Total timing
 

Engine builder set the timing and carburetor for me today. He found that engine seemed to run best with 27 degrees of total timing with the vacuum disconnected. It seemed strange to me but he says it’s optimal tune and not to mess with. Engine cranks easily and sounds great. How does 27 with no vacuum sound to y’all.

No surprise mine is best at 29 degrees total

On par with the 1970 455 specs and about where my 462 runs best.

Let’s get the terminology right here for your benefit.

The builder told you that 27 degrees of total mechanical advance works best, not total timing which would include vacuum advance.

It’s fine to just run 27 degrees if that’s what works best in terms of power while also being safe for different weather conditions.

So now you just need to run ported vacuum to bring in another 8 to 12 degrees above idle.

This you will need to determine on your own by driving the vehicle to see how much the motor will accept without pinging at part throttle and under changing ( hills and such ) road conditions

Sounds like it's going to get terrible gas mileage.

You'll want that vacuum advance hooked up and dialed in properly if you plan to put a lot of street miles on it and want at least a semi efficient engine on gas.

As far as total mechanical timing there is no absolute way to know what is best unless you either run it on a dyno or hit the dragstrip. Otherwise it's just a guess.

I’ve always tried to run some vac advance either ported (off idle) or straight from the intake (always on) but some cans have way too much, I normally end up with about 10 degrees additional. Adjustable can or by a can for your style distributor hei or point style or even tack weld a stop in the arm movement slot . The last Pontiacs I worked on were 14 initial 19 from my mechanical all In by 2800 and 10 from my vac .


Sent from my iPhone using Tapatalk

Do you run vacuum advance

My machinist is a hardcore racer and I don’t think MPG ever enter his mind

Total Advance, while under load:
27* indicates both an optimal fuel atomization and optimal combustion chamber. Vaccum signal providing ZERO*.

Adding Vacuum Advance would be for partial load, like cruising, so i wonder what an optimal Vac Adv* value would be for combined mileage and part-throttle power?

Supposing a Highway eval can occur with Mech control of the vac pod (which i tried awhile back), with or without the help of cruise control.

People need to come up with a uniform way to express their timing numbers ... something like.

Initial - mechanical - vacuum .. in my case 9-24-15

Then maybe two numbers for start - end rpm of Mechanical like (800-2900)

So my engine (YS-400) would be 9-24-15 (800-2900) ... doesn't tell the whole story by a long shot, but gives a basic idea.

Sorry, back to the OP's question.

We 455 engined folks just might have a complete disregard for Initial Advance. 6* thru 18* being a broad window after having Dial-In the Total ADV for ET.

Whereas the 400 and 350 engine folks really got to optimise Initial ADV after learning the optimal Total ADV.

Well, then again the initial can be optimized, and should be, which involves changing the Mech ADV to set the Initial and sustain the desired Total ADV. This is called "Tuning", that which is a lost effort for many folks across the hobby.

Hmmmm. For those 1/4 mile folks that run best with 38*+ Total Adv; Aside from declaring the Head Combustion chambers as grossly inferior, i wonder if trials on carb emulsion changes, ( atomization changes ) would bring the Total ADV down, perhaps to 27*.

Afterall, the lower Total ADV combined with optimal power assures least compression loss ( - HP), perhaps even least Octane requirement, oh and least Starter motor stress.

Low Octane "kernel" taking less time to "burn to pressure" whereas High Octane "kernal" takes longer to "burn to peak pressure" will also factor in to the Total ADV under load, needed for optimal ET.

All that fuel chemistry considered to get with Ping-Free power for a set Total ADV and Compression also begets getting the CYL pressure aligned/optimal over the Stroke's "Torque-arm efficiency" range.

Reminds me that the 4.21" Stroke has the earliest TQ-Arm eff crank angle, compared with 4.00", 3.75", and lesser Strokes. Should provide cause to have least Total ADV for big Strokes.

In conclusion, there ought be a Sticky chart made from compiling the MOTOR's manuals for optimal Initial and Total ADV vs Stroke and Compression (all Iron Head Data), along with notable modern (Aluminum) head Initial and Total ADV included for these Stroke and Compression spreads. THAT would be a great aid, and revealing.

Coffee was good this AM.

So, I'm like a few more folks. What is the initial timing set at?

That's not surprising given the amount of people I run into that still don't have an idea how the vacuum advance works or why it's beneficial and choose to just run without it rather than learn or spend the time dialing it in.

I'm a racer too, but that doesn't mean I won't run a vacuum advance. While racing it has no influence but it's beneficial with everything else the engine does.

Was he talking about idle timing of 27? If that’s the total mechanical advance timing, how did he determine that the motor ran the best at 27? Dyno?

At small cruising type throttle openings every gas powered motor needs more timing because of the simple fact that it takes more time to burn those small amounts of fuel in the most complete way possible.

The OPs builder stating that 27 degrees was the best was for wide open throttle one would have to assume.

I just wondered if the builder had it on a dyno. Seems to me it would be difficult to tell if it was running better at say 27 than 29 at WOT without some diagnostics.

Well at any rate if it was not determined on a chassis dyno then that 27 degrees is just a good starting point because things will always be different once you have it on your vehicle.

I like this thought. It would be great to build a small database with members' settings, adding block, stroke, Head Casting & CR. So mine would be:

12-21-10 (1000-3000) 1970 model 350, 4.25" Stroke, 6X-4 Heads, 9.13CR

Just on the engine stand. If I get it in tomorrow I will have a week of shakedown. If something is off I will be able to adjust it