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I need a lesson in Timing vs. Octane


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1973 351C 4V Q Code

I'm hoping for some clarification as to how octane relates to timing.

My understanding is (feel free to pick apart!):

1. Base timing on my car is 16* BTDC at 87 octane.

2. Higher octane gas (I run 93 octane) burns slower and longer, thus you can run more advance (20* BTDC?).

Are those statements correct? And if so, is it the case that if you run higher octane you should run more advance? In other words, if I run 93 and the car is timed for 87, I should be advancing my timing (by some amount) to take advantage of the higher octane.

Am I correct?

 

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I'm definitely not an expert by a long shot, but here's what I did faced with the same dilemma back in the early days of owning my 71 M code.

Now, I'm dealing with higher compression, supposedly 10.7:1 with 4V quench heads. The thing would spark knock badly on 91 no ethanol fuel, best I could get in my area. I started to learn about timing after I joined the group here. I was able to get the car running better after a friend played around with it. After an engine rebuild in 2012, I had a lot of issues with timing. Most of which turned out to be a bad choice of Pertronix Ignitor III module. 

There has been many threads on this subject in "search". I suggest doing some more research on fuel octane numbers. If you're set at 16* initial, it ought to run fine. The "curve" of the timing is probably more important as well as the type of distributor you're using. There is a lot more info needed before anyone can make a solid suggestion imo. 

I'm out of time right now to go further, but will continue later. Search my posts on timing as well as others.

It is a frustrating thing I know only too well.

 

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Geoff.

 I learn something new every day!

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Geoff-

Thanks for the info!

My questions are more "theory" I guess . . . not necessarily tied to my setup.

Modern cars have knock sensors and advance timing (I think) until knock/ping is detected. Running higher octane allows the computer to advance the timing further. Again . . . I think. :)

If that's correct, I was wondering if we should automatically advance initial timing higher than base when 93 octane will be used. Or maybe I'm overthinking things.

To answer your questions, I have a Pertronix disty, Pertronix II module and Pertronix II Coil. Using ported vacuum advance.

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1 hour ago, 73inNH said:

1973 351C 4V Q Code

I'm hoping for some clarification as to how octane relates to timing.

My understanding is (feel free to pick apart!):

1. Base timing on my car is 16* BTDC at 87 octane.

2. Higher octane gas (I run 93 octane) burns slower and longer, thus you can run more advance (20* BTDC?).

Are those statements correct? And if so, is it the case that if you run higher octane you should run more advance? In other words, if I run 93 and the car is timed for 87, I should be advancing my timing (by some amount) to take advantage of the higher octane.

Am I correct?

 

I think you might be looking at it backwards.  It is true that higher octane allows more timing advance.  I don't necessarily agree you should advance the timing just because you move from 87 to 93 octane.  Ideally you figure out the timing your engine is happy at and try to get it to run without detonation with the fuel you have available.

With Clevelands, they tend to like a lot of initial timing.  16 degrees BTDC is usually a good place to start.  However if your distributor is curved with a 15L advance slot, your all in timing will be 46 degrees, which is probably too much.  You are better off running a 10L advance slot, for 36 degrees of all in timing.  You also have to consider vacuum advance and the springs, which control the rate of mechanical advance.

Jason (71 Mach 1, 351C 4V, 4 Spd. Toploader, Grabber Blue)

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6 minutes ago, 73inNH said:

Geoff-

Thanks for the info!

My questions are more "theory" I guess . . . not necessarily tied to my setup.

Modern cars have knock sensors and advance timing (I think) until knock/ping is detected. Running higher octane allows the computer to advance the timing further. Again . . . I think. :)

If that's correct, I was wondering if we should automatically advance initial timing higher than base when 93 octane will be used. Or maybe I'm overthinking things.

To answer your questions, I have a Pertronix disty, Pertronix II module and Pertronix II Coil. Using ported vacuum advance.

HA, I think you have this backwards again.  Knock sensors are there to detect detonation and retard timing if detected.  For example, if your car calls for premium fuel and you put in 87 octane fuel, it may cause the car to detonate.  The knock sensor would detect this and retard timing as a result.

Edited by trillizo_y_uno
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Jason (71 Mach 1, 351C 4V, 4 Spd. Toploader, Grabber Blue)

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There is only so much timing any given engine will "like" regardless of octane. Every engine is different, based on any number of variables - compression, cam specs, etc. Higher octane can be used to reach "ideal" timing for your engine - but it is very easy to advance your timing too far, which is just as bad as running insufficient timing - and if you go far enough, no amount of octane will help.

My approach would be to fill up with the highest grade available, then tune for best all-around performance with no detonation. After you are satisfied, then back off the octane a grade at a time if for nothing else just to save a few pennies at the pump.

Black 1971 Mach 1

351C/FMX/TrickFlow Heads/Lunati Retro Roller Conversion

Classic Auto AC, Manual Front Discs, Upgraded Springs/Shocks/Close-Ratio Steering

 

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Reading through other replies, I agree with Trillizo_y_uno. Again, you do NOT want too much advance. On a stockish motor, 36 degrees mechanical is about all you want. On mine, I reworked a slot as a 10L (.410" width) and curved with different springs at different tension. My initial is 14 deg. plus 20 crank advance with about 4-6 degrees vacuum on top and the all in is around 3000 rpm. I am using 13cc dish pistons for a comp ratio of around 9.8:1 at zero deck height. On 91 non ethanol gas, this thing pulls like a train now, with zero spark knock. Before, I found that with the flat top pistons, zero deck height and a 14 degrees of initial timing, it knocked like crazy even with a 10L slot width. That's why I dropped the compression and that cured the issue. Too much compression with the fuels we have is not a great idea. Higher octane fuel is actually less volatile than low octane 87, so it does not pre-detonate under compression. I wish we still had 100 + octane like the good ol' days, then we could really have some fun!!

I commented recently on distributor slot width, but off the top of my head, not sure who's post it was, but it was just a few days ago. I have an email from Shell Fuels somewhere in my stash where they define Octane. I'll see if I can find it, scan it and post for you. I also have a sheet on "Timing 101". That ought to be on here somewhere, if someone knows where to find it.

Geoff.

 I learn something new every day!

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If your timing marks are off it is likely the outer ring on your harmonic balancer has slipped, and will continue to move around. It's also possible your pointer has been damaged, but that should be fairly obvious.

Adjusting the timing controls when your engine achieves maximum combustion pressure in the cylinders. You actually want maximum cylinder pressure right after TDC. It takes a few micro-seconds for the spark and complete combustion to occur. Therefore, initial timing needs to be somewhere between 6°, or so , and around 16° BTDC. Because the delay time until complete combustion and maximum cylinder pressure is fairly constant the ignition has to advance even further as the engine speed increases, which means the total advance will be around 34° to 38° at 2,500 to 3,000 RPM. These are general values and engine type and build will determine actual timing advance. You do not want maximum pressure when the piston is still rising.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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21 minutes ago, 73inNH said:

Geoff,

I believe my timing marks are off, which makes precision difficult. I'll plan on fixing that.

 

Yeah, well that's not good lol! 

I found the Shell email, but it was not on Octane, but ethanol content, sorry. I know I have something on Octane somewhere though. I did find a couple of articles on timing. One is Timing tips for Ford (and other) distributors. However it is copyrighted. The link is http://www.bob2000.com/dist.htm The other is Timing and Vacuum advance 101 by Lars Grimsrud. I think that is or was on the Forum somewhere. If not you might find it on Google. It's several pages long and too long to scan. Good info there.

Good luck moving forward.

Geoff.

 I learn something new every day!

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You will likely hear it under light acceleration at slow to moderate speeds. The problem is you likely won't hear it at WOT, and that is where the real damage occurs. If 91 or 93 octane gas is available where you live, use it. Chuck

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