[Cliff R]

You are welcome.

[nUcLeArEnVoY]

Listen to Cliff, OP.

I'm a victim of Cliff's sworn enemy: one of my car's POs went the, "hey lemme toss in this rumpety-rump badass sounding cam into this anemic low comp 6X-4 head-havin' motor" route. I'm JUST barely escaping having to mod the idle circuit on my original carb, since with vacuum advance I'm pulling around 15-16" of vac at idle. Ill tell you one thing, i couldn't do it without vacuum advance. My motor is a W72, but even they are still pretty low compression. For the record, I use the factory vacuum modulator valve for my vacuum advance. It's actually a cool little doodad- it provides exactly 9" of manifold vacuum to the distributor during idle (which already throws 17 degrees of advance from my stock vac can, making my total timing at idle a whopping 35 degrees - this is STOCK, mind you), until port vacuum exceeds 9"; at which point whatever the port vac is, is what the distributor gets. I love it since it's sort of the best of both worlds.

Still. Because of the cam I have combined with the low comp, it's been challenging tuning this thing, but it's an ode to how much cam choice and compression influence timing. I use the stock base timing of 18 BTDC and don't get starter kickback at all, and I also use the factory vacuum can which adds in a whopping 25 degrees of advance all in by 11". Where things balance out is I have the stock weights and very heavy advance springs that don't put in all the timing until near redline, and so my mechanical advance is very gradual which is why I'm able to get away with so much vac advance without pinging - this is how it was setup from the factory. My mechanical curve doesn't even start until around 1200 RPM. I don't really care, anyway. I don't race my car- I just like to take it out on drives and cruise it. Sometimes I'll gun it and it does have some oomph, a lot of that because of gearing AND vacuum advance.

So much timing was added to the late 70's low compression motors because of not only their compression but also emissions equipment, especially EGR. EGR cools combustion temps when in use (which is during cruise conditions) by introducing inert exhaust gases to the intake charge, which allowed for very high vacuum advance without fear of ping. Vacuum advance is what made those anemic motors even somewhat liveable.

I deleted EGR and I'm in the process now of putting a limiter on my vac advance can to limit it to 20 degrees, which should have me between 48-53 degrees total timing between 2250-3500RPM and cruising, which should be about perfect for my low comp, overcammed motor. My cam is contributing to an even worse DCR due to bleeding compression cuz of the overlap and tight LSA, requiring even MORE timing. Even now with my vac advance can unmodified throwing in 25 degrees, I never hear pinging on REC-90, but I want to err to the side of caution since I deleted EGR; not to mention I do get some bucking and surging when cruising around 35-45 MPH. Not going to hurt anything to just shave 5 degrees of advance off my cruise timing.


Point is, keep your vac advance on. If you choose manifold vacuum, it'll smooth out as well as cool down your motor at idle, add in some kick during acceleration, smooth out off-idle transition, and provide the advance needed to sufficiently burn the hyper lean mixture during cruise. There is nothing but benefits to it as long as you know how to appropriately meter it for your engine's needs.

[Mr Anonymous]

I had a combo that was happy with "full manifold vacuum" at idle. It was under 8.0:1 compression, and 2.41 rear gear. I had years and tens of thousands of miles invested in the tuning efforts, and it ran extremely well for what it was. The driveability of a well tuned HEI/Qjet combo is its own reward.

[sdbob]

Thanks Cliff,and all you guys for all the info. Yes yrs ago it was the light springs. I've always run vaccum advance even in my SD I race,its stock.

[Shiny]

Originally Posted by Cliff R

.....

Here, a simple and quick method to determine or get close to the ideal timing the engine is going to want at idle is to make sure it has adequate idle fuel/bypass air FIRST. Then simply loosen up the distributor and advance the timing till it smooths out and makes highest vacuum (going further it starts to "buck and kick" in protest}. Then slowly retard the timing until the engine just starts to slow down a tad and develop a nice exhaust note (deep/heavy sound), stop, and take a gander at it with a timing light....

... hopefully some of the info helps a bit........Cliff

Yes, yes it does help. Thanks for sharing an understandable approach to setting the initial timing.

I was planning on maximizing the vacuum but the next "back it off" step is noted and I expect it will avoid issues when the engine is hot.

Thanks to all for this thread.

Mike