My engine vacuum was 10-12 on carb. Now that I have Holly Sniper it seems to run around 8. I can't find any leaks but my headlights work extremely slow. Will EFI vs carb lower overall vacuum due to 4 barrels cracked open instead of 2? I really thought the opening would have sort of equaled out.

 

Absolutely not. Your EFI will definitely NOT change engine vacuum. You definitely have a leak somewhere.
Vacuum is created by the downward movement of the pistons and is greatly affected by cam overlap.
On your EFI at idle your 4 butterfly's are nearly closed. And most of your idle air is controlled by your IAC. Which is in essence, bypass air. If anything your 4 butterfly's are flowing less air than just the 2 on the carb.
You indeed have a issue. But you can't blame it on your EFI. Perhaps a poor installation of the EFI. Perhaps vacuum leaks inside the lifter valley at the intake manifold gaskets. Perhaps a poor seal at the throttle body to manifold. Perhaps your losing vacuum elsewhere in the car? Perhaps a shot PCV valve or incorrect PCV valve for the cam in your engine?
plug ALL lines. Read manifold vacuum again.
If still low. Pull and reseal that intake manifold.

 

What other mods have you done that 12 inhg was your starting vacuum?

 

carb or efi if all dialed in should have the same amount of vacuum. If I create a vacuum leak like pulling off a capped port my idle speed actually increases a minor amount. I'm against using any type of PVC that flows water vapor, oil mist, and unburnt bad gasses back into the manifold. Valve covers only need a vent or some type of vacuum pump going to a canister trap.

Did you change the ignition timing or is the efi way off at idle?

 

Did you plug any unused vacuum ports on the Sniper?

 

I installed the EFI a long time ago. This was the first time I had tried my headlights since. I virtually never drive it at night. The lights work but very slowly now. I still suspect I have a leak somewhere but haven't found it yet. Just happens that this was the first time checked the vacuum since EFI installation. I'm pretty sure the vacuum was still good at that time. I'm probably going to have to do some serious searching to find a leak Lights were just what got me doing a vacuum check in a very long time.

 

Just plug the vacuum line source going to the headlights and see if your vacuum increases. In other words eliminate the entire headlamp vacuum system to narrow down the source. If your vacuum increases the leak is in the headlamp side of the system. BTW never mentioned the year. I assume it's later than 1972 which was the last year for the wiper door.

 

Has your timing been altered? This can change your vacuum

 

Disconnect and plug off ALL vacuum sources at the engine to determine what your MAX baseline vacuum level is for your engine. Unless you have a pretty aggressive cam, you should get something between 18-21 in. Hg. on the vacuum gauge. Then add the vacuum lines back into the system ONE AT A TIME. If the vacuum level drops when you add something back into the system, THAT has a vacuum leak somewhere in that "branch". Search it out and fix that leak before continuing. NOTHING in a vacuum system SHOULD leak at all, unless there is a problem somewhere. With older components (vacuum relays, vacuum actuators, etc.) some internal seals could be worn and allow a bit of leakage. But, more than 1 in. Hg loss in any "branch" is really too much, IMO.

P.S. Ignition timing can make a big difference in how much vacuum an engine can produce. It may be beneficial to adjust your timing for maximum vacuum at idle when doing the above tests. Once the vacuum issues have been solved, you can then reset ignition timing wherever you wish it to be (max performance, max economy, etc.)

 

This is why I asked.

 

You obviously have a cam with similar duration and overlap to my 30-30 cam at 10-12" @ idle.
I found idle rpm made a huge difference in vac level. Even 50 or 100 rpm.
I wound up at 950 for the most stable idle.
You?
And the timing at idle also made a huge difference.
Any changes to either of these?
Are you letting the efi control the timing?

 

 

​​​​​​My 383 before proflow 4 was 11 to 12 inches of vacume I had the same 383 rebuilt same everything except went injection, I get now 9 inches now, everything Is capped my brake booster and vacume to headlights, no change, thought maybe intake gasket, changed it, it's the same..
I used propane around the throttle body and everywhere I thought I could get a leak nothing...

 

There are progressive throttle kits available for the Sniper, both fixed rate and adjustable. This won't fix the vacuum issue, but might help with part throttle and off idle driveablity. I've got (had) a healthy, but not huge cam and the initial tip in is pretty aggressive. I like it, but it could be considered touchy.

 

The Carburetor or the throttle body doesn't make or delete vacuum. When I switched from a vacuum secondary Holley Carburetor to a 4 barrel throttle body my vacuum did not change at all. Not a bit.

 

To echo some of the other forum members:

If all engine parameters are the the same, there should be no change in idle vacuum. If the engine idle vacuum has changed after the swap, then some other parameter has also changed. As mentioned, the EFI system simply delivers fuel and air but doesn't create nor destroy engine vacuum.

What I have seen after a swap:

1. Timing changes especially when utilizing a hyperspark or similarly controlled timing. People have a tendency to undertime their engines because they believe 15 degrees should be in the base timing area at idle. That setting completely ignores the normal added timing due to vacuum advance. Since the table takes into account both RPM and engine load (vacuum) the number generally needs to be around 20-26 degrees depending on engine combo. Most undertimed engines will show a drop in manifold vacuum.
2. Vacuum leak introduced during physical swapping process.
3. PCV issues.
4. Incorrect throttle blade setting. However, this will also cause IAC issues and idle RPM issues. If your engine is idling correctly at the same or similar timing areas, your vacuum should be the same.
5. Too low of an idle RPM setting. Unlike a carb where you typically set RPM based upon maximum idle vacuum and a good fuel mixture (and proper timing) EFI will force the engine to idle at the user's desired setpoint. That may not be where the engine itself desires to be. That also usually shows up as a lower than normal vacuum.
6. Other engine issues

 

I have seen factory timed cars with 4* initial advance and no vac at idle due to ported vac.
The we switch to 12* at idle and add 10* due to vac on manifold advance, for 22* total.
Idle vac jumped a lot, like 4-6"
Idle rpm jumped 200 also, forcing you to turn it back down. And vac was still up.
Factory timing settings are very retarded, at idle, and at low rpm, for early pre-computer emissions control attempts.
They tried to make it run hotter, so it would burn off HC in the hot exhaust pipes.

 

Yes! Best emissions aren't necessarily the best running conditions. As Leigh mentions ported vacuum was a crude way to retard timing to jack up combustion temperatures to burn up hydrocarbons. You can easily simulate that with EFI controlled timing, however it's not necessarily the most efficient place to be.

You could as an example, run a lower timing, yet program the EFI system to maintain a higher RPM which would open the IAC more to compensate. That combination may also cause a decrease in vacuum depending on the engine characteristics. Advanced timing during slower combustion events will always produce a more efficient combustion.

 

Yes I have seen an EFI timing table, but would not know how to go about programming one.
Something like 22-26* at idle right?
Then 36* total above 3000.
Those parts I get. That is the same as std.
But being a 3-D chart it is the rest of it that gets confusing.
Still needs to drop to like 15* or so near idle once the vac drops.
It's all the curves and slopes in between that make me feel like I am skiing! LOL

This 2-D chart:

OEM 1964 GM factory L76 mechanical only advance curve

Turns into something like this in 3-D:

The typical 2-3 numbers we always talk about are in there somewhere! Plus a whole lot more!
Yellow is 36*
Orange is above, up to 50*
Blue is below 15* and below

I guess the mechanical & vac system in our cars does the same thing, I just have never seen someone try to map it that way.
Deg* advance, RPM, and Vacuum level.

Oohh I basically "see" the L76 curve on the near edge of that 3D map!

 

I don't want to hijack the thread and turn it into another EFI timing thread, I've done that here: Sniper for Dummies Level 2
Yes, as you can see the mechanical curve is the 'X' horizontal, and the engine load or vacuum curve is the 'Y' vertical axis. As our older cars move through the 'table' it is being affected by both axis simultaneously. EFI timing tables are just like that, but you can customize that with a much greater level of control. You aren't limited to a vacuum canister, centrifugal weights and springs. There are numerous ways to generate that 2D / 3D table and they aren't hard if you understand what your distributor and vacuum advance are doing.

Here is one method. Using a spreadsheet (that I did not develope) and knowing your mechanical timing properties, you plug in those numbers and it generates the table for you.



What it is doing is integrating the X and Y axis numbers together, just like your mechanical advance and vacuum advance would be doing mechanically. If you notice, the idle area timing in this example with a base timing of 16 is actually 26 degrees. That would be exactly where the mechanical setup would be idling. What I see a lot of, users will fill the idle area with 15 degrees, and there car's engine wants more like 22 degrees. We discussed ported vacuum, but that is an emissions timing level not a good idle timing number. The chart above would be the equivalent to a manifold vacuum sense, not ported.

Another method to build good EFI timing tables, is to use the graphs 'fill' function in the EFI software. This is not difficult, but it would require too much time to reproduce here. It is explained in greater detail in the timing section in the above hyper link.

Here's an example of an EFI timing table that integrates both in the Holley software:



Furthermore, if you wanted to have a 'conditional' timing table that modified the base table above, you could build in conditions that the timing table had modifiers. For example, if you wanted less timing during engine warmup in order to get engine temperatures up faster, you could do that. You could use engine coolant temp as a condition, and pull out timing in the idle area only until the engine reached a desired temperature.

Anyway, back to the original purpose of the thread. If the engine is under-timed, it will have an effect on engine vacuum.