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More Top End?

ARTZ

By ARTZ,
in General Automotive Discussion

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djr81 Grand Master

djr81

Posted
Posted

The first reply to the thread nailed it. Marc's post there has some pretty good information too.

It all depends on what you're aiming for. You set a goal, then you change all the variables to shoot for that goal. If I want to drag people to 100km/h I'll use the standard 4.11 diff in my Skyline because 2nd gear tops out around that and if it was any shorter I'd need another gear change, which will cost me time. If we're dragging to 80km/h I'll put a 4.36 in the car, because even though 2nd gear will now top out around 90-95, I only need 80 of that and the shorter ratio will give slightly better acceleration.

If you calculate it, a standard R33 GTS-T has the gearing to achieve ~277km/h (forget the exact figure). Realistically you'll only get to around 250 because you'll run out of torque. Shorten the diff a bit and you might get 255 thanks to the extra torque of the gearing. Shorten it too much and you'll only get 230 because your car will redline in top gear at this speed. Make it too tall (say 2.5:1) and you'll have a theoretical top speed of 400km/h+ with only enough torque to get to 160-170. But 1000hp behind that ratio and you'll propel the car to around 320. There are lots of variables to it all.

All hypotheticals for the purpose of examples^

Actually there arent alot of variables. It is pretty simple.

You have two factors.

1. How many horsepowers your car can produce at a given speed.

2. How much drag you car generates at a given speed.

When these are equal you are going as fast as you can.

The gearing dictates where abouts on the powah curve the engine is running - hence it influences the top speed. Excessively tall gearing means you dont make all the hp you can and you are therefore slower than may otherwise be possible. Too short on the gearing & you have introduced a new, mechanical limit to things.

To go twice as fast you need eight times as much power. Yes, boys and girls, eight times. It is a cubic relationship.

Birds Grand Master

Birds

Posted
Posted

lol, way to over-simplify it by stating umbrella variables. I can do that too, watch:

There's actually only one variable to it - how fast your car can go!

Or we can just not play devils advocate for the sake of it...

Reality: there are tons of underlying variables that influence factors such as drag, gearing, torque etc.

Birds Grand Master

Birds

Posted
Posted

Be careful with loading up 5th gear at low speeds, because it is an overdrive gear and you will now have increased resistance from the taller ratio. The standard ratios are pretty well setup so that the engine and gearbox are never laboured, so just make sure you're staying within a healthy torque band :)

WHITER33GTS-T Veteran

WHITER33GTS-T

Posted
Posted

Be careful with loading up 5th gear at low speeds, because it is an overdrive gear and you will now have increased resistance from the taller ratio. The standard ratios are pretty well setup so that the engine and gearbox are never laboured, so just make sure you're staying within a healthy torque band :)

If this was for me, dont worry mate I only have 3 gears :whistling:

mad082 Grand Master

mad082

Posted
Posted

yes definatly one of your better posts Marc. While i have your attention, do bike speedos work in cars?

short answer: no. long answer: no they won't.

Im changing from a 4.1 to a 3.7 in my 33 for more top end and less revs on highway at 110kph

skylines are a small displacement engine, so they won't handle the taller diff ratio as well as a big cube engine. you may find that a smaller hill on the highway may require you to change down a gear as it is putting you into a lower torque range and taking you into an area of the rev range where you might not be making full boost. it will also make the car harder to get off the line (including reversing), so you will see increased clutch wear as possibly worse fuel economy. also as for gaining "more top end", unless you were pushing serious horsepower (enough to max out 5th gear, so up around 350 to 400kw at the wheels) you will lose top end and have a lower top speed than with the stock gearing.

EDIT: just saw you are running a C4, so you would be maxing out top gear pretty easily, so you can disregard my post, but it still is worth posting for other people thinking of changing diff ratios with the stock 5sp gearbox.

WHITER33GTS-T Veteran

WHITER33GTS-T

Posted
Posted

short answer: no. long answer: no they won't.

skylines are a small displacement engine, so they won't handle the taller diff ratio as well as a big cube engine. you may find that a smaller hill on the highway may require you to change down a gear as it is putting you into a lower torque range and taking you into an area of the rev range where you might not be making full boost. it will also make the car harder to get off the line (including reversing), so you will see increased clutch wear as possibly worse fuel economy. also as for gaining "more top end", unless you were pushing serious horsepower (enough to max out 5th gear, so up around 350 to 400kw at the wheels) you will lose top end and have a lower top speed than with the stock gearing.

EDIT: just saw you are running a C4, so you would be maxing out top gear pretty easily, so you can disregard my post, but it still is worth posting for other people thinking of changing diff ratios with the stock 5sp gearbox.

Im running a tricked up C4 but would still like to use a 5 speed again one day. I think the extra torque from a 3lt would still be fine with a taller diff but my turbo would be a little laggy. lol

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