Modenaperf

hello to all, a question remains a mystery to me to this day and no answer seemed to be relevant.

let me explain, if acceleration is linked to torque at the wheel, the vehicle that has more power and torque for an equivalent weight accelerates stronger, but I've always wondered why our turbo vehicles "feel" so fast and gives you that punch in the back that I haven't found on na much more powerful cars with more torque.

Where does this feeling of acceleration come from? is it the shape of the torque curve or the power delivered suddenly by the pressure that makes the turbo so "violent".

codrus

Humans perceive "jerk" more than we do acceleration. That is, the rate of change of the acceleration, or the 3rd derivative of position (position => velocity => acceleration => jerk).

Cars with big V8s or big turbos let you go from low levels of torque to high levels very quickly just by putting your foot down, and that feels fast.

Cars with smaller, high-revving NA motors don't have that same rush as the torque climbs (at least, not unless have a very fast transmission and downshift), so they feel slower even if they're turning the same times.

--Ian

Modenaperf

however it would seem that even cars with big v8s don't feel as brutal as less powerful turbo cars, I've seen quite a few people somewhat disappointed in the acceleration part of some full torque v8s that haven't been experienced, in comparison of their turbo cars. for example according to an owner a zo6 c5 will not provide as much punch as a less powerful Impreza sti etc many examples are possible and go in the direction where even the big v8 does not feel as fast as the turbo (but more than high rpm engines though) does this come from the progressive delivery feature na?

Modenaperf

on the v8s that I have experienced with a fairly large displacement, they say. at the go-around in the right part of the regime it seemed to react instantaneously and very briefly then continues and continues linearly so that the acceleration is finally "little felt". turbo look like I'm accelerating: a little, a little a little more, well, a lot, and boom all the power comes at once.

Ted75zcar

iTrader: (2)

All the tq all the time is faster than all the tq half the time... period. Tq is acceleration. 250lbft at 2500RPM for a given gear is the same as 250lbft at 6000 RPM in the same gear, even though the hp numbers are vastly different.

Panici

I think Ian hit this on the money.

Constant acceleration (corresponding with a flat torque curve) puts you back in your seat, and is objectively faster.

A turbo car with a higher boost threshold will be slower until the turbo spools, then you have a sudden increase in the rate of acceleration (Jerk) which feels subjectively faster then constant acceleration.


Sheds some light on why a manual is so fun to drive (and generally feels faster then an automatic). You have that pause in acceleration on each shift and then the rate of Jerk is high when you come back into the throttle in the next gear.

MrJon

I agree with Codrus about the perception of jerk vs acceleration being a big component of this.

BUT, one HUGE thing it looks like you are missing is gearing. You mention comparing an STI to a C5 Z06, well do the math on the gearing, you don't say what year STI but I'll just choose a new one, and I'm going to choose second gear to compare, STI 2nd gear 2.23, C5 Z06 1.78, so 2.23/1.78 is about 1.25, so right off the bat the STI is getting a 25% bump to acceleration in 2nd gear. Then the rear end ratio is STI =3.9, C5 Z06 3.42, so 3.9/3.42 is 1.14, so that's another 14% more. Then the rear tires on the vette are also taller so 26.7/25.8 = 1.03. The total gear advantage for the STI is about 47% looks like an STI is rated at 290 torque, and I think the vette is rated at 400 torque, well 290*1.47 is 426. The STI is a little heavier roughly 10% so then the acceleration you would expect between the two cars in second gear to be pretty much even.

This is far from a complete analysis of this, but I think it makes the point about gearing and what it's going to feel like. If you look at gearing those cars with big V8s usually have very LOOOONNGGG gears, whereas cars with small turbo engines usually have very short gears, which is going to make them feel like they are accelerating harder... for a little bit, until they have to shift up.

Modenaperf

I took the example of the vette, which often has very long gear ratios, it's true. but it could be another big auto boxed v8 like the 6.2 amg 7 box ratio. I understand the principle of jerk in perception, but as said in the first answer on the big v8s as soon as you press the accelerator you go from 0 to a big torque instantly, that should push you back into the seat, I not say that it does not but its still less violent than the turbo in general. does the fact that it is brief and instantaneous on na detract from the feeling? obviously once the first shock has passed the acceleration is constant and gradual so we feel it less, but I'm just talking about when we accelerate full throtlle, the instantaneous side of the na seems less brutal than the turbo which starts for similar or even lower torque levels why?

Erat

iTrader: (5)

There's something about a big cube small block pushing you back in the seat from 1800rpm all the way to 7000rpm that a turbo car just doesn't give you. C6 Z06 comes to mind. Even my measly LS3 has this to some effect.

Ride in a supercharged vette or 5.0 mustang and you wont have this same opinion.

Modenaperf

Aside from huge engines and horsepower with huge torque and horsepower, na engines aren't known for "pushing in the seat" at least there isn't the typical turbo back shove. I'm not the only one to think that. it's a fact and if it wasn't the case nobody would have found a c63 w204 which seems "slow" or slightly disappointed with the power compared to a turbo tuned car like focus rs, 335i less powerful and not much less light etc etc while it is a big v8 with torque. that said, I agree, with a c6 zo6 or even zr1 we reach completely crazy weight/torque+hp ratios.now look at how a big turbo gtr33 of equivalent power works... it feels awfully brutal once on boost however, at low revs there isn't the torque of the 7l ls

stratosteve

iTrader: (1)

I got to do a ride along at a porsche club autox. It was a Tesla M3 performance. I have never ridden in something that pulled that hard. Ridden in some cool **** over the years. Just plain stupid. I giggled the whole ride. Jfc that was awesome.

Dr.Sep

in 3 rolls out of 3, from 30 40 and 60mph my miata pulled away from a Tesla M3 performance; from a dig there would be no chance, but I walked him from a roll each time (cheers to me I guess)

stratosteve

iTrader: (1)

Was from a dig. I dont know how it does from a roll. Glad I was smart enough to keep my head against the seat.

Panici

Ah but supercharging adds another variable.

Generally a supercharged car ramps up in boost/torque as the engine speed increases, as opposed to a flatter torque curve of an NA V8.

Ted75zcar

iTrader: (2)

Not for PD style blowers.

sonofthehill

iTrader: (1)

Hilarious!
Was just telling someone how slow my car feels next to a fast car, literally feels slow. On the other hand, next to a comparable car, it feels damn fast when I am pulling a tiny bit on them. Next to a normal car, my car feels boring.
The point is, it's all relative at a given moment in my opinion anyway.

Panici

I'm not familiar with that term?
To my knowledge the main types are Roots / Twin Screw / Centrifugal.

I've never owned a supercharged car so am still learning.

wherestheboost

iTrader: (-1)

The jerk concept is correct. The last factor in all of it comes down to mental surprise. When you have a high torque n/a car - you slam the gas pedal down and the torque is delivered. With a turbocharged car there's the SLIGHTEST lag (or big lag) from when you slam your foot down to when you accelerate. I think that's enough to play with the brain to make you feel faster. It's almost the "unpredictableness" of the torque delivery. n/a is like a stepwise function. Turbo can often have that asymptotic curve followed by tapering off. Maybe you can mimic the "feeling" with a corvette by rolling onto the throttle vs stomping.

PD = roots. Centrifugal is like closer to a turbo than no (increasing torque as the rpm's increase). Roots (or PD) torque curve is more akin to an n/a car.

codrus

A positive displacement blower is one that fully encloses a volume of air during the pumping process, thus preventing air from flowing backwards across the pump at low speeds. Roots and twin screw are positive displacement, centrifugal is not. PD blowers tend to have broader ranges of RPMs over which they produce boost, but the centrifugals are more efficient.

--Ian