New Mk2 6 Speed ratios ?

[fordboy1981]

I also find it hard to believe that the T would have a higher 1st gear then the 8....

oh well we'll just have to wait and see...

Dave.

[Lawsy]

Well, with the 3.73 diff, quarter times should be quite a bit faster, maybe 0.1 - 0.15 even.

My reasoning is that the 2nd - 3rd and 3rd - 4th shift is shorter.

This is based on that the current turbos run 3.46 centers, not the 73's.

So, because there is no longer a shift at about 97km/h, the 0 - 100 time should easily be faster, and down the strip you will have the same number of shifts, but instead of running at say, 4750rpm(rough guess, no calculation was made) as you cross, this would probably be closer to 5000.... Or more even, is your velocity at this stage should be about 3 or so k's higher.

My 2 cents, but that's how I read the figures.

I could be so far off the mark it isn't funny though, im open to correction if this is the case.

Edited 15/10/04 01:34 PM by Lawsy

[Lawsy]

1 thing I never quite understood is this.

What is the reason for always having a 1:1 4th or 5th gear (depending on the box)?

Is it simply easier to construct?

Reason I ask this is because if I was to construct a gearbox, only if it coincidently happened to work out better, I wouldn't choose to have a 1:1 ratio at all. My 4th gear would be around about 1.05 or 1.10, while lowering third a further 2-5 points. 5th could then be slightly shorter by say, 2 points and 6th can remain the same as above.

We can't have a perfect box I guess....

What im about to do is write up a quick excell doc to work it all out for us, so we can punch in whatever ratio we want at any rpm on any rim size with any diff ratio and get a speed reading.

[Lawsy]

I dunno whats giong on, but if im calculating it correctly, then we most definately aren't going to make it over 100 in 2nd gear....

with 17/235/45's on, and a 3.73 center, 91 is all we are going to get......

Heres my working.

Tyre diameter = 17 + (235 * 45/100)/2.54 = 21.16338583.

Then, 21.16 / 2.54 * PI which is 168.8763131.

Now its as simple as working out rpm and ratios, so lets take the 73s and 2nd gear.

((rpm*60{per hour})/(diff*gear)*circumference)/100000 (cm in a kilometer).

((6000*60)/(3.73*1.78)*168.876)/100000

which is 91.56755128

..... So um, where did I go wrong?

With 18/235/40's on and the 3.46 center, you get 101.22

I dunno, I think you need to multiply the output tyre profile value by 2 to get the total tize, I just didn't think you would have tyres that damn big.... 235/40 profile tyres most certainly don't look near 25 inches in total diameter... that's a 4 inch sidewall, surely its not that much....

If it is then it blows out the other way, then we are doing 109 in second....

Why did I bother.......

Edited 15/10/04 04:16 PM by Lawsy

[WBT56]

1 thing I never quite understood is this......What is the reason for always having a 1:1 4th or 5th gear....

Because you basically get the 1:1 gear for free, as you are only coupling the input shaft and output shaft together. Any other combination requires an additional gearset/torque path, and hence a bulkier box. Ben.

[Lawsy]

I'm sure I knew that at some stage....

[bcl]

I dunno whats giong on, but if im calculating it correctly, then we most definately aren't going to make it over 100 in 2nd gear....

with 17/235/45's on, and a 3.73 center, 91 is all we are going to get......

Heres my working.

Tyre diameter = 17 + (235 * 45/100)/2.54 = 21.16338583.

Then, 21.16 / 2.54 * PI which is 168.8763131.

Now its as simple as working out rpm and ratios, so lets take the 73s and 2nd gear.

((rpm*60{per hour})/(diff*gear)*circumference)/100000 (cm in a kilometer).

((6000*60)/(3.73*1.78)*168.876)/100000

which is 91.56755128

..... So um, where did I go wrong?

Using a tyre calculator 245/40-18 are 25.72" in diameter, 240/35-17 are 25.33" in diameter, so the 18" are 1.56% larger.

Also I use a 5800 rev limit... that's my cutout.

You have "Tyre diameter = 17 + (235 * 45/100)/2.54 = 21.16338583.

The correct expression is:

17 + ( (235 * 45/100)/25.4 ) *2 = 25.33"

Just over 100kph in 2nd with the 3.73 diff is quite correct.

A 3.46 diff instead of 3.73 would give around 8kph extra in 2nd at 5800 revs.

Brian

[Turbo6man]

I dunno whats giong on, but if im calculating it correctly, then we most definately aren't going to make it over 100 in 2nd gear....

with 17/235/45's on, and a 3.73 center, 91 is all we are going to get......

Heres my working.

Tyre diameter = 17 + (235 * 45/100)/2.54 = 21.16338583.

Then, 21.16 / 2.54 * PI which is 168.8763131.

Now its as simple as working out rpm and ratios, so lets take the 73s and 2nd gear.

((rpm*60{per hour})/(diff*gear)*circumference)/100000 (cm in a kilometer).

((6000*60)/(3.73*1.78)*168.876)/100000

which is 91.56755128

..... So um, where did I go wrong?

With 18/235/40's on and the 3.46 center, you get 101.22

I dunno, I think you need to multiply the output tyre profile value by 2 to get the total tize, I just didn't think you would have tyres that damn big.... 235/40 profile tyres most certainly don't look near 25 inches in total diameter... that's a 4 inch sidewall, surely its not that much....

If it is then it blows out the other way, then we are doing 109 in second....

Why did I bother.......

Your calculation of wheel diameter is incorrect as you have added the sidewall height only once - you have to add it twice to get diameter then divice the result by two to get your radius. Then you can calculate the circumference and divide the circumference into 1000m to know how many revs per kilometre the wheel will do. (from memory for a 235x45x17 it should be around 505)

Then your formula for calculating kph per thousand revs becomes:-

60,000 over 505 over gear ratio over final drive ratio.

Hope this helps

[Lawsy]

As said, I had already figured it out

I dunno, I think you need to multiply the output tyre profile value by 2 to get the total tize, I just didn't think you would have tyres that damn big.... 235/40 profile tyres most certainly don't look near 25 inches in total diameter...

but it still doesn't seem right as it stretches 2nd gear so far it isn't funny.

btw, I measured the tyres on the Ghia, and sure enough top to bottom its nearly 26 inches....

I thought I would spam excel with some random crap and managed to come up with.

=IF(tp="",IF(rim=16,rim+((tw/10*60/100)/2.54*2),IF(AND(rim=17,tw<235),rim+((w/10*50/100)/2.54*2),IF(AND(rim=17,tw>235),rim+((tw/10*45/100)/2.54*2),IF(OR(rim=18, 19, 20),rim+((tw/10*40/100)/2.54*2))))),rim+((tw/10*tp/100)/2.54*2))

Well, it works..... lol.

btw, that actually doesn't do anything much, it simply works out the circumference of the tyre, but accounts for people who don't know their profile.... I was going to stick it in VB6, that's why I did it but then couldn't be bothered......

Edited 16/10/04 01:07 PM by Lawsy

[Grass]

I dunno whats giong on, but if im calculating it correctly, then we most definately aren't going to make it over 100 in 2nd gear....

with 17/235/45's on, and a 3.73 center, 91 is all we are going to get......

Heres my working.

Tyre diameter = 17 + (235 * 45/100)/2.54 = 21.16338583.

Then, 21.16 / 2.54 * PI which is 168.8763131.

Now its as simple as working out rpm and ratios, so lets take the 73s and 2nd gear.

((rpm*60{per hour})/(diff*gear)*circumference)/100000 (cm in a kilometer).

((6000*60)/(3.73*1.78)*168.876)/100000

which is 91.56755128

..... So um, where did I go wrong?

Using a tyre calculator 245/40-18 are 25.72" in diameter, 240/35-17 are 25.33" in diameter, so the 18" are 1.56% larger.

Also I use a 5800 rev limit... that's my cutout.

You have "Tyre diameter = 17 + (235 * 45/100)/2.54 = 21.16338583.

The correct expression is:

17 + ( (235 * 45/100)/25.4 ) *2 = 25.33"

Just over 100kph in 2nd with the 3.73 diff is quite correct.

A 3.46 diff instead of 3.73 would give around 8kph extra in 2nd at 5800 revs.

Brian

OK Guys, what rev limit are we talking about. I tried it last night in my T and I got to 110 in second before changing up and before the rev limiter hit....

Seemed strong and pulling all the way to 6000+ RPM's