Mechanical Advance Restricted?
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Mechanical Advance Restricted?
Following good advice of the forum yesterday to set up my Mighty Demon 750 on the 454.... I set initial timing at 18 degrees (vacuum advance plugged) at 1000 RPM. Ran the engine up to 2500 and only pulled another 10 degrees for a total of 28. I'm running an HEI distributor with stock weights and one light spring and one medium spring (advance mechanism is lubed and free). Shouldn't I be getting more advance by 2500 RPM? What's the recommended fix?
Never got around to tweaking the Demon......tonight
Never got around to tweaking the Demon......tonight
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
Check the distributor and see if there is a bushing on the limiting pin. These bushings can significantly limit the number of degrees of advance the distributor allows. I believe there are timing kits available for the HEI distributor that gives you a choice of bushings. You will have to find out which bushing works best by trial and error. As a check you might want to remove the current bushing you have and see how much advance you get without any limiting bushing. You may find that by dropping the initial a few degrees you are right on target without a bushing. Good luck.
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
I don't know, the distributor was set up on the bench while the motor was apart. Just ordered the advance kit and did the spring install according to recomendations on the forum (one light one medium). I didn't have time to switch to both lights last night, but I don't think that will give me another 7 degrees? Or will it?
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
As far as the bushings, I have a whole pile of those that came with the kit..was led to believe by historical forum posts that they really don't have much impact on overal advance so I left the "stock" one in.
Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
Originally Posted by Hvymtlc5
I don't know, the distributor was set up on the bench while the motor was apart. Just ordered the advance kit and did the spring install according to recomendations on the forum (one light one medium). I didn't have time to switch to both lights last night, but I don't think that will give me another 7 degrees? Or will it?
Good Luck
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
check for max advance at 3500 that way you know that it is all in. Some stock ignitions have pretty high rpm total advances.
I've run high performance motors with 3000 or 3200 before all the mech. is in. The reason being that I had such high DCR and modern high swirl fast burn technology heads that make max power at @ 32 degrees of total advance. I'm now using the MSD dist. That i have 100 choices of total advance and advance curves. All set with switch positions. No moving parts.
Lower compression motors can use lower rpm total advance. Because EGR and lower compression motors have a high amount of residual exhaust gas left in the cylinder. Which in turn retards the flame travel requiring 38+ degrees of total advance to run right
I've run high performance motors with 3000 or 3200 before all the mech. is in. The reason being that I had such high DCR and modern high swirl fast burn technology heads that make max power at @ 32 degrees of total advance. I'm now using the MSD dist. That i have 100 choices of total advance and advance curves. All set with switch positions. No moving parts.
Lower compression motors can use lower rpm total advance. Because EGR and lower compression motors have a high amount of residual exhaust gas left in the cylinder. Which in turn retards the flame travel requiring 38+ degrees of total advance to run right
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Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by gkull
I'm now using the MSD dist. That i have 100 choices of total advance and advance curves. All set with switch positions. No moving parts.
Pro
Joined: Sep 2004
Posts: 595
Likes: 0
From: Boston MA
Originally Posted by gkull
check for max advance at 3500 that way you know that it is all in. Some stock ignitions have pretty high rpm total advances.
I've run high performance motors with 3000 or 3200 before all the mech. is in. The reason being that I had such high DCR and modern high swirl fast burn technology heads that make max power at @ 32 degrees of total advance. I'm now using the MSD dist. That i have 100 choices of total advance and advance curves. All set with switch positions. No moving parts.
Lower compression motors can use lower rpm total advance. Because EGR and lower compression motors have a high amount of residual exhaust gas left in the cylinder. Which in turn retards the flame travel requiring 38+ degrees of total advance to run right
I've run high performance motors with 3000 or 3200 before all the mech. is in. The reason being that I had such high DCR and modern high swirl fast burn technology heads that make max power at @ 32 degrees of total advance. I'm now using the MSD dist. That i have 100 choices of total advance and advance curves. All set with switch positions. No moving parts.
Lower compression motors can use lower rpm total advance. Because EGR and lower compression motors have a high amount of residual exhaust gas left in the cylinder. Which in turn retards the flame travel requiring 38+ degrees of total advance to run right
You might have a curve that has a slight plateau at 2500 and then goes up again at 2700 or 3000. Especially if you are using 2 different spings. Pull the throttle back until you hit 3000 or 3500 just to make sure it is fully advanced. If it is and you are still only getting 10 degrees of advance, I would say that something is binding. As mentioned before, check to make sure the bushing is installed correctly. I have seen those installed upside down....10 degrees of mechanical advance seems low to me.
Thanks,
Joe
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Originally Posted by bobs77vet
hows that working for you???? i need a distributor for my l48 thats going in the hot rod and i am waffling between a $69 ebay HEI "specail " and a nice MSD one lile you have. this admittedly will be a "nothing special" motor perhaps the MSD unit would be overkill
IMO - It's a good idea to have an electronic dist. Just dial in your settings.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by gkull
It's not cheap! I just went down to Summit and bought it along with Taylor pro wires and it's $500. But then you also have to have a coil. I use the Crane coil that is used in conjunction with my Crane digital multi spark HI-6R. So you end up with @ $850 dollars to fire your spark plugs. I also had to buy a new comp cams polymer gear for my billet cam because my shaft size was different from my previous Mallory Unilite dist.
IMO - It's a good idea to have an electronic dist. Just dial in your settings.
IMO - It's a good idea to have an electronic dist. Just dial in your settings.
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Put the light springs in last night and double checked the advance mechanism.....now I have 12 degrees mechanical for a total of 30 degrees at 3000...still not enough (?).
How much advance should the mechanical provide? Any suggestions?
How much advance should the mechanical provide? Any suggestions?
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Hvymtlc5
Put the light springs in last night and double checked the advance mechanism.....now I have 12 degrees mechanical for a total of 30 degrees at 3000...still not enough (?).
How much advance should the mechanical provide? Any suggestions?
How much advance should the mechanical provide? Any suggestions?
Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
Originally Posted by bobs77vet
take the all springs out and measure it....i would think it should be in the 16-22 range
Also, considering I have a DUI, Street/Strip HEI in there, and the old springs "looked" very light, will that make a difference?
Last edited by kb2fzq; Jan 12, 2006 at 09:55 AM.
Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Hvymtlc5
Bob, I can do that, but what would cause the limiting factor now?
there would be no limiting factor except for broken parts, and the limits of how far the weights expand and you would only do this to see what the mechanical advance was....more of a test proceedure then anything.... you would not want to run the car with this because the advance would come in immediately but it will tell you what the most advance you can get out of the distributor is
Race Director
Joined: Sep 2004
Posts: 11,874
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by kb2fzq
Don't want to steal this thread, but while we're talking about it and you're a green lite, thought I'd ask...put a light and a medium in last Tuesday...I didn't put the lite on it, but seat of the pants "seems" better...how much of a noticable change should I expect, aside from actual timing light advance data??
Also, considering I have a DUI, Street/Strip HEI in there, and the old springs "looked" very light, will that make a difference?
Also, considering I have a DUI, Street/Strip HEI in there, and the old springs "looked" very light, will that make a difference?
when it is set up right it will be very noticable....it is the quick acceleration that you will notice the pull from say 1600-3000 rpm
Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
Originally Posted by bobs77vet
when it is set up right it will be very noticable....it is the quick acceleration that you will notice the pull from say 1600-3000 rpm
Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
you know the way to do this is put in springs that will give you the advance you want all in by 3000 rpm....Lars makes this so simple....just put lighter springs in until you get 36* by 2500-3000 rpm and don't worry about initial timing as long as the car starts ok...i'm using one light and one mdeium spring...the real key is to make sure the distributor is shimmed correctly and the mechanical advance shaft rotates fully and the bushings are all in place.