Diff's

[Bedfordcrazy]

hey guys, can someone confirm a terminology I have heard. Ever since i have had my van, everyone that has looked at it, or the diff, they ask if it is a highway diff. Is there such a thing as a highway diff or is that just a slang term, if there is what is a hyway diff. My understanding of a hyway diff is, a diff that can cruise on the freeway, and not rev hard. Is this correct ?
George.

[hotrod]

yup

[BusyKiwi]

it's basically the diff ratio

2.75 to 1 or the other end 4.11 to 1

good for the drag strip or the highway

[rossie]

yes George your right, one that cruises the hwy without reving the crap out of it, another way of redusing the revs on the hwy is with an overdrive trans. Put the hwy 2.78 or similar with an overdrive can cause alot of strain

[Warren]

George

To understand the "highway diff" you need to understand the "diff ratio"

as said the diff ratio ranges from High 2's to low 4's. This ratio is the relationship between the number of revolutions the engine turns compared to the turn of the rear wheels which is determined by the crown wheel and pinion in the diff.

For every turn of the rear wheel you have X turns of the engine, where X is that relationship of the crown wheel and pinion.

How to find out what ratio your diff has can be as simple as looking at the dust cover disk on the yoke of the diff (the ratio is usually stamped on the disc) or by counting the numbers of turns of the tailshaft for a full rotation of the wheel.

So if you have say 3 tailshaft rotations to 1 wheel rotation you get a 3 to 1 ratio or as it is more commonly writen is 3.0:1.

This ratio can also be looked at like leverage the higher the ratio, the less force is needed to do the same work.

OK so now if we have two diff ratios (lets use BusyKiwi's numbers) what will be the different behavior.

So we have the same van same engine but all we do is change the diff how will the van proform.

Lets start with the 4.11:1 one first, the engine will have to turn over over 4 times for for one rotation of the wheel, now this means that for a given distance the engine will have to turn over more times so it would travel the distance slower.
But this greater leverage means that it could also do more work, as in move greater weight over that same distance.

So to move large weight, a sacrafice of top speed is made for increased load carrying, which is what the Beddys were designed for, also in England the likely speed the van would do is only up to 80kph.

With the 2.75:1 for the same distance the engine will turn over less or alterantly go faster for the same engine revs. This is what we want for the "Highway".
This ratio has less leverage and therefore would not be able to do as much work as the 4.11:1.

The 4.11:1 diff will get up to maximum speed quicker than a 2.75, but it will have a lower top speed.

In drag cars they want to get up to speed ASAP, and they have heaps of HP and only a short distance to do it so the High ratio is best.

Where as in a touring van you wan a balance between power, speed and fuel economy and the lower ratios suit that more.
 
Warren

[ben]

hey george just to add to what everyone else says..

and im not sure if im correct so correct me if im wrong..


no one talks about hwy diffs except round bedfords because of the standard diff issue..

so when we talk about a hwy diff generally talk about a bedford diff being cut and moded for a different centre in it to create a "hwy" diff..


if you just wak a holden diff in a van or something similar you will achive the same result but i wouldnt call it a hwy diff, more call it a standard diff with a reasonable ratio and not enough width for the bedford..


thats what i think anyway..

cheers
ben

[Bedfordcrazy]

Thanks Guys, I have learnt a lot in the last few days, between my mechanic and you guys here i'm learning a lot about mechanical and fabrication, i have always had a passion for working on cars, one reason why i have my mechanic do the work in my workshop, so i can get my hands dirty as well. My hope is that when I start to show my van later this year, I will be able to answer all the Qs. that people may ask, because i helped on every part. All going well now, we should have all the mechanicals done by the end of april, and then we start sheet metal work.
George.

[Mark B]

Another question....

Do you have to change the tailshaft when you change diffs or modify the centres? I have a diff here from a Kingswood, haven't measured it yet or looked at the ratios..so if i did fit it (not sure if it will be the right one tho) would i need a tailshaft from a kingswood?

[SkippyThBushKangeroo]

 i think  at one time  when corresponding with u ben , u refered to my 9 inch. . .diff that is  as a highway one after i had worked out it had a 67 bango casing an a 65 front piece   a sort of hybrid  of two different years put together to give it that ratio it needs for hway cruisin, made sense to me anyway mate, another gem from yor wealth of knowledge for which i am eternally grateful. 

[SkippyThBushKangeroo]

hey mark have sussed out different diffs in th holden range some can fit though it is extremly tight.. joe has done it an explained enough to me to to realise it woz easier to leave the ford 9 in an just bung a few bannanaskins in it to shut it up  . . only jokin bout the latter . but for sure joe is in the know on thatun