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Home > Technical Chat > 3.7 or 3.9 Final Drive choice ?

alaskanow0

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Mansfield

Hi guys,

I'm just about to order a semi helical crownwheel and pinion from Swiftune, but I'm not sure if to go for 3.9 as per the original plan with the 1293cc or a 3.7 due to the increased torque and lower rev limit of the 1460cc

3.9 would give me 108mph at 7500 rpm v
3.7 would give 113mph at 7500rpm.

I plan to do mainly track days. I'm just concerned I will exceed 108mph at the end of the straights. I did 117mph at castle Combe with the turbo. theorically the new build will have a higherbhp/tonne. 150bhp v 499kg.

Any real life experiences ??

Class C 3rd Place Avon 2011 14.18 @101mph


wil_h

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Betwix Harrogate and York

For circuit use I'd be using a 3.4 with that size motor.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

On 2nd Jan, 2013 fastcarl said:

the design shows a distinct lack of imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


alaskanow0

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Mansfield

It's not turbo though Will. Is 7 Port N/A 300degree cam. 4-7500rpm power band. Plus the Dogbox gearing will need at least a 3.7 to pull off the line.

Edited by alaskanow0 on 5th Mar, 2016.

Class C 3rd Place Avon 2011 14.18 @101mph


cossierick

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wakefield West Yorks

3.7 at least if not 3.4


cossierick

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wakefield West Yorks

I'm sure your calcs will be better than mine but if I remember correct my 3.7 was way too short , I was going over the line at York drag way on the rev limiter at 7k at 93-94 mph !! That was on 12" wheels


alaskanow0

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Mansfield

My 1380 miglia spec engine was about 95mph at 7k on 10's and that had a 4.2FD but it revved to 8.5k.

Class C 3rd Place Avon 2011 14.18 @101mph


wil_h

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Betwix Harrogate and York

Well a diff isn't that difficult to change. So if you go with the 3.7 then you'll be able to gauge it eitehr way and only need to change it once.

At least with the 3.7 you could go hillclimbing as well..

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

On 2nd Jan, 2013 fastcarl said:

the design shows a distinct lack of imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


alaskanow0

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Mansfield



On 5th Mar, 2016 wil_h said:
Well a diff isn't that difficult to change. So if you go with the 3.7 then you'll be able to gauge it eitehr way and only need to change it once.

At least with the 3.7 you could go hillclimbing as well..


Sorry ignore my ignorance why a 3.7 for hill climbing?

Edited by alaskanow0 on 6th Mar, 2016.

Class C 3rd Place Avon 2011 14.18 @101mph


wil_h

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Betwix Harrogate and York

The 3.4 would be too tall for hillclimbing, especially as you need to do an emergency start every run.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

On 2nd Jan, 2013 fastcarl said:

the design shows a distinct lack of imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


D4VE

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lowestoft suffolk

Amuse my ignorance... emergency start??

On 24th Oct, 2015 jonny f said:
Nothing gets past Dave lol

NOTHING GETS PAST ME!! *tongue*

1/4 mile 14.7 @ 96mph 12psi boost
Showdown class A 2nd place 18.6 @ 69mph


JT

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Hertfordshire

O


On 5th Mar, 2016 D4VE said:
Amuse my ignorance... emergency start??


Opposite to a emergency stop! Lol

My build thread..

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=542985


mw3

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Derby

I ran a 3.7 in mine and I found it good all round.

Matt W




On 14th Mar, 2012 mw3 said:
Got a nice big delivery from Carl at Force Racing today.


minimole23

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Wiltshire

I vote 3.7 with that rev limit

On 7th Oct, 2010 5haneJ said:
yeah I gave it all a good prodding


alaskanow0

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Mansfield

Thanks guys. I guess the real question should have been 'what speed can you get up to on a track day' and I guess the answer is depending on which track.

The 3.9 or even a 4.1 would be best suited to 1/4 mile for sure but it's never going to compete with the big boys. I guess a low 14 at best.

So I may as well make it a all rounder. The 3.7 seems to be the sensible choice. It will make it far more pleasant on the road.

Class C 3rd Place Avon 2011 14.18 @101mph


D4VE

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lowestoft suffolk

Low 14 at best?

Hadnt this car in heavier mode already done that n/a?

On 24th Oct, 2015 jonny f said:
Nothing gets past Dave lol

NOTHING GETS PAST ME!! *tongue*

1/4 mile 14.7 @ 96mph 12psi boost
Showdown class A 2nd place 18.6 @ 69mph


alaskanow0

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Mansfield




On 6th Mar, 2016 D4VE said:
Low 14 at best?

Hadnt this car in heavier mode already done that n/a?


Yes 13.91 at 560kg, but that was using nitrous. It only managed a 14.6 without. I don't fancy running nitrous again after it killed my engine on the Sunday at Shakedphere after a 60bhp shot trying to beat Rich1380 for 2nd place.

Class C 3rd Place Avon 2011 14.18 @101mph


alaskanow0

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Mansfield

A 13sec quarter from a NA is a major challenge. There's only Paul W & Rich1380 done it without cheating gas I think.

Class C 3rd Place Avon 2011 14.18 @101mph


dion

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Cape Town, South Africa

How about a 3.7 and a set of MS SC drop gears? Then a simple swap of primary gear you can effectively change your diff ratio between 3,6 ; 3,7 ; 3,9


alaskanow0

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Mansfield




On 7th Mar, 2016 dion said:
How about a 3.7 and a set of MS SC drop gears? Then a simple swap of primary gear you can effectively change your diff ratio between 3,6 ; 3,7 ; 3,9


That's a good idea.

Class C 3rd Place Avon 2011 14.18 @101mph


alaskanow0

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Mansfield

3.7 ordered

Class C 3rd Place Avon 2011 14.18 @101mph

Home > Technical Chat > 3.7 or 3.9 Final Drive choice ?
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