Hello everyone and welcome in this video
we're gonna be talking about why Porsche puts the engine in their 911 in the back of the vehicle and
really, this is just an excuse for me to show you some footage of myself driving around in the snow in Porsche 911s
but I figured we'd make it educational while we're at it
so we're gonna go over four different scenarios here and talk about why it actually could be
logical why it is logical to put an engine behind the rear axle
now of course there are disadvantages to doing this there's reasons why other companies don't do it. There's a high polar moment of inertia
that's just a fancy way of saying that it makes the car less eager to turn and it also gives it a
tendency to oversteer where that heavy rear end wants to come out ahead of the front of the vehicle
but we're going to go over some of the scenarios which are actually pretty advantageous
which makes it look like a pretty good idea especially for sports cars. That's kind of the focus here
is that for sports cars putting the engine in the back actually can make a lot of sense.
So the first thing we're going to talk about is braking and so we have a scenario here
we've got two different sports cars the one on top is obviously if you can't tell by looking at it very obvious a Porsche 911
with the engine in the back and the bottom one is a red sports car
you know it's a sports car purely because it's red the body shape does not indicate that it is a sports car
regardless with the engine in the back these are both rear-wheel drive
vehicles
the Porsche has 60% of the weight on the rear axle
I made up these numbers just to simplify it make it easy to understand and
the other sports car has 45 percent of its weight on the back because it's got that engine up front
okay, so first of all we're gonna look at braking so the Porsche is slowing down the other sports car is slowing down
the weight transfer of course is going to go to that front axle and so with the Porsche you're gonna move
let's say we move 20 percent of the weight to the front axle well now
we've got 60 percent of the weight is on that front so the front end of the car is
responsible for 60% of the braking the rear is responsible for 40% whereas in the other car
75% because it's front heavy is in the front
25 on the rear and what's the tendency with sports cars?
Well the tendency is to have larger wheels in the rear because they're rear-wheel drive
to help with traction so the skinnier tires
are responsible for the most of the weight the ideal scenario is you want your weight evenly
distributed across all of your tires for whatever it may be cornering,
decelerating, accelerating, you want it to be even
for the tires that are trying to do the action that you're trying to do in this case braking
so the Porsche is gonna have an advantage here
because it's got less weight on that front axle than the other car so it's got that big rear tire
that's got a larger percentage of the braking responsibility than the other sports car very simple just
decelerating you have weight transfer occur you want that to be evenly
distributed rather than this heavy front bias like you see with the red sports car
now for acceleration the story changes a bit because now we want as much weight as possible on that rear axle so that we have
the maximum amount of potential grip to
accelerate with and so with the engine in the rear as you start to accelerate you're going to have load transfer to that rear tire
so for example here, we're going from a 40/60 weight distribution split once
we're hard on the accelerator pedal that g-force transfers the center of gravity is now transferring weight to the rear of the car,
now we've got a 25/75 split so
75% of the vehicles weight is available for accelerating the vehicle so the maximum acceleration
force this car has will be greater than this car here because it has less weight transferring to that rear tire
so starting with a
55/45 just like we did here we accelerate at about 15% to that rear tire now you're at a
40/60 split so 60% of the vehicles weight
available for accelerating now
this is assuming you have the power to do so if neither of these have enough power to get to this number right here
then it doesn't matter you know they're both 50 horsepower vehicles both of these are gonna accelerate at the same rate
but as you start to increase the amount of power
they have once you become traction limited this one will be able to accelerate
faster than this car because it has more weight on the wheel that's doing the acceleration okay now
let's get into all-wheel-drive a versus rear-wheel drive
and why a rear engine makes sense in an all-wheel drive sports car and Porsche has been all-wheel drive
911s for 30 years now
I actually got to drive both the all-wheel drive and the rear wheel drive out in the snow in Canada super exciting
but why does a rear-engine, all-wheel drive sports car make sense and the key word there is sports car?
because for other scenarios it might not make as much sense for a sports car you want it
rear-wheel biased because sports cars tend to be rear-wheel drive
that's the most fun form of a sports car at least
you know I would say the majority of people agree that that's the best way to go is to have it rear-wheel drive
and so because of that if you're going to have an all-wheel drive system
you're going to want it to be rear-wheel biased
and then as you need it send power to the front and that's exactly what Porsche does in this very simple manner so you've got
your engine at the back
it sends it through the transmission through the differential to your rear wheels and then there's a drive shaft going to the front a
multi-plate clutch that it's electronically controlled
and you can send up to 40%
to the front axle if you want to now you don't have to you can send the vast majority at 95% to those
rear wheels and just a little bit to the front. Just let the front do the braking through the steering
allow the rear to do the majority of the accelerating keep the car a little bit tail-happy
fun to drive a very sports car friendly and there are cars that'll do this with front engine
but it's much more complicated because it means sending a shaft all the way to the back
then sending another shaft back to the front and then splitting it if you want it to be rear-wheel biased
that's why most cars with front engines that are all-wheel drive will actually be front-wheel biased and then that will send torque to the rear
as needed what Porsche has done with the rear engine makes a lot of sense for an all-wheel drive sports car
now for a rear-wheel drive sports car it also makes a lot of sense to have the engine
at the back or in the middle either way
it's going to simplify where that placement is for efficiency and for weight purposes
so you don't have an engine at the front
then a long drive shaft going all the way to the back and and all of that
complication that added rotational inertia that added weight that loss of efficiency
very simple layout here
we just goes directly to those rear wheels the path that the power travels is very short relative to a front-engine rear-wheel-drive vehicle
it also does a nice job of dividing up the tasks so the front of your car is mostly responsible for
braking and for turning and you know the more you
add to those tires the less they can do of each individual task
so the rear is doing the accelerating the front is doing the majority of the braking
and the cornering and that helps you have those more balanced tires remember you want to split up the tasks have the vehicle have you
know even distribution of weight and pressure on the different tires for maximum grip
another cool thing about having the engine on the rear for a rear-wheel drive vehicle. Is that in poor surface
conditions if you're out on the dirt or for example if you're sliding around on ice and snow
it's nice to have that weight on that rear axle because that means your maximum grip is greater for acceleration
than if your engine was up front so the peak acceleration
G-force that you can accelerate with with a rear-wheel drive vehicle is equal to the coefficient of friction
between the tire and the ground let's say you're on ice and this is something like point two
multiplied by the weight percentage that's on that rear axle
so let's say we have sixty percent of the weight on the rear axle we have a drag coefficient or a
coefficient of friction of 0.2 that gives us 0.12 Gs which isn't a whole lot but remember
we're on a really slick surface here
0.12 G's of acceleration force that were capable of with that static scenario
with our other vehicle. We're only capable 0.2 times 0.45 0.09 G's so
33% more acceleration force you're able to accelerate 33% more have
33% more useful acceleration traction by having that weight on the rear of the vehicle rather than at the front
so there are a lot of logical reasons for the engine being placed where it is even though there are disadvantages to it
so hopefully you guys have enjoyed watching this video
and if you have any questions or comments of course feel free to leave them below. Thanks for watching.