Top speed vs improved acceleration
Discussion
I daresay this has been done a multitude of times before, so forgive me. Off the back of another thread/discussion, and it's something I've pondered a few times, what are the driving reasons (no pun intended) behind chasing top speed for cars, is it just a function of usable gear ratios?
Now, before people jump down my throat as is their wont on GG, I'm not necessarily advocating lower top speeds, I'm just asking if there are any mechanical reasons to not reduce top speeds while improving "punch" for want of a better word, throughout the resulting smaller speed range.
Personally I'd happily trade top end on pretty much any car that was to be used solely for road use beyond, say, 100mph, for better response/economy depending on application.
What would be the sort of "improvement" in acceleration by gearing a Golf GTi to max out at, let's be generous, 120mph?
Is it marketing that prevents it? Are people not interested in it? Is there a mechanically tricky balance to hit (regarding drivable gear ratios, I realise it's not difficult per se)
Or have I simply got completely the wrong end of the stick on this?
Now, before people jump down my throat as is their wont on GG, I'm not necessarily advocating lower top speeds, I'm just asking if there are any mechanical reasons to not reduce top speeds while improving "punch" for want of a better word, throughout the resulting smaller speed range.
Personally I'd happily trade top end on pretty much any car that was to be used solely for road use beyond, say, 100mph, for better response/economy depending on application.
What would be the sort of "improvement" in acceleration by gearing a Golf GTi to max out at, let's be generous, 120mph?
Is it marketing that prevents it? Are people not interested in it? Is there a mechanically tricky balance to hit (regarding drivable gear ratios, I realise it's not difficult per se)
Or have I simply got completely the wrong end of the stick on this?
doogz said:
Quick example, my 106 is geared for about 120mph, flat out in 5th.
Acceleration for a 1.6L engined car is pretty impressive.
But at 70mph, it's doing over 4500rpm. Not ideal for noise and fuel economy.
Yeah, can see...so it's a tricky gearing balance? Unless something like approaching an overdrive gear were to be used? (I know I probably don't mean a literal overdrive, but a tall top)Acceleration for a 1.6L engined car is pretty impressive.
But at 70mph, it's doing over 4500rpm. Not ideal for noise and fuel economy.
DrTre said:
What would be the sort of "improvement" in acceleration by gearing a Golf GTi to max out at, let's be generous, 120mph?
Don't know, but you'd end up with an extraordinarily close-ratio gearbox and depending on torque curve could find you lose more time changing gears than you gain from in-gear acceleration.Back in the day when car speed topped out around 100 mph they might have only 4 gears. A modern 6-speed box enables higher top speeds (although generally in a gear lower than top gear) with the extra ratios allowing big gains in motorway fuel economy.
It's no accident that the latest auto boxes are 8-speed. It's also no accident that 6-speed are likely to remain the max for manual gearbox applications (although there will no doubt be some weird exceptions). A decent auto box can handle the changes much better than a wet-driver.
....because no one wants to be driving down the highway at a 80mph (or whatever the allowed top speed is) at close to max revs in the highest gear. It's already annoying enough in smaller cars that rev above 3500 rpm and generate a lot of noise...
If you make your car that the highest gear maxes out at a 100mph (and the engine revs to 7500rpm), you'll be doing 3250rpm at 50mph already in that gear. If the same car however is geared to reach 155mph, at the same speed of 50mph, you'd be at 2420rpm.
There's also a limit as to how much acceleration you can achieve on gearing since changing gears cost time too. This may change with double clutch automatic gearboxes, but there's still a limit.
If you make your car that the highest gear maxes out at a 100mph (and the engine revs to 7500rpm), you'll be doing 3250rpm at 50mph already in that gear. If the same car however is geared to reach 155mph, at the same speed of 50mph, you'd be at 2420rpm.
There's also a limit as to how much acceleration you can achieve on gearing since changing gears cost time too. This may change with double clutch automatic gearboxes, but there's still a limit.
Edited by Phil-CH on Friday 24th February 13:38
While I generally agree with you that, for a road car, it doesn't really make a difference whether it tops out at 100, 130 or 160 mph and using lower gearing would, without doubt, improve acceleration, there is something you're missing.
That is cruising on the motorway.
If your Golf GTi (or whatever) were geared to top out at 100 mph, you would be pulling significant revs at speeds between 70 and 80 which would be noisy and ultimately tiresome, for most people at least!
That is cruising on the motorway.
If your Golf GTi (or whatever) were geared to top out at 100 mph, you would be pulling significant revs at speeds between 70 and 80 which would be noisy and ultimately tiresome, for most people at least!
DrTre said:
I daresay this has been done a multitude of times before, so forgive me. Off the back of another thread/discussion, and it's something I've pondered a few times, what are the driving reasons (no pun intended) behind chasing top speed for cars, is it just a function of usable gear ratios?
Now, before people jump down my throat as is their wont on GG, I'm not necessarily advocating lower top speeds, I'm just asking if there are any mechanical reasons to not reduce top speeds while improving "punch" for want of a better word, throughout the resulting smaller speed range.
Personally I'd happily trade top end on pretty much any car that was to be used solely for road use beyond, say, 100mph, for better response/economy depending on application.
What would be the sort of "improvement" in acceleration by gearing a Golf GTi to max out at, let's be generous, 120mph?
Is it marketing that prevents it? Are people not interested in it? Is there a mechanically tricky balance to hit (regarding drivable gear ratios, I realise it's not difficult per se)
Or have I simply got completely the wrong end of the stick on this?
As a point of note have a read of any US road test (Motor Trend, Car & Driver etc.) and almost none of them will mention or list top speed.Now, before people jump down my throat as is their wont on GG, I'm not necessarily advocating lower top speeds, I'm just asking if there are any mechanical reasons to not reduce top speeds while improving "punch" for want of a better word, throughout the resulting smaller speed range.
Personally I'd happily trade top end on pretty much any car that was to be used solely for road use beyond, say, 100mph, for better response/economy depending on application.
What would be the sort of "improvement" in acceleration by gearing a Golf GTi to max out at, let's be generous, 120mph?
Is it marketing that prevents it? Are people not interested in it? Is there a mechanically tricky balance to hit (regarding drivable gear ratios, I realise it's not difficult per se)
Or have I simply got completely the wrong end of the stick on this?
Frankthered said:
While I generally agree with you that, for a road car, it doesn't really make a difference whether it tops out at 100, 130 or 160 mph and using lower gearing would, without doubt, improve acceleration, there is something you're missing.
That is cruising on the motorway.
If your Golf GTi (or whatever) were geared to top out at 100 mph, you would be pulling significant revs at speeds between 70 and 80 which would be noisy and ultimately tiresome, for most people at least!
Which is why an "enthusiast box" would be 4 or 5 close(ish) ratios and a long 5th/6th for the motorway.That is cruising on the motorway.
If your Golf GTi (or whatever) were geared to top out at 100 mph, you would be pulling significant revs at speeds between 70 and 80 which would be noisy and ultimately tiresome, for most people at least!
Doesn't this come down to more than just gear ratios but where in the rev range the maximum torque is delivered or at least the shape of the torque curve. Is this not why in a modern diesel you can get some respectable mid range acceleration but without a huge maximum power output or top speed. I could be very wrong here?
The top speed is purely limited by the mechanical and aerodynamic drag against the maximum power output. Assuming that the gear ratios were such that 2 vehicles with very different engines but the same power output and same drag would achieve the same top speed. How quick they got there would be dependent on the gear ratios and the engine power curve and also the vehicles weight.
The other aspect as already mentioned in this thread is that we don't want to be driving around at high rpm all the time. Again why in the real world a lot of diesels feel quicker than they are compared to a similar power output petrol. You can use everything an oil burner has to offer more of the time without realising it (until you come to overtake and end up swearing)
weight is also the killer in acceleration hence why a light weight caterham with relatively little total power is quick to accelerate but has a relatively low top speed (in this case due to the areodynamics)
The top speed is purely limited by the mechanical and aerodynamic drag against the maximum power output. Assuming that the gear ratios were such that 2 vehicles with very different engines but the same power output and same drag would achieve the same top speed. How quick they got there would be dependent on the gear ratios and the engine power curve and also the vehicles weight.
The other aspect as already mentioned in this thread is that we don't want to be driving around at high rpm all the time. Again why in the real world a lot of diesels feel quicker than they are compared to a similar power output petrol. You can use everything an oil burner has to offer more of the time without realising it (until you come to overtake and end up swearing)
weight is also the killer in acceleration hence why a light weight caterham with relatively little total power is quick to accelerate but has a relatively low top speed (in this case due to the areodynamics)
Gotcha doogz.
Perhaps I made the point a bit extreme with 100 mph, and to be fair I am talking of road useonly...I realise a long top would blunt acceleration a huge amount.
Asking the same question in a different way, golfs used to have a top speed of 120 but weren't busy on the mway, so ... what's changed? is it just the power? the gearing HAS to be dictated by the power available?
i know these are doofus questions
Perhaps I made the point a bit extreme with 100 mph, and to be fair I am talking of road useonly...I realise a long top would blunt acceleration a huge amount.
Asking the same question in a different way, golfs used to have a top speed of 120 but weren't busy on the mway, so ... what's changed? is it just the power? the gearing HAS to be dictated by the power available?
i know these are doofus questions
I had an early Mk1 2.0 Mondeo. It had a 'close-ratio gearbox'.
It reached a maximum speed of an indicated 129mph at ~6500rpm in top and it was turning over at 4000rpm at 80mph, which was quite high.
I suspect that the shorter gearing was used to make it appear more urgent than the 1.8.
It reached a maximum speed of an indicated 129mph at ~6500rpm in top and it was turning over at 4000rpm at 80mph, which was quite high.
I suspect that the shorter gearing was used to make it appear more urgent than the 1.8.
Edited by MC Bodge on Friday 24th February 14:36
DrTre said:
Gotcha doogz.
Perhaps I made the point a bit extreme with 100 mph, and to be fair I am talking of road useonly...I realise a long top would blunt acceleration a huge amount.
Asking the same question in a different way, golfs used to have a top speed of 120 but weren't busy on the mway, so ... what's changed? is it just the power? the gearing HAS to be dictated by the power available?
i know these are doofus questions
It takes remarkably little power to "cruise" on the motorway at 60-80 mph. THink of whatever car you drive? Do you have the throttle wide open to maintain speed on the motorway? It takes perhaps 30-50bhp to actually maintain 70ish Mph (many factors permitting, of course).Perhaps I made the point a bit extreme with 100 mph, and to be fair I am talking of road useonly...I realise a long top would blunt acceleration a huge amount.
Asking the same question in a different way, golfs used to have a top speed of 120 but weren't busy on the mway, so ... what's changed? is it just the power? the gearing HAS to be dictated by the power available?
i know these are doofus questions
So you may as well gear to not have a huge amount more than that coming from the engine at "cruising" speeds. Need more power to accelerate faster? Change down and do so, even in a pretty weedy car.
I've ridden in a Japanese Spec Impreza Sti Type RA - The ones that were specced with steel wheels and NO sound deadening, to go rallying in. Most important for this discussion though, is that they came with a short ratio box geared to top out at about 120 mph. On the motorway, at 80, you were doing about 4-4.5k Rpm, and the noise was horrific. As was the fuel economy.
Oh, and an "overdrive" gear is any gear which Reduced the output shaft speed from the input shaft speed. Most cars 5th or 6th gears are in fact overdrive gears. In many cars, 4th gear is 1:1.
I agree that most cars should have a very leggy 6th gear - for motorway cruising at high efficiency. I think that would do more for economy then all this stop start stuff!
DrTre said:
Thanks all, and thanks for the explanations...though I'm still left pondering a tall top to cruise with, like the Lotus Carlton which couldn't pull its top gear to maximum, but I think it's me being thick.
I have gone directly from one extreme to the other:I sold a JDM Impreza 22B to buy the Lotus.
Subaru claim that the 22B should just about hit 150mph. To do that it would have to hit the red line in 5th (top). I reckon 145mph would be absolute max. Different gearing (like the UK version) would easily take it above that. Steady Motorway speeds were buzzing along at 4000rpm in 5th. You need to change gear very often, but that's part of the fun. The push in the back is severe, but punctuated by the frequent gear changes.
The Lotus cruises in overdrive 6th at 1600mph at 70mph, which is presumably good for economy! Top speed runs (max. 177mph) can only be achieved in 5th. Gear changes are required less often in general driving, since the wave of power hauls the car forward at great velocity for a relatively small increase in rpm. This is also good because the ZR1-sourced gearchange ain't the best.
HTH
doogz said:
Quick example, my 106 is geared for about 120mph, flat out in 5th.
Acceleration for a 1.6L engined car is pretty impressive.
But at 70mph, it's doing over 4500rpm. Not ideal for noise and fuel economy.
Yeah, can see...so it's a tricky gearing balance? Unless something like approaching an overdrive gear were to be used? (I know I probably don't mean a literal overdrive, but a tall top)Acceleration for a 1.6L engined car is pretty impressive.
But at 70mph, it's doing over 4500rpm. Not ideal for noise and fuel economy.
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