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  #1  
Old 12-27-2008, 02:34 PM
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Default Vacuum advance - heat problems possible?

Looking through other threads, we can agree the distributor's vacuum comes from the carb's ported vacuum nipple. Some people have made things work okay by using manifold vacuum, so be it. I run my distributor to the TQ's ported vacuum.

An image in the Carter Carb books shows the location of a ported vacuum hole in a throttle plate, I can find similar openings in one of my spare TQ's. The system of idle fuel feeding is plain, the blades cover the transfer slot openings at idle and as they open the transfer slots get manifold vacuum to draw more fuel. Nowhere can I find a venturi effect acting on any of the slots in the throttle plate body, so it seems this is a manifold vacuum only situation. So, manifold vacuum gradually pulls all it can from the idle fuel feed passages until the primary boosters experience enough air flow to start pulling fuel through the primary circuit and "starve" the idle circuit.

Somewhere in there, the slot for distributor vacuum is getting a simliar signal to what the idle fuel transfer slot is feeling and is pulling in added degrees of advance.

Here's my problem.....I've always run the vacuum on my cars, performance wise that was a mild 360 in a '67 Barracuda, then that motor in my A-100 van, then that motor in my '67 Belvedere, finally with a 440 in the B-body. Cams in the 360 were in the 230deg @ .050 range and that's what I started using in the 440. This past summer I installed a Racer Brown SSH-44 with 242deg @ .050. I've got an early version of a F/A mixture gauge connected to an oxygen sensor in the driver's side header collector, and use that to gauge needles and jets on the TQ. I never had much difficulty with the runnings of the 360 in any vehicle (except the A-100 with 2.94 gears) nor the 440 with the milder cam. However, with the bigger cam I found the car would stumble in steady-state cruising to the strip with 4.30:1 gears. I tried varying speed between 45 and 70mph, same problem.

When I'd get surges previously and see the A/F meter go blank, I'd adjust the needles out a little or swap to thinner ones and re-tune from there to get that cruise mixture in a good range. However, the A/F meter wasn't showing anything drastically wrong with A/F with the big cam/tight gears. As you can imagine, the throttle's pretty open with 4.30's at 70mph...turned just about 3800rpm. So, I disconnected the vacuum advance and voila the surging went away. There wasn't really any pinging I could hear but the car is pretty loud (dynomax welded street mufflers that you can see right through...like oval muffler-shaped glass packs). Sure felt the surging, though. Since I was disconnecting the vacuum advance at the strip anyhow, I just left it disconnected. However, one day driving back the car overheated and would not cool down. It runs pretty hot anyhow, but the electric fan was always able to cope before.

So, does a lack of vacuum advance initiate heat problems sometimes as well as reducing fuel mileage and causing all of the other issues for which it's usually given credit?

Also, I've taken apart another distributor for the 440 to clean it up (gunked on grease galore) and look at changing springs. FBO says they weld the vacuum advance plate to keep it from moving as it must cause a lot of ignition fluctuation in normal operation. I'm going to buy their guide and see what it says, but have any of you welded up the vac adv plate for strip use?

BII
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Old 12-27-2008, 03:09 PM
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Quote:
Originally Posted by BelvedereII View Post
So, does a lack of vacuum advance initiate heat problems sometimes as well as reducing fuel mileage and causing all of the other issues for which it's usually given credit?

FBO says they weld the vacuum advance plate to keep it from moving as it must cause a lot of ignition fluctuation in normal operation.have any of you welded up the vac adv plate for strip use?

BII
The lack of vac. adv. wont cause any heating problems at all. Your surge is cause by too much timing at the wrong time. That's why no one uses them on performance cars. There's no need to use one cause your are running higher than stock intial timing specs to begin with and your mechanicals are all in before 3000 so the vac. adv. adds too much timing. No need to use it. And in a performance engine you wont see any diff in milage. I have welded the plate in one of my circle track engine dist. Works fine. Still have the mechancial advance though.
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Old 12-27-2008, 04:15 PM
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The vacuum advance when adjusted as per specs will pull in about 20+ degrees of timing at cruise rpm's. Therefore with your maximum mechanical set at it's maximum, the lack of vacuum advance may have the timing too retarded for high cruise rpm's which over distance may cause additional heating in the combustion chamber and exhaust ports. fft MAYBE?
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Old 12-27-2008, 06:02 PM
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What was your total timing without the vacuum advance hooked up?
Must have been under 30*, even under 34* for a big block is low.
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Old 12-27-2008, 06:46 PM
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PY, almost always at 35, and doubt it was different that day. Timeslips were the same as previous trips.

AAR, I was wondering if less advance at speed will have the charge burning too late and bringing up temps as it continues to burn through the exhaust stroke. An egt probe would tell for sure, I suppose.

I understand the vac diaphragm can be tuned via allen screw, and have read that the amount of vac advance is changed by doing this. I was always under the impression the adjustment changed vac requirements to pull in advance and the arm itself determines how much. For instance, the arm on the distributor I just took apart is marked with a 9 which I figure for distributor degrees and 18 deg at the crank.

It's winter of course, so all this will have to wait a couple months to test.

BII
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Old 12-27-2008, 08:32 PM
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I think you may have the timing just a tad too retarded in the initial/mechanical, and the vacuum advance pot is too sensitive. First I'd try adjusting the vacuum pot a tiny bit at a time until the cruising surge disappears, then bump up the initial maybe 2 degrees or so to be on the safe side.
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Old 12-28-2008, 03:27 AM
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I have a 72 service manual for my Cuda. It lists possible causes of overheating: blocked radiator air passages, incorrect timing, low engine oil, incorrect valve timing, bad temp gauge, restricted overflow tube, faulty rad cap, frozen heat control valve, dragging brakes, excessive engine idle, frozen coolant, faulty fan drive, faulty temp sending unit, faulty vacuum bypass valve, overfilling, insufficient corrosion inhibiter, blown head gasket, broken or shifted lower hose spring, low coolant level, collapsed rad hose, fan belt lose, glazed, or oily, air leak through bottom hose, bad thermostat, water pump impeller broken or lose, restricted rad water passages or restricted engine water jacket.

Bad timing is one cause.
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Old 12-28-2008, 10:51 AM
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Quote:
Originally Posted by BelvedereII View Post
AAR, I was wondering if less advance at speed will have the charge burning too late and bringing up temps as it continues to burn through the exhaust stroke. An egt probe would tell for sure, I suppose.
At cruise the cylinders are not filling efficiently as as a result the spark has to come much earlier to get maximum cylinder pressure in the 20-30 degrees ATDC where max power and efficiency occur.

With a 242 @ 50 cam you will have much lower manifold vacuum and combine that with the throttle position to maintain hiway speeds with 4.30 gears I can't imagine you are seeing any vacuum advance while cruising.

You definitely can see elevated operating temperatures if you don't have enough timing. I don't know what you have for initial and total mechanical but I would start dialing in the initial then recurve the distributor to maintain the 36-38 the carb. The cam you are using will likely like something in the range of 15-20 degrees BTDC for initial timing. Until you get that sorted out the carb adjustments you have made are suspect.
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  #9  
Old 12-28-2008, 02:07 PM
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Yeah, lots of ways to overheat! My service manuals are a 1967 and one for the B-van which should be about 1974, maybe 1973. This overheating happened all of a sudden, when I disconnected the vacuum advance and left it that way for the drive home.

It was odd that the car bucked/hesitated like that on the highway as I'd have figured on there being no vac advance either at that point, but the fact is it all stopped when I disconnected the vac advance. It was a borderline thing, it wouldn't do it right away, and would come and go by changing throttle pressure, best way to describe is it sort of crept in at steady pedal. I'm hoping turning the adjustment on the vac pot will cure it.

The distributor in there now is the one they include in a conversion kit. I bought it new but didn't take it apart or check advance/mechanical advance markings. I also haven't paid too close attention to timing at idle, I just disconnect the vac and set it for max around 35. I'm not trying to see just how fast the car will go, it's a bracket racer. Whatever I run that day is okay by me---as long as it's consistent. This year I expect I'll get it into the 7's (1/8th mile of course) so it might get real important at the 8.01 range.

So, whatever mechanical is in the car's current distributor is kind of back-setting the idle timing. If it's got 26 through the mechanical, my idle would be about 9. I doubt that's the case, though, as it idles smooth at 720rpm when warm (digital tach, that's the number I see most at idle).

DGC, you're right about the carb tuning being suspect, and it was probably worse back when I didn't have an A/F indicator. I agree the car could probably idle fine with around 18 intitial degrees, so getting the mechanical to add in 17 might be the goal. For all I know, the distributor in there has only 15 to 17 mechanical. Anybody using one of the factory "conversion" distributors and know what theirs has?

The distributor I have apart is original to the motor, which came from a Winnebago. I've run it in the motor and it works, but didn't fiddle with it much before putting the MP catalog item back in.

BII
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Old 12-28-2008, 05:04 PM
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Out of the box, I believe the MP distributor has WAY TOO MUCH mechanical advanced dialed in and this should be adjusted according to the instruction sheet. You can use drill bits as gauges for dialing in the total mechanical advance and then go to the vacuum advance adjustments.

Any chance the bucking/hunting is from TOO SMALL primary jets?
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Old 12-28-2008, 05:14 PM
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Is it possible the vacuum at cruise is on the verge of pulling the needles down, then having them raise quickly and repeatedly? I saw this happen (jiggling up and down) when learning to set up my TQ. I can see that resulting in the bucking/hesitation you mention. Just a thought.
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Old 12-28-2008, 06:51 PM
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I got a reman'd stock LA V8 electronic distributor from Checker, and even for my messed-up stock 318 it was too slow on the advance curve (slower than the stock points one I took out).
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  #13  
Old 12-28-2008, 06:56 PM
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The problem's gotta be advance based....with vac adv connected, it acts up. With vac adv disconnected, no problems at all. Except for that incident of overheating at the end of the racing season.

I finally broke open a TQ strip kit I've had sitting around since about 1992, and installed the .104 primaries. Largest I'd had until then were .098's, so I'd just been messing with needles and their position. I raised them at one point to as high as they'd go without having the cam press the bar into the choke linkage, which is when the shrink wrap came off the strip kit. Had thinner needles in there, too!

At one point, I took a spare spring from another TQ and shortened it to let the needles raise at the lower vacuum settings, so I have tried a variety of things to tune it in. Wish I had a lot more experience with these, I was very lucky with the TQ (cracked outer body, but never leaked) that I used on the 360 from 1992 until 2004/2005. I think I changed the needles and jets once in all that time, although I only raced it in the Barracuda '92-94 and in the 'dere from '02-'04/'05. It was fine in street running through all that time, and even worked on the 440 in the 'dere until I started messing with this Winnebago-application version.

Can't complain for the most part either because it was consistent enough for me to win the series I ran this past year. Seemed I only lost once on a stumble, all other problems were on runs I won or lost due to other reasons. However, I want to tune it in better this go 'round--starting with ignition. Kinda see the attraction of living in the south where this can be done about any month you'd like.

I've looked through the TQ sticky on this website, and the parts included in my Strip Kit aren't exactly those that are listed. I think mine was factory sealed, but does anyone know if they were supposed to come with a sliding box or other solid means of closure? This one's styrofoam, open top except for the shrink wrapping to keep it all in.

BII
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Old 12-28-2008, 09:31 PM
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Guess this is pertinent enough not to start another thread, but I have to ask...when it comes to welding up mechanical advance slots, everyone says to go with the outside limits. Why not weld up the inside limits and go from there...that way even with light springs the weights will be under some tension at their rest position and you'd never have to worry about them not being pulled fully "closed"?? There must be some reason it's always the outers, I've just not seen why.

BII
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Old 12-29-2008, 05:52 AM
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My experience with Mopar vacuum advance units is they are not reliable!
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Old 12-29-2008, 12:22 PM
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Quote:
Originally Posted by BelvedereII View Post
DGC, you're right about the carb tuning being suspect, and it was probably worse back when I didn't have an A/F indicator. I agree the car could probably idle fine with around 18 intitial degrees, so getting the mechanical to add in 17 might be the goal. For all I know, the distributor in there has only 15 to 17 mechanical. Anybody using one of the factory "conversion" distributors and know what theirs has?
I have purchaed two MP distributors in the past couple of years; one was the vacuum advance unit and the other was the billet unit with no vacuum advance.

Both had 24 degrees of mechanical advance in the distributor which is fine for a stock motor. The billet unit had two of the lightest pink springs and the vacuum advance unit had two of the orange springs which are the next to the stiffest.
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Old 01-03-2009, 01:39 PM
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Welding the interior appears to work. The slots had .205" of travel (.445" total slot length minus the .240" peg) and the shaft was marked on the underside with the number 13 (for deg). I figured the "value" in the slots should be altered to result in 9deg at the distributor, and 9/13ths of .205" is .142", so I had the inner ends of the slots welded up and I filed them down. Looks like it was close enough as it wound up with 17 crank degrees.

I'd say the distributor had its original springs, typical thinner spring on one side and wide-slot heavier gauge second spring that comes in later. I left the original "primary" spring and installed a very light one on the opposite weight, it being under tension as installed now.

It's been warm enough here so I pulled the car out of the garage and fired it up to warm up the motor with the existing MP distributor. I swapped in the modified distributor and set the idle at 750rpm to 18deg using my dial-back timing light. Moving up 100rpm at a time, it stayed at 18deg until 1550rpm where it made 20deg, then 23deg at 1650, 26deg at 1750, etc. to 33deg at 2050. At 2700 it showed 34deg. I'll have to run it again to 3500 to make sure it's showing max, but while warm enough it was still a bit cold as I did this after dark. All seems okay, and with a relatively loose 9.5" Dynamic converter the motor doesn't do much work at the sub-2000rpm range--drag strip launches are always over 2k anyhow, gears are 4.30's.

I didn't check the MP distributor's timing, but the more I think about it I've got to find out how much mechanical advance it's got. Now I'm not so sure I'd set it at 35 total degrees--I'm thinking I advanced it to get it to idle better at some point and wound up with 40deg or more total. Bad oops.

BII
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Old 01-03-2009, 04:12 PM
Century Cp Guru Century Cp Guru is offline
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Advance works at light part throttle When ya get into seeing on you advance light it gives ya a false impression, Best way i have found over the years is, unplug vac advance and see how fast in comes in, then pull it apart and since ya aint got a Sun Dist machine use the Mr Gskt or Mp Perf dist springs Most Mopar Vac advance distributors have 14 degs of mechanical advance built in A big block usual lkes 38 deg total set at 10 deg Initial. and see that it will be in at 2200 to 2800 with the light springs that Makes 38 total if its to fast coming you can use the combo of the old heavy spring on were the fact light was and the lighest of the spring kits on the original heavy spring postion. i have never had to go with welding and grinding etc. Try it it works. 9.5 converter you need no vac advance.
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Old 01-03-2009, 07:34 PM
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CP, do you really consider it to be "light part throttle" when the car's turning nearly 3800rpm to maintain 70mph?

I don't have the vac advance connected when I'm checking advance, never have, so not sure what point you're trying to make.

BII
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Old 01-04-2009, 09:58 AM
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Quote:
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Advance works at light part throttle When ya get into seeing on you advance light it gives ya a false impression, Best way i have found over the years is, unplug vac advance and see how fast in comes in, then pull it apart and since ya aint got a Sun Dist machine use the Mr Gskt or Mp Perf dist springs Most Mopar Vac advance distributors have 14 degs of mechanical advance built in A big block usual lkes 38 deg total set at 10 deg Initial. and see that it will be in at 2200 to 2800 with the light springs that Makes 38 total if its to fast coming you can use the combo of the old heavy spring on were the fact light was and the lighest of the spring kits on the original heavy spring postion. i have never had to go with welding and grinding etc. Try it it works. 9.5 converter you need no vac advance.
Your post is some what confusing. Mechanical advance is strictly rpm dependent. The more rpm the more advance until you hit the limits. The vast majority of mopar distributors have around 24 degrees of mechanical advance built.

The vacuum advance arm is stamped with a number which is 1/2 the number of crank degrees of advance built into the module. The most common number is 8.5 which represents 17 degrees of vacuum advance. The vacuum advance only operates during light load cruising when there is high manifold vacuum. It does not operate at all during WOT. The vacuum advance improves efficiency by adding timing under the light load conditions where the cylinders are not filling efficiently and a lot more timing is required to start the burn early enough to make power.

The springs in the distributor control the rate that the mechanical advance comes in. The stock factory springs are quite stiff to prevent engine damage by drivers that abuse the engine. From a performance standpoint you can almost always improve the power with a faster advance rate. The medium stock spring and one of the light MP springs from the two spring kit will have all your advance in around 2500 rpm which is a good point for most street cars.

Welding of the advance slots to limit the amount of advance is necessary when you start using cams with more duration than stock. The more duration the more over lap and over lap causes inefficient filling of the cylinder at idle and lower rpms. This inefficient filling requires more initial advance to start the burn sooner to ensure max power and efficiency, plus good idle & off idle throttle response. When you start increasing the initial advance past the 15 degree range you need to limit the mechanical to maintain the same 34-36 that open chamber small block heads like or the 36-38 that open chamber big block heads like.
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Old 01-04-2009, 10:07 AM
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Quote:
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CP, do you really consider it to be "light part throttle" when the car's turning nearly 3800rpm to maintain 70mph?

I don't have the vac advance connected when I'm checking advance, never have, so not sure what point you're trying to make.

BII
BII, its not so much the throttle position as it is the manifold vacuum. Typically at light load cruising the throttle is barely open and you have high manifold vacuum.

At 3800 rpm and 70 mph you are going to have the throttle opened quite a bit and the manifold vacuum will be very low, likely below the point at which the vacuum advance will add any timing at all.

BTW, you want to weld the slots at the end of the travel not the beginning. The reason you are impacting to parameters at the same time which makes it difficult to dial in. Welding at the end of the travel has no impact on when the timing starts to come in welding at the beginning will impact he starting point as well as the total. You want the timing to start coming in approx 200 rpm above your idle rpm. If you weld at the beginning of the slots you could raise the beginning poit to 500 or more rpm above your idle rpm.
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Old 01-04-2009, 02:03 PM
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manifold vacuum has nothing to do with the advance of the dist.Please explaine why the dist. advance doesn't advance when you close the throttle at 70 mph.The manifold vacuum would be very high but the advance in the dist. has very little.
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Old 01-04-2009, 06:39 PM
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Its manifold vacuum that makes the vacuum advance work!

The reason you don't get any advance with the throttle closed is the timed port the vacuum advance is connected to is above the throttle plate which is very near atmospheric pressure when its closed. That's why you don't get vacuum advance at idle either.

If the vacuum advance was connected to a direct manifold source it would go to full advance when the throttle is closed.
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Old 01-04-2009, 08:38 PM
Century Cp Guru Century Cp Guru is offline
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Doug The dist usually have 14 degs in the dist and 14 degrees in dist = 28 degs at crank .what i meant is watch the Vac advance with the light you will se how much it pulls in at crank , most people go nuts and start adding the 3 figures, i alway use the iniatial and dist mechanical and set with no vac advance with a light that has Degrees of advance Knob.
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Old 01-04-2009, 08:50 PM
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You do get vacuum at idle at the timed port.Thats why you are to disconnect hose before timing engine.To prove that manifold vacuum has nothing to do with vacuum advance,take the carb off the engine and blow air by the hole in the venturi and you will get vacuum at the timed port.In fact all the functions of the carb will work off the engine except the idle circuit.the idle holes are below the throttle blades and are exposed to manifold vacuum.The reason you don't get very much vacuum when the blades are closed is because very little air is flowing past the timed port hole in the venturi.So now you are going to say how come the advance stops working when you floor the engine and I am going to tell you why.The faster the air is moving thru the carb,the air tends to go to the middle of the venturi away from the hole for the timed advance port.Just like a fast flowing river the water in the center of the river is moving fast while the water at the shore is moving very slow or sometimes backwards.When you flow a carb you don't do it on the engine you do it with air blowing thru the carb,then you measure how much gas was drawn thru the carb.
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Old 01-05-2009, 03:45 PM
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The ported vs. manifold vacuum debates seem to pop up quite a bit in automotive discussions, and some people seem to tune their cars up well enough using one or the other.

Rusty, I've looked into the primary bores of my Thermoquad and cannot find any holes near the venturi. To be clear, my understanding is that the venturi are the narrowed portions of the plastic bowl where the boosters dump their fuel at this lower-pressure area. The only ports I can see are in the base plate/throttle body, which of course is about an inch below the actual venturi area. An image from my Carter Carburetors book shows ports in that lower base/throttle body, identifying them as a round idle port, vertical transfer slot, and horizontal "source of ported vacuum". By blowing through the external fitting, I find air flows through a hole in the throttle body, not through the venturi area that you've identified.

My understanding of a "manifold vacuum" source would be one that gets its signal purely from the underside of the throttle body, not within any of the 4 bores of the carburetor.

You're right, there must be a potential for air flow past the holes in the throttle body to generate a vacuum signal that will activate vac advance and that's why we disconnect/plug the vac fitting when setting initial timing. From the image in the carb book, it looks like that horizontal "source of ported vacuum" slot gets uncovered about the time the transfer slot comes into play, and it gets a strong signal pretty quick as it's oriented horizontally to expose a large area fast. It'll have that signal to some extent or other up to "full strength" as long as the throttle blades are in a position to expose the ports to the relatively lower air pressure inside the manifold as compared to air pressure above the blades, until those values equal enough that the signal at the port isn't sufficient to do any work. It seems to me that at WOT or even mostly open throttle, there will always be some residual signal, whether it's caused by air rushing past the opening/port in the throttle body or whatever difference remains between pressure/vac in the manifold itself and pressure/vac of the air in the throttle body at the point of the opening/port.

I haven't seen that fully explained, that's just my swipe at it, but it seems the important thing is to find out vacuum, in inches of mercury, at the port chosen to control the distributor throughout the range of the car's operation. That could be done with a long hose and gauge inside the car; it'd be great to have a way to determine actual position of the blades as you drive around, to know what the port is "seeing" to make it show the vacuum you find on the gauge.

I think I've seen a reference to a brass tube at mid-point of a TQ bowl body being a true port at the venturi that shows flow through the venturi/booster, but I don't think the porting for distributor vacuum advance is ever in that area.

BII

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Old 01-05-2009, 03:58 PM
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Quote:
BTW, you want to weld the slots at the end of the travel not the beginning. The reason you are impacting to parameters at the same time which makes it difficult to dial in. Welding at the end of the travel has no impact on when the timing starts to come in welding at the beginning will impact he starting point as well as the total. You want the timing to start coming in approx 200 rpm above your idle rpm. If you weld at the beginning of the slots you could raise the beginning poit to 500 or more rpm above your idle rpm.
DGC, I've already posted the numbers for the results of testing; the beginning point is 700rpm above idle. I saw 18deg from 750 to 1450rpm, then it threw in 15deg by 2050rpm and another degree by 2700rpm although I didn't run it in steps to find exactly where that one was added in. With the set of springs in there right now, I'd guess it should be pretty much all in, or maybe I wasn't reading the dial exact enough by flashlight in the relative dark to see the right mark on the gun and it's truly all in at 2050.

Not a big deal either way from 1550rpm on up, though, since your suggestion is that my car will be difficult to dial in with 18 degress, as measured, from idle to 1450rpm. Could you clarify, taking into account the gears, converter, etc. of my car?

BII
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Old 01-05-2009, 07:18 PM
dgc333 dgc333 is offline
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BII, having the advance start at 1450 rpm when you are idling at 700 is kind of high. Having it start at around 1100-1200 rpm would be better with it all still in at 2050. In any case if you wanted to reduce the timing added from your current 16 degrees and weld up the slots at the beginning it will preload the springs even more than they are now and will raise the the point at which the timing comes in even higher than the 1450 its at now. Lets say you want it coming in at 1450 you will need to reduce the spring preload.

Its this changing of two parameters (starting rpm for the advance and the total mechanical timing) with one change that I refer to as making it hard (as in difficult not impossible) to dial in. If you weld at the end of the slot it won't change the 1450 rpm starting point. Ideally
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Old 01-05-2009, 08:52 PM
dgc333 dgc333 is offline
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Quote:
Originally Posted by rusty duster View Post
You do get vacuum at idle at the timed port.Thats why you are to disconnect hose before timing engine.To prove that manifold vacuum has nothing to do with vacuum advance,take the carb off the engine and blow air by the hole in the venturi and you will get vacuum at the timed port.In fact all the functions of the carb will work off the engine except the idle circuit.the idle holes are below the throttle blades and are exposed to manifold vacuum.The reason you don't get very much vacuum when the blades are closed is because very little air is flowing past the timed port hole in the venturi.So now you are going to say how come the advance stops working when you floor the engine and I am going to tell you why.The faster the air is moving thru the carb,the air tends to go to the middle of the venturi away from the hole for the timed advance port.Just like a fast flowing river the water in the center of the river is moving fast while the water at the shore is moving very slow or sometimes backwards.When you flow a carb you don't do it on the engine you do it with air blowing thru the carb,then you measure how much gas was drawn thru the carb.
If you want to split hairs yes there will be some vacuum on the timed port at idle but its so so much lower than the vacuum required to move the vacuum advance mechanism that you can say for all practical purposes there is no vacuum. The only way there would be absolutely zero vacuum would if there was a tight seal but then the engine wouldn't run.

If you are blowing air through the carb from the choke side you will see positive pressure on the timed port with the throttle closed because now the throttle is preventing air flow and pressure will build higher than atmospheric right where the port is. Once you open the throttle past the port the signal will drop until its reading a very very small.

The reason its recommended to remove the hose when timing the engine is not all engines use the timed port and some are connected directly to manifold vacuum or due to a long duration came the throttle has to be opened past the timed port to get the engine to idle. I have never worked on an engine in my 30+ years of wrenching that had enough vacuum on the timed port to move the vacuum advance that could not be attributed to the throttle blade being opened past the port. Even BII's post of a page from a carb manual confirms my statement that there is only atmospheric pressure on the transfer slot when the throttle is closed which is also a timed port.

If the vacuum advance was truly operated by vacuum generated by air flow over the port then the vacuum advance design would have to vary for every variation in the diameter of the port the size of the throttle bore and placement if the bore because all those things would impact the air flow across the port and the signal generated. Basically it would be unique for every carb. That is not the case they are rated on what manifold vacuum they start to add timing and the amount of manifold vacuum that is required to obtain full advance.

Your river analogy is is exactly opposite of whats happening in a carb. The edges of the river has all kinds of shape changes and obstructions that impact the flow resulting as you stated more velocity in the center. The carb is opposite, the obstruction (throttle blade, booster, choke) is in the center slowing down the flow and along the surface where the port is the velocity is not impacted.

I assume when you state "flow a carb" you mean measure the flow capacity in CFM. It doesn't really matter whether you blow air through the carb or suck it through like the motor does but most flow benches for doing this type of testing suck it through. In any case because air is compressible you have to define the pressure drop you are going to test at and at what pressure the CFM rating is being expressed at in order to get comparable results. The standard for 4bbl carbs is 1.5" of Water (approx 1/27 of a psi) pressure drop and the CFM is at atmosperic pressure. So they start a pump sucking air through the carb until they have a 1.5" of H20 lower pressure (ie vacuum) on the suction side compared to the side exposed to atmospheric pressure. The air flowing through the carb you measure the velocity and you kown the pressure so you can calculate the CFM rating of the carb.

NOW, there is two ways of doing this one is dry and the other is wet. Demon and some new Holleys are rated wet but most carbs ar rated dry. The difference is a dry rating is just that, there is no fuel in the carb and its strictly a measure of the air flow. In a wet flow there is gas (or more likely a non combustible equivalent) being feed into the bowls so it is being drawn into the air stream just like it is in actual use so you have a mixture of air and liquid. Remember liquids are not compressible and will take up space that would be occupied by air which is compressable. The result is the same carb measured wet will come up with a lower CFM rating than one measured dry.
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Old 01-05-2009, 09:59 PM
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BelvedereII BelvedereII is offline
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DGC, I don't know how much you're reading in this thread, I already wrote that
Quote:
so I had the inner ends of the slots welded up and I filed them down. Looks like it was close enough as it wound up with 17 crank degrees.
That means I've already done it and have given the results. To go back a little further, the distributor was giving 26 degrees before, and now, after welding on the insides of the slots it's giving 16 degrees.

Still, can you describe exactly what difficulties I will encounter in dialing this in as it stands? Remember, the welding's already done, and the curve is there for you to examine at your leisure. We've moved from theory to an existing condition, so tell me please what is going to be difficult.

BII
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