Final drive

Anonymous · Post by **Anonymous** » Sun May 14, 06 11:24 am

Where can I get a lower final drive from??

I know its the opposite approach to setting up for racing but I'd like to possibly drive to the South of France and initial runs to work on the m1 reveal an acceptable cruising speed/noise level of about 55-65...(as well as an ability to hit over a ton without much effort

)

But seriously....if it is possible raise that to 70-75, I'd be good to go I reckon.

I have no idea what rpm I'm turning over as my tach doesnt work but I'd like to achieve 2500 at cruise....

Dave-R · Post by **Dave-R** » Sun May 14, 06 12:11 pm

Yet again. As in other posts on here. If you have Microsoft Excel you can dowload and use this table which will show you what speed you will be doing in each gear and at what rpm for any given axle gear or tyre size.

http://challenger.mpoli.fi/web/AUTOMATICchart.xls

The most common rear gear is the 3.23:1 ratio but you can buy higher and lower ratio gears.
It is not a straight forward or cheap task fitting these gears.

Dave-R · Post by **Dave-R** » Sun May 14, 06 12:13 pm

Forgot to say you probably want something like a 2.76:1 rear gear.

Anonymous · Post by **Anonymous** » Sun May 14, 06 12:24 pm

So top whack is about 140....impressive.

Great chart Dave....so whats intailed in swapping the fd??

Can I view the relative rpm/mph difference with a 2.76:1 using the chart?

Cheers

Anonymous · Post by **Anonymous** » Sun May 14, 06 1:33 pm

Remember that gears act as torque multipliers. Reducing the final ratio will result in requiring more torque to reach the higher speeds. Also a factor is the aerodynamic drag force which increases greatly at the higher speeds, and less so the rolling resistance of the tyres (but it still adds in).

Basically it probably won't do 140. That is an idealised theoretical speed, but still an excellent guide tool.

Dave-R · Post by **Dave-R** » Sun May 14, 06 6:36 pm

steve of doom wrote: Great chart Dave....so whats intailed in swapping the fd??

If you buy a complete diff unit that takes the same size driveshaft UJ it is just a case of dropping the driveshaft, pulling the axles out, and swapping the diff.

If you just buy a ring and pinion it is a lot of work and very complicated for someone that has not done it before. Requiring some accurate measurements and some tools you are unlikely to have. I have done a few now and am only just starting to feel confident about it as each time I have done it better and better. That last one I did for Trev Young was spot on.

Can I view the relative rpm/mph difference with a 2.76:1 using the chart?

Open two versions and switch between the two.

As Bratfink was saying. Higher grears will increase top speed up to a point. They will also drop your rpm for each gear which is what you want.

However the higher the gear the slower the car will accelerate.

Also remember that for best fuel economy you want to cruise at the same rpm as peak torque for the engine. You will burn more fuel either side of peak torque. The rpm you make peak torque depends on the camshaft duration.

Anonymous · Post by **Anonymous** » Sun May 14, 06 10:31 pm

I'm not really interested in top speed but at the same time I was still curious.

I realise that it would more than likely reduce the top end, and that it would require more toque to drive it(the things already got 450 odd foot pounds...how much do you want

)

but I was impressed by the speed @ 6500rpm with my existing 3.23 ratio and judging by the way the motor revvs at high speed I'd say actually, it probably would (although I wouldnt like to be in it at the time I tell thee'

) get up to 6500 in D

Yeah Dave, ...I'll look out for a complete diff I think.

Anonymous · Post by **Anonymous** » Sun May 14, 06 10:54 pm

Oh now I see...you can input your own spec and the chart displays the speeds in gear. Cool.

I have done a bit of research and...If I have the 509 cam the motor said to have then operating rpm range is between 2600 rpm and 6000 rpm (looks like I'm not going to be revving to 6500 and making my land speed record attempts after all Bratfink

)

So then with 2.76 fd, (with 14" rims)according to the chart, 66-79 mph is achieved at 2500 and 3000 rpm respectively. Thats about what I'd like for me' French riviera trips

...I'm just off to the 'wanted' board.

Cheers Dave, Bratfink

Dave-R · Post by **Dave-R** » Mon May 15, 06 8:51 am

steve of doom wrote:the 509 cam the motor said to have then operating rpm range is between 2600 rpm and 6000 rpm (looks like I'm not going to be revving to 6500 and making my land speed record attempts after all Bratfink )

That rpm range is the "sweet spot" and does not indicate the full rpm range of the engine.

Anonymous · Post by **Anonymous** » Mon May 15, 06 4:59 pm

Okay, cool but a good range to be in for cruising no?

Blue · Post by **Blue** » Mon May 15, 06 5:25 pm

A pair of 15" rims with some taller tyres might be a cheaper option...

Anonymous · Post by **Anonymous** » Mon May 15, 06 11:06 pm

Yep, good idea Blue. Wont that put the speedo out though?

Dave-R · Post by **Dave-R** » Tue May 16, 06 9:22 am

steve of doom wrote:Yep, good idea Blue. Wont that put the speedo out though?

So will changing the rear gears. You just fit a speedo drive gear to suit. The spreadsheet tells you how many teeth it needs. They cost about £10 to buy I think?

Try inputing different tyre sizes on the spreadsheet and see what you get.

Anonymous · Post by **Anonymous** » Tue May 16, 06 5:00 pm

Excellent Dave cheers