Couple questions about timing curve, mileage, and octane
 

Hey so now I'm working through tuning up my engine. It feels like it lacks torque pretty badly at low-RPM despite being a setup that should get some substantial torque off-idle. I built basically this engine, but with aluminum 64cc heads. It really struggles to get the RPMs up off idle, but if I'm in second and step on the gas it'll move forward pretty quick when it the trans kicks down.

Right now I've got my timing set to 14 degrees initial with a stop plate limiting me to 34 degrees total. Of course, I plan to step that up by 2 degrees as Lars' paper recommends 36 total. My question is, how do I figure out what to set my initial to? In addition, I see this article linked here suggesting 24-26 degrees of initial timing and limiting total to 34-36 degrees. If I did that, would I be killing off my gas mileage for performance or any similar drawbacks? I'd love to get more performance out of the engine down low, but I also need to be mindful of keeping a not-too-terrible gas mileage for the time being.

Last question: how do I figure out what octane I should be using? I have aluminum heads and will be running either 34 or 36 degrees of total advance which makes me think I should do relatively well on 89. Gas prices around here are getting a little out of control so anything I can do to keep my cost-per-mile lower would be pretty nice. 91 is getting close to $4 per gallon over here :ack:

Others will chime in, as lots of additional questions and various answers to yours. The Aluminum heads will help you, but what is your compression? That will help determine octane. Also, what camshaft? With aluminum heads and lower compression (9.5 give or take?) you should be fine with 89. If you have a very tame cam, possibly not.

on my SB I am running 35-36 degrees with 20 out of the distributor, so 16 degrees initial. All your timing should be in by 2500-2800rpm, so if it isn't it may feel lazy. I would add a couple more initial for 36 total and check with a dial light to see where it is all in.

Also, is your throttle opening completely? With someone in the car and foot to the floor, make sure the butterflies are opening completely. Are the secondaries working/opening under load? If not, may be lazy.

Compression isn't terribly high. I don't remember the specifics, but it's either 9.5:1 or 10:1. Cam is a Comp XE274h which I'd say is pretty decent, not quite tame.

I do have the medium springs in my MSD streetfire, so that might be part of the problem. I'll switch them out to the light springs, looks like it should be all-in by 3k with those.

Throttle is definitely good, first thing I checked. Secondaries, now that you mention it I totally forgot to check. Next time I drive I'll stick a paperclip to my secondary diaphram arm, as I've seen some people suggesting on the forums before.

When it comes to your spark curve, there's no conflict between fuel economy and performance. The whole point of the spark curve is to maximize the combustion efficiency. The proper curve will maximize/improve the combustion efficiency over the whole load/RPM range.

For fuel economy purposes you need a vacuum advance to bring in 40-45* of timing.

The older the engine, the more prehistoric the combustion chamber/engine, and the worse it breaths, the more timing it wants.

For example newer (2005~) Sefi LS engines use around 24* of timing total for approx 500rwhp naturally aspirated around 6L.
The cylinder is full, the combustion chamber is newish, well designed.
If we fill the cylinder even further say 200% (15psi of boost at ambient air temps) the timing would be closer to 10* for pump 93. Or more optimal 15-18* for E85 fuels.

I digress
Older engines making half as much power could use 30-36* at WOT. it depends again on compression ratio and head manufacture. CC design. Plug placement. Some do better than others, and by better I mean use less timing.
How do you tell? You can either measure EGT or take it to a dyno. If you want elaboration ask...

But. For fuel economy. The name of the game is completely different. 36* total WOT timing has nothing to do with economy.
When you cruise with an engine the situation is completely different, new conditions.
Conditions:
1. The cylinder is mostly empty (high vacuum)
2. Combustion is leaner (cruise economy DEMANDS 14.7:1 to 16.8:1 air fuel ratio Or it won't be right!)

With such a high vacuum in the cylinder (it means air molecules are more spaced out) and limited fuel molecules (lean air fuel ratios) it means you can fire the ignition really early (45* is typical, some new engines run 50* of timing though) to get the burn 'started' earlier.

cliffs
Use a lean air fuel ratio (14.8:1+) verify with wideband
Use vacuum advance (non EFI engines) to bring in additional timing for cruise economy

With that compression and cam and aluminum heads, I think you'd be fine burning 89 octane. I'd add those 2 degrees to get to 36 on your timing (don't forget to plug your vacuum advance when you check/set timing), reattach your vacuum advance and then readjust the idle. 2 degrees might wake it up a bit. Checking to see that it's all in before 3K will help too.

Well you did pick a cam with an 1800-6000 rpm powerband -- itll be weak down in lower rpms to compensate for the power it makes 5000-6000 rpm.

Didn't catch in your writeup, manual or auto trans -- as CC also recommends a min 2200 stall on the TC


:seeya from another MN guy

You have modern aluminium heads. You don't need 36* total. Aluminium head makers such as AFR recommend 32-34*. Iron heads 36*. Are you using ported or full manifold vacuum for advance can? Most experts recommend manifold vacuum. I would try 16* initial with your build and go up from there. You will need a means to limit mech'l advance so you don't exceed 32-34* total. Gas mileage? Keep your vacuum advance connected and hope for the best, lol.

True - the cam has 230/236 duration @ 50 so powerband will be higher up. It is more responsive in 2nd as you're approaching power at that point. Gearing will also make a big difference with that cam in a 350.

Whenever you go to WOT, manifold vacuum drops to nearly zero. The vacuum advance isn't involved with low rpm, WOT conditions. However, the advance can is important for normal driving, low power, low rpm use; and if you use the car for normal activities, you need the vacuum advance system in place. The vacuum advance system does influence engine performance as manifold vacuum increases with increasing rpm at WOT conditions.

Need more detail about the heads..those referenced in the article are vortec and no info on their site about intake cc. stock vortec heads were never cast in alum. so they could be anything from 180 cc Jegs heads, 210cc fastburns or 230cc dart for all we know.

On my78'L-82,I adv/turn dist till I get spark knock then back it off for the intended gas. I take the dist tool with me for testing. When you find the sweet spot put the t/light on it to find out what total actually is, for ref. Hdal

That is a cam with 60* of overlap. Time when both valves are open. That Is alot of overlap and is going to make for a soggy bottom everything else being the same.

i would recommend 19* to 20* of initial limit mechanical advance for total of 34* and limit vacuum to achieve 51* at cruise.

what is the intake volume on the heads? What is the quench distance? What intake manifold is being used? What rear gear and how much stall on the torque converter? ALL of these things matter.

you can't do it this way because knock is inaudible before it is audible.

Knock sensors detect inaudible knock.

Many knock sensors will not detect knock once it is audible.

Complicated answers aside...The Initial timing is not much of a concern. The initial timing is what you get when the distributor centrifugal advance mechanism is fully retracted back down to zero. You do not "set" initial, it happens because that is where it falls. Setting your distributor at 36 degrees at full centrifugal advance at high RPM without any vacuum advance is all you can set with the adjustment. The lighter springs should be selected to allow full centrifugal advance at around 2600 - 3000 RPM or so. The centrifugal advance should be completely out at idle. If the springs are too loose, the centrifugal advance will start too early.... so ideally you want it to begin advancing just a couple hundred RPM over your idle speed. About 1000 or 1200 RPM or so as a beginning point. Helps keep idle steady and prevent "hunting" RPM at idle speed.

If you want to change from where your initial advance falls, you MUST change the amount of centrifugal advance available in the centrifugal advance mechanism in your distributor. Reduce the amount movement to increase your initial advance at idle, or increase the amount of movement if you want to lower the advance at idle. The top stays at 36 degrees. Total sum of initial plus all the centrifugal is 36.

Bottom line - Adjust available centrifugal to change the initial.

this only applies to stock motors, and not really even then as the whole timing system was designed to comply with emissions control of the era.
I really wish people would stop saying it . Leads to much confusion and many miss-tuned engines.

you absolutely set the initial timing if you want a decent idle and any off-idle performance.

:iagree:
There is a method to determine initial timing using a vacuum gauge which I did. As I have said in this post and before, there is a good probability that the initial and mech'l will exceed 36* for iron heads and 32-34* for modern aluminium heads. The mech'l advance will then need to be limited to attain the full advance. I have a programmable ignition so its easily done electronically. Other means are available. Vacuum advance is for cruise.

IMO

The initial timing is perhaps the one aspect of combustion engine tuning which is somewhat a personal preference situation.

I set it by listening to the engine and working towards the idle quality and response that I prefer for my driving.
As long as it 'makes sense'. Idle timing is heavily RPM dependent (below 1k rpm). Consider an engine at 900rpm vs 500rpm vs 200rpm vs 50rpm vs 1rpm
Connect the dots for idle timing:
RPM = timing
900 = 18* BTDC
500 = 12*
200 = 3*
50 = -7* ATDC
1 = -35*~

-Notice how the slower the engine is rotating the later we wait to fire the plug
-Negative timing is "after top dead center" i.e. when the engine is moving so slow (1 revolution per minute) it has to wait until the crankshaft lever arm is in position before the plug can ignite the mixture

What I was saying applies to modified distributor curves on high performance engines. If anyone is confused, the search function is thier friend. The initial advance can be changed to improve idle but the way to do it does not involve setting it with your timing light. Additional timing helps to a point. Getting the curve right is part of that. Just turning the distributor to add initial advance is not the answer. If you do that, you are doing it wrong.

I’m not confused. Initial timing is a critical part of tuning for idle and off idle performance.
Specifically and especially for a larger cam with more overlap on a street driven engine.

obviously you can’t add initial timing without effecting the total and cruise timing. That is where limiting the mechanical and vacuum advance comes in to play.
So, no , you can’t just add initial without changing the or effecting the other two.

but initial timing is not just “ it is what it is “ by some default because you set total timing and the curve. Ideally we want the engine to run well in all conditions not just WOT. Since significantly more time is spent at idle , coming off idle and cruise power settings then those areas should be addressed. This requires modifying the total movement of both mechanical and vacuum movements.
The OP may also find that his vacuum advance may not pull sufficiently, or at all, at idle , requiring a vacuum can that pulls at a lower vacuum in order to get it to idle well.