Ideal Final Drive ratio for Harrop/low-end torque

[crzystve42]

Had a thought cross my mind while researching the Harrop supercharger for some more low-end grunt

My original plan was to go with a stock FD (3.81 with the 6spd) and a Harrop, but I'm not too keen on the 3k rpm while cruising on the highway.

So my thought is, would having a Harrop (and a theoretical stroker) with a lower (3.15 or so) FD differential make the car behave more like a Coyote 5.0 or LSX with low-end torque and lower cruising rpm?

Current plans due to funds is to swap out for a 4.10 to create the illusion of low-end torque for daily driving, but it would raise my cruising rpms for highway roadtrips.

Not sure if anyone has done the math on this (or could lead me in the right direction to calculating it myself).

[wyatth]

Not sure anybody has or wants to swap to a longer FD, if that’s the Q...
What math is there to do? The gearing and SC torque are not comparable, though they can be complimentary.
3K cruising with ~8500 redline isn’t bad at all IMO.

[//steve\\]

I don't understand the concern for cruising at a higher RPM? Stock gears are already atrociously long IMO. The last thing I'd want to do is make them longer. They feel long even with the increased output the Harrop provides....My car has one (at least for a little longer) and I'd still happily shorten the gearing.

My geared S2000 did 4500rpm at 75mph on the highway and even stock gearing was 4000rpm.....this doesn't bother me clearly

[pbonsalb]

How long have you had the car? With all the bolt on mods it does pretty well with the existing torque multiplication. I can light up my 295/35/18 rear tires off the line, and spin them on the 1-2 shift. I have probably added 15% power.

I might try a 285/30/18 rear tire to see I would like a 4.10. 285/30/18 essentially turns a 3.85 into a 4.10.

I have put 70k miles on mine over 9 years so I have a good feel for it.

I do understand your logic, though. I also have an E36M3. I supercharged it in 2006 and always wanted a little more power. I went from a 3.23 to 3.38 to 3.64 diff. With the 5 speed it was not much fun at the highway speeds I like, around 80, so I wanted to swap in a 6 speed. I also still wanted more power so I changed to a turbo and made so much torque that I dropped down to a 3.15 and would be happy with a 2.93. The 5 speed was very relaxed with the 3.15, but the 500-600 lbs rwtq kept breaking them so last year I swapped in a GS6-53 from an 07-10 335i, which is the same 6 speed internally as in my E90 M3 (and also the F10 M5, so it can handle big torque). Now highway cruising is really relaxed.

Honestly, though, the magic of the E90 M3 is the high revving V8. By going to a positive displacement supercharger you are altering that magic. If you want to be true, consider the much more expensive stroker option.

Also consider buying a different car, especially if you don’t really care about the character of the S65 engine. I personally don’t — it’s been a good car but I’d be happier with an F80 M3. My next car will be a twin turbo 6 or 8 with AWD and auto/DCT/pdk.

[Helmsman]

Originally Posted by pbonsalb

Also consider buying a different car, especially if you don’t really care about the character of the S65 engine. I personally don’t — it’s been a good car but I’d be happier with an F80 M3. My next car will be a twin turbo 6 or 8 with AWD and auto/DCT/pdk.

Ha! Long time owner and solid contributor, still with a pragmatic and honest view, love it. Sorry to say mate but I don't look forward to the day you manage to change that old M3 though...

[SYT_Shadow]

Originally Posted by pbonsalb

Also consider buying a different car, especially if you don’t really care about the character of the S65 engine. I personally don’t — it’s been a good car but I’d be happier with an F80 M3. My next car will be a twin turbo 6 or 8 with AWD and auto/DCT/pdk.

I was going to say the same thing.

[Richbot]

http://www.bgsoflex.com/auto.html - for a decent RPM/MPH/gearing/tire calculator, and some other fun ones

Stock FD is 3.85

If you want to experience what it will be like with a 4.10 gearing-wise, put a 285/30-18 on the rear

You don't want a shorter rearend if you're going to supercharge IMO, and the 3.15 is too long, especially if you start using a 27" rear tire - you might find a decent compromise from the 3.46-ish R&P?