What is the correct setting for timming on my '75. The tag in my engine compartment is worn right where the timming setting should be. Any help would be appreciated.
:flag

 

Stock is about 8 BTDC which is fine for a smogged out GM emissions motor.

For better power and response, set the curve so all in by 2800 rpm or so. 36 BTDC when all in, idle at whatever (mine is at 14 but I never drive at idle anyway :D ). Adjustable vacuum can for another 14 degrees at cruise.

Here are some docs that Lars wrote up. Worked great for me.
http://www.bokonon.net/corvettefaq/c3/HEIPrimer.doc http://www.bokonon.net/corvettefaq/c...oSetTiming.doc

 

SteveG75;

I dowloaded the info you supplied but could not open them. I have a Performer intake and headers, those are the only mods. The timing is currently set at 6degrees with the idel at 1300rpm with vacumme blocked off. When I connect the vacumme my rpm goes up 2000rpm. I think thats a bit high because when I'm ideling the car is doing about 20mph.
:confused:

 

SteveG75;

I downloaded the info you supplied but could not open them. I have a Performer intake and headers, those are the only mods. The timing is currently set at 6degrees with the idel at 1300rpm with vacumme blocked off. When I connect the vacumme my rpm goes up 2000rpm. I think thats a bit high because when I'm ideling the car is doing about 20mph.
:confused:

 

Ok, here is my initial ideas without seeing the car. At 1300 rpm, you are already getting advance. Need to bring idle down to 800 (mine is a manual) when car is warmed up.
Disconnect vaccum line to set timing obviously. Make sure it is plugged so you are not fighting a vacuum leak.
I used the springs only from an aftermarket set to get all my advance in by 2800 rpm. Then set advance at 36 BTDC at 300 rpm using a dial back light.
To set the vacuum adavnce, I used an adjustable can that I set for 16 degrees of advance (checking before and after) making sure it is hooked to a ported source (i.e. zero at idle and vacuum available at cruise). Lars pointed out to me that GM connected it to a constant source. Good source is top front passenger side of carb.

Let me try to post Lar's info.

 

How to Set Your Timing for Peak Performance

by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. A 2-7-01


This tech paper will discuss setting the timing on a Chevy V8. This procedure also applies to other GM V8s.

The procedure outlined here differs from the Service Manual, and is based on my years of experience doing this work in the quickest, least painful, most economical way while keeping the level of quality high. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use.

How to Set the Timing
When you think about it, setting the timing at idle speed makes no sense at all: You don’t operate your car at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple way for most people to set the timing, and is not a good procedure for optimum performance.
Small block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance disconnected) is all in by 2,500 – 2,800 rpm and is set to 36 – 38 degrees. If you have an adjustable timing light, this is very easy to check. If you don’t, you need to scribe a 36-degree mark on your harmonic balancer. Here’s how:
Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is the distance to 36 degrees. Measure this distance CLOCKWISE from your existing harmonic balancer timing mark and place a clear mark on the balancer.
Remove your distributor cap and rotor. Remove the 2 centrifugal advance springs. Install the rotor and the cap (without the springs). Disconnect the vacuum advance.
Start the engine. It may kick back a little due to the advance coming in immediately without the springs. If you’re using an adjustable timing light, set the light to 36 degrees advanced. Now rev the engine just a little while observing the timing marks with the light. It shouldn’t take much rpm to peg out the advance without the springs installed. With an adjustable light set at 36 degrees, align the stock timing marks with “0” when the timing is “pegged out.” With the non-adjustable light, align your new 36-degree mark with “0.” Rev the engine a little to make sure the timing will not advance any further. Shut it down.
Pop the cap and rotor and re-install the springs. Put everything back together, but leave the vacuum disconnected. Start it up. For future reference, make a note of the timing setting at idle. This is your new curb idle timing spec. Now give the engine a few quick rev’s past 3,000 rpm and verify that the full timing (36 degrees) is coming in. If it’s not, you need to change to a softer set of springs until you get full 36-degree advance before 3000 rpm. (NOTE: A stock set of springs will usually not allow full centrifugal advance to come in before redline rpm. If you have stock springs installed, don’t rev the engine beyond its limits to try to force full advance in.)
Shut it down and hook up the vacuum. Now do a road test.
The 36-degree 2500 rpm advance curve is optimum for performance, but may require premium fuel. Lug the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”). If you’re getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until engine knock stops. Engine knock will seriously damage engine components if not corrected. If you get no knock, you may see slightly improved performance at 38 degrees total timing. This is particularly true if you’re running at high altitude.
If you have no engine knock under acceleration, but the car “chugs” or “jerks” at cruising speed (light throttle application), you are getting too much vacuum advance on top of the mechanical advance. You may need to change out the vacuum advance diaphragm with an adjustable unit available from aftermarket sources. Adjust these units so that you get the most vacuum advance possible without any “chugging” or “jerking” at cruise speed.
Your timing is now set for best possible performance. Make note of the new setting, and use this for your future tune-up work.

Questions, Comments & Technical Assistance
If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an e-mail. Also, if you need any technical assistance or advice regarding this process, or other maintenance issues, feel free to contact me:

lars.grimsrud@lmco.com

 

A quick “timing and tuning primer” for those of you just joining this thread on optimizing timing for your HEI car.

First, locate and check out my topic folder “How to Set Your Timing for Best Performance.” This contains a lot of the background information regarding ignition timing, but is applicable specifically for the point-style distributors.

Okay, here we go with some basics…

Ignition timing has a marked effect on both engine performance/power output as well as on vehicle emissions. Unfortunately, the HEI-equipped C3 Vettes built in the ‘70s were more tailored for emissions than for performance, so there are significant performance gains to be made by “tweaking” the calibration of the ignition system.

At idle, most engines produce the smoothest running operation with about 6 – 12 degrees of initial timing. The more retarded the timing is, the lower the Hydrocarbon emissions will be. However, the more retarded the timing is, the more power you lose…

As rpm comes up, the engine demands more timing advance in order to produce peak power. To control emissions, the HEI distributors are designed to bring the additional timing in very slowly, and to limit the total timing in such a way that emissions are minimized. This is not conducive to producing peak power…

Typically, GM V8s will produce peak torque when a total timing of 36 degrees is brought in at the engine’s torque peak rpm – about 2500 rpm. If you have your initial timing set at 10 degrees BTDC, this means that you need the centrifugal advance to bring in an additional 26 degrees of timing by 2500 rpm in order to get best power. What we see, however, is that the stock HEI setup usually produces about 15 – 18 degrees of maximum centrifugal advance, limiting the total maximum advance (initial timing plus centrifugal advance) to about 25 – 28 degrees. This total advance will come in close to redline rpm – not at the torque peak. This is 10 degrees short of producing peak power!

So to properly tune the car for best performance, we do a couple of things: First, the advance curve length must be corrected and/or verified. If the curve is too short to produce acceptable timing results, it must be lengthened. Second, we must change the rpm at which the total advance is allowed to occur. Since the engine’s torque peak occurs at 2500 rpm, we must change the advance curve to bring the total timing all in by 2500 – 3000 rpm. Finally, we must set and adjust the timing to produce a total timing setting of 36 degrees. Once these parameters are properly set, the car will perform its best.

Keep in mind that these parameters will not produce best emissions. In order to produce low emissions, the timing must be retarded.

Finally, we have the issue of vacuum advance. Vacuum advance is added on to the mechanical advance curve discussed above, and is a method for improving fuel economy and throttle response at light, part-throttle settings (at cruise speed). A GM V8 at light throttle cruise performs best when timing is at 52 degrees BTDC. If timing goes much beyond 53 degrees at cruise, the car will start to buck and jerk due to too much timing advance. But 52 degrees is just about right.

Since the factory designs the total mechanical part of the advance curve with about a 26 – 28 degree limit, they install a vacuum advance control unit that adds on about 24 degrees of vacuum advance at cruise. This puts the timing right up at 52 degrees max, and is a perfect cruise setting. But, if we now go in and change our advance curve for peak Wide Open Throttle performance (36 degrees all in at 2500 rpm), we see that the factory vacuum advance unit will now add another 24 degrees on top of our 36 degree setting, giving us a total ignition advance of 60 degrees. This is too much advance for any car to handle, and will result in the car jerking and bucking severely at cruise speed. Not a good thing.

So to compliment the performance curve of our distributor, we must now replace the stock vacuum advance unit with a unit that will give us a total vacuum advance of not more than 16 degrees. By carefully researching the specifications of the vacuum advance units, it is possible to produce a perfect setup.

This method of tweaking and tuning will result in a significantly improved level of performance and throttle response, from off-idle through redline rpm. Mind you, it’s not like bolting on a supercharger or anything like that, but it will produce a noticeable, seat-of-the-pants difference in performance and “feel” on the throttle. My past tuning and performance measurements show that in order to produce a seat-of-the-pants difference in performance, you have to change horsepower by at least 10hp. Getting 10 horses by tweaking your timing curve is a pretty neat mod to do.

 

SteveG75, I'm finding this pretty confusing. What happened to the good old days when you hook up a timing light and set the timing at a certain degree and it's all set? What is a dial backlight? and why do I have to change springs? I'm used to setting the timing to a specific degree and then advance it another degree or two and it's all set. Have I been doing something wrong all these years? I'm the show me type of guy, where you show me how to do it and I'll always be able to do it. I just read Lars papers on timing and I'm more confused now??

:confused:

 

The good ole days went bye-bye with emissions-clean cars.

You can still set your timing to 8 deg. It just won't be the best performance that you could get.

I picked up an old engine tuner's book (published in 1967) at a flea market a couple of weeks ago and the author talked about the very same thing that Lars paper discusses. So it's not a new thing, just more important with the engines we are dealing with.... Smog years....

I did the mods to my car and it performs so much better. This is something you can feel in the seat of the pants when driving!

 

These instructions are so that you get the most performance out of your engine. You can just set your timing at 8 degrees at idle and be done with it, but your engines performance will not be optimal. The timing pointer on your car/damper is only good for setting timing up to about 16 degrees BTDC. A dial back light will allow you to dial in 36 degrees BTDC on the light, then just set the timing so that the marker on the damper is at "0" on the pointer, and you'll be at 36 degrees BTDC. There are other ways of doing this like degreeing your damper, but the easiest is to shell out the $$$ for an adjustable timing light. The 36 degrees should come in about 3,000 RPM's. HOWEVER, you can always just set your initial timing at idle to 8 degrees and close the hood. It wont do any damage to the car.

 

Another SteveG here, I guess I could be SteveG74 :D

The springs act against the natural tendancy for the counterweights in the distributor to swing outwards and advance the timing. Stiff springs will resist the centrifugal force until maybe after 3000RPM, weak springs will let the weights fly out at a lower RPM so you have to find a combo that works around 2500 to 2800.

A dial back timing light allows you to dial in the advance on the light and then you line the timing mark up with the zero mark rather than the 8deg, 10deg, 12deg mark etc. Since most timing tabs only have 8-12degs on them you would otherwise need a timing tape wrapped around the harmonic balencer or you would have to physically work out where 36 degrees is per Lars' instructions.

Hope that helps.

 

RX -
Feel free to drop me a note if you have questions on optimizing your timing - it's really not as difficult as it sounds, and the performance payoffs are quite noticable. Not only that, but what I usually see is that when people understand the whole concept behind the advance curve and the vacuum curve, they usually end up being able to identify and correct problems with the system that they didn't know they had up until then. I'll also be more than glad to give you a call and chat with you about it if it would be easier to explain things "in person."

Correct timing and optimized timing advance is one of the most overlooked maintenance areas of our older cars, yet it's one of the easiest things to correct for an incredible performance improvement with virtually no cost. Setting it up right is really worthwhile.

 

I think we all missed Racer X's question though which indicated he had lost his timing mark/tab. So he may have to establish #1 TDC and then install a new tab and notch the balancer no??

 

I think he said "tag", not "tab" He means the label that told you the stock timing is damaged. I think the timing "tab" on the engine is still there.

 

I was curious, how does this work when using a unilite distibutor with no vaccum advance?

 

Distributors with no vacuum advance are primarily intended for racing applications where the vehicle is constantly operated at or near wide open throttle (WOT). Under these conditions, there is little or no manifold vacuum, so vacuum advance becomes unusable. For street-driven cars, vacuum advance is essential for good fuel economy and good part-throttle throttle response at cruise speeds.

If you are running a non-vacuum advance distributor, the total mechanical timing spec for peak performance remains the same: You set the distributor up to provide a maximum full centrifugal timing of 36 degrees as a good starting point. Most car/engine combinations perfrom best when this full max timing of 36 is slammed in at about 2500 rpm. Heavier cars need it to come in at a higher rpm to avoid detonation, whereas lighter cars can have it come in even quicker. This will produce the best power at WOT with your non-vac distributor, but cruise economy will not be the best without the additional timing advance provided by a vacuum advance control unit.

Most stock distributors will produce an initial timing at idle of about 12-16 degrees when the total is set up for 36, but this can vary quite a bit. Depending on the type of cam you're running, you want to try to tailor the length of your centrifugal curve to produce an initial timing that provides a good idle with good throttle response. This will usually be something in the 8-16 degree range. If this cannot be obtained with the total set to 36, you need to modify the distributor's advance curve to achieve the numbers you desire. How much work you put into this depends on how fanatical you want to get about tuning and getting the car to "feel" just right. Some of us think that these essoteric excercises are the fun parts of this hobby, while others consider this type of stuff a pain. You can set your car up to an initial timing at idle of 8 to 12 degrees just like the data plate says and get a nice running car. But the guys with the cars that are really FAST are the guys who spend the time working out and fine-tuning the little details like this. Timing and advance curve makes a noticable difference in the throttle response of the car.

 

I think I'm starting to get the picture. With in the next couple of days I'm going to buy a dail back timing light. I'm pretty excited because I think I'm starting to get the concept. I'll keep you guys posted. Lars, if I encounter any problems I'll call you, again thanks for all the valuable info.
:flag

 

Everyone:
Please note that the "How to Set Your Timing" article copied and posted above is an early revision of this article. Later revisions include some specifics regarding the HEI systems....

Basically, the HEI systems CANNOT be set up for total timing by removing the advance springs - this ONLY works on the point-style distributors. If you remove the springs on the HEI systems, you will obtain an artifically over-advanced condition that will never be obtained with the springs in place. So to set TOTAL timing on an HEI system, install a random pair of very weak springs instead of the tight factory springs. Then briefly rev the engine up into the 2500 - 3000 rpm range while observing the timing with your adjustable ("dial-back") timing light. With a set of weak springs, the full centrifugal timing should come all the way in and stabilize out at some peak value. Set up for 36 degrees total timing at this point. Then, re-install your stock springs or a set of custom springs to tailor the curve to your desired specifications. But don't set the timing with the springs removed on an HEI.

 

Lars,

Got the reprint from the CorvetteFAQ site. You may want to send them a revised version.

Steve