67Dart273
Well-Known Member
I don't know why we need an "article written by a GM engineer." NO these cars DID NOT use manifold vacuum. My 69 RR, my 64 426 neither used manifold vacuum "bone stock" "back then."
Plugging Timed vacuum advance hose
- Thread starter stumblinhorse
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I don't know why we need an "article written by a GM engineer." NO these cars DID NOT use manifold vacuum. My 69 RR, my 64 426 neither used manifold vacuum "bone stock" "back then."
65-440
Well-Known Member
All you can do ? Ha, that was pretty damn good ! Thank you for that read, I should just have you build my next engine ...
65-440
Well-Known Member
does it matter who he was an engineer for ? I've read a lot of what you've written on here and even asked your advise on some electrical stuff, but do you honestly believe gm timing operation differs from Chrysler and even ford ? I think not, the principal and operation is the same across the board ...
Agree - and there are many different engineering jobs within each of the projects, so that alone doesn't make him an expert on the subject in question. Its interesting that I've been looking at various OEM Chrysler carbs, mostly the 4 bbls from the late 60s and every one has provision for ported vacuum. A few were set up with a delay valve for decel using manifold vacuum for emmisions.
Too bad he doesn't know his history. It's only partially true. I used to think that too, and I said something similar on another forum and the guys had plenty of examples of pre-emmissions era use of ported vacuum for timing.* The choice of which one to use is a matter of which one works the best for what the engine wants and what you are trying to do.
Awsome. Depending on the vacuum generated at idle sometimes its hard to setup to do that, sometimes not.
Some can be, some can't. If the nipple sticks out of a hex shaped end, it may have an adjuster screw. If its barrel shaped, it doesn't.
However, the adjustment only controls how much vacuum is needed to move the advance mechanism. It doesn't change the rate or the total. The total can be altered with some tinkering. Probably this is getting way beyond what you were looking for, so I'll leave it there.
*archived Innovate Forum "Carb Issues", scroll to #11, 17 (me) and responses #13 and #19-22.
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To the other parts of your question. Yes. It's generally best to get the PCV valve in place before tuning the carb. The PCV valve will be a restricted, but significant, air source at idle.
The valve doesn't need to be sealed well to the valve cover if you really want to get going on this without waiting for the grommet. But it does need to be connected well to the carb base.
How you go about the timing will depend on the engine. Knowing what year, the rest of the setup, and your goals, then its possible to make some suggestions or point you in a direction.
harrisonm
Well-Known Member
This is all very interesting. Many years ago I rebuilt my 69 4 speed Barracuda's 340 with a Comp XE268H, Eddy Air Gap, headers, Eddy 750 carb and a Mopar Performance Electronic ignition setup. I followed the instructions for the ignition. The instructions said to unplug the vacuum advance and set total timing at 32 - 34 degrees at 2000 - 2500 RPM, and I did that. I did not fully understand timing at the time, so when I later hooked the timing light up at 2000 - 2500 with the vacuum hooked up and saw 50 degrees, I was concerned. I called the Mopar Performance tech line and the guy told me that was not good and to just unhook the vacuum line. He said the engine would run better and that a performance engine did not need vacuum advance anyway. So, since I apparently made the mistake of assuming the guy on the tech line knew what he was talking about, I've been driving it with 34 degrees timing (no vacuum) at about 2500 RPM ever since. It seems to run OK, but from what you guys are saying, 50 degrees total (vacuum plus mechanical) at 2500 RPM is OK and I SHOULD hook up my vacuum line. After reading that article, I now understand that I will still have some vacuum advance at 2500 RPM so the 34 degrees I set with the vacuum unhooked will become about 50 when the vacuum is added in. I also now think I understand that and the vacuum (and the associated vacuum advance) will disappear at WOT, and I will then be left with 34 degrees of timing. Does that sound about right?
Basically yes.
And believe me, I'm pretty sympathetic having gone through numerous attempts way back when. I still have the original instructions for the Chrysler built conversion kit. I'll dig 'em out tommorrow if you want. They recommend around 34 degrees at 2700 rpm for an LA, and then hooking up the vacuum advance and adjusting it's running at 50 degrees at your normal highway cruising vacuum.
Now here's the catch. Whether you use ported or manifold will depend on both your vacuum at idle and the initial timing. The Mopar Performance distributors generally came with relatively long advance curves. My guess is that your XE268 may pull decent vacuum at idle, 12-15" ?????
If so, ported will probably be best if your initial timing is already in at 12 - 18 degrees BTDC. If your initial timing is under 10 degrees BTDC, then try either way.
Too much advance will want leaner mix than will produce best power and you may have to slip the clutch more. But having the vacuum advance while cruising will result in better milage and cooler running engine. Most hot -rods seem to be happy with anywhere from 14 to 20 degrees initial depending on how radical the engine.
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SKnight
Member
Lots of discussion here.
The reason you pull and plug the ported vacuum line is when you’re setting your timing you set the base, then you’ll want to rev it to check your mechanical advance operation. If the vacuum advance is hooked up it’ll advance when you open the throttle covering up the mechanical operation.
Then, when you’re happy with the mechanical side, you can temporarily hook the vacuum canister to manifold vacuum and check it’s operation and amount of advance while it’s idling. Then reconnect to ported vacuum and drive happy.
The reason you pull and plug the ported vacuum line is when you’re setting your timing you set the base, then you’ll want to rev it to check your mechanical advance operation. If the vacuum advance is hooked up it’ll advance when you open the throttle covering up the mechanical operation.
Then, when you’re happy with the mechanical side, you can temporarily hook the vacuum canister to manifold vacuum and check it’s operation and amount of advance while it’s idling. Then reconnect to ported vacuum and drive happy.
67Dart273
Well-Known Member
Regardless of whether you run manifold vacuum or ported, vacuum goes away with your foot in the throttle. So "firewalled" the vacuum drops off under power and you are left with whatever initial + mechanical you put in it AKA somwhere around 35, and that is what you want
On problem that can occur with ported is when you (and us) start to swap around carbs, etc you can run into a situation where the carb produces too much vacuum at part throttle, and you can get into ping. I used to have this problem running either GM QJ or Holley spreadbore on my TQ manifold on the old 340, and later, on a 360 with a TQ manifold. These were both mild engines with either stock or stock 340 cam
It might be helpful to keep in mind the goal of timing with respect to vacuum. Vacuum is a good indicator of load and throttle position. More manifold vacuum indicates lower load, leaner mixtures and therefore more time required for the flame to grow.
However at idle, using manifold vacuum is not always the best indicator of mixture. This can be due to exhaust dilution and related factors. So the way to decouple the vacuum signal at idle is to internally place the vacuum port so it is just above the throttle blade at idle position.
However at idle, using manifold vacuum is not always the best indicator of mixture. This can be due to exhaust dilution and related factors. So the way to decouple the vacuum signal at idle is to internally place the vacuum port so it is just above the throttle blade at idle position.
I've also run into this problem of part throttle and high speed cruise pinging when both the mechanical and vacuum advance comes in too quick for the fuel. With more recent fuel (past 15 years) I had to back off when vacuum advance came in, and with the Mallory built MP distributor, also slow the upper part of the mechanical advance curve.
I am experiencing the same thing with my 318 Swinger. Just the stock 2 barrel and Pertronix conversion. I need to see if the vacuum canister is adjustable. If not I need a replacement. Last summer I filled up at a out of town car show. **** gas pinged right away. I pulled over and plugged the vacuum advance hose so I could make it home.
65-440
Well-Known Member
Some good info here. Last year while tuning the new carb on my 340/4spd dart I drove around for a few days with the vacuum gauge hooked up to see what was happening during all different loads, and adjusted accordingly ... I think I will drive around now with the gauge hooked to ported vacuum, I'm interested to see the readings.......
65-440
Well-Known Member
Just went through my 1970 service manual, shows vacuum advance hooked to ported vacuum with a description .... looks like a got some tweaing to do once the weather gets above 30 degrees ! .... on both cars ....
But your cars are no longer stock. So your two setups, especially the 340 with the XE274 cam may not act as AJ described in this statement "Obviously, as soon as the throttle is opened, the vacuum falls and, with it, the low-speed vacuum-assisted timing."
The manifold vacuum from idle (at say 700 - 800 rpm) to off- idle may go up. That would largely due to the valve overlap causing pretty low vacuum at 800.
65-440
Well-Known Member
If I remember right, the most vacuum I could get at idle was 11"
That's about what I was thinking.
So depending on the vacuum advance's internal spring, it may or may not be contributing anything at idle. I posted an example a little while ago - here it is in Adj Vac Advance Can
If you get a chance, measure and plot the mechanical timing curves of your current setups. Then note the amount the vacuum adds and what vacuum moves it.
Even if you decide to experiment with it, you'll have a baseline.
It will be interesting to see if you've achieved a timing at idle that the engine likes with some vacuum advance or not.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I don't remember iron heads, but with Eddies
it's almost impossible to over-time the part throttle and cruise, with factory type parts. I have run over60* at hiway speeds. My can is a 22*. I run 14* at idle. Beginning at 1000rpm this increases to 28* at 2800. And then it slows to 34@3400. So to these numbers, the can will add it's 22* at any time the sparkport is activated, say from 1600rpm and up.
A little math says that on the fast part of the cam, it brings in 14* in 1800rpm so that's 7.78* per 100 rpm. My od cruises at 2250=65, so that's 9.7 degrees in the mechanical, plus 14 initial, plus 22 in the can, for a grand total of about 46.This is still not enough for my Eddy-headed 367 at 10.9Scr. I have a dash-mounted adjustable timing gizmo, that has up to a 15 degree electronic dial back in it. So I could add that 15 to the 46 for a total of 61. And I used to often play with it. I would hold the throttle steady on flat level ground, and add timing in 4* increments and watch the speedo. If the speed went up. I would back out of the throttle, back to 65mph, stabilize it, and add 4 more. And so on. That's how I came to the 60* at cruise. That is also how I bugged out the part throttle timing.
My 14* initial, is now governed by the 28* at 2800, a nice safe number for 87E10 gas. And by the 34* at 3400, also a nice safe place to bring it in,for 87E10. I have far more torque than I need, so sacrificing a few footpounds ( with a bit of delayed timing), in the name of dollars saved over the life of the engine, for me was a no-brainer. I was desperate to run 87 .
.What I discovered at hi-way speeds, was that the timing made very little difference from 55* to 61*. So eventually I re-indexed that device to the center of its range, and so now it has about a +5/- 10 range.And a maximum of 46+5= 51@2250
..Of course, adding the 22 in the can at various part-throttle rpms, makes the engine quite a bit smoother and seemingly quieter. And part-throttle acceleration, with the can fully to partially engaged, seems stronger as well.
..And with the 4-gear and 3.55s parading was a chore, as the minimum speed at 700 was 6mph with a 2.66low. So I installed a 3.09 low, and lowered it to about 5mph. Now with the timing device, I can take 10 degrees out at idle, or to 4*initial, without affecting the T-port sync. This reduces the engines power, such that it idles down to 550in gear,on flat level hard ground, and so the parade speed can be as low as 4mph.A little slip on the clutch and 3 is possible.
..Ah the joys of owning a stick.
I would encourage you to get such a device, and begin a new tune.You can use this device to add or subtract power-timing in 400rpm increments, and so develop an exact power-timing curve. Then the building of that curve becomes an exercise in averaging, Sometimes it happens that you have to sacrifice 2 or 3 degrees from the max timing, to get around a mid 2000rpm requirement. On the street, you'll never notice that with 3.55s, cuz you won't get to it until perhaps 30mph, and at WOT you will have been spinning all the way, and if you stay in it, a loss of a few horsepower; how can you tell? Now if you strapped a sled on the hitch, took her out to 4000rpm and then played with the gizmo,..... then you might find it,lol.
Like I said, I've been away from iron heads. Since 1999, with just a few tunes between then and 2012, after that I quit.
In any case HappyHotRodding.
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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So far I agree with every thing you have said, especially this part. This characteristic can be very advantageous to know if you have a manual trans. If you set your can just right, you can really pump up the part-throttle take off, as the vacuum continues to rise, up to about 2200 rpm (with typical street cams) as the overlap period stops pumping air back into the intake. This can make a marked improvement in low rpm driveability.
Automatics, not so much,lol.
My examples were with stock to very mild cams, which the OP seems to have.
65-440
Well-Known Member
I should really start taking notes when I'm making adjustments, im pretty good when tuning my carbs but never wrote down timing numbers. I'll double check hopefully this week some time. I'm pretty sure I adjusted the can to open at 9" vacuum, because I was using manifold vacuum and wanted it full open with the low idle vacuum. I will raise it up some with the switch to ported vacuum but I Need to drive with a gauge to see what my ported vacuum readings are under different throttle conditions. I guess my question would be after figuring all that out, where to set the can to start advancing the timing ?
Notes are good!
When I get in, I usually rewrite the mechanical timing & rpm numbers into a graph.
I would not touch the vac advance with a change to ported source of vacuum.
* One change at a time, unless there is some particular relationship that demands otherwise.
In this case, the only change will (should) be the loss of advance at idle. As soon as the throttle is slightly off idle, it will bring the vacuum advance the same as before.
* If anything, you may decide to change the initial timing.
Example:
Lets say with your 340 has the same generation Chyrsler built MP distributor that I used in the test linked above. If it was set to begin adding vacuum advance around 9", then by 11" Hg it will be adding 4 to 5 degrees. At 15" and higher it will be adding around 16 degrees advance.
Lets say it currently idles at 800 rpm with 12 degrees initial and that distributor's mechanical curve begins advancing at 850 rpm and adds 20 degrees before 3000 rpm.
Therefore,
It generally idles with 12 initial + 5 degrees from vacuum = 17 deg BTDC. That's ballpark for tuning in that size engine/cam/compression.
Full mechanical advance would be 12 initial + 20 advance = 32 degrees by 3000 rpm. That's decent, safe, although a little more advance might result in quicker WOT acceleration at the strip. It also might result in quicker response off the line - but that also depends on the slopes and shape.
Interstate cruising will probably be around 2600 - 3000 rpm and 16 to 20" Hg vacuum.
So timing will be 12+20 degrees BTDC mechanical, plus 16 degrees BTDC from that vacuum advance. Cruising 48 degrees BTDC. Again not bad, engine will nice, although maybe not quite as sharp as it possibly could with a little more.
Now if all that is done is change the vacuum source to ported. The only change will be in the idle timing. It will drop to 12 degrees. Nothing you do with the vacuum advance adjustment will offset that.
The engine may run 50 to 150 rpm slower at idle, and it may like a slightly richer mix. If it runs too slow and wants to die, you will be tempted to increase the throttle opening with the idle speed screw. However this will change the transfer slot opening. If its opened too much, it messes up the entire off -idle and carburation (AJ has a whole write up on that).
So, IF you find the engine is running to slow, and your measurements show it likes more timing, give it the timing back by adjusting the initial.
IF, as in the example I gave above, the total timing and cruise timing and rate of advance may allow for additional advance, go ahead and test everything.
However, IF the total timing is already 35 degrees, adding 5 more degrees into the initial will put the total at 40. DO NOT run it at WOT like that.
Even changing from 32 to 35, you want to be pretty careful about your next outing at the drag strip. If the mph go up and there's no pinging, great! But do check the spark plugs for any visual signs of detonation.
If there is any audible pinging, get out of the gas. Finish the run at less than full throttle and reduce the timing. In this scenario, the mechanical advance will need to be shortened or at minimum slowed down.
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