Hey guys and gals,

I just purchased a vacuum guage and I have some questions about some of my findings.

First, I hooked the guage up to the manifold and found that I have 16-17 inches of vacuum. Is this normal for a 1985 Ford with 460 and C6 tranny? I have the motor timed at 10 BTDC. My guage says "timing set late" in this range. Normal on the guage is in the 20 range.

Next, I noticed that the vacuum to the carb is not hooked up as the decal under the hood shows. It is a Holley 4180(all emissions equip has been removed by previous owner-thus the incorrect vacuum hookups) with the vacuum advance from the distributor hooked up to the manifold through a 3-way tee threaded into the intake. From this 3-way, another vacuum line goes to the top left hand side of the carb. The third stem of this 3 way is capped off. My truck seems to run good with this setup, but should'nt the advance work off ported vacuum, not manifold vacuum. Also, there is a VCV valve on the water neck that is not hooked up to anything. Is the vacuum this way because emissions equip has been removed?

Sorry so long, but I'm confused!

 

Anyone have any experience with this?

 

If your engine is running fine why worry about it. My haynes manual says that the Duraspark II requires Manifold vaccum and not ported.

My 2 cents....

 

youngguy,

The tree on the t’stat housing is a thermal switch. It only allows vacuum to pass through it when the motor has reached a certain temp. The distributor vacuum advance may well have run through it, but it is for emission purposes so I would not use it if all the other emission stuff is off the motor.

I would hook up the distributor to ported vacuum. Cap everything else. The timing set at 10 is a little low. Try moving it up a bit, 12 – 14 range, and see how it runs and the impact on the vacuum readings. Also, the manifold is really two in one. The vacuum tree on the front of the manifold is from the front four cylinders while the back tree (at the very back of the manifold and where the power brake booster is connected) is from the rear four cylinders. See how the two differ.

Also make sure the idle speed is set high enough. As it increases so will manifold vacuum idle readings.

Have you checked for vacuum leaks? To do so, first check that anything still using vacuum is not leaking. This would include the auto trans, brake booster, EGR, HVAC, and anything else that you can trace. The best way is to see if you have any device leaking is to read the vacuum with it connected (as it is now) and then disconnect one device at a time making sure the port it came from is properly capped off. Only do one device at a time.

Then, after you know that all devices are not leaking, spray carb cleaner all around the union of the intake to the cylinder heads, around the carb, carb base, EGR plate, vacuum trees and anywhere else that manifold vacuum could leak. Look for a brief increase in vacuum and idle speed and if you get it you have found a leak. Often the manifold will need to be retorqued to stop leaking.

My motor had similar readings as yours, but I have been able to get it up to about 21 by tracking down all leaks, setting the timing correctly, and dialing in the carb.

Good luck,

Mark

 

The following may be helpful: http://www.centuryperformance.com/vacuum.asp

 

I agree with both mchild and FordF150Fixer,

I run my advance on ported vacuum only because it runs better than from manifold. Since most of your emission controls are missing, I would use mchild's suggestion to start capping unused ports. 20" of vacuum at idle is normal, 15-17" will normally indicate a small leak.

 

Thanks Fellas,

This is a good starting point. I will let you know what I find. Probably next week as my boys and I are heading for Vegas for the Monster Jam World Finals.

 

Anyone care to explain "ported vaccuum" vs. manifold vaccuum to a guy who has NO clue what you are talking about?
Thanks,
Zac

 

Manifold vacuum is vacuum directly from the manifold and has normal vacuum at idle.

Ported vacuum is vacuum from above the throttle plate on the carb, thus there is very little if any vacuum until the throttle is operated opening the throttle plate.

I believe this is the best way I can describe it-correct me if I'm wrong.

 

Thanks a bunch. I was beginning to wonder if I was ever going to learn all of the terminology surrounding the internal combustion engine.

 

aka,

Stay here for a little while and we will get you up to speed in no time.

 

So let me get this right. If you hook a vacuum gauge to the manifold vacuum it should be constant.
When I hook it to my engine it seems that as I increase the rpm's the vacuum increases. Isn't this the way is should be?

 

Manifold vacuum is present when the motor is drawing in air (and fuel) to the combustion chambers against the closed throttle plates. Think about sucking on a straw with your finger over the open end. With the open end completely closed off the vacuum in the straw is rather high. Now, if you slowly open the other end while providing a constant level of suction then the vacuum will drop as air is able to be drawn through. But, if the suction level is increased – as in increasing the engine rpm’s – then the vacuum can increase even higher. That is what occurs with an internal combustion engine. There is a point where the throttle plates are open so much that the vacuum level is very low even with very high rpms.

So what you are seeing happen with your vacuum gauge is correct.

The issue on ported versus manifold vacuum is where the vacuum is being read. Manifold vacuum is on the manifold side of the carb (i.e. inside of the straw) and thus is whatever the motor is creating. Ported or timed vacuum is just above the throttle plates, and therefore, does not “see” vacuum until the throttle starts to open. At some point, with the throttle plates opened enough, the two are the same.

Hooking the distributor to ported vacuum provides timing advancement as the throttle is slowly opened and the port “sees” the vacuum. If the throttle is fully opened – as in flooring it – then the vacuum drops until the rpms get high enough to create some vacuum. The vacuum advance is designed to give more advance under light load acceleration when the motor can handle it. Under heavy load it would probably experience pre-ignition with the amount of advance that the vacuum advance typically provides.

I hope that helps but feel free to ask questions.

Mark

 

Hey, thanks for the reply....that clears up a lot of questions I had.