Changing Gears at a High Rev Count?

dealgan

Going back to the original question .. I heard "somewhere" that the forces within th eeingine components increase at roughly the square of the rpm ?
i.e. The forces working on your moving engine parts at 4000 rpm will be 16 times the forces at 1000 rpm.

Mabye more realistic figures would be comparing 4000 rpm to 2000 rpm
in no specfic units, take forces at 2K as being 4
then forces at 4k would be 16, which is 4 times the force, and thus 4 times the "wear and tear" ?

This may be just anecdotal .. but it makes sense to me.

Cheers

BrianD3

Most of what has been posted in this thread is just speculation. The truth is that no-one here really knows the wear caused by reving a car to 3000/4000/5000 or whatever rpm. It makes sense that there would be greater wear at higher rpm but how significant would this be? Perhaps the oil used in car engines is designed to lubricate more efficiently at higher rpms? Also, what about the effect of oil pressure? Who the hell knows?

Personally, I don't ever rev the engine in my car to the limiter. Rev limit is 6200 on my car and I don't think I've ever had the engine above 4800 and even then it's only been when overtaking with a fully warmed up engine. I wouldn't go above 2500-3000 on a cold engine.

In contrast I drove a Ford Focus 1.4 recently and it was so gutless that I had to rev it to close to the red line when overtaking. The engine in the focus may has crap power but it does rev freely and smoothly.

Also what about Honda engines - most of these love to rev yet we all know how Honda engines have a great rep for reliability and lasting a long time.

BrianD3

davkav

Many honda owners I know top up there oil once a month due to the high rev nature of these engines especiallythe Vtec. Higher revs puts added pressure in the engine which cause some of the oil to leak through seals that where not designed for constant high revs.
On my odl motor bike I used to drive it into the ground. If i drove around maxing every gear Id usually have to top up the oil tank at the end of the week, but if i drove it at low revs and just cruised there was no need to top up the oil.
Just thought of this now, ive been in a coulpe of fairly suped up cars like skylines and the likes and their oil pressure guages would increase as the engines revs came close to red line

Dagon

So then Hondas can take more revving than most cars? Cause I have a Honda and it definitely is rev happy. Only a 1.4 but when revved it is at it's best.. but of course I'm weary of doing 4500+ too much incase I cause damage to the engine.

Duckjob

Yes indeed, Honda engines are designed to rev higher than most others, most likely due to the companys racing heritage.

I've a CRX that doesn't really wake up until 5000rpm, when it flies, and it's not even a VTec. I've had a similar to Alun with his MX-5 - if I pootle around for a while then when I do call on her to move, she feels rough and sluggish, whereas I find regular blasts up to the redline keep her smooth and eager, and so are a necessary part of the cars maintenance. That's my excuse, anyway

Esel

Engines are designed to give maximum power at high revs. An engine is happy at the engine speed at which is designed to make max power. A properly maintained and serviced engine will not suffer any damage if operated within its rev limits. Fuel and oil consumption will suffer, however. Rev limiter or not, most engines will stop making any more power before the engine approaches its redline.

Google 'powerband'.

Anyone remember the glorious (but narrow!) powerband on two-stroke twin-cylinder motorcycles?

causal

esel wrote:

Anyone remember the glorious (but narrow!) powerband on two-stroke twin-cylinder motorcycles?

The 1993-2001 Honda CB500 twin 4-stroke has a truly glorious powerband from about 8000-10000rpm
It is narrow because the revs snap through that range so fast and the redline is at 10600.

causal

ds20prefecture

esel wrote:

Engines are designed to give maximum power at high revs. An engine is happy at the engine speed at which is designed to make max power. A properly maintained and serviced engine will not suffer any damage if operated within its rev limits. Fuel and oil consumption will suffer, however. Rev limiter or not, most engines will stop making any more power before the engine approaches its redline.

This seems to be the first accurate piece of information in this thread.

My old Alfa was happiest between 5 and 7000 rpm. The red line was 7250, I think. It sounded glorious and went like stink in that range. While accelerating I would nearly always rev that high, not because the car "needed it" but rather that it was a much more involving drive if I did. As soon as you reach your desired speed, change into an appropriately high gear, but not too high. For e.g. cruising at 30mph, fifth is the wrong gear. Fourth is fine, and third has you ready to accelerate away from trouble. In my wife's Skoda, the car will rev to 6000rpm, but it sounds strained and unpleasant much above 5.

In my DS, anything above 4500 is like making your granny compete in a 100m dash. She wheezes, bitches and complains then just ignores you and goes back to her knitting at 3500.

AMurphy

Ernie Ball wrote:

This site seems to suggest that you're wrong about that and it makes some sense. Just looking at the torque peak doesn't take the gearing of the transmission into account.

Ahrrrrr, here we go again.
A gearbox is a torque amplifier, that's all. So what would you expect when you gear down, lowered torque at the wheels?. read the article again...

So irrespective of what gear you are in or even if you just weld two wheels directly to each end of the crankshaft, stick the blasted thing in a frame and accelerate.... peak acc occurs at peak torque, doesn't matter whether it is a honda 50 or a 5L Ford. Doesn't mean a H50 will outaccelerate a 5.0L.

And/or if you have a perfectly flat torque band, the acceleration is the same irrespective of the rpm.
(I have taken the liberty to assume that all other noise items, like bearings, wind, friction, etc are nil or neglected.)

Ernie Ball

AMurphy wrote:

Ahrrrrr, here we go again.
A gearbox is a torque amplifier, that's all. So what would you expect when you gear down, lowered torque at the wheels?. read the article again...

OK, I just did. I don't claim to understand this as well as your exasperation suggests that you think you do. So maybe you can help me. One way you could do that would be to tell me where you think that the article in question confirms your view. Citing it might help. Because I see much in it that seems to contradict you.

For example, you claim:

So irrespective of what gear you are in or even if you just weld two wheels directly to each end of the crankshaft, stick the blasted thing in a frame and accelerate.... peak acc occurs at peak torque

and the article says:

But, you ask, isn't your acceleration greatest at the torque peak? Yes, it is! But only within that gear. The next gear down will give you even greater acceleration at the same speed, unless the vehicle speed is too high for that gear.

To use engine torque to understand how your car performs, you MUST include the effects of the transmission.

So please explain...

SouperComputer

causal wrote:

The 1993-2001 Honda CB500 twin 4-stroke has a truly glorious powerband from about 8000-10000rpm
It is narrow because the revs snap through that range so fast and the redline is at 10600.

causal

used to race single cylinder 100cc karts, peak power 28-30bhp @ 13500 RPM, peak torque at the same speed, more or less. Without a gearbox, revs drop as low as 9500 on some corners and as high as 22000 on streights!

I think thats about as narrow as you could get, yet the engines still pull well from about 10000 up.

AMurphy

But, you ask, isn't your acceleration greatest at the torque peak? Yes, it is! But only within that gear. The next gear down will give you even greater acceleration at the same speed unless the vehicle speed is too high for that gear.

Or put another way. within any selected gear, peak acceleration occurs when the engine is at peak torque.

It's right in that sentence. Changing the gear up or down, will amplify the torque, for example, either double it or half it,depending on the gearing.
If you double the torque by gearing "down" you'll accelerate faster and if you amplify by <1, by gearing up, you'll not accelerate as well as before.

And I take "speed" to mean road speed, mph, not rpm.


To use engine torque to understand how your car performs, you MUST include the effects of the transmission.

This is the bit I qualified, ie I choose to ignore these side effects completely for simplicity.
It is possible to conceive a large gearbox full of 1:1 gears, theoretically this would give the same power and torque at the output as the input, but in reality, the grinding of the gears, churning heavy oil, etc would mean the entire power could be absorbed, just turning the gears and leave nothing available for useful work at the output.

Ernie Ball

OK, I can now see that there are two separate questions:

1) When is peak acceleration?

2) When should one shift gears for maximum speed? (assuming that this--and not engine longevity or mileage--is what one cares about).

I accept that the answer to (1) is that peak acceleration occurs at peak torque.

But the answer to (2) is not, as some have suggested (if memory serves), that one should shift at peak torque. Usually, one will have to shift at some point after peak torque in that gear because, although torque is declining, power output to the wheels is still greater than it would be in the higher gear. In other words, if you plot the transmission torque curves for each of the gears, the optimal shift point is where the torque curve for one gear and the higher gear intersect. This is normally after the torque peak.

This is confirmed here and here.

AMurphy

Makes perfect sense.

But most of us dont really know where peak torque occurs....settle for whatever transfers you from one range to the other without hitting the "screeming banshee" in the lower gear or stalling & lugging in the higher gear.
I had a Ford Focus Ghia rental last time, and noted it seemed to work best about 3500~4000rpm. Might be 2500~3500 in a TD Avensis.

davkav

max acceleration does not occur by changing at peak torque. you achieve max acceleration through the gears by changing at the maximum level of bhp, different with all cars but its usually about 500-1000 before the redline

RicardoSmith

davkav wrote:

max acceleration does not occur by changing at peak torque. you achieve max acceleration through the gears by changing at the maximum level of bhp, different with all cars but its usually about 500-1000 before the redline

I'd have assumed that max BHP gives you peak torque at the flywheel. Is this not correct? I'd assume that the Redline is simply a limitation of engine tolerances and not an indicator of max anything. In my experience with big capacity engines and diesel engines quite often don't go near the redline to achive max acceleration.

Changing gears cause a loss in acceleration, as theres no torque applied to the wheels as you change gear, and BHP drops as the revs decrease.

So max acceleration occurs when you minimise this loss of torque and bhp when changing gears. You do this by having the gear ratios closer together and speeding up the gear change. There must also be a point where the time taken to regain lost torque and bhp is minimised.

Depending on the gearing of the gear box and flywheel, this would mean changing up earlier or later (holding on to a gear) so as to minimse the time spent in a dead/flatspot in the power delivery/torque curve (power band). Where the BHP and torque is increased more slowly than in other parts of the power band.

The only way to work this out accurately is by the use of timing gear and data logging equipment. Most drivers do it by feel. Which is probably close enough. For racing though it wouldn't be. There would be a whole range of factors that change the optimal change up point. Air temp, the fuel mixture, gearing, weight, resistance etc.

I could be (very likely) completely wrong. But thats my 2 cents anyway.

AMurphy

RicardoSmith wrote:

......
Changing gears cause a loss in acceleration, as theres no torque applied to the wheels as you change gear, and BHP drops as the revs decrease.

So max acceleration occurs when you minimise this loss of torque and bhp when changing gears. You do this by having the gear ratios closer together and speeding up the gear change. There must also be a point where the time taken to regain lost torque and bhp is minimised.

Depending on the gearing of the gear box and flywheel, this would mean changing up earlier or later (holding on to a gear) so as to minimse the time spent in a dead/flatspot in the power delivery/torque curve (power band). Where the BHP and torque is increased more slowly than in other parts of the power band.

........

This would be a more complete and accurate explaination.

davkav

back to the question asked, to achieve the maximum acceleration out your passat change gear at 5700 rpm according to the VW website performance specs. At 5700 rpm the car is at its maximum bhp output anything over this and the cars acceleration decreases anything under will do fine pending on how you want to drive your car. Your original question was will reving your car high do any damage to the engine, the engine will wear more-so when driving at high revs but it will not damage your just decrease its life span but you'll probably have sold the car before you notice any difference i.e 10yrs time. here is th link of the site that i got the specs from im not sure if that is your engine but the specs are the same on all the 1.8T's.VW website select the 1.8T from the drop down menu

Balfa

The whole power vs torque debate is enormous and not easy at all to understand, and a little off topic. As for damage to your car, the engine is designed to run up to the redline/rev limiter, so it certainly shouldn't be damaged by this, but certainly higher RPM means more friction, more forces, more heat and more wear inside the engine. Something will eventually go wrong, sooner or later. Higher RPM will tend to push you towards sooner.

However, it's not drastic. The square the RPM to find the engine wear thing may or may not be accurate, but it's probably a fairly good indication. Think about this scenario. You floor it up to 6,500rpm, change down to 3,000rpm, and up again to 6,500rpm, in each gear to get good acceleration. We'll pretend your average engine speed is 5,000rpm. At this engine speed, you'll be wearing your engine twenty-one times quicker than at 2,000rpm. But we'll say you only spend 10 seconds accelerating this hard, then you cruise in 5th gear at 2,000rpm. So you're doing the equivelant of 210 seconds (3.5 minutes) "normal" wear in 10 seconds.

If you floor it hard from time to time and spend most of your time cruising, that's fine. If you floor it every chance you get between red lights, your engine won't last as long. Depending on who you believe, it might also be an idea to floor it from time to time just to clean out the throttle body and stuff like that. It's not the end of the world, and if you want to accelerate hard now and then, go right ahead.

If I'm accelerating, I'll often use a lot of the rev range (driving a daihatsu domino, i had to use the full rev range to get the thing to move at all!), but if I'm driving at a constant speed, it'll almost always be in 5th gear, even if it's just at 30mph.


edit: as for cold engine, i treat this REALLY gently. At all costs I try to keep the engine below 2,500 until at optimum temperature. Btw, my Hyundai Accent has never ever run at any temperature other than exact optimum (it has a little mark to show were the needle should be) once it gets going. It's a far cry from the daihatsu domino, I'm really impressed Are all newish cars like this?

BrianD3

Balfa wrote:

edit: as for cold engine, i treat this REALLY gently. At all costs I try to keep the engine below 2,500 until at optimum temperature. Btw, my Hyundai Accent has never ever run at any temperature other than exact optimum (it has a little mark to show were the needle should be) once it gets going. It's a far cry from the daihatsu domino, I'm really impressed Are all newish cars like this?

Different gauges behave in different ways. Some are more sensitive than others. Your Hyundai's gauge might give exactly the same reading if the coolant is at 75 degrees or at 95 degrees, OTOH your Domino's gauge needle might be seen to move if there is a change of 5 degrees in the temperature. My old renault has a very sensitive gauge and it seems to be quite accurate judging by an experiment I did using a digital thermometer immersed in the expansion tank. Car has exhibited this behaviour from new. Whereas with the gauge on my newer Laguna the needle is much more stable, even when the fan comes on/off the needle barely moves. The needle also flies up to the midpoint very quickly (~3 mins) after the car is started from cold which I am suspicious of. There's a bit of psychology involved in designing temp gauges, people get worried if they see a needle moving about or not sitting at the midpoint or taking a while to reach normal temp. Therefore designers make the gauges less sensitive and set them up so that the driver will only notice them doing anything if there is an actual problem i.e. overheating.

There is plenty of discussion on the web about these "fake" or "need to know" temp gauges. Hondas and Fords are often mentioned as having fake gauges, many other modern cars probably do too. If these gauges are designed to be fakes, then they're really no different from a temperature warning light (aka idiot light)

BrianD3

ando

davkav wrote:

you achieve max acceleration through the gears by changing at the maximum level of bhp

ok that maybe for a petrol but what about diesel. Diesel as everyone knows gets the peak torque WAY down the RPM. My car gets peak torque of 210ft/lbs at 2000RPM, using 80BHP and declines from there and max's out at 4000rpm with 150ft/lbs using 117BHP. If I want to get to 0-60 the fastest, should I be changing gears at 3000RPM? or leaving it go till 4000RPM? As the BHP increases the torque decreases? that supprised me



BHP: http://homepage.eircom.net/~monza1999/TDI/tdp2.jpg

causal

Notwithstanding the fact a lot has already been said on what determines max acceleration...

Put simply, the optimal* point to change gear is when you will get more acceleration from the gear you are changing into, than the gear you are currently in.

So it's not enough to look at the powerband for the current gear, you have to look at the powerband of the gear you're changing into.

causal

*optimal for max acceleration, not fuel economy

SouperComputer

an engine is at its most effective if gear is enagaged at max torque and changed up at max power, this is what is referred to as the powerband in racing.

So....... Peak torque 3500 RPM, Max power 5600 RPM, redline 6200: change gear at 5600 and the car should be gear so that it does not drop below 3500 when the next gear is engaged.

as road speed increaces, empahsis is no longer on acceleration due to frictional and aerodynamic losses, so the gears become "shorter". Eg 5th and 6h may only drop to 4500 and 5000 RPM respectively when engaged.

there are some exceptions, for example engines that produce peak power and peak torque at near enough the same revs. In that case its a trade off as to how much time you spend in neutral, vs how much acceleration is available below peak torque.

This is one of the reasons for hydraulic gearchanges in f1, as the powerband is very small.

causal

SouperComputer wrote:

So....... Peak torque 3500 RPM, Max power 5600 RPM, redline 6200: change gear at 5600 and the car should be gear so that it does not drop below 3500 when the next gear is engaged.

That example sums it up nicely

Garda: "Do you know you were speeding, the motorway limit is 120kph?"
Me: "No officer, I was investigating the relationship between torque, power, gearing and how they affect my powerband"

causal

commited

unkel wrote:

What I do about revving high: I never go over red line minus roughly 2k rpm until engine is fully warmed up

:eek:

I dont go above 2500rpm (1.2 petrol) until the engine has warmed up. Revving an engine whilst it's cold will shorten its life span.

Esel

commited wrote:

:eek:

I dont go above 2500rpm (1.2 petrol) until the engine has warmed up. Revving an engine whilst it's cold will shorten its life span.

Agree. This should be a hard and fast rule.

Also, always drive away immediately after starting a cold engine - never let it 'warm up' first. This causes massive wear.

unkel

esel wrote:

Agree. This should be a hard and fast rule.

Also, always drive away immediately after starting a cold engine - never let it 'warm up' first. This causes massive wear.

Aye. To clarify my previous statement, I usually stick to <2000 rpm on a cold engine. In an extreme case (necessary overtake or something like that) I would take it to red line - 2000 rpm

AMurphy

ando wrote:

..... As the BHP increases the torque decreases? that supprised me



BHP: http://homepage.eircom.net/~monza1999/TDI/tdp2.jpg

Shouldn't. The Power is given as 2*Pi*RPM*Torque, Therefore, assuming the torque is flat or relatively flat at least, then the power will increase with increasing RPM.

However, as the torque goes "over the top", the power will slow or stop increasing and if and when the torque goes into a steep decline, the power will begin to drop with increasing RPM... but remember the torque is dropping even faster. So under those circumstances there is no point in continuing in that gear, unless you have alreached top and terminal velocity.

So the previous statment;

Put simply, the optimal* point to change gear is when you will get more acceleration from the gear you are changing into, than the gear you are currently in.

is significant, when the next gear up is at the correct ratio so as to take advantage of the situation.

Dub_Ster

JB123 wrote:

I like changing @ 8500rpms .Is this a petrol car?

mines going to rev out at 10500 when the ecu is remapped i cant wait any way are engines are desinged to rev to 12500 out of the factory due to the marvolous varable valve timeing ( any one wondering why higher lift cams for track days )

simple man 3000 revs a suberu impreza doesnt come oon boost at 3000 ,
its perfectly fine to put your foot down and leave it there untill you come on the red then change gear procsley and gentaly and itle be fine ...cars are designed to be drivin hard as well as slow ...

Ernie Ball

How would one say all that in English?