Quadrajets and full manifold vacuum.
 

I’m mostly just soliciting general conversation and information for the group here. I don’t necessarily have an issue. There have been the ported vs manifold arguments here before but I don’t recall seeing it in reference to specific carb styles

I’ve always ran off ported vacuum. But like most I have read the myriad articles and arguments that say ported spark is a remnant of the smog era and isn’t necessary. So out of curiosity I decided to experiment today. I was mostly interested in getting my car to run cooler at idle. That’s the one place I still have trouble is coolant temps at a dead stop.

Sean Murphy who built my carb told me he recommended ported spark because full manifold can lead to a hesitation when you stab the throttle due to the vacuum signal drop.. And sure enough that’s what I experienced today.

Moved the hose, adjusted down the idle. Went out and from a stop I hit it. Big stumble before it found itself. I didn’t floor it. I just sorta aggressively rolled into it. I wasn’t trying to lay 100 yards of rubber in front of my neighbors houses.

Mostly I’m just curious, are Q Jets just more prone to this? While they are well represented on the forums they do have the Quadrabog moniker. Are these carbs inherently better with ported spark?

I will say when I was letting it idle it seemed at a glance to be warming slower when I let it sit. I also think I could hear it pinging slightly, but I know my canister needs to be limited.

As per how it effects transition off the idle circuit in a q-jet, I have no idea.

However if you look at the timing maps of modern vehicles, you can see that even at idle rpms and no load, they are adding big amounts of timing. You can find some timing maps of stock or near stock vehicles that demonstrate this.

Ostensibly from an 08 (LS3 Corvette)
https://cimg2.ibsrv.net/gimg/ls1tech...c4660a3b70.png

Your top row is your map reading in kPA and your left side row is rpm. This is basically a load vs rpm relationship. Hi load, higher kPA etc.

So taking my personal car as an example, it idles at about 800 rpm and the map reading there is typically around .36. If you extrapolate that data to the above table, that engine would be running about 25 degrees of total timing. Based on where my initial is set and 10 degrees in the can, I'd be at 21 degrees, so that table isn't way far off. But you have to also look at things compared to total timing. I've got me engine setup to run 31 degrees total. The LS3 map shown above shows that engine in most of the loaded range is only wanting 21-23 degrees. So the fact that a stock LS3 at idle will want about 23 degrees of timing, tells you something about why you may want to be on manifold vacuum over ported vacuum.

the is of course effected by the volumetric efficiency of your engine at idle. Larger cams will produce less efficient idle, lower kPA and as a result will like even more timing at idle.

I'm not a carb expert, nor a q-jet expert, but I'm assuming if you're running manifold vacuum with one of those carbs, you can tune it to compensate for that loss of vacuum signal with a sharp throttle stab off-idle.

This may not be the answer you are looking for. But a mentor of mine when I began tuning and working on engines 40+ years ago, had this to say. "The engine will tell you what it wants, you have to be smart enough to listen to it and respond to it's voice". I have always kept this in the back of my mind whether tuning a stock engine or our nitro funny car engine. And it's true. More specific to your post, I don't think there is anything inherently different to how a Q-jet responds to timing vs another carburetor. The Q-jet is very precise on the primary side with the triple venturi, and may be a tad leaner right off idle. But this can be tuned around. Holley carburetors tend to cover their transitions by pouring on extra fuel, something the OE carbs couldn't do. That's why in almost every apples to apples situation, a Q-jet will get better mileage. The camshaft, amount of vacuum advance and the spring rate in the canister all have a real effect on "what is best" . Experiment with the vacuum advance and give the engine "what it wants". Step one is to buy and install an adjustable vacuum advance

I have a Crane adjustable canister with a vacuum limiter on my 396 with a QJet, stock cam and a points distributor. I have it hooked up to manifold vacuum. All the tuning was done with the distributor and the QJet wasn't touched with the exception of lowering the idle back down to 750 RPM. The car pulls without hesitation at all speeds plus a bit more MPG to boot.
10% ethanol, 91 octane, no pinging.
As mentioned above, it takes a little bit of listening to get it right.

https://www.summitracing.com/parts/crn-99601-1/

Yeah, at about 800 RPM I was seeing 30* or a little more when connected to manifold vacuum. (Base setting of about 12* at the crank) And I know on part throttle Im seeing deep 50s which ive talked about in other threads. I know I need to limit it. I have been waffling between that Crane product you showed and limiting it manually with a screw or something as others have documented.

That is kinda my plan. Limit the canister so it only allows maybe 12* added instead of closer to 18-19. Then maybe give the manifold thing a shot again.

Ive read all the arguments, at least on paper I agree that manifold maybe the way to go.

Very true.

I'm having the same problem with my 72 car.

It doesnt like full vacuum. I tried everything; adjustable vacuum can to reduce the amount of vacuum at idle, idle speed, mixture screws, more timing, less timing. Its a mild cam, around .450 lift. 18" of vacuum at idle.

Nothing works with full manifold vacuum. So I unplugged it, and it runs awesome with no vacuum advance at all. So be it. I wont waste time messing with ported vacuum. I know its benefits, but for now, no vacuum advance is what she likes.

All these old engines will like vacuum advance. It's just crucial to set them up properly.

The adjustable units need work. Can't just turn the screw on those things and toss in a limiter. The problem with the screw adjustment is that if you tighten that spring to increase the vacuum needed to pull it, it also limits the amount of movement of the rod, limiting the amount of advance and vice versa.

To start with, limiting the travel needs to be done on the pull side, not the push side like some instructions explain. It only needs to be limited about .200" give or take. That'll give you about 10-12 degrees of advance at the crank when it's all in.

The second critical part of those adjustable units is the screw adjustment. I find that most engines really like that screw pretty much all the way loose with no extra spring tension. This begins the vacuum advance pull around 6-7 inches and is all in by 10-12 inches. This is pretty much how the OEM cans worked on the performance engines of the 60's.

From there it's up to the individual whether they want ported or manifold. The unit will work basically identical with the only real difference being on manifold you have the extra timing at idle and light throttle or coast conditions. When you whack the throttle, if everything is set properly, it shouldn't matter to the carb which way it's hooked up, because at full throttle that unit is taken completely out of the equation anyway and running solely on the mechanical advance.

Here are a couple of Pontiacs with Q-jets, both running vacuum advance on manifold vacuum that I set up like I described above. Both run a pretty basic curve of about 14 initial and 34 total, and the vacuum advance is tossing in another 12 degrees or so maximum. They idle rock solid with very little rpm change in and out of gear, and throttle response is crisp and clean.

https://youtu.be/AOqwz3QlO5k

https://youtu.be/Iesvy0R3isA

https://youtu.be/bavv6KgUcmw

https://youtu.be/er1z7PpqsnY

There is no right answer.

If you have it on manifold vacuum and have high idle vacuum, running good at part idle or cruising just off idle, and you punch it. you are going to lose some of the advance during that time of low manifold vacuum when you punch. That sudden vacuum drop results in the bog from the retarded timing..

But a big cam with low vacuum idle isn't going to lose much advance at idle or off idle anyway especially if the vacuum cannister is adjusted to only come in at higher vacuum or only add a little extra advance.


A low vacuum engine needs a different cannister (or adjustment) than a high vacuum engine. And my opinion is that idling with the vacuum advance on the manifold defeats the purpose of adding more vacuum only at cruise., but manifold vacuum may be the best option if you don't have the 'perfect' cannister for your application.

People are bound to disagree on which is best, because what is best is determined by the parts you are running. The factory seemed to be all over the place on vacuum advance settings for all their various engines.

Despite all the good info out there folks still do not understand how VA really works.

The factory was much better at the source location for ported vacuum advance than the aftermarket. If you look very closely at the source location in a Q-jet for example it will provide FULL MANIFOLD VACUUM to the distributors Vacuum Advance with the slightest movement of the throttle off idle.

One has to realize that there are numerous other ported sources on many Q-jets that were NOT designed to operate the distributor VA. They were for other things like the EGR valve and the source location is higher and/or they have a "bleed off" slot cut vertically to lower the signal so the EGR didn't fully apply and lean out the engine in the "normal" driving range.

If you are using a correct source location for ported vacuum you will have FULL MANIFOLD VACUUM to the Vacuum Advance for all "normal" driving scenarios and it will have the exact same vacuum reading as all other manifold sources below the throttle plates.

So the only real difference between ported and manifold vacuum will be no VA idle and coasting.

Some engine don't care at all where you source the signal to the VA, others are finicky if you add too much timing idle/coasting and others thrive on it.

Since I use higher compression with my engines and camshafts with improved vacuum at idle (wider LSA/less overlap) they do not typically like, want or need any additional timing at idle speed. So in most cases I hook the VA up to a well located ported source. I also don't need to add much timing with the VA since I'm using tight squish and efficient dynamic compression at lower RPM's, etc.

I take in a LOT of troubled combinations here for custom tuning and find that despite all the attempts to make them happy before they got here the biggest problem with most are the distributor not solely with the carburetor. The root evil is often a poor cam choice for the CID and compression ratio as well.

A lot of engine "builders" and "guru's" still stick to "old school" methods to set up these engines and eliminate the VA and install super-quick spring/weight kits with a LOT of initial timing to offset the negatives from camshafts on tighter LSA's and more overlap that the engine likes/wants/needs at idle speed to be happy.

The cure for nearly all of them is to add idle fuel to the mixture screws so the engine can get plenty for complete combustion at the lower vacuum reading, and a little more timing at idle speed. NONE of them really like, want or need these super-quick mechanical advance curves well meaning tuners have put in them so I slow things down there as well.

I ALWAYS use the vacuum advance, and in most cases will test it both on a ported and manifold source to see how it responds. I also find myself reducing the amount of advance it adds "on the pull side" as mentioned above and use and prefer cans that start early and all in around 10-12" vacuum or so.

At this point I've cured so many piss poor running engine combo's I've lost count as I opened up my Saturday mornings to custom tuning troubled set-ups a couple years ago. It sort of got out of control and I found myself working almost every weekend and getting vehicles in here from great distances. To date the furthest cam from Atlanta Ga and most recently one from Canada.

I've only ran into a couple that I couldn't completely cure but was able to get most of them spot-on without a lot of effort. Few that I couldn't nail down had piss-poor cam choices, either WAY too small and/or LSA was too tight.

Something that's hard for me to get my head wrapped around is that with all the excellent info out there folks still think a 454/455 is going to "make more low end power" with a tiny cam in it on a tight LSA. What's really going to happen is that you cut the balls off of it and spike cylinder pressures up pretty high, and pinch off any potential for good upper mid-range and top end power using a cam in it that would be better suited for a 350 build with 8-9 to 1 compression in it.

Anyhow, what I recommend when it comes to this topic is to make absolutely sure that your distributor adds NOTHING below about 900-1000 RPM's with the mechanical advance, nice smooth curve all in around 2800-3200rpm's or so, and about 10-14 degrees from the VA. Set the initial timing so the total timing isn't going to pound the rod bearings out of it, then start testing manifold and ported VA sources to see how the engine responds. Some engines will enjoy 20 degrees or so timing at idle, others will not. Another thing that deal can get you on is that adding a bunch of timing at idle requires that you lower the idle speed screw and nearly close the throttle plates. With some combo's this leads to difficult restarts as the timing is retarded a bit and throttle plates closed down and not supplying enough air until it fires up and gets the additional timing to the VA via manifold vacuum. This is something you may encounter but is seldom discussed with this topic.......Cliff

I bolded that one section because I think that is likely my combos issues with full manifold. My cannister adds the better part of 20*. I know that is an issue. So my RPM in park jumped 500 RPM when I switched vacuum ports. At 1500 RPM I was seeing like 35*, probably some mechanical advance and all the vacuum advance. When I lowered the idle back down to 900ish I was seeing 30* on the crank, which is what I would expect given I know my initial is about 12*.

But because I had to back the idle screw off so much it likely had the throttle plate issue you described.


I remember in another thread you mentioned that there was a good one size fits most starting point as a measurement between the vacuum advance arm and a physical stop. But I cant find it. I think it was .120? Or something like between .110 and .150? Can you refresh our memories there?

One other oddball note, it seemed to me that the RPM drop from park to gear was greater when I had it on the full manifold. I get a little drop normally but its pretty tame. 100 RPM give or take. It was probably 250 or a little more on the full manifold. It actually died on me the first time I put it in drive.

Several reasons you can get a greater drop in RPM's with manifold vacuum. With some set-ups the load placed on the engine reduces vacuum just enough to drop out some timing from the VA.

In other cases the throttle plates are nearly closed and the load just isn't quite a stable as if they were open a little further with more transfer slot exposed when you place a load on the engine.

My last two engines don't like, need or want any additional timing from the VA. I can set them to do OK with it, but it doesn't fire up nearly as well after shutdown and it develops a very slight "skip" or miss in the idle quality with 24 degrees at idle vs 10.

Driving the vehicle really no difference either way, same power, throttle response, efficiency and never even thinks about heating up at stoplights or any length of time at idle in any weather.

Around 1/8" travel will be a good starting point with the VA. I use the MIG and prefer to provide a stop for it using a round file and drill bits to measure how much.......Cliff

Was watching a documentary on the 32 Ford V8 ... they were one of the first to use VA .. they considered it a timing LIMITING device under load.

In other words it was not intended to ADD timing, it was intended to limit or reduce timing during high load situations. At first it was a simple mechanism that locked out the mechanical advance under high load. Then transitioned to a device that would reduce timing under load.

I've always looked at modern VA as a method to reduce timing under load rather than a device to ever add additional timing ... because it's natural state while the engine is running is usually "all in" ... it's "active" state is when vacuum decreases and it removes timing.

I think because the spring in a VA tries to keep it at zero ... people think that's it's normal position ... but forget that in it's normal operating environment it would almost never be in the zero position unless there was zero vacuum.

I mean in reality it's function is a timing REDUCTION device, to reduce TOTAL timing when needed, not a device to ADD timing ... correct?

I remove the vacuum advance and install one of these cable timing controls to the vacuum advance plate, that way if I get a tank full of low grade fuel, I'll take a few degrees out...

I run the initial around 20-22, with a slow/late mechanical, bringing 12-14 in by 3000. I prefer to have a stable idle and not have it dancing around depended on some fickle vacuum dashpot..

Semantics I guess. You are looking to add timing at part throttle, or part throttle and idle depending on your setup. Keep extra timing out at full throttle. But I see your point.

Much like a power valve for a Holley carburetor was called an economizer valve in carburetors used by Ford.

Clay

I provided that measurement in my post Jeremy. Approximately .200" of movement is all you need from the vacuum advance. This will give you roughly 10-12 degrees of timing at the crank. 20 is too much for your combo and will cause all kinds of idle issue on manifold vacuum, and you almost won't even be able to bring the idle down and cause throttle blade position issues as described above, which gets into all kinds of other tuning and drivability problems..

You won't get wonky idle problems and inconsistency if you set them up as I described above. Watch the first video and you'll see, that engine, even with it's 239 @ .050 cam, doesn't change in and out of gear more than 150 rpm.

The old canisters from the mid 60's on some of the solid lifter performance engines, like the B29 (if memory serves) were my favorite to use for these applications, because they worked with 6-7 inches of vacuum and were all done by 10-12 inches, and they didn't throw a ton of advance at you. For a while I was still getting these from Napa through the Echlin line but they have long been discontinued. Lucky for us the adjustable units can be modified to work in the same way.
Like I said lighten the spring tension as far as it will go. That makes the can start working around 6-7 inches. You then limit the travel as I have done in the pic below. I prefer to weld the slot. Then I file it until I have about .200" of movement. This provides 10-12 degrees at the crank. Simple.

If you have mechanical advance set up properly, you won't get any stumble, as the vacuum drops out with throttle input, the mechanical should be taking over in a linear fashion. At no point would there be "no timing" because vacuum dropped, it just doesn't happen that way. And there is no extra timing at full throttle because there is no vacuum pulling on the can. If a stumble is occurring something is fundamentally wrong with the setup.

This quote from Cliff explains why some people see wonky idle. This is especially true when you use a vacuum canister that adds a ton of timing and doesn't even begin to work until 12-15 inches of vacuum, then at 20 inches it's throwing a crap load of timing at you.

With engines that idle with 12 inches in gear, then vacuum readings jump to 15 inches out of gear, this just won't work. When the engine is put in park that extra vacuum starts throwing a ton of timing at it and the idle jumps considerably, and as idle rises so does the vacuum and timing and it compounds the problem.

This is why you want the canister to start at the lower vacuum setting, and have it all done by 12 inches or so, just like the old performance cans used to do, so it's not as sensitive in and out of gear.

The 455 I built in the first video above idles up here at 5,000 feet elevation with 12 inches of vacuum. That's going to be about 15 inches at sea level. So with the vacuum advance setup like I described, the engine isn't even dropping enough vacuum in and out of gear to affect the vacuum advance, so you have a rock solid stable idle, even when I turn on the AC.

Not trying to be nitpicky, but are you sure the units in that graph are Kpa- most likely Bar? Also, just looking at the table, this is a boosted engine and they are pulling timing at high loads?

Am I missing something?

http://www.crankshaftcoalition.com/w...ance_Specs.pdf

I know ive seen lists like these before. Some of those "VC" numbers are still available. Just as an experiment I might try one that looks like it will fit my needs, rather than an adjustable unit.

https://www.napaonline.com/en/p/ECHVC1827 like this one, which is listed as starting at a low signal and only adding 5* at the distributor. 10-12 at the crank is likely where I want to be.

Kind of makes you wish someone produced a servo control VA ... vacuum sensor to a small microprocessor .. to distributor mounted servo. Dial in your start, climb rate and max. I found you can map a particular can pretty easy with a hand vacuum pump ... at least fairly easy to find start vacuum and when it's maxed out.