Base Timing Questions

[Bewy]

MV8,
Post #20.

I am replying to your post first. I used many Chrys pick ups over the years with GM 4 pin modules & never had a problem. The Chrys, GM & MSD pickups generate a small AC voltage [ millivolts at cranking speed ] which triggers the module. As rpms increase, this voltage also increases to a few volts. Module doesn't know or care about the name on the p/up....they all trigger fine. The adjustable air gap on MSD & Ch p/ups changes the voltage; smaller the gap, larger the voltage generated.
You do NOT want a series resistor in the p/up line. If using the Ch p/up, one wire will be black; it goes to the 'G' terminal on the module.

The current limiter in the 4 pin module is to limit coil current, NOT p/up current.

 

[Bewy]

VA is being confused with VS.
VA means vacuum advance & it is that pod on the side of the dist. So ign related.
VS refers to carburetion. Some Holley carbs [ & clones ] open the secondary throttle blades by using engine vacuum.
An engine with 12:1 comp ratio will need less idle & WOT timing than a 9:1 CR engine, all else being equal.

 

[340inabbody]

VA is being confused with VS.
VA means vacuum advance & it is that pod on the side of the dist. So ign related.
VS refers to carburetion. Some Holley carbs [ & clones ] open the secondary throttle blades by using engine vacuum.
An engine with 12:1 comp ratio will need less idle & WOT timing than a 9:1 CR engine, all else being equal.

Understood about needing less advance for higher compression set ups. I did a bunch of reading on this and it makes sense. Thank you.
Regarding VA vs VS was confused. For some reason I thought they required each other as a system not being independent of each other. I now realize that you can operate one and/or the other and they are independent…….

 

[4spdragtop]

 

[MV8]

MV8,
Post #20.

I am replying to your post first. I used many Chrys pick ups over the years with GM 4 pin modules & never had a problem. The Chrys, GM & MSD pickups generate a small AC voltage [ millivolts at cranking speed ] which triggers the module. As rpms increase, this voltage also increases to a few volts. Module doesn't know or care about the name on the p/up....they all trigger fine. The adjustable air gap on MSD & Ch p/ups changes the voltage; smaller the gap, larger the voltage generated.
You do NOT want a series resistor in the p/up line. If using the Ch p/up, one wire will be black; it goes to the 'G' terminal on the module.

The current limiter in the 4 pin module is to limit coil current, NOT p/up current.

Thanks for the feedback. You are right; I was thinking of the coil side limiter. I've installed these modules on fords and bikes and never had this issue before. A couple things I have not checked are if I am getting the necessary threshold voltages needed for switching the coil off and on at terminal G and the bias voltage at W from the module. Bottom line is no spark cranking including with a jumper from the batt to eliminate any volt drops. I need some time to tinker.

 

[MV8]

Ok, so I got off my rear. The distributor installed is a new china that had a bad pickup. I had replaced with a NOS chrysler pickup that was 300 ohm, but had trouble with maintaining the reluctor gap because the Y clip that holds the plate down was broken and I had to improvise. Wasted more time on that. I ordered an actual chrysler distributor that was a reman and had not gotten around to testing until today. I also ordered a new china small cap hei they have now thinking I would swap the male terminal cap for the standard female cap and keep my new superstock plug wires I put together. Nothing interchanges.

I checked W and found the hei putting out 1.31vdc bias (typical 1.25vdc but it varies), spun the engine over and G showed a little less through the winding (1.25vdc) but did not fluctuate.

Plugged in the reman (426 ohm pickup), spun it by hand and got a fluctuating voltage on G plus I could hear it arcing inside the installed distributor's cap. Stay away from China junk and I don't mean the boat.

I apologize for the thread hijack and will try to stop doing that.

 

[340inabbody]

What is your thinking as to why I shouldn’t use a vacuum secondary on this set up?

OK, I’ve read some of crackedback previous threads on this. Seems like he’s more concerned about power than drivability. I could be wrong and don’t want to speak for him. But I understand his point that you can probably make a mechanical secondary work just as well as a vacuum, secondary with tuning and I won’t argue that. The question is am I capable of all tuning and would vacuum secondary be, more useful to me with my application.

For me, I’m trying to make the best of what I have. And what I have is not ideal at all. I have an automatic with an engine that was really designed for a four-speed racecar. And I’m trying to tame it. The engine has a 12 to 1 compression ratio And I do not have the cam specs. I’ve gotten this to run before and it was fair. What I’m trying to do is to get it to run better. I am willing to sacrifice 25% power for drivability if need be, I have on hand a vacuum secondary carburetor , i can get my buy a vacuum advance distributor to work with my MSD 6AL. But for now I’m just trying to make what I have work better. And learn as much as I can from you guys thanks, Ron.

 

[RustyRatRod]

Can you get us some compression PSI measurements?

 

[AJ/FormS]

V-secs don't suck, lol.
IDK about @crackedback, but
When I stomp it, I'm stomping it cuz I want "warp-drive"...... N-o-w ! I am perfectly capable of mimicking a V-sec with my right foot. and with a combo like yours; fuel-economy was long ago put on ignore.
I tried them both on my Eddy-headed, 11.3/1, 367, and I really tried to like that 750 V-sec, but in the end, it ended up on the shelf with all the other also-ran stuff. Long live the 750DP! Having said that, if you already have a 4bbl carb, anything 750 or smaller, just run what you got, it will be fine.

and yes, a properly dialed in Vacuum Advance distributor, will make that hot motor so much more pleasurable to drive on the street. I feel so strongly about this, that no matter how broke I was, this would be my next purchase.
And the reason is;
that with an automatic, and ignition locked at 34 degrees, running on the Primaries, this modest amount of timing is always going to be retarded; anywhere from say 10 to 20/25 degrees, depending on load and rpm.
This is gonna cost you;
low-rpm performance, fuel-economy, spark-plugs, oil changes, and;
your headers may burn out, and/or your engine may run hot, and this can/will negatively affect the overlap cycle.

If you really are willing to sacrifice 25% power to get drivability, I suggest a much smaller cam . I went thru this very thing with my car over two decades ago. It's a way better idea than trying to tame a race engine.

EDIT
I reread this thread and have changed my mind.
For what you are trying to do, a V-sec or AVS-type carb would be better, and a spread-bore carb would be best. Something easily tuned, and for me, that means a TQ; but the Holley is fine, if you already have it..
But
I would do a compression test, and figure out what cam is in there, and probably swap it out, if it was possible, as regards cylinder pressure and stall-speed; and, no matter what, I would install a VA distributor.

 

[340inabbody]

What type of compression test should I do guys ie cold hot etc? And how this wrt tuning?

Down the road I plan to lower the compression with different pistons and change the cam. But not right now…

 

[Woodys_Cuda]

What type of compression test should I do guys ie cold hot etc? And how this wrt tuning?

Down the road I plan to lower the compression with different pistons and change the cam. But not right now…

The compression test is best done with a warmed up engine (google is your friend). What is wrt tuning?

I also forgot to mention that my distributor has an adjustable Vacuum Advance Canister, VAC. It's an older Accel for Mopar that I think can add up to 20 degrees.

I'm running an MSD 6AL ignition with my setup and it really helps the idle and my plugs are cleaner.

There is a love/hate relationship here with the MSD ignition. Mine is an older analog version that seems to be less problematic than the newer digital boxes.

 

[Newbomb Turk]

What type of compression test should I do guys ie cold hot etc? And how this wrt tuning?

Down the road I plan to lower the compression with different pistons and change the cam. But not right now…

Reading through post 34 I realized I had forgotten to mention another pitfall of locked timing is that you don’t know EXACTLY what your timing is.

All electronic ignition’s regard with rpm. If you lock out the timing and don’t account for retard the timing could be anything.

Of course, you can set total timing at max rpm but most guys just set it at idle and go.

Even if you do set total timing at max rpm you have no idea what your timing is between idle and any point that you check timing.

 

[340inabbody]

The compression test is best done with a warmed up engine (google is your friend). What is wrt tuning?

I also forgot to mention that my distributor has an adjustable Vacuum Advance Canister, VAC. It's an older Accel for Mopar that I think can add up to 20 degrees.

I'm running an MSD 6AL ignition with my setup and it really helps the idle and my plugs are cleaner.

There is a love/hate relationship here with the MSD ignition. Mine is an older analog version that seems to be less problematic than the newer digital boxes.

WRT means with respect to… sorry I use it a lot

 

[92b]

Reading through post 34 I realized I had forgotten to mention another pitfall of locked timing is that you don’t know EXACTLY what your timing is.

All electronic ignition’s regard with rpm. If you lock out the timing and don’t account for retard the timing could be anything.

Of course, you can set total timing at max rpm but most guys just set it at idle and go.

Even if you do set total timing at max rpm you have no idea what your timing is between idle and any point that you check timing.

Really it's the same process for locked timing as it is for unlocked timing. If want to know your timing curve you would check timing at each rpm interval just like you would with unlocked timing.

 

[Newbomb Turk]

Really it's the same process for locked timing as it is for unlocked timing. If want to know your timing curve you would check timing at each rpm interval just like you would with unlocked timing.

Yes. That’s how it SHOULD be done if you are using locked out timing.

But how many guys actually do it.

I was showing a guy (I was on a video call with my buddy’s kid) showing him what he was dealing with.

At 7200 he tapped out. He is shifting at 8k but he couldn’t tolerate zinging the engine that high for timing it.

And most guys won’t do it.

So they have no clue what their exact timing is.

If you really want to tip over your customers try this.

Pick an rpm that you think is pretty close to peak torque (just an example but you can do it at any load/rpm) and then park the engine there and by hand turn the distributor while watching torque and vacuum.

Once you find what the engine wants at that rpm/load you raise the rpm and do it again.

I only did that for one engine. Then I made a little handle to clamp to the distributor so I didn’t burn the hell out of my hands.

Of course, the handle got hot too but not as bad.

Now I have a 3 stage retard wired into the dyno ignition and I can sit at the desks and change the timing with a turn of the knob.

Waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay better than burning my hands doing it.

 

[340inabbody]

Yes. That’s how it SHOULD be done if you are using locked out timing.

But how many guys actually do it.

I was showing a guy (I was on a video call with my buddy’s kid) showing him what he was dealing with.

At 7200 he tapped out. He is shifting at 8k but he couldn’t tolerate zinging the engine that high for timing it.

And most guys won’t do it.

So they have no clue what their exact timing is.

If you really want to tip over your customers try this.

Pick an rpm that you think is pretty close to peak torque (just an example but you can do it at any load/rpm) and then park the engine there and by hand turn the distributor while watching torque and vacuum.

Once you find what the engine wants at that rpm/load you raise the rpm and do it again.

I only did that for one engine. Then I made a little handle to clamp to the distributor so I didn’t burn the hell out of my hands.

Of course, the handle got hot too but not as bad.

Now I have a 3 stage retard wired into the dyno ignition and I can sit at the desks and change the timing with a turn of the knob.

Waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay better than burning my hands doing it.

I have the MSD remote variable timing module inside my car. I have keep it centered but can get +/- 7° when needed at my fingertips…. It works really well and is accurately calibrated/marked..

 

[crackedback]

OK, I’ve read some of crackedback previous threads on this. Seems like he’s more concerned about power than drivability. I could be wrong and don’t want to speak for him. But I understand his point that you can probably make a mechanical secondary work just as well as a vacuum, secondary with tuning and I won’t argue that. The question is am I capable of all tuning and would vacuum secondary be, more useful to me with my application.

For me, I’m trying to make the best of what I have. And what I have is not ideal at all. I have an automatic with an engine that was really designed for a four-speed racecar. And I’m trying to tame it. The engine has a 12 to 1 compression ratio And I do not have the cam specs. I’ve gotten this to run before and it was fair. What I’m trying to do is to get it to run better. I am willing to sacrifice 25% power for drivability if need be, I have on hand a vacuum secondary carburetor , i can get my buy a vacuum advance distributor to work with my MSD 6AL. But for now I’m just trying to make what I have work better. And learn as much as I can from you guys thanks, Ron.

Drivability is in the tune whether it's a vac or mech secondary carb. If you can't tune one style, you can't tune the other.
Mileage, your right foot controls that. Stand on it all the time, mileage suffers. The primaries on the carb don't care how the secondaries open and in both cases your right foot controls that.

 

[340inabbody]

So another question I have is this: if too much base advance makes starting more difficult then doesn’t a vacuum advance distributor add in a bunch more advanced (full vacuum) at idle? Wouldn’t that make the engine more difficult to start or at start up the vacuum delay makes it not add in until it starts?

 

[crackedback]

Vacuum advance benefits in one state, cruise/constant state rpm driving. It's a mileage thing. Lean mixtures are hard to fire, so you have to give it a bunch more lead, enter the vac adv distributor. If you want to run the cruise at 15:1 or higher, by all means, get a vacuum advance distributor. Be VERY careful because that lean mixture, 12:1 compression, big cam, lots of advance may not play nice. It can be a headache getting it right and I've had cars that refused to accept any vacuum advance. As much as getting economy is a goal, I will not waste my time on certain deals that are potentially tempermental. It's a race engine, not a mileage master.

It shouldn't add any advance when starting because the can will require in excess of about 12-14" of vacuum to activate, unless it has a very generous adjustment in it. Where you hook it up, manifold or ported throws another wrench in that discussion.

You have a high winder and want to put a bunch of stuff on it that makes it a runner trying to run at the olympics breathing with a soda drink straw.

Getting to the Carb, I did 750 DP carb for a friend, 360 350hp/400tq, 3.00 gears IIRC, 904 auto with a 3K ish converter. Car got 20mpg on the freeway and ran around 108 in the 1/4.

 

[Newbomb Turk]

So another question I have is this: if too much base advance makes starting more difficult then doesn’t a vacuum advance distributor add in a bunch more advanced (full vacuum) at idle? Wouldn’t that make the engine more difficult to start or at start up the vacuum delay makes it not add in until it starts?

If you hook the VA to ported vacuum then at idle and very low throttle openings there won’t be any vacuum there.

If you do have vacuum on a ported port and it does have vacuum there the throttle blades are open too far.

If you have the VA hooked to manifold vacuum there won’t be any vacuum while cranking. Or not enough to pull timing.

 

[340inabbody]

Vacuum advance benefits in one state, cruise/constant state rpm driving. It's a mileage thing. Lean mixtures are hard to fire, so you have to give it a bunch more lead, enter the vac adv distributor. If you want to run the cruise at 15:1 or higher, by all means, get a vacuum advance distributor. Be VERY careful because that lean mixture, 12:1 compression, big cam, lots of advance may not play nice. It can be a headache getting it right and I've had cars that refused to accept any vacuum advance. As much as getting economy is a goal, I will not waste my time on certain deals that are potentially tempermental. It's a race engine, not a mileage master.

It shouldn't add any advance when starting because the can will require in excess of about 12-14" of vacuum to activate, unless it has a very generous adjustment in it. Where you hook it up, manifold or ported throws another wrench in that discussion.

You have a high winder and want to put a bunch of stuff on it that makes it a runner trying to run at the olympics breathing with a soda drink straw.

Getting to the Carb, I did 750 DP carb for a friend, 360 350hp/400tq, 3.00 gears IIRC, 904 auto with a 3K ish converter. Car got 20mpg on the freeway and ran around 108 in the 1/4.

I hear you loud and clear I can’t count how many threads I have read where you said “you’re right foot controls the mileage” lol.
I am only considering a vacuum advance to improve idle quality and overall drivability. So in that light does your opinion change at all? I know tuning will be a pain in the *** to try and accomplish what I’m trying to do. And I’m definitely not saying you’re wrong. I’m just trying to understand whether it’s worth moving in that direction.

 

[340inabbody]

If you hook the VA to ported vacuum then at idle and very low throttle openings there won’t be any vacuum there.

If you do have vacuum on a ported port and it does have vacuum there the throttle blades are open too far.

If you have the VA hooked to manifold vacuum there won’t be any vacuum while cranking. Or not enough to pull timing.

Respect you and crackedback a lot. I appreciate your inputs and help! But man you guys are on the opposite sides here, aren’t you? Lol would love to see you guys go at this directly lol and have me sit back and eat popcorn and learn what I can from the discourse.

 

[Newbomb Turk]

Respect you and crackedback a lot. I appreciate your inputs and help! But man you guys are on the opposite sides here, aren’t you? Lol would love to see you guys go at this directly lol and have me sit back and eat popcorn and learn what I can from the discourse.

I think we both agree that on a street car you need VA. Whether that is ported or manifold vacuum could be deter by how the engine is built.

At 12:1 on pump gas, I’m going to try and get VA to work off ported vacuum. I don’t see a scenario where I would need manifold vacuum at idle.

I’m also using an electric fuel pump with a return like and the regulator is boost/vacuum referenced.

Hooking manifold vacuum to the regulator will drop fuel pressure as manifold vacuum increases.

This will lean out the carb at a cruise.

I’m thinking between VA and the vacuum referenced regulator I’m hoping to get my cruise A/R about perfect.

Of course, I could be bat crap crazy and wasting my time.

 

[MopaR&D]

I hear you loud and clear I can’t count how many threads I have read where you said “you’re right foot controls the mileage” lol.
I am only considering a vacuum advance to improve idle quality and overall drivability. So in that light does your opinion change at all? I know tuning will be a pain in the *** to try and accomplish what I’m trying to do. And I’m definitely not saying you’re wrong. I’m just trying to understand whether it’s worth moving in that direction.

It depends on whether the engine wants MVA or not. I don't think tuning will be that hard, vacuum advance is the kind of thing that's best dialed in with driving. However the idle vacuum will be much lower than cruise so it may be a case where using MVA doesn't give much advance at idle unless the can is adjusted very "loose" (brings in advance at low vacuum) which may make it ping during cruise.

 

[Bewy]

340,
What is important to remember is that the engine will:
- idle best [ smoothest, highest rpm, highest vacuum ]
- use less fuel
- run cooler
- be most efficient
when it has a LOT of timing at idle.......more than the 10-12* the engine had on the showroom floor.
The actual amount will vary engine to engine, mainly dependant on cam duration/overlap & CR to a lesser extent. Some engines will need as much as 50*. MVA is one method of obtaining the reqd idle timing; it is used a lot because it is simple to hook up: effective, adjustable, reliable & load sensitive. Load sensitive is good, because under load the timing drops way. Using locked timing gives the reqd idle timing, but is there all the time, & does not drop off with load. When I hear comments like 'MVA' didn't work for me, 90% of the time it wasn't set up right. Then MVA gets the blame....

Good move getting a dist with VA. Make sure it is adjustable; it will have the hexagon end on the VA unit.
I could give you lots of good MVA stories, here is one....