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Old 06-29-2004, 06:13   #1 (permalink)
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Need Help With 9 Inch Gear Ratios

Looking at changing diff ratios. My 3 that I have narrowed it down to are
3.7, 3.89, and 4.11 .
The car is xa gt coupe 4 speed top loader 393 stroker engine and tyres will be around the 295/50-15 size . car will be 50% road 50% drag/club event car
Any advise or experience would help
thank you
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Old 06-29-2004, 06:19   #2 (permalink)
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Re: Need Help With 9 Inch Gear Ratios

with a tyre of just over 26.5 inches it would be good to know what revs you intend to rev the engine to for it to run a good time on the 1/4

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rod
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Old 06-29-2004, 06:24   #3 (permalink)
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Re: Need Help With 9 Inch Gear Ratios

The engine is not finished and the cam has not been brought yet . But I am between 2 different cams that work 3800-7000 and 4000-7200 not sure if this will help (new to this)
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Old 06-29-2004, 07:15   #4 (permalink)
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Re: Need Help With 9 Inch Gear Ratios

The 295/50x15's will give you a noticeable RPM drop @ 100kph. Doesn't seem like much but with 4.11's @ 3200rpm an extra 200rpm needed at 100kph with 265's feels like heaps! Around town 4.11's are fine. They suck on the freeway with toyota hatchbacks passing you. You'll generally cruise at 3000rpm doing about 90.

Even with the 4.11's @ 7000rpm (295/50's) you'll be doing 134mph. At 6500 you'll be doing 125mph. EITHER one of those is a huge MPH for crossing the finish line at the strip.....reflective of a 10 sec car. SO..........with a higher diff ratio, you will definitely be passing the line well below your peak RPM and not optimising your set up.

Personally I say go the 4.11's!! sounds like you'll be doing a bit of racing and I reckon you'll only kick yourself with anything higher. Get that big heavy XA hauling some mail!

Here's a driveline link you can play with. (transfer case ratio = 1, 4x4 thing!).

http://www.chrismans.com/Calculators.html

Cheers.

PS: let us know when you'll be running it!
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Old 06-29-2004, 07:30   #5 (permalink)
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Re: Need Help With 9 Inch Gear Ratios

Thanks TruBlu351 4.11s sounds good just a bit worried about driving down to the motorplex and on club runs thats why I was thinking about 3.89 .But also want to run good 1/4 times
Also the car has just got back from being painted so long time until its back on the road
Hope to be ready for xmas or at least by big als poker run
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Old 06-29-2004, 08:23   #6 (permalink)
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Re: Need Help With 9 Inch Gear Ratios

I have 3.89's in mine, on the freeway at 100 km/h its sitting on 3000 rpm, and thats right on the verge of being too high for my liking.
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Old 06-29-2004, 15:58   #7 (permalink)
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Re: Need Help With 9 Inch Gear Ratios

4.11's is roughly 3600rpm @ 100kph. Which isn't a drama for a decent engine. Going by the cams your looking at your minimum stall is 3500rpm anyway. I've done plenty of k's on 4.11's. The difference between 3.89 and 4.11 is two tenths of f'all but on the strip is means doing the numbers.

Just to be a pain I'd also speak to an engine builder and see what they think the finalk ratio should be. I am betting 4.56 is better suited 4.11 is a comprimise.

My mate drives around with 4.11's and a 3500rpm convertor driving all over Vic. Drives to and from the strip It'll be at the next FF drag day.

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Old 06-29-2004, 20:38   #8 (permalink)
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Re: Need Help With 9 Inch Gear Ratios

Remember that a 393C will make around 10% more torque across the rev range, and will peak at around 10% less revs.
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