Timing question, just checking my numbers

[deja]

My initial timing no vacuum is +8.

Initial timing with vacuum hooked up it stays at +8.

Revved to full advance with no vacuum is at +22.

Revved to full advance with vacuum is +35.

 

The engine is a old Gen1 350 with an unknown cam, but not stock and HEI distributor, I don't know if its had a performance weight kit installed.

 

Do these numbers look ok? The engine seems to run fine, just curious if its timed properly.

[Lason]

Could be aftermarket weights. Usually just a genral figure I shoot for is 10 degrees at idle and about 32 degrees at full advance. Just a general figure though.

[LeeMS]

Deja, sounds like you're close for timing. Now you can tune by ear, that is with the eng. up to operating temp. go out and lug the engine, high gear up a hill. Advance the timing until you hear eng. ping, a marbles in a tin can rattle then back the timing off 2 degrees or so. Looks like you're running a single-plane intake manifold,this the carb the cam and exhaust all factor in timing. After market advace weights have a cad plating shine, stock have a darker more finished look. Hope this helps you get the most out your engine. Lee

[jakeshoe]

My initial timing no vacuum is +8.

Initial timing with vacuum hooked up it stays at +8.

Revved to full advance with no vacuum is at +22.

Revved to full advance with vacuum is +35.

 

The engine is a old Gen1 350 with an unknown cam' date=' but not stock and HEI distributor, I don't know if its had a performance weight kit installed.

 

Do these numbers look ok? The engine seems to run fine, just curious if its timed properly.[/quote']

I would look for more total timing (full advance/no vaccum). 32 is usually a minimum, most conventional non fast-burn style chambers will like it around 36*.

You have 22* now, move this to say 34*, this will move your initial timing to 20* which is fine and will actually improve idle characteristics and of idle response.

Add the vacuum in, as long as it doesn't create any sprak knock it is fine, if it does you can use a Crane adjustable vaccum advance kit to limit the amount of vac advance.

 

Also,

Just a note, your distributor has less than normal amount of mechanical advance built into it, so be sure all the weights are not sticking, bushings are OK, etc.. An HEI will typically have at lest 20* of mech advance built in, so a 8* initial will result in 28* or more of total timing.

If yours is working properly this is good, as usually enthusiasts have to use manifold vacuum instead of ported to get a decent idle but not over-advance the timing at WOT and cruise.

[deja]

Deja, sounds like you're close for timing. Now you can tune by ear, that is with the eng. up to operating temp. go out and lug the engine, high gear up a hill. Advance the timing until you hear eng. ping, a marbles in a tin can rattle then back the timing off 2 degrees or so. Looks like you're running a single-plane intake manifold,this the carb the cam and exhaust all factor in timing. After market advace weights have a cad plating shine, stock have a darker more finished look. Hope this helps you get the most out your engine. Lee

 

I will try pushing it until it pings, assuming I can hear that over the loud exhaust, LOL. I certainly havn't heard any so far. This engine won't be in the car much longer, LT1 swap in pending, just trying to see what this one's capable of.

[deja]

If yours is working properly this is good' date=' as usually enthusiasts have to use manifold vacuum instead of ported to get a decent idle but not over-advance the timing at WOT and cruise.[/quote']

 

Thanks. Its been a long time since I messed with this stuff. When I got the car initial was set at 4, I bumped it to 8 but didn't want to go too far with today's crap octane levels and not knowing a lot about this engine's specs. Back in the day (late 60's - early 70's) we didn't ran vacuum at all, just pushed the initial and relied on after market weights to make it to full. But we also had Sunoco 260 for octane!

I'm a little confused about your last statement. Right now the distributor is hooked to the ported vacuum. Are you saying I should try hooking it to manifold instead?

[jakeshoe]

Probably not in your case,

You have a relatively small amount of mechanical advance, this usually works better for a performance oriented engine.

OEMs went to very low initial timing to reduce hydrocarbon emissions at idle (retarded timing creates hotter exhaust temps, thereby burning the charge further as it exits the exhaust and contacts somewhat more oxygen rich air), this makes an engine not idle as well as it could.

What you will see on many combos is that a guy is running a fairly decent combo, it could be anything but we'll say it's a 355 SBC, with a properly matched aftermarket intake, heads, cam of around 230* of duration at .050, 9.5-10-1 compression, headers, decent exhaust, etc.

This combo will not idle very well at all at 4* or even 8* of intial timing, it can be MADE to idle decently by cranking the idle speed screw to open the throttle plates. This causes the idle transition slots in the carb to be over-exposed and creates an off idle stumble.

The real solution is to advance the timing.

You set the idle timing on a performance combo (emissions not a consideration) for max idle speed and vacuum. The first time you do this you will realize that most of these combos will idle best (and have best throttle response) at WELL over 20* of initial. I have seen some combos that would idle great at over 50* of intial...

So the above combo we'll say really seemed to idle best at 26* of initial. We'll assume it has a decent chamber design and flattops, so would need a maximum of 36* of total timing.

This means that you would want a distributor that only has 10* of mechanical advance.

If you were using a stock HEI (even with aftermarket weights) it would usually create 20+* of mech advance. We'll use 20* for this example.

With 26* of intial, adding in 20* of advance you now have 46* total timing. Too much for WOT, it is almost guaranteed to cause detonation.

So you can either limit the mechanical advance (very frustrating for most people without a distributor machine), install an aftermarket distributor, or run with overly slow initial timing and the poor idle, lack of vacuum, and driveability problems it creates.

OR you can cheat...

You can use the vacuum advance to create "initial timing" greater than what the distributor will allow.

We know that at WOT we lose almost all vacuum, so the vacuum advance is no longer in play at WOT.

If you hook the vacuum advance to manifold vacuum, it will pull in quite a bit (sometimes too much) timing at idle. Vacuum advance usually adds an additional ~20* of timing.

So if you have the initial at 8* and add that 20*, then now you are very near your best idle timing of 26* on the combo I am using.

 

Let me tell you how I set up a distributor on a fresh combo to simplify it.

This is using a stock type points or HEI and maintaining a vacuum advance function.

Get out the dialback timing light, get the motor running and a "rough tune" for cam break-in if necessary, etc.

Get all the idle mixture screws set to attain a good idle. Just tune it well enough to run and idle as smooth as possible with no vacuum hooked up.

Get engine idling, set the timing for maximum idle speed, re-adjust idle speed as necessary to compensate for added timing. Your aim should be the least amount of idle speed screw.

Now if it seems to idle best at 32*, I back it off about 2*, so record 30* as being the best idle.

Now rev the motor to bring in full mechanical advance. Record the difference. We have the distributor set at 30*, at 3000+ rpm it achieves max advance at 54*, so we have 24* of mechanical advance.

Most typical SBC combos will like 34-38* of total, the Vortec headed or similar combos will often not like anything over 32*.

We'll set this combo for 36* total as an initial setting.

So obviously we have way too much mechanical.

We now hook up the vacuum, record how much vacuum advance we have.

If we had 30* initial and plug in the vacuum and it jumps to 50* we have 20*, you may want to rev it up a bit and watch for any additional advance with vacuum increase above idle.

So the combo as it stands has 24* of mechanical advance, 20* of vacuum advance.

With the vacuum plugged in we can back the distributor back down 20* to the 30* idle setting. It should be idling at 30* with the vacuum connected to manifold vacuum. This gives us an actual initial timing of 6*, idle timing of the ideal 30*, and total timing of 30* at WOT (which leaves us a bit slow).

What you will need to do is use a Crane adjustable vacuum advance to limit the vacuum advance. We could pull 6* of vacuum advance out leaving only 14*, this will allow us to pull the actual initial up to 12*, leaving the total at 36* and the idle at 30*.

This also keeps us from having over-advanced timing at cruise.

 

Its really not as complicated as it sounds... Its a pretty easy process once you do it once. The Crane kit is about 20-25 bucks.

It workes pretty well, only potential drawback is that in some cases if vacuum drops off alot on gear change with an auto trans, you will get a stumble or engine will die.

Usually combos that this is an issue will work very well with locked out timing.

Hopefully this will help you.

[deja]

That is quite a lot of processs on one read. I will give this a try and see what happens. Sure was a lot easier when I had the tuner dude with the Sun distriutor machine, LOL. As I rememeber my '70 AMX was set at 10* initial, NO vacuum used at all, not even connected and total of 36* at 2700 RPM. Like you said tough to attempt without an distibutor machine to set the weights.

I've never heard of the Crane thing http://store.summitracing.com/partdetail.asp?part=CRN%2D99600%2D1&N=700+115&autoview=sku, this looks like the same thing by Accel http://store.summitracing.com/partdetail.asp?autofilter=1&part=ACC%2D31035&N=700+115&autoview=sku.

 

Thanks for taking the time with this.

[jakeshoe]

Ya,

it is a lot of numb ers and steps mixed up.

 

Let me try to summarize it.

 

Determine ideal idle timing,

determine amount of built in distributor mechanical advance,

determine vacuum advance amount,

adjust distributor setting for best total timing without vacuum advance,

then adjust vacuum advance for ideal idle timing setting.

[deja]

Ya' date='

it is a lot of numb ers and steps mixed up.

 

Let me try to summarize it.

 

Determine ideal idle timing,

determine amount of built in distributor mechanical advance,

determine vacuum advance amount,

adjust distributor setting for best total timing without vacuum advance,

then adjust vacuum advance for ideal idle timing setting.[/quote']

 

That does simplify things. Found adjusters at Summit, see my post above.

[jnjdragracing]

Deja,

 

On my Monte Carlo, which is mainly a street car. I run a msd dist with locked out advance, you can take the hei dist. and weld it up so you have no advance. Set your timing to what is best I would start at 36 and work up to 40.

 

You will have to have a seperate starter button with a seperate ingnition. Turn the engine over and then throw the ingnition to it.

 

Of course you will have to listen for spark rattle depending on how much compression you are running verses octane level.

 

To make sur the dist advance is locked with a timing light check your timing and turn the roms up and make sure it does not advance on you, if so then the advance is not locked.

 

It should run strong form idle to high rpm.

 

John

[deja]

LOL, little over the top for what I'm trying to do. Sounds awesome but this engine is only a temp in my Z so I'm not putting a lot of money or major effort in this engine. Just seeing what its capable of with the correct setup. LT1 is in the plan, hope to start the conversion in a couple months.

[jakeshoe]

Deja,

Looks like you're on track. Yes the Accel is probably the same part even... either should work, I have used and am familiar with the Crane so that is what I use as an example.