Which aftermarket distributor to buy, 351C 4V

[Stanglover]

The vacuum advance is not figured in the total advance equation. Vacuum advance should only be active during part throttle, high vacuum low load engine operation. The fuel mixture is leaner at part throttle. Sometimes as lean as 15-17:1. Leaner mixtures require the spark plug to fire earlier to properly ignite than richer mixtures due to the greater percentage of oxygen. The vacuum advance provides the earlier spark firing and makes it possible to run leaner mixtures thus improving fuel economy. When you put your foot in it, at about 70% of load, the mixture needs to richen (power valve open or metering rod retracted) to provide power and prevent engine damage and the additional spark advance is no longer needed. This is where the vacuum the engine makes will begin to drop and where the vacuum advance should no longer be adding advance. The exact vacuum reading where this should occur varies from engine to engine. Generally a high compression high performance engine will want to be rich and retarded sooner than a low compression low performance engine.

As to how much vacuum advance should be added, in general you should run as much as the engine will tolerate without pinging. This takes some trial and error to determine.

Thanks TommyK, this explains a lot. So if I'm getting it right, I'll set up for 34-36* total mechanical with an initial setting of 14-16* ie L10 slot = 20* + 14-16*, vacuum disconnected and plugged. Then adjust the vac can to add no more than another 5-6* depending on pinging. (I have dropped the comp ratio to about 10:1) So if my timing light sees 42* WITH vac advance, I'm still OK??? My understanding was that I should see no more than 36* WITH vac advance, but that would mean setting my initial at 10* or less. Factory spec for the '71 351C 4V manual trans is 6* initial, which is where I'm at right now. I do not know at this point, what slot is being used, but I suspect an L15 as I have 36* without vac. I plan on stripping the reman and welding up 1 slot and go from there. This is why I spent a few bucks on another dist, either for parts or to learn with and allow me to drive the car while I'm getting other stuff done.

Am I on the right track now??

Thanks again,

Geoff

 

[TommyK]

The vacuum advance is not figured in the total advance equation. Vacuum advance should only be active during part throttle, high vacuum low load engine operation. The fuel mixture is leaner at part throttle. Sometimes as lean as 15-17:1. Leaner mixtures require the spark plug to fire earlier to properly ignite than richer mixtures due to the greater percentage of oxygen. The vacuum advance provides the earlier spark firing and makes it possible to run leaner mixtures thus improving fuel economy. When you put your foot in it, at about 70% of load, the mixture needs to richen (power valve open or metering rod retracted) to provide power and prevent engine damage and the additional spark advance is no longer needed. This is where the vacuum the engine makes will begin to drop and where the vacuum advance should no longer be adding advance. The exact vacuum reading where this should occur varies from engine to engine. Generally a high compression high performance engine will want to be rich and retarded sooner than a low compression low performance engine.

As to how much vacuum advance should be added, in general you should run as much as the engine will tolerate without pinging. This takes some trial and error to determine.

Thanks TommyK, this explains a lot. So if I'm getting it right, I'll set up for 34-36* total mechanical with an initial setting of 14-16* ie L10 slot = 20* + 14-16*, vacuum disconnected and plugged. Then adjust the vac can to add no more than another 5-6* depending on pinging. (I have dropped the comp ratio to about 10:1) So if my timing light sees 42* WITH vac advance, I'm still OK??? My understanding was that I should see no more than 36* WITH vac advance, but that would mean setting my initial at 10* or less. Factory spec for the '71 351C 4V manual trans is 6* initial, which is where I'm at right now. I do not know at this point, what slot is being used, but I suspect an L15 as I have 36* without vac. I plan on stripping the reman and welding up 1 slot and go from there. This is why I spent a few bucks on another dist, either for parts or to learn with and allow me to drive the car while I'm getting other stuff done.

Am I on the right track now??

Thanks again,

Geoff

34-36 degrees of total timing (initial + mechanical only) all in between 2500-3000 rpm should be safe. 6 additional degrees of vacuum advance on top of that should also be a safe place to start. I ended up with about 10 degrees of vacuum advance. Anymore than that and my engine would ping going up hills at light throttle.

 

[Stanglover]

The vacuum advance is not figured in the total advance equation. Vacuum advance should only be active during part throttle, high vacuum low load engine operation. The fuel mixture is leaner at part throttle. Sometimes as lean as 15-17:1. Leaner mixtures require the spark plug to fire earlier to properly ignite than richer mixtures due to the greater percentage of oxygen. The vacuum advance provides the earlier spark firing and makes it possible to run leaner mixtures thus improving fuel economy. When you put your foot in it, at about 70% of load, the mixture needs to richen (power valve open or metering rod retracted) to provide power and prevent engine damage and the additional spark advance is no longer needed. This is where the vacuum the engine makes will begin to drop and where the vacuum advance should no longer be adding advance. The exact vacuum reading where this should occur varies from engine to engine. Generally a high compression high performance engine will want to be rich and retarded sooner than a low compression low performance engine.

As to how much vacuum advance should be added, in general you should run as much as the engine will tolerate without pinging. This takes some trial and error to determine.

Thanks TommyK, this explains a lot. So if I'm getting it right, I'll set up for 34-36* total mechanical with an initial setting of 14-16* ie L10 slot = 20* + 14-16*, vacuum disconnected and plugged. Then adjust the vac can to add no more than another 5-6* depending on pinging. (I have dropped the comp ratio to about 10:1) So if my timing light sees 42* WITH vac advance, I'm still OK??? My understanding was that I should see no more than 36* WITH vac advance, but that would mean setting my initial at 10* or less. Factory spec for the '71 351C 4V manual trans is 6* initial, which is where I'm at right now. I do not know at this point, what slot is being used, but I suspect an L15 as I have 36* without vac. I plan on stripping the reman and welding up 1 slot and go from there. This is why I spent a few bucks on another dist, either for parts or to learn with and allow me to drive the car while I'm getting other stuff done.

Am I on the right track now??

Thanks again,

Geoff

34-36 degrees of total timing (initial + mechanical only) all in between 2500-3000 rpm should be safe. 6 additional degrees of vacuum advance on top of that should also be a safe place to start. I ended up with about 10 degrees of vacuum advance. Anymore than that and my engine would ping going up hills at light throttle.

Ok, sounds like I am getting close to understanding it all. At 6* initial, it is a bit 'flat' and blogs a bit when I stomp on it. I KNOW I will need to do some carb tweaking as well, but one thing at a time, get the timing closer and better responding, then the carb. My plug read looks close, so I'm not too concerned about jetting.

I'll post the results later when I get the work done, which could be a few days.

 

[TommyK]

I think your target of 14-16 degrees of initial timing will work well. In general I like to use as much initial timing as my engine will tolerate. That means as much as I can dial in and still have the engine start reliably on a hot day when fully heat soaked. This will help get the t slot exposure correct with a reasonable idle speed which is important for off idle and part throttle operation.

 

[Stanglover]

I think your target of 14-16 degrees of initial timing will work well. In general I like to use as much initial timing as my engine will tolerate. That means as much as I can dial in and still have the engine start reliably on a hot day when fully heat soaked. This will help get the t slot exposure correct with a reasonable idle speed which is important for off idle and part throttle operation.

Thanks for the continuing support.

WHEN it stops raining, I'll get at it. I do now have a much better understanding. Hope others will benefit as well.

Thanks my friend,

Geoff.

 

[Stanglover]

UPDATE:

I just stripped and took the rust, yes rust, out of my so-called reman distributor that I have had in the car for about 5 years. There is no way it could have got water in it between the cam and center shaft. Perhaps this is why it always was trouble, The base plate was also rusty, while the cam plate, with the slots, was bright and clean. I took it to my buddy and got the L13 slot welded so I could change it to an L10, .410" = 20*. Next I changed back to the PerTronix III I have to eliminate the points. I also used a set of 925D springs and set to equal tension as best I could. Set the base timing to 16* vac off, but I can't rev it at home, so took for a drive. Got some pinging, backed off the timing , tried again, still pinging, set it back some more. Now just light pinging on hard throttle, but timing is back to 6* initial with 10* on the vac can. I need to take the car someplace where I can rev the snot out of it and no complaints from neighbors!!

QUESTION: Do I need different springs or tension? I'm definitely getting better with this, but not sure what I should do next. I am only running 10:1 pistons, but would Octane Boost still be needed?

All the basic stuff seems OK, plug read etc, rpm @ 850 idle (hot).

Geoff.

 

[TommyK]

UPDATE:

I just stripped and took the rust, yes rust, out of my so-called reman distributor that I have had in the car for about 5 years. There is no way it could have got water in it between the cam and center shaft. Perhaps this is why it always was trouble, The base plate was also rusty, while the cam plate, with the slots, was bright and clean. I took it to my buddy and got the L13 slot welded so I could change it to an L10, .410" = 20*. Next I changed back to the PerTronix III I have to eliminate the points. I also used a set of 925D springs and set to equal tension as best I could. Set the base timing to 16* vac off, but I can't rev it at home, so took for a drive. Got some pinging, backed off the timing , tried again, still pinging, set it back some more. Now just light pinging on hard throttle, but timing is back to 6* initial with 10* on the vac can. I need to take the car someplace where I can rev the snot out of it and no complaints from neighbors!!

QUESTION: Do I need different springs or tension? I'm definitely getting better with this, but not sure what I should do next. I am only running 10:1 pistons, but would Octane Boost still be needed?

All the basic stuff seems OK, plug read etc, rpm @ 850 idle (hot).

Geoff.

First, I will assume that you are using the highest octane pump gas available in your area. You will need it.

You need to map the curve that is in the distributor to see exactly what you have.

Disconnect and plug the vacuum advance.

Hook up you timing light.

Start the car.

Using the curb idle adjustment screw increase the idle to 2500 rpm. Note the timing. Increase the idle to 3000 rpm. Note the timing. The goal here is to identify at what rpm max advance is achieved and what that advance is.

IMO ideally it should be 34-36 degrees at 3000 +/- rpm to start with. You will need to tweak the initial timing and the tension on the springs to achieve this.

Leave vacuum advance disconnected for now.

Test drive.

Report back.

Edit: Typically one of the oem heavy springs and one of the light Mr. G springs will get you close to the correct curve. Since it seems you are using two light Mr. G springs, I suspect the mechanical advance is coming in too soon. Hence my suggested course of action above.

Don't remember what cam you have but you might want to also check to see what your cranking cylinder pressure is. This plays a big role in detonation tolerance.

 

[Stanglover]

UPDATE:

I just stripped and took the rust, yes rust, out of my so-called reman distributor that I have had in the car for about 5 years. There is no way it could have got water in it between the cam and center shaft. Perhaps this is why it always was trouble, The base plate was also rusty, while the cam plate, with the slots, was bright and clean. I took it to my buddy and got the L13 slot welded so I could change it to an L10, .410" = 20*. Next I changed back to the PerTronix III I have to eliminate the points. I also used a set of 925D springs and set to equal tension as best I could. Set the base timing to 16* vac off, but I can't rev it at home, so took for a drive. Got some pinging, backed off the timing , tried again, still pinging, set it back some more. Now just light pinging on hard throttle, but timing is back to 6* initial with 10* on the vac can. I need to take the car someplace where I can rev the snot out of it and no complaints from neighbors!!

QUESTION: Do I need different springs or tension? I'm definitely getting better with this, but not sure what I should do next. I am only running 10:1 pistons, but would Octane Boost still be needed?

All the basic stuff seems OK, plug read etc, rpm @ 850 idle (hot).

Geoff.

First, I will assume that you are using the highest octane pump gas available in your area. You will need it.

You need to map the curve that is in the distributor to see exactly what you have.

Disconnect and plug the vacuum advance.

Hook up you timing light.

Start the car.

Using the curb idle adjustment screw increase the idle to 2500 rpm. Note the timing. Increase the idle to 3000 rpm. Note the timing. The goal here is to identify at what rpm max advance is achieved and what that advance is.

IMO ideally it should be 34-36 degrees at 3000 +/- rpm to start with. You will need to tweak the initial timing and the tension on the springs to achieve this.

Leave vacuum advance disconnected for now.

Test drive.

Report back.

Edit: Typically one of the oem heavy springs and one of the light Mr. G springs will get you close to the correct curve. Since it seems you are using two light Mr. G springs, I suspect the mechanical advance is coming in too soon. Hence my suggested course of action above.

Don't remember what cam you have but you might want to also check to see what your cranking cylinder pressure is. This plays a big role in detonation tolerance.

Thanks TommyK, I was hoping you'd chime in. I am using Shell 91, no ethanol, best I can get without ethanol here. It is fresh gas.

I was thinking that I might need 1 stronger spring as this was the original set-up. I'll do as you suggest tomorrow. It's easy to change things now I got rid of the points. Does it matter which side I use a stronger spring?

The cam is a Melling MTF2, which is just slightly better than stock.

I'll get back later. Thanks so much for your clear approach so even I can understand it!!

Geoff.

 

[TommyK]

With a 10:1 SCR that cam is going to produce some fairly high cylinder pressure.

On 91 octane it may be a challenge to get the timing dialed in and not ping.

 

[Stanglover]

With a 10:1 SCR that cam is going to produce some fairly high cylinder pressure.

On 91 octane it may be a challenge to get the timing dialed in and not ping.

I'll play with the curve an bit more before adding O/boost, but you may be right. The whole idea of giving up performance, was to get something that would run on straight pump gas, but still be a strong runner.

Next step of course will be a compression pressure check now that the rings should be seated, I hope! Not as easy now I have a power booster and power steering stuff in the way on the left side.

I'll work on it later today if I get time and report back. Like I said, I do feel I'm getting a grip on how to get what I want, so hopefully it's just a matter of working with it.

Thanks Tommyk,

Geoff.

 

[Stanglover]

Tom, Here's where I'm at right now.

Timing is now at 15* initial no vac. 25* with vac. 34-35* @ 3000 but I missed checking it at 2500, so I need to re check again and because of the situation, I didn't get to drive it with vac off.

Today I did a cylinder pressure test, and put in Autolite 25's,( instead of 24's),gapped at .050" as I have the Pertronix Flamethrower coil @45,000 V. It ran smoother than it has for a long time. After a quick trip around the block, country roads, it seemed a bit lean, but as I didn't drive it very far, hard to tell. Still minor pinging on hard throttle, so I may need to change tension on the springs or put the OE heavy spring back in. I did change 1 spring that did help.

Cylinder pressures were: 1- 180psi, 2- 190, 3-190, 4-190, 5-192, 6-185, 7-187, 8-195. Don't know why #1 was so low. I thought #4 would be as it was fired up for cam break-in with a dead plug (I didn't do it!!) Anyway, good to know that cylinder is NOT damaged. The initial pressure given at build time was 165 psi.

I'm getting close and enjoying learning. Next will be Octane boost as I'm off to a show on Sunday, 50 miles each way, so I should get a good feel for performance and mileage.

Thanks for you continued help Tom,

Geoff.

Did I mention that the new pistons are KB148's 13cc dish top, but zero deck. The one's I took out were KB177 flat top's also at zero deck.

 

[midlife]

At idle, you should have no vacuum and your timing should be the same as with the vacuum plugged. Either I've mis-interpreted things, you've tapped the wrong vacuum port, or your curb idle speed is too high.

 

[TommyK]

At idle, you should have no vacuum and your timing should be the same as with the vacuum plugged. Either I've mis-interpreted things, you've tapped the wrong vacuum port, or your curb idle speed is too high.

I agree with Randy. For your engine with the way your distributor is set up I would use ported vacuum, not full manifold vacuum.

Actually because you are still trying to get the mechanical advance curve dialed in I would leave it disconnected for testing purposes. Once you have the mechanical timing curve dialed in you can reconnect it to a ported vacuum source and get the vacuum advance dialed in.

 

[Stanglover]

At idle, you should have no vacuum and your timing should be the same as with the vacuum plugged. Either I've mis-interpreted things, you've tapped the wrong vacuum port, or your curb idle speed is too high.

Thanks. To be clear, I am checking with no vacuum and plugged at idle. It is connected to the base port on the carb, NOT timed port. Idle speed is around 850 rpm. That's not the problem, getting the curve right is and that takes trial & error.

Geoff.

 

[Stanglover]

At idle, you should have no vacuum and your timing should be the same as with the vacuum plugged. Either I've mis-interpreted things, you've tapped the wrong vacuum port, or your curb idle speed is too high.

I agree with Randy. For your engine with the way your distributor is set up I would use ported vacuum, not full manifold vacuum.

Actually because you are still trying to get the mechanical advance curve dialed in I would leave it disconnected for testing purposes. Once you have the mechanical timing curve dialed in you can reconnect it to a ported vacuum source and get the vacuum advance dialed in.

Tom, I had not read your response before answering Randy's, As for ported versus non-ported vacuum source, there continues to be much debate on the subject.

I have an article by Lars Grimsrud, Timing and Vacuum Advance 101 in which he states that ported vacuum was solely an emissions thing. I'm not quoting anything, but in essence he calls ported vacuum an "aberration". Hmmm!

As for my case, I will be working on it again today. I am going to try that OE spring again with 1 925D springs and I will plug off the vacuum source till I'm closer. I think my vac can is close to pulling the amount of vacuum advance I'll need, so I can then check it ported and non-ported and see which runs better. I'm close now, it's just a matter of fine tuning it. Also, I did buy a Cardone Select distributor as a back-up. Budget is shot for now, so I'm not about to drop 300 or more on a better brand.

I'll get back later with the results,

Thanks again to all who have helped me understand the mystery's of "timing advance"

Geoff.

 

[Don C]

I've always used manifold vacuum, for various reasons, including driveability and controlling pre-ignition. Back in my racing days I would run straight mechanical advance for races.

 

[Stanglover]

In one word Eureka!!! I put the OE heavy spring back in plus 1 of the Mr.G 925D's, changed nothing else, but ran it without vacuum. Shortening the advance slot was key to getting the curve needed without doubt. The engine has NEVER run better, but I will need to play with secondary spring and or jets to get it even better.

Next will be Octane Boost as I've got to fill it up anyway. There is still a hint of ping under certain throttle conditions. If it goes away, great. If not I can still play with timing.

Awesome help from everybody, Thanks.

Geoff.

 

[Don C]

Perseverance won again. Good job.

 

[TommyK]

That's great!

Run around on the octane booster for a while and see how she drives.

I think if you wanted to invest a little more time you could get it to where you wouldn't need the octane booster anymore.

One word of caution on the octane booster. There are very few out there that actually raise the octane appreciably. You need to read the manufacturer info carefully. Some claim to raise octane by 2 or more "points". An unsuspecting buyer thinks this means 2 octane numbers when in fact it is .2 octane numbers.

 

[Stanglover]

That's great!

Run around on the octane booster for a while and see how she drives.

I think if you wanted to invest a little more time you could get it to where you wouldn't need the octane booster anymore.

One word of caution on the octane booster. There are very few out there that actually raise the octane appreciably. You need to read the manufacturer info carefully. Some claim to raise octane by 2 or more "points". An unsuspecting buyer thinks this means 2 octane numbers when in fact it is .2 octane numbers.

Yes, it is still a work in progress, but I'm close enough I won't do any damage to the motor.

As for O/boost, yes I have that info. Funny thing is I have tried $16 a bottle stuff and I have tried $2 "Dollar Store" SBM? brand that is made in the US, with the same results, a marginal improvement. I guess you pay for marketing hype. Anyway, got a show tomorrow so I'll be putting over 100 miles on it, so I will get a good read on the performance and mileage.

Geoff.