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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Transmission and average HP!

If we can increase our average HP per gear we end up with a faster car.

There are several basic ways to increase average HP, make more peak hp, narrow the gear spacing to stay at a higher HP level, or build a motor with a wider power band.

Just changing the first to second gear spacing can add about 30% more HP at the 50mph shift point! Try to get that with any single motor mod.

Assuming a shift point of say 6500rpm and assuming 400chp-- I think we see 'about' the following relationships with a stock transmission:

At 15mph when we are accelerating from a stop we are at 2000rpm and making only about 60hp (no boost).

By 25mph we are pulling 3200rpm and on a good quick spool motor making about 180hp (full boost?)


With 1st to 2nd shift, we drop from 6500 to 3800rpm at 50 mph and from about 400hp down to about 260hp. Thus a total of 2700rpm drop and a 55% reduction in HP with the shift. Again, our HP runs from 260 to 400hp in second.

2nd to 3rd: drops to 4600rpm at 90 mph and 330hp with a 1900rpm drop. Thus our HP runs from 330-400hp. Not bad.

3rd to 4th: drops to 4700rpm at 130 mph and to 340hp with a total 1800rpm drop.


The biggest challenge seems to be the big rpm drop between first and second gear. Some 75hp or 30% could be gained if we had the same rpm drop going to 2nd as to 3rd and 4th. This should make a big difference in acceleration just past 50mph.(Freeway entrance speed.)

I have heard is said often that gears can "transform a car"! This seems to help me understand why this is.

Just something to think about.

Last edited by 911st; 02-24-2010 at 08:52 AM..
Old 02-19-2010, 10:00 AM
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