Pick your brains !!

[Mossy68]

Just want to run through a few things with you clever lot if I may.

I may have a diff coming cheap with 3.90 gearing. Not exactly what I'm after but beggars can't be choosers. Should give me the opportunity to rebuild diff, then swap , and if I'm not happy I can swap back.

I have roughly calculated that at the moment with a tyre diameter of 26.4" and my current 2.76 gearing I should be at 2124 rpm at 60mph ?

By changing just the gearing I should be at 3024 rpm at 60 mph. Sound about right ?

Still on a stock convertor , but am I correct in thinking for race purposes one would aim to get a convertor near to the rpm at 60 mph for a good " launch" set up ?

Also. Engine is pretty much stock apart from carb, intake and exhaust etc. stock convertor.
How will it behave by adding the 3.90 gearing ??

I understand it will be a good test of bottom end strength !!!

Cheers.

[Pete]

I think you will be pulling 3200 rpm at 60 mph with 3.91:1 gears, allowing for transmission losses....

[Pete]

Convertor choice is determined by the gears, cam and the amount of torque the motor makes - it will stall later in a small block than in a big block........

[Mossy68]

Cheers Pete. Still much to learn

Worth a try then ?

[Pete]

Of course its worth a try - it''s not my car

It will liven it up a bit, which is nice...

[Mossy68]

Thank you Pete. Any advice appreciated.

[John]

Give this a go Gary

http://vexer.com/automotive-tools/speed-rpm-calculator

[Mossy68]

Give this a go Gary

http://vexer.com/automotive-tools/speed-rpm-calculator

Cheers John. Looks like a good site for info.

[MilesnMiles]

It'll be screaming its nuts off! I had those gears in my Dart for a bit. Maybe get taller tyres to relax the gearing a bit. I settled on 3.55s but like you my 3.91 set came very cheap.

[Mossy68]

It'll be screaming its nuts off! I had those gears in my Dart for a bit. Maybe get taller tyres to relax the gearing a bit. I settled on 3.55s but like you my 3.91 set came very cheap.

Thanks Miles. I've been looking at tyre size and how it effects the gearing.
3.55's are what I want , but I will try the 3.90's. Worth a laugh !!

[Pete]

I run tall tyres on the Bee - with 4.10:1 gears it works out at 1000 rpm per 20 mph... you can live with that...........

[Mossy68]

I run tall tyres on the Bee - with 4.10:1 gears it works out at 1000 rpm per 20 mph... you can live with that...........

Mmmm. Got a mate with a tyre shop at the top of my road. I'm sure experimenting will be fun !

[Dave-R]

This is the calculator I made myself. It's the only one I would use.

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

[Mossy68]

Thanks Dave.

[Les Szabo]

Just want to run through a few things with you clever lot if I may.

I may have a diff coming cheap with 3.90 gearing. Not exactly what I'm after but beggars can't be choosers. Should give me the opportunity to rebuild diff, then swap , and if I'm not happy I can swap back.

I have roughly calculated that at the moment with a tyre diameter of 26.4" and my current 2.76 gearing I should be at 2124 rpm at 60mph ?

By changing just the gearing I should be at 3024 rpm at 60 mph. Sound about right ?

Still on a stock convertor , but am I correct in thinking for race purposes one would aim to get a convertor near to the rpm at 60 mph for a good " launch" set up ?

Also. Engine is pretty much stock apart from carb, intake and exhaust etc. stock convertor.
How will it behave by adding the 3.90 gearing ??

I understand it will be a good test of bottom end strength !!!

Cheers.

Generally yes, but not in your case. If your powerplant made good hp from say 2800>6000, you would put a 3000>3500 verter in it. Your engine makes power from around 1500>4500, so leave the stock verter and with 3.91's/27" tyres you'll go through the top end in 3rd at around 4400 which is bang on max power and ideal to give you your best shot at a good ET.

For street, it'll be harder on your box, your wallet and your motor, but the feel factor may just be worth it to you?

Les