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Old 09-12-2006, 09:11 PM
67 Deuce 4 Me's Avatar
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SBC total timing question

Im trying to set my timing on my 388. I have a good advance timing light. My distributor is a HEI with a curve kit(lightest springs). I set the base timing approx 18 deg idle. But the most I get is 28 deg total @3000 rpm (vac. plugged) If I set total @ 35deg then base is way up in the 20s. The car runs great. But I think something is wrong. I just want to put the best tune up on it possible. I checked the mechanical advance and it moves freely (not sticking). Maybe its sticking when its running?? I dunno. I searched old threads but none really touched on this problem. Thanks guys

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Old 09-12-2006, 09:34 PM
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My MSD distributor came with four advance stop bushings to limit advance to 18, 21, 25 or 28 degrees of centrifugal advance. MSD does have an advance kit for HEIs that has two pairs of these bushings. The MSD part number is 8428. Maybe these will help.
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Old 09-13-2006, 07:18 AM
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Quote:
Originally Posted by 67 Deuce 4 Me
Im trying to set my timing on my 388. I have a good advance timing light. My distributor is a HEI with a curve kit(lightest springs). I set the base timing approx 18 deg idle. But the most I get is 28 deg total @3000 rpm (vac. plugged) If I set total @ 35deg then base is way up in the 20s. The car runs great. But I think something is wrong. I just want to put the best tune up on it possible. I checked the mechanical advance and it moves freely (not sticking). Maybe its sticking when its running?? I dunno. I searched old threads but none really touched on this problem. Thanks guys
Stock GM HEI distributors for non computer controlled vehicles usually have 20 degrees of mechanical advance built in. If your HEI has a vacuum advance cannister on it you have the correct distributor. Check to see that the weights are not being restricted by the new springs you have installed. Using the lightest springs will allow the mechanical advance to open very quickly at a lower RPM. Are you certain that they are not already open some at idle? This will make it appear to have less total mechanical advance because you will already have used up some of the movement.

What is your idle RPM?

Try using the medium springs.

The point behind using a "curve kit" is to get the full mechanical advance to be "all in" somewhere between 2500-3000 RPM. The weights should not have begun to open at idle RPM. Some combination of the springs ie. light/light, light/medium, medium/medium, etc. should enable the advance weights to slowly transition from the closed position (at idle) to maximum open (in the RPM range mentioned). Too light of a spring combination will cause the total mechanical advance to occur before 2500 RPM and too heavy of a spring combination will cause the total mechanical advance to occur after the suggested 3000 RPM.

NOTE:

The 2500-3000 RPM range is the usual RPM range used for most street application for best performance. Other applications are used for varying conditions.

Last edited by Frisco; 09-13-2006 at 07:33 AM.
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Old 09-13-2006, 08:19 AM
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Here is another thought that you may have missed (I've done this):

A lot of those kits it's very easy to install the ramp upside down when you install the curve kit (one side is CCW rotation and the other is CW rotation) - when you have it installed upside down the advance works kind of like how you are describing.
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Old 09-13-2006, 10:00 AM
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Quote:
Originally Posted by Frisco
Stock GM HEI distributors for non computer controlled vehicles usually have 20 degrees of mechanical advance built in. If your HEI has a vacuum advance cannister on it you have the correct distributor. Check to see that the weights are not being restricted by the new springs you have installed. Using the lightest springs will allow the mechanical advance to open very quickly at a lower RPM. Are you certain that they are not already open some at idle? This will make it appear to have less total mechanical advance because you will already have used up some of the movement.

What is your idle RPM?

Try using the medium springs.

The point behind using a "curve kit" is to get the full mechanical advance to be "all in" somewhere between 2500-3000 RPM. The weights should not have begun to open at idle RPM. Some combination of the springs ie. light/light, light/medium, medium/medium, etc. should enable the advance weights to slowly transition from the closed position (at idle) to maximum open (in the RPM range mentioned). Too light of a spring combination will cause the total mechanical advance to occur before 2500 RPM and too heavy of a spring combination will cause the total mechanical advance to occur after the suggested 3000 RPM.

NOTE:

The 2500-3000 RPM range is the usual RPM range used for most street application for best performance. Other applications are used for varying conditions.

The light spring comments makes complete sense Because of my cam size my engine idles around 900 to 1000 rpm. So like you said i'm probably going into mechanical advance at idle. I found a great deal on a Mallory unilite dist today from a friend. Like new 75.00. It the photo optic one. will work great with my MSD and coil. I will install it and tweak on it more thanks for the input guys!
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Old 09-13-2006, 10:58 AM
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Also the shape of the weights that come with advance kits varies from the original HEI weights. The small end arm has a different angle and curvature.

Try heavier springs to determine if the weights are beginning to advance at too low an rpm. You might even have to use lighter springs on the factory weights to get the curve you desire.
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Old 02-03-2010, 04:38 PM
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Chevy 350 Ethanol

Im trying to find out the best timing for my engine - SBC 360 (bored 0.060", 13.6:1 domed pistons, procomp heads and a Magnum Comp Cams. Runs with 100% Ethanol (dominator 750). My doubt is: the engine has good idlle with 30 but the MSD pro billet distributor has 18 minimun mechanical advance (it is still so much timing to advance!). So the distributor goes total timing (30+18) 48!!! This is bad for the engine. My next try is to lock out the distributor around 33 using a start retard module. Am I doing in the right way?
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Old 02-03-2010, 07:54 PM
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if you keep getting troubling readings you may want to verify TDC and make sure the harmonic balancer hasnt slipped.
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Old 02-04-2010, 02:35 AM
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Im using a ATI harmonic balancer. I re-check the timing (TDC) mark at the pulley and it is ok. My problem really is if I should lock out the distributor at 33-35.
Thanks!
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Old 02-04-2010, 03:02 AM
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Quote:
Originally Posted by streetrod350
Im using a ATI harmonic balancer. I re-check the timing (TDC) mark at the pulley and it is ok. My problem really is if I should lock out the distributor at 33-35.
Thanks!
What cam specs and how (what type racing) are you using the engine.
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Old 02-04-2010, 07:09 AM
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the lightest springs don't work very well because they start to advance at idle and frequently don't have enough force to return to the same position at idle every time. a tuning nightmare.

use the middle size springs and make sure the bushings are in good shape.
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Old 02-04-2010, 11:36 AM
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Quote:
Originally Posted by cobalt327
What cam specs and how (what type racing) are you using the engine.
Street/Strip Comp Cams solid 294/294 (248/248@0.50) .525/.525 lift. Street darg racing.
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Old 02-04-2010, 11:48 AM
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that cam needs at least 20 degrees at idle. any less and the headers will glow red at idle. should be ok with 25 or 30 at idle if the starter doesn't complain (slow cranking when warm).

if you are using stock gm heads then run about 38 to 40 degrees total mechanical advance.

if you are using after market fast burn type heads (that includes vortec) then use about 36 degrees of total mechanical timing. Unless the engine is low on compression (less than 9.5:1 cr), then run 38 to 40.
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Old 02-04-2010, 11:54 AM
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Quote:
Originally Posted by 454C10
that cam needs at least 20 degrees at idle. any less and the headers will glow red at idle. should be ok with 25 or 30 at idle if the starter doesn't complain (slow cranking when warm).

if you are using stock gm heads then run about 38 to 40 degrees total mechanical advance.

if you are using after market fast burn type heads (that includes vortec) then use about 36 degrees of total mechanical timing. Unless the engine is low on compression (less than 9.5:1 cr), then run 38 to 40.
Im using 13.6:1 cr (100% Sugar Cane Ethanol right out the gas pump) with aluminum Pro Comp Heads and 1 3/4" headers. Im very happy to find out someone to talk about my doubt. Here in Brazil they are all dumbs and provide fake information.
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Old 02-04-2010, 02:43 PM
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ask on the MSD site (or summit or jegs etc) if there is a adjustable vacuum advance unit available for that specific model distributor

using a distributor vacuum advance unit connected to the manifold vacuum port on the carb,,,, you can set the base to only 20* (for while cranking much easier on the starter and battery when hot)
once the motor does start the vacuum advance will/can be set to add 10* more for 30* at idle

Wide open throttle (low HG/high load condition) the vacuum advance timing drops out completely and you are only operating on 20 base plus 18 centrifugal =38 mechanical total
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