brotherGood

Sort of piggybacking off of my engine thread, but I feel this could be better suited as a general question.

I am currently averaging around 7-8in of vacuum at idle with my 400. The general consensus by a majority of people is that is due to the cam. While I'm still not 100% sold on that, it does lead to this question though.

For that low vacuum, what would be the better option? For context, this will be a majority street driven car, with PB and AC, as well as the Trans Modulator. Would hooking the reservoir up to the brakes help alleviate not only the braking, but also ensure everything else has what it needs? Or should I be looking into a pump of some sort? I get the pump is more expensive, and runs the risk of possible safety issues if it stops working, but would the low vacuum be enough to run the trans/AC if the reservoir is hooked to the brakes?

Just trying to get an understanding,

Thanks in advance.

Olds64

I wouldn't depend on a vacuum reservoir to supply vacuum to power brakes on a street driven car.

The 6.9l IDI in my Ford truck uses a vacuum pump to supply vacuum to the power brakes and transmission modulator. In later years Ford went to hydroboost brakes on their trucks. That's another option you might consider. Of course, you might have to change the transmission modulator too.

VC455

A vacuum reservoir has capacity for a few brake applications. After that you need to recharge it by running the engine at high idle for 15 seconds or so.

Unless everything in the system is completely tight, you will lose vacuum in a few days--meaning that you will have no brakes when you first start the car.

A vacuum pump will start running when you turn on the key and within about 5 seconds you will have full brakes.

I went the vacuum pump route when I changed the cam. I leave the key in run position until the pump shuts off and then I start the car. That way I'm never without brakes when the engine is operating.

BangScreech4-4-2

I'm not convinced that your engine should have such low vacuum, even with the cam specs that you listed in your other thread. Something ain't right.

However ... if it can be established that you have enough vacuum to run the transmission, why not ditch the PB booster altogether? I've driven lots of A-bodies equipped with manual brakes, and they don't stop any worse than their boosted counterparts. Just takes a little more muscle, is all.

brotherGood

Without breaking into the engine, I'm not sure where the vacuum issue is coming from. I had thought about rebuilding my Qjet to verify its not carb/spacer related, but I don't see that making up that big of the difference. Even the amount of tuning that carburetor may need, it doesn't have that much in it. I also don't really know who around here I can trust to open the engine back up to verify, as quite frankly I've been burned by "the good guys" in the area with this excursion. If this were my small block Mopar that I built, I'd have no issues opening it up because if I screwed it up bad enough, I could just grab another and try again. I can't afford to risk it with this being the original engine.

In regards to the booster, I'd like to keep PB if at all possible. Beyond the headache I had when rebuilding the brake system trying to find the correct booster/master cylinder, I also like the idea of PB for both whenever my wife decides she wants to drive it, as well as if I ever go to sell the car down the road.

joe_padavano

A vacuum reservoir cannot increase vacuum level beyond what the engine provides. If the engine only makes 8" of vacuum, the reservoir will never have more than that. A vacuum pump is independent of the engine and will make whatever vacuum you want (up to atmospheric pressure), depending on how long it runs. The brake power booster "assist" is a function of the vacuum level and the booster diaphragm diameter. 8" of vacuum will not provide sufficient brake assist even with an 11" or dual 9" booster.

The AT modulator needs direct connection to manifold vacuum to operate. The modulator uses manifold vacuum as an indication of engine load (high vacuum = low load, low vacuum = wide open throttle). You cannot connect the modulator to a reservoir or independent pump - that defeats the whole purpose of the modulator. If your vacuum is consistently low, you need a recalibrated modulator to account for that.

The bigger question is why the vacuum is low. If you are not "100% convinced" the cam is the problem (and seriously, you don't know what cam is in that engine?), then you'd better figure THIS out first, before spending money on anything else.

brotherGood

I know what cam is in the engine, but everyone I talk to aside from those on this forum are saying with the size of the cam-I'd expect low vacuum. I have had a few conversations with Mark about this, and trust I'm getting good info..which is why I'm at a loss as to why I can't obtain it.

If I had someone around me I'd trust to get into the engine and verify the cam was installed correctly, and that the valve timing isn't off..I'd go that route first. The only other thing I could think of would be a leak at the intake due to mating surfaces not lining up since the block was zero decked. But that'd be something I would've expected both shops had addressed, or more importantly I'd be able to hear. It does sound like its got blower whine the more throttle you give it, and it sounds like its coming from behind the engine based on how I hear it moreso at the doors than the nose, but I can't figure out what'd be leaking back there if that'd be the case.

OLDSter Ralph

Joe P pretty much laid everything out for you. What did Cutlassefi tell you about the cam and vacuum ?
Perhaps post:
your idle RPM,
Initial timing (with vacuum advance disconnected and line plugged)
Distributor type (points or HEI)
Camshaft specs
Carburetor and jetting.

One thing that crossed my mind........did you check for a vacuum leak at the rear of the engine where the intake manifold mates to the top of the engine block ?

brotherGood

Cam should not be an issue in regards to vacuum. Best idle RPM I can get to is about 1100-1200 in park. Initial timing is 20 degrees, timed via total minus mechanical. Running a rebuilt and recurved points dist. (original to the car). Cam is Erson TQ50H (228/235 @ .050). Carb is new OOTB Summit M2008 (750 cfm). I've not adjusted the carb much more than Idle screw and Idle air mix screws, as I am not confident things are to that point (i.e. lack of vacuum)

That is the one place that we had not sprayed, as we didn't really see much of a way to get to it. That being said, as mentioned earlier it sounds like its got a freakin' blower and I hear it most when everything is shut and its as if its coming from behind the front tires (rear of engine would be about there)

As touched on in the engine thread a few minutes ago, I'm going to pull the intake off and swap the valley pan for gaskets, as well as verify things are actually lined up (the block was machined but I'm not sure the intake was cut to match)

matt69olds

For an engine to idle with only 8 inches of vacuum, it would definitely shake the fenders off the car.

And the cam specs you listed should provide way more vacuum than what you’re seeing. Something isn’t right.

A vacuum tank will only store the vacuum it sees. Meaning if the engine never makes more than 8 inches, that’s all the tank will store. Lucky for you, there are times the engine will make much more than 8 inches, namely when slowing down. Assuming the tank has a check valve, it will store the higher vacuum.

brotherGood

I agree with that. The idle doesn't get much lower than 1100 without shaking pretty fierce. I'm hoping the issue is the intake leaking underneath, but until I try it-I won't know for sure.

Any reservoir I was looking at does indeed have a check valve..so fortunately as long as I was able to build while driving it should be able to store more than the 8..but I didn't think it'd compound.

70sgeek

I tried the Leeds Bandit series electric brake pump on my 350 - was already making about 16-18lbs vacuum on a Howards street performance cam but wanted to see if it might have helped boost the consistency a bit more.

Worked great for a few weeks but one day as I'm approaching an intersection stop sign the fuse on it blew and left me practically no braking power. Luckily had enough pedal pressure to bring it to a stop and spare fuses to get it going again for the ride home.

But after that I removed it and went back to an add-on Summit Racing vacuum can. No issues since.

OLDSter Ralph

Was the block "decked" ? I would think the amount should be on the invoice.

If you can't get it to idle lower than 1100 without shaking fiercely, you have a massive vacuum leak.

brotherGood

I'm timing it via total timing, so using my dial back light and revving it to where it stops moving on the balancer, then move the dist. back to line it up at zero again (34 total)

I have not changed jets or PV's yet. I am trying to get the vacuum situation resoved first-meaning either reservoir/pump or a potential leak

The block was decked, just checked the invoice and all it says is "zero deck". He did balance it, but it was discovered at shop #2 that it wasn't assembled when balanced, as the pistons were hitting the counterweights on the crank and there was a bent rod.

I guess where I ultimately am with this would be...my experience is limited to bigger cams, and to vacuum leaks that aren't glaringly obvious. I may also be overthinking a chunk of this as I'm quite paranoid because it is the original engine, and I am working on a car worth more than 2k. I wish all the times I'd had the thing running trying to chase vacuum, I would've thought to capture the sound I hear, as well as the engine shaking at 1100 rpm.

Koda

See if you can kill it by putting hand on the primaries. If you can't, you have a massive leak.

I have a very well hidden can on my H/O. The cam is a little rowdy; the brakes are good except when warming up initially, so maybe the fast idle needs a little tweak. Once it's ready to run after a couple min, the brakes are fine, but, like others said, that may not be riding on the can and it may perform the same way without it. Point being, what then, is the point of a vacuum can in series with the booster, which is already a vacuum can in and of itself?

brotherGood

Had my first instance yesterday morning of how I need to address this now that it's a for sure thing that is necessary.

In looking for cansiters, is there anything in particular I'd need to look for?

Thanks.

70sgeek

Something like this may be all you need - can be purchased at (or thru) most local part stores or online - Ebay probably your friend in that regard. Get a vacuum gauage for it as well (mounts on top) so you can monitor the pressure

joe_padavano

As noted in Post #6, a canister doesn't increase the vacuum. If the engine is not making enough vacuum to provide sufficient power assist, the canister won't change that.

brotherGood

Right, but if vacuum is registering 15# (purely as an example at this point) won't the canister hold 15#? Or does it hold strictly what is being made at idle only? The car stops, the issue is just after a lot of stop/hold/creep forward/stop etc (similar to the line at the show yesterday).

joe_padavano

Vacuum is usually highest at idle - closed throttle typically causes the highest manifold vacuum. If repeated stops are the problem, then yes, more vacuum storage volume will help.

brotherGood

True, I had it backwards (it's been a long few days). But yeah, sitting in line was the first I'd noticed it feeling like it was lurching.