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By Col O/S
#50011
Hi,

The engine is a Chev 350 running Holley TBI and MSD 8 Pin HEI, this is all working, just in the process of optimising. The information I am getting on timing curves is all based on mechanical distributors, so just want to run it past someone who knows . V0.4 board

The timing information I am getting is 12 deg initial, 32 deg total with the vacuum disconnected. For the vacuum advance it is variable, but seems to be 10 deg is about average.

So if I am getting this right the bottom left cell will be 22deg (12+10) @600rpm, 30kpa and the top left 12deg (12+0)@600rpm, 100kpa. For the next part I will be showing linear advance all the way for simplicity of description. Far right bottom cell will be 42deg (32+10) @5500rpm, 30kpa. Far right top cell will be 32deg (32+0) @5500rpm, 100kpa. A simplistic table below.

100kpa 12deg ----- 32deg
30kpa 22deg ------42deg
-------- 600rpm -----5500rpm

The base tables have nowhere near this amount of advance which is why I am concerned I am crossing things up somehow.

Cheers Col
User avatar
By jonbill
#50013
the distributor uses "ported" vacuum, so doesn't add timing at idle.
it might help to understand that the vacuum advance is to increase economy when cruising, say 2000 to 3500 rpm with only slightly open throttle.
I'd start with a table with 12 to left and 32 to right as per the stuff you've found with vacuum disconnected.
Then add upto 30% more in that cruising region.
By Col O/S
#50020
theonewithin wrote:
Mon May 03, 2021 7:16 am
You need a dyno session.
A dyno is used to optimise tune and generally starts the run at 2000+ Rpm, not sure how this will resolve what is a basic initial timing issue?
By Col O/S
#50021
jonbill wrote:
Mon May 03, 2021 7:23 am
the distributor uses "ported" vacuum, so doesn't add timing at idle.
it might help to understand that the vacuum advance is to increase economy when cruising, say 2000 to 3500 rpm with only slightly open throttle.
I'd start with a table with 12 to left and 32 to right as per the stuff you've found with vacuum disconnected.
Then add upto 30% more in that cruising region.
I had been looking for an answer on/off for 2 weeks, nothing sensible found, A half hour after posting talking to a friend and he sends me a link, all about the SBC, but in a Ford forum by a GM engineer. The ported vacuum is for pollution engines to reduce emissions, not needed in my application as going into a pre-pollution application. My system is full vacuum coming of the TBI and going to Speedy, then the spark table will be used to control the HEI distributor advance.

So I am using 6deg during cranking, it looks like 23deg is about right for initial timing with 15deg due to vacuum, so 8deg at 100kpa. This then goes up to about 50deg at cruise, about 3500rpm+, with the same 15deg to come off for wide open throttle(100kpa).

I will approach this a bit slowly and not dial straight in as I am using 95 octane with 10% ethanol, so may burn a bit quicker.

Link to article referenced https://www.460ford.com/threads/timing- ... er.117504/


Cheers
User avatar
By PSIG
#50028
Hi Col - You are on the right track from the get-go. Some of this is for others following. Timing is always set to produce peak cylinder pressure at your engine's geometrical sweet spot (crank angle) for producing maximum efficiency and performance (including max power, torque, HP, economy*, etc) under any and all conditions (throttle, VE, temperature, manifold and barometric pressures, fuel type and grade, etc). This is why timing is variable, as conditions are always changing, but the target is always the same.

The timing values you posted will be different for every project, and are only generalized theoretical targets. As to tuning and dyno's, no two engines are identical, and no two fuels or conditions are either. If it was all the same, we wouldn't need dyno's or tuning. I see you get that, and are on your way to tuning for performance. I know which article you read, and John was right on the money. Follow its concepts. After that, we also need to consider how we use fuel and timing together in order to accomplish certain goals.

In the basic search for efficiency and performance, we can use our logs as data indicators, sort-of like a dyno. For example, we have efficiency indicators such as MAP. Under identical conditions, if the MAP indicates lower (more vacuum), then the efficiency is higher. You can figure-out why that would be true, and use that info to search for best performance, also incorporating other data indicators.

Likewise, an example of traditional efficiency measurement for drag racers is trap speed. This simply means the maximum speed attained on the run, where a tuning improvement results in a higher trap speed, and is typically used to find and set maximum effective timing at full-throttle, above peak torque. We can do the same thing with our logs, simply noting the time and speed in repetitive short runs at less than max speed, and in specific ranges that you can't do on the track. Tuning tricks. Find these timing end-points, then begin playing connect-the-dots as trends and understanding develop.

Finally, there are concepts in order to achieve goals, such as idle stability, cruise economy or throttle response. The timing you use at idle may be different, depending on what scheme you intend to use. For example, here is a random ignition timing table (Spark Table) manually set to lower-than optimal idle timing (timing 'valley' or 'trough'), in order to allow idle recovery. Here we can see that it creates a de-tuned "torque reserve", where timing (and torque) is increased if rpm falls lower than target idle speed, working to push the rpm back up automatically:
Idle_Timing_Torque-Reserve.jpg
Idle_Timing_Torque-Reserve.jpg (430.28 KiB) Viewed 298 times

This is for concept, as Speeduino also has an automated feature for this that may be employed once the concept is clear. As you can see, you must first tune to find the max idle efficiency timing before you can begin modifying to meet your goals.

Other considerations may be incorporated into your timing, such as coping with det-limited fuel, or smooth starting from cranking transition. These are considerations beyond the basic target of peak efficiency, and different for every engine setup. While much of this can be sensed for felt for tuning improvements, analyzing info from your logs can be a big help in determining best direction to tune, and for finding issues or opportunities you didn't sense or know existed:
Timing_Table_EXAMPLE-Low-RPM.jpg
Timing_Table_EXAMPLE-Low-RPM.jpg (465.51 KiB) Viewed 298 times

There are of course many more considerations throughout the range of operation, but I hope this adds to the conceptual direction you have already set yourself. The basic timing curve is only the beginning step in ignition tuning. Have fun!

David

* Note that economy is based on maximum efficiency of course, but some additional coordinated fuel and timing techniques are used to also introduce other energy-conserving schemes, e.g., pumping losses and effective compression. Beware—tuning may be addictive. 8-)
By Col O/S
#50046
PSIG I very much appreciate the detailed response. It is a long time ago and in a different country where I was last playing around with this sort of timing issues. I have brought the idle timing up from 12 to 21deg and the vacuum has improved 4kpa. I am just easing it up a deg at a time and letting it settle, then go again. I had to stop at 21deg as I had a problem with my fans not coming on and the engine getting hot. Not a real problem as I had been completing the wiring in the engine bay and had removed a temporary wire that the fans needed thinking I was finished with it. To be continued tomorrow.

Cheers
Col
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