I understand the whole principal of setting up timing.

All about the centrifugal and vacuum timing.

Also the magical 36* all in by 2600-3200 RPM .......

What i would like to understand is the initial timing, please.

Why some cars are set up at 6* and others as much as 18*? (I understand that 18* will ignite sooner than 6*)

What determines Initial Timing????

thank you

 

The distributors have different amounts of mechanical advance built into them for pollution and fuel economy reasons. Some engines needed long advance curves (those set at 6*) and others short curves (those set at higher numbers). Every engine has an ideal amount of advance depending on load, RPM, throttle position and fuel economy requirements. That's why my wife's Honda's computer controlled fuel injected and ignition puts out more power with lower pollution that the old corvettes that had much larger engines.

 

In regards to our corvettes it has a lot to do with the flammability of the mixture and the pressure developed during the compression stroke.

Each intake stroke ends up having a bit of exhaust still present in the combustion chamber once both valves close. This effects the flammability of the mixture.
A cam with high duration, lots of overlap or poor flowing exhaust ports or system, to list a few reasons, is going to have more exhaust contamination in the final charge to be burned. Ideally we want the max cylinder pressure reached at about 15* to 20* or so after top dead center and not have the majority of the mixture still burning much later in the stroke. In order to accomplish this it may be necessary to ignite the mixture sooner due to the level of contamination creating a slower burning mixture at lower RPM's.

Another reason is lower pressures present in the combustion chamber during the power stroke.
Running lower CR's or again having higher overlap periods creates lower pressures in the combustion chamber at lower RPM's. With less pressure the intake charge burns slower. This means we need to ignite it earlier to achieve our ideal pressure point for power.

Under light throttle cruise conditions the vacuum advance lights the mixture even earlier to try to achieve getting the max pressure at the right time.

At WOT throttle vacuum drops out as lighting the mixture too early at high power settings causes excessive pressure prior to TDC and can cause a loss of power or detonation.

 

Here's what I did to set my initial timing after doing some research. I don't have any pollution controls on my engine. Plug vacuum advance hose. Adjust idle mix screws to get a decent idle. Connect vacuum gauge to a manifold port. Now set your idle speed to the lowest rpm where the engine will run without stalling. For my setup it was 650 rpm with a manual transmission. Now advance your timing while observing your vacuum gauge. Stop advancing the timing at the point where you just reach maximum vacuum. Your idle rpm should have increased. Turn you idle speed back down to your start idle. Again for me it was 650 rpm. Repeat these steps until you reach a point where no more ignition advance will increase vacuum. Now, check initial advance with a timing light. Mine was 24*. Set your normal idle speed and adjust idle mixture to attain highest vacuum. Reset normal idle speed if idle increased. Now check your initial timing with a timing light. Lets say you get 20* for your setup. Then you now have to limit your mech'l advance to attain say 36* at 3000 rpm which would be 16*. My dist'r is a Mallory fully programmable for ignition advance so it was easy for me. You'll need to figure out how to restrict total mech'l advance on your dist'r. Hook up your vacuum advance, re-adjust idle if needed. This method worked well for me and no more eye watering when my initial advance was set at 12*.

 

You might find this post to be very helpful.
https://www.corvetteforum.com/forums...long-post.html

 

What, if any, are the disadvantages to running a high amount of initial advance and a low amount of mechanical?
My 69 427 is basically stock with a very mild roller cam and the original smog system. It has the original distributor but it has been recurved. Mechanical advance is limited to 17* so I also run 17* initial for 34* total which is all in at 3000 rpm and 16* vacuum using a B26 vacuum can. It runs really nice like this and exhaust smell is greatly reduced but are there any cons to doing it this way?

 

If you have crap low octane gas running 34* may help prevent detonation. Your so close it probably doesn't make much difference.

Lars' paper: http://www.lbfun.com/warehouse/tech_...t%20Timing.pdf

 

An engine is most effecient when vacuum is at its maximum. This relates to what REELAV8R explained in his post. When you find that sweet spot in the combustion process, you will have maximum vacuum. If I were to set my initial timing to the factory spec for a stock '76 L-82 pollution controlled engine, it would be 12* initial advance. Mine is no longer stock. I'm running ~10.5:1 compression, hyd roller .560" lift with a duration of 227* @ 0.05. The article referenced by Peterbuilt no longer holds true for initial advance settings for modified engines. I tried 12* and my eyes watered and the engine stumbled and lacked power. Then I tried the method explained by me in my previous post. There is a significant improvement in driveability. Oh, the best gas I can get here is 91 octane. I have total advance set to 32* and there isn't any pinging during wot. I did need to reduce my vacuum advance down to about 8* advance for a total of 40* at cruise to reduce ping.

If I may I'll add the following although its getting off topic. By setting up my initial timing this way, it ensured that my carb was idling from the idle fuel circuit only on the primary side and not drawing fuel from the transition fuel circuit. I have a Holley 650 DP Street HP with four corner idle. All my idle speed adjustments are now made from the secondary idle adjustment screw only. I actually took the carb off after I set my initial timing to check the transition slot relative to the primary throttle blade. Lo and behold it was a nice square!

 

Yes, I understand but what is the reasoning for reducing the amount of centrifugal advance and increasing initial? I believe in Lars paper he says best performance for a GM motor is around 18* centrifugal and 18* initial. I understand the 36* total but why is reducing the mechanical advance (I think the stock 427 was 24*) to add more initial a benefit? What I am getting at is why would it be better to run 18* and 18* over 12* and 24*?

 

For my setup, the reduction in centrifugal was directly influenced by the initial setting. If the initial setting is high because it was determined that this is what gives the highest vacuum and therefore efficiency, then I must reduce my centrifugal to maintain my total of 32*. You still want total advance to be limited for the setup of the engine. I'm not sure it makes any difference in how the two are added up to reach the total advance except that more "traditional" initial advances like 12* was not helping my engine setup. 36* total is a nice ball park number that works on most iron block/headed engines, I guess. For newer aluminium head designs such as my AFR 195's, it is not needed to run 36* total and it's recommended to retard the timing a few degrees. The same holds true for my K-B hyper pistons that reflect more heat back into the combustion chamber. Again, retard timing a few degrees because they improve the overall combustion process so there's no percentage in having the timing set to 36*. So for my setup, I don't need 36* but I can't confirm this but I do run 32* on crappy 91 gas and I haven't had any issues. Anyway, it's better for my engine to run 24 initial and 8 centrifugal because it idles smoother and accelerates better. YMMV but I would give that method a shot for setting initial and adjust centrifugal to attain max advance for your setup.

Maybe Lars can offer his thoughts on why he recommends 18* but I think this maybe a good starting point for tuning but may not be optimal for everyone.

 

I think the reason the factory chose the initial timing settings they used involved many factors such as compression, vehicle weight and the way they expected the vehicle to be driven. You also need to factor in the blend of gasoline you are burning today which is quite different form the leaded gasoline from the 60s and 70s.

The guidelines I use for determining what initial timing is best for an engine is in the Barry Grant Inc. catalog and/or web site in the Demon carburetor selection guide; they recommend 10 to 12 degrees of initial timing when the camshaft duration is less than 220 deg. @ .050, 14 to 16 degrees of initial timing with less than 240 deg @ .050 and 18 to 20 degrees of initial timing with a cam with less than 260 @ .050. The total ignition advance must be checked and set (both mechanical and vacuum) when you increase the initial timing or engine damage will result if the total advance is excessive for the engine.

I hope this helps Henry @ oles carb

 

This is really interesting read.
Thank you so much for taking time and explaining it to me!

 

If you want to read a bit more this is where I got most of my info from for setting initial timing.

http://www.burtonmachine.com/tech

 

The simple answer is every engine is slightly or greatly different than other engines, and the ambient atmosphere is different in different parts of the country.
For our C3 antiques GM had to come up with one common timing curve for each engine RPO, despite all the manufacturing tolerances when making tens of thousands of those engines. Some are going to have slightly higher compression than others (ideally requiring a slightly less advanced curve), and some of these engines in these cars are going to be driven in low altitude, low humidity conditions (requiring a slightly retarded timing curve compared to high altitude or humid locations). GM was forced to set the timing curve very conservatively to prevent engine damage (and warranty claims) in those vehicles driven in low altitude (high barometric pressure), low humidity conditions.
For our own cars we can tailor the timing curve to more closely match what the engine actually wants/needs, as we know the conditions it's driven in, and the compression/octane personality of the engine. We don't have to set our timing curve so that it will necessarily work safely in a similar vintage Corvette driven on the opposite side of the country.

Cliff note: We can customize the timing curve on our one vehicle fleet. With tens of thousands of Corvettes built each year, back then GM had to settle for the worst case situation (manufacturing tolerances and ambient conditions) that would determine the timing curve.

 

As a rule....just throwing this out there......the more overlap the cam has, the more initial advance you will need.
Real radical cams (260+@.050) usually just want to be "locked out"....no advance curve period.

My 244@.050 cam likes 21 initial and 35 total with the Dart fast burn chambers.....

Jebby

 

My cam is 262@.050 and likes 18 initial and 36 total, L88 BBC heads. I run a B28 vacuum advance can in my MSD distributor as well (all in at 8" of vacuum)

 

Kind of off topic, but I bought one of those Mallory programmable distributors off a friend who never used it. How do you like it? I haven't used it yet. It will be going in my 327ci I'm putting in.

 

I have a 327ci with an old school 30-30 cam in it. I *think* the 30-30 is in the 250-255 range @0.050, so based on what I am reading in here it would need more initial and less (if any) advance?

 

You can't really compare late 60's lobe profiles to modern lobes unless you use the higher lift numbers like duration at .200 inches not the .050 numbers. IMO they sound radical and have poor manners and they really don't make HP like modern solid or roller cams

For the Comp cams solid lifter reproduction Duntov 30-30, P/N 12-673-4, specs are:

advertised duration: 284 intake/291 exhaust,
duration @ .050 : 247 /254 ,
lift : .504" / .498",
lash: .012"
lobe center: 112 degrees

Another concept I see in the above posts is 36 degrees. That is a wrong assumption with modern heart shaped fast burn combustion chambers. Dart and AFR claim max of 32-33 degrees and some of the shallow valve angle heads of 18 degree and less are more like 28 degree max.

 

This is exactly right.....the lobes on the 30-30 are sooooo...lazy that they cannot even be lashed like a modern flat tappet (TDC method)
I could not begin to tell you what would be ideal for that cam as I have zero experience with it.

Jebby