Exactly!!
Vacuum advance information and myths
- Thread starter halifaxhops
- Start date
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Let's take your GTO example and apply it to modern unleaded fuel which burns faster and has lower octane than when you pulled up to the pumps in 1968. Hypothetically, what would you expect to have to do with the factory idle timing to survive on the fuel available today?
So you admit that Chevy didn't do what you claim all GM did but you continue to beleive it.
Wait! You don't beleive anything I've posted but you want me to tell you why your car is setup differently than the Chevy?
Keep repeating it, but that won't make it true.
Post 100. Lots of Kool-aid in there. Tidbits of information in a gonzo-journalist writing style. We already have seen the evidence that the assertions are not true. If you want to continue beleiving the rest, that's on you. Yea maybe the author was a corporate engineer at GM. Or maybe he worked for GM the same way I worked for Jaguar, which is a half truth. He clearly didn't work at Chevy in 1965.
You all have a good weekend.
Rat Bastid
Dunamis Metron
Go BUY the books and learn for yourself. I’m not posting **** for you.
You just aren’t worth my time.
Rat Bastid
Dunamis Metron
That’s been proven wrong. Many times.
No offense but you did not make any attempt to indulge any of the physics points in that last post from Bewy. If you are correct, you should be able to explain in detail why any of that is wrong. I was waiting for a response from your end to continue to provide thought provoking analysis of the conversation. The physics reasoning bevy provided in that post are correct from my experience. Rather than stomp and walk away because you think people don't BELIEVE you, you should be able to explain away any faults the post has.So you admit that Chevy didn't do what you claim all GM did but you continue to beleive it.
Wait! You don't beleive anything I've posted but you want me to tell you why your car is setup differently than the Chevy?
Keep repeating it, but that won't make it true.
Post 100. Lots of Kool-aid in there. Tidbits of information in a gonzo-journalist writing style. We already have seen the evidence that the assertions are not true. If you want to continue beleiving the rest, that's on you. Yea maybe the author was a corporate engineer at GM. Or maybe he worked for GM the same way I worked for Jaguar, which is a half truth. He clearly didn't work at Chevy in 1965.
You all have a good weekend.
I have already decided to test both theory for temp, mileage, fA ratio and manners this spring in a controlled study. As far as getting his Pontiac to run on new fuel with MV. My 67 300 10-1 runs perfect on today's fuel timed that way and I get over 17 mpg on the hwy.
Last edited:
Rat Bastid
Dunamis Metron
You claim how well your tune works but you’ve given no details.
How about how much initial you have with MVA, what your total is at WOT and the RPM that occurs.
Compression ratio.
Gear ratio.
Intake manifold.
Cam timing numbers.
With that information it can be determined if you can do something different that may (or may not) work better.
You won’t know that unless you consider at least that there may be other options.
Mattax,
Post #94 is correct & you continue to be wrong on the reason for the switch to PVA. So Chev used PVA on one engine. It could be because I believe PVA was used on some man trans cars, but later man & auto got MVA. What about the millions of others that used MVA? And then switched to PVA?
Post #103. Just more nonsense. How do you know Lars did NOT work for GM in 1965 or even earlier?????
Where is the PROOF that the assertions are not true?? Your earlier post which is just your opinion, but no supporting quote from an informed source like I provided???
Sorry, not getting the reference to my GTO. Are you saying it didn't use MVA?
From the 1967 Pontiac Service Manual, page 6E-31: 'The vacuum advance is connected directly to manifold vacuum advance so there is full vacuum advance at idle.
Post #94 is correct & you continue to be wrong on the reason for the switch to PVA. So Chev used PVA on one engine. It could be because I believe PVA was used on some man trans cars, but later man & auto got MVA. What about the millions of others that used MVA? And then switched to PVA?
Post #103. Just more nonsense. How do you know Lars did NOT work for GM in 1965 or even earlier?????
Where is the PROOF that the assertions are not true?? Your earlier post which is just your opinion, but no supporting quote from an informed source like I provided???
Sorry, not getting the reference to my GTO. Are you saying it didn't use MVA?
From the 1967 Pontiac Service Manual, page 6E-31: 'The vacuum advance is connected directly to manifold vacuum advance so there is full vacuum advance at idle.
440 Mike Post #102.
When my car was built, leaded fuel was in use which allowed higher comp ratios. Nothing needs to be done to the idle timing because the engine doesn't ping at idle! The centri curve &/or total timing at idle may need tweaking on a stock engine.
On the LS1 engines, WOT timing was about 27*. These engines idled at 22*BTDC!!!! So MVA is alive & well in modern FI engines!!
I have an article where two 1970s engines on PVA were converted to MVA. They were Pontiac 455 engines, one with man trans the other auto & the specific aim of the project was to see what changed with the switch from PVA to MVA. If you like, I could post the results, which were veeeery positive.
When my car was built, leaded fuel was in use which allowed higher comp ratios. Nothing needs to be done to the idle timing because the engine
On the LS1 engines, WOT timing was about 27*. These engines idled at 22*BTDC!!!! So MVA is alive & well in modern FI engines!!
I have an article where two 1970s engines on PVA were converted to MVA. They were Pontiac 455 engines, one with man trans the other auto & the specific aim of the project was to see what changed with the switch from PVA to MVA. If you like, I could post the results, which were veeeery positive.
Rat Bastid
Dunamis Metron
Reading comprehension is a real bugaboo for you.
I’m not posting any more books or articles that you won’t read anyway. And IF you did read them, you wouldn’t understand what you read.
The books are WELL KNOWN BOOKS. Some of them you claim you have and you claim you have read them.
Crack open a book and see what you can learn.
At your advanced age I figured you know this simple stuff already.
RB,
The reason you are not posting anything is because there is nothing to post. Full of baloney as usual.
You said in post #90, smart people build engines that do not need 30* of [ idle ] timing to run.
When I asked in a subsequent post what the 'formula' was, you said 'Math' in post #98.
Second time asking: what book as the Math?
I know I will get some bullshit answer because there is no book, no Math & never was...
The reason you are not posting anything is because there is nothing to post. Full of baloney as usual.
You said in post #90, smart people build engines that do not need 30* of [ idle ] timing to run.
When I asked in a subsequent post what the 'formula' was, you said 'Math' in post #98.
Second time asking: what book as the Math?
I know I will get some bullshit answer because there is no book, no Math & never was...
Initial timing (seen on the crank at idle), Mechanical Advance (measured at about 3000 RPM or when springs are fully extended), and Vacuum Advance all contribute to total advance which must vary based on engine speed and load. Best power on a SBM is usually achieved at 34° Crankshift advance. Detonation is the limiting factor, but maximum power (MPH in the quarter is a good measure) is the goal.
Using best power timing, know the mechanical advance amount in distributor degrees (usually found on the distributor advance plate shown in Distributor Degrees). Initial timing will then be Total Advance minus 2X Distributor Advance. I have seen electronic distributors with 11, 12, 14 and 15 degree advance plates which means 22 to 30 degree crankshaft advance. Taking the 15° advance plate example, total timing of 34 degrees less 30 degrees means initial advance of 4 degrees. This is fairly typical for many pre-pollution controlled cars.
To get more initial advance which often helps idle quality and engine idle temperature, you need to reduce the mechanical advance to say 11 distributor degrees. This will allow 12 degrees initial advance and 11X2 or 22 degrees distributor advance. The FBO advance limitor plate is a good way to handle this issue. FBO Mopar Distributor Limiter Plate
Total advance at cruise conditions is controlled by the vacuum advance canister. The degrees advance are on the arm inside the distributor so you need to remove the canister to see the value in distributor degrees. Double that value will be the crankshaft degrees the canister will deliver. The desired advance is that value which delivers best fuel economy without detonation on tip-in acceleration (usually the worst condition for detonation).
I set up my 340 this way back in the '70's during the Fuel Crisis and got 24 MPG out of the '68 Cuda with careful driving but still durned 14.5's with 2.94 gears.
Good luck with tuning your ride!
Using best power timing, know the mechanical advance amount in distributor degrees (usually found on the distributor advance plate shown in Distributor Degrees). Initial timing will then be Total Advance minus 2X Distributor Advance. I have seen electronic distributors with 11, 12, 14 and 15 degree advance plates which means 22 to 30 degree crankshaft advance. Taking the 15° advance plate example, total timing of 34 degrees less 30 degrees means initial advance of 4 degrees. This is fairly typical for many pre-pollution controlled cars.
To get more initial advance which often helps idle quality and engine idle temperature, you need to reduce the mechanical advance to say 11 distributor degrees. This will allow 12 degrees initial advance and 11X2 or 22 degrees distributor advance. The FBO advance limitor plate is a good way to handle this issue. FBO Mopar Distributor Limiter Plate
Total advance at cruise conditions is controlled by the vacuum advance canister. The degrees advance are on the arm inside the distributor so you need to remove the canister to see the value in distributor degrees. Double that value will be the crankshaft degrees the canister will deliver. The desired advance is that value which delivers best fuel economy without detonation on tip-in acceleration (usually the worst condition for detonation).
I set up my 340 this way back in the '70's during the Fuel Crisis and got 24 MPG out of the '68 Cuda with careful driving but still durned 14.5's with 2.94 gears.
Good luck with tuning your ride!
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 25,866
- Reaction score
- 13,175
Ok here's something for your perusal;
My alloy-headed 367 has had 3 cams in it since 1999. At .050 they were: 249, 223, and 230 now. The only things that I changed between those combos, was the Scr, and the Q, to keep the Dcr at about 9.0, and the pressure to, in the range of 180 to 185psi which I knew ran on 87E10 without detonation.(altho I ran the 223* at 194psi for several weeks).
All of them ran on the same 750DP.
The only changes made to it, was the size of the Idle-Air bypass holes that I drilled in the primary plates.
The only adjustment differences were to the amount of Transfer-slot exposure and a minor change in mixture screw adjustment.
They all ran the same distributor, and altho I tried many many different timing curves; I kept coming back to the same total-timing, the same initial-timing, and the same ported V-can.
--------------------------------
I like to idle thru the A&Ws (etc) at walking speed.
To help with this, I have a 3.09Low manual trans with 3.55s in the back, for a starter gear of 10.97 (this is like having 4.11s with a regular 2.66low trans). To get to 3.5mph, with this combo, requires the idle to be ~500rpm. The only way to do this with a Dcr of 9.0 (or better) without the chassis getting jumpy, is to retard the timing, until peak cylinder pressure occurs way down on the powerstroke, and the engine becomes smoother, and has just enough power to pull itself and none extra.
In my combo this comes at ~5* Idle timing.
But, it's normal idle-timing 12>14, with 20* in the Distributor-cam, for a maximum mechanical timing of 32>34*
The Vacuum advance is modified to deliver 22* .
The timing curve is a two-stage, with a kink in it, like the one Mattax posted.
The rate of advance is .8* per 100rpm, beginning at around 900rpm, and it's all finished advancing on the primary spring, at 2800; the amount being ~14>15 over base. So I usually see 28/29* @2800. From there to all in, then is ~4/5 more.
Knowing this, It is possible to graph this out., and see exactly how much timing the engine is seeing at any rpm. And, It is possible to set the Vcan to add it's timing to that amount. At low rpm with a big cam, the vacuum is low, so it pulls in only a small amount of V-can timing. As the throttle is opened, the engine vacuum rises, bringing the V-can timing with it, until about 2000>2400, where the vacuum peaks. and so my V-can is adjusted to roll in smoothly, and be all in at about that time.
Now then, my combo cruises at 65=2240 rpm in overdrive.
The mechanical timing will be ~11*, add the base 14* = ~25*, add the Vcan 22*, for a cruise timing of ~47*.
2240 in Second gear is 26mph and the Power timing is 25*, and you might think my 367 is gutless, but think again; (your 340 might be, lol that's joke guys). Well cruising at 2240 she has 47*. Dialing in the throttle bit by bit, it falls steadily to 25*. But because the engine makes 180psi CCP at cranking and who knows how much more at 2240, and is running in 1.92 x 3.55=6.82 Second gear, she has gobs and gobs and stinking GOBs, of torque and easily spins the 295s. Do I need more timing? Not hardly...
At 2800 in Second gear (33mph), the timing can be varied from 50>28, by the gas-pedal.
At 3400 in Second gear, (40mph), the timing can be varied from 56>34*
You can imagine my opinion of running a locked distributor ...on the street; and you can imagine my opinion of not running the Spark-Port.
>Oh but you say, it works the same.
Sure it does, but at different gas-pedal openings, because the Spark-port works differently than the Manifold-port. And with a manual trans, and a high-pressure design, it is easy to have too much power at idle/off-idle, and then it gets very hard to drive slowly, with a manual trans..
>To fill-in my modest timing curve, I have a spark-delay box in the cabin with an adjustable range of 15*. I set it in such a manner as to get up to 9* retard and 6* advance. This makes it possible to set the base timing from the cab, from ~14* down to about 5*, and the cruise timing from ~47 to ~53@2240 rpm. The engine likes cruise timing of up to 60* at 2850rpm, but with overdrive, that is pushing 85mph, which is well over the 68MPH max speed-limit in Manitoba.
Sure I could recurve my distributor one more time, and get rid of the Timing Box, but it's working so fine as it is.................
>Finally, the 292/292-108 cam was in my 11.3Scr version of this 367. It was none too excited to idle at 500 in gear and pulling itself. But it did pretty good at 550 (4mph), so I learned to toe the clutch a lil. This is one of the reasons I pulled that cam out after just a few weeks.
> Here's my point:
but my elevator goes to the top. And
I know that a 292/292/108 cammed, 11.3Scr 367, can be tuned to idle at 550 rpm in First-gear , and pulling itself , with just 5 measly degrees of Idle-Timing........ because I had one that did this. and
My [email protected] will do it at 500rpm@5*
Listen to what the engine is telling you....... Don't force it. Be patient.
BTW:
If you care to try it;
Just run your tach up to 2400 rpm and keep it there. Then, without regard to the actual numbers, advance your timing and return the fast-idle to 2400. Repeat until additional timing does not cause any increase in rpm. When you get there, read the timing at 2400rpm.
Don't be surprised to see numbers in the 40s or even 50s. That is the optimum timing for your engine at 2400 unloaded. Put the timing back.
You can do this at say every 400 rpm from 3600 down to idle, and plot the numbers on a graph, then connect the dots and smooth it out.
Now you have the Optimum no-load timing curve for your engine. Use it as a guide to build up your PowerTiming curve, to cruise-timing; plus/minus say 200rpm. Adjust the Idle Timing up or down to best make it happen. Remember, the closer your cruise timing is to optimum, the higher is your potential for best fuel-economy at that rpm.
What your Idle-Timing is relatively unimportant because it can NEVER be optimum anyway ...... because if you try to make it so, the Idle-Rpm will run away on you, ever higher, and the transfers will be closed or nearly so, and the mixture screws are likely to be WFO, in compensation. Oh sure, she's "idling" all rightee...... but just try putting her into Drive and taking off ....
Anyway
These are my experiences, offered for perusal. What anybody actually does/does not do, with this information is of no concern to me.
Happy HotRodding
My alloy-headed 367 has had 3 cams in it since 1999. At .050 they were: 249, 223, and 230 now. The only things that I changed between those combos, was the Scr, and the Q, to keep the Dcr at about 9.0, and the pressure to, in the range of 180 to 185psi which I knew ran on 87E10 without detonation.(altho I ran the 223* at 194psi for several weeks).
All of them ran on the same 750DP.
The only changes made to it, was the size of the Idle-Air bypass holes that I drilled in the primary plates.
The only adjustment differences were to the amount of Transfer-slot exposure and a minor change in mixture screw adjustment.
They all ran the same distributor, and altho I tried many many different timing curves; I kept coming back to the same total-timing, the same initial-timing, and the same ported V-can.
--------------------------------
I like to idle thru the A&Ws (etc) at walking speed.
To help with this, I have a 3.09Low manual trans with 3.55s in the back, for a starter gear of 10.97 (this is like having 4.11s with a regular 2.66low trans). To get to 3.5mph, with this combo, requires the idle to be ~500rpm. The only way to do this with a Dcr of 9.0 (or better) without the chassis getting jumpy, is to retard the timing, until peak cylinder pressure occurs way down on the powerstroke, and the engine becomes smoother, and has just enough power to pull itself and none extra.
In my combo this comes at ~5* Idle timing.
But, it's normal idle-timing 12>14, with 20* in the Distributor-cam, for a maximum mechanical timing of 32>34*
The Vacuum advance is modified to deliver 22* .
The timing curve is a two-stage, with a kink in it, like the one Mattax posted.
The rate of advance is .8* per 100rpm, beginning at around 900rpm, and it's all finished advancing on the primary spring, at 2800; the amount being ~14>15 over base. So I usually see 28/29* @2800. From there to all in, then is ~4/5 more.
Knowing this, It is possible to graph this out., and see exactly how much timing the engine is seeing at any rpm. And, It is possible to set the Vcan to add it's timing to that amount. At low rpm with a big cam, the vacuum is low, so it pulls in only a small amount of V-can timing. As the throttle is opened, the engine vacuum rises, bringing the V-can timing with it, until about 2000>2400, where the vacuum peaks. and so my V-can is adjusted to roll in smoothly, and be all in at about that time.
Now then, my combo cruises at 65=2240 rpm in overdrive.
The mechanical timing will be ~11*, add the base 14* = ~25*, add the Vcan 22*, for a cruise timing of ~47*.
2240 in Second gear is 26mph and the Power timing is 25*, and you might think my 367 is gutless, but think again; (your 340 might be, lol that's joke guys). Well cruising at 2240 she has 47*. Dialing in the throttle bit by bit, it falls steadily to 25*. But because the engine makes 180psi CCP at cranking and who knows how much more at 2240, and is running in 1.92 x 3.55=6.82 Second gear, she has gobs and gobs and stinking GOBs, of torque and easily spins the 295s. Do I need more timing? Not hardly...
At 2800 in Second gear (33mph), the timing can be varied from 50>28, by the gas-pedal.
At 3400 in Second gear, (40mph), the timing can be varied from 56>34*
You can imagine my opinion of running a locked distributor ...on the street; and you can imagine my opinion of not running the Spark-Port.
>Oh but you say, it works the same.
Sure it does, but at different gas-pedal openings, because the Spark-port works differently than the Manifold-port. And with a manual trans, and a high-pressure design, it is easy to have too much power at idle/off-idle, and then it gets very hard to drive slowly, with a manual trans..
>To fill-in my modest timing curve, I have a spark-delay box in the cabin with an adjustable range of 15*. I set it in such a manner as to get up to 9* retard and 6* advance. This makes it possible to set the base timing from the cab, from ~14* down to about 5*, and the cruise timing from ~47 to ~53@2240 rpm. The engine likes cruise timing of up to 60* at 2850rpm, but with overdrive, that is pushing 85mph, which is well over the 68MPH max speed-limit in Manitoba.
Sure I could recurve my distributor one more time, and get rid of the Timing Box, but it's working so fine as it is.................
>Finally, the 292/292-108 cam was in my 11.3Scr version of this 367. It was none too excited to idle at 500 in gear and pulling itself. But it did pretty good at 550 (4mph), so I learned to toe the clutch a lil. This is one of the reasons I pulled that cam out after just a few weeks.
> Here's my point:
I'll admit that I ain't the brightest bulb in the box,
but my elevator goes to the top. And
I know that a 292/292/108 cammed, 11.3Scr 367, can be tuned to idle at 550 rpm in First-gear , and pulling itself , with just 5 measly degrees of Idle-Timing........ because I had one that did this. and
My [email protected] will do it at 500rpm@5*
Listen to what the engine is telling you....... Don't force it. Be patient.
BTW:
If you care to try it;
Just run your tach up to 2400 rpm and keep it there. Then, without regard to the actual numbers, advance your timing and return the fast-idle to 2400. Repeat until additional timing does not cause any increase in rpm. When you get there, read the timing at 2400rpm.
Don't be surprised to see numbers in the 40s or even 50s. That is the optimum timing for your engine at 2400 unloaded. Put the timing back.
You can do this at say every 400 rpm from 3600 down to idle, and plot the numbers on a graph, then connect the dots and smooth it out.
Now you have the Optimum no-load timing curve for your engine. Use it as a guide to build up your PowerTiming curve, to cruise-timing; plus/minus say 200rpm. Adjust the Idle Timing up or down to best make it happen. Remember, the closer your cruise timing is to optimum, the higher is your potential for best fuel-economy at that rpm.
What your Idle-Timing is relatively unimportant because it can NEVER be optimum anyway ...... because if you try to make it so, the Idle-Rpm will run away on you, ever higher, and the transfers will be closed or nearly so, and the mixture screws are likely to be WFO, in compensation. Oh sure, she's "idling" all rightee...... but just try putting her into Drive and taking off ....
Anyway
These are my experiences, offered for perusal. What anybody actually does/does not do, with this information is of no concern to me.
Happy HotRodding
Great info !
AJ/FormS, That was quite a discussion with lots of excellent information. You definitely had time on your hands to make all those measurements and good skills to know how to chang the springs and such to make the distributor do what you wanted it to do. What spring kit did you use? And what Vacuum Canister? How did you modify the canister?
This post shows the need for someone to make distributor machines available again. Sun? Please come back?
David Vizard has posted a new video, #120, on ignition timing. He prefers manifold vacuum vis ported vacuum in the carb. Not sure this will work well for all street applications, but he has a lot of experience that we can all learn from.
I would love to read comments from FABO experts on both of these discussions. I believe that most people leave a lot of performance and economy on the shelf by not paying attention to ignition timing.
This post shows the need for someone to make distributor machines available again. Sun? Please come back?
David Vizard has posted a new video, #120, on ignition timing. He prefers manifold vacuum vis ported vacuum in the carb. Not sure this will work well for all street applications, but he has a lot of experience that we can all learn from.
I would love to read comments from FABO experts on both of these discussions. I believe that most people leave a lot of performance and economy on the shelf by not paying attention to ignition timing.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 25,866
- Reaction score
- 13,175
>As to springs; I had collected about a dozen old distributors from the 60s>80s, and just robbed what I needed. Same with cams, and same with VA cans.
But
>as to Vcans, I soon found out 2 things. 1) that as pirated, none of them were compatible to run with the power-timing curves required to be used with 87E10 and 11.3 Scr, and 2) any/all of mine were suitable to be modified, up to about 22>24 degrees, which thus made them compatible.
> as to how this is done, I simply filed off the stops until the arm stopped pulling in any further. And I set the vacuum delay screw, inside the hose-nipple, to the minimum preload.
>as to ported or not, here's where it gets tricky.
For an automatic transmission equipped car;
Firstly; you have to know that if your Auto equipped engine has a high stall, then it almost doesn't matter whatever your timing is below stall,
and if your stall is 3500 or more, well then almost anything goes.
But
> if you have a manual-trans car, AND even a modest cam, AND are a streeter like me; you have a problem.
Secondly, you have to understand why you might need the extra advance at idle and or at low rpm. I challenge you to start your car and bring it up to operating temperature and check the timing.
> then idle it down to just before it stalls, then add fuel via the mixture screws until she gets happy. Then check the timing.
Next, grab your distributor, and arbitrarily pull in some advance. Then reduce the idle speed until just before it stalls, and add mixture screw fuel as may be required. Then check the timing.
Repeat as many times as necessary, until adding advance no longer causes the rpm to rise, or the mixture screws are maxed out.
Then idle it up to 750, and see if she'll maybe take a lil more timing. and check the timing one last time.
If at any time, adding timing causes the Rpm to drop, then back up! that was too much.
Now; compare the timing between the start of this test, to at the end.
What you got at the end, is the timing that the engine needs, to make max IDLE-power, in neutral/park.. Don't be surprised to read timing in excess of 25>30 degrees.
Now, if you mod your Vcan to pull in 22* like I did, and hook it to manifold vacuum, then you can only run about 3* of mechanical timing...... Which means that when you floor it, that vacuum Advance will drop out and yur stuck with turtle-acceleration due to not having enough power-timing. So in this case, I wouldn't want full-time Manifold-controlled VA.
but why in the world did I mod my Vcan for 22*?
Well, I have an overdrive manual trans. and I can gear the back to run 65mph down to 1600, and my, at-that-time 11.3/1 high-compression 367, will pull that gear no problem, but it does it best if I give her the timing that she needs and wants. What I discovered is that no matter how hard I tried, with factory distributor parts, I just could not give her that, and so, my hiway fuel-economy suffered terribly. My car was a DD and, MOST of the mileage on the odometer is accumulated while on the hiway; so I had to do something!
So I bought/installed a dash-mounted, dial-back, timing module that had a range of 15 degrees, and began experimenting.
What I found was that, running 65mph at 1600, at optimum timing, and optimum fueling, the 367 got very little better fuel-economy, than at 2200, similarly optimized. And, the starter-gear( ~11/1) while running 2200 was dynomite, whereas the one to run at 1600(~8/1) , was pretty lazy.
So it was a no-brainer to run the 65=2200.
But
in order to run there with optimized timing, she needed about 56* of cruise timing, which means 56 less 22=34 degrees of mechanical timing. But the 367 did not like 34* of power timing below 3000. So I had to make a compromise to save the engine.
With more experimentation, I found that the 367 had no problem and liked 28*@2800, and was fine with most of that, even at idle. So I built a timing curve to start at 14*@1000rpm, and end about 28*@2800. this is a range of 14*, at slope of 1.29 degrees per 100 rpm. So at 2200 this amounted to 15*. and adding the 14* initial, plus the 22* of Vcan timing it now totals 51*at 2200; just a little shy of the 56* optimum.
To get the 56* I just crank in the timing module.
So that is how I ended up with the 22* Vcan. ..... which is still 5* short, to run without the module.
>another thing that happens with a manual-trans running a lot of idle-timing on the manifold VA, is that the engine has just too much idle power to drive slowly, without slipping the clutch.
I found out that my 367, would idle down to 500, in gear, with an 11/1 starter gear, and would thus pull itself at 3.66 mph; that was slow enough for me. But, it required the Idle timing to be about 5*advanced, or less, which meant that I could not run the VA unless the base timing was 22 less 5=17* retarded. Any more than 5* advance, and she would start to get jumpy; and if I didn't clutch it, she would get very jumpy, and it did not matter what her idle AFR was.
So in my case, Manifold-controlled VA was impossible.
Automatics get jumpy too, but generally, the convertor sucks it up. and street-guys tend to throw a lot of low-speed timing at their engines anyway, to try and get some torque back, that was lost in the combo, due to it not having enough cylinder pressure caused by the combination of too low an Scr, and a late-closing intake valve. And so their engines idle sounding like crap; but that's what they like; and it ain't my money paying for their gas.
Now
>as to automatics and low-stalls. Pretty much everything I said for a manual-trans, also applies to these; and here's why.
Most Normally-Aspirated SBMs, no matter how they are built, are gonna accept about 28>32* of power-timing @2800. How you get there is a crap-shoot.
Suppose your Vcan is hooked to manifold vacuum, and the Vcan makes 14* . So then cruising at 2800 that would be 28 mechanical plus 14 in the can equals 42 degrees, which could be between, 14* and up to 18* short of optimum. For example, my engine liked close to 60* to cruise at 2800 with IIRC 3.55s.
So then, right off the line, with only 42*, the engine is running very retarded at cruising speed.
>Why is this important?
Well, when your peak cylinder pressure occurs at a certain place in the piston's travel from top to bottom, there will be exactly one place, where the crank will be in the optimum position to transfer that pressure peak.
Too soon and the pressure will drive the conrod onto the crank, hammering the crap outta the soft bearings; not to mention what happens if the engine gets into detonation.
Too late, and peak pressure never occurs, and almost worse, is that the heat in the chamber chases after the piston, ever lower down the cylinder wall, and instead of that heat pushing on your driveshaft, it heats up the cooling system. And the possibility exists that at after a certain rpm, the combustion gasses have not yet finished burning when the exhaust valve opens, and now that heat goes into the head-port and heats the cooling system from there. All of this is bad news for your engine and for power, and for fuel economy. So you never want to run too far retarded, and 14 to 18 degrees is extremely retarded, and that's gonna cost you fuel-economy big time..
Now then, still working on the low-stall automatic.
Suppose it stalls at 2000 rpm, and suppose the engine wants not more than 24*, but she also wants 28* at 2800 and 32* at 3200. So that is a nice straight slope of 4* per 400 rpm = 1* per 100. That means at @1000 rpm she will Get not want, 14 degrees, and at 800 it would get 12*.
So then, maybe you get the idea that 12>14 is not nearly enough, and you want to hook the 14*Vcan to the manifold, but not to exceed 24*@2000 rpm@WOT
So then, at 800 idle, the total Idle-timing will be 12initial plus 14 in the Vcan =26
Sounds good right? she idles great the Transfer slots are about right and to get the Idle-Afr on the money, you have leaned the mixture screws right out. Great.
Right up until you step on the gas, and the engine stumbles because of the lean mixture screws. So you richen that up and Good to go right?
But now, whenever you are cruising along at a low throttle opening and below stall, the engine is continuously rich, and you blame the lousy fuel economy on anything but the manifold-plumbed Vcan. And yur scared to install a higher stall because you may think that your fuel-economy will be worse.
Here's a secret, the Vcan on ported vacuum usually begins coming in around 1600 to 1800 in first gear, depending on the base timing, and the actual manifold vacuum. This is a function of where the throttles are, relative to the signal port, which is a function of how much power is being called for. So if you have a 4000 pound boat, 2.76 gears, and a 2000 stall, even modest acceleration will call the Vcan into play. But with a 3000 pound car, 3.91s, and a 2800 stall, a similar throttle opening may call for more VA, because the vacuum is staying higher. In other words, I cannot tell you exactly when your particular combo will begin pulling vacuum advance, so 1600 to 1800 is just a guess.. That's where mine began @3650 pounds with a starter gear of 10.97, and the throttle set to "barely accelerating."
The point is this, the Vcan on the Spark port only begins operating at very low throttle openings, after that, it operates pretty much the same as when directly plumbed to the intake.
BTW
I'm no expert on this.
But I am an expert when it comes to my combo cuz I tried just about everything you can imagine ..... on it.
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