Will changing the exhaust, carburator or any other non-transmission part change the speed vs. rpm relation?

Question

Is it possible that changing an exhaust or chip or carburator or injection or any other non-transmission related part will allow a car or motorbike to travel at a given speed but at a lower rpm?

I believe that you can change EVERYTHING on a car or motorbike but if the transmission is kept untouched (including wheels) the speed vs. rpm relation is going to be the same... youre still travelling at 100 mph at 7000 rpm, you will never be doing 100 mph at 6500 rpm by changing non-transmission parts ... you might get there faster, you might maintain it with less throttle or greater ease but the speed vs. rpm relation is the same... am I wrong?

Answer 1

The speed is determined by the gear ratio's. So those things that you mentioned can not change the RPMs for a given speed.

However they make it possible to use a higher gear sooner. Which would change the RPM's.

Do you consider the sprocket part of the transmission, I would.

Answer 2

Well, not exactly.  You're right, on a general scale, but there are a few very specific changes that could have an effect similar to what you're asking about.  You left out the differential, which is a non-transmission part but IS a gear reduction, and changing that gear ratio will change your speed at any given RPM.

If you wanted to get very technical, it's also possible to use the computer to force or eliminate the use of a lockup torque converter on an automatic transmission.  Locking a lockup torque converter allows a direct connection through the transmission.  Unlocking it uses what's termed a "fluid coupling".  To incredibly oversimplify this, it's a lot like pointing two fans at eachother, with only one doing the spinning.  The moving air will cause the other fan to start spinning also.  This is what a torque converter does, but uses transmission fluid instead of air.  Back to the point...  leaving this unlocked (in "fluid coupling" mode) will result in a higher RPM for a given speed (usually around 200rpm for most stock applications, but can vary greatly).  Locking it will force a direct mechanical connection that will maintain a steady and predictable lower RPM to Speed relation, in comparison.

Normally, the vehicles computer will control when to lock or unlock the torque converter, but this can be controlled through tuning, and therefore COULD change your RPM to Speed relation via a chip upgrade.  It's unlikely (and impossible on anything but automatic transmission cars with locking torque converters).

If you've driven both an automatic and a manual, you can notice an obvious difference between the way they drive.  In a manual, there is a direct relation between RPM and Speed.  The higher your engine is turning, the faster you go, and they both increase steadily.  In an automatic, when you take off from a stop, you'll usually notice your RPMs increase (typically to around 2000rpm or a little higher), and the engine will stay at 2000rpm for a while until your vehicle speed catches up.  That RPM point is a critical number for that particular converter.  High performance converters go to a much higher RPM while taking off.  Once the vehicle gets close to matching it's speed with the RPM, the RPM will finally start to increase, but always stay higher than the speed (again, 200rpm higher is typical) until the converter is told to lock up.

For what it's worth, my Saleen now has an electronically controlled automatic transmission that has been converted to run in a full manual setup (it never shifts automatically).  Since the computer no longer controls the electronic lockup function of the torque converter, it's now controlled by a switch that I activate when I'll be at a steady speed long enough to justify locking the converter (which is when your computer would force the converter to lock on it's own).

Aside from these two possibilities, nothing that I'm aware of could change the RPM/Speed relation.  Sorry for the extra-long response, but I didn't want to leave you confused!  Hope I didn't make it worse. ;)

Answer 3

em d..... Nothing you do to a bike except changing wheel diameter or gear ratio will change rpm to actual speed.. You can change the motor all you want. It's not going to effect it.... Hope that helped..God bless you !

Answer 4

Agreed. The relationship between rpm and mph would remain the same. When you said (including wheels) I presume you included the size of the tyres. Clearly increasing the effective wheel diametre by fitting bigger tyres while maintaining the same rpm would increase the vehicle speed.