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> Peaky Turbos, And letting off the throttle.
DigiBunny
  Posted: Nov 15 2011, 06:33 AM


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I had a generous amount of time inbetween classes, so I rode with a friend who drives a Turbo, first Gen Jazz. We went back to her place to set the foundation for a tower defense game.

Now I know precious little about the car, aside from the fact that it's been turbocharged, and gleaned that as she pushed and shattered the legal speed limits on an empty highway, that there was a Blow off valve in there.

It also had a very violent forward lurch whenever the throttle was let off. At least compared to the NA I drive.

All of this has me thinking: How would a peaky, turbocharged vehicle react when the driver releases the gas pedal far enough for it to fall out of boost; especially while cornering? I understand how lift off oversteer works, and I suspect there would be a good deal of it here, but would it be drastically greater than a Naturally Aspirated car?

Meteor
Posted: Nov 15 2011, 07:34 AM


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I just want to be sure of one thing. Do you think the suspension on the turbo Jazz is set softer or stiffer than the one on your car (or, how does the forwards, backwards and sidewards weight transfers feel compared to your own car's)?

I know that the power of a turbo engine falls significantly when it loses boost pressure. That would affect acceleration, but would it affect throttle-lift-deceleration? Forwards weight transfer is mainly caused by deceleration, but I don't see how the drop in power output in a no-boost situation would affect the car's deceleration during simply letting off the gas. Neither NA or turbo engines use any engine power when coasting on zero throttle (assuming of course, that the engine is only working to move the car forwards and not simultaneously running the A/C or anything).

I can also think of a few other variables that might affect a car's lift-off deceleration. Right now, I'll just wait for more info before trying to answer this.
DigiBunny
  Posted: Nov 15 2011, 08:36 AM


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The whole theory is based on my concept of having more power is equal to greater potential acceleration, which means a potentially much more violent weight shift when this greater acceleration suddenly vanishes.

As for it's suspension; I can say it's more gentle, which I would think means it's softer, if the way it deals with humps and potholes is anything to go by. I can't judge the weight shifts, as my friend doesn't drive smoothly.
Spaz
Posted: Nov 15 2011, 08:42 AM


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My car makes maybe 100whp out of boost. Once the needle on the gauge creeps into positive pressure that obviously increases drastically, but the point is that any car set up for a turbo isn't going to have much oomph out of boost.

The fact that it lurched is possibly due to the BOV, I know my spring is too tight for lower boost levels and if not driven correctly will lurch forward violently going from low boost to vacuum because the valve won't open far enough to let the air out so it instead surges out through the compressor. But if I use a looser spring, then the car won't hold boost up top as the valve forces open. Simple solution is to ease off the throttle and the car won't lurch, just by making corrections to my pedal usage I've been free of the issue for almost a year now. Best solution is to get a BOV that matches the setup better, provided that is what's causing the problem.
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Meteor
Posted: Nov 15 2011, 09:39 AM


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QUOTE
The whole theory is based on my concept of having more power is equal to greater potential acceleration, which means a potentially much more violent weight shift when this greater acceleration suddenly vanishes.

I could see that happening, but only under particular conditions, said conditions being:
.The car with greater potential acceleration suddenly switches from a lower effective acceleration to a greater effective acceleration, suddenly shifting its weight backwards.
.The acceleration quickly falls to zero before the nose naturally comes back down due to the whole car gaining momentum.

But otherwise, I can't see this hypothesis working. Also, the greater a car's acceleration, the more time it'll take for that same acceleration to fall to zero under just the absence of throttle, making it harder for that greater acceleration to suddenly vanish.
To illustrate: Suppose the frictional forces acting on two identical cars at a certain point would produce a deceleration of 8 m/s-square. If at that same point, one car has an acceleration of 4 m/s-square, the other an acceleration of 8 m/s-square and both cars switch to zero throttle (which, for the sake of this example means no force to overcome that friction), the 4 m/s-square acceleration would fall to zero in 0.5 seconds and the 8 m/s-square acceleration would fall to zero in 1 second.

You're saying the power loss from losing boost pressure could affect a car's lift throttle deceleration, but I'm still not sure about that. I don't know if an engine burns any fuel or not when it's running at above idle-RPMs with the throttle shut off, and I hope someone else here can answer that.

What I know about suspensions tells me that stiff means quicker weight transfers and soft means slower but greater weight transfers. But I can't get an answer out of this yet.

But anyway, while I was typing all this, cmspaz ended up explaining why turbo cars might lurch forwards under zero-throttle. I hope that helps you.
MetalMan777
Posted: Nov 15 2011, 02:26 PM


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QUOTE (Meteor @ 6 hours, 51 minutes ago)
I know that the power of a turbo engine falls significantly when it loses boost pressure. That would affect acceleration, but would it affect throttle-lift-deceleration?

No. Boost doesn't make it past a closed throttle. There's only a couple factors that actually have anything to do with engine braking.

When you close your throttle, your intake manifold quickly harbors vacuum. If you don't have a BOV, sure a little boost could leak past the plate, but it's negligible. Your engine is an air pump with a built in power source. Take away the air (ergo power stroke) and your engine effectively becomes an air-spring (in this case vacuum, but it's the same thing) brake.

My guess is that the Jazz, like most Hondas, has a fairly light flywheel and revs freely both up and down. There's not a lot of momentum in that engine (nor a small car like that overall), so it simply has an easier time accelerating/decelerating than a heavier car with a heavier engine. Keep in mind an engine with more rotating mass would also likely have a much larger displacement, thus be a much more effective brake (lots of factors involved in this, let's just assume the Honda has a good combination for engine braking and be done with this).

Suspension would also affect your perception of the forces. You won't feel a lurch unless you are actually decelerating. The liftoff oversteer wouldn't be greater than a comparable NA car, except for the fact that a turbocharged could potentially reach the corners at a higher speed.
Mazda ina Ford guy
Posted: Nov 15 2011, 06:43 PM


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I suspect what DigiBunny was experincing was the absence of boost, felt as a substantial drop in forward acceleration, caused by transition from boost to vacuum, often mistaken for decceleration, when in fact the car is still accelerating, or at least "coasting". Still this will result in front suspention load-up, and some oversteer just from weight transfer. Most advanced turbo driving techniques are a result of trying to overcome/minimize this "drawback". Turbo vs. N/A, totally different animal, one more edge to be ridden and mastered, like the limits of your tires and brakes on your N/A, you have to ride and master that nomansland between boost and vacuume to truley drive a turbo near the limits. That being noted, I'd bet your friend just needs some more "stick time" to get a handle on her "peaky turbo".
Spaz
Posted: Nov 15 2011, 10:17 PM


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QUOTE (Mazda ina Ford guy @ 3 hours, 33 minutes ago)
Most advanced turbo driving techniques are a result of trying to overcome/minimize this "drawback". Turbo vs. N/A, totally different animal, one more edge to be ridden and mastered, like the limits of your tires and brakes on your N/A, you have to ride and master that nomansland between boost and vacuume to truley drive a turbo near the limits. That being noted, I'd bet your friend just needs some more "stick time" to get a handle on her "peaky turbo".

Or if you drive a rally car you take advantage of it and nail the brakes once the front suspension starts loading to initiate turn-in rotation...
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sideways
Posted: Apr 16 2012, 11:31 PM


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QUOTE (Meteor @ Nov 15 2011, 10:39 AM)
But otherwise, I can't see this hypothesis working. Also, the greater a car's acceleration, the more time it'll take for that same acceleration to fall to zero under just the absence of throttle, making it harder for that greater acceleration to suddenly vanish.
To illustrate: Suppose the frictional forces acting on two identical cars at a certain point would produce a deceleration of 8 m/s-square. If at that same point, one car has an acceleration of 4 m/s-square, the other an acceleration of 8 m/s-square and both cars switch to zero throttle (which, for the sake of this example means no force to overcome that friction), the 4 m/s-square acceleration would fall to zero in 0.5 seconds and the 8 m/s-square acceleration would fall to zero in 1 second.

If you own a vehicle, please conduct an experiment for me- And report back with the results.

Accelerate at half-throttle and then let off as quickly as possible.
Accelerate at full-throttle and then let off as quickly as possible.


If you dont have a access to a vehicle to safely test this experiment, I can just explain why there *would* be a noticeable difference.

We have car A and car B. Car A and car B both decelerate when off the throttle at the same rate. For arguments sake well say they both decelerate at a rate of .1 g when off the throttle.

Car A can accelerate at 1 g (yes, this cars fast)
Car B can accelerate at only .5 g.

If both cars accelerated at full throttle and then suddenly let off- Whats going to happen? Car A is going to from +1 g to -.1 g. Car B is going to go from +.5g to -.1g. Its this difference youre going to feel.

(yes I know this is a semi-older thread, just finally got up enough caring to make a response to help anyone who might be curious if/why there was a difference)

This post has been edited by sideways on Apr 16 2012, 11:32 PM
Meteor
Posted: Apr 17 2012, 12:44 AM


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That. . . Actually makes sense. Thanks for posting it up.

Looking back at that quote, I made a big mistake regarding the car's acceleration falling. A deceleration of x per second makes the speed fall by x per second, not the acceleration facepalm.gif

This post has been edited by Meteor on Apr 17 2012, 01:01 AM