What is the best Diff for a Bec kit car ?

Depends on many factors.
1; rear tyre diameter
2; engine
3:what the vehicle is used for
4:rear axle type
5: the list goes on.

As a rough rule - a 3.38-1 would suit most engines for most things on an IRS setup
A 3.54-1 on a live axle
A 3.14-1 would be OK with a hayabusa or similar if road use and big journeys were likely
A 3.21-1 freelander diff is a good unit with an Atb lsd. And light
But if a circuit based car it'll be better to gear for max rpm at the end of the longest straight your likely to do!

Irs
Zx9 engine
Seven style car

Currently running a 3.62


What are the benefits of a 3.38

I had an ir's car on 185/60/14 rears and a 3.62 diff. It was slightly under geared for road use.and was pretty noisy at cruising speeds above 60mph

[Edited on 5/3/16 by daniel mason]

Yeah this does sound noisy but the sound doesn't come from the Diff

It's running on 196-45-15S is there any acceleration or speed benefit ?

Everyone raves about fitting a 3.38 on BECs here instead of 3.62.

From my own experiance.

The car will accelerate a little slower, have a theoretical higher top speed. And at say 70mph will be reving a little less.
My old 919cc fireblade MNR would do about 115mph flat out just tapping onto the limiter but it needed about 2 miles of Swiss Alps straight road to get there. This was with a 3.62

My old 1000cc R1 5jj with 3.62 would hit 120mph on the straight at Spa on the limiter. With a 3.38 fitted it was pretty much identical top speed but obvs not on the limiter. It did take more of the straight to get to 120mph though.
The carbed blades and R1s just don't have enough power to pull through the wind.

The only benefit in a 3.38 I see in a 7 with 7 style body is dropping the revs 500rpm or so whilst cruising along in 6th.

My new car is full body. 2013 fireblade engined and I've still gone for a 3.62. I do have a 3.38 sitting around just in case but I'm not convinced it'll be required

If noise and high revs bothered me I wouldn't have got a car with a bike engine :-)

[Edited on 5/3/16 by gaz_gaz]

Have you had a lookat the speed calculator thread (top sticky in this section) ? That'll give you the relative speeds for each diff.

I'd guess the 3.62:1 is about right for that motor. Prs... Will give good acceleration. Cons... Shorter gears (more changes required). Restricted the top speed a bit (but given the (non) aero properties of a 7 that'll be low anyway unless you have a lot mre power than the zx9). The revs might be a bit high for motorway cruising
A 3.38:1 will improve the cons at the expence of a little accelleration (though fewer gear changes will balance that out a bit.

On paper acceleration might be slower with a 3.14 But in reality my drag times inc 0-60 dropped quite a bit when I ditched the 3.62
Less wheelspin off the line and you can hold the gears longer.
Makes no difference to setting off either, so definitely better.
This is with a ZX9 engine and 13" wheels.
70mph is approx 7000rpm in top gear.

An Elite MX200.

Built in reverse
Can change the ratio in about 10 minutes simply by taking off a cover and changing two gears.

And I happen to have one that I was just about to post for sale

I had a zx9 in my striker I built many years ago had a3.64 diff it used to rev out in top 12 k and that was 120mph genuine speed I was running 185/60/14 tyres so it was 10mph per 1000rpm I think I did a quick check and a 3.38 diff would give you 11mph per 1000 revs in top gear