Gear ratios, and driving in the real world!

Black Bug

Black Bug

Well-Known Forum User
OK, I understand what ratios do, but I'm wondering how much of a difference they would make in the real world. Say we've got two boxes, ratios of box 1 are 3.5, 2.1, 1.4, 1, 0.75. Box 2 has ratios of 3, 1.85, 1.3, 1, 0.75. We'll stick a 3.9 diff on, 7000 rpm redline and 195/70R14 tyres. Assume an ideal engine with perfectly flat torque curve with say 200bhp at 7k rpm, if you need to worry about an engine.

Playing on the toy at http://geocities.com/z_design_studio/ the max speeds per gear at 7k are, box 1:
38, 63, 94, 132, 176
box 2:
44, 71, 102, 132, 176

Anyway to calculate what the times would be to reach these speeds?

Cheers,
Rob
 
..on the subject of gear boxes......what does this symptom mean?.

Whilst accelerating in say, 2nd (very noticable in 2nd...) if (IF) I take my foot off the clutch and DONT go into 3rd, its as if the brakes come on, the slowing down is so prevalent!....I've smelt the brakes straight after and there's no smell so i dont think its them ....could it be lack of fuel getting to the carbs/...my set up is:

2.8 lump
standard 260 fuel pump
twin SU's with K un N's
electronic ig.

Nice one, Si.
 
If you are looking for which box to use with less than 250hp fwbhp then box 1 is the best if you have more than 400fwbhp then use box 2. the calculations are too much to go into here but with a small power plant you need the lower gears to act as torque multipliers to get you moving and give you the kick off the line. if you have loads of power and torque you dont need the low gears this justs wastes time having to change. Try the program desktop dyno.
 
Ta mate,

I believe I fall very much in to the under 250bhp at this point in time. I've been playing with CarTest2000 software since I originally posted here, I suspect that it's similar to the desktop dyno you mention. Interestingly it seems that although gears do make a difference it much, much less than I thought it would be, 1/10s of a second on a perfect 1/4mile run. However I really need to plug in accurate hp/torque figures from a proper dyno run for the software to work best. Of course that would mean having wheels on the car, not to mention a gearbox ;)

gav sbc said:
If you are looking for which box to use with less than 250hp fwbhp then box 1 is the best if you have more than 400fwbhp then use box 2. the calculations are too much to go into here but with a small power plant you need the lower gears to act as torque multipliers to get you moving and give you the kick off the line. if you have loads of power and torque you dont need the low gears this justs wastes time having to change. Try the program desktop dyno.
Click to expand...

Cheers,
Rob
 
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