Engine Vacuum reserve capacity question

Mcmahst · Apr 24, 2025

I have a 73 Mustang with an 87 5.0 drivetrain, with all stock systems, except the TAB/TAD system is removed. I was wondering if I could plumb the vacuum needs of the EGR regulator and the AC system by using just one vacuum canister? I would use at least one check valve (between the intake and canister). Do you think that is sufficient capacity for those needs?

Noobz347 · Apr 24, 2025

It depends on the size of the canister.

Mcmahst · Apr 24, 2025

7 cubit inches

Noobz347 · Apr 24, 2025

That sounds really small but... Does the A/C really need one?

90trunk · Apr 24, 2025

Are you going to use a large 70's era Ford "Coffee Can" vacuum reservoir or the Foxbody HVAC "Sphere" and TAB/TAD "Tuna Can" combo?
As mentioned above, size/capacity matters (as it always does...).

If you are using the Foxbody reservoirs and not using the "Tuna Can" for the TAB/TAD you can "T" them together. There's no penalty for having a larger vacuum reservoir capacity on your HVAC vacuum control lines.

Mcmahst · Apr 25, 2025

Yea, I kinda screwed up. When I did the layout of the 73 and 87 components I needed, I only made room for one small vacuum canister. It’s a small round, cylindrical plastic unit with the strap at the bottom. It’s not stock 73 or 87, one I had from a previous project that fit in the space I had. I may try and fit the stock 87 ball shaped unit in the same space, under the vapor recovery can. It’s got to be twice the volume of the one there now. And yes, while I’m unsure how much vacuum the EGR regulator would consume while at cruising speed, the 73 AC unit has 3 or 4 vacuum motors on the HCAV unit. Really, I figured my only concern would be under WOT conditions. With the check valve in place, can’t see reaching over to change the settings on the HVAC unit with my right foot buried on the pedal. But, the EGR regulator is something I’m not well versed on, in typical operating situations. So, once I get back to my car in a few weeks, I’ll post a pic. I will see if I can fit the larger unit in its place. Thanks for the helpful questions.

Mcmahst · Apr 25, 2025

If this diagram can be used for the 87/73 integration, it seems I can get away with not using the vacuum reserve on anything but the HVAC unit. I will need two check valves though. Am I reading that correctly?

Noobz347 · Apr 25, 2025

Mcmahst said:
my only concern would be under WOT conditions.

Right, when you floor it, you'll probably get a blast of air from the defrost vents.

Mcmahst · Apr 25, 2025

But that’s what the check valve prevents, as long as the vacuum circuit is closed and not leaking. I’ve tested all of the components and I have a sealed system. Now, if I was to move the controls during WOT, that would be a problem.

General karthief · Apr 25, 2025

What reason you have to move the controls during WOT?
A vac can with a check valve on the engine to can line and the controls don't move when vac drops.
Am I missing something?
Since this is a transplant and no emissions (right) you don't need vac to the EGR valve I think.
it's normally closed right?

Mcmahst · Apr 25, 2025

I was being sarcastic. I was exaggerating the circumstances.

I’m thinking I’m good with the small reserve I have, but I’m gonna explore if the ball shaped stock one from the 87 will fit. I posted that diagram to illustrate the stock setup doesn’t use a reserve on the EGR regulator. Now, if that diagram is wrong, it would be nice for someone to inform me. So, thanks for the questions.

nicholase · Apr 26, 2025

That schematic you posted is correct. I just took off my EGR and Smog pump vacuum solenoids and used that same schematic. This is what it looks like in reality. Single black hose goes to vacuum can. EGR top, TAB,TAD bottom.

Looking close you can see the red vacuum line for the EGR is bypassing any check valves and hooked straight up. That red line splits and goes to a check valve, then to a vacuum can and then to TAB,TAD.

This whole assembley is seperate from the climate control vacuum. It comes out just like it looks in the picture. The climate control uses a little vacuum ball the size of a baseball. Maybe a little smaller.

The EGR seems to use no vacuum reserve. Probably because its only operated under light throttle amd steady cruise to the bestof my knowledge. It's not used during WOT or heavy throttle. So the lack of vacum reserve makes sense.

Mcmahst · Apr 26, 2025

Thank you @nicholase for the confirmation. I’m gonna put that baseball reserve on, if I can get it to fit.

Mcmahst · Apr 27, 2025

I’m sorry to the community that I created a confusing posting. It happens sometimes that my mouth is two steps ahead of my brain. I couldn’t have made my car what it is without your help. Again, thank you for taking your time to help a stranger. Steve

Engine Vacuum reserve capacity question

Mcmahst

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

Mcmahst

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

90trunk

Active Member

Mcmahst

Mcmahst

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

Mcmahst

General karthief

wonder how much it would cost to ship you a pair

Mcmahst

nicholase

I lubed and pushed, until it was all the way in

Mcmahst

Mcmahst

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Engine Vacuum reserve capacity question

Stangnet Facilities Maint Tech... Er... Janitor

Stangnet Facilities Maint Tech... Er... Janitor

Active Member

Stangnet Facilities Maint Tech... Er... Janitor

wonder how much it would cost to ship you a pair

I lubed and pushed, until it was all the way in

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