and i was wondering if its bad for the car to rev up the engine a bit before letting the clutch go to get a boost. Also, what exactly does torque do? Ive tried searchin the internet for the answer but all i can come up with are scientific explanations. What does having torque mean performance-wise? Thanks.

 

assuming you have a rotary it will become natural to rev it a bit - 1500 - 2000rpm - to start in first gear. That's the nature of the engine. If it's not a rotary you have to give a bit of gas at the time you release the clutch - but the amount just varies with the car. A Honda Civic or Mazda Protege will require less than a Porsche Boxster. You don't have to search the Internet for torque - just search on this forum. I can't explain torque well myself. But I know this - a truck that needs to pull a lot off the line needs torque. That's why 18-wheelers are almost all if not all diesels - diesel engines can generate lots of pulling power. I had a VW TDI engine - one 'can' shift through all the gears starting from a stop without ever hitting the throttle - the engine has so much torque it can pull the car through all gears and not stall. Most cars can't do that - they'll stall. Hope that helps a little! oh, and this. A buddy in TX got to finally drive an RX-8 a couple weekends ago. When we discussed how he liked it,he went on and on about the torque of the Renesis. He's driven RX-7s and previous Rotary-engined Mazdas for years and currently owns my former 90 Convertible that has been street ported, and had my former 13B from an 84 GSL-SE in his GSL until he messed up the rear diff. At any rate, he was exuberant about how much torque they finally got in the rotary by doing the side porting, etc.

 

The units on torque are force x distance. Like pound-feet or newton-meters.

In short: torque is the amount of rotational force. So torque at the wheels is the amount of force the car can push on the road to get it to go forward. Engine torque is the amount of force the engine exerts on the tranny (in a turning motion though, not in a line).

So torque is the amount of force you have at a certain radius. So for the renesis: it makes a peak of 159 lbs-ft of torque. So if you attached a rod to the crank that was 1 foot away from the center of the crank, you'd feel 159 lbs at the end of the rod.

Torque is multiplied by a gear reduction. So if you're in first gear (3.76:1) with the rear diff at 4.444:1 you're making 159*3.76*4.444 = 2657 lbs-ft at the wheels. The 8 comes with 225/45 R18s which end up having a total diameter of 26" or a radius of 13" or 1.08 ft. So in first gear, the 8 is pushing the car forward with a force of 2657/1.08 = 2542 lbs!

Since force = mass*acceleration (and neglecting areo and other forces) you're accelerating at 8 m/s^2 or about .815g.

So why not just have crazy high gear reduction so you have lots of force and therefore lots of acceleration? Two reasons: one, your tires have limited grip, and more importantly, when you multiply torque you reduce speed. You can have really high ratios but then your top speed is compromised. Hence the need for a transmission with multiple gears.

So when the engine is making its 159 lbs-ft its spinning at 5500RPM. So you divide the engine rpm by the gear ratios to obtain wheel rpm: 5500/3.76/4.444 = 330 rpm. And we know from above that the wheel radius is 13", so the circumference = 2pi*R = 81.6. So for every revolution of the wheel the car moves 81.6 inches = 6.8 ft. Since the wheels are spinning at 330 rpm the car is moving 330*6.8 = 2246 ft per minute or 25.5 mph.

What I'm getting to is that only looking at the torque of an engine can be misleading. When comparing two vehicles the torque at the flywheel doesn't mean anything. You need to look at the power (and therefore the torque at the wheels).

Power is torque x rpm. Since the 8 revs really high, although it has low torque it can still make a lot of power.

So you take an E36 M3 for instance. They make tons of torque, but can't spin very high. So where the 8 lacks in torque it makes up in RPM.

Why is this important? Because you can gear the renesis lower (i.e. a high number) to reduce the speed and multiply the torque of the engine. So what you end up with is the same amount of torque at the wheels. So the M3 makes 240 hp and the 8 makes 238. So they should be equally as fast given the same weight (remember Force = mass * acceleration). The M3 weights 3175 lbs. But I bet the 8 isn't geared as low as it could be because too few people acutally want to listen to the sweet rotary buzz along at highway speeds. So the 8 is probably a tad slower than the E36 M3.

Anyway, This was kinda long and drawn out. But I hope you have a better idea of torque. And power.

I want RX8 owners to be armed for combat against those who rag on the Renesis because it doesn't make a lot of torque. Just calmly explain that torque at the flywheel isn't important. Torque at the wheels actually moves the car. And the 8 makes plenty of that because it's geared low (4.444 diff is pretty low) and revs high.

cheers

 

Great explanation, Berserk. Thanks for laying it out so clear for those of us less mechanically inclined.

 

maybe a stupid question but im not an engine buff. if you add more hp to a rotary engine or any engine to that matter, is it possible to add more torque at the same time or do you have to do other things to an engine to get more torque that you wouldnt do for hp.. can you even add more torque to an engine