Explain what makes car go faster

"PartyPooper2012" (PartyPooper2012) 12/23/2016 at 06:30 • Filed to: None

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So yesterday on The Grand Tour they had 2 cars. One with a big engine and many cylinders. Other with half the cylinders and half the engine.

Yet.... there is no significant speed or acceleration difference.

The curb weight of the smaller one is 3459lbs. Big one is at 3705. That’s a difference of one healthy American adult.

What is the performance advantage of getting more cylinders and liters? Anyone?

vondon302 > PartyPooper2012 12/23/2016 at 06:56

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Torque!

PartyPooper2012 > vondon302 12/23/2016 at 07:01

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400 in big engine.. 350 in small engine

random001 > PartyPooper2012 12/23/2016 at 07:15

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I had a really long reply, with details and numbers, that got kinja’d.

Torque and gearing, rotating mass, aerodynamics, traction...

vondon302 > random001 12/23/2016 at 07:27

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Stripes and stickers too!

XJDano > random001 12/23/2016 at 07:27

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So, math. All the stuff we said we’d never use in the real world.

Jordan and the Slowrunner, Boomer Intensifies > PartyPooper2012 12/23/2016 at 07:37

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Grip, low end torque, that blue color, etc.

PartyPooper2012 > random001 12/23/2016 at 07:41

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if one doubles all parameters, shouldn’t it go twice as fast in half the time?

but no... it doesn’t

PartyPooper2012 > random001 12/23/2016 at 07:43

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Try again? It’s a day before holiday weekend. it’s slow as molasses anyway 

PartyPooper2012 > Jordan and the Slowrunner, Boomer Intensifies 12/23/2016 at 07:44

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You’re telling me, if I placed that big engine into that small car, it would go twice as fast as original big engined car?

It would obviously already have the blue coloring package that makes it go fast and grip well

Levitas > PartyPooper2012 12/23/2016 at 07:47

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The difference is in the nitty-gritty engineering numbers that most people don’t care about.

The smaller engine will have higher cylinder pressures, possibly hotter combustion temperatures, possibly hotter exhaust gasses, possibly better efficiency, more stresses on the materials/internals of the engine.

The larger engine (5.0L V8) has a higher compression ratio (9.37:1 for the FoRS vs. 11.0:1 for the GT) but doesn’t have the added bonus of a turbocharger; the FoRS sustains 23.2 psi of boost pressure which really helps with the whole turn liquid dinosaur into tingly-pants feelings.

TL;DR Turbos are effectively black magic and it’s possible to write entire books on them and that’s why. And if you built a 5.0L V8 from a mustang just like in the FoRS, it would make way, way more power and torque. (See: GT500 or others)

DipodomysDeserti > PartyPooper2012 12/23/2016 at 07:49

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Newton’s 2nd Law of Motion:

F=ma

F is determined by fuel, air, and spark. M is determined by how fat your car is.

Newton’s 3rd Law of Motion: For every action there is an equal and opposite reaction.

In other words, you also need grip.

random001 > PartyPooper2012 12/23/2016 at 07:50

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Well, no. I mean, it can’t. You’re making an assumption that all things are linear. They are not. You can double the displacement and cylinders, and now you have a much larger rotating mass, which takes more power to get going, and makes more power. but now you have to control that power to the ground. Put that double engine in the same car geared for the 1/2 engine, and all you’ll do is strip everything in the drivetrain, so you have to beef up the drivetrain, which adds mass, which saps power and takes longer to get moving....

This crap ain’t so simple, hoss.

random001 > vondon302 12/23/2016 at 07:51

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I always forget that part!

random001 > XJDano 12/23/2016 at 07:52

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I use too much. It hurts...

PartyPooper2012 > DipodomysDeserti 12/23/2016 at 07:53

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why couldn’t newton make better laws? I vote for amendment to his laws.

DipodomysDeserti > PartyPooper2012 12/23/2016 at 07:56

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I’m waiting for someone in the incoming administration to actually say something like this.

Also, in regards to other scientific findings: it’s just a theory , man.

PartyPooper2012 > Levitas 12/23/2016 at 07:56

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Well, that’s nice and all... but why not compare apples to apples. Take foRs with turbo/alternator. Compare it with the tang with turbo/alternator.

But of course... who in the right mind would compare apples to apples?

OPPOsaurus WRX > PartyPooper2012 12/23/2016 at 08:11

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an apple fell on his head and messed his brain up

Frenchlicker > PartyPooper2012 12/23/2016 at 08:13

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Their comparison was apples to apples in a way the average buyer would car about: the price tag.

PS9 > PartyPooper2012 12/23/2016 at 08:24

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The universe is totalitarian. It makes the rules we have to follow, and we don’t get a chance to vote it out of office no matter how shitty things get.

PS9 > PartyPooper2012 12/23/2016 at 08:26

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This is a super-broad question featuring only one point of data an no consideration for potentially mitigating factors. I’ll let xkcd explain why such extrapolations aren’t valuable for drawing meaningful conclusions;

MasterMario - Keeper of the V8s > PartyPooper2012 12/23/2016 at 08:32

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It’s all about how much fuel you burn and how efficient the engine is. A turbo charged engine forces more air into the engine allowing you to burn more fuel and make more power per liter than a NA engine. Why not make everything a turbo? Well, NA engines tend to be much less stressed (longer life), produce more torque at lower RPMs (better driveability), and we have that american mentality of “no replacement for displacement”

PartyPooper2012 > PS9 12/23/2016 at 09:00

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Can’t wait to have many husbands... someone to help me finally LS swap my ford pinto

PartyPooper2012 > PS9 12/23/2016 at 09:00

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but....Trump! Trump All things!

PartyPooper2012 > Frenchlicker 12/23/2016 at 09:01

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when have you seen blue apples being compared to red apples?

PartyPooper2012 > OPPOsaurus WRX 12/23/2016 at 09:02

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lazy shmuck is what he was.... who has time to sit under an apple tree?

Frenchlicker > PartyPooper2012 12/23/2016 at 09:21

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They are cars meant for proper reasoning so I say it’s fair to compare.

DynamicWeight > PartyPooper2012 12/23/2016 at 11:39

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Who is healthy at 246 pounds!? Maybe a competitive weight lifter.

But to answer your question, more cylinders means less rotating mass which, if you can spin them fast enough means more power. That’s how motorcycles can get almost 200 HP out of a 1 liter engine.

More displacement means more air which means bigger booms. And the booms are what make the wheels go all turny.

PartyPooper2012 > DynamicWeight 12/23/2016 at 11:42

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.... mmmkay... why doesn’t tang spread its wings and fly away at million miles per hour?

you’re obese at 250 ;-)

DynamicWeight > PartyPooper2012 12/23/2016 at 11:44

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Tang? I didn’t watch the show btw so I don’t know what car’s you’re referencing.

PartyPooper2012 > DynamicWeight 12/23/2016 at 11:46

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“MUST” I reveal everything?

DynamicWeight > PartyPooper2012 12/23/2016 at 11:51

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So your question is why the mustang can’t just keep accelerating for ever? The answer is two fold:

1. Air resistance is hyperbolic.

2. The reason the redline exists is because you can only spin thing so fast before they fly apart. It’s a “materials problem”.

PartyPooper2012 > DynamicWeight 12/23/2016 at 12:34

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no. thats not my question.

Why doubling cylinders and liters doesn’t translate to double the acceleration or velocity... I understand the whole more engine = more weight. OK. Lets pretend I gain 25% in acceleration even.

The video presented to us yesterday, both vehicles are on par. In theory, vehicle with bigger engine and cylinders should outperform... but it don’t

Shift24 > PartyPooper2012 12/23/2016 at 12:43

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Ok real talk here as no one has pointed out a couple things here. Now I know % is not the best for power loss figures but it will work for now. RWD is generally 15-20% power loss where AWD can be 20-30% because you are sending the power through more gears and more wheels. Also if you look at whp there is about 100hp difference (see graphs below). Mustang 15% loss at 373hp and RS 23% loss at 270hp. That big difference here that played out on Grand Tour.

Now the other thing that played out is top end, now I am not great on explaining or even understanding it but larger displacement V8s will always have better top end pulling power than Turbo 4 cylinders. Turbos help increase torque numbers vs a NA 4 cylinders (i.e. vtec = no torque). You can look at the RS graph vs the LT1 Corvette graph in torque. Look at the graph (black is torque on LT1) Focus starts above 180 tq and ends at 180 tq. LT1 starts at 300 tq and ends 400 tq. There is a bigger drop off with small 4 cylinders.

Now gearing also matters but im not explaining that because to me thats black magic...

Mustang

Focus

LT1 Corvette

J_P_Cars10s > PartyPooper2012 12/23/2016 at 12:46

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I took a senior level course in my mechanical engineering program all about internal combustion engines. The only time in history I’ve enjoyed reading a textbook....

Long story short, the Focus, thanks to its forced induction, is burning fuel at roughly the same rate as the Mustang. Displacement has not a lot to do with peak power. There’s a fixed amount of energy in fuel, and modern engines turn about a third of that into usable torque.

It’s all about the mass flow of air. The more air that’s moving through the engine, the more fuel that’s being burned. A small, high RPM engine with inlet pressures well above atmospheric can move the same amount of air as a much larger engine drawing in air at roughly atmospheric pressure.

PartyPooper2012 > Shift24 12/23/2016 at 13:08

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Thanks... I think... lots of charts and technical jargon... but if my grandma was buying one of these two cars without understanding torque ratios and grounded to the ground clearances - you offer her a car with 4 cyls and 2 liters and a car with 8 cyls and 5 liters, surely the latter should go twice as fast compared to the first....Their weights aren’t crazy different. Little more weight and frictions contribute to slowerness... so it should go maybe a bit slower.... but you look at them two racing around... and they are on par. neck and neck. I am going to pretend that because RS had the extra alternator and tang didn’t, adding the alternator would make it go faster... but by how muches? 2x? Twice the motoring and same breathing tubes

PartyPooper2012 > J_P_Cars10s 12/23/2016 at 13:13

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so you saying if a turbo alternator was added, tang would gallop quicker than RS by say 50%? 25% Whole integer %?

Shift24 > PartyPooper2012 12/23/2016 at 13:20

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But again your forgetting something. You need to add that alternator (turbo) as almost another element to the drivetrain. Easy way to put it an alternator can double a cars power so like adding 4 more cylinders. So 2.3L +2.3L is 4.6L so it looses to a car with 5.0L

PartyPooper2012 > Shift24 12/23/2016 at 13:22

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I like your math!

J_P_Cars10s > PartyPooper2012 12/23/2016 at 13:23

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It’s far more complicated than a fixed percentage. But yes, adding forced induction to the Mustang would (could) increase the mass of air flowing through it far beyond what the smaller displacement engine could handle.

The complications come when you consider the different cylinder geometry (compression ratios, etc), heat transfer, and frictional loads.

Die-Trying > PartyPooper2012 12/23/2016 at 16:10

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What is the performance advantage of getting more cylinders and liters? Anyone?

you get an engine that is able to do the same work, but at lower rpms, and more reliably, with less broken parts. you can usually build for cheaper, depending on how large someone goes.

PartyPooper2012 > Die-Trying 12/27/2016 at 07:18

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so if I had 36 cylinders and 50 liters, engine would do same work but without even spinning?

Die-Trying > PartyPooper2012 12/28/2016 at 19:06

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not quite, it is able to do the same work (make the same amount of torque) but at a much lower rpm, for example idle speed, as a high strung,short stroke, small displacement engine, which can only make its peak torque high in the rpm range.

PartyPooper2012 > Die-Trying 12/29/2016 at 07:28

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so then if I had 36 cyl and 50l engine, i could in theory idle the engine at 80 mph?

Die-Trying > PartyPooper2012 12/29/2016 at 14:32

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with enough gears, and enough rotating mass( heavy flywheel), yes.......

PartyPooper2012 > Die-Trying 12/29/2016 at 14:58

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in conclusion, more cylinders and more liters translates to torqueier idling. Doesn’t actually make car faster.

Die-Trying > PartyPooper2012 12/29/2016 at 15:07

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right........ by building and engine to take advantage of of those cylinders, and litres, you can optimize the REST of the set up to make better use. just like an engine which was very small, but revved very quickly, would take advantage of its rpm advantage and gear it around THAT powerband.