diffgears / mph

FS5 · 20-06-2006, 04:48 PM

do diff gears effect your mph ?
i have a set of 3.45's in my au 5spd and i will be changing them to 3.9's soon .i currently have to grab 4th gear about 50m before the line. will going from 3.45 to 3.9's increase my mph or will the mph stay the same ?
i know going s 3.9's will give me a better ET but just not sure about the mph.
thanks ,jake

**Stav** · 20-06-2006, 05:30 PM

Gears will boost you off the line and reduce top end by the same amount.

Racecraft · 20-06-2006, 05:34 PM

Quote:

Originally Posted by useless

Gears will boost you off the line and reduce top end by the same amount.

Are you sure,, mph is power I dont see how changing the diff will drop mph unless you are running out of puff before the 400m

FS5 · 20-06-2006, 06:00 PM

Quote:

Originally Posted by Racecraft

Are you sure,, mph is power I dont see how changing the diff will drop mph unless you are running out of puff before the 400m

my peak hp comes on at 5400 rpm and i'm crossing the line @ 4400/4500 rpm in 4th

OED666 · 20-06-2006, 06:00 PM

you will be fine... i got 4.11's, and there is no way mine slows down at the top end. i agree with tibbo, unless your running out of puff you will be fine.

3.9's are good gears. in a way i wish i got them, but only for highway cruising. you will find that your first gear with 3.9's is shorter than mine with 4.11's.

BTW, useless, have you got that nice cam in yet...

Mark351 · 20-06-2006, 06:04 PM

Why do you need to change into 4th? I can hit the line still powering down in 3rd, and mine's also a 4spd manual with 3.45s....

**Stav** · 20-06-2006, 09:13 PM

Quote:

Originally Posted by OED666

BTW, useless, have you got that nice cam in yet...

Hheeh not yet mate.I am giving this setup one last chance to see what difference the cold weather will have down the 1/4 mile.

Also I have nearly finished my super head that ive been porting and fighting with for a while.I am thinking of doing the head and cam at the same time or if I feel lazy just stick the cam in and deal with the head at a later kilometres.

: .

FS5 · 20-06-2006, 09:52 PM

Quote:

Originally Posted by DeathXR

Why do you need to change into 4th? I can hit the line still powering down in 3rd, and mine's also a 4spd manual with 3.45s....

when it had 3.23 i used to hit the line right on the limiter ,now after fitting 3.45's i need to grab 4th .
what rims and tyres do you run ?
i run 225 /16 / 50's

FS5 · 20-06-2006, 09:54 PM

Quote:

Originally Posted by OED666

you will be fine... i got 4.11's, and there is no way mine slows down at the top end. i agree with tibbo, unless your running out of puff you will be fine.

3.9's are good gears. in a way i wish i got them, but only for highway cruising. you will find that your first gear with 3.9's is shorter than mine with 4.11's.

BTW, useless, have you got that nice cam in yet...

oed what gears did you run before the 4.11's ?
did you notice any difference in your trap speed after fitting the 4.11's?

OED666 · 20-06-2006, 10:51 PM

i had 3.9's before. i didnt notice any difference in trap speed. maybe even faster now...

i spose i never really unleashed the full potential of it with 3.9's. plus i didnt have my au motor in when i had 3.9's

Mark351 · 20-06-2006, 11:16 PM

Quote:

Originally Posted by FS5

when it had 3.23 i used to hit the line right on the limiter ,now after fitting 3.45's i need to grab 4th .
what rims and tyres do you run ?
i run 225 /16 / 50's

I'm running 17"s, but didn't notice any difference when I ran 15"s

FS5 · 21-06-2006, 08:44 AM

Quote:

Originally Posted by DeathXR

I'm running 17"s, but didn't notice any difference when I ran 15"s

what mph are you getting when tou hit the line in 3rd ?
i have tested mine a couple of times crossing the line in 3rd and it dose around 90mph .

Mark351 · 21-06-2006, 10:31 AM

Up to 95 mph, but I've got intake/exhaust mods and a great tune.

OED666 · 21-06-2006, 10:38 AM

plus you have nice cold vic air... us qld'ers arent quite as lucky...

maybe a road trip to vic is in order... :P

FS5 · 21-06-2006, 05:42 PM

Quote:

Originally Posted by DeathXR

Up to 95 mph, but I've got intake/exhaust mods and a great tune.

crikey thats a bit of difference ,are you hitting the limmiter in 3rd.
considering we are both running the same gearbox and diff ratio's you wouldn't think there'd be 5mph difference running 16 inch wheels against 17's .

Mark351 · 21-06-2006, 05:52 PM

I can't remember ever hitting the limiter in 3rd, and of late that's less likely to occur.

jonbays · 21-06-2006, 06:28 PM

4th in your manuals is like third in our autos and even with 3.9's you should pull 5500rpm at around 28kmh/1000rpm thats 154kmh a good TS for most hot sixes.

jonbays · 21-06-2006, 09:32 PM

Diff Tyres Kmh/1000rpm Speeds in gear
Ratio Third 3000-5000-5750rpm
3.08 225/50R16 36.59 110 183 210
3.23 225/50R16 34.97 105 175 201
3.45 225/50R16 32.74 98 164 188
3.73 225/50R16 30.28 91 151 174
3.89 225/50R16 29.04 87 145 167

FS5 · 21-06-2006, 11:20 PM

Quote:

Originally Posted by jonbays

Diff Tyres Kmh/1000rpm Speeds in gear
Ratio Third 3000-5000-5750rpm
3.08 225/50R16 36.59 110 183 210
3.23 225/50R16 34.97 105 175 201
3.45 225/50R16 32.74 98 164 188
3.73 225/50R16 30.28 91 151 174
3.89 225/50R16 29.04 87 145 167

so 3rd in an auto is the same as 4th in a manual .

deathxr ,yours is a manual isn't it?

**Stav** · 22-06-2006, 12:51 AM

Quote:

Originally Posted by OED666

BTW, useless, have you got that nice cam in yet...

Ah stuff it I am tempted to throw it in this weekend.

FS5 · 22-06-2006, 08:31 AM

Quote:

Originally Posted by useless

Ah stuff it I am tempted to throw it in this weekend.

dooooooo it ya know you wanna

Mark351 · 22-06-2006, 08:31 AM

Quote:

Originally Posted by FS5

deathxr ,yours is a manual isn't it?

Yeah, mines a manual. Just ignore the auto guys. ;)

jonbays · 22-06-2006, 09:16 AM

Quote:

Originally Posted by FS5

so 3rd in an auto is the same as 4th in a manual .

deathxr ,yours is a manual isn't it?

Yes

1:1 gearing in third gear in a 4 speed BTR Auto and in 4th gear in the 5 speed manual.

jonbays · 22-06-2006, 09:20 AM

Even with 4.1 gears you should never hit the rev limiter in third/fourth in the qtr. Well not unless you are genuinely getting under 13 sec ET's.

Speed limiter is differnent. In non XR models if the ecu thinks its doing 180 it will shut down the fuel. Without speedo adjustment that's maybe as low as a true 141kmh

jonbays · 22-06-2006, 10:07 AM

In my opinion for drag racing with a falcon six auto or manual you want third or fourth gear respectively to take you to the line at peak power rpm leaving enough room to cross the line and just hit the rev limiter.

Now in a stock falcon or mildly modded one you will see peak power at around 4900-5000rpm and with most road going cammed cars this doesn't get much above 5250-5500rpm.

So for best drag strip performance you want to use all three or four gears fully in the qtr so for a mild car 3.73 gears are good which with 225/50R16 tyres give 30.28kmh/1000rpm and a power band from 91kmh to 151kmh and a rev limit at 174kmh.
Few mild falcons will do more than 151Kmh TS in the qtr!

For a bit of a hottie or absolute max qtr performance 3.9 gears give 29.04kmh/1000rpm and a powerband from 87kmh to 145kmh and rev limit at 167kmh.

See how well frdgal is going with 3.9's and a stallie and you see this is a good option.

I am sticking with my 3.45 gears which gives me 32.74kmh/1000rpm and a powerband from 98kmh to 164kmh with a rev limit at 188kmh. This is nice for a road car as it has nice pull in third from 100k and revs to the speed limiter before rev limiter! Also for a wagon it doesn't spin up the extra long tailshaft too much.

FS5 · 22-06-2006, 02:19 PM

3.9's will be the perfect match for my car as it is now.
i'm still interested to know if changing the gears will increase my mph ?

OED666 · 22-06-2006, 03:01 PM

i was reading a mag recently, and these guys fitted 3.9's to a vy ss and it did picked a few mph at the end.

i am sure the gears will help overall acceleration.

jonbays · 22-06-2006, 09:19 PM

Quote:

Originally Posted by OED666

i was reading a mag recently, and these guys fitted 3.9's to a vy ss and it did picked a few mph at the end.

i am sure the gears will help overall acceleration.

From what I can tell the lower diff gears really make a difference in the middle range.

A stallie helps out in the 60' in an auto.

The diff works well to improve the 330 and 660. 660 on the guys and girls with lower diffs than me arent making any ground on me so i don't know that the gearing will add mph to your TS. I would say they will cut tenths from the mid range 330 and 660.

jonbays · 22-06-2006, 09:31 PM

here are the splits between me and useless
useless runs 3.73 diff and a hi stall converter
I run 3.45 gears and stock converter

60' 330' 660' 1000' ET
-0.1670 -0.0760 -0.0200 -0.0030 -0.0040
The big differece is the 60' and thats the hi stall
The small but significant difference in the 330 and 660 is the diff gearing
The 1000 and qtr not much difference.
you can see by the fact we both have similar modded falcon auto wagins and the similar TS MPH the cars power to weight has to be pretty close.
So the differences are the stall and gearing.
Oh and the drivers too.