confused about power & torque on a couple of cars

confused about power & torque on a couple of cars

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danlightbulb

Original Poster:

1,033 posts

105 months

Thursday 26th November 2015
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Hi guys,

I'm a bit confused. I've searched the net and read up on it but don't really have an answer so asking here.

I'm looking at a Honda Accord which has has 188bhp, which seems good, but the torque figure seems really low at 164 lb.ft. It's 0-60 time is 7.9s, which also sounds good. My mondeo makes 210 lb.ft, and I know its a diesel which make higher torque at lower revs, and obviously its 0-60 time is alot longer at 10.8s. But still I'm confused about how the torque works. What I'm most concerned about, as its a VTEC engine, is having to red-line the engine all the time to get the acceleration out of it.

Another car I'm looking at is a Saab 9-3, the 150bhp 2.0 litre turbo version. Despite being lower power it makes 177 lb.ft of torque which is more than the Honda. I assume the turbo is helping here. For an £80 remap, that torque figure climbs to 265 ft.lb at 220bhp.

I've driven a Saab 9-3 but the engine was a dog so I couldn't get any useful information from that test drive, and I haven't driven a remapped version. I should be seeing the Honda shortly. I'd like an idea of what to expect from the Honda and how it might compare to the Saab.

Can anyone help?






Edited by danlightbulb on Thursday 26th November 23:46

Renovation

1,763 posts

120 months

Thursday 26th November 2015
quotequote all
You basically have it, without getting too technical:

Highly tuned NA engines make high power at high revs and relatively low torque

Diesels generally have high torque at low revs and relatively low power

Which is why when driven in normal use diesels feel fast but when you need to overtake you find they aren't as fast as you thought.

Turbos generally respond well to a remap.

Vtec - yes you have to thrash the nuts off them - rewarding on the right roads, tiresome in City traffic.

Edited by Renovation on Thursday 26th November 23:36

DS197

992 posts

105 months

Thursday 26th November 2015
quotequote all
Tuned Hondas and Toyotas are worse in terms of torque compared to other tuned naturally aspirated cars imo. They sacrifice too much torque for hp.

danlightbulb

Original Poster:

1,033 posts

105 months

Thursday 26th November 2015
quotequote all
Renovation said:
Highly tuned NA engines make high power at high revs and relatively low torque
Right so does this mean they are significantly slower at low revs? The Honda has a 0-60 time of 7.9s, which is pretty good, but to get this is it driven right up above 6k revs which is where the VTEC kicks in?


Renovation said:
Which is why when driven in normal use diesels feel fast but when you need to overtake you find they aren't as fast as you thought.
Yeah my current car is pretty quick off the lights but dies by the time its hit 25mph.


Renovation said:
Vtec - yes you have to thrash the nuts off them - rewarding on the right roads, tiresome in City traffic.
This is what i'm worried about, that the car will feel dead during normal driving.

lostkiwi

4,584 posts

123 months

Friday 27th November 2015
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If you drive a diesel currently the Honda will likely feel flat and slow as you won't get the kick as the turbo comes on boost at low revs.
In general revvy petrol engines are lower geared so as to increase the torque at the wheels. To a certain extent this masks the lack of flywheel torque. To get that performance figure they will be using the top end of the rev range.

Fastdruid

8,623 posts

151 months

Friday 27th November 2015
quotequote all
Assuming two "identical" spec cars, one diesel, one petrol. If you are using the correct gear then the petrol will have more torque at the wheels everywhere outside of 1st gear, where the diesel will have far more but for only a very short time.

That does however mean that at times you can be 2 gears lower than the diesel and with some even three, however it also means that if you are *not* in the right gear then the petrol will have far less torque. You will also have more flexibility in your gears, for example 3rd in mine will do from ~10mph up to ~90 and mine doesn't even rev that high!

Personally I find diesels far more picky about gears than petrols, a combination of boost threshold, a narrow powerband and a lack of revs all conspire to require far more gear changes to keep them "on the boil" and there is very little choice of gears. I could drive round town at 30 for example in 2nd,3rd,4th or 5th in my car while you'd probably be limited to 3rd/4th in an equivalent diesel (without dropping off boost).

Where however a diesel does "win" is when you stick the car in a high gear for economy and then want power, most non-turbocharged petrols are making sod all torque/power at ~2k but that's prime diesel territory and so they feel massively strong. Turbo'd petrols are even better and have the advantage of a strong bottom end *and* revs but of course come with a fuel cost that few are prepared to pay.

danlightbulb

Original Poster:

1,033 posts

105 months

Friday 27th November 2015
quotequote all
Thanks. I'm not going to be buying another diesel was just using it for comparison. But yeah, it has a very small power band but does pull well at low revs.

In the two cars I'm referring to, the Honda is a 2.4 litre Vtec and the Saab is a 2.0 litre turbo. The Saab gets quite a bit better mpg than the Honda according to the official figures. According to the guy who remaps them, fuel economy is improved further after a remap as well (assuming you don't then thrash it everywhere I guess).

I'm concerned the VTEC will have nothing at the low end and everything at the high rev end which I'll mostly never use. I do need to drive it obviously and see for myself. But the headline torque number seems quite low.

It would also be nice to drive a car that can just rev and rev in one gear all the way up the speedo. I'm wondering whether the turbo'd Saab will suffer from a drop off of power at higher revs once the turbo is exhausted, maybe not as bad as a diesel does. Whereas the VTEC will (I assume) rev all the way up without breaking a sweat, and even kick up when the VTEC kicks in at 6k. Problem is I'll never be up at those revs.

Confusing stuff.







Edited by danlightbulb on Friday 27th November 01:46

jamieduff1981

8,022 posts

139 months

Friday 27th November 2015
quotequote all
Torque is real. It's the force with which the crankshaft tries to twist whatever it's connected too.

The amount of torque an engine produces at any given time varies depending on throttle position and timescale in which each cylinder can do its thing. The actual power stroke itself is not so much under the influence of the engine RPM, but the ease of charging the cylinders through the inlet valves and expelling it through the exhaust valves very much is - and without adding loads of complication a naturally aspirated engine can only be tuned perfectly for one RPM and operating either side of this is sub-optimal for torque.

In short, maximum torque output is a curve which assuming WOT throughout, varies with RPM. The figures you've got, OP, are peak figures which the engine can usually produce at a single point whilst being worked through its rev range. Your VTEC produces its peak torque at high revs due to valve and manifold design.

Turbochargers improve the cylinder charge as you know, which directly increases maximum torque at any RPM where the flow of exhaust gases is suffienct to spin the turbo fast enough such that its delivery of inlet air overcomes the losses its very existence in the exhaust system causes. Turbocharges typically hurt torque at very low revs then come good delivering strong torque for most of the rev range.

Power is just the rate of work done. You can exceed your car engine's torque easily with a breaker bar, but you can't spin the breaker bar at 6,500rpm whilst pushing so hard on it. The BHP produced anywhere in the rev range is the product of torque x revs x fixed number. If torque remains constant throught the revs, power is higher the higher it revs. Torque isn't constsnt though, so power is a curve too and peak power usually occurs as torque starts fizzling out as the revs climb.

If you draw a power curve for each engine, then the one with the most area under the curve will typically result in the fastest accelerating car (subject to silly gearing, traction etc etc and of course the car's mass being equal).

SidewaysSi

10,742 posts

233 months

Friday 27th November 2015
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So OP I take it you haven't bought a car yet?! smile

Get the Accord, you know it makes sense. Just learn to drive according to the engine. VTECs are great engines but you need a very different driving style than you are probably used to.

delta0

2,334 posts

105 months

Friday 27th November 2015
quotequote all
Diesels have very long gears compared to petrol so torque at the wheels is more important as that takes into account the gear ratios. Or you could just us power as that combines rpm and torque together which is why the Accord is quicker.

Edited by delta0 on Friday 27th November 07:40

griffin dai

3,194 posts

148 months

Friday 27th November 2015
quotequote all
I'm guessing the Hondas a nicer car (Saab geek here, the 9-3's decent enough but the interiors below par....for a Saab!!)

How much mileage are you doing? You can pick up a nice 2006 2.8 v6 Aero for around £3k now, £450 remap on this gets you to 320bhp & 400ft/lbs (different tuners give less/more torque so that's just a ball park figure) very thirsty though. I was getting around 28mpg on mine, down to around 20 now though smile

Martin_Hx

3,951 posts

197 months

Friday 27th November 2015
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With 188hp the Honda wont be slow, i wouldn't get hung up about torque! You will not have to "rev the tits off it" to go 30mph to the shops biggrin

SonicShadow

2,452 posts

153 months

Friday 27th November 2015
quotequote all
DS197 said:
Tuned Hondas and Toyotas are worse in terms of torque compared to other tuned naturally aspirated cars imo. They sacrifice too much torque for hp.
Toyota and Honda 4cyl NA's put out about the same amount of torque as other 4cl NA's of the same configuration and capacity.

IanCress

4,409 posts

165 months

Friday 27th November 2015
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Despite being slightly down on torque, the Honda will probably be the nicer drive. It won't have that 'flat' area at low revs before the turbo starts to work, and the power delivery will be more linear.

alistairolsen

75 posts

103 months

Friday 27th November 2015
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Ok, first of all, forget torque, power is ALL that matters, anyone who tells you otherwise doesn't understand.

Second, forget peak figures, as again, theyre almost meaningless.


Here is a fairly typical naturally aspirated plot. Its not perfect as theres a dip in the torque and therefore power curves, but flat torque and power rising to the redline

http://jetboatbase.lamp.wiredgroup.com/wp-content/...

This is a VTEC dyno plot, clearly showing the cam changeover. You can see how introducing the second cam gains power at the top end of the rev range:

http://image.automobilemag.com/f/features/6710874/...

And here is a typical TDI chart, showing the very flat power curve:

http://i121.photobucket.com/albums/o229/shortyscli...

You can see the way the whole curve is 'swollen' above that of the naturally aspirated engine. The N?A engine makes more power overall ( 167 against 156, but apart from a single point in the range this is meaningless. Take half of the available rev range for each, the n/a petrol at 3000rpm makes 90bhp and the tdi at 2500rpm is making 130bhp. This is what makes the difference, the power available over the bulk of the range at real world engine speeds.

Bradley1500

766 posts

145 months

Friday 27th November 2015
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danlightbulb said:
I'm concerned the VTEC will have nothing at the low end and everything at the high rev end which I'll mostly never use. I do need to drive it obviously and see for myself. But the headline torque number seems quite low
I haven’t driven an Accord but own and have spent a lot of time in various Civics and Integras with VTEC units. They easily keep up with traffic in normal conditions without the need to rev the nuts off them.

I find the power delivery fantastic, although do appreciate everyone is different. Unlike turbocharged cars which tend to have flat spots, VTEC engines build more power the more of the rev range you use.

As you mention you really need to drive and experience one for yourself to decide whether it is for you or not.

tedman

368 posts

103 months

Friday 27th November 2015
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I reckon the Honda engine will last longer, too.... they rarely ever go wrong.

danlightbulb

Original Poster:

1,033 posts

105 months

Friday 27th November 2015
quotequote all
Martin_Hx said:
With 188hp the Honda wont be slow, i wouldn't get hung up about torque! You will not have to "rev the tits off it" to go 30mph to the shops biggrin
I can remap the saab though for a measily £80 and it will have 220bhp and 360nm of torque.

If the honda needs revving high to get the most out of it, wont that kill the fuel economy as well? Wheras more power out of the turbo engine at lower revs will help the economy?

Ive been looking for power curves for both cars but cannot find.

SonicShadow

2,452 posts

153 months

Friday 27th November 2015
quotequote all
danlightbulb said:
I can remap the saab though for a measily £80 and it will have 220bhp and 360nm of torque.

If the honda needs revving high to get the most out of it, wont that kill the fuel economy as well? Wheras more power out of the turbo engine at lower revs will help the economy?

Ive been looking for power curves for both cars but cannot find.
The turbo increases the amount of air in the cylinders, the amount of fuel also needs to be increased proportionately. It's not as simple as that.

TA14

12,722 posts

257 months

Friday 27th November 2015
quotequote all
SidewaysSi said:
Get the Accord, you know it makes sense. Just learn to drive according to the engine. VTECs are great engines but you need a very different driving style than you are probably used to.
That's my vote as well. (IMHO for the SAAB, or any car, thinking that you can get a 50% increase in both power and torque and better mpg is living in dreamland.)