What octane fuel for factory stock 318?

I recently got my 71 Barracuda back on the road. It is factory stock, except for the points being replaced by a Petronix Ignitor. I started out with 87 non-ethanol gas. The initial timing is set to 8 degrees BTDC. It runs pretty good. I have access to 110 racing fuel and want to blend it with the 87 in order to boost the octane. What octane rating would you recommend?

Thanks,
David

As little as possible to prevent pinging and knocking. More is detrimental to efficient combustion.

Robert

More octane than needed produces less power, not more. Stock 318 will do fine on 87 as stated above.

If you are running 8 degrees BTDC and it's not pinging on 87 octane , then you don't need any high octane fuel. I would just leave it alone and stay with the 87.
You can use any octane fuel you want; but, you won't get any performance gains by just switching to higher octane fuel. The high octane fuel will keep the engine from pinging when you have your timing
advanced.

8 degrees initial and 33 total should run fine on 87 e10. No need for expensive higher octane gas.

I run 10 degrees initial, 33 total, full advance at 2500 rpm on my 67 318.
I get a small amount of ping during full throttle on a hot day with 87 e10, but normally no ping at all.

On this engine combo 85-87 should be enough unless its all carboned up inside.

Altitude that you drive at also matters. At sea level you may run 87 octane but take the same car up to Denver CO and it will need 85 octane.

Octane posts can get as interesting as an OIL thread. So many get top quality confused with Premium. You can get some crappy 94 octane that runs just like a Top Tier 87 octane.......

And 'Seat of the pants" dyno will lie to ya.

Dave_J wrote: "You can get some crappy 94 octane that runs just like a Top Tier 87 octane......." Many misunderstand what Top Tier fuel is as well. I hear on Q&A shows the owner of a new gas direct injection car recites the manufacturers mandatory required use of Top Tier fuel ONLY for their car so they put 92 octane in it. I learned top tier is where a blends quality certifiably goes above and beyond "regular" fuel. Has nothing to do with octane. Similar to organic produce vs regular produce.

Every Mopar OEM stock production V8,from 273 2 barrel to Street hemi and 440 6 pack, motors are no where close to having the engine specs like deck heights and combustion chamber volumes correct for the stated combustion ratio
Their from 1.0 all the way up to as close as .3 ratio from the blueprint specs, their always lower
Your good on using todays lowest price and lowest octane pump swill is my main message, don't waste our precious race gas or your money to use in that motor

If it's a non Magnum, I'm surprised it doesn't rattle with 8 deg initial. I had a 69 Belvedere with a 318 many years ago and factory timing spec was TDC. If you tried to add any timing at all on regular gas, it rattled and that was before unleaded fuel.

Kevin

From day 1, my Dad and I always set the timing to TDC as specified on the green EC info sticker (and owner's manual). As a kid, I had the job of laying across the fender and stretching to reach the distributor in order to set the points. What a pain! Now 48 years later, I decided that the points needed to go. That is why I went with the Petronix Ignitor. After installation and the timing set to TDC, the plugs kept fouling. I starting reading on this forum and others about advancing the timing. Started with 5, then 8 degrees. Car runs pretty darn good and and no more plug fouling.

By the way, I appreciate all the responses. Tuesday, I took everyones advice and bought 15 gallons of non-ethanol fuel from a reputable station. They also sell the 110 race fuel, but I passed on that option.

Thanks again,
David

For just driving around I'd run 87. I had a 69 318 2bbl coronet 440. I took it to the track flipped the air cleaner and advanced the timing. tossed in 5 gallons of 93 and ran 15.69's. My 95 5.2 Dakota w/ MP ECM would rattle it's brains out if you didn't run 91. I'd get unleaded 107 at the track it ran 14.07. I think the 71 has lower compression than the 69.

A dirty little secret is the electronics eat up some of the timing. Every transistor switching takes time. A tiny amount of time but it adds up - especially at high rpm when small fractions of a second are degrees of crank motion. At idle this delay is not enough time to show on the timing. So that shouldn't be the reason it was insufficient with the same setting. But something to be aware of, and one reason it should accept more timing at idle without pinging.

The timing specs for idle were a part of the cleaner air package (CAP). They can be very sensitive to idle speed with the light primary spring used get the timing to advance with increasing rpm. The idea of the quick advance was to get the timing close to the ideal established in the pre-CAP era.

Below is a graph comparison of 318 timing based on the specs in the '68 Plymouth FSM.
Non-CAP vs CAP for a manual transmission equipped car.

I’ve been experimenting with this 74 Dodge truck I picked up last February. I replaced the intake and carb to a Performer and a Carter 9636. I also added a fresh electronic dizzy. The truck already had headers. Started using mid grade fuel and eventually bumped the timing up 5 degrees from TDC. The motor seemed to respond favorably to the additional timing without any detonation. I went ahead and bumped up another 5 degrees for a total of 10 BTDC. Again, the motor seemed to respond well. I then went ahead and dropped in 87 just for shits and grins. So far so good, no detonation. I still need to put on a fresh cap, rotor, plugs and I have a set ofTaylor wires to install.

David - good looking B-Fish. Important to run non-ethanol fuel too in my estimation. Sta-Bil has a fuel treatment for ethanol as well - you might look into that. I have no idea how well it works though.

Thanks. Because the Barracuda has been in the family for so long, I obviously have a huge emotional attachment to it. And yes I am biased too.....I think the 70 and 71 Barracuda's/Cuda's are one of the best looking cars to ever come out of the "Big Three".

I have never used Stabil. Even though StarTron's primary use is for treating ethanol fuel, I have been using their Enzyme Fuel Treatment as a stabilizer...and adding it to 87 non-ethanol.

Dave I have a 10.5 - 1 iron headed 327 vette and it runs fine on 87 ethanol as long as I don't jack the timing up to race. I've run ethanol free premium,and all 3 grades of ethanol in it and around town it could care less. Car sits all winter and I've never added anything to the gas and it's been fine now for over 35 years.

I love your car and agree on the 70-71 Cuda's being great looking cars.

Mike

Mike,
I would imagine the terrain is mostly flat in central il. What kind of timing does that blue car like?

Hemi locked out @ 34

Thank-you. Is that with 110 octane?

It has been over a year now since I posted my octane question. About 6 weeks ago a new Chevron station opened in the neighborhood next to mine. It offers 91, 95, and 98 octane fuel...no 87. A big bonus for me is that I no longer have to drive 14 miles for non-ethanol fuel.

So this past Sunday, I went by and filled the tank with 91. Should I now advance the timing or leave it at 8 degrees BTDC?

Thanks,
David

Incredible gas station! Send one my way. Wow

If you are using the Chevron 98, then you can bump timing to 8 degrees tdc. You can even go up to 10 degrees btdc and still have no pinging.

Same with my 68 Coronet with 318. Factory timing for my 68 was 2 1/2 degrees ATDC, that is stated on the green tuning spec sticker on the fender. It actually runs good and no pinging with that timing.
However, I keep it set at TDC, but if I set it to 5 degrees or 10 degrees BTDC, the 318 pings and have to use 100 octane avgas to stop it from pinging. So I just stay at TDC on 87 octane.

at what rpm does the pinging occur ? in all gears ?