11:1 compression. They muust be using different heads or crankshafts.

    ChuckH262 likes this.
  10. SMH! I hesitate to chime-in here because I always end-up getting beat-up when I've either posted this or opened this dialogue with friends and/or automotive enthusiasts. There is so much myth about gasoline and how it impacts a MODERN engine. I assure ALL of you that you are flushing money out your tail pipes.

    I'll explain...

    • The 3.9L engine in your Thunderbird is a high-compression (performance) engine, with a compression ratio of 10.75:1. Regardless of what gasoline you put into the tank, the compression is NOT going to change. ALL gasoline has the same energy content...PERIOD!

    • ALL gasolines sold in the United States have fuel additives which clean the combustion chamber. There is no increase or decrease in the amount of (nitrogen-based) cleaning additive in any given gallon of U.S. gasoline. These additives are required by FEDERAL law. These additives will clean carbon deposits as the additive in the gasoline comes into contact with the internal engine surfaces. Though, different fuel retailers will label their fuel additives by differing marketing terms, it's ALL THE SAME cleaning performance. Adding a bottle of fuel injection cleaner, etc. to a tank of modern gasoline is wasted money (and depending upon the chemicals in that fuel-injection cleaner, potentially harmful to engine sensors). The same cannot necessarily be said if you fill-up in Mexico.

    • Gasoline-air vapor will self-combust (that is, ignite/explode) when compressed. Modern gasoline has additives in the fuel which helps it resist this phenomena...dependent upon the level of additive in the fuel, the more resistance that fuel-air mixture has to combustion under compression. That level of resistance is known as 'octane'. There are several different ways to measure it's resistance, but in the United States we use the (R+M/2) method. This is the number posted on the pump.

    • All MODERN engines have acoustic "knock" sensors installed on them (your Thunderbird is no exception) which detect preignition/predetonation (a.k.a. "knocking" or "pinging") constantly (hundreds of times per second). If the sensors detect engine/spark knock, the ECM automatically and instantaneously adjusts the spark timing to back-off of the ignition advance to eliminate knock.

    • If you put 'regular' (87-octane) gasoline in your tank, and start your car, it will run and idle just fine. As you drive it gingerly down the road, it will run and perform just fine. As you accelerate hard (put a demand on the engine for more performance), the ECM will advance the ignition timing to account for the milliseconds of expanding ignited fuel energy so that the engine can get the most efficient use of that energy to push the piston down. At the same time, the ECM is listening for spark-knock. If the ECM detects spark-knock, it will back-off of the ignition advance (slightly) until the spark-knock is no longer detected. This happens hundreds of times per second, and is indicernable to the human ear or other senses. When the ignition timing advance is 'retarded', the engine is not operating a 'peak' performance, and therefore, you are not able to eek the maximum engineered horsepower out of the engine at that moment. This would be known as a performance 'degradation'. Since your Thunderbird engine is naturally-aspirated (not turbo-charged or super-charged), the instances of pre-ignition/detonation is minimal (comparatively-speaking).

    Now, with all that said, what does this really mean to a lay-Retrobird-driver? Well, if you decide that you don't want to pay an additional ~50¢/gallon at the pump and opt to fill the tank with 'regular' (87-octane) fuel, you're going to save about $9.00 per fill-up. If you drive your Thunderbird as a cruiser, you're going to realize little to no difference in the performance of your car. You will NOT gain or lose any MPG (remember, there is no difference in energy content of premium fuel versus regular fuel (or anywhere in-between))! Now, if you drive your car like you stole it, accelerate hard from a stop, and put your foot into it when your "accelerating" on the highway, you may be able to notice, or I should more accurately say "measure", a minor horsepower reduction as opposed to the same driving-style with a tank of 91-octane. Why? - because the ECM is detecting spark-knock and retarding the ignition advance to eliminate it, which takes away from the maximum-engineered engine performance potential. So, if you don't drive your Thunderbird hard, you can opt for 87-octane fuel and save yourself approximately $450 annually which you can put into your maintenance fund, or spend otherwise.

    Ford has included this information (from it's engineers) in your operator's manual:

    "Your vehicle is designed to use 91 “Premium” unleaded gasoline with an (R+M)/2 octane rating of 91 or higher for optimum performance. The use of gasolines with lower octane ratings may degrade performance. We do not recommend the use
    of gasolines labeled as “Premium” in high altitude areas that are sold with octane ratings of less than 91. Do not be concerned if your engine sometimes knocks lightly. However, if it knocks heavily under most driving conditions while you are using fuel with the recommended octane rating, see your dealer or a qualified service technician to prevent any engine damage." (keyword(s): "optimum", "may").

    If you understood all of that, consider yourself a wiser consumer. Don't let myth, marketing, or outdated information ruin your wallet.

    If you prefer, you can spend 15-minutes watching this video (which reiterates everything I've explained) and save yourself THOUSANDS of dollars over the ownership lifetime of your Thunderbird (or other automobile)...

    If you're one of those people who puts 100% nitrogen in your tires, you'll probably still put 91 octane in your tank, and please DO! My diversitized retirement plan holds stocks in Exxon, Chevron, Shell and other energy companies, so I'm not offended in the least.
  11. That is a pretty thorough analysis there ^^^

    I've tried a little experiment since taking ownership of my 03.

    Two tanks of "regular" which in my corner of Texas is 87 Octane - idles fine, pings slightly under even modest acceleration - averages of 14.9 and 14.4 mpg

    I'm on the second tank of "Plus" which is 89 or 91 Octane IIRC - idles fine, no ping, throttle does seem a little peppier under acceleration but my driving style stays relatively the same. One time two nights ago, I started the car and it idled very erratically for about 10 seconds then 'normalled out' and ran fine ever since. First tank average 12.9 mpg :eek: - I'm 119 miles into tank #2 at the halfway mark, but the "3rd quarter" seems to go the fastest so that doesn't mean much.

    I'll be trying 2 tanks of "Premium" which is 93 Octane here for comparison the next two.

    Basically, I was looking at about 6 dollars or so per tank extra at 89/91 ... probably about 9-10 for 93 octane. While I like the extra pep in the step with proper fuel, I'm curious about the MPG change, though I suspect it's possible I may have left my foot in it a little longer when I noticed the leap. :D
  12. vinnie

    vinnie Aways Learning Lifetime Donor

    In my neighborhood the difference between mid-grade (89-91) and premium (93) octane is 30-35 cents per gallon. Since I've been thoroughly seduced by the Shell Tier One V- power claims to clean everything I tend to spend the extra 5 or 6 bucks per fill up for piece of mind, real or perceived. My Retro is pleasure/cruiser maybe 3k per year. Besides, I have more money than I know what to do with! :rolleyes:
  13. Mach5

    Mach5 2004 Merlot Lifetime Donor

    I've always used Shell 87 Octane in my 2004 daily driver since purchase back in May. Runs smooth, no problems or issues at all. My mechanic basically echoed JRM above. He said the only thing that causes issues is if you switch back and forth between 87 and 91 octane - it confuses the computer. I tend to drive gently - keep my foot out of it.

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