StealthElephant

How much timing do I want @ part throttle cruising around town for the best gas milage?

When do I want full timing to come in during WOT?

ede

you'd want full timing to come in around 3000 rpm. you'd want total advancve to be around 34* to 36* for most all SBC applications. there isn't really anything for part throttle or full throttle it's rpm dependent.

StealthElephant

Well right now my distributor has 10* advance at 10" vacuum. It also is set at 24* mechanical.

I'm going to need to run about 16* initial advance, which means 16*+24* @ WOT = BOOM. So I'm going to bring my mechanical down to about 18*.

My second issue is the vacuum advance. I was told I either need to run a vacuum canister to control vacuum, or get rid of/limit my vacuum advance. Basically, 16* initial + vacuum advance = pinging under load. Since my compression has me running alot more initial then normal, I need to drop vacuum advance.

My question is how much advance do I want driving around town for the best gas milage? Another thing is when exactly does mechanical advance start to come in? Because if I have vacuum, and I'm cruising around town at 1600RPM, is my mechanical starting to come in? Because 16* initial + vacuum advance + partial mechanical will also cause me problems correct?

Basically, I want to a shop that has been building race engines for like 40+ years, I buy my parts through them. I told them my problem with my engine, and the old man instantly told me that vacuum advance was hurting me with the high compression I have. He said I needed about 16* initial, which meant I had to lower my mechanical advance as well.

F-BIRD'88

vacuum advance is not hurting you.

But.... the entire advance curve needs to be tailored to your engine and what it needs to operate at best efficiency.

If you want to use 16degrees intial timing then limit the mechanical advance travel to add 18degrees at full advance.
Then...
You can buy an adjustable vacuum advance and taylor it to your engines needs.

You want to add about 10 to 20degrees of aditional vacuum advance
at (high vacuum) hyway cruise speeds. There is no definitive set amount that your engine will need. You'll have to experiment to find what works.

An engine with EGR or a long overlap cam will awant more vacuum advance at cruise than an engine with a smaller cam or no egr.

Streetiron85

I posted the same question a few months ago, and it was still unclear to me what is best. I bought an MSD adjustable timing control, that's a computer that's rigged to a dial that mounts to your dash, and it allows you to retard or advance your timing up to 15* while you're cruising.
What I found out is that you can add a lot of advance at cruise (18" vaccum / 2000 rpm). And I was never sure whether or not it improved the fuel economy or not, because I was able to dial so much advance in electronically, that it was outside of the limits of the distributor to take it. In other words the electronic box would add so much that it would upset the cap to rotor phasing and it would misfire because the spark was firing with the rotor midway between two cap terminals. If you can picture that.
If you're using vaccum advance or electronic, it changes the cap to rotor phasing, and reaches the point where that's the limiting factor.
If you have a spare cap, you can drill a hole in it and observe the cap to rotor phasing with a timing light and see what I mean. You'll see that vaccum advance alters the phasing.

Hope this makes sense...

five7kid

iTrader: (14)

Any reference to "total" means initial plus mechanical only - no vacuum.

Any reference to "initial" means w/o the vacuum attached.

You set total for WOT performance. You dial in the amount of mechanical advance produced so that the initial is low enough to prevent starter kick-back. After all that's done, you adjust the vacuum advance to max it out w/o producing partial throttle loaded pinging.

StealthElephant

Well my buddy is going to make me a piston stop and I'm going to remark TDC on #1 for the 3rd time. Hopefully with timing tape I can watch my advance curve as the engine revs.

I'm pinging under load at part throttle going up hills barely and pinging slightly at WOT.

I'm guessing I need to drop around 6* of mechanical advance off my 24* down to 18*. I backed my vacuum advance back and it seemed to help. I dunno though, it's really hard to hear very slight pinging.

Someone told me that a retarded engine gets very bad gas milage (makes sense, since combustion is happening to late). Does even slight pinging kill gas milage as well? It kills power so I image it hurts fuel economy as well.

I dunno, I'm just wondering if I'm 2-4* too far advanced under part throttle, would that acount for as much as 5MPG? I mean, I was getting 7.7MPG. It just seems like my gas milage is so bad that it has to be more then just timing.

I pulled the #1,2,3, and 4 plugs. #3 was running rich, so was #2. The other 2 plugs looked perfectly normal. The 2 plugs that were rich had clean electrodes though, so I know they were sparking, but all around was black. I'm running the 42 ACdelco heat range, are my plugs too cold? I dont' know why 2 were clean and 2 were covered, I didn't pull the back 4 cause they are a PITA to get to. Didn't surprise me though, 7.7MPG, the fuel has to be going somewhere.

How fast does a cat clog up from running rich? I don't know if running rich while I was tuning the car clogged the cat, and now even with better tuning the cat is doing it. Before the engine was retarded, so I can understand bad milage there, but now the timing is much better.