I used to be in the "no" camp and told myself that the cobra was basically a street legal racing car and doesn't need any fuel saving nonsense but after doing a bit of reading I've decided there's nothing to lose so may as well fit it. I highly recommend reading David Vizard's book "How to build max-performance Chevy Small Blocks". If vac advance is good enough for someone of Vizard's experience and knowledge, then it will do for me.
Vacuum advance is load sensitive as opposed to RPM sensitive (centrifugal advance) so it won't affect all of the good careful work I've done on my mechanical advance curve. Vac advance will not affect the timing at heavy and wide-open throttle because there is insufficient vacuum to activate the mechanism. It only comes in at light loads when you are cruising along. Just a quick refresher before I get in to fitting it - the basic premise is that at light loads the spark needs to occur earlier because the lean mixture and partially filled cylinder need more time to burn fully. The way I imagine it is that the fuel is more spread out so the flame front travels slower, hence you need to give the engine more time to burn the mixture in each cycle. There, that made perfect sense didn't it!
Well anyway, a whole load of people a lot cleverer and with a lot more experience than me have written reams and reams on the topic and its all out there for the inquisitive mind to digest. I would politely suggest ACTUAL BOOKS or articles by people with proven experience and knowledge as a source for information rather than what some completely unknown person on an internet forum (or blog for that matter!) says.
Right, on with the fitting. My dizzy has a plastic jobbie where the vac advance canister goes so that was removed and the advance can fitted in its place. There is an arm which hooks into a tab on the rotor. This is what pulls the timing in/out.
Here's my distributor before any mods (ignore the completely different advance weights/springs). The black plastic piece screwed in at the bottom of the picture is the blanking piece for the vac advance.
I removed the rotor housing thingy for a better view.
The kit comes with a fancy looking plate that you can just about see in dark grey below the top fixing screw of the vac advance unit. It looks a bit like a cog-wheel and has an off-centre hole. This is the limiter plate. I set it to give the smallest amount of vac advance possible so I could start safe and increase if required.
When I got the advance kit I also bought a hand vacuum pump so I could determine the vacuum at which the advance comes in, when its fully deployed and also apply known amounts of vacuum with the engine running and measure the advance.
First of all I hooked up the hand vac pump with a spare bit of hose and applied vacuum until the mechanism moved. This was at whatever setting was straight out the box with no adjustment. The mechanism moved at approximately 7 inches of mercury.
I then kept applying vacuum until the vaccum mech stopped moving and the maximum vac advance had been deployed.
You can see in the right hand picture that the arm has moved to the bottom of the slot when fully deployed (just above the Made in Mexico sticker). The limit plate adjusts the start position. This highlights the slight drawback of the limit plate - if you rotate the plate to limit the advance, the spring in the diaphragm is compressed and it will require more vacuum to stat moving. It would make more sense to have the limit plate on the other side of the arm so the mechanism moves into the limiter and the vacuum at which the advance starts moving remains the same.
I can see a potential future modification on the horizon - move the limt plate to the other side of the arm, but that's a task for a rainy day. Back to the task at hand though, I noted down the values and found that the vacuum advance started at 7.5in.hg and finished at 13.5in.hg. I know from the vac gauge on the dashboard that when I'm cruising along at around 2000rpm I get 15in.hg vac which means this setting is ok - all the vac advance will be in so it will be steady and not wandering around (I assume that's the aim) so I may as well go for a drive and see what its like.
The only thing left to check was exactly how much vacuum advance I had. The limiter plate just has a bunch of nobbles in it, it doesn't say what the value is so you have to measure it. With the vacuum pump still attached I got my timing light hooked up and started the car. I let it warm up for 5 minutes then checked the initial timing which was 17°. I pumped 15in.hg vac on the hand pump and the engine picked up a few revs which showed that the vacuum advance was working. Checked the timing and it was 32°. This meant that I had 15° vacuum advance.
I found a bit of hose and attached it to the vac can, then ran the other end round to the front of the carb. Now here is a dilemma. Ported vac or manifold vac? Ported vacuum on a carb comes from above the throttle and is only present above idle. At idle there is no vacuum on the ported source. Full manifold vacuum is present all the time (apart from wide open throttle of course) as it comes from below the throttle.
Full manifold vacuum is present all the time. After thinking about it I put it in the ported vac. The reason - my idle vacuum is quite low because of the cam, approx 9in.hg. If I hooked up the vacuum advance to manifold vacuum the vac canister would be partially deployed at idle and may lead to an unsteady idle which hunts around because the timing is constantly changing. The weight of opinion is that full manifold vac is the "correct" way to do it - you set your base initial to stock at around 12° or so then you get your extra timing from the vac advance to take it up to around 25° so you get a good clean idle, but this needs to be fully deployed at whatever your idle vacuum is. That means I need a vac advance can that is limited to around 8-10° and is fully deployed at idle vacuum i.e. somewhere around 9in.hg. I may be able to set up this adjustable can to get that setting in the future when I have done some more adjustment but for now I'm just interested with measuring the total vac advance in degrees and seeing what its like when running - hence I went for ported vacuum.
Ported vacuum on an Edelbrock carb is on the left looking from the front. (UK drivers side).
Went for a drive, about 40 minutes or so with lots of cruising but also some varied acceleration to see if I could detect any pinging. Everything seemed fine and things were noticeably smoother at lower revs. I didn't look much at the AFR but will have a look at the data later on. Next thing to do is to lean out the carb in cruise mode and see what I can get away with. I think at least one step leaner (4%).
