Explanation of rear axle gear ratios?

Anonymous · Post by **Anonymous** » Mon Jul 19, 04 1:11 pm

Hey there... Sorry for my extended absence..

Could anyone explain to me the difference/benefits of the different gear ratios for an 8 3/4 489 rear axle..

I'm trying to understand which is better for acceleration/top speed, I'm looking to eventually to get a gear set that will suit Street/Strip.

Cheers guys..

Kev · Post by **Kev** » Mon Jul 19, 04 1:34 pm

Cruising gears are lower numerically, ie; 2.76:1
Acceleration gears are higher numerically, ie; 4.10:1
The "1" refers to one revolution of the wheel and the first number (eg; 4.10) refers to the number of revolutions of the propshaft. Therefore the higher numeric gears will give you stonking acceleration but not much topend and lower numeric will give you a higher speed for the same revs but at the expense of acceleration. To work out what gear you need, you have to take into consideration max revs, convertor slippage and tyre height. DaveR has a fantastic spreadsheet for ya to pulg yer numbers into. I'm sure he will post a linbk here.

Dave-R · Post by **Dave-R** » Mon Jul 19, 04 1:35 pm

The lower the gear (higher number) the more torque you have at the rear wheels and the faster you will accelerate.

The higher the gear (lower number) the faster your top speed will be but the less torque you will have at the rear wheels so acceleration is slower.

Drag racers need to pick a rear gear that will get them accross the finish line at the top of their power curve in top gear.

For economy you want to cruise at whatever rpm your peak torque is at.

The best gear for your car depends on the cam in the engine and the size of your rear tyres.

Dave-R · Post by **Dave-R** » Mon Jul 19, 04 1:38 pm

Ha! You beat me by 1 min Kev!

What gearbox are you using Gary? I will give you a link to help you out as Kev suggested.

Kev · Post by **Kev** » Mon Jul 19, 04 1:41 pm

http://www.davediana.free-online.co.uk/start.htm
Scroll down to Math Spreadsheet.

Dave-R · Post by **Dave-R** » Mon Jul 19, 04 2:35 pm

The link from there is an out of date version. Use this one instead.

http://www.challenger440.pwp.blueyonder.co.uk/maths.xls

Dave-R · Post by **Dave-R** » Mon Jul 19, 04 2:37 pm

Or you can just use this if you have an automatic;
http://www.challenger440.pwp.blueyonder ... Cchart.xls

Or this if you have a manual trans;
http://www.challenger440.pwp.blueyonder ... Dchart.xls

Anonymous · Post by **Anonymous** » Mon Jul 19, 04 2:52 pm

I've got a Torqueflite 727 which I hope to get re-built by Blue at some point in the future (looking distant right now)...

The cam that's going in the 360 is a Class II Street Performance, I can give you all the numbers if that helps..

As for tyres, trying to work out how to squeeze my set of 275/60/15s under the back arches..

Dave-R · Post by **Dave-R** » Mon Jul 19, 04 3:19 pm

You need to play around with that AUTOMATICchart.xls link above then.

Give me the cam spec. I will be able to see what sort of rpm you are making power at.

What rpm stall speed converter does the cam manufacturer say you should use?

Anonymous · Post by **Anonymous** » Mon Jul 19, 04 3:49 pm

Engine is going to get a .30 overbore if that makes a difference, I have a complete .30 re-build kit...

http://cgi.ebay.com/ebaymotors/ws/eBayI ... 2462301428

I went and upgraded to the performance cam, plus some flat top high compression pistons.

Clevite Performance - Part Number 229-2023 S2053

BTC ABC BBC ATC
-1 35 48 -4

Int Exh

Duration @ .50" 214 224

Advertised Duration 288 298

Lobe Spread = 112

Lash = Hydraulic

Valve Lift .444 .466

Cam Lift .296 .311

No idea what rpm stall speed converter is supposed to be, let alone what it's supposed to do..!!!!

Guy · Post by **Guy** » Mon Jul 19, 04 4:59 pm

I have the perfect convertor for you for 2400 stall and its for sale if you need it bargin price for the nats only.

Dave-R · Post by **Dave-R** » Mon Jul 19, 04 5:33 pm

Guy is right that his convertor will make it launch great. But you will also get away with a stock converter if you want better door to door economy instead of a better 0-60.

This is a pretty mild cam you have although still better than stock. You are going to have about 330hp at around 4,500 to 5,000 rpm with stock heads and a good carb.
The torque curve is going to be pretty flat so you will get best ecomomy anywhere between about 2,500rpm and 4,000rpm.

So your rear gear choices should be a 3.23:1 or a 3.55:1 depending on which you want more - economy or peformance. If your car is heavy lean towards the 3.55:1.

Dave-R · Post by **Dave-R** » Mon Jul 19, 04 9:59 pm

Put the numbers into Desktop Dyno for you.

This is with stock heads (1.88 intake valve size) a dual plane intake manifold, 10:1 compression ratio, 750cfm 4bbl carb and headers with full exhaust system.

Hope this helps.

Anonymous · Post by **Anonymous** » Tue Jul 20, 04 12:47 pm

Dave,

That all looks great, thanks for doing that (now I've just got to try and understand it..!!!).

You say the cam is a mild one, could I get a better one then ? I'm not too worried about economy as the car will not be driven that often. The more horses I can squeeze out of the 360 the better.... one day in lottery winland I'm hoping to stick a Procharger and maybe some laughing gas on it too...

I'm looking to make it a bit of an animal, so any suggestions now before I get near to the engine rebuild stage would be great.

I'm most likely to get Alex to do the re-build as we have spoken and tentatively agreed a rate

The car is going to be lightened as much as I can, I'm playing around with bit's I can lose (maybe not as severe as Blue though).

I want a car that can cane all the Boy Racer's round my way (just to satisfy my inane hate of them). Plus I wouldn't mind a respectable strip time and 0-60.

Suggest away please.....

Dave-R · Post by **Dave-R** » Tue Jul 20, 04 2:15 pm

It boils down to money and how fast you want to go.

You wanna buy ported ally heads? Re-build the trans/axle with race parts?

How much have you got to spend? ?500 or ?15,000? You can easy spend the latter if you want?

How fast do you want to go?

Alex is more than capable of sitting down with you and going over all the options with an idea of the sort of money each option will cost.

I make no secret of being a big lover of Hughes Engines cams and heads. In fact you can learn a lot just from reading their web site at http://www.hughesengines.com/

If it was my 360 I would have fitted a 4" stroke crank and a pair of Edelbrock heads.
Cam would be Hughes HEH 3742 AL which has 237/242 duration and 0.548/0.555 inch valve lift. This is the more modern version of the cam I have in my 440 and I like it a lot.