3.9 diff

[TBG]

Better not mention 3:1 diff at this point! Running for cover!! D

[Bigbaldybloke]

I originally did lots of longish motorway or dual carriageway journeys in my Plus 2 with a lightly modified big valve engine. I changed to the 3.55 diff and found the car more pleasant at motorway speeds on 175/70 x 13 tyres. I didn’t notice a real drop off in acceleration in normal road use, might be different if I did track days, but I don’t. I’m fully rebuilding the car now and have no plans to refit the original diff. I have the standard gearbox.

[berni29]

Hi Guys

I was just stripping my +2 (ahead of a chassis change) which I bought as a bit of a project and have yet to drive, and I have discovered that it has a 3.9 diff. This is mated to a type 9 gearbox with repositioned gear lever.

Is this combination going to be OK for road use, or am i going to have to source a higher geared diff? Its an ex-race car.

Anyone have experience of this combo?

Many thanks

Berni

[andyelan]

Hi Berni

Don't know much about the Type 9 gearbox but I assume it's 5 Speed with a direct forth gear same as the Lotus S130/5.

The problem with the 5 speed Plus 2, if you want to call it that, is that even with the std 3.77 diff it's very high geared. 6500 revs/min forth gear will take you to 120 mph and fifth a whopping 150 mph (theoretical of course). Now, although the Big Valve twin cam appears to be very flexible, it is after all, still a small capacity highly tuned unit and with peak torque around 5500 revs/min, so it really need around 30000 revs/min before it gets into it's stride. That means that anything below 70 mph in top gear and the engines torque is really starting to fall away.

So, In my opinion, a 3.9 diff should be fine (in fact I've been thinking a 4.2 might be ideal), the engine will rev a bit more but then that's what it's supposed to.

Regards
Andy