My car is running at about 18 degrees initial and 34 degrees at 2800 RPM with the vacuum advance disconnected and plugged. When I plug it in it idles at around 40 degrees. Is this too much? I run 93 octane gas.

 

Not if it's idling well.

 

It actually idles really well once you plug that in. It cleans up the idle a lot. I just thought maybe 40 was a bit much. Can I bump my timing up any farther at the track?

 

Find out what your engine likes. 32-36 is the typical range for SBC's (assuming the timing mark and tab are accurate).

 

So it doesn't matter with the vacuum advance hooked up how advance it gets? I have a non adjustable vacuum advance for my dist.

 

By "at the track" I assumed you meant wide open throttle. At WOT, there isn't enough vacuum to pull any vacuum advance, so the mechanical is all you set.

Part-throttle situations, such as autocross, may be a different story. Again, see what it likes.

 

You're running ~22* vacuum advance? With 18* base? Aside from autocross, the thing that might cause a problem that I see is slight hills. Cruising up a bit of a hill you'll start to load the car in high gear, with high vacuum and say 2500RPM cruise speed you'll have lots of timing. Maybe 50* or more. That's ok with VERY light load, but if you notice it pinging under load and light throttle, use a stop to limit the vacuum travel. It's a little half circle thing with teeth on it, comes in distributor timing curve kits usually.

 

What are you running timing wise? Mechanical and total. How about with the vacuum advance hooked up?

 

Vacuum advance is only for part throttle operation under load. Typically you never play with the vacuum advance. There are different pots which allow different amounts of vacuum advance and there are also adjustable pots.

All you really need to know and adjust is the initial advance and the mechanical advance to limit how much total you get. An engine could like 12* BTDC but also like 36* total. If the mechanical advance isn't 24* in this situation then your initial or total will be off. Bumping your initial up to increase your total may cause other problems because the initial is too high.

With your car at 18 initial and 34 total, are you at full advance? Will the total go higher at higher rpms? A good performance ignition should be at full advance by 3000 rpm. According to your numbers, you only have 16* of mechanical advance. If your initial timing of 18* starts to kick back when cranking the engine over, you'll need to drop it back a few degrees and increase the mechanical advance to increase the total.

A distributor recurve kit includes weights and springs. The weights control how much advance, the springs control the speed of the advance. You can mix and match parts to fine tune the curve. One heavy and one light weight and one heavy and one light spring if needed.

You can also pull weights, springs and center cams out of junkyard distributors for more tuning options. Not all HEI distributors are the same.

 

I use the crane kit. It came with springs, a vacuum advance limiter, and an adjustable vacuum can for amount and rate.

Personally? I'm running 24* base, and i've welded the slot in the mech advance to only give 12*. I think i've got about 12* or so of vacuum advance. I run ported advance.

 

Boy I am learning about this very thing in trying to tune my HEI also.

I bought a summit HEI replacement Dissy and have added an adjustable vac advance canister to it.

Origionally I couldent get the weights to give any more than 11* of mech advance. So I modified the weights arms so they were shorter and now I get way to much mech advance (+24*). Luckily the weight and spring sets are cheap so I have some more sets that I will grind to get this darn distributor to give a more reasonable 16-20* of mech advance.

To help me to answer the question that you asked though -- I got myself this MSD knock sensor thing that alerts you when you have knock. Super handy (but a little pricey). So far it has helped immensly in part throttle tuning my brand new setup. I just go out and drive and note where the thing knocks. then I can go back home and change the amount of vac advance or its rate for part throttle knock. Or I can change the weights and springs if I have WOT knock.

Good luck.