FlyLS6

Nothing exciting, just a gauge.

Ok....I've worn some out a couple of previous threads on timing.

Without rehashing all of that, I decided I'd try timing the car with a vacuum gauge. My understanding is that one wants the "smoothest" needle. This is about as good as I could get:

http://www.youtube.com/watch?v=oat9Z...ature=youtu.be

This was at 18* initial, at between 850 and 900 rpm on the car tach. The car has 21* mechanical ("dial it in" timing lights are cool, I should have bought one a long time ago). The needle action is at the smoothest I could get it, up to that much advance. At 15*, for example, the range of motion was almost two segments, instead of the one shown here.

Motor is 454; 10.2; open chamber 188 heads; factory exhaust manifolds, 770 SA, and this cam: http://www.cranecams.com/product/car...detail&p=23804

So, what I've read is that by using a vacuum gauge to set timing, look for the least amount of motion out of the vacuum needle. Does the above look like what I want? Never having used one for this purpose before, I have no idea.

7T1vette

Hmmmm.... I don't think you can 'time' the ignition with a vacuum gauge. You might be able to 'optimize' timing at an idle condition [depending on your point of view as to what 'optimize' means]; but you can't set ignition timing throughout the rpm range with vacuum data.

cardo0

Yep 67* of overlap only makes 13-14"Hg vacuum (i'm surprised that much). That's barely enough to run accessories.

For carb tuning try Cliff Ruggles w/s:http://cliffshighperformance.com/sim...orum/index.php
Beware u may lose a lot of sleep reading all the carb tuning threads there.
I have to say even Cliff's posts really can help u more than anyone here.

Is this what your looking for? http://cliffshighperformance.com/sim...sg1010:thumbs:

Good luck LS6,
cardo0

FlyLS6

After I figured out the 21* timing in the distributor I realize I could have just set the initial in the 36* - 38* range which is the "norm". What started this was that after a recent dyno run (below) I checked the initial timing (as an after thought) and discovered it was set at somewhere around 19* to 20* (it's a long story), giving me 40* or more total advance (no detonation issues). I thought that might be a little excessive and not best for most hp.

I was all set to pull it back to the 36* - 38* range when I remembered "tuning by vacuum gauge" comments I had seen, and so looked into that as perhaps away around the "well, that looks good" school of setting initial timing. The key, for me, was a comment about "steady" maximum vacuum, which I interpreted differently from just "maximum". The video was when I was messing with it at lunch that day and I ran out of time. I've since pulled the initial down to 17*, for a total of 38*, because there I got the needle to maintain 1/2 in swing in value, better than the 1 in swing of value I had at 18*, 16*, or anywhere else. The difference is that I normalized the idle speed at 850 rpm, each time I moved the distributor. I hadn't done that at lunch because I was hurried. When I got back to it after work, I watched the needle for a good one to two minutes, just to verify swing. So, in MY interpretation of "steady", 17* works best. Does all this mean squat? I have no idea at this point. Although, it's interesting to me that the most stable reading I got on the gauge is in the "standard" that is recommended.

In the fall I'll have a set of new dyno runs to see the difference, if any, when I can get in there on a cooler day (after a jetting change also). Thanks for the links, I'll check them out.

CA-Legal-Vette

I've certainly heard of using a vacuum gauge to set timing but I thought it was usually to set idle at the point of highest vacuum rather than the smoothest. It'll be interesting to see what the dyno tells you.

63split63

I have never heard of setting the timing with a vaccume gauge and I have slept at Holliday Inn Express and been to a few rodeos in my time .

Bill

63mako

18 degrees initial and 21 mechanical = 39 total. You were running over 40 with no issues or detonation. I would leave it there. BBC 39 is ok.

pws69

Timing light works great if you know how to use it... I'll stick with that!

resdoggie

So, what do you use your timing light for? Use the right tool for the job!

midyearvette

i have run as much as 42-43 degrees total with no ill effects, watch out for detonation even if you cant hear it....read your plugs with a scope light to look for any type of metal inside the plug well.....open chamber heads will tolerate more timing than closed
timing with a vacuum gauge could be risky, use your light with a dial back, all you are doing is optimizing timing at an idle with a vacuum gauge......

FlyLS6

Thanks for all the comments, but I hope everyone realizes that of course I use a dial back timing light; that's how I confirmed the 21* mechanical advance. I'm only using the vacuum gauge to read the vacuum signal within the range the motor could be initially timed, to decide exactly where I want to try it. This, based on the commentary I'd read that could be worth a million bucks, or a million pesos.

I may in fact kick it up, but before I do I want to get it back to the dyno to measure it quantitatively...curiosity won't let me do otherwise. I'll post the chart here when I do.

Once I see the results, then I'll decide what to do.

imariver

[QUOTE=CA-Legal-Vette;1580846161]I've certainly heard of using a vacuum gauge to set timing but I thought it was usually to set idle at the point of highest vacuum rather than the smoothest. It'll be interesting to see what the dyno tells you.



I've been using a vacuum gauge for many years, but only to adjust the carb jets. If the needle is bouncing, or iractic, more than likely you have a timing and or valve issue. When adjusting the jets you want to acheive the highest possible vacuum reading.
Back in the day almost all tune-up manuals had some sort of "chart" showing how to read a vacuum guage.

69427

A timing light only tells you what the timing is, not what it's supposed to be. A vacuum gauge (like an engine dyno) shows you what the engine actually wants.
This type of test would be productive at all RPMs and loads too, but the difficulty factor certainly goes up significantly.

to the OP.

Ralphbf

[QUOTE=imariver;1580857024]When you install an after market cam you need to forget the stock timing and see what the engine wants.

Vacuum gauge will tell you about valve timing and Carb. settings, and even ignition timing.

http://www.secondchancegarage.com/public/186.cfm

Ralph

cardo0

Nice effort here but i believe what the engine really wants is as much advance it can get without pre-ignition. What your trying to avoid is peak cylinder pressure before the piston is TDC. Yes u want peak cyl press just after TDC.
What i thought u were looking for was to optimize the idle circuit of carb sorry. And i don't mean to bust your bubble after sharing your honest efforts. But i believe the best tool to set up a timing curve is a knock sensor - MSD sells them <$200.

Good luck,
cardo0

Ralphbf

To find out how much your engine really needs is pretty simple.
First put the gas in it you want to run.
Warm it up to operating temp.
Set the distributor so you can just turn it, NOT loose.
Accelerate if it doesn't ping, stop and advance the distributor by turning it.
Set it distributor so there is no pinging.

If it's so advanced that it's hard to start increase the total advance of the distributor.
Let's say your engine likes 28 degrees of total advance.
So 28 will be your target, but with more total advance you can retard the distributor, easier starting, plus you can reach your targeted full advance.

So all that is left is setting your advance rate.
A weight and spring set can run less than $10.00

Ralph

69427

A knock sensor only tells you that the fuel octane is insufficient for the instantaneous cylinder pressure. (It will also tell you if you have excess piston to wall clearance.) It does not tell you if the timing is correct.

cardo0

I think u missed my statement Peak Cylinder Pressure. Yes ignition does start before TDC but again if Peak Cylinder Pressure occurs before TDC your attempting to bend rods or bust pistons.
Your welcome to believe what u want but please share with us how u optimize your adv curve?

No flames, just ready to learn here,
cardo0

69427

Professionally I've had access to some pretty nice equipment (dynos, LPP sensors and knock sensor signals displayed on oscilloscopes, spark and fuel calibration hardware, etc) when doing engine work. After a few years and thousands of miles on test tracks ya get a decent feel for what's going on in an engine.
In my own garage I don't have access to hyperexpensive lab equipment. I make do with a WBO2 sensor, a homebuilt knocksensor display unit, a timing light, and an electronic stopwatch. Dialing in the timing at low RPM and load (as the OP has done) is productive as that's where the car and engine spends most of its life. It's also less stressful, as things happen slower at these light loads and there's less chance of damaging the engine. Once you've got the low RPM stuff dialed in you've now got the starting point of your timing curve. The high RPM stuff is a combination of experience (34*, 36*, 38*?), your engine combo, and any acceleration time tests you are able to safely do. Without actual dyno results or acceleration times, we're pretty much just getting the timing in the productive neighborhood.
That's it. I don't have any disagreement with whatever timing you decide for your engine. I just disagree with some of your tuning statements.

vettebuyer6369

Guys, I'm all for differences of opinion, but let's keep it civil and stick to arguing about ideas, not personalities.

thanks, guys... carry on!