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Streetracing.co.za • View topic - Diff ratio and turbo lag
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 Post subject: Diff ratio and turbo lag
PostPosted: Wed May 04, 2016 11:26 am 
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Just arrived..

Joined: Fri Aug 14, 2015 1:41 pm
Posts: 22
Location: Pretoria
Does anyone have experience with this? My question is basically if say my car starts to boost from 3000rpm on a 3.15 final diff ratio. If I now change to 4.1 diff ratio will it still boost from 3000 rpm? Maybe it will just feel quicker? I know I will loose some top end but if I can rev higher will the car feel more "alive" on the street.

There must be a reason why most drag cars run 4.1 and higher diff ratios on huge tires!

Thanks


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 Post subject: Re: Diff ratio and turbo lag
PostPosted: Wed May 04, 2016 11:32 am 
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Resident Clown
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Joined: Wed Jul 27, 2011 8:33 am
Posts: 4493
Location: Centurion
im not a boffin on this but the only way you will change the RPM the turbo spool/boost from is by changing the turbo's specs. The diff will only assist with keeping you in higher rpm range when shifting between gears.

The car will also be heavier on fuel with the 4. ratio. But with power who worries about fuel consumption

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 Post subject: Re: Diff ratio and turbo lag
PostPosted: Wed May 04, 2016 11:37 am 
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teacher's pet

Joined: Sun Feb 09, 2014 6:47 pm
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 Post subject: Re: Diff ratio and turbo lag
PostPosted: Thu May 05, 2016 10:03 am 
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Just arrived..

Joined: Fri Aug 14, 2015 1:41 pm
Posts: 22
Location: Pretoria
Yip, it make sense what you are saying. My car actually starts to boost from 2800 rpm and limit is currently 6800. But I am busy with a cam, springs and retainer change, so now I can do 8000 rpm all day long ( I can probably go to 9000rpm but I don't think the bottom-end is going to like it at this stage). I was thinking of a slightly larger turbo but the drawdown is more lag. My idea is to offset the lag with a shorter diff ratio on the street. Maybe around 3.7 from 3.1?


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 Post subject: Re: Diff ratio and turbo lag
PostPosted: Thu May 05, 2016 11:26 am 
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2 Bar BOOST!
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Joined: Wed Feb 09, 2011 4:50 pm
Posts: 2625
Location: Die Hoewes, Centurion
You can always convert to a VNT (variable nozzle turbo)

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- Daily: 2007 Hyundai Getz CRDI Euro spec (81kw, 235NM)
- 1994 Nissan 1400, 4AGZE, SC12 Supercharger, 20v Black top flywheel, T51 gearbox, Spitronics Titan advance ECU, SpeedSpot 4 into 1 branch and 63mm exhaust, 120 wHP, 260 NM on 93 ulp (Uncorrected mechanical dyno).
- 1985 Golf 2 GTS with 2 liter ABF and Spitronics Titan advance.
- 2002 Honda CBR 600 F4i with Stealth exhaust pipe, DNA air filter.
- 1982 Leyland Mini 1275E.


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 Post subject: Re: Diff ratio and turbo lag
PostPosted: Thu May 05, 2016 1:10 pm 
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Joined: Fri Aug 14, 2015 1:41 pm
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Location: Pretoria
I am not aware of anyone that have done this successfully in SA? How to control the vanes? What about high EGT's? And lastly to find a tuner to take the time to map it all!


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 Post subject: Re: Diff ratio and turbo lag
PostPosted: Thu May 05, 2016 2:28 pm 
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2 Bar BOOST!
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Joined: Wed Feb 09, 2011 4:50 pm
Posts: 2625
Location: Die Hoewes, Centurion
The VNT works with vacuum that works with an electronic tap between the intake and the turbo actuator that can be controlled by most new aftermarket ECU's.

_________________
- Daily: 2007 Hyundai Getz CRDI Euro spec (81kw, 235NM)
- 1994 Nissan 1400, 4AGZE, SC12 Supercharger, 20v Black top flywheel, T51 gearbox, Spitronics Titan advance ECU, SpeedSpot 4 into 1 branch and 63mm exhaust, 120 wHP, 260 NM on 93 ulp (Uncorrected mechanical dyno).
- 1985 Golf 2 GTS with 2 liter ABF and Spitronics Titan advance.
- 2002 Honda CBR 600 F4i with Stealth exhaust pipe, DNA air filter.
- 1982 Leyland Mini 1275E.


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