scootchu

I had some issues a while back with timing and detonation and in a nutshell solving the cooling issue made it go away. Today in an effort not leave anything perfectly good and working alone decided to replace my springs in the dist, limit the mechanical advance to the space of 10L with some shrink tubing, run 16 degrees initial timing and Holy Crap! The difference is amazing. I did have some small detonation when just getting on the gas, so I disconnected the vacuum advance and that went away.

I have about 34-35 degrees total advance. With the vacuum connected there was just too much.

I am happy, so what is the downside to my happiness and a disconnected vacuum advance???

As long as my detonation doesn't come back and my engine temp stays normal, should I worry???

Starfury

16* is waaaay too much initial advance, especially if you don't have a really wild cam. It should be closer to 10*. Total timing is ok, but you'll need to up the total mechanical advance when you retard the inital. 30-34* all in by 3k should be about right, depending on the cam.

scootchu

I have no idea of the cam that's in there now, but it's not stock. The engine seems happy though.

67mustang302

I'd idle it with whatever it likes best, 10, 14, 20, whatever. That said, most street engines will work best between 10(stock) and 18(large cam/intake). No vacuum advance may hurt mileage, or it may not. It might hurt part throttle power or it might not. As far as detonation just off idle at part throttle, that can be solved by going up in power valve rating. The sooner the valve comes on to deliver fuel at part throttle, the cooler the combustion process, the more timing you can run. I've found in most cases a carb'd performance car likes a power valve about 2-4" below idle vacuum with a quicker timing curve, but every engine is different.

Starfury

My Magnum 280 cam wants 14* of initial (accord to Comp Cams) to make up for the low cylinder pressure at idle caused by the valve overlap. It'd have to be a pretty hefty cam to want any more than that.

I suspect the detonation problem will be fixed by dropping the initial advance and increasing the total mechanical advance. You'll probably be able to hook the vacuum advance back up at that point for better cruising fuel economy.

67mustang302

How rapid is the advance curve also? 16* initial with a rapid curve could result in too much timing too soon.

scootchu

Thanks for that info. I will play around a bit with adjusting the vacuum canister tomorrow to utilize it without bringing too much advance in.

I will also look at the Power Valve. It's a Holley 600cfm.
I found this video on Holly Power Valves.
http://www.youtube.com/watch?v=6vKlAnKdcB4

scootchu

I used a Mr. Gasket kit with two springs designed for full advance between 2000-2800 rpms. I will have to get my helper to read the tach while I check for the total advance.

67mustang302

Which is it, 2,000 or 2,800rpm? There's a huge difference. General rule of thumb is to give the engine as much timing as soon as you can without detonation or any loss of power(within reason, you can burn crap if you're not careful).

As for the power valve rating, the 1/2 of idle vacuum rule is a good rule of thumb and a basic starting point, but it's not always true(yes, yes, I know..."But Holley said in the video..."), particularly in more modern performance engines that are more efficient at part throttle. The 1/2 idle vacuum rule was instituted back in the day of flat tappet cams and heads that didn't flow all that well, so it's not set in stone(it really comes down to the individual setup). On my setup for instance, if I run a valve rated for 1/2 my intake vacuum, I end up lean as **** at certain part throttle positions and the car won't run well. It's not so much an issue with Holley's out of the box feel good rich tunes, but when you start tuning for lean best power it becomes more critical, since the jets/IFR's alone end up not being able to cover the gap between light throttle and power enrichment, so you drop the PV rating to compensate.

scootchu

I understand, I just have to be able to see the tach and the timing at the same time, which I will have to wait for help from the wife to check later today.

The only time I got any detonation was that little blip of the accelerator off of idle. Just a tiny little blip. At anything close to WOT it was all power and no detonation. Disconnecting the vacuum advance made that little bit dissappear.
After disconnecting the vacuum advance it was push you back in the seat smooth power. In fact it was like night and day.
I started doing all of this when I noticed that the car felt like (and I am having a tough time finding a word for it) it was being held back when cruising. Kinda like holding a dog on a leash. Pulling, sort of like tugging. Now that is gone and it's all power increase.

I have a power valve that was in the carb, first I will check that since I believe it is the same in the carb.