Quick question about distributor advance

My engine is a 350 with mild cam, 650 edelbrock and headers. My HEI distributor only gives me 18 degrees of centrif. advance. I've read that it you should be getting 20 to 24 degrees. My target is 36 total, so in setting my timing, my initial ends up being 18, which seems high. So my question is: Should I play with my springs and weights to got more dist. advance or is 18 from the dist ok as long as the engine can handle 18 initial? Thanks in advance for your advice.(sorry for the bad pun).

 

you want 52-53 total,16-18 initial,with around 20-22 distributor total no more,and 14 vacuum cannister advance all in by 2800 rpm.No more it'll ping like crazy,you should get 52-53 full vacuum connected,all AFTERMARKET cams have some advance built into them from the factory,stock cams do not, that's why most are 8-10 initial,they don't tell you that when you get em!this combination yields the best street performance,power and economy ALL aftermarket cams regardless of manufacturer have advance built into them.You should get 36-38 (initial+centrifugal distributor)+14 distributor vacuum cannister hooked up for around 52-53 all in by 2800 rpm.

 

First, I haven't worked on sbc's for some 30 years, but my first question is why do you think you need 36 degrees total advance. I think the initial of 18 is about right on, but when you take off the vacuum lines and run the motor up to 3000rpm what does your timing show......................
A vacuum advance only works at slow speeds, and as the engine RPM increases it is dropped off and the mechanical advance takes over.....................
Take a look at this video and listen carefully....................I am running a drag car with
radical cam/racing only and am only running 38 degrees total advance................

As for an internal combustion engine being able to utilize 50+ degress of advance...........
the motor would self destruct before you got to the end of 1/4 mile.....................


http://www.summitracing.com/parts/MSD-85805/?rtype=4


IMHO

 

uh dont think so.

 

Hey Betnik- A GM HEI should have bout 20 degrees of advance in it. If you set your initial advance at 12 degrees for example you would have 32 degrees total.The trick is where it comes in.Use a dial back timing light-if you don't have one-buy one or get degree tape on your balancer.Start the car ,warm it up-check your timing at idle.Have buddy operate your throttle and bring it up 500 rpm at a time checking the timing as you go-write it down so you can keep track-you want your timing "all in" by about 2800 rpm.I had 32 degrees at 4000 rpm-changed one spring in the dizzy to a lighter one and had 32 at 3000 rpm.
Test drive with vacuum advance unhooked and plugged-mash on it and make sure it don't knock-if it does back it off a couple degrees.
Get an adjustable vac can-(hook it up to full manifold vacuum) and adjust it until the engine starts to ping when it starts to work(like on a small grade) back it off a couple turns and you are set.(A lot of the factory pre-set Vac cans will give you too much vacuum advance(like 20 degrees) at part throttle causing the engine to ping when it's under load.)
Sometimes you have to dial your idle way down(like 600 rpm) to set your initial accurately.A performance curve can start advancing as early as 800-1000 rpm.-(So you would get an inaccurate initial timing reading)
You will also need to re-set your idle after your vac advance is hooked up and adjusted.
A little bit of tinkering but you will see the difference in throttle response and fuel mileage at part throttle.Your plugs won't load up as fast either!
Just keep track as you go so you can map your changes and results.
Jim

 

Just to clarify things, I am not including vacuum advance when I said total advance of 36. I set my timing w. vacuum disconnected. Centrif. advance started coming in around 1000 rpms, and was all in by 2500. My understanding is that vac. advance is basically only for improved fuel mileage and goes away with full throttle. My car is now running fine with 14 initial and 18 from the distributor, but I wanted to advance the timing a bit to improve qtr. mile time.

 

That's right-leave the vac advance disconnected from the dizzy and plugged-A golf tee works perfect. I leave the vac advnce disconnected until I get the mechanicl advance set the way I want it. Full throttle pulls will tell you if the advance is too much-you will get knocking.
The factory weights in an HEI are pretty good-I use a little 3 in 1 oil and make sure the pivots and where the weights slide is lightly lubbed and nothing is sticking.A spring kit is like 5 bucks-you may only use 1 of them-it's okay if the springs aren't the same tension-like a medium and a stiff or whatever combintion works to give you the result you want.
For a street motor use full manifold advance(which I am sure some will disagree with)Vac advance works it's majic at part throttle to advance the timing to allow the long burn time necessary when the engine is not under load.Mash the pedal -the throttle plates open-manifold vacuum goes to atmospheric and your vcuum advance disappears.But at part throttle you will get a better vaccum signal and more of it from manifold vacuum hookup rather than "ported" hookup. The adjustable Vac can will allow you to control it.
If you hook up the timing light with the vac advance hooked up you will see high numbers(In the mid to upper 40 degree range) at 2500-3000 rpm-The engine doesn't detonate at this high advance because it is under partial(light) load.

 

Looks like you are at 32 now-just advance a little at a time until it knocks then back it off-That will give you your optimum mechanical advance for the track.If you are using the same gas on the street you could continue on to tune the vac advance curve.

 

Thanks for the tips. Some of my friends agree with you that full manifold vac advance should be used. But I switched mine over from port to manifold vac and the engine wouldn't idle right. So I switched it back and it runs fine. Two articals I read in car mags suggested that using ported vac was preferable. I don't know. I only have my own (limited) experience to go on.My buddy's 350 runs fine with the vac advance attached to the manifold vac.

 

There are 2 types of vacuum advance one is for ported. That means as the motor increases in speed it starts to draw vacuum thus advancing the timing. The other one is Manifold and this is when the motor is running normal the dist is advanced to give better mileage around town. When you step on it the advance goes away as the RPM increases so you don't have detonation.
My self I run full mechanical that way I can adjust it when I need to. I run 20 initial with 12 in the dizzy for 32 total. If we drag race I bump it up to 36.

 

Yep I know what you mean-sometimes the camshaft profile can give a somewhat erraatic vacuum signal at idle which can play hell with your idle characteristics. The adjustable vac cans usually correct this.Some factory non-adjustable vac cans need a higher vac signal before they work-others start working at a lower signal and advance to a much lower amount-like 6 degrees of a vac advance.If ported works better for your car-that's where you should run it. When an engine that has been set up with ported advance is switched to full manifold-the idle will often have to be re-set 'cause it will increase to a higher rpm than you want.
Now go get that new track ET record!

 

Probably get some arguing over this but I'll throw it out there anyway. If it starts easy with the 18 initial, and doesn't ping when you tip into the throttle, the 18 wont hurt a thing. You don't need the vacuum advance at all. Just leave it unhooked and to prevent the plate from moving you can get a 5 dollar lock out and just pull the can off altogether. Probably don't need the lock out, they don't usually float around anyway. The lock out makes it look a little prettier though. If you've got the money buy a fully mechanical MSD pro billet ready to run and forget about the HEI. Not saying the HEI aint a good part. The pro billet is just the best street distributor. I'm not going to get into a long argument about "theory" with everyone on here, about why you need a vacuum advance. I'll just tell you how I've tuned a ton of small blocks on my very own engine dyno. And by the way, I hate to sound arrogant, I'm really a very humble and open minded person. I used to keep my mouth shut untill I bought the dyno and can now without doubt say I know the outcome of my tuning efforts. So I'm much more free with my opinions. 34 to 36 total is always the magic number for the average iron headed small chevy. More might feel snappier on the street when you nail it but you're not making more power throughout the curve and I promise the car doesn't pull harder through the midrange. If you've got old school, open chamber small block heads set the total timing to 36. Eliminate the vacuum advance and let the initial fall where it may. As long as it starts ok, all is well. If it doesn't you may have to increase the mechanical advance in order to let it start with a little less initial. Or just buy a high torque starter and forget about it. It'll start with any amount of initial then. We've built race car engines with no advance at all. Just 36 degrees all the time. They start fine. Have it all in by 2500 in your case by the way. 2 degrees too much timing can cost as much as 15hp. 2 degree too little can be the same. You can't always hear detonation. Learn to read plugs and you can see it. Good luck. PM me with any questions and I'll do my best to help.

 

Alot of good advice already given I'd check to see how fast your mechanical is coming in at many times you can gain some performance by having the mechanical advance come in sooner than what the factory springs do (use softer springs). Usually 32-36 degrees works well on mild sbc's sounds like you are in the ballpark does it crank over good when hot at 18 initial?

 

Thanks for all the good advice and information, guys. Right now it seems good with 14 initial and 18 from the distributor, and I'm tired of messing with it, so I think I'll drive it around like this for a while and in a few weeks when I take it back to the track, I'll advance it in 2 degree increments and see how that impacts my e.t.
p.s.(before I switched distributors, I had the initial timing set at 18 for a couple of days and I was surprised to find that it started up fine and didn't ping, using 93 octain Sunoco)

 

in my experience if it aint broke dont fix it (it sounds like it is already working ok)

 

Changing the valve timming doesn't change the ignition timming. Sorry to put mud in your bucket.

Yes the dizzy is driven by the cam shaft but you are still setting it by the marks on the crank. Your ignition timming is set in relation to where the piston is at in the compression stroke and doesn't know what the valves are doing at the time.

The GM HEI is a pollution control dizzy, it is not designed for max performance. You can buy an an advance kit for it that will have different springs and weights. That will make the mechanical advance quicker if you want it to be. You can also get an adjustable vacum advance canister to also set how quick it advances but you cannot get any more advance from the dizzy than it makes. They do make hot rod pieces to go in your HEI but by the time you do that you could own a trick dizzy.


18 degrees initial is a little extreme. You should be around 12 on a stock motor even on a hot street motor I would go much higher than 14-16.

You run it the way you want, I'm just giving you what I would do. If you want performance ignition you will need to do something different.

They do make hot rod pieces to go in your HEI but by the time you do that you could own a trick dizzy.

 

You are wrong. Max of 38 degrees adavnce. 52-53 is way to high. As for aftermarket cams having advance built into them, again it looks like you do not understand the operation of an internal combustion engine.

SBC's normally run 32-34 degrees total Mechanical advance. Should be "all-in" by about 2,500-3,000 RPM's. Disconnect and plug the vac. advance line, hook up a timing light and read the timing while you or a friend opens the throttle a little. Check to see what your total timing is when it's fully advanced. Adjust to the aforementioned 32-34 degrees. Also record the RPM's at which it stopped advancing. If it's way higher than the 2,500-3,000 start trying lighter advance springs to get it down in that range. You will probably end up with 8-10 degrees at idle, but the important is not the idle timing...it's the fully advanced total as well as the "advance curve"...ie: the rate that it will advance as RPM's increase.

 

WOW good post with some variation in answers !!
This is a function of just plain old "knowing where we are at" and i see this every day doing distributors.

As per the posts above there are quite a few answers to the orginal question , and more than one way to get where we need to be.

Method one : On any engine we MUST know where we are at in regards to timing and timing advance etc. There are a couple ways to determine this.
One way (racers/sprint cars etc) would be to just lock out the mechanical advance , take engine to 3000-3500 rpm and set the timing to 36-38 degrees and lock er down. Most sprint cars , dragsters are tuned this way.
This would require knowing exactly where tdc is on the engine and use a degreed balancer or advance timing light. Most of these cars are set at 36-38 degrees maximum timing .

Method two: Use and tune the factory automatic mechanical advance to allow lower advance rates at low rpm and higher rates at higher rpm.
There are two issues here, one is the factory limit set with the advance units and the other is the rate of advance adjusted by springs and or spring tension. Using gm as a example ( ford and chrysler will vary) the factory limits the maximum advance usually by a slot in the cam assembly to stop the advance or allow additional advance per engine / gear ratio etc per car.
Pictures below show different cams for different advance i am presently putting together on the bench. Note they are numbered as to the number of degrees each offers etc. The two cams are for a contact controlled distributor and the simgle pic is for the GM HEI distributor.

Lets discuss the two cams from the delco distributors. One has a number 524 and the other is a 738. This would be 24 degrees engine ( 12 degrees distributor) and 38 degrees engine (19 degrees distributor) and would offer two entirely different advance curves per engine.
The 524 engine spec card shows a intial timing setting (balancer) of 12 btdc and the 738 spec card shows a 0 degree ( tdc) setting . Both engines show a 36-38 total advance at xxxx rpm. These intial settings and advance are figured at the factory for proper starting and good driveability.
36-38 total advance has been proven to be the number for the small block chevrolets....

Now as far as WHEN this advance happens most street driven vehicles seem to like all in at 2500 to 3000 rpm.

Vacuum is another story in the chapter of spark advance!!!! For this discussion lets not even go there ....

Hope this helps..........

 

AMEN !!!

 

This guy sounds pretty bright. Think I might like to attend his distributor class. Check out his website.

 

You obviuosly have never worked on a sbc!

 

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,52-53 degrees was the number of your timing including the vacuum cannister,since he's looking for improved et I would leave vacuum disconnected and run distributor advance tocome in earlier say around 1000-1200rpm that will give you wot performance.I've used thi combination on BBC and sbc throughout the years and wouldn't do it any other way.And to say that camshaft profile won't change timing is just an ignorant statement no offense,high lift camshafts affect vacuum signal and idle especially when driving around town and part throttle when vacuum signal is connected.It's a trade off performance vs. Economy you'll have to determine which one you want to go with.

 

This guy is beyond pretty bright; he spent years doing training for Allen Electric and has logged more time on a distributor machine than most of us have behind a windshield......take one of his classes and you'll quickly learn how much B.S. is floating around ......fats

 

You must be talking about gmcbubba,I'm glad he interjected his opinion in the conversation he is well respected here he's the man when it comes to engines!!!

 

Then there are guys like me that run 36 degrees locked down on a mild blown SBC. It loves it starts good and no pinging but that kind of timing is not uncommon for a blower motor.

 

A friend just sent me this link, which I found to be enlightening. Maybe it's not the definitive answer, and some may disagree with some of it but some may find it helpful. I don't want to start a shit storm about it. Thanks to all for your input.

http://www.gofastforless.com/ignition/advance.htm

 

Vacuum advance can be your friend on street driven or heavier vehicles. I too have stared down the barrel of a timing light for forty years.
Initial with "manifold" vacuum advance should not exceed 25 degrees. This enables a smoother idle and the use of power brakes. The stroke of the vacuum plunger is controlled with a bushing on the rod or a snail washer that comes in some of the advance kits. That is different from setting the spring pressure inside the can.
Using vacuum advance gives better mileage and cooler running.
Yes, a total of 45 to 52 degrees is not uncommon on most factory distributors.
Learn to read spark plugs. Have someone show you what to look for.
Aluminum specs on the porcelain is too much advance. Throw away the plugs and start again.
Or, it could be too lean a mixture.
But, that is another topic.

 

Hey Pirate,
It looks like a ride to Macungie this weekend is just what you need for your tune.
Mid 80* for temp. and long,mild inclines...
Smokey

 

Total timing degrees, without vac advance hooked up, should be in the mid 30's, depending on what your engine combo likes. Don't even think of using anything in the 50's !

I almost always set the timing around 36 degrees (depending on how the engine is built and the fuel I am using at the time) with the engine revving about 2,700 rpm, then find the sweet spot by acceleration runs. You almost certainly will not have to adjust it by more than just a few degrees.

What I do to get the "lower rpm" advance just right is to buy a Mr Gasket HEI Curve Kit. I believe that most parts stores have a version of that hanging on their pegboard rack near the aftermarket coils, plug wire looms, etc. So far I haven't found any stock weights that allowed enough advance weight movement for what I like. The aftermarket kits I have worked with solve that.

I usually use the lightest springs in the kit, or one light spring and one medium spring, and lightly grind the spot where the weights contact the center "bar" in the curve kit (where the weights pivot against the center piece) so that I get the timing to fall back a little bit farther back at idle speeds than the unmodified kit would. Not much, just a bit. With a little grinding I get a little bit more weight-travel and a slightly earlier advance movement.

I do that mostly for easy starting and reasonably good idle.

That way I have less spark timing at cranking speed, and a less heavy load on the starter during fire-up.

Sometimes I experiment with the different springs in the kit, but that is the setup I almost always return to.

Ported vacuum. I have never seen a "manifold vacuum" setup that worked the way I thought it should. (extra advance at idle, that goes away immediately when you open the throttle? that sounds like a makeshift patch over a different problem to me)

That's what works for me.

 

I just read what you wrote.....BUT a Total timing of 36-38 is very good for most all naturally aspirated small black chevies.

^^^^^<TOO p and performance poor in result timing.....will total of amount high detonation<>
Too much timing

Total timing equals===initial, mechanical advance and vacuum advance.

This may work if he was drag/street racing but fuel mileage will suffer.

I am glad you method has worked for you, but the thing is...the grind of the camshaft will not change "timing" read at the crankshaft. Neither will the lift of the valve.Early opening, Overlap and late valve closing Will effect idle vacuum.

True