WOT Braking

[OVERKILL]

Originally Posted By: 2Fast4U
Originally Posted By: stephen9666

I think you might be confused. When these cars "cut out," they don't shut off the engine. I think they just reduce engine RPM.

Also, I believe an engine at low RPM produces more vacuum than an engine at high RPM, so there wouldn't be a problem with vacuum.



OP states "but the engine did not cut out" and "2007 Mustang V6 (ours): Engine cut out under WOT". I've been around/ working on cars for years. When someone tells you to "cut the engine".... they don't mean let off the throttle. We are to believe a 2007 Mustang at WOT then full brake goes to idle and does not stall or cut out?



He's saying that when he's holding the gas to the floor and SIMULTANEOUSLY hits the brake, that the Mustang goes to idle. Since it is DBW, that is not surprising.

 

[Nick R]

Originally Posted By: OVERK1LL
Originally Posted By: 2Fast4U
Originally Posted By: stephen9666

I think you might be confused. When these cars "cut out," they don't shut off the engine. I think they just reduce engine RPM.

Also, I believe an engine at low RPM produces more vacuum than an engine at high RPM, so there wouldn't be a problem with vacuum.



OP states "but the engine did not cut out" and "2007 Mustang V6 (ours): Engine cut out under WOT". I've been around/ working on cars for years. When someone tells you to "cut the engine".... they don't mean let off the throttle. We are to believe a 2007 Mustang at WOT then full brake goes to idle and does not stall or cut out?







He's saying that when he's holding the gas to the floor and SIMULTANEOUSLY hits the brake, that the Mustang goes to idle. Since it is DBW, that is not surprising.


Yes, thank you overkill lol. The surprising thing is that all except the cavalier were dbw. The reason I did this was to test the safety programming. All should return to idle if the brake is depressed hard under acceleration. However, only two of them did.

 

[ARCOgraphite]

Originally Posted By: Volvo_ST1
Originally Posted By: Landrew
You can actually put the car in N or neutral to the lay person - essentially cutting engine power to the transmission


You see, Nick wasn't talking about cutting power to the wheels, he was talking about the engine automatically "cutting out." I asked "Why?" because I found his expectation to be unreasonable.
Most new cars with ECT and keyless ignition will or should have throttle cut out if a conflict of 1/4+ throttle open and brake application is indicated by the ECU - Not unreasonable by any means. Sounds like good failsafe software and hardware programming. My humble toyota car has ECT and electric PS; I'm waiting for the next solar flare event car to cause me to drive off a cliff

 

[Trav]

Remind me never to loan you my car.

 

[Nick R]

Originally Posted By: Trav
Remind me never to loan you my car.


They all knew EXACTLY what I was going to do, they wanted to know what would happen. Good god, it's not like I do this to every car I drive.

 

[ARCOgraphite]

Originally Posted By: stephen9666

...
Also, I believe an engine at low RPM produces more vacuum than an engine at high RPM, so there wouldn't be a problem with vacuum.
Nope - about the same with nearly closed throttle. The vacuum is caused by the absence of air at full 1ATM in the cylinders; to keep air out of the cylinders (engines are big air pump) the throttle valve should be, um, throttled. UNder no load you can easily run an engine to 4000 rpm with 8th throttling.

 

[ARCOgraphite]

Originally Posted By: Nick R
Originally Posted By: Trav
Remind me never to loan you my car.


They all knew EXACTLY what I was going to do, they wanted to know what would happen. Good god, it's not like I do this to every car I drive.
If a car cant handle this - its a pice of junk. Similar to flooring a car thru second gear. My car gets wailed on mercilessly multiple times a day and it's still running at 45K miles (sort a ...)

 

[Trav]

Who said anything about the car not being able to handle it?
If you want to wail mercilessly multiple times a day on your car that's your business.I wouldn't loan you anything i drive either.

 

[cchase]

Originally Posted By: Trav
Who said anything about the car not being able to handle it?
If you want to wail mercilessly multiple times a day on your car that's your business.I wouldn't loan you anything i drive either.


Really. Just because it can "take it" doesn't mean it's doing it any good. I could probably punch myself too but it doesn't mean it would be good for me.

 

[stephen9666]

Originally Posted By: ARCOgraphite
Originally Posted By: stephen9666

...
Also, I believe an engine at low RPM produces more vacuum than an engine at high RPM, so there wouldn't be a problem with vacuum.
Nope - about the same with nearly closed throttle. The vacuum is caused by the absence of air at full 1ATM in the cylinders; to keep air out of the cylinders (engines are big air pump) the throttle valve should be, um, throttled. UNder no load you can easily run an engine to 4000 rpm with 8th throttling.


I'm not trying to insult you, but I can't really understand your post.

Wikipedia seems to agree with me, that there is more vacuum at low RPM/low load conditions.

http://en.wikipedia.org/wiki/Manifold_vacuum

Quote:
If the engine is operating under light or no load and low throttle, there is high manifold vacuum. As the throttle is opened, the engine speed increases rapidly. The engine speed is limited only by the amount of fuel/air mixture that is available in the manifold. Under full throttle and light load, other effects (such as valve float, turbulence in the cylinders, or ignition timing) limit engine speed so that the manifold pressure can increase -- but in practice, parasitic drag on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburetor, means that a low pressure will always be set up as the engine's internal volume exceeds the amount of the air the manifold is capable of delivering.

If the engine is operating under heavy load at wide throttle openings (such as accelerating from a stop or pulling the car up a hill) then engine speed is limited by the load and minimal vacuum will be created.

 

[eljefino]

An engine at high RPM and the same load will make more vacuum than lower, or the same in/HG measurement but at a larger volume, for faster recharging of the brake booster. The best vacuum you can get is hauling down a hill engine braking in a lower gear.

My Dad had a vacuum gauge in his stick shift Fairmont which I watched as a kid from the back seat.

 

[Craig in Canada]

Originally Posted By: Landrew
I've heard there is another way to get the car safely slowed down when the engines at WOT. I'm going to do a bit of research and see if I can find it again someplace........


I can think of a bunch without doing any searching:

Turn off the ignition?

Put it in neutral?

Correctly install the proper floormat? (sorry, couldn't resist)

 

[Craig in Canada]

Originally Posted By: ARCOgraphite
I never had a car that would overcome its brakes** - even a '70 455 Buick Electra convertible. One 4th of July evening, many moons ago, in town Andover, MA, I attempted a "burnout" or "smokeshow" using the brakes to hold the car. The car had Posi. The front brakes held, but when the rear tires got stickey, the rear traction was great enough to slide the front tires - even though they were locked! That's Torque. 2&1/2 tonne car too.

** if they were in good service order, 89 caddy was an issue.


This scenario wouldn't be a factor in a typical FWD car owned by most people with "runaway" cars or "stuck" throttles. Apparently on these cars the ignition switches and gear selectors don't work either.

Second, unless you had much wider tires in the rear, the amount of static grip available to your front and rear tires should have been identical before the weight transfer of acceleration came into play. If you had some muscle-car layout with double-wide rear tires that was your problem with that experiment - not the brakes. Again, not the typical situation on a Cavalier or Toyota.

Anyways.... I've had a number of cars where on a couple of occasions I've simultaneously applied throttle and brake during a brake bedding process. This would usually be in second gear, not first, and oscillating between, say, 50mph and 15mph. As I said, it wasn't absolute WOT but generous throttle while braking nonetheless, and it was second gear, but the brakes had no problem overcoming the engine power. One was a FWD 1990 Integra. Another a RWD 1992 Porsche. I can't remember if I've done it on my RWD BMW or not (probably not).

 

[Volvo_ST1]

Originally Posted By: ARCOgraphite
Most new cars with ECT and keyless ignition will or should have throttle cut out if a conflict of 1/4+ throttle open and brake application is indicated by the ECU - Not unreasonable by any means. Sounds like good failsafe software and hardware programming. My humble toyota car has ECT and electric PS; I'm waiting for the next solar flare event car to cause me to drive off a cliff

Thank you for admitting the dichotomy between increasing complexity in an effort to reduce the chance of things going wrong.

I am not sure I see much more than a solution in search of a problem. I shall test how my non-DBW vehicle behaves if I floor it while stomping on the brake. I will report.

 

[Rix]

LOLLERSKATES

If my cars couldn't take a failry regular flogging, I'd never own them.


















(Toyotas of course)

 

[ARCOgraphite]

Originally Posted By: stephen9666
Originally Posted By: ARCOgraphite
Originally Posted By: stephen9666

...
Also, I believe an engine at low RPM produces more vacuum than an engine at high RPM, so there wouldn't be a problem with vacuum.
Nope - about the same with nearly closed throttle. The vacuum is caused by the absence of air at full 1ATM in the cylinders; to keep air out of the cylinders (engines are big air pump) the throttle valve should be, um, throttled. UNder no load you can easily run an engine to 4000 rpm with 8th throttling.


I'm not trying to insult you, but I can't really understand your post.

Wikipedia seems to agree with me, that there is more vacuum at low RPM/low load conditions.

http://en.wikipedia.org/wiki/Manifold_vacuum

Quote:
If the engine is operating under light or no load and low throttle, there is high manifold vacuum. As the throttle is opened, the engine speed increases rapidly. The engine speed is limited only by the amount of fuel/air mixture that is available in the manifold. Under full throttle and light load, other effects (such as valve float, turbulence in the cylinders, or ignition timing) limit engine speed so that the manifold pressure can increase -- but in practice, parasitic drag on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburetor, means that a low pressure will always be set up as the engine's internal volume exceeds the amount of the air the manifold is capable of delivering.

If the engine is operating under heavy load at wide throttle openings (such as accelerating from a stop or pulling the car up a hill) then engine speed is limited by the load and minimal vacuum will be created.

I was talking around the question to encourage more thought. An engine is an air pump, The faster it turns, the more vacuum it can create - but in a practical sense at nearly close throttle the engine would register about the same vacuum (air pressure) on a gauge in a vacuum port, but could have more capacity to maintain a partial pressure at a higher rpm under low load at near closed throttle - just by the fact that it is displacing (evacuating) more L/sec due to the piston speed.-
Another way to look at it is, the gauge vacuum reading is related to how far the throttle is open: Less throttle angle = more vacuum, wide open throttle = no vacuum. So it's not the engine speed (RPM) thats the prime determiner of vacuum, but more at the throttle angle that determines vacuum (within a system of moderate loss or leak-down).

 

[mechtech2]

Brakes are powerful.
Maybe 500 HP , give or take.

But there is substantial torque multiplication in first gear, making the engine's power a lot more.
This explains the results.

 

[Craig in Canada]

Originally Posted By: mechtech2
Brakes are powerful.
Maybe 500 HP , give or take.

But there is substantial torque multiplication in first gear, making the engine's power a lot more.
This explains the results.


Talking HP simply confuses things. Reducing the discussion to torque (brake torque, and torque at the wheels after multiplier) will make more sense.

The fact remains that brake torque is virtually always greater than even first gear torque at the wheels. Any vehicle with decent tires can probably brake at 1g. Most vehicles cannot accelerate at 1g, even in first gear. Almost any vehicle should be able to lock wheels during braking (excluding ABS action) but fewer can peel out at will (especially the subject vehicles of the Toyota debacle).

Lastly, brakes are installed on all four wheels producing brake torque and using tire grip to stop the car. Most vehicles only drive two wheels, limiting power transfer to the road to two tires worth of grip and not four.

 

[SteveSRT8]

I have never owned a car with properly working brakes that could not easily hold the car in first gear under full throttle. This includes many many street rods and hopped up cars.

Might get a bit smokey, but it didn't move till I let go!

And growing up in the 60's and 70's meant you'd better be ready for an occasional throttle sticking wide open! Just turn the key!

I for one would like a master disable button in my car to disable ALL the electronic nannies except abs. There's an option I'd pay extra for.

 

[CBR.worm]

Many of the modern cars do not cut out the engine, but reduce power when the brakes are applied. Most cars brakes can overcome the engines power even if they destroy themselves in the process.

Where it becomes more difficult is in some of the new cars that have a button to start/stop the engine and a button to change from drive to neutral as well as an electronic throttle control. You can push the neutral button or the stop button, but what if they don't work? As I recall some of the cars will not let you stop the engine with the button if you are at speed or in gear. I believe Toyota you have to hold the stop button in for some number of seconds before it turns off if you are in gear.

My infiti cuts power to about 30% with the brakes applied, but you can still do a rolling burnout with that 30% power. If you push the brakes harder it will stall the engine (it's a manual)