Hey guys, I just had a thought, the 240ci Straight-6 in my 1966 F100 runs like a Swiss watch, but I was reading in my owner's manual and it said it required 94 Octane gas. I know this was back when the gas was leaded, and modern gas is much different, but I'm wondering, with the 9.4:1 compression ratio, and timing of 6 degrees BTDC, what gas I should be running in it. The engine sounds fine as far as I can tell, but I'm not too familiar with this awesome engine.

 

I'm looking forward to the answer as well. I have the same motor and have been crossing my fingers while filling it with 87. Try to buy marine gas (non-ethanol) when it isn't an empty pump.

 

On these old cars and trucks I usually stick with running 92 octane for piece of mind. they were made to run on the old gas before they started all this ethanol crap. I can definitely tell the difference on my Mopars between the low octane and high. Just like the oils now days I always use a zink additive everytime I do an oil change. I've seen multiple rebuilt engines fail due to not using any additives and wiping out cams and lifters.

 

I use 87 in "Shrek" my 66 F100 with the 240, course he has unknown miles, valve guide seals are shot and so are the rings,,, but he still purrrss and runs like a champ, albeit with a bit of smoke on the downhills, and uphills,,, and flats He still gets 18+mpg to Fresburg and back. I do have a 302 in the garage waiting to go in while I rebuild the 240. But I run 87 in all my older trucks,,, all the fuel here in Kalifornia sucks.

 

Everything in my yard drinks the same cheapest on the corner 87 octane. Spend your money any way that you want but you don't need to waste your money on higher priced gas. IMHO

I aint got no race cars.


John

 

Something interesting I noticed, I put 92 ethanol free gas in my truck with a bottle of octane booster because i had 1/2 a tank of 87 in it, and maybe its just me, but it sounds smoother, and had slightly better throttle response and smoothness at lower RPM.

 

May I ask a question? How much extra did it cost you to drive 100 miles?



John

 

Hehe, thankfully there is a place in town that sells the ethanol free stuff, lots of ATVs, dune buggys, and dirt bikes run the stuff. I figure the 1/2 tank of 87 + 1/2 tank of 92 + 1 bottle of octane booster made it 93-95ish. I'm guessing the timing adjustment + better gas is making the engine run so well, its almost whisper quiet compared to before.

 

Good gas will makes a difference on how well it runs. If ever rebuilding one of these old engines it would be worth investing in getting hardened valve seats installed in the heads.

 

I put 89 in my 240 i6 the other day. Ran like crap after. Had to put sea foam in it and then 87 on top. Running good again

 

It is a tough question to answer. If it's not knocking, then you probably don't need a higher grade or you might try a lower grade.

If it is a real 9.4:1, then that generally means that high octane is required, but that also depends on the camshaft and a few other things. You will need to calculate the dynamic compression ratio to find out what you really need and you may not have all of the necessary info for that. If you google dynamic compression ratio calculator, many links should come up for that.

If it is a pretty stock rebuild and Ford advertised it at 9.4:1, then it is probably lower than 9.4: 1, and premium may not really be required.

On the subject of octane booster, read the bottle carefully. It will say something like increases the octane number by 10 points. So you think hmm, 92 octane plus this octane booster = 102 octane, sweet! Wrong, 1 point = .1 octane number, so that 92 octane with the octane booster is about 93 octane.
The 1/2 and 1/2 of 87 and 92 with the octane booster probably resulted in something lower than 92.

 

Very informative post, thanks! As far as I know,. the engine has never been rebuilt, so I'm figuring its very close to that 9.4:1 figure. I didn't know that about the octane booster, thats very interesting! Since I really would only need at most 94 octane gas, 92 + a decent octane booster should be fine from what I'm figuring.

 

If it's not knocking you don't need a higher octane, a pricier higher octane fuel is simply wasted.

You're lucky that you have a local ethanol free station, that fuel would be very good for your truck. As would a lead additive in the fuel and a zinc additive in the oil.

That said you could likely bump your timing up and get more power and economy. This may make it knock and that knock solved with higher octane fuel. This may actually pay off, going from 87-92 is likely only a 6% cost increase, if the MPG increase with a timing bump is more then 6% then it pays off.

 

Yeah, the ethanol free is about $1.69 more per gallon compared to the regular 92. I'm not sure about finding a lead substitute additive, I'll have to look around. And yes, I'm running Shell Rotella-T 15W-40 for the ZDDP My timing was initially set at 15 degrees BTDC according to some shop paperwork i found in the glove box, and now its set to the factory spec of 6 degrees, its running a lot smoother, especially since I've moved to 92 octane gas. To be honest, I'm not entirely sure what spark knocking sounds like on a Straight-6...

 

At that price I'd expect it to be high octane VP? And no that's not worth it.

Lead substitutes are around, I bet your local store has some. FWIW Tetraethyllead wasn't just for octane, it also did a lot of good for the exhaust valve. From Wiki.... "Tetraethyl lead works as a buffer against microwelds forming between the hot exhaust valves and their seats. Once these valves reopen, the microwelds pull apart and leave the valves with a rough surface that would abrade the seats, leading to valve recession. When lead began to be phased out of motor fuel, the automotive industry began specifying hardened valve seats and upgraded exhaust valve materials to prevent valve recession without lead" Basically if you don't have hardened seats then you should use a lead additive regardless of octane.

Timing is complicated, such a thing should be another thread, book, college major, etc.

Knock in your engine will sound like a quiet 12V cummins engine.