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great point.... I agree.. Anything is possible.. but what you forget is that when your at wot and apply your brake and the engine goes under extreme load, it will again build vacuum, because the throttle is wot but the rpm will be only at2200 or so. And to add that what your saying would not be true under al conditions... With my old Grand national under boost(19 psi) which is the 100% opposite of vacuum, I still had brakes to stop the car.. And they were vacuum brakes too. So I can def say that you do not need vacuum and with 19 psi of boost , vacuum brakes still stop a car..
Actually you're wrong. You need to do some reading on manifold vacuum properties in gas engines. The vacuum force is built up in the intake manifold and is directly related to engine load and throttle plate position. In the case you are sighting low load (referenced by low RPM's), but the throttle blade stuck at WOT, the engine will generate very little vacuum, since the motor is capable of ingesting all of the atmospheric pressure air it needs. The throttle blade needs to be able to close in order to generate any significant amount of vacuum.
In the example of your Grand National, I will assume that you know that the car doesn't operate under permanent boost? The 19psi you reference is peak boost under WOT acceleration. As soon as you release the gas pedal the throttle butterflies shut...generating vacuum...and the waste gates (or recirc valves depending on the design) open venting the boosted air to the atmosphere to prevent turbulence from backed up air damaging the turbo impellers or blowing the pipes off. This is the Psshhhhtttt sound that you here come out of turbo cars when they lift their foot off the gas.
I will also assume that you didn't routinely drive your car at WOT and then stomp on the brakes to see if it would stop.
Avoid hitting anything expensive, Place transmission in Neutral, pull over and stop car using brake, turn off engine after stopping. Hope the rev limiter worked or somebody owes you a new engine.
Wrong
That was the standard setup on the sportsman cars I used to race. A power brake system without the booster active, with absolutely no problems stopping the car from around 80 mph. The same for the power steering box without a PS pump.
Right
But if your car was running away from you, why would you pump the brakes? Why not push with all you've got to try to stop the car?
Also, a properly operating vacuum booster will have at least three brake pedal pumps of vacuum in reserve.
On the issue of your sportsman with manual brakes, I will also assume that you adjusted the brake pedal ratio to gain the leverage that you need without the booster assistance. A standard power assisted brake setup uses a pedal ratio of about 4:1 or less in some vehicles. Manual brake setups use a ratio of 6:1 or even 7:1 to gain the needed leverage. If you convert from a booster assisted system to manual, you need to adjust the pedal ratio in order to have acceptable brake performance. There are also far more differences between a sportsman (or any track oriented car) as opposed to a plain Jane passenger car. I will also assume that the sportsman was significantly lighter and had high performance brake materials. The track and the road are very different places.
You are absolutely right that IF someone simply pushed the brake pedal to stop the car, they could. The problem is that you are dealing with average people in an anything but average situation. I can easily see people pumping the brakes or hitting them multiple times in an effort to get the car to stop, thereby working against themselves. Where in the average course of driver education are people taught how to deal with these situations. I'd like to think that I could get a car to stop in this situation, but its never happened to me, so I don't know. I have a ton of driving time in everything from Spec Miata's, to dragged out GTO's, to performance street cars, to everyday vehicles. I would like to think I know what to do, but who knows what happens in that situation.
Avoid hitting anything expensive, Place transmission in Neutral, pull over and stop car using brake, turn off engine after stopping. Hope the rev limiter worked or somebody owes you a new engine.
All sound advice if:
1. You can actually place the car in neutral.
2. You can actually turn off the engine.
The Toyota issues as discussed at length in this thread have proven that people are not able to do the logical things to get the car to stop. If you aren't able to do any of the normal steps your only chance is to use your one or two applications of boosted brake assist to bring the car to a complete stop. You will literally have seconds to analyze what is going on and take your one chance at getting the car stopped.
Actually you're wrong. You need to do some reading on manifold vacuum properties in gas engines. The vacuum force is built up in the intake manifold and is directly related to engine load and throttle plate position. In the case you are sighting low load (referenced by low RPM's), but the throttle blade stuck at WOT, the engine will generate very little vacuum, since the motor is capable of ingesting all of the atmospheric pressure air it needs. The throttle blade needs to be able to close in order to generate any significant amount of vacuum.
In the example of your Grand National, I will assume that you know that the car doesn't operate under permanent boost? The 19psi you reference is peak boost under WOT acceleration. As soon as you release the gas pedal the throttle butterflies shut...generating vacuum...and the waste gates (or recirc valves depending on the design) open venting the boosted air to the atmosphere to prevent turbulence from backed up air damaging the turbo impellers or blowing the pipes off. This is the Psshhhhtttt sound that you here come out of turbo cars when they lift their foot off the gas. The waste gate controlled maximum boost they did not bleed off boost pressure when I let off the gas..
I will also assume that you didn't routinely drive your car at WOT and then stomp on the brakes to see if it would stop.
I do not need to be informed of this. Did you read what I wrote? I said UNDER BOOST my vacuum brakes could stop the car.. And there was NO blow off valves on these cars. Nothing ever ever vented to the atmosphere.
Thats a ricer accessory that was not used on TR's.. You need to read up on how the turbo Buicks worked The wastegate on the TR bleed off boost pressure into the exhaust system when maximum booth was reached. There were no blow off valves on GN's
I do not need to be informed of this. Did you read what I wrote? I said UNDER BOOST my vacuum brakes could stop the car.. And there was NO blow off valves on these cars. Nothing ever ever vented to the atmosphere.
Thats a ricer accessory that was not used on TR's.. You need to read up on how the turbo Buicks worked The wastegate on the TR bleed off boost pressure into the exhaust system when maximum booth was reached. There were no blow off valves on GN's
OK, so I don't know what I'm talking about...
You're absolutely right, they didn't have "blowoff" valves to vent to atmosphere. I said wastegates in my original post and they certainly had them, the technical aspect still holds true regardless of where the boost is vented. IIRC they used two seperate wastegate systems. A default closed configuration, except on the '81's which used a default open setup. I wonder why the open setup only lasted one year...anyway...
You seriously routinely stopped your car under WOT at full boost, by hitting the brakes? I seriously doubt you did, because that would be stupid. When you were going WOT generating boost and you wanted to stop, you took your foot off the gas. Doing so closed the throttle butterfly building vacuum. The vacuum pressure in this case powers the brake booster and also actuated the wastegate to vent all that boosted air into the exhaust system. You then put your foot on the brake and the car stopped. At no point was your car running at 19psi of boost and you used the brakes.
Unless...of course, you are talking about launching the car at the drag strip. In that case you are taking an already stopped car, applying full brake pressure and revving the engine (which generates boost) in preperation of launch. In that case, you really should have been using a trans brake or line lock as the car will tend to creep up. However, this scenario is totally different than driving the car down the road and trying to stop it while going WOT, something I guess you didn't do to often.
a wastegate does not blow off the boost pressure in the intake if you suddenly shut the throttle. Its only function is to open at a specified boost pressure to control boost. When you snap off the throttle at 20 lbs boost. the boost went no where. In fact it slugged back and forth from the closed throttle blast to the turbo., It never hurt anything either. Made a cool flutter sound( NOT the blow off sound) that GN's are famous for ..
a wastegate does not blow off the boost pressure in the intake if you suddenly shut the throttle. Its only function is to open at a specified boost pressure to control boost. When you snap off the throttle at 20 lbs boost. the boost went no where. In fact it slugged back and forth from the closed throttle blast to the turbo., It never hurt anything either. Made a cool flutter sound( NOT the blow off sound) that GN's are famous for ..
I hope you know that I'm not trying to be a jerk about this, just enjoying a discussion that turned very technical. I'm from South Jersey as well and if you were any bit active in the car scene over the past 10 years, we probably know each other, lol.
I think the one piece that you are missing here is that the vacuum boost is generated inside the intake manifold. When you close the throttle butterfly, the only air the engine is able to get is from the idle pass through. This restriction is what helps generate the vacuum. In the case of the GN regardless of where the boosted air goes (BTW I'm pretty sure the wastegate opens and vents the boosted air as soon as you lift off the gas...that's the fluttering sound you hear) when you take your foot off the gas, your engine has just gone from breathing with its mouth wide open, to trying to breath through a straw. This restriction is what generates the vacuum in the manifold and powers the boosted systems.
The air on the otherside of the butterfly can be at atmosphere (NA engine) or boosted to 19psi (or 400 it really doesn't matter) the engine can only get what it needs to maintain idle and this disparity is what generates the vacuum.
Here is a quick and dirty example:
Outside air /// intake manifold ---> engine --->exhaust
Outside air represents the air on the outside of the engine, on the other side of the throttle butterfly. It is technically "inside" the cars piping. This is essentially an unlimited amount of air under all circumstances.
The /// represents a closed throttle butterfly. This limits the amount of air the engine can recieve regardless of the pressure of the outside air trying to get in.
The intake manifold has a volume of air that it can handle. When a car is at idle with the throttle butterfly closed it is consuming air faster than the volume of the intake manifold can be filled and this is what generates a vacuum.
Back to our Toyota example...if the butterfly is stuck at WOT, regardless of engine speed, you cannot generate any vacuum in the manifold to keep the brake system boosted. If the guys in the video had pumped their brakes a couple times and then tried, the cars wouldn't have stopped.
I am sure the CHP officer, his wife, his child, and his brother that were killed at 100 mph a year or so ago was a hoax too!!
This entire conversation is unbelievable not the runaway Prius.
I don't think its a hoax, I think its driver error. Thats case where those people died was definitely a floor mat issue. Here is the 29 page report. Floor mat issue not a sticking throttle issue
Last edited by frankgn87; 03-09-2010 at 10:45 AM..
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The wastegate on the TR bleed off boost pressure into the exhaust system when maximum booth was reached. There were no blow off valves on GN's
I'm from South Jersey as well and if you were any bit active in the car scene over the past 10 years, we probably know each other, lol.
