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Diff ratio / Torque converter questions

Started by _Viper_ , Oct 15 2018 08:37 PM

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8 replies to this topic

#1 _Viper_

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Posted 15 October 2018 - 08:37 PM

Hey all, So got my Torana going now and been out on a road trip, and the Rpm is a little high for what I would like cruising at 100kph being 3500rpm

Now this car is going to be more of a cruiser not a drag car so would like to reduce it a little.

So I punched my info into this gear ratio calc http://www.apexgarag...ar_ratios.shtml

Holden 355 - TH350 - 9" 3.7:1 - 645mm/25.4" OD wheels and it says I should be doing 100kph @ 3000-3100rpm

It was a long long time ago when I got the Torque converter so not sure what rpm it was supposed to be but would this mean I have a 3500 hi-stall? And its constantly slipping while im cruising, creating alot of heat and hurting fuel consumption?

Should I be reducing stall to 2500-3000?

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#2 rodomo

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Posted 15 October 2018 - 09:32 PM

3.08 diff will bring it down to 2800rpm ish

Edited by rodomo, 15 October 2018 - 09:32 PM.

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#3 Bigfella237

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Posted 15 October 2018 - 09:47 PM

Those calculators are only really accurate for manual transmissions, they don't take TC slippage into account.

My calculator agrees with a figure of 3034 RPM @ 100km/h with a 1:1 top gear, but again, the converter will likely be the difference you're seeing.

How mild / wild is the motor / camshaft? Does it need that much converter?

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#4 _Viper_

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Posted 16 October 2018 - 10:50 AM

Yep I understand its the convterter slipping resulting in the difference but isnt it bad to be slipping the whole time while cruising long distance? Doesnt it continually create alot of heat while slipping and not good for it?

Plus reving higher then needed = excess fuel Usage?

Or is it normal for a tranny to slip while cruising

Id only call it a mild 355, it does have a crane 280 solid cam

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#5 myss427

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Posted 16 October 2018 - 12:18 PM

Best conversion would be to go an extra gear, cost more but gives you the best of both words. Could sell your Turbo 350, put in a 700r and knock a 1000 rpm off.

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#6 gtrboyy

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Posted 17 October 2018 - 12:18 PM

My vh is 355 10:7cr f280 harrop single 750dp 4500rpm stall/trimatic 4.56 gears/27" tyre

Light throttle slips a little as expected up to 2000-2200rpm....stomp on it in top gear flash stall is 4400rpm...with better tune/sorting flash point should come up more although for racing I'd want it higher with taller tyre.

But for street 3500-4000 stall would be as low as I'd want with 25-26" tyre & 3.7 gears.

Calculate slippage % & if possible hook up temp guage to see what is actually going on....4500rpm stally in mine cruises nice at light throttle untill you stomp it although my cousin had cheap 2800 stall behind a 360/727/3.7 gears that slipped to 2600rpm flash point.

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#7 Redslur

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Posted 17 October 2018 - 05:47 PM

3.5 gears is probably a better for your set up anyway with that cam It will drop things a tad too.

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#8 _Viper_

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Posted 17 October 2018 - 06:47 PM

Just reading up on the topic a little bit and says even though it could be say a 3500rpm Stall converter this is only when under full load and at cruise it shouldn't slip up to that speed?

So question is why is my converter slipping 12% while at cruise?

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#9 gtrboyy

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Posted 18 October 2018 - 01:11 PM

Engine out of tune can affect stall speeds too.

Drive along in top gear at a steady speed then boot it....look at rev tacho to see where it grabs,that will tell you what flash point/stall is for a better idea.

Some cheapie convertors or ones with wrong parts in them will sound like they're slipping all the time.