j.kenzie@sbcglobal.net

Now that you mention it, Fred, I have noticed one drawback, maybe related to the non-Venturi arrangement. I'm not sure, because I've been driving without the Venturi action for years. The Venturi was attached but the side arm was plugged. Today I actually removed it and ran a hose directly from manifold to booster.
What I have noticed for some time, is that if I just jump in my car and drive away, I need to press pretty hard on the brakes to stop the car, like when backing out of my garage. Now it's on the warm up cycle, so it's at 2000 RPM, and it is an automatic. I just wrote it off to high revs, and cold ATF, because the brakes seem very normal otherwise.
I will continue to watch the situation, and in fact it would be no big deal to re-install the Venturi.
Thanks again,
Dave

j.kenzie@sbcglobal.net

Do you run the brake booster off of manifold vacuum, or off the electric pump?
Thanks,
Dave

FredR

Dave,

The entire purpose of the venturi is to ensure that a vacuum is always available no matter what the pressure in the inelt manifold actually is. The pressure drop at the venturi nozzle will be pretty much a full vacuum no matter what the pressure recovers to. As I am aware the check valve in the nozzle attaching to the booster is there to ensure that vacuum is retained in the booster to the extent possible.

Under normal circumstances when the throttle closes a deep vacuum is established immediately and most of the time that will be adequate. However when the throttle is open the vacuum pulled will be depleted significantly and more so on a boosted motor.

Each to his own of course but I would not run without the venturi fitted. I am not sure if there is any permutation of circumstances that can result in the servo being left with no vacuum but should it happen most likely you will not be able to stop such is the difference.

Rgds

Fred

V2Rocket

Jumping in here, warning: anecdotal advice ahead.

I am inclined to agree with the premise that the existence of the check valve is to still provide vacuum "power" to the various accessories (including the brakes) while running at sustained large throttle openings (autobahn cruising).

Reason for my autobahn-agreement is that my other car, a Subaru, has nothing more than a reinforced rubber hose connecting the intake manifold to the power brake booster. No check valve, no venturi device - just a 10mm ID hose clamped between two barbs (one on booster, one on manifold)...Any time the throttle closes (unless you have a BIG vacuum leak) you should have sufficient vacuum in the booster to slow down "easily".

j.kenzie@sbcglobal.net

How does the Venturi work in the boosted car. My understanding of it suggests that it would pull a vacuum through it as long as there is flow past the jet. This should occur boosted or not, due to the throttle restriction. Maybe the Jet pump, or Venturi, concept is a great idea for boosted cars. I'm kind of leaning toward putting mine back, but frankly I spend 99% of my time normally aspirated, and I have a check valve and a vacuum reservoir for the other 1%.
Thanks for the continued consideration.
Dave

FredR

The venturi is designed to pull a vacuum at the "vena contracta" position and this sucks on the connected port from the servo- there is no flow to speak of from the servo- just that the line is constantly under vacuum as long as there is a flow through the venturi from uptream of the throttle to the plenum area. If the back pressure in the plenum is higher due to boost as I am aware that does not impact the ability of the venturi to drop pressure local to the nozzle inside the venturi then as the gas stream expands and slows down the perssure increases to that of the plenum [Bernouilli theory].

j.kenzie@sbcglobal.net

Thanks, Fred.
My Latin isn't as good as yours. Do you think the Venturi arrangement should work well with a boosted car. I think it should (as long as the is a difference in pre-throttle and post-throttle pressures, regardless of actual pressures.)
Dave

kombatrok

Just to make it clear, even with no vacuum to the booster at all you will still have full braking ability... It will just require more pedal force for the same braking effect. So for those saying you will not be able to stop if you lose vacuum to the booster, that is simply false in every respect.

99% of all cars out there, including plenty of race and performance cars just run straight manifold vacuum to the brake booster with perfectly acceptable booster performance in all situations.

and to j.kenzie, I have full power from the booster immediately on starting the car, as should you if you run manifold vacuum to the booster. My car for whatever reason has never had the high idle function work when starting a cold engine however, I always get about 800rpms when I first start it, no matter the temperature.

ptuomov

It'll stop without Venturi jet pump for sure, at least if going forward. The braking action loads your entire body on the brake pedal. I don't know about reverse, though...

In my experience, when you are driving fast with a boosted car on a high gear for a long time, sometimes vacuum operated stuff gets a little lazy. Cruise control and pop-up lights, by my experience, which I assume are run by power vacuum.

FredR

Dave,

The Venturi is there to give a more secure source of vacuum or so I concluded when I studied the system. Thus whenever the motor is spinning it will generate a deep vacuum at all times no matter where the throttle is. Just remember Porsche did not design the 928 as a boosted application and I have not driven a force fed 928 so I cannot give you a cast iron guarantee but my expectation is that it will work on boosted applications. The logic behind this is that on a boosted application there will be more differential pressure across the throttle so it should work. What I do not know is how much overpressure the venturi assembly can take before it "blows apart". I dare say it will take a lot more pressure than the supercharger can deliver but...? All I can say is that on mine I felt I had to seal it as ht eoush fit of the assembly was "marginal".

As for comments to the effect that the brakes will work without vacuum on the servo [if that was the intent] all I can say is when you are driving down the street turn the engine off and see what happens to the braking- just do not do this going down a hill or in front of a brick wall! As I am concerned the Venturi system gives an additional level of security to the system possibly intended to protect the system if the check valve on the servo inlet fails.

With no vacuum in the servo the brakes will still work but are marginally better than useless by relative comparison.

j.kenzie@sbcglobal.net

Hi Fred,
Thanks for the explanation. I ordered more hose and will probably just re-install the Venturi or Jet pump soon. In your scenario where I turn off my engine and try my brakes, won't the vacuum reservoir and check valve provide vacuum to the brakes for a while? How long do you think?
Also re-establishment of the Venturi system will probably raise the idle, but It doesn't look to me like it's unmetered air, so it should not affect the AFR. Would you agree?
Thanks again,
Dave

ptuomov

I think the hose and passage diameters are such that I wouldn't expect any problems with the Venturi jet pump under the normal supercharger installations. Going from 17 psi to low twenties psi, a lot of stuff started popping up. However, everything that had any issues that needed to be remedied were large cross-section pipes and large area surfaces. Never had any issues with that venturi jet pump in terms of it not being able to handle the pressure. All the small diameter stuff is holding up just fine and completely stock. The force that the thing sees is pressure times area, so it all makes sense.

By the way, my car with the blue engine has all the stock vacuum functionality retained, including the charcoal canister evacuations, venturi jet pump, manifold pressure referenced fuel dampers, etc. It all works just fine, you just need to reroute the crankcase breather system and then add one check valve.

dr bob

The brake booster housing typically holds enough to give you two maybe three assisted stops without "new" vacuum applied to it.

The reservoir in the fenderwell does nothing for the braking system. It's isolated by the little check valve. Reservoir clients include HVAC and cruise control actuators only.

j.kenzie@sbcglobal.net

You are so smart Dr. Bob. The brake booster has its own check valve, but I forgot about the little blue check valve which does indeed isolate it from HVAC and vacuum reservoir.
Thanks for the information,
Dave

j.kenzie@sbcglobal.net

Well, I reinstalled the Venturi system today. Subjectively, the brake assist is improved at all light to moderate speeds. Don't know about full throttle braking yet. I don't see any ill effects so far. I did have to reset my idle speed and idle mixture.
Thanks for all the help,
Dave