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340 timing question

JG1966

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First off, I want to thank everyone who has helped me on this board. It's the best site on the web, bar none. Here is my question. I just put a timing light on my engine for the time since I re-purchased my old car from college. 340 was running fine but wanted to know where I was at. I was at 6 initial. Way too low. I moved to 16 advanced. Still ran smoothly. But that moved my total advance to 40* all in at 3800 rpm. Is 40 too high? I've read 36* is optimal. Should I reduce initial to get to 36 or adjust distributor mechanical advance to get to 36? Or take her out and see if maybe she likes 40?
 
Total initial + mech advance of 36 is generally considered good and safe for most engines. Certain engines may accommodate a little more depending on how they are built, altitude, gas quality, etc. Above 36 you would just need to experiment and see if you can avoid detonation and pinging. I would say 40 is too high but you have a pretty slow advance curve if it doesn't top out until 3800 rpm. I think I would try 12 degrees initial and see if I couldn't change out the springs on the weights to bring the total mechanical in a little sooner - say around 2500 rpm or so. A set up of 12 initial with 24 mechanical is a pretty good all around set up for a street driven car IMO.
 
Total initial + mech advance of 36 is generally considered good and safe for most engines. Certain engines may accommodate a little more depending on how they are built, altitude, gas quality, etc. Above 36 you would just need to experiment and see if you can avoid detonation and pinging. I would say 40 is too high but you have a pretty slow advance curve if it doesn't top out until 3800 rpm. I think I would try 12 degrees initial and see if I couldn't change out the springs on the weights to bring the total mechanical in a little sooner - say around 2500 rpm or so. A set up of 12 initial with 24 mechanical is a pretty good all around set up for a street driven car IMO.
Thanks GTX. I'm honestly not sure at what RPM I reach my total advance. I was under the impression that you rev to 3500-4000 to make sure you are all in. I had someone sit in car and bring up to about 3,800 RPM and I was at 40 degrees on dial back timing light. For all I know, I would have been at 40 at 2500 or 3000. I don't mean to sound stupid (maybe I am) but does it matter if you're mechanical advance is maxed out at 3000 or 3800? Or is total advance when you're maxed out what really matters?
 
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First off, I want to thank everyone who has helped me on this board. It's the best site on the web, bar none. Here is my question. I just put a timing light on my engine for the time since I re-purchased my old car from college. 340 was running fine but wanted to know where I was at. I was at 6 initial. Way too low. I moved to 16 advanced. Still ran smoothly. But that moved my total advance to 40* all in at 3800 rpm. Is 40 too high? I've read 36* is optimal. Should I reduce initial to get to 36 or adjust distributor mechanical advance to get to 36? Or take her out and see if maybe she likes 40?
Are you running a vacuum advance? If so, make sure you are plugged into manifold vacuum, not ported advance as you have probably been told (as I was). With the vaccum adavance plugged in, set the timing at around 30. It should idle well there but use a vacuum gauge to get the highest vacuum reading you can get and it should be right. Benefits are that the initial will be low enough to start easily, then on start up, it will come back up to your 30 or so. Under throttle, the vacuum advance will fall off because of lower manifold vacuum but the mechanical advance will kick in. Total timing all in should end up around 36 or so. You can adjust accordingly to get to the magic 36 but it works.
 
Are you running a vacuum advance? If so, make sure you are plugged into manifold vacuum, not ported advance as you have probably been told (as I was). With the vaccum adavance plugged in, set the timing at around 30. It should idle well there but use a vacuum gauge to get the highest vacuum reading you can get and it should be right. Benefits are that the initial will be low enough to start easily, then on start up, it will come back up to your 30 or so. Under throttle, the vacuum advance will fall off because of lower manifold vacuum but the mechanical advance will kick in. Total timing all in should end up around 36 or so. You can adjust accordingly to get to the magic 36 but it works.
No vacuum advance. It's always plugged.
 
Hmmm, well I have a different viewpoint on this one. There certainly are times when manifold vacuum would be a good idea (.750" lift roller cams come to mind), but Mopar designed the "stock" engines to run with venturi/ported vacuum. How is your motor built? It's true that when you floor the pedal the vacuum advance stops almost immediately & almost completely (vacuum at WOT is maybe 2" in. Hg or so). The mechanical advance will help....eventually.... as the rpm's increase. If you floor it from idle, you'll notice.

Regardless, when setting the initial & total timing, the vacuum advance on the distributor should be blocked off while you're checking/setting the timing. I agree with Bill that you should set your initial timing (and idle fuel mixture for that matter) by maximizing the vacuum and rpm's at idle. That's how you know you hit the "sweet spot" in initial timing that your engine likes. Next, you rev the engine up slowly, while watching your timing marks, to see what your total advance is. 36-degrees +/- 2-degrees is typical total advance. Also note at what actual rpm you're "all in".

Now the fun stuff..... IF your total advance is more than 36-degrees, then you'll likely need to limit the total advance. You can do this with little advance limiter plates, built-in adjustment screws on some distributors or the old "weld up the advance slots & then file them away until you hit the limit you want" technique.

Then the good part.....now, if you like, you can take a look at your advance curve. This is basically swapping out the two mechanical advance springs stronger/weaker or adding/subtracting weight. That allows you to custom tailor exactly when & how much mechanical advance comes "all in".

Lastly, re-connect the vacuum advance (if you like) and see how the car drives.
 
Agree with monk&AR- but you'll find it runs better on the street with vac.advance hooked up. With the gas we have nowadays too quick mech.advance might ping.
 
Agree with monk&AR- but you'll find it runs better on the street with vac.advance hooked up. With the gas we have nowadays too quick mech.advance might ping.

Thanks guys. I will try hooking up the vacuum advance and see how she does. It's always been plugged since I got her and she runs fine, so I didn't want to mess with anything. But can't hurt to try it out.
 
I'm also a fan of vacuum advance. It smooth's out the idle (provided it's plugged into a non-ported outlet) and generally results in a cooler running engine. You will probably get a bit better fuel mileage also.

On the mechanical advance, the engine will produce more power with more advance up to what it can tolerate (doesn't ping or detonate) so the earlier it can be brought in, the more power the engine is making at low to mid-rpm. I like to have strong enough springs that the mechanical advance is locked down at idle and maybe a 100 rpm or so above, so the idle doesn't hunt around and the engine doesn't stumble with a manual transmission when taking off from a stop. But then you want it to come in fairly quickly as the engine can tolerate. On a race car they don't car about idle smoothness or street matters so they can just bring the advance in fast and make power. You can get a Mr. Gasket spring kit and experiment with it. Also you don't have to run the same springs on both weights. You can run a light spring and medium spring for instance - the cam coordinates the weights so they operate in unison. That gives a little more tuning flexibility.
 
JG remember that everything is a trade-off. My previous 440 had mech. only but a 4000 stall conv,4.88 rear and 2" Hookers; race only with a manual valve body that would jar ur teeth out on the street.Course the M&H slicks on the rear weren't really good on the street anyway!
 
First off, I want to thank everyone who has helped me on this board. It's the best site on the web, bar none. Here is my question. I just put a timing light on my engine for the time since I re-purchased my old car from college. 340 was running fine but wanted to know where I was at. I was at 6 initial. Way too low. I moved to 16 advanced. Still ran smoothly. But that moved my total advance to 40* all in at 3800 rpm. Is 40 too high? I've read 36* is optimal. Should I reduce initial to get to 36 or adjust distributor mechanical advance to get to 36? Or take her out and see if maybe she likes 40?

Every serious engine buff should have one of these under the cap. Firecore gives you stops when you buy their distributor so you can custom set your advance curve
 
Agree although I just checked my total adv(mech) on my Summit Mopar dist and ended up with 12* initial for a 38* total. With vacuum it's about 50* but not worried about it.
 
Hmmm, well I have a different viewpoint on this one. There certainly are times when manifold vacuum would be a good idea (.750" lift roller cams come to mind), but Mopar designed the "stock" engines to run with venturi/ported vacuum. How is your motor built? It's true that when you floor the pedal the vacuum advance stops almost immediately & almost completely (vacuum at WOT is maybe 2" in. Hg or so). The mechanical advance will help....eventually.... as the rpm's increase. If you floor it from idle, you'll notice.

Regardless, when setting the initial & total timing, the vacuum advance on the distributor should be blocked off while you're checking/setting the timing. I agree with Bill that you should set your initial timing (and idle fuel mixture for that matter) by maximizing the vacuum and rpm's at idle. That's how you know you hit the "sweet spot" in initial timing that your engine likes. Next, you rev the engine up slowly, while watching your timing marks, to see what your total advance is. 36-degrees +/- 2-degrees is typical total advance. Also note at what actual rpm you're "all in".

Now the fun stuff..... IF your total advance is more than 36-degrees, then you'll likely need to limit the total advance. You can do this with little advance limiter plates, built-in adjustment screws on some distributors or the old "weld up the advance slots & then file them away until you hit the limit you want" technique.

Then the good part.....now, if you like, you can take a look at your advance curve. This is basically swapping out the two mechanical advance springs stronger/weaker or adding/subtracting weight. That allows you to custom tailor exactly when & how much mechanical advance comes "all in".

Lastly, re-connect the vacuum advance (if you like) and see how the car drives.
Mother also recommended initial timing at 5 btdc and some at tdc but we all know that doesn't work for optimal performance. All I can tell you is that manifold vacuum works for me....but hey, if yours starts and runs like a scalded dog, who can argue with that? By the way, the limiter plate from 4secondsflat is cheap and works great.
 
Mother also recommended initial timing at 5 btdc and some at tdc but we all know that doesn't work for optimal performance. All I can tell you is that manifold vacuum works for me....but hey, if yours starts and runs like a scalded dog, who can argue with that? By the way, the limiter plate from 4secondsflat is cheap and works great.
Thanks for all of the input. I checked again and I don't go all-in until about 3800 rpm. I must have a slow advance curve. I will try hooking up my vacuum advance (I'm gonna use ported not manifold) and see how she drives and what total timing moves to. I imagine it will go to 50* or so, at which point I will lower mechancial advance in distributor to reach 36* total. From all I've read, I'd like to have vacuum advance hooked up even if it means tinkering with my distributor (something I've never done and am a bit intimidated about). Thanks again.
 
Thanks for all of the input. I checked again and I don't go all-in until about 3800 rpm. I must have a slow advance curve. I will try hooking up my vacuum advance (I'm gonna use ported not manifold) and see how she drives and what total timing moves to. I imagine it will go to 50* or so, at which point I will lower mechancial advance in distributor to reach 36* total. From all I've read, I'd like to have vacuum advance hooked up even if it means tinkering with my distributor (something I've never done and am a bit intimidated about). Thanks again.
Good luck, we're here for ya brother
 
Thanks for all of the input. I checked again and I don't go all-in until about 3800 rpm. I must have a slow advance curve. I will try hooking up my vacuum advance (I'm gonna use ported not manifold) and see how she drives and what total timing moves to. I imagine it will go to 50* or so, at which point I will lower mechancial advance in distributor to reach 36* total. From all I've read, I'd like to have vacuum advance hooked up even if it means tinkering with my distributor (something I've never done and am a bit intimidated about). Thanks again.

You'll get it. I would tend to start at lower advance and work my way up instead of the other way around to avoid ping/detonation/damage, but you can do it that way if you're not too rough with it while tuning. And don't be intimidated by distributors. Once you get inside you'll see they have like 6 parts total in them....piece of cake.
 
Thanks for all of the input. I checked again and I don't go all-in until about 3800 rpm. I must have a slow advance curve. I will try hooking up my vacuum advance (I'm gonna use ported not manifold) and see how she drives and what total timing moves to. I imagine it will go to 50* or so, at which point I will lower mechancial advance in distributor to reach 36* total. From all I've read, I'd like to have vacuum advance hooked up even if it means tinkering with my distributor (something I've never done and am a bit intimidated about). Thanks again.
Speaking of Distributors, I don't think you mentioned what type was in your engine? Some of them, including the Mopar performance replacement distributors, have an adjustable plate already in them so it's really not hard to make adjustments if needed... here's a picture of a gutted MP unit, you can adjust the amount of advance by loosening those two Torx head screws, they are connected to a plate underneath that will slide and close the holes up some then just retighten them... by the way you don't need to gut the whole distributor to do this,
20170427_163617.jpg
this is an old one I was using for parts at one time
 
Thanks for all of the input. I checked again and I don't go all-in until about 3800 rpm. I must have a slow advance curve. I will try hooking up my vacuum advance (I'm gonna use ported not manifold) and see how she drives and what total timing moves to. I imagine it will go to 50* or so, at which point I will lower mechancial advance in distributor to reach 36* total. From all I've read, I'd like to have vacuum advance hooked up even if it means tinkering with my distributor (something I've never done and am a bit intimidated about). Thanks again.
If you hook to a ported vacuum opening you will lose some of the benefits of having vacuum advance - smoother, cleaner idle, and cooler engine temps in stop and go traffic. The engine can tolerate the advance at idle since it isn't under load. And some of the vacuum advance units are adjustable with an allen wrench so you can reduce or increase the total amount of vacuum advance without having to mess with the internals of the distributor. It sounds like you currently have 24 degrees mechanical advance set in your distributor which isn't bad. That's what I have set up in my old Corvette with mostly L88 parts in the BB. I installed a rebuilt NAPA distributor in my 440 years ago and it had 30 degrees - bad, bad, bad. I welded up the advance plate and ended up with 20 mech and 16 initial which has been working well with vacuum advance (stock engine except 800 Edelbrock).
 
If you hook to a ported vacuum opening you will lose some of the benefits of having vacuum advance - smoother, cleaner idle, and cooler engine temps in stop and go traffic. The engine can tolerate the advance at idle since it isn't under load. And some of the vacuum advance units are adjustable with an allen wrench so you can reduce or increase the total amount of vacuum advance without having to mess with the internals of the distributor. It sounds like you currently have 24 degrees mechanical advance set in your distributor which isn't bad. That's what I have set up in my old Corvette with mostly L88 parts in the BB. I installed a rebuilt NAPA distributor in my 440 years ago and it had 30 degrees - bad, bad, bad. I welded up the advance plate and ended up with 20 mech and 16 initial which has been working well with vacuum advance (stock engine except 800 Edelbrock).
Agreed! Edlebrock is also recommending manifold vacuum for non emissions vehicles and ported for emissions on their carbs. The logic on the emissions vehicles on the ported advance is to have a retarded initial timing. For performance, manifold vacuum is the way to go...period. No doubt there will be many that don't agree but like I said, if it works for you then who am I to argue!
 
I'm also a fan of vacuum advance. It smooth's out the idle (provided it's plugged into a non-ported outlet) and generally results in a cooler running engine. You will probably get a bit better fuel mileage also.
Only if your cam is mild enough to pull enough vac at idle to pull the can in. I have a motor that idles at 8" vac and hooking it to manifold vac won't work.
 
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