Merc63

Ok, this isn't for the car, but for the bike.. I require some assistance figuring this out...

Scenario: Im racing a bike at the 1/4mile. Its a 5spd manual.

When I hit the traps at the end, Im running through them just as Im about to red line 4th gear, with a 6250rpm red line and 4th gear maxing out at 115mph.

Say I have 100Hp and I double my HP to say 200. How will this affect my trap speed??? Will I be hitting red line in 4th gear as I come up to the traps like before when I had 100hp?

What Im thinking is my gearing is the same, only my power is more, so I will be at the same spot as before when hitting red line in 4th gear, but getting there quicker, so my MPH will remain the same, but I will be lowering my time?

Merc63

I think my logic is flawed, only because of the horsepower formula that uses trap speed as a factor. IE: an increase in trap speed directly relates to an increase in HP... So if my HP doubles and my trap is the same, the formula wouldn't work.

What Im thinking is for example, say I let the clutch out at 1500rpm in 1st gear and when I hit red line Ive traveled 50ft. Now if I doubled my power, traction is the same and I do the exact same thing, but with double the power, wouldn't I still hit 50ft by redline, but get there quicker?

Merc63

anyone?

propain

My brain hurts reading this as I have been drinking. My feeling is you would reach and top out 4th gear long before the trap due to the extra power. So you would be well into 5th as you finished the 1/4. Sounds to me like if you added 100 more HP you should also change the gear to make better use of the extra power.

In the end what I am saying is your ET will be lower your trap will be higher but you will need 5th gear because your ratios haven't changed.

Merc63

If Im switching gears at the same RPM everytime, wont the same gearing take me the exact same distance, despite whatever horsepower I have?

IE, First gear off the line is going to take me to 100ft at redline, now if I add 1000 hp, Im still going to hit 100ft at redline in first gear, but get there way quicker???

Am i thinking about this wrong?

The engine creates mechanic leverage which is translated through the gearing to the tires so they turn. If I record how long it takes me to travel 100 ft at a certain RPM, and now I double my power, im only changing the amount of time it takes to hit that 100ft by the same RPM... I doesn't change the distance that gear will travel in the same rpm...??? Right?

Confusing!

Mort

I think if you have the same traction then yes you will cover the same distance faster. Gearing is mechanical so that the same number of rotations will always equal the same distance if traction remains consistent.

propain

The more torque and HP the less distance traveled to max out the RPM in each gear.

Lets say you have a first gear ratio of 2.87 and rear end gear ratio of 3.55. If you shift at 7000 RPM in a car with 100 torque you will travel 371 feet in first gear. Take the same ratios and double the torque and you will travel 185 feet.

So much to take into account though. Max RPM, tire diameter and car weight.

No matter what though the answer to your question with all thing remaining constant except the power you will cover less distance and reach the speed quicker with more power.

So the question becomes if you double the HP of the bike do you have enough gear?

Mort

The greater the TQ and HP, the distance will be the same to redline in each gear, but the Time required to reach that distance will be less as the engine will reach redline in less time.

propain

This is not true.

Go out and see for yourself. Put your car in 1st gear and use 1/2 pedal (less power) to redline. Take note of when you reached redline and your position and distance covered.

Now do the same thing from the same start position. This time use as much throttle as possible without spin (More Power) and take note of your position and distance covered when you reached redline.

You will see that not only did you reach redline quicker but you also reached redline way before your 1st position when using half pedal with less power.

Merc63

Lol this is so confusing.

What mort says makes perfect sense to me.

But I think propain is right, but I just don't understand it.

Dads C63

Its based on the speed you reach.
The tires will reach the same distance only quicker which brings up the speed. When the speed increases the gear changes are quicker. The rate of acceleration increases.

1/4 mile with stock C63 = 12.2 @ 114mph (mid way though 4th gear)
Now add LTH and tune for 100hp: 1/4 mile is 11.0 @ 127mph.
(almost to the top 4th gear)
Now add nitrous or SC 200 more hp. 1/4 mile is 10.1 @ 140mph. (out of 4th gear unless you change the gearing or use taller tires.

You are covering the same amount of ground but the speeds have increased to the point you need taller tires or more gear. Its the speed(acceleration) thats changing everything.

Merc63

Ya I think that's what Im missing is the acceleration is also changing and Im not taking that into consideration.

propain

Well said. The more power the more acceleration the more speed the more gear you will need.

So back to the original question, and back to my statement you will most certainly be shifting into 5th gear. You might even run out of gear if you double your HP and you are already topping out 4th.

Merc63

Thanks guys, got it.