What rear diff ratio?

GJUK · Post by **GJUK** » Wed Mar 11, 15 9:32 am

Hi,

I currently have this in my Abody.
400 big block +30 engine
Comp Cam UG 3878 501 lift (intake and exhaust)
2800-3000 Jegs stall convertor
1975 'heads'?!
727 auto transmission

At the moment there is a 4.1 diff in the back.
It changes in to top gear at 17mph :-/. On the road it revs to like 4500 rpm at 60mph. you have to cruise at 50mph, even then it's revving its nuts off.

I have just bought some 295/50/15 wheels for the car which are 26.7" I understand in diameter.

What diff would you fit? 3.9? 3.8?

The car will be used 99% on the street, think fast road car.

Thanks,

Jon

charger01 · Post by **charger01** » Wed Mar 11, 15 9:54 am

3.55's

GJUK · Post by **GJUK** » Wed Mar 11, 15 10:26 am

Wouldnt that bog the engine down a lot?

Dave-R · Post by **Dave-R** » Wed Mar 11, 15 10:32 am

Use this spreadsheet I made years ago.

It will let you play around with the axle ratios so you can see what rpm and speed you will have with different ratios.

If I were you, and if you drive any distance on motorways, I would find out at what rpm your engine makes the most torque. Then fit the gears that allow you to cruise at 75mph at that rpm.

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

Les Szabo · Post by **Les Szabo** » Wed Mar 11, 15 1:13 pm

No mention of that cam but its almost equivalent to an XE284 which runs from 2300>6500, and needs at least 9.1CR, the verter matches the cam and gears you have but you haven't got enough CR, (7.8:1?), or head flow with bone stock heads. So you need something like an XE256H.

http://www.compcams.com/Company/CC/cam- ... d=705&sb=2

Fit a stock verter or 1 just above and 3.23 or 3.55 gears, then you will make torque low down where you want it for a street car, it will be a different animal. Putting just the gears in will help some, but the parts in that motor are all wrong for pure street, and I would have a look at your box as well.

http://www.moparuk.com/forums/viewtopic.php?t=49868

Les

GJUK · Post by **GJUK** » Wed Mar 11, 15 1:33 pm

I'm not sure on the comp ratio of the engine, is there any 'quick' way of telling - guessing spark plug out and do a compression test?

I know the head is ported.

information from the guy I bought it off:

It is a 400 low deck with a .030" over bore. That makes it a 406 c.i. I don't know what your knowledge is on Chrysler engines but all their big block wedge engines were either categorized by low deck, or block and raised block. Only difference is all the low block engines had a 3.38" stroke and the raised blocks had a 3.75" stroke.

So back to your Dart, that engine is fresh but it could be one of two engines. Internally either factory cast pistons or forged higher compression pistons. Depending which one will also let me tell better about the rotating assy.
The rear end is a 489 case with 4:10-1 gear ratio. I don't know what kind of axles are in it. In reality, a 68 Dart in that shape, with all the wheels already converted to the big bolt pattern and even without any engine or transmission

*shrugs*

GJUK · Post by **GJUK** » Wed Mar 11, 15 1:34 pm

double post sorry

Les Szabo · Post by **Les Szabo** » Wed Mar 11, 15 2:13 pm

No, no quick way, you can check cylinder pressure but that will only tell you condition of bores/valve seal and depending on how much you have cam choice as well but we won't go into that now.

Only way to tell CR is pull a head off, CC it, see how far down the bores your slugs are, h-gasket thickness and calculate, you get an idea from that 400 motor build on the link. Info you got sadly is no use whatsoever. Just change to a 3.55 and see how you go, you might be happy with it as it is.

Les Szabo · Post by **Les Szabo** » Wed Mar 11, 15 3:10 pm

You can follow this 400 build as well?

http://www.hotrod.com/how-to/engine/ccr ... -the-cost/

GJUK · Post by **GJUK** » Wed Mar 11, 15 4:46 pm

Thank you. I will take a look at the link, cheers for everyone's help.

Mick70RR · Post by **Mick70RR** » Wed Mar 11, 15 5:42 pm

I'd fit 3.23 gears.

Adrian Worman · Post by **Adrian Worman** » Wed Mar 11, 15 6:14 pm

Mick70RR wrote:I'd fit 3.23 gears.

I'm with Mick, if you're gonna drive to shows, 2-3 different tracks etc, 3.23's are prrfect, even better with tall tyres.
You can always take some smaller diameter wheels/tyres to the track.

I can drive 80-85 all day and the engine don't sound like a grenade all the time.

I know plenty of blokes who build weekend drag strip warriors and they have a blast on the street, it's all about what you want or expect from your car .......all a compromise tho innit?

GJUK · Post by **GJUK** » Wed Mar 11, 15 7:01 pm

Hi guys, thanks for your continued input.

I know a lot of you have questioned what I want from the car in the past and why it's had the work done to it that I've had done, Full cage, tubbing and now axle shortening with large (heavy) steel wheels.

I want the car to be used on the road, on a warm day go for a blast. Drive it to one of my mates house and go to a show in it. Honestly it probably won't ever make it on to the strip again with its current engine anyway.

I still want it to go well but I know you can't have your cake and eat it. I guess it's important at this point to work out what the long term plan for the car is.

It's staying with a big block, it will probably be stroked next year or so.

I don't think I've revved the car past 4.5k, though the cam is only getting going at this point I understand.

Maybe it's best for me to fit a 3.8 /3.9 ratio diff or so, to retain some of that 'get up and go' but allow a better (not great) cruising speed. I mean anything is better than it is at the moment, it changes up in to top gear at 17mph!!!

The new rear tyres are taller slightly which will help also I understand.

autofetish · Post by **autofetish** » Wed Mar 11, 15 7:25 pm

Is the kick down cable adjusted correctly ?

I mean is it normally for it to shift into top gear at 17mph

I don't know just a thought ?

GJUK · Post by **GJUK** » Wed Mar 11, 15 7:29 pm

autofetish wrote:Is the kick down cable adjusted correctly ?

I mean is it normally for it to shift into top gear at 17mph

I don't know just a thought ?

Yeah it has been checked, at 55mph or so it's revving at just over 4k. It's not great on the road at the moment. Up to 50mph though it's very nippy