The V8 Owners Forum -

I’m starting to have some thoughts about my ignition system and whether I’ve set it up correctly, this has all come about because of a discussion that I’ve had with someone on the Cobra forum, he reckons that a vac advance system should be connected to the non-timed port, he runs 15 degree advance at idle plus another 15 that the vac system adds to the timing making 30 in total at idle on his SBC!!! I don’t think this is the right way to set up the timing but it has got me thinking!!

Basically I have read that an engine always makes most torque when the peak cylinder pressure happens 15-20 degrees ATDC but in order for that to happen the spark has to happen many degrees BTDC. The actual firing point depends mainly on two things, the speed of the engine (RPM) and the charge density (basically the VE). The turbulence in the cylinder also plays a part in all of this.

Now I know that my engine runs best with the idle timing at 20 degrees BTDC, the tickover speed is at its highest with this figure, I pull the idle speed back with the idle screw on the carb and a vac gauge shows that the engine is pulling as much vaccum as it can with this setup. Let’s just say that tickover is 1000 RPM (its a bit less but it makes the numbers easier to work with). If we assume that my engine runs best when the peak cylinder pressure is at 15 degrees ATDC then at tickover it must be taking 35 degrees of crank rotation before the peak pressure has been reached at 15 degrees ATDC (20+15=35). 35 degrees is quite alot of time in terms of crank rotation at the low speed of 1000 RPM which must mean that the flame speed is low, this must be because the charge density is very low at tickover, after all the throttle is virtually shut so it would be.

So what happens when the throttle is floored at 1000RPM? The VE must shoot way up and therefore the flame speed must also go up, does this mean that I am now running with too much advance? This situation must happen all along the rev range, the timing required for cruising and for WOT operation being totally different, the vac system deals with this but if I have already added too much static timing it cannot deal with that.
Maybe at 1000RPM under WOT the engine only needs 5 degrees of advance in order for peak pressure to happen at 15 degrees ATDC?
Maybe at 2000 RPM (That being twice the speed of 1000RPM) the engine only needs 10 degrees of static timing?

So how to test all of this? Well I was thinking that I could set the timing to say 5 degrees by twisting the dizzy and program my MSD system not to advance the timing at all, it would be a flat line. I would also disable the vac advance system, then say in third gear I would time how long it takes to go from 1000 RPM to 2000RPM under WOT conditions. I would then advance the timing a couple of degrees and repeat the test until I had found the best timing for WOT for 1000 to 2000 RPM. The test would then be done from 2000 to 3000RPM with various advance figures for WOT. This would be repeated up the rev range until I had figures for the whole RPM range for WOT conditions
This could then all be programmed into my MSD system (I run a locked out dizzy) and in order to get an idle timing figure of 20 degrees assuming that the best static figure for WOT was found to be 5 degree I would set the vac system to give 15 degrees and connect it to the non-timed port. If my vac system is setup to give 15 degrees more advance this would be active all the time that the car is cruising but the timing would back off to suitable WOT timing figures when the throttle is booted.


EDIT...I've had another thought! Maybe I should not 'flat line' the ignition timing, maybe I should program the MSD to give zero advance at 1kRPM, 10 degrees at 2k RPM and 20 degrees at 3k RPM. Then I can twist the dizzy to add more timing to the above figures, for example I could add 5 degrees to the zero figure at 1k RPM then carry out the 1k to 2k WOT timing test.

What do you guys think of all of this gumph?

Hi
Your "flat line" technique is the one I used to use on Minis. It works pretty well just takes time to do and you need to know you are hitting the same bit of hill at the same speed each time. Really you need to do it all as quickly two or three time minimum as you can on one day as air temp, humidity and atmospheric pressure will have significant effects as well. once you are done you can go back and jet your carb the same way. . . get that sorted and you can have another go at the ignition you can have days of fun this way, and think of all that fresh air .
best regards
Mike

When you say testing....

I presume you mean you're going to test this on a dyno so there are actually quantifiable results ?

Dizzy's are limited by their mechanical design, so can always be improved upon. Vacuum cans are an attempt to give some more control, but again it's still quite limited.

I dont see your idle timing of 15-20deg as unreasonable.

I dont see say 28-30deg of timing at WOT and 1000rpm as unreasonable either....but every engine will be different.
Different designs, intakes, exhausts, chambers, CR, cams, burn rates etc So some may be happy and even need 30deg here, others may not tolerate it at all.
But can you realistically achieve this with a dizzy ? Not so easily.

Light throttle and cruise, lean mixtures etc...30, 40 and even near 50deg may not be unreasonable in a lot of engines
Again...is this easily achievable with a dizzy at say 1500rpm, 2000rpm ? and without vac advance...nope, I doubt it.

But unless you have some means of actually testing, and measuring output to see what is best, it will always be a best guess compromise.

Hi Guys that for the coments. my dizzy is actually locked out, all teh timing figures are programable via the MSD system, I can advance, or regards the ignition as much as I like via a laptop.

I was not going to dyno my engine, I don't really need to have an actual BHP figure, I was hoping that the testing with a stop watch would find me the best timing settings for each RPM.

I agree, this could all take ages, I really need an old runway!

sidecar wrote:
I agree, this could all take ages, I really need an old runway!

Take ages....with questionable results, and cost a fortune in fuel ! lol

Given how small some gains might be...a stopwatch is really going to struggle to be a good enough indicator.

Light loads you might be able to feel that timing is helping, and less throttle is needed. But for power runs...it'll be much harder to quantify.

Pete this sounds interesting. With your programmable MSD advance for the vac system as well as engine speed I think it will be worth the effort. Just need to find that old runway now!

Pete, have you tried running your engine with your standard ignition timing but full vacuum to see what happens yet? It's something I keep meaning to do after "the chat" on the other side but my cobra is away for a few days so can't do it yet.

Cobratone wrote:Pete, have you tried running your engine with your standard ignition timing but full vacuum to see what happens yet? It's something I keep meaning to do after "the chat" on the other side but my cobra is away for a few days so can't do it yet.

Hi Tone, I've not done that because I know via testing that the engine wants 20 degree at tickover, if I just plug the vac pipe into the non-timed port the timing will shoot straight up to 35 degrees which is way too much for idle.

I suspect that one of two things will happen when I carry out my tests, it will either make no detectable difference because the changes in power are too small to make enough difference for the stopwatch to measure or it will make a huge difference because the engine is over advance for WOT all along the rev range up to the point where all the advance is 'in'.

I'm curious to see the result of your tests. Shame you're not closer as we could do the testing quite easy on my car just with a few clicks on the laptop rather than having to twist the dizzy each time (no, that wasn't a plug for MS)

Cobratone wrote:I'm curious to see the result of your tests. Shame you're not closer as we could do the testing quite easy on my car just with a few clicks on the laptop rather than having to twist the dizzy each time (no, that wasn't a plug for MS)

LOL, I can do it with a few clicks on my laptop to but I just thought that it might be easier to twist the dizzy a couple of degrees between each test! To be honest I'm still just mulling it all over at the moment! The main reason for my post was to see what others thought about my idea that the timing may well be far too adavnce for WOT operation seing as the starting point was set at 20 degrees and that was determined with the engine just about as far away from WOT operation as its possible to get!