14 replies to this topic

[oliver122]

hi guys , im looking at changing my final drive form a 3.4 down to a 3.2/3.1 but was wondering what people opinion's are , it will be on a std box but the engine has got a kent 276 cam and a ported, polished, big vave head . im going to fit a lightend flywheel aswell so i thought i might aswell pull the engine and change my final drive while im there . any help would be great tar 

[Dusky]

You ll run off cam. Why fit a high rev cam and then want comfort if a 3.1?

[nicklouse]

as above. if you want to change the diff to a 3.1 i would also change the cam to a more suitable one.

 

or stick with the 3.44 and the ´276.

[Cooperman]

as above. if you want to change the diff to a 3.1 i would also change the cam to a more suitable one.
 
or stick with the 3.44 and the ´276.

Excellent advice. With a 276 you will be getting maximum power at around 6100 rpm and at those revs a higher ratio FDR will not be as good.
With a 3.44:1 at, say, a 70 mph cruising speed you'll be using about 4200 rpm which is right in the best torque band for a 276.

[oliver122]

well i built the engine and was happy with the spec i choose , however on the road i dont like the shortness of first gear ( pretty much none exsistant ) also i only get about 20mpg ( which i dont think is right ) i have had it setup on a rolling road at mikeanics in congleton. so was thinking that with a lower final drive i would increase the cruising speed not reving it as hard 

[nicklouse]

 also i only get about 20mpg ( which i dont think is right )

so what is the full spec?

 

my 1293 is in the teens for MPG but i did not build it for MPG

[Carlos W]

If you're always dropping into 3rd as the engine is off cam your fuel economy wont improve by changing the final drive

[oliver122]

• this is the spec mate
• 276 kent cam
• duplex timing gears
• Stage 3 smart performance head ( polished and ported , 35.6mm x 29.4mm fast road valves , 175lb double valve springs, 3 angle seats )
• stage 2 maniflow lcb with rcm exhaust system ans single box centre

std flywheel

std gearbox

3.4 final drive

[Cooperman]

That FDR would seem ideal for the rest of the engine spec.

An A-series at that level of tune needs to rev as that is what the 276 cam needs to do.

With a high FDR like 3.1:1 you will be over-cammed with a 276.

[oliver122]

ahhh cheers cooperman 

[carbon]

Oliver,

 

If you're using sintered A+ rockers with the 276 cam you might be able to use 3.1 diff. Does the car run smoothly in top gear at 30mph, or does it prefer third?

 

The first gear in standard helical A+ depends on which model it came from. Lowest first gear is 4:1 from Metro HLE cluster. For a higher first gear but keeping 3.44 diff your other option is to see if you can fit a closer ratio helical set, these can give first gear 3.35:1 and were used in rod box for 1275GT.

 

20mpg is poor fuel consumption. What carb set-up are you using?

[KernowCooper]

I was just about to ask the same carb question ? mines a 1330 BV head, 10.2cr, 276 cam, aldon yellow with vac advance, maniflow LCB and single side exit silencer on a HIF 44, and I can get low 40s on a run and 25-30 on A roads wound up, are you sure its set up correct on the mixture? you make no mention of what distributor your using? if a standard one the advance curve wont get the best out of the spec.

Edited by KernowCooper, 09 November 2015 - 06:15 PM.

[Cooperman]

One might assume that if a 276 cam and a BV head is part of the chosen engine spec, then the intention is to use the cam for best performance, which, in turn, means fitting a FDR which will allow it to pull to the revs for max power in top gear. A 3.44:1 FDR will give c.16.5 mph/1000 rpm in top gear which is a speed of 100 mph at peak power. If you have about 95 bhp, then that is about right. With a higher final drive ratio, like a 3.1:1, the max speed will be lower as it won't pull through the 'drag curve'.

The 276 is clearly not an 'economy road cam'. For that you need am MG Metro cam (or the similar Kent 266). You fit a 276 for best 'go'. So fit a 3.44:1 or even, my choice for this, a 3.76:1, but with the original Cooper 'S'/Clubman 1275GT gear ratios. Now that would be great for acceleration and a cruise of 70 mph @ 4600 rpm. Remember a classic Mini is at its best on twisty roads where good acceleration between bends is what is needed for maximum fun.

With a 3.1:1 you will hardly ever be truly 'on the cam' when normal driving, which would be a shame.

[nicklouse]

I have been reading through old posts.

Mmmm using a metro turbo "plenum"

Mega jolt

So I am guessing a poor flowing SU carb based set up.

And the possibility of a interesting ignition map.

But again there may have been some sense seen on the inlet set up and hopefully someone who knows what ignition curve is needed.

[oliver122]

I have been reading through old posts.

Mmmm using a metro turbo "plenum"

Mega jolt

So I am guessing a poor flowing SU carb based set up.

And the possibility of a interesting ignition map.

But again there may have been some sense seen on the inlet set up and hopefully someone who knows what ignition curve is need

, ive had lots of free time at work and i come up with all sort of thoughts , i was going to use a plenum so i could remote the air filter due to the k and n with the stub stack i current run being noisy , i do run megajolt which was "set-up" at mikeanics and never touched since but to be honest i was always abit doubtful of his work