WBBC:n hurja isolohko esittäytyy!

Originally Posted by tapio

Tässä kaksi hyvää artikkelia ajoituksesta ja HEI-jakajasta by Lars Grimsrud.

How to Set Your Timing for Peak Performance
(Non-HEI)

by Lars Grimsrud
Colorado Corvette Crazies (CCC)
The Ultimate Corvette Tuning & Beer Drinking Fraternity
Lafayette, CO Rev. B 4-18-01


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.

NOTE: This procedure cannot be used on the HEI ignition systems. Removal of the springs will cause an artificially over-advanced condition that will never be achieved with the springs in place. You can use the basic technique described in this paper with the HEI units (setting timing up to 36 degrees), but to check total timing, you must install a set of soft springs. You cannot remove the springs altogether. With the soft springs in place, rev the engine until the centrifugal advance is pegged out. Adjust for 36 degrees total. Then re-install your original springs.

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:

V8FastCars@msn.com

***

An HEI Ignition & Timing Primer

by Lars Grimsrud
Colorado Corvette Crazies (CCC)
The Ultimate Corvette Tuning & Beer Drinking Fraternity
Lafayette, CO Rev. New 1-12-03

The following two short articles are excerpts of some posts I did on the CorvetteForum.com regarding the timing curve on the non-ECM GM HEI Ignition Systems. Although not organized as “how-to” technical articles, I think you may find some interesting and useful information here….

First, be sure to get a copy of my technical article, “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. If you do not have a copy of this article, be sure to drop me an e-mail request for it. Also complimenting this article, and essential reading to understand the vacuum advance system is my article titled “Distributor Vacuum Advance Control Units Specs and Facts.” Drop me a note for a copy if you don’t have one available.

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.

For assistance on comments on this subject, drop me a note to:

V8FastCars@msn.com


The following CorvetteForum post discussed my development of a curve kit for the HEI distributors. It contains some interesting information about the HEI curve and how to change it. I am not currently actively marketing or selling these kits, but I can build them to order for people interested:

Development of the HEI Ignition Curve Kit is complete, and I’ll have the first kits ready to go by the weekend. Parts are trickling in slowly from the various suppliers, and I’m hand-fabricating the special advance weights that are required to make some of the HEI systems work right. I can build one kit every evening, so I will fill orders as they come in just as quickly as I can build kits. For more information, drop me an e-mail. The kits are $45 including postage, and I trustingly accept non-rubber personal checks. Please keep in mind that I do this for the passion and love of the hobby, so I’m not putting a worthwhile profit margin on this – if you bounce a check on me it’ll hurt… so please don’t…. Payable and mail to:

Lars Grimsrud
1285 Cressida Court
Lafayette, CO 80026

V8FastCars@msn.com

I highly recommend that you buy yourself an adjustable timing light – it makes this job much easier and more precise. Use this as an excuse to go visit your friendly Sears Craftsman store…

The development of this kit was very interesting, and revealed several things…

First, the stock HEI setup leaves HUGE room for improvement. It responds very well to the correct tuning technique, but this is the key: As important as the parts you use to modify your car is the TECHNIQUE you use to set it up.

The HEI systems are primarily tailored to provide low vehicle emissions. In most cases, this means that the ignition curve is slow and retarded. Retarding the timing reduces Hydrocarbons, but it also retards your performance…. Typically, we find that the HEI systems are set up for an initial timing of about 8 degrees. The “length” of the advance curve varies greatly from one distributor to the next, but typically the centrifugal advance will allow an additional 15 – 18 degrees of advance, with the maximum advance coming in at about redline rpm. This gives you a total mechanical advance of about 26 degrees on your HEI ‘Vette.

26 degrees. You gotta’ be kidding. This is 10 degrees short of best power, and the rpm curve is off by 2500 rpm. Pretty significant.

The advance curve on the HEI systems is limited by the design of the centrifugal weights. The weights have a unique geometry which causes the weights to “bind up” and not advance any further once they hit a certain advance point. You can advance your timing to achieve 36 degrees total advance, but on many cars, this will result in an initial timing setting of about 18 – 22 degrees. This works well for radical cams, but does not work well for most stock or near-stock setups.

What we find, then, is that the total length of the advance curve must be increased for most applications, and the curve must be brought in at a much lower rpm: 2500-3500 rpm is the target range for full 36-degree advance. To do this, your tuning technique must include a verification of your stock advance curve length, and then a thoughtful modification and correction to this curve length. For best throttle response and performance to stock and near-stock cars, the initial timing should be around 12 – 14 degrees (as measured with vac advance disconnected), centrifugal curve length should be about 22-24 degrees, with total mechanical timing at about 36. If you can get close to this, you’ll get a noticeable performance improvement – anything is better than the stock 25 degree max setup…

In doing the tuning we also found that none of the aftermarket kits provide good instructions, and none of the kits produce the correct curve length. Some of the kits improve things slightly, while others actually make your car run worse than stock. No kidding.

Suur kiitos tapio, tämä lisätään varmasti tech artikkeleihin!