Quick question about distributor advance

sounds like moter has an axe to grind with me??if you'd read what he's trying to accomplish,you would see he's trying to increase his ET's(ex:not worried about economy)to do as such you must make a choice sir,either WOT acceleration pedal mashed to the floor for 1320 ft or part throttle,put put around the fairgrounds and look cool to your girlfriend while going 5 mph,the choice is yours!High lift cams affect idle,idle will affect power accesories (ieower brakes)not to mention will load up like a bitch when just crawling around town,been there done that!cam timing alone will not change the vehicles charachteristics,you must compensate initial+mechanical advance to suit the camshaft grind you are using in turn driveability will suffer if you plan on using a .500" lift cam in a SBC vs.a .400" lift you cannot tell me that there is no difference in those grinds!!? and how different they would be in the same engine combination.

 

Please folks, timing in the 50 degree range is nuts.

Please don't tell me to add them all together (initial plus mech plus vac). That's not at all how it works.

When you open the throttle to get the revs up to check max mechanical advance timing- what do you think happens to the vacuum? It kinda goes away somewhat doesn't it? (oops!)

Where's your max "combined" advance again? How did you say to get there? Kinda meaningless isn't it?
What engine condition would put them all at max at the same time anyway? Describe how to use a timing light under the hood under those conditions. !!!

How in the world do you set it up to read all three at max at the same time when you hook up your timing light, and then set your timing to that number?? How do you even get there? You don't. So don't tell people they need to do that.

Leave the vac advance completely out of it until everything else is completely finished and locked in place.
Ideal timing will be somewhere in the thirties unless something is really out of whack.

Then, and only then, should you play with the vac advance, tuning it for best part-throttle drivability and part-throttle mileage.
Do NOT set the distributor timing with it or you will wander around- forever lost..

 

<TABLE border=0 cellSpacing=0 cellPadding=6 width="100%"><TBODY><TR><TD style="BORDER-BOTTOM: 1px inset; BORDER-LEFT: 1px inset; BORDER-TOP: 1px inset; BORDER-RIGHT: 1px inset" class=alt2>Originally Posted by primeisnotacrime
Ok here's the deal sounds like alot of you misunderstood what I said.You want an optimum of 36-38 degrees total for timing with a mild small block chevy .
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QUOTE- "I just read what you wrote.....BUT a Total timing of 36-38 is very good for most all naturally aspirated small black chevies."


I've used that timing number on small white Chevies, and small blue Chevies. It works well there too.





Sorry I just couldn't help it.

 

The lips of wisdom are closed but to the ears of understanding.
Hermetic Axiom

 

Oh I dunno the Snake used to run about that much and sometimes more on his hemis.

 

ditto

 

hehehehehhe

 

If it runs good, and doesn't crank like a dead battery when you start it, leave it. I run that much initial on most all my "performance" stuff

 

OK no offense but what you said is that all aftermarket cams are advanced. You set your timing with the vac advance disconnected and it is determined by crank rotation not valve timing.

On an altered cam that the tomming is changed from stock it is the valve timing that is changed. It doesn't change the ignition timing at all. The tuner changes the ignition timing by turning the distributer on the motors that we run or by tweaking the prom on a computer controlled ignition on a late model car. It is measured in relation to crank rotation.

BTW not all aftermarket cams run the valve timing advanced, yes it is common but it isn't always the way it is done. Sometimes it is beneficiary to retard the valve event for a purpose built engine. But that is an entirely different discussion.

I understand what you are saying in that it is commonly excepted to advance the ignition timing on an engine that has the cam shaft changed. I don't agree with your numbers. Perhaps if you actually showed the equation with an explanation of your variables we could cypher what it is that you are trying to say. Never the less if I measured the timing on a street motor using a timing light and it came up above 50 degrees advanced all in I would be concerned that something was amiss.

 

Ok I understand kinda where your thinking is coming from,I shouldv'e been more clear in my explanation but since I felt attacked I had no choice but to defend myself you understand?...he should use between 34-38degrees total timing initial plus centrifugal the 50+ figure came from something and old timer showed me years ago.I start with16-18 degrees initial this figure seems high but trust me it's not!the engine with excessive overlap and high lift and slower burning 92+ octane gas,the engine will run smoother and cooler in traffic no heavy starter draw and no excessive cylinder temps that somtimes cause run on in hotter temps.I've always seem to find the sweet spot right around 18 degrees initial and 18 degrees centrifugal all in by 2500 rpm the extra 14-16 degrees comes from the vacuum cannister.Since the cam plate that mounted to the base of the distributor cannot be measured in degrees we'll use vacuum instead.I f you attach a mity-vac to the cannister and measure in hg, most are 18-24 fully retracted plunger(baseplate cam advance at full vacuum conditions)I limit the plunger to 14-16 in hg so as not to detonate under light throttle conditions especially with 18degrees advance initial.Everything above 2500rpm the mechanical advance starts to take effect and limits everything to whatever timing is set for the engine be it 34-38 degrees.

 

Ok ,, this post has gotten out of shape and in reality there is no one absolutly incorrect. Its all in how its said and how its figured !!!!
Heres the deal;
Lets take a common hp engine spec card and study the factory specs. I will use a 1968 350 hp Corvette with 4 BBL Carb.




Start with the intial timing setting on the left. Two distributor numbers are listed one uses 4 BTDC at idle and the other uses 8 BTDC at idle so lets use the 8 degrees for a balancer intial timing setting.
Now move down and look at the ignition advance "degrees at 2500 rpm " thats 5000 engine rpm and is listed as centrifical ( mechanical) and vacuum combined at 35 to 39 degrees (using the largest of the two.
Even when modified with changes etc usually athe engine builder wont increase this number. They may wish it to happen sooner but the basic engine design dictates the number of actual degrees.
If you move over one section you will see the distributor specs listed as well. Mechanical advance is 15 @ 2350 ( 30 degrees engine at 4700 rpm) (thisn number is stamped on the cam assembly) and vacuum setting of 7 1/2 ( 15 engine) at 15 1/2 inches of vacuum.
As the engine cant turn 4700 rpm and still maintain 15 inches of vacuum you can never have all this advance at the same time. When rpm is high vac is low and when vacuum is high rpm ( and load ) is low. So there a overlap . Shown on the card with the maximim advance number above.
The vacuum number is also stamped on the advance unit as well.

Makes it simpler doesnt it ....???
Now

 

THANK YOU.



By the way, the barbecued Spam was good.Thanks for the hospitality.
Nice little shop.

 

I'm offering one more bit of advice to this thread then I'm done with it. I once read this on a sign and it must have been written by a wise fellow.

Never argue with an idiot. They pull you down to their level and beat you with experience.

 

He was a very wise man indeed!

 

What's a golf tee?

 

I've got another one for you.."It's better to keep your mouth closed and have them think you're an idiot than to open it and remove all doubt!"

 

Hey wait a minute I resemble that remark.

primeisnotacrime,

You are supposed to quote instead of doing a quick reply so we know which one of us is supposed to be offended and as they say in ""The neighborhood,"get all up in your business.""

Thanks for explaining where your numbers came from, I think I can sort it at this point.

I still don't set my initial very high as a light would show as a rule. But the one thing that we all should bear in mind is that so many SBCs have been molested at this point that you almost never know if they have the correct balancer and one should actually use an adjustable pointer and a degree tape.

 

A Golf tee is that little pointed wooden thing (looks like a spike)that golfers use to set their golf ball on when they Tee off.

Just the right shape to plug a vacuum line and don't take up a lot of room in your tool box

Jim

 

prime obviously didn't understand where my last post was directed

 

I understand who your post was directed at and I see it as an insult from you but I'm a humble good natured kind of person and it's easy to fling shit in someones direction your never gonna smell!.the 50+ figure is an arbitrary number to help remember that the14-16 degrees is actually in hg for my own reasons to remember.Whether it be 18 initial/18 centrifugal,16 initial/20 centrifugal I always seem to rely on 36 total for a base

 

So you can tell me it is irrelevant, but what advance might I read with 10' initial, 20' centrifugal and 10 vacuum adv at say 3K RPMs' for either ported or manifold vacuum? I know I don't want to set it this way, I am just looking to know

 

Prime I owe you an apology. It was wrong of me to insult you like I did. I'm not obligated to think you know beans about engines, nor you me. You may be brilliant in areas I'm not aware of. I sincerely apologize.

 

How would you know that? I spend every minute of free time I have in my garage.

 

This has been a very interesting thread with lots of good info. However 2 things hsven't been mentioned:
First- avoid the new digital "dial back" or advance reading timing lights if you are running a high output,solid state ignition system such as a capacitive dischage multi spark unit such as MSD sells. MSD and HRM have repeatedly warned that they may give erratic or even worse, incorrect readings. If you have a good "dial back" of the analog variety, take good care of it, as a new one is almost impossible to find, although I must confess I haven't checked Snap-On. Instead, check and make sure the TDC mark on your balancer is correct, then put on a timing tape and use a regular timing light, reading your advance on the timing tape like we did before "dial backs" came along.
Second- Considering Manifold vs ported vacuum? One issue not mentioned is that ported vacuum does drop to zero at idle, and if you're having engine run on after igniton is switched off and are on manifold vac. a switch to ported often cures the problem. But keep an eye on temp during extended idle. But then if you're running a flat tappet cam, extended idle is about the worst thing you can do to it!
Dave

 

If you notice-I didn't say that I used one to hold up a golf ball!
Actually learned that trick from my mechanic brother-who does play a little golf too.
I'm like you-don't ever seem to make the time to take part in that game-would rather be out in the garage.
AA

 

And i thought this was going to be easy...

 

36 to 38 is fine if your running racing gas or alcohol like a sprint car but if your running 87 octane unleaded you better not go over 33 or 34