Why the low torque?

[Quentin]

The absolute torque spec would matter if Toyota said bollocks to a gearbox and just had the final drive multiplier. Thankfully, we have 4 gears in our gearbox that increase torque. Thankfully, again, Toyota and Subaru saw fit to make those 4 gears pretty aggressive with an aggressive final drive. A lot of vehicles with more engine torque have taller gearing, too, for the sake of fuel economy and that nullifies some of the performance advantage. The non track pack '14 Mustang V6 is a good example of that.



I honestly think the ratios are pretty spot on. Driving like an animal keeps you in the happy upper range. Normal daily driving (up to 3500 rpm or so) is nice with the aggressive gears, too. The car isn't a stoplight hero, but it just feels right.

[pushrod]

Originally Posted by Target70

call me slow, but I would have to see the math on that for it to make sense to me.

I'm not going to call you slow.

Try this:

Start with a 90mm bore, and calculate the surface area of the face of this piston. Then, take 500cc and divide by the surface area to get the stroke of this cylinder. Half of the stroke is the crank pin offset.

Then, choose a cylinder pressure. Apply it to the piston face area to get force (F = Pressure / Area).

Take F and multiply it by the crank pin offset. This is piston work (W = F*d)

Repeat for an 80mm bore. The piston work will be the same.

Actually, don't do any of that. Instead, note that Work is F*d, but it is also P*V, pressure times volume. We said both are 500cc, and we said both cylinders have the same pressure, P, so in both cases we have W=500*P. Done.

HTH

[pushrod]

Originally Posted by Quentin

I honestly think the ratios are pretty spot on. Driving like an animal keeps you in the happy upper range. Normal daily driving (up to 3500 rpm or so) is nice with the aggressive gears, too. The car isn't a stoplight hero, but it just feels right.

The FR-S needs a whole other gear to hit 60. If 0-60 was everything, this would be a big liability.

When I drive on the track, I really see and appreciate that the car was geared to get every horse out of the engine, and that a lot of it is my responsibility.

That said, I don't imagine the auto FR-S is a very nice ride. I base that on nothing.

[Quentin]

Originally Posted by pushrod

The FR-S needs a whole other gear to hit 60. If 0-60 was everything, this would be a big liability.



When I drive on the track, I really see and appreciate that the car was geared to get every horse out of the engine, and that a lot of it is my responsibility.



That said, I don't imagine the auto FR-S is a very nice ride. I base that on nothing.

Good thing that 0-60 is completely arbitrary, huh? That chart goes a long way to show why the car gets the numbers it does, though. It is geared correctly to keep the car on boil. The fact that we have 5 gears in the space of the Stangs lower 3 says a lot about the intentions of the car. The non Track Pack Stang is a car meant to hit spec sheet numbers (31mpg and 300hp, OMG!) Just more proof, to me, that you can't have everything and you have to compromise somewhere. The big question: Did Toyota and Subaru compromise the right things? (Peak torque for HP, 34mpg rating in the 6MT for good gear spacing.) I believe so.

I've spent some time in the AT FR-S, but not enough to get a good feeling for the overall matching of the gearing to the engine. I figure 1-3 is fine but it gets a little long legged on 4th, 5th, and 6th.


Sent from Tandy 400

[Twin4Life]

Im doing the first ever Mr. Fusion swap. No torque dip at all, and 88mph is a cinch to hit in seconds.

[stonenewt]

Originally Posted by Quentin

Good thing that 0-60 is completely arbitrary, huh?

Going off topic but 0-60 is a fairly meaningless performance metric. I want to know is 15-60mph through the gears (starting from 2nd gear) & 50-80mph in the top two gears. Those figures give me a much better idea of how fast a car is A-B & how easy it is to drive on a motorway.

[Ubersuber]

Originally Posted by pushrod

Emphasis mine:



1,2) There isn't really such a thing as "more torque than power", or any other comparison between the two, really. The crankshaft has power, in the form of torque about a rotating shaft. More torque for a given RPM, more power. More revs for a given RPM, more power. Torque is one of the components of power, do you can't compare them like that.

3) You might be shocked at how small the effects of bore to stroke ratio really are. Manufacturers have converged on roughly square bore:stroke ratios probably because a square cylinder has the highest volume to surface area ratio, which makes the engine block lighter. It also happens to give a good valve area. Speaking of valve area, if you think of a 2-valve V8, most of them have very big bores, so that the two valves that are in each cylinder can be nice and fat. Check out the bore and stroke of various engines like Chevy 350, etc. Then look at the Honda F22C1.

4) The stroke is only half of the equation for the piston putting torque on the crank. The stroke is the leverage, and the piston area (from the bore), with the cylinder pressure acting on it, is the force. The way the math works, any 500cc cylinder will make the same torque with the same cylinder pressure. There are secondary differences: friction, etc, but theoretically inches are inches, regardless of bore and stroke.

There are two aspects you may be missing that affect real world power output.

One is piston speed as has been noted by others and the other is volumetric efficiency.

The larger the bore the more valve area is possible. The more valve area the higher the potential volumetric efficiency. For this reason naturally aspirated shorter stroke engines of the same measured capacity will ALWAYS deliver more power because they will draw in more air than longer stroke engines.

If you like, increasing bore has a similar effect to increasing capacity or lightly supercharging an engine.

Multiple valves increase effective valve area for reasons that become obvious if you draw a circle and then fill that circle with smaller circles. The limits on the added volumetric efficiency of multiple valves are mechanical. At some point it is not feasible to gain efficiency by adding more valves. Five is the most I've seen in an ordinary road engine, three intake and two exhausts in Ferrari and Audi engines. Both have since gone back to four.

A third aspect is cylinder numbers (not precisely applicable to the topic as we are comparing similar engines but the comparison between our theoretically less efficient flat four and Porsche's flat sixes reinforces the argument that this Subie engine is very impressive), the more cylinders you have the more bore area is possible for the same measured displacement so a 6 should be more powerful than a four of the same measured capacity and same bore stroke ratio. The highest number of cylinders tried for the purpose of exploiting this aspect of engine design was the 64 valve H16 BRM engine: two flat 8 engines on top of each other driving two cranks geared to one output shaft. Specific power was awesome as was the sound made by the engines as they exploded (up to that point they also sounded awesome but the disintegration caused by destructive harmonic vibrations between the two cranks was equally impressive).

In general, high bore stroke ratios (over square) produce more power and low bore stroke ratios produce higher torque at low rpm. In addition, low bore stroke ratios are much more emissions friendly because the combustion chamber is more compact.


If you supercharge an engine all manner of volumetric issues can be overpowered by the force of the supercharger, including the valve timing constraints that produce the torque "dip" in our engines. In reality what you are seeing is two torque peaks resulting from a two stage variable valve timing system optimized for two overlapping rpm ranges. This produces higher torque than other choices commonly used. If you compare the torque curve of our engine to the torque curves of the Porsche flat 6's fitted to the Boxster Cayman you will see almost exactly the same twin peak torque curves. Even the much vaunted 911 flat 6 shows a similar pattern. This makes sense when you realize that Porsche consulted to Subaru in the design of Subaru's variable valve timing system which is very similar to that used by Porsche.

Superchargers can simply overpower volumetric inefficiency although since road engines rarely use the forced induction in normal operation the practice is to make a fuel efficient and powerful normally aspirated engine and then supercharge that, using electronics to control detonation issues and selecting a compression ratio that represents a good compromise between efficient off boost operation and good boosted power. Supercharged road engines can be made with smaller bore stroke ratios and still produce good peak power. VW Audi has gone this route and I suspect peak piston speeds are the limiting factor in selecting the most favourable bore stroke ratio for these VWAG engines.

FIAT has developed their range of "Multi air" engines which use true continuous variable valve timing and produce very useful flat torque curves".

Next time you feel your BRZ engine drop into that "dip" try thinking of it as transition from one torque curve to the next, an advantage not a deficiency. On those rare occasions when you actually need to drive at that "dip" point just tough it out and try harder next time to drive as the engine and gearbox are intended: low rpm for low speeds and high rpm for peak acceleration. Then the only time you will be in that dip will be in first gear at about 15 mph or so.

Drive to the engine and gearbox, not against it. Satisfaction awaits.

[Ubersuber]

Originally Posted by stonenewt

Going off topic but 0-60 is a fairly meaningless performance metric. I want to know is 15-60mph through the gears (starting from 2nd gear) & 50-80mph in the top two gears. Those figures give me a much better idea of how fast a car is A-B & how easy it is to drive on a motorway.

This aspect of engine performance is completely overlooked in North America but is generally regarded as at least equally important to standstill acceleration in Europe and other places.

Generally speaking, the smaller your engine the more you care about in gear performance and less about standing starts. Same decision making affects the decision to supercharge or not.

European drivers spend a lot of time with the steering wheel off centre and for that engine response is paramount and brute power is less important. Our little cars are optimized for these conditions. Over here the challenge is finding a road twisty enough to test the capabilities of our cars. No sane person would buy a BRZ to drag race....

[stonenewt]

Originally Posted by Ubersuber

Next time you feel your BRZ engine drop into that "dip" try thinking of it as transition from one torque curve to the next, an advantage not a deficiency. On those rare occasions when you actually need to drive at that "dip" point just tough it out and try harder next time to drive as the engine and gearbox are intended: low rpm for low speeds and high rpm for peak acceleration. Then the only time you will be in that dip will be in first gear at about 15 mph or so.

Drive to the engine and gearbox, not against it. Satisfaction awaits.

Problem with this is that it doesn't cover back road driving at a medium fast pace naturally. I found that I was constantly driving off the top of the lower torque bump just before straightening the car up properly. I was always wanting another 500-750rpm in that lower zone, I didn't mind if it was at the bottom or top of that rev range really, I just wanted a few more revs. Result was it became hard to make fluid progress without resorting to a >7/10ths gear change pattern but at 3 to 5 10ths pace.

I much prefer the two-zone I got with an Elise. Torque started to fall off at the 4500 rpm range & then kicked in at 6200 rpm. This meant one had the option to be swift but not aggressive on the back roads at 5/10ths.

Eyeballed figures every 250rpm from the last dyno run I did on my Lotus (sorry no scanner or camera to hand).

Could drive quickly but not needing to be on top of the gears at all the time at on the bottom bump then the top area for when you wanted to fly. But needed to be careful over 8250 rpm as the engine didn't really like it up there.

[stonenewt]

Originally Posted by Ubersuber

This aspect of engine performance is completely overlooked in North America but is generally regarded as at least equally important to standstill acceleration in Europe and other places.

Yeah but you try getting those figures out of a car manufacture

[Mikem53]

Originally Posted by Ds3ire

Good day guys and girls,

I was wondering about the torque figures on the Toyota86/ Scion FRS. Can somebody explain to me why they are so low at 205 Nm? I dont have an issue with the power as 147kW is sufficient for daily driving as well as fun on weekends. But the low torque figure indicates to me that this car has extrememly low pulling power.

Any thoughts on why this has been done? Initially thought that it was due to the low body weight and to resist burning out the tyres on take off.

This leads to higher revs required to spin the crankshaft to get the car moving initially. The gear ratio helps to even this out (3.58:1) but its still low torque going to the crankshaft. The other downside of this is that we have to change down a couple times when overtaking as the pulling power is just too low.

The concern is that cars with similar mass vehicles, especially diesels, the torque and pulling power is much higher. A 1.2l polo diesel has 220Nm torque.

So my question is: Why is the torque so low on these cars and what are the effects of it?

TQ numbers and other specs don't tell the whole story. You have to take into
Account the total "package".
Thanks to the high CR, I find the car to be quite responsive and peppy considering its diminutive size and displacement.
Keeping the car in the proper gear at the proper revs provides adequate power for just about any situation. Owned much more powerful sports cars than this one over the years.. But they weren't as much fun as this car..
Especially considering its use in the real world... The more I drive it.. The more I realize just how good they did with this car...

[pushrod]

Originally Posted by Ubersuber

There are two aspects you may be missing that affect real world power output.

Are you sure about that? I made references to valve area, and also said "on paper" in my subsequent posts.

[Ubersuber]

Originally Posted by pushrod

Are you sure about that? I made references to valve area, and also said "on paper" in my subsequent posts.

Not sure what you meant by "on paper". On paper a bigger bore engine will have bigger valves.

Undersquare engines produce peak torque at lower rpm than oversquare engines so I'm not sure your on paper description applies.

As an afterthought: the other important variable with bore/stroke ratios is piston area and the effect of bmep on that area.

Basically, the larger the bore the higher the total force on the piston computed by pressure times area. The shorter stroke means the piston moves under this higher force for a higher percentage of total combustion cycle time, although utilizes expansion force for a smaller percentage f the cycle. Longer stroke engines develop lower piston force but exploit it for longer duration of the combustion cycle time, but of course at lower average pressure. Longer stroke engines utilize the higher pressure portion of the combustion gas expansion for less time as a percentage of the total. Longer stroke gives an engine with "pulling power" for this reason. The short stroke exerts higher pressure but over a shorter lever arm than the long stroke engine.

So, power and fuel efficiency are opposing performance conditions, undersquare engines produce higher torque at lower rpm than oversquare engines partly for this reason. In other words, peak torque in an undersquare engine will generally occur at lower rpm than in an oversquare engine although the peak torque may still be less. Undersquare engines use their fuel more efficiently because they exploit expansion for a longer time segment in the combustion cycle.

No free lunch in other words. Square engines are a good compromise between the two opposing characteristics.

[Ubersuber]

Originally Posted by stonenewt

Problem with this is that it doesn't cover back road driving at a medium fast pace naturally. I found that I was constantly driving off the top of the lower torque bump just before straightening the car up properly. I was always wanting another 500-750rpm in that lower zone, I didn't mind if it was at the bottom or top of that rev range really, I just wanted a few more revs. Result was it became hard to make fluid progress without resorting to a >7/10ths gear change pattern but at 3 to 5 10ths pace.

I much prefer the two-zone I got with an Elise. Torque started to fall off at the 4500 rpm range & then kicked in at 6200 rpm. This meant one had the option to be swift but not aggressive on the back roads at 5/10ths.

Eyeballed figures every 250rpm from the last dyno run I did on my Lotus (sorry no scanner or camera to hand).

Could drive quickly but not needing to be on top of the gears at all the time at on the bottom bump then the top area for when you wanted to fly. But needed to be careful over 8250 rpm as the engine didn't really like it up there.

That Rover K series was a very good engine. Undersquare as it was bored out almost beyond its design capacity so a longer stroke was the only answer. to the more power requests. It seems an even longer stroke is possible.

Undersquare engines tend to develop peak torque lower in the rpm range than oversquare engines. All other things being equal an oversquare engine will have higher peak torque and higher bhp due to better breathing. It is more difficult to make an oversquare engine comply with emissions and fuel economy requirements.

The later Lotus Elise with the Toyota engines went back to a square bore/stroke.