SS454 timing questions

[Grapejuice1998]

I have just completed a rebuild of my hei distributor and have a couple of questions.

 

First, base/initial timing. Where do I need to be on that?

 

Next, it has 3 different sets of springs, so it will advance in 3 different curves. (see: attachment)

 

Last, it also has an adjustable vacuum advance. The instructions say to turn it all the way clockwise to start with. How do I go about the fine adjustment?

 

Thanks!

 

 

[Grapejuice1998]

*crickets*

[Rock]

I have a small block 400 so I can't help with the timing setting but as far as the spring in the distributor I tried all 3 and picked the one that ran the smoothest and mine turned out to be the medium spring. I think my timing was 11 Degrees BTDC when all was said and done. Some times it's just trial and error.

[MCfan]

Alan, you might get quicker/better answers if you provided more information with your questions.  

 

Which engine do you have (assume 454 from your avatar)?  Is it stock or modified with cam/intake/carb enhancements (important for manifold vacuum levels)?  If stock, you should have base timing info on a sticker in your engine compartment.  What vintage of HEI (assume it pre-'75 if it has manual vacuum advance)?  What type of driving will you be doing?  Is performance or economy more important?

 

When I rebuilt and installed a pre-'75 HEI on my modified 402, I messed with an adjustable vacuum advance unit for awhile.  Basically, you aren't changing the total range of advance because that is fixed and the degrees (i.e. 20) should be stamped on the advance linkage arm in plain sight. You can limit the range a bit with properly sized bushings on the "L" of the actuator arm, but that is another subject.  

 

If your vacuum advance unit is adjustable, you can only adjust WHEN the vacuum unit begins introducing that amount of advance into and out of your timing circuit by adjusting the level of vacuum required to operate the advance unit.  It would be most helpful to put a vacuum gauge on your intake manifold or carb port (where ever the vacuum advance hose normally connects) and see how many inches of vacuum your engine pulls at idle and partial (i.e. cruise) throttle (should be zero at WOT, of course).

 

IMO, there is much more art than science involved in "curving" a distributor and setting up your overall engine timing so be prepared for a LOT of trial and error if you really intend to optimize engine performance.  If you are going to use your vacuum advance (it's optional in some cases), you need to disconnect it and securely plug/cap the vacuum line or port.  You need to look up base timing specs for your engine but it will usually be between 4 and 8 degrees BTDC.  Your big block will idle terrible at this setting until you connect your vacuum advance unit again.  Be sure to remember that each timing mark on your stock degree tab is 2 degrees or you will set your base timing too advanced.  Check your timing at idle with the vacuum connected to see how many degrees the unit is adding to your base timing at that vacuum level.  It really helps to have a digital read-out timing light so you can always bring the timing mark on the crankshaft back to align exactly with the "O" mark on you timing tab and accurately read the degrees of advance on your timing light display panel.

 

Basically, your vacuum advance should be operating at idle and during steady throttle cruise conditions.  During acceleration, your engine vacuum drops and your mechanical advance (controlled by the springs and weights) take over.  So you can basically tune them independently for those two conditions.  The adjustable vacuum advance theoretically addresses the TRANSITION of timing control between the vacuum and mechanical advance modes.  By adjusting your vacuum advance unit (mine used an allen wrench up through the vacuum port) through its range you are changing it's reaction to the level of vacuum your engine is creating at that time.  One end of the range will kick it in or drop it out at a somewhat lower vacuum level than the other end of the range (I don't know which end of the range your instructions are giving you initially).  

 

Any change in transition from vacuum to mechanical advance will be far more applicable to normal driving around town so that is where you will want to test the results of any adjustment you make.  You want your total advance (base+vacuum+mechanical) to stay in the upper part of its 34-36 degree total range for best performance, operating temperature and economy but it must not be too high or you will get detonation (pinging) on acceleration/load.  The quickest way eliminate detonation is to back your base timing down a couple degrees at a time but you may also need to change to heavier springs so your mechanical advance does not come in too quickly under load.

 

"Curving" a distributor is an old school art and I looked far a wide before I found someone who could do it for my new HEI.  My friend is a NHRA Pro Stock driver and speed shop owner from Sweden.  He worked with the adjustable vacuum advance for quite awhile and concluded that completely eliminating it was best for my modified engine and style of 4-speed driving.  He went with 14-16 degrees base timing with 20 degrees added by the mechanical all in by 2800 rpm.  He used a combination of springs to get a smooth advance curve.  Note that you do not have to use the springs in matched pairs so you can experiment with a medium and a light spring or a medium and a heavy spring, etc.  The other variable you can change is the mechanical advance weights - there are many to choose from - different profiles and weights.  He didn't change mine but I had just put new bushings on the weight studs and you surely did also in your rebuild.

 

Not sure any of this will be helpful to you.  For many of us, I suspect engine tuning is one of the best places to seek professional help.  Good luck.

[Grapejuice1998]

Alan, you might get quicker/better answers if you provided more information with your questions.  

 

Which engine do you have (assume 454 from your avatar)?  Is it stock or modified with cam/intake/carb enhancements (important for manifold vacuum levels)?  If stock, you should have base timing info on a sticker in your engine compartment.  What vintage of HEI (assume it pre-'75 if it has manual vacuum advance)?  What type of driving will you be doing?  Is performance or economy more important?

 

When I rebuilt and installed a pre-'75 HEI on my modified 402, I messed with an adjustable vacuum advance unit for awhile.  Basically, you aren't changing the total range of advance because that is fixed and the degrees (i.e. 20) should be stamped on the advance linkage arm in plain sight. You can limit the range a bit with properly sized bushings on the "L" of the actuator arm, but that is another subject.  

 

If your vacuum advance unit is adjustable, you can only adjust WHEN the vacuum unit begins introducing that amount of advance into and out of your timing circuit by adjusting the level of vacuum required to operate the advance unit.  It would be most helpful to put a vacuum gauge on your intake manifold or carb port (where ever the vacuum advance hose normally connects) and see how many inches of vacuum your engine pulls at idle and partial (i.e. cruise) throttle (should be zero at WOT, of course).

 

IMO, there is much more art than science involved in "curving" a distributor and setting up your overall engine timing so be prepared for a LOT of trial and error if you really intend to optimize engine performance.  If you are going to use your vacuum advance (it's optional in some cases), you need to disconnect it and securely plug/cap the vacuum line or port.  You need to look up base timing specs for your engine but it will usually be between 4 and 8 degrees BTDC.  Your big block will idle terrible at this setting until you connect your vacuum advance unit again.  Be sure to remember that each timing mark on your stock degree tab is 2 degrees or you will set your base timing too advanced.  Check your timing at idle with the vacuum connected to see how many degrees the unit is adding to your base timing at that vacuum level.  It really helps to have a digital read-out timing light so you can always bring the timing mark on the crankshaft back to align exactly with the "O" mark on you timing tab and accurately read the degrees of advance on your timing light display panel.

 

Basically, your vacuum advance should be operating at idle and during steady throttle cruise conditions.  During acceleration, your engine vacuum drops and your mechanical advance (controlled by the springs and weights) take over.  So you can basically tune them independently for those two conditions.  The adjustable vacuum advance theoretically addresses the TRANSITION of timing control between the vacuum and mechanical advance modes.  By adjusting your vacuum advance unit (mine used an allen wrench up through the vacuum port) through its range you are changing it's reaction to the level of vacuum your engine is creating at that time.  One end of the range will kick it in or drop it out at a somewhat lower vacuum level than the other end of the range (I don't know which end of the range your instructions are giving you initially).  

 

Any change in transition from vacuum to mechanical advance will be far more applicable to normal driving around town so that is where you will want to test the results of any adjustment you make.  You want your total advance (base+vacuum+mechanical) to stay in the upper part of its 34-36 degree total range for best performance, operating temperature and economy but it must not be too high or you will get detonation (pinging) on acceleration/load.  The quickest way eliminate detonation is to back your base timing down a couple degrees at a time but you may also need to change to heavier springs so your mechanical advance does not come in too quickly under load.

 

"Curving" a distributor is an old school art and I looked far a wide before I found someone who could do it for my new HEI.  My friend is a NHRA Pro Stock driver and speed shop owner from Sweden.  He worked with the adjustable vacuum advance for quite awhile and concluded that completely eliminating it was best for my modified engine and style of 4-speed driving.  He went with 14-16 degrees base timing with 20 degrees added by the mechanical all in by 2800 rpm.  He used a combination of springs to get a smooth advance curve.  Note that you do not have to use the springs in matched pairs so you can experiment with a medium and a light spring or a medium and a heavy spring, etc.  The other variable you can change is the mechanical advance weights - there are many to choose from - different profiles and weights.  He didn't change mine but I had just put new bushings on the weight studs and you surely did also in your rebuild.

 

Not sure any of this will be helpful to you.  For many of us, I suspect engine tuning is one of the best places to seek professional help.  Good luck.

 

While I knew quite a bit of it, some of it I didn't, so all of it was helpful and appreciated.

 

I kinda thought the title stated the motor size, but it's mostly stock. It has Hooker headers and an Edelbrock intake (still has a QJ though), but as far as I know, it's otherwise stock.

 

I daily drive it for the most part (N. Texas) and I do like to get on it from time to time. The car was really running badly when I got it earlier this year, so I set about fixing things as I found them. I went to put a timing light on it recently, an found it had NO timing marker, or scale. It had the required hash mark on the balancer, but nothing to reference the timing off of. Nothing. Can you beat that?! Anyway, I marked the distributor base with a sharpie and advanced it a bit and it ran a whole lot better, which led me to a complete distributor rebuild and the installation of a proper timing pointer.

All of that is done now, so I thought I'd get some timing advice from people who have already been there and done that. I could have looked up the base stats online, but all that would have given me was a starting point. I was hoping for (and got) some base and total timing numbers to try now.

The adjustable advance instructions said to go all the way clockwise to start with, so that's where it is now.

The advance curve tables and associated springs, are each very different in how much total timing comes in and how quickly. Take a look at that attachment, please.

 

Thank you so much for taking the time to explain in such detail. I truly appreciate the effort.

[Grapejuice1998]

Sorry, there was an attachment at one time. Here is the graph of the 3 sets of curve springs.

 

[MCfan]

Alan, thanks for the additional information and for sharing the spring curve plots.  It's a shame they didn't use a consistent vertical axis scale and plot all three curves on the same graph.  I consolidated their data into a single graph using PowerPoint and it gives a much clearer visual comparison of the performance of the three spring pairs (see photo below). I also don't know why they chose a different ending rpm value for each of their graphs so I just ended them all at 6000 which is probably more than your engine will see anyway.

 

 

As you can see, the pair of Lt Silver springs begins adding mechanical advance early and quickly at lower rpms and is almost all in by 3000 rpm.  That may be too much, too fast for your type of driving and also using the vacuum advance (which will be adding several degrees of advance to your base timing at idle).  If you go with an aggressive mechanical advance curve like that you will probably want to adjust your vacuum advance unit to drop off sooner as the mechanical advance comes on because your engine is seeing total advance (base + vacuum + mechanical) at any and all rpms.

 

My big blocks like around 34-36 degrees of total advance but you have to consider your driving style and your normal acceleration rpm range from idle (when manifold vacuum drops off).  I know my 402 4-speed without vacuum advance needs and has a more aggressive mechanical advance curve (lighter springs) because it is only adding to the base advance of 14-16 degrees.  I haven't specifically checked it, but I feel certain the normal acceleration rpm range for my SS454 is higher than for my 402 4 speed primarily due to the automatic transmission.  Compression ratios, rear gear ratios and tire sizes are also very different in both.

 

Your challenge is to find the right combination of vacuum advance and mechanical advance curves to keep your big block happy with your particular drive train and type of driving.  Just looking at the three curves, I'm guessing the HD Silver springs would make your engine feel sluggish, especially under heavy acceleration.  I suggest starting with the pair of Blue springs and then compare them to one Blue and one Lt Silver spring which should result in a curve between the Blue and Lt Silver pairs.  That would still give you zero mechanical advance at idle (all vacuum advance) and yet build rather nicely as you accelerate. I can't imagine that you would need a less aggressive curve than the Blue pair, but you could always try one Blue and one HD Silver spring to compare the results.  

 

Basically, set your mechanical curve (by spring/weight selection) to give you enough total advance during acceleration/load and then set your vacuum advance unit to "fill in" during idle (no load) and cruising (low load) when manifold vacuum is higher. There are so many factors to consider that it comes down to a lot of trial and error to find the best combination.  Having good data like idle speed, manifold vacuum, normal acceleration range, compression ratio, gear ratios, etc. can help get you in the ball park but it will still come down to how responsive and healthy your engine feels under typical driving conditions in your particular vehicle.  

 

There is always a "sweet spot" between sluggishness (too little total advance) and detonation (too much total advance), you just have to find it.  Good luck!