Auto 5 Speed - more torque needed?
 

I've been reading about how the 5 speed auto version should be out in the 2006 RX8. Some people have talked about how the current 4 speed auto transmission can't handle the revs from the 238bhp engine type and was paired with the 197bhp engine type.

Thing is, the lower engine type has more torque than the higher engine type (164lb-ft vs 159lb-ft). So if the 5 speed auto transmission is paired up with the higher engine type, would the performance suffer from the lower torque?

Does the extra torque in the current 4 speed auto really make much of a difference anyway?

Anyone have any opinions?

Dude, it's 5lb-ft. I don't think it makes any difference at all.

The torque curves are nearly identical - as stangmatt66 says, it's only 5 lb-ft difference. The lower mechanical efficiency of a torque converter vs. a direct connection clutch will soak up that 5 lb-ft anyway, so at the driveshaft the current 4 spd AT cars will never make more torque than the 6 spd manual cars.

BUT - torque at the rear wheels is a function of gearing. 5 speeds on the future 5AT trans will make it much easier for the 5AT car to keep the engine in the powerband, and it will have the higher redline and greater power of the high-power engine anyway. In short, the 5AT/high-power combination will always have more torque at the rear wheels than the current 4AT/lo-power combination.

Regards,
Gordon

stangmatt66: That's what I was thinking too, but it just got me thinking since a car review on Top Gear (UK) made a comment about how the extra torque on the auto would be handy around town.

Gordon: thanks for that explanation, that pretty much answers my question!

I have to disagree with you on this one Gord :). No torque is ever "soaked up" in the torque converter, rather it is power. In fact, torque multiplication occurs across the converter when there is significant slip between turbine and impeller. This is due to the redirection of fluid flow by the stator. So, in fact, there is significantly more torque at the driveshaft in the A/T vehicles under launch conditions than in M/T vehicles. The lower mechanical efficiency of the torque converter is related to energy/power loss. Basically, this means that even though torque multiplication occurs, turbine speed is much lower than engine/impeller speed and thus the power at the driveshaft is lower.

So to answer the original question, the lower torque of the 238 hp engine won't be very noticeable because of the torque multiplication that takes place across the torque converter, and also, as Gord mentioned, the 5-spd transmission will allow the engine to more easily operate within its powerband. You might be wondering why A/T vehicles aren't faster than M/T vehicles if they produce more torque at the driveshaft. The main reason is simply that this extra torque comes at the expense of driveshaft rotational speed, and thus horsepower.