Low manifold vacuum?

[cdr]

AS SAID BY others, bump the initial up & the vacuum will come up !!!!!

 

[hunt2elk]

Second - and this is HUGE - you only have about 90 lbs of cranking pressure!?!

I'm not talking about a compression test Meeps, but rather a leak down test.

 

[moper]

I have a different view of FBO products and nothing persoanl against Don. I have talents with these dinosaurs many don't and have never found a need for his products or manifold vacuum to make my cars run better.

Admittedly I missed the leakdown numbers. You have some issues there if you have 14-15% leakdown on 3-4 holes. That's why the vacuum is low. You need to figure out why the rings or valves are not sealing. You should have somewhere around 6-8% leakage.

 

[Ratroaster]

Interesting. Trying to sort out the same issue on my '63 Plymouth. 5-6" of vacuum @
900 rpm at idle. Cam is an Engle; .594 on In & Ex. 298 duration on 108 lobe center. Not running power valve due to vacuum and still appear to be running rich. Not sure which direction to head. Probably do a compression test and go from there.

 

[Meep-Meep]

I'm not talking about a compression test Meeps, but rather a leak down test.

HA! What happens when I read fast wile working. Leak down not great but I'll bet most of your problems are cam related - as in too late closing or just too much. Do a cranking compression test.

I'm not talking about a compression test Meeps, but rather a leak down test.

HA! What happens when I read fast wile working. Leak down not great but do a cranking compression test also. Try to lock the distributor at 30 deg or so and see what happens and hope it will not kick back with that much advance. Did you degree the cam? Seems to me the cam might be the biggest factor here.

 

[moper]

The problem is the engine is losing a lot of the cylinder pressure the cam is trying to build. That's what the leak down test indicates. So while the cam "might" be a contributing factor - the mechanical health of either the rings or the valve job are diagnostically indicated problem areas that require further investigation.

 

[747mopar]

A couple of things I see right out of the gate causing me to raise a big red flag. First, why are you concerned about hooking up vacuum advance to manifold vacuum? Second - and this is HUGE - you only have about 90 lbs of cranking pressure!?! Stop everything and check the following:

1. The cam is way retarded (intake closing way too late).
   
2. The cam is just too much for 10:1 CR
3. You don't actually have 10:1 CR as you thought.

The above assumes you don't have any major assembly malfunctions, etc.. Get some answers and report back.

To expand on my manifold vac advance comment, forget about that widely publicized article that suggests you need to run your vac advance off the manifold vac source. BS! Either lock the distributor to full advance (high overlap cam race engines like this) or run a quick curve and ported vac advance with as much initial as your starter can stand. For the street ALWAYS run the vac advance off the ported port. Always.

Good call, I guess when I read the OP I didn't realize those numbers were the compression test results..... definitely something wrong there.

O.k. I'd like some clarification on your thinking regarding the manifold vacuum. Not questioning you just like hearing everyones thoughts. In my case I had a motor that cranked hard at 30 degrees but ran awesome off idle (anything less and it was very sluggish) so by running manifold vacuum and having the vacuum advance pull in 10 as soon as the motor starts I've been able to get the best of both worlds (20 for starting but 30 at idle)? I don't see a right or wrong here just different ways of achieving our goals.

Sorry, don't want to hijack the thread just curious.

 

[747mopar]

Simultaneous replies...... nevermind haha. You guys are getting me confused.

 

[moper]

O.k. I'd like some clarification on your thinking regarding the manifold vacuum. Not questioning you just like hearing everyones thoughts. In my case I had a motor that cranked hard at 30 degrees but ran awesome off idle (anything less and it was very sluggish) so by running manifold vacuum and having the vacuum advance pull in 10 as soon as the motor starts I've been able to get the best of both worlds (20 for starting but 30 at idle)? I don't see a right or wrong here just different ways of achieving our goals.

Timing is a fickle thing. It's very individualistic so I agree it's not a right or wrong, but more of a owner's satisfaction deal. If you as the owner are happy, then the situation "must be" fine. However, from my perspective, what you call "best of both worlds" I call worn parts, or a mismatched setup. I would say you fell prey to a savvy marketer who has convinced you that his snake oil is your cure. It's more and more common as the enthusiasts get younger and less aware of how these dinosaurs work.
I would equate it to my knowledge of computers. I buy one. And I use it. I can't fix it. I don't know how they work. Ignorance may be bliss but it limits my ability to determine if my machine "is fine". I should say I'm told my laptop is a POS by those in the know - but I'm blissfully ignorant of what "could be". So I ignore them...lol.
I would put it this way... Does ibuprofen permanently cure pain, or allow you to feel better while giving your body time to heal? Which is the "best case" there? I submit that from my experience and point of view manifold vacuum is your pain relief, only the engine won't ever heal. Kind of like running Av-gas or octane boosters. You are covering a problem rather than addressing and repairing it, and in that covering, you ultimately are losing out on performance of some kind.
I am curious - When does the centrifugal advance start and finish? What does your total timing end up at? How's your gas mileage?

 

[hunt2elk]

I am curious - When does the centrifugal advance start and finish? What does your total timing end up at? How's your gas mileage?

No idea on the milage as I haven't put more than 1/2 tank through it and most of that was at idle.
We were setting the timing at 16* to start, and then the vac advance puts in another 10*, so it will idle at 26*. With the vac unhooked and plugged my total mechanical is 34*, and hooked up it is 42*.
After talking to Dev and hearing that his motor starts fine at 20* with a milder cam, maybe I will try that. I think I got a starter kickback when hot at anything over 16*, but I will try this again.

 

[superfragl]

The can adjustment won't go that low and there is none made for a Mopar distibutor that will. Don says they like to have at least 10" of idle vac to get the cans to work.

Yes it will. I adjusted mine last night to start pulling at 1" and all in by 7"

 

[hunt2elk]

Yes it will. I adjusted mine last night to start pulling at 1" and all in by 7"

Where did you get that can?

 

[moper]

Remember, vacuum level is a symptom. More precisely it's a "side effect". A result of the mechanical parts not producing the desired affect. Just as the high percentage of cylinder leakage is. Neither are causes of hard parts not working, and neither in and of themselves can address the cause. Diagnostics tell you the story if you're willing to read and understand.

 

[superfragl]

It is a regular can off one of the distributor I had. I got two cans - one will start pulling at 5" and all in by 10" and second one is the one I use - it can be adjusted all in by as low as 5"

 

[hunt2elk]

It is a regular can off one of the distributor I had. I got two cans - one will start pulling at 5" and all in by 10" and second one is the one I use - it can be adjusted all in by as low as 5"

Sounds like what I need. The lowest one I have starts pulling @ 7".

 

[mentor70]

A couple of things I see right out of the gate causing me to raise a big red flag. First, why are you concerned about hooking up vacuum advance to manifold vacuum? Second - and this is HUGE - you only have about 90 lbs of cranking pressure!?! Stop everything and check the following:

1. The cam is way retarded (intake closing way too late).
   
2. The cam is just too much for 10:1 CR
3. You don't actually have 10:1 CR as you thought.

The above assumes you don't have any major assembly malfunctions, etc.. Get some answers and report back.

To expand on my manifold vac advance comment, forget about that widely publicized article that suggests you need to run your vac advance off the manifold vac source. BS! Either lock the distributor to full advance (high overlap cam race engines like this) or run a quick curve and ported vac advance with as much initial as your starter can stand. For the street ALWAYS run the vac advance off the ported port. Always.

if he is running an FBO kit, he HAS to run the vacuum off the manifold, the distributors Don sells are modified to run vacuum off the manifold.

 

[superfragl]

To expand on my manifold vac advance comment, forget about that widely publicized article that suggests you need to run your vac advance off the manifold vac source. BS! Either lock the distributor to full advance (high overlap cam race engines like this) or run a quick curve and ported vac advance with as much initial as your starter can stand. For the street ALWAYS run the vac advance off the ported port. Always.

Please explain why it is BS??

 

[Meep-Meep]

Please explain why it is BS??

Pretty much every distributor advance mechanism is set up to advance the timing as RPM increases. At idle you set your initial and the idea is you should have no mechanical advance action at this point. The vacuum advance provides some amount of advance at some level of vacuum and only works when vacuum is present. Manifold vacuum at idle is usually enough to pull the diaphragm to it's fully rated advance setting but what happens when you open the throttle quickly? The manifold vacuum drops suddenly and that will almost always cause the timing to RETARD right off idle and that gives the impression of a flat spot causing people to blame the accelerator pump and whatnot. So, some might suggest very light advance weight springs to avoid this timing retard during the drop in vacuum. The weights fly out quickly to try and counter the effect of sudden timing retard, which may or may not work. Or just lock the distributor to full advance less ~10 degrees and use the vacuum can to pull the rest in once you generate manifold vacuum. This might be the best application of using manifold vacuum on the advance can but at this point you have no curve and your total timing is subject to a vacuum advance can working off a manifold vacuum signal against a spring. I call this a band aid and subject to problems. In my opinion if I wanted to retard the timing on a locked distributor to start the car (without killing the starter) I'd just do it electronically and MSD makes just the thing to do this with electronic precision.

My take for a street driven car is this: Select the components that produce proper cylinder pressure with a static compression ratio that is suitable for the available fuel - usually pump premium 91 or 92. For the street you are 9.5-10.5:1 depending on the engine and heads. Set the mechanical curve for performance, usually all in by 2500 and never cheat the timing! Meaning, run the maximum advance for that engine (MoPars are 36 - 38 deg, again, depending on heads, etc..) with a curve that works. Now connect the vacuum advance to the ported source and this is the brilliant part so pay attention. I call this a mechanical computer. Why? Because it gives you added advance during the times that you need it most and is dictated by engine load and throttle position.

During light loads like flat freeway cruising when your throttle plates are cracked open slightly you want more advance than what the mechanical curve can provide at that RPM. Remember, the mech curve is RPM dependent so adding about 10 degrees of advance during that point in the curve you get a pretty crisp running engine. 45 degrees may seem high at 2800 RPM, and it is under load, but this is the brilliant part. As you roll the throttle and the vacuum signal starts to fall off, the advance can defaults to the mech curve at whatever point it is in the RPM range!!! At WOT you have no vacuum advance so no danger of over advancing your engine and causing damage. The vacuum advance is a "smart switch" and poses no threat to your basic performance inspired timing curve because your engine does not rely on the vacuum advance to run properly. It's just an enhancement for that sustained light load cruising to the next car show. If you race you will likely leave with your foot to the floor or darn close to it, right? Well, the advance can won't even come into play because the throttle blade will pass the ported port so fast no significant vacuum signal/time will be generated. Hook it to the ported port, set timing @ in/Hg, forget it and reap the benefits.

 

[mentor70]

Thats a very sound, traditional theory. The only problem is, is every engine is different even with the same parts, and the "out of the box" vacuum advance on the distributor will need to be tuned on a case by case basis. all the Mopar ones ive encountered had well over 50* total even will only 8 to 10 initial. The procedure is covered in the ignition section of the old Mopar engine bible, but no one ever reads that section.

most all modern electronic controlled engines run like how FBO sets up their ignitions.

 

[moper]

I am with MEEPMEEP on this one. There is a "tipping point" where the "traditional" ported vacuum setup becomes useless: when the parts mismatch or poor execution produces a big enough negative effect on performance and it can't be tuned out given available options. However in those cases, a properly matched driveline removes the tendency for any issues when running timing locked or strictly initial/mechanical. That means when you have a raspy cam, you need deep gears and a high stall convertor to make the PACKAGE work and the understanding that your engine is not designed to run well at idle. You sacrifice idle and drivability for full throttle max power output. That works for a lot of people BTW but there is a reason you don't see a Cup car engine idling through Burger King's drive through. I'll add that a lower output engine, in a matched package, will usually outperform a higher powered engine in a mismatched package. That happens quite a bit on the street.

Thats a very sound, traditional theory. The only problem is, is every engine is different even with the same parts, and the "out of the box" vacuum advance on the distributor will need to be tuned on a case by case basis. all the Mopar ones ive encountered had well over 50* total even will only 8 to 10 initial. The procedure is covered in the ignition section of the old Mopar engine bible, but no one ever reads that section.
While I agree that different engines are different animals, the basic theory on these is the same up until one reaches the threshold I note above. If you are using a factory or the majority of aftermarket distributor "out of the box" there should have been a 7/32" allen key in that box. Usually along with bushings and/or springs to tune the distributor. So not only are you correct - tuning is expected to be needed. I also agree that Mopar liked a lot of total timing. However - that was not done in the vacuum can - but rather the centrifugal advance plate. There are instructions available online and in print back to the 60s on how to modify that to limit the total, enabling among other things more initial and better tuning of the vacuum advance unit.

most all modern electronic controlled engines run like how FBO sets up their ignitions.

Equating anything produced by FBO for these dinosaurs is akin to equating an Ipad to an abacus. I was a tech during the change-over from carburetors and solid state ignition to electronic carbs to TBI, EFI, and full ECM systems. Any distributor is only reactive based on the physical properties of the engine and can only respond with a singular output for that input. Modern management accounts for everything from fuel type to weather/altitude to driving style and is loaded with the ability to vary the performance in a variety of ways. No comparison.

What FBO does in running manifold vacuum is to provide a rigged cover-up for a parts mismatch or poor tuning or assembly or all of the above, during an engine's idling state, while sacrificing the part and light throttle drivability that the ported vacuum design provides. Engines, especially street and steet/strip engines, idle about 10% of their life and spend about 2% of their running life at WOT. So why would anyone want to get a distributor that maximizes idle quality and equals WOT performance by sacrificing the performance of the other 88% of the time? Because an enthusiast was not realistic through the whole process and the combination is mismatched. So now it idles nicely. Good for you .
What Don has made a success at is promoting this work around, and timing that work around with a rising ignorance factor of the enthusiasts as the older experienced ones drift away. FBO manifold vacuum distributors, such as they are, are nothing but way overpriced crutches and his ported distributors are overpriced and include nothing that cannot be done with 2hrs, $30 or $10 and a MIG welder, and some basic knowledge.