What rear differential would be recommend and what ratio to get my RPMS down from 4K at 75mph to 2.5-3k?

Thanks,
Rob

 

You would need somewhere in the neighborhood of a 2.50 ratio to get that kind of RPM at 75 MPH with an AP2 gearbox.

Just so you know, you wouldn't hit VTEC in first gear until over 40 MPH in first gear, and wouldn't hit redline in first gear until over 55 MPH. That would make for a VERY slow car around town. On the freeway, it would probably be roughly as fast as any other car since you could downshift to second gear or so to get into the power band, but getting the car rolling from a start would be tough. Basically it would be like starting your stock car from second gear all the time.

 

Better off changing your sixth gear to something other than stock to attain that lower RPM.

 

what exactly is your reasoning for wanting to do this? Do you think it's going to be more fuel efficient? It won't be.

If you haven't noticed you practically get 30mpg doing 75mph at 4k rpm. Do you think it's going to be better if you reduce the RPMs? 4k is pretty much a torque peak in the s2k and it's actually the most efficient to run it at that RPM (the least amount of effort for the engine, which means the least amount of fuel required). Trying to get your car to cruise at 75mph and 2.5krpm is just going to cause a lot of bogging and you'll probably have to increase throttle angle and ultimately would suffer from worse fuel economy....

 

And if you are just wanting lower RPMs because you don't like cruising at 4k rpm...well, you bought the wrong car. A Vette might suit you better.

 

lower rpm=higher throttle opening

 

I was thinking it would give it more top end. Just thinking

 

You *will* get better fuel economy at lower revs at the same speed.

Torque peak rpm has nothing to do with operating efficiency at very low output. What rpm produces the maximum torque at WOT has little/nothing to do with what rpm will be most efficient at very small throttle openings (cruise). Cruising at 6500 rpm (peak torque rpm for an AP1) will yield significantly worse fuel economy than cruising at ~4000rpm in a taller gear at the same speed.

Also, while lower rpm *does* mean a larger throttle opening is required, it will still be more fuel efficient. Less pumping losses, less internal frictional losses, + same external load (rolling resistance and aero drag) => greater efficiency, higher mpg.

All that said, the mpg gain won't be enough to pay for the mods over any reasonable length of time. And overall performance will suffer.

 

??????

 

I agree a taller sixth gear would be welcome.. You never hit 6th on the track, unless you modified your final drive, so having an overdrive 6 would not hurt track performance.

The car is most efficient when it comes to generating power near the torque peak, but that doesn't mean it gets the best MPG.

The short six is there so that people dont have to downshift when changing lanes, something that i dont care about...

Since the first 5 gears operate normally you would still get your deserved acceleration around town without having to mash the throttle.

I bet that some other milder honda has exactly the same mounting scheme as the s2000 and the sixth could be supplanted...

Beating on the car on the track is excusable but there is no need to beat on it on the highway when your sipping a redbull commuting 5 hours to school or to a race track.