Flywheel Weight VS Torque

Submitted: Wednesday, Feb 07, 2007 at 09:04

ThreadID: 42072 Views:5386 Replies:12 FollowUps:23

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Member - Hughesy (SA)

Is there anyone out there with a strong mechanical background that can enlighten me on the above topic.

I was looking in the CARS section the other week and Peugeot have developed a twin turbo diesel for its new 407. This 4 cylinder 2.2L TTD has 125KW and 370Nm of torque at just 1500rpm. It has 200Nm @ just 1000rpm, 250Nm @ 1250rpm and still has 355Nm @ 3000rpm.

Now on paper that looks like the best "4x4" diesel engine you could get!!! But I can't imagine those figures translating to real life performance lugging a 3 tonne cruiser/patrol around. If i gave those same figures for an engine of twice the cubic capacity we'd all want one......

My slug 1HZ has no where near that torque or at those low revs but I know it will slug its way through soft sand doing 600rpm at times and not stall.

So does that translation to "offroad" performance have something to do with the weight of the flywheel?? Obviously the weight of a cruiser flywheel would probably be double of that of the Pug. But torque is torque (is talk) so how does it work??

Don't want to debate the reliability or life of such a poket rocket engine (we all know when that will end up!!!)

Come on, fill my brain with everyones point of view.........

(PS if that engine was good in a cruiser/patrol just imagine how much room you'd have under the bonnet to fit all the accessories.........Roachie would be able to fit a SPARE engine in there...ha ha)

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Reply By: Member - MrBitchi (QLD) - Wednesday, Feb 07, 2007 at 09:22

Wednesday, Feb 07, 2007 at 09:22

IMO torque is torque, doesn't matter how many cubes are makin it. Only issue with such a small donk may be longevity.
The weight of the flywheel will affect throttle response and therefore driveability. The lighter the flywheel the more responsive the engine. In a 4WD in the rough a bigger flywheel will give less response to throttle input and therefore smoother performance.

AnswerID: 220337

Follow Up By: Member - Hughesy (SA) - Wednesday, Feb 07, 2007 at 10:33

Wednesday, Feb 07, 2007 at 10:33

Thanks John but here's a q for you - if you were towing a large camper/van/boat around (ignoring engine longitivity) would you prefer a Ute Turbo cruiser motor (one without intercooler) or the 3L nissan motor which from memory have similar numbers on paper?????

Do the same comparison with some of the other small capacity diesels that have similar or more power than a cruiser motor (I'm only using cruiser motors as a comparison cause I know them well) ie Jeep, Navara etc.

I haven't driven most of these new diesel vehciles but I'd put my money on the large capacity engines being better for slow or stop start work.

So does this have something to do with flywheel weight or maybe cubic inch??

I'm not arguing the numbers that the manufactureres release just how that relates in the real world.

FollowupID: 480906

Follow Up By: garrycol - Wednesday, Feb 07, 2007 at 10:36

Wednesday, Feb 07, 2007 at 10:36

There is not an issue of longevity in a small engine if it is designed from scratch to take the designed power and torque outputs. The issue with longetivity in smaller engines comes from engines that were designed to develop a certain power output and have been developed further to deliver more without the internals being beefed up.

A small how output engine that has been designed from the start will have the big beefy crankshafts, the big beefy conrods, pistons etc - if a 2 liter 4 cylinder engine is producing the same power as a 4 litre 6 and it was designed that way - ergo the crankshaft, conrods etc on the smaller engine will be beefier than the bigger one because the power on each stroke is greater.

So if the subject engine was desugned from scratch to put out its power its longevity should not be worse than a similarly designed larger engine.

Gazzz

FollowupID: 480907

Reply By: Robin - Wednesday, Feb 07, 2007 at 09:25

Reply By: F4Phantom - Wednesday, Feb 07, 2007 at 09:27

Wednesday, Feb 07, 2007 at 09:27

As far as I am concerned torque is torque. The wheels dont know if the engine is 25cc or 7.3L as long as the prop shaft turns with a certain torque & speed then thats all that matters. Only problem is that its hard to get your mind around a 2.2L giving the same torque as a 4.2L. The only possibe thing I can think of is engine braking, it could be that smaller powerful diesels may be easy revvers but I cant see why when they are not getting fuel. We need a diesel mechanic!

AnswerID: 220342

Follow Up By: Member - Hughesy (SA) - Wednesday, Feb 07, 2007 at 10:22

Wednesday, Feb 07, 2007 at 10:22

The point I was trying to make is that with all these new High Tech small capacity diesels coming out the biggest complaint that I hear from people is that at low revs they have no "pull". The old 2.8 TD Patrol motor and probably pretty much the same for the 3L Nissan - on paper the numbers and torque look good but this doesn't reflect how they perform at low revs.

I had a work 2.8 Nissan and it was an absolute gutless wonder from standstill but once it got going it was remarkable (haven't driven the 3L). I would have hated to have towed with it in a city environment.

If torque is torque and a little 2.2L can produce that much power then why are Toyota putting more cylinders and cubic inch into their diesel engines???

FollowupID: 480902

Follow Up By: F4Phantom - Wednesday, Feb 07, 2007 at 10:31

Wednesday, Feb 07, 2007 at 10:31

Then you have to look at the torque figures vs rpm. Old style diesels dont give you more torque or more power and sometimes not even better economy, but they do give the same or less torque at lower rpm. If the 2.8 was a crap box to drive at low rpm it could have been for two reasons, 1 it didnt give the torque low enough in the rpms because it had no boost, or 2 you were not actually after torque, you wanted KW which comes from rpm, and kw translates into acceleration. EG my wrx accelerates like a hiundai under 3000rpm but over that the kw come to life. It has crap torque so cant just accelerate from high gear ratios. Around town kw is good for accelerating off the lights, and torque is good at 100km/h to keep the speed up a long hill.

FollowupID: 480905

Follow Up By: Member - Hughesy (SA) - Wednesday, Feb 07, 2007 at 10:40

Wednesday, Feb 07, 2007 at 10:40

Yep but the 1HZ has a max torgue of only 285Nm at 2300 or 2400rpm which is pretty high. Most of these new diesels have max torque reached below 2000rpm.

With the 4.2L diesel you can be fully loaded and let the clutch out without increasing revs (750rpm) and it will move off slowly without a problem. If you did that in the 2.8L Nissan (or some of the other small TD) empty it would stall.

Definately give that the larger capacity engines don't have the response or power but neither of those really help you in the slow/stop start/towing stuff.

FollowupID: 480908

Follow Up By: F4Phantom - Wednesday, Feb 07, 2007 at 10:47

Wednesday, Feb 07, 2007 at 10:47

Yeah again what is the torque value a 750rpm. Thats all that matters, if a 25cc engine gives more torque than a 7.5L v10 at 750 rpm then the 25cc will be easier to start off with. Perhaps idle rpm torque in the 4.2 is better than the 2.8 which is why it feels better off road. Also take into acount auto trannys, they can convert kw into torque so a thrashy 2.8L could be much better off road in auto than man, this point is interesting but beside the point to this decussion. we are just talking measureable torque from the output shaft at given rpms.

FollowupID: 480910

Follow Up By: Member - Hughesy (SA) - Wednesday, Feb 07, 2007 at 11:00

Wednesday, Feb 07, 2007 at 11:00

Definately won't argue about the advantages of an Auto matched up with a diesel.

I'd like to have at hand the torque curves for some of these engines. I'd bet that "in real world" what your saying about torque isn't correct (not trying to pick an argument:). Ie that there would be graphs that show lower torque at lower revs than other engines yet when actually driving them you don't get it in drivability terms. So is this because of engine displacement or flywheel weight or....????

I understand that gearing has an effect but we're just talking high range 1st / 2nd which in terms of final ratio most vehicles are around the same - if anything the large diesels are lower ratio (? - less revs for same speed) which is a disadvantage.

FollowupID: 480917

Follow Up By: F4Phantom - Wednesday, Feb 07, 2007 at 11:26

Wednesday, Feb 07, 2007 at 11:26

the flywheel is only there to smooth out power delivery, eg a v8 has a smaller flywheel than a 1 cylinder static engine. it takes away the peaks from the combustion cycle. If you revv an engine hard and slip the clutch you are only getting inertia built up by the rpm and only then you dont get more torque from the engine, but the clutch slipping does deliver a higher torque figure a lot like the impellers slipping in the slidge box. So in the end it does come down to nm and rpm. The engine size is irrelivent. If a big 4.2L is giving a more comfortable feel at a certain rpm then it must have the right figures at that rpm. I would imagine a 2.0L modern diesel may give more torque at low revs (1000 - 1500) but i doubt it would give as much torque as a 4.2L at idle (750). What they need is the 70's porsche sportomatic transmission in a 4x4 modern small diesel, then we would solve all the problems.

FollowupID: 480926

Follow Up By: Member - Matt Mu (Perth-WA) - Wednesday, Feb 07, 2007 at 12:04

Wednesday, Feb 07, 2007 at 12:04

You are only looking at MAX torque with the old diesels and Max torque with the new diesels.....what you are missing is that the old diesels have LOTS of torque at all revs.....not just at MAX.

The new ones will have nothing, nothing, nothing, then boom...up to Max at their rated RPM!! Not very driveable with a load. The rev range of an old diesel is useable from low to high RPM. Thats driveability and hard to show on paper without a torque curve and just figures!

FollowupID: 480937

Follow Up By: F4Phantom - Wednesday, Feb 07, 2007 at 17:27

Wednesday, Feb 07, 2007 at 17:27

I think as part of this discussion we actually have discussed rpm. Your right in your example but the small diesel given in the first example actually says that it has more torque at the given low rpm. The only way the large engine could possibly be better is if the large engine has more torque at idle (750rpm), because at 1000 and 2000rpm it does not. CI is irrelevant. If it turns out the small engine has less torque than the 4.2L, the only conclusion (besides the whole reliability longevity issues) is that LC's should have 2.2L modern diesels in them.

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Follow Up By: PajeroTD - Wednesday, Feb 07, 2007 at 21:00

Wednesday, Feb 07, 2007 at 21:00

if the power and torque throughout the rev range are equal, and we know we can't measure structural integrity of an engine from cylinder displacement, everything is exactly the same in performance then. BMW have a larger and more solid engine block in a 3L diesel than they do a 3L petrol. their 3L diesel is heavier than their V8 petrol too. For structural integrity. It is a good towing engine. a 2.2L Landcruiser would have one drawback if its power and torque were equal to a 4.2 That is a marketing issue. Would it be hard to convince people that a 2.2 diesel Landcruiser is a good thing?

FollowupID: 481053

Reply By: Member - Oldplodder (QLD) - Wednesday, Feb 07, 2007 at 09:55

Reply By: PhilZD30Patrol - Wednesday, Feb 07, 2007 at 11:02

Reply By: Gerhardp1 - Wednesday, Feb 07, 2007 at 11:14

Reply By: Kiwi Ray - Wednesday, Feb 07, 2007 at 12:28

Reply By: rod2101 - Wednesday, Feb 07, 2007 at 12:30

Reply By: Eric Experience - Wednesday, Feb 07, 2007 at 22:03

Wednesday, Feb 07, 2007 at 22:03

Hughesy.
You are spot on with your prediction that the 2.2 would be a better motor.
the fly wheel is chosen to tune the resonance of the crank shaft and has nothing to do with torque,[ resonance in a crankshaft causes fatigue and vibration] The new diesels are far superior to the old bangers for off road driving. The reason they are so good is the new very fast injectors, with the old motors you get one squirt per stoke but the new units can inject up to 5 times in one stroke this gives you smooth running and therefore less stress on the mechanicals. In order to have enough air in the cylinder to fire 5 times you must have a good turbo hence the twin turbo. The torque figures you mention are correct and are greater than the 4.2 but that torque is not available in the first half second as it takes that long for the small turbo to spin up. The new Merc Sprinter uses this 2.2 twin turbo motor and it is available up to 4 tonne capacity, it pulls like v8 and returns about 8L/100k bloody amazing. I have both a Toyota and a Merc my girl has the Peugeot and it will pull away in top gear from very low speed, the difference in performance is huge in favor of the small motor. Eric.

AnswerID: 220491

Follow Up By: Member - Hughesy (SA) - Thursday, Feb 08, 2007 at 16:31

Thursday, Feb 08, 2007 at 16:31

Hi Eric,

So you've driven one of these sprinters with that 2.2L motor in them?? And they pull like a V8 even from low revs?? I might have to pretend I'm shopping for a new car next time I'm in the city and try one of these pocket rockets.

With an Auto these little engines sound like the ducks nuts. Just don't want to be the first to try the reliability issue out......or the cost of parts for one!!!!!!

FollowupID: 481215

Follow Up By: Member - MrBitchi (QLD) - Friday, Feb 09, 2007 at 09:13

Friday, Feb 09, 2007 at 09:13

Have to agree with Eric re the Merc Sprinter vans. Drove the college one a while ago, dam near does wheel stands off the lights once you get the hang of it. So much torque it's amazing!!
Also agree with the comment about the 1st half second of no torque. Means it's actually very easy to stall until you've driven it a bit and got used to the clutch.