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diffs - please explain! 

 

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 Post subject: diffs - please explain!
Posted: Mon Jan 22, 2007 5:52 pm 
Getting Side Ways
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ok, from what i thought, the higher the diff ratio the quicker you get thru your gears, but top speed is lower.

ive got a manual with 3.45 gears and a big cam, so im lacking a little down low, if i wanted to get a quicker 1/4, id assume that a set of 3.9's, or 4.11s, would get me there quiker.

now ive heard some people on here swear by lower gears, and some on higher gears.

how would a set of 4.11's slow you down on a 1/4 mile?

 

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 Post subject: Re: diffs - please explain!
Posted: Mon Jan 22, 2007 5:58 pm 
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FAST-XR wrote:
ok, from what i thought, the higher the diff ratio the quicker you get thru your gears, but top speed is lower.

ive got a manual with 3.45 gears and a big cam, so im lacking a little down low, if i wanted to get a quicker 1/4, id assume that a set of 3.9's, or 4.11s, would get me there quiker.


In a word, Yes.

 

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Posted: Mon Jan 22, 2007 6:06 pm 
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4.11's = more gear changes over the 1/4!

 

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Posted: Mon Jan 22, 2007 11:07 pm 
Getting Side Ways
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so with 4.11s your saying that ill be be hitting 5th?

 

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Posted: Mon Jan 22, 2007 11:08 pm 
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You already use fourth? Then I'd be looking at why you have such little low rev torque, nor the high rpm power (& revs) to justify this big cam.

 

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Posted: Mon Jan 22, 2007 11:19 pm 
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well if i just hit fourth at the line, its still a gear change, and if it happens earlier with the 4.11s is that a bad thing?

 

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Posted: Mon Jan 22, 2007 11:36 pm 
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I take it this is for the 97 EL XR6, right? What is your current terminal speed? At what rpm do you change gears? & at what rpm are peak power & peak torque achieved?

You need to make the most of your combination & that means matching all components from the intake & exhaust, cam, head (ported or not), gears etc. You may find avoiding the change into fourth saves more time, or you may decide to go with 4.11s & make some use of that change into fourth.

I still don't see why you are changing into fourth at the line instead of just holding third, especially since a big cam that has sacrificed torque would require more rpm to be effective.

 

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Posted: Tue Jan 23, 2007 10:09 pm 
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I would suggest advancing the cam timing first, try 2dgs then 4dgs, to see if this helps. its easier to do this than change diff gears.
There is a formula for workin this out, as in revs & gears.

RPM x total outside tyre dia in inches x 3 (this is a constant)
______________________________________________ = speed MPH

Total gear ratio ( g'box ratio x diff ratio)

Eg; 3,000 rpm becomes 3.000 or
2,500 rpm becomes 2.5 ok?

So 2.5 (rpm) x 26" x 3 = 195
________________________ _________ = 56.52mph

( 4th) 1 x 3.45 (diff) 3.45

56.52mph x 1.61 = 90.99 kmh



Use this to sort out rpm verse tyre size and diff gears.

 

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Posted: Tue Jan 23, 2007 10:36 pm 
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going from 3.45 to 3.9 is putting in a lower ratio.
3.08 is higher than 3.9.
higher ratio = long legs
lower ratio = short but quick
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