l48 timing question
Intermediate
Joined: Jun 2011
Posts: 31
Likes: 1
From: Calgary AB
OK I've read all the material on this site and I think I'm ready to get started. Before I do, should I assume that I will need lighter springs for the mech advance than the stock ones currently on my '75 L48? It seems that that in the interest of greener emissions the timing is severely retarded with the factory set advance curve and maybe I should be prepared with alternate parts?
Drifting
Joined: Oct 2010
Posts: 1,972
Likes: 6
From: Chester Virginia
I bought a kit for that from summit which includes springs, little round new washers for the weights and new centrifugal weights. Since I found mine is at 3000 rpm not 4600 from the factory I haven't put them in but I'd like that kick down around 2500 rpms so this winter when it's cooler I plan to change the springs one at a time and see what I get. I don't plan to change the weights but haven't opened the kit to see the instructions yet either.
Lance
Lance
Intermediate
Joined: Jun 2011
Posts: 31
Likes: 1
From: Calgary AB
I bought a kit for that from summit which includes springs, little round new washers for the weights and new centrifugal weights. Since I found mine is at 3000 rpm not 4600 from the factory I haven't put them in but I'd like that kick down around 2500 rpms so this winter when it's cooler I plan to change the springs one at a time and see what I get. I don't plan to change the weights but haven't opened the kit to see the instructions yet either.
Lance
Lance
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Your math depends on your vacuum source! Ported vacuum is sourced from above the throttle plates, and produces no vacuum at idle. This, combined with very low initial settings from the factory (book spec on my 1968 327 is 4* initial) was meant to raise EGTs (it does!) for pre-cat emission control. If he has ported vacuum as opposed to manifold vacuum, he will see his initial 16* at idle, even with a fully functioning vacuum advance.
Manifold vacuum will usually give you FULL advance at idle, ported vacuum advance will usually give you NO advance at idle. So, if someone says that they're vacuum canister pulls 16* of timing, and only shows 8* of advance at idle, something is wrong somewhere, no matter what the vac adv is hooked up to, because it should either be 16 or 0.
Scott
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I bought a kit for that from summit which includes springs, little round new washers for the weights and new centrifugal weights. Since I found mine is at 3000 rpm not 4600 from the factory I haven't put them in but I'd like that kick down around 2500 rpms so this winter when it's cooler I plan to change the springs one at a time and see what I get. I don't plan to change the weights but haven't opened the kit to see the instructions yet either.
Lance
Lance
FWIW, I used the 929g advance kit, and installed the medium silver springs on the stock weights, and the timing came in at 2800, so I called it good enough for government work.
Scott
Drifting
Joined: Oct 2010
Posts: 1,972
Likes: 6
From: Chester Virginia
Scott...you're probably right...something like that is likely. I need to read the instructions in the kit. Right now my engine is running so well that I am not even taking the wire shields off and opening the distributor up to see what springs are actually in there since it appears to be curved at 3,000 rpm now.
I do have a question....if you only replace a spring on one side does that make the symmetrical weights off balance and do some longer term harm to the distributor with one weight flying out faster than the other or are they somehow tied to one another so it doesn't matter? That thing whirls at a pretty good pace.
Lance
I do have a question....if you only replace a spring on one side does that make the symmetrical weights off balance and do some longer term harm to the distributor with one weight flying out faster than the other or are they somehow tied to one another so it doesn't matter? That thing whirls at a pretty good pace.
Lance
Burning Brakes
Joined: Sep 2008
Posts: 805
Likes: 5
From: Naperville Illinois
My point is that if you set the timing at the all in point of the mechanical advance, and then check it at idle, whether you have ported vacuum, manifold vacuum, or NO vacuum advance, it makes it easy to do a quick check of timing, without having to remove air cleaner, block off hoses, rev throttle, etc., that's all.
Manifold vacuum will usually give you FULL advance at idle, ported vacuum advance will usually give you NO advance at idle. So, if someone says that they're vacuum canister pulls 16* of timing, and only shows 8* of advance at idle, something is wrong somewhere, no matter what the vac adv is hooked up to, because it should either be 16 or 0.
Scott
Manifold vacuum will usually give you FULL advance at idle, ported vacuum advance will usually give you NO advance at idle. So, if someone says that they're vacuum canister pulls 16* of timing, and only shows 8* of advance at idle, something is wrong somewhere, no matter what the vac adv is hooked up to, because it should either be 16 or 0.
Scott
Similarly your point above is still not correct--that is to say, a quick check at idle is not useful if your vac advance is hooked up to ported vacuum. With ported vacuum, your vac advance could be malfuntioning and you wouldn't know it by checking timing only at idle, because no vacuum is present.
Last edited by Rally68; Jul 22, 2011 at 04:12 PM. Reason: repeated word
Burning Brakes
Joined: Jan 2003
Posts: 755
Likes: 32
From: Roswell Georgia
Since the individual spark plug wire contacts in the Chevy distributor cap are set 45 degrees apart (360/8), how can you have 52 degrees of advance?
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Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I don't think that really matters, tho, you could have the advance at 180* (not really, your engine wouldn't run that way! Just exaggerating to make a point.), all the cylinders would be getting the spark at the same advance, so what difference would it make?
Scott
Last edited by scottyp99; Jul 22, 2011 at 05:24 PM.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Scott...you're probably right...something like that is likely. I need to read the instructions in the kit. Right now my engine is running so well that I am not even taking the wire shields off and opening the distributor up to see what springs are actually in there since it appears to be curved at 3,000 rpm now.
I do have a question....if you only replace a spring on one side does that make the symmetrical weights off balance and do some longer term harm to the distributor with one weight flying out faster than the other or are they somehow tied to one another so it doesn't matter? That thing whirls at a pretty good pace.
Lance
I do have a question....if you only replace a spring on one side does that make the symmetrical weights off balance and do some longer term harm to the distributor with one weight flying out faster than the other or are they somehow tied to one another so it doesn't matter? That thing whirls at a pretty good pace.
Lance
Be sure to check the weight pivot pin on Hei's, they wear quite a bit if not lubed and will give erratic spark. Check for the glide buttons under the weight too.
Beware of too light a spring, that can give you a little mechanical advance at idle when you don't want it.
Drifting
Joined: Oct 2010
Posts: 1,972
Likes: 6
From: Chester Virginia
Scott...that's why I say I want to read the kit's instructions when it's cooler and I mess with it. I know there are new little round nylon type or substitute washers which I think are for the pivot pins for the weights but I want to read what they say and think about it. I'll do that before I mess with it.
I haven't changed it at all yet under the theory that I have 95% of the benefit at 3,000 rpm all in now and it runs wonderfully and messing with it I might violate that old dictum: "do no harm." Thanks for the answer. Didn't know there were glide pads either but I'll check for wear.
could be one of those, "while I'm in here I might as well......" deals when I do open it up.
Lance P.
I haven't changed it at all yet under the theory that I have 95% of the benefit at 3,000 rpm all in now and it runs wonderfully and messing with it I might violate that old dictum: "do no harm." Thanks for the answer. Didn't know there were glide pads either but I'll check for wear.
could be one of those, "while I'm in here I might as well......" deals when I do open it up.
Lance P.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
The 52° only comes into play when cruising. For example you have 36° centrifugal advance (because of dist rotation speed) when driving down the highway at 2500 rpm, your vacuum at that speed may be anywhere from 24in (if you let off the gas completely) to 8in if you give it slight gas, and the vacuum advance adds to the centrifugal to give you a max of 52°. When cruising at any speed your vacuum is constantly changing and the advance added is also changing accordingly.
Try running a vacuum gauge when driving, you will see it.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I'm sorry, I thought your earlier post said 16 initial plus 16 vac should equal 32 at idle for a quick check? My point was that's not true if using ported vacuum. I wasn't addressing the earlier poster's 16 +16 = 24.
Similarly your point above is still not correct--that is to say, a quick check at idle is not useful if your vac advance is hooked up to ported vacuum. With ported vacuum, your vac advance could be malfuntioning and you wouldn't know it by checking timing only at idle, because no vacuum is present.
Similarly your point above is still not correct--that is to say, a quick check at idle is not useful if your vac advance is hooked up to ported vacuum. With ported vacuum, your vac advance could be malfuntioning and you wouldn't know it by checking timing only at idle, because no vacuum is present.
Scott
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Scott...that's why I say I want to read the kit's instructions when it's cooler and I mess with it. I know there are new little round nylon type or substitute washers which I think are for the pivot pins for the weights but I want to read what they say and think about it. I'll do that before I mess with it.
I haven't changed it at all yet under the theory that I have 95% of the benefit at 3,000 rpm all in now and it runs wonderfully and messing with it I might violate that old dictum: "do no harm." Thanks for the answer. Didn't know there were glide pads either but I'll check for wear.
could be one of those, "while I'm in here I might as well......" deals when I do open it up.
Lance P.
I haven't changed it at all yet under the theory that I have 95% of the benefit at 3,000 rpm all in now and it runs wonderfully and messing with it I might violate that old dictum: "do no harm." Thanks for the answer. Didn't know there were glide pads either but I'll check for wear.
could be one of those, "while I'm in here I might as well......" deals when I do open it up.
Lance P.
Scott
Burning Brakes
Joined: Sep 2008
Posts: 805
Likes: 5
From: Naperville Illinois
Huh??
Because I suggested that you're not right and try to explain my opinion?
If I'm wrong please tell me what I missed. All I was doing was trying to add some useful information to the thread; sorry that led to me giving offense.
Because I suggested that you're not right and try to explain my opinion?
If I'm wrong please tell me what I missed. All I was doing was trying to add some useful information to the thread; sorry that led to me giving offense.
Drifting
Joined: Oct 2010
Posts: 1,972
Likes: 6
From: Chester Virginia
Scott...thanks. for me, I first need to understand the theory then the mechanics then the choices then I take a baby step then a bigger step or leave it as is. I have now made 62 changes in 9.5 months of ownership and I'd say all to the good so it must be working okay.
This forum and people like all of you with enormous knowledge are invaluable for a dummy like me though I must admit I know a heck of a lot more now than I did in the fall. Just enough to be dangerous.
Lance
This forum and people like all of you with enormous knowledge are invaluable for a dummy like me though I must admit I know a heck of a lot more now than I did in the fall. Just enough to be dangerous.
Lance
Burning Brakes
Joined: Jan 2003
Posts: 755
Likes: 32
From: Roswell Georgia
Has nothing to do with term spacing.
The 52° only comes into play when cruising. For example you have 36° centrifugal advance (because of dist rotation speed) when driving down the highway at 2500 rpm, your vacuum at that speed may be anywhere from 24in (if you let off the gas completely) to 8in if you give it slight gas, and the vacuum advance adds to the centrifugal to give you a max of 52°. When cruising at any speed your vacuum is constantly changing and the advance added is also changing accordingly.
Try running a vacuum gauge when driving, you will see it.
The 52° only comes into play when cruising. For example you have 36° centrifugal advance (because of dist rotation speed) when driving down the highway at 2500 rpm, your vacuum at that speed may be anywhere from 24in (if you let off the gas completely) to 8in if you give it slight gas, and the vacuum advance adds to the centrifugal to give you a max of 52°. When cruising at any speed your vacuum is constantly changing and the advance added is also changing accordingly.
Try running a vacuum gauge when driving, you will see it.
Le Mans Master
Joined: Aug 2006
Posts: 7,411
Likes: 1,551
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
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52 Crankshaft Degrees.
If you set your initial timing at 12* before Top Dead Center it means the spark plug fires when the crankshaft is 12* BTDC.
When you hook up your ported vacuum advance you ad 16 more degrees-your spark plug now fires (12* + 16*=28*) when your crankshaft is now 28* BTDC.
When the distributor is turned CW or CCW the spark is fired earlier or later (advance or retard).
Does that help or is it more confusing?
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Scott