70_Shark

I was reading some material I found a couple years back from another forum. They were discussing vacuum timing.

Spark timing does nothing to address the issue of ‘motor condition’, only when the spark occurs in relation to crank position. Vacuum timing a motor sets the spark according to the condition of the motor, as compression, mileage, cam, etc., all become a factor.

I’ve been playing around with timing and keep getting frustrated and setting it back to where I thought it should.

ZZ502/502
8 degrees BTDC @ 800rpm, 36 degrees Total @ 5000rpm,
8 degrees initial
6 degrees vacuum
21/22 degrees mechanical
36 total

With my vacuum gauge, I was trying to adjust my idle mixture screws I was always getting a ~2” flutter around 12”.

OK, It’s only an engine what do I have to loose? I hooked up my vacuum gauge to manifold vacuum, and went for it.

It feels like I gained 50 rear wheel! Runs much better, smooth hard pull, not as loud at idle! Basically it does not sound like a NASCAR anymore.

I checked the timing mark and 30 initial! I did not hear any detonation so I took it for a long spin. Probably its best outing. I did notice a slight miss when she was wining up(58 degrees?). I pulled the plugs and they looked great. I gained 5”(then backed off 1” as suggested) of vacuum and lost the flutter.

I checked that the balancer lined up with TDC.

Anyone ever run into this?

Thanks,
Dan

81 Vette

*ponders*


Could be engine specific, but I wouldnt mind looking into that more.

Vetterodder

"Total" advance is initial plus mechanical. In your example, you have 30º total. 30º at idle with vacuum advance connected to manifold vacuum is not excessive. That's about how factory hipo engines were set up before smog regs created ported vacuum advance. I'm surprised that you vacuum advance is adding only 6º. Typically, it's 16º or more. If I'm understanding you correctly, you are runniing 24º initial + 6º vacuum advance to get that 30º at idle? How does that add up to 58º with 21/22 mechanical?

81 Vette

I wanna keep this thread going hahaha, how exactly does one go about changing vaccum like that anyhow?

oregonsharkman

Sounds very similar to my post...........running 16 degrees at idle but hits around 45+ degrees at revs...........and runs great and doesn't like less timing???????????

70_Shark

I have a modified vacuum canister that I set to 6°. I will double check tonight when I get back home, but I'm pretty sure I was checking initial without the vacuum advance connected. So 30 + 6 + 22 = 58°

70_Shark

81 Vette,
Basically, as your car is in driving condition(all vac hoses, air filter, etc.) with vacuum gauge installed, advance to peak manifold vacuum, note timing, then retard 2-3" of vacuum. Go and have fun.

I started with vacuum advance pugged, just reading off the manifold to get initial.

I will repeat tonight to see if I can fine tune it better.

-Dan

81 Vette

definitely interested in that. good luck. thanks.

Fevre

What do you mean by this? Did you check your #1 piston tdc and the balancer registered tdc? Balancer may have slipped giving you a false reading when you time it.

70_Shark

Yes, That is what I suspected. It is right on the money TDC.

I'm thinking that maybe my springs are too light and I'm getting mech. advance right away.

-Dan

bobs77vet

For what its worth I have always fine tuned my cars (last 20 yrs) with the vacuum gauge, especially the carburetor settings. I have never had a bad experience where the vacuum gauge misled me.

70_Shark

Thanks Bob for the reassurance. As a newbee to performance tuning I'm finding the vacuun gauge very helpful. There is so much to learn.

exdrag77

I tried that this afternoon with my car and it was interesting. I have the timing set at 16 initial with 20 mech advance in the dist (no vac advance on the dist im using). I hooked up a vac gauge to manifold at idle and have 9". Advanced timing until 13" which was max I read on gauge then backed off 2" until 11" vac was read on gauge (the articles I read said to back off 1" but that seemed too close for me!). Sounded ok when I revved so checked timing now and have 22 degrees. The total timing is 22 + 20 = 42 degrees. I thought I read that 36 degrees total was what a small block should run so backed the timing back down to 16 initial although it sounded good at 22. Now I'm wondering if my stock balancer has slipped?! I'll be checking that tomorrow. Maybe I'll run the timing back up a bit tomorrow and see how it performs, although a bit paranoid about breaking parts!

bobs77vet

I wonder what if everything wasn't indexed correctly when the engine was built? how many degrees could that account for??

let me think about this for a while.... but until then if it is running better and smoother what are you afraid of breaking? For giggles and grins you should do a MPG check prev setting vs. new setting

glen242

Where is Lars when we need him?

Vetterodder

Setting timing based upon idle vacuum readings is only of use if the engine never does anything but idle. Doing so doesn't take into account advance that the distributor adds and you're as likely to wind up with too much or too little total as with the right amount. 42º is too much total. If you want the benefits of more idle advance (including higher vacuum) recurve the distributor with less mechanical or use vacuum advance connected to manifold vacuum.

70_Shark

I do take into account advance that the distributor adds. I'm ordering a spring kit on a hunch. I already use vacuum advance connected to manifold vacuum. I'm finding the vacuum gauge a useful tool.

Exdrag77, I'm working on the same problem. I'll figure it out.

exdrag77

After thinking about it last night, using the vac gauge told me what initial timing I need (in my case, 22 degrees at idle). Therefore, since I would assume 36 should be the total for Small Block, I need to reduce my mechanical to 14 degrees (if it will go that low). If I was using vac advance, my vac advance plus initial should be 22. Gonna check and try that as soon as I get the balancer back on (another story!).

Bobchad

Initial plus mechanical plus vacuum equals total advance.

You want about 36 degrees total initial and mechanical advance in at around 2,500 - 2,800 rpm. This measurement is taken with the vacuum advance disconnected. This is controlled with the initial advance setting, 8 degrees BTDC in my car, plus adjusting the advance characteristics of the distributor. I have the MSD Pro-Billet. Following their instructions, I added the limiting spacer that gave me an additional 28 degrees of advance and used the spring set that allowed this to occur just under 3000 rpm.

Started the car and hooked up the timing light and the setting was dead on.

Hook up the vacuum advance and at a rpm level above the point where your total mechanical advance kicks in and under no load, you should get a total advance of 52 degrees. Mine came in exactly at that number with the vacuum canister on the distributor. Some of the vendors sell adjustable vacuum cans.

Here are links to Lars' 2 papers. http://home.comcast.net/~chadwick.ro...istributor.pdf
http://home.comcast.net/~chadwick.robert/howto.pdf