why is it that an m6 can take a much bigger cam without idle problems than an a4 can? also what are the limits of both as far as streetable idle? by streetable i mean lopey but not shaking the car apart at stoplights, sub 1000rpm idle also.

 

I am not sure I am right, but I would imagine the stalling (and mine does at start up from time to time) is due to the load the engine has on it...hooked to the tranny and such. With an M6 and it in nuetral, there is no load. Thats what i think, but am not sure....would love to know also.

 

ttt.

 

The factory torque converters do not have a very high stall speed and are matched pretty mucht to the design engine torque. When you put in a radical cam, you move the HP range up much higher and the torque at the low end, especially idle will drop. This drop will either cause stalling or force you to raise the idle to a speed that there is sufficient torque to keep the engine idling. The M6 as was stated allows the engine to idle in free wheel mode, with basically no load on it. However, when driving a radical cammed car in a stick shift, you will notice that the off the line torque and get away will drop, but you can punch it to a higher RPM to get the torque up to compensate. Changing the rear gears to a numerically higher set allows the load that the engine sees when starting from a stop to be less to compensate for the drop in torque. But all is not lost with an A4, you can do the same thing by going to a higher stall torque converter. This allows the engine to go to a higher RPM which gives it more torque to get off the line, and at the same time lowers the load that the engine sees at while stopped. In fact, if you go high enough with the stall speed, you will notice basically no drop or slowing down of RPM when the car is shifted from neutral to drive. But, one must be careful as high stall converters produce more heat, and when putting one in that has a stall speed of around 3,000 RPM or higher, a trans cooler must be used or the trans will see higher than designed temperatures and will most likely self destruct early. Also, going to a very high stall speed will affect gas mileage to a certain degree, as you will have more slip the further below the stall speed that you are. Once you are cruising on the highway it will sort of average out, especially if the converter is locked up. But you will notice the Tach will rise and fall a lot more with the high stall than with the stock converter due to the slip. In the past, the only way to run a wild cam in an automatic was to use a high stall converter. Otherwise you had to idle at almost 1,500 to 2,000 RPM to get the car moving, and in the winter it was a nightmare. Rule of thumb in the old days when Turbo 400's were the hot number and third gear was 1:1, was to choose a converter with a stall speed slightly below or equal to the RPM that you would get at your most common cruising speed on the highway. Changing the rear gears say from 3:55 to 4:10 would raise the RPM you would have on the highway and allow the use of a higher stall speed without encountering excessive slip.
Hope this helps....