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Old 12-06-2005, 14:32   #1 (permalink)
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What diff gears for my combo?

My combo is:
351
238 cam @.050
3300-3500 stall
3.5 gears
235/60/14
Runs 12.9ET with flat 2sec 60’s on part throttle launch due to lack of traction.

Question: Would changing from 3.5 to 3.9 or so improve my ET? Or would I lose top end speed and run out of revs? I change at 6000rpm.
Would tyres or gears be the better investment?
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Old 12-06-2005, 15:14   #2 (permalink)
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Re: What diff gears for my combo?

Gears = even less traction. 4.11's work well with F238's. They also cross the line @ max rpm which seems ideal.

I would suggest tyres to help with your 60's. Another problem you may experience once you get traction is bog down. It's like a delay it'll sit there reving wasting time and hurting your 60's. I used to have it when I ran a 3500rpm convertor on the old 351 setup. The 4500rpm convertor solved it.

Basically bog down is pulling the cam down below it's power range on take off.

I also experienced it for the 2nd time with a 3300rpm convertor wihen I converted to a glide. 1.9-2.2 60fts but still managed 12.1/12.2's. A larger convertor solved it once again.

[edit]what rpm do you stop making power?
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Old 12-06-2005, 15:17   #3 (permalink)
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Re: What diff gears for my combo?

Quote:
Originally Posted by northiam
My combo is:
351
238 cam @.050
3300-3500 stall
3.5 gears
235/60/14
Runs 12.9ET with flat 2sec 60’s on part throttle launch due to lack of traction.

Question: Would changing from 3.5 to 3.9 or so improve my ET? Or would I lose top end speed and run out of revs? I change at 6000rpm.
Would tyres or gears be the better investment?
The best thing for your combination would be a 1.60 sixty-foot time. You need to improve traction first and foremost if you're looking to improve your ET. Add a pair of Caltracs or SOMETHING to get it under control. You'll be really, really glad that you did. You can easily drop that ET down to the mid to lower 12s with improved traction.

Once you make more traction than your engine can overpower, then start thinking about gear combinations that will increase your launch (lower your sixty). Good traction may require different tires, perhaps even slicks. When you start breaking parts, it is time to go on a diet (car and, at least in my case, driver!).

Lose 100# in the car. The preferred way is re/moving weight from in front of the COG (center of gravity) point. Lower unsprung weight wherever possible. If relocating weight, move it behind the rear axle or directly over the rear axle if possible.

Be sure to make your safety checks along the way to ensure that you're safe, eg: driveshaft loop, SFI-rated axles...and then borrow a set of slicks from a mate and have a blast down the track.

If you're short of funds, play around with your launch so that your tires do not slip at all as you're accelerating away. Use less and less throttle until traction is on that precarious edge of going away. At that point, you can't accelerate any quicker. If your tires are slipping "now," you could be accelerating more quickly with proper driving of it so that there is no traction loss.

If you still can not gain traction, change to a higher gear ratio (lower numerical value). Try moving from a 3.50 to a 3.23 or a 3.00. You'll get better fuel economy and perhaps not overpower the tires so easily. If you can afford a gear change, you can afford Caltracs (assuming you have leaf springs). Caltracs really work and will help your car hook. I ran 11.02 in a 2950# car with Caltracs, adjustable Rancho shocks and BFG drag radials. The car did have a full cage and subframe connectors.




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Old 12-06-2005, 18:51   #4 (permalink)
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Re: What diff gears for my combo?

Quote:
Originally Posted by davis
The best thing for your combination would be a 1.60 sixty-foot time. You need to improve traction first and foremost if you're looking to improve your ET. Add a pair of Caltracs or SOMETHING to get it under control. You'll be really, really glad that you did. You can easily drop that ET down to the mid to lower 12s with improved traction.

Once you make more traction than your engine can overpower, then start thinking about gear combinations that will increase your launch (lower your sixty). Good traction may require different tires, perhaps even slicks. When you start breaking parts, it is time to go on a diet (car and, at least in my case, driver!).

Lose 100# in the car. The preferred way is re/moving weight from in front of the COG (center of gravity) point. Lower unsprung weight wherever possible. If relocating weight, move it behind the rear axle or directly over the rear axle if possible.

Be sure to make your safety checks along the way to ensure that you're safe, eg: driveshaft loop, SFI-rated axles...and then borrow a set of slicks from a mate and have a blast down the track.

If you're short of funds, play around with your launch so that your tires do not slip at all as you're accelerating away. Use less and less throttle until traction is on that precarious edge of going away. At that point, you can't accelerate any quicker. If your tires are slipping "now," you could be accelerating more quickly with proper driving of it so that there is no traction loss.

If you still can not gain traction, change to a higher gear ratio (lower numerical value). Try moving from a 3.50 to a 3.23 or a 3.00. You'll get better fuel economy and perhaps not overpower the tires so easily. If you can afford a gear change, you can afford Caltracs (assuming you have leaf springs). Caltracs really work and will help your car hook. I ran 11.02 in a 2950# car with Caltracs, adjustable Rancho shocks and BFG drag radials. The car did have a full cage and subframe connectors.




:davis:
gday davis,
i know im getting off the topic, but, i was wondering if you can answer a couple of Q's?
did you use the top or bottom hole?
did you have an air gap or preload on the bars?
what did you have your ranchos set at?

cheers,
justin
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Old 12-06-2005, 20:00   #5 (permalink)
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Re: What diff gears for my combo?

Quote:
Originally Posted by northiam
My combo is:
351
238 cam @.050
3300-3500 stall
3.5 gears
235/60/14
Runs 12.9ET with flat 2sec 60’s on part throttle launch due to lack of traction.

Question: Would changing from 3.5 to 3.9 or so improve my ET? Or would I lose top end speed and run out of revs? I change at 6000rpm.
Would tyres or gears be the better investment?
Are you going to only run street tyres, or step up to slicks?
What rpm/mph do you cross the line at now. With that height tyre and crossing at 107mph, i would guess you are crossing the line at 5800rpm??Which means 3.9 would be a good idea, but especially with some sticky tyres which would be heigher height than you have now.
26 inch slicks, with 3.9 gears would be a good start, and still good to drive in street trim.
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Old 12-06-2005, 20:26   #6 (permalink)
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Re: What diff gears for my combo?

Quote:
Originally Posted by debacle-racing
gday davis,
i know im getting off the topic, but, i was wondering if you can answer a couple of Q's?
did you use the top or bottom hole?
did you have an air gap or preload on the bars?
what did you have your ranchos set at?

cheers,
justin

Sorry Justin...memory leaks. I don't recall the slightest. I probably knew exactly at one time, but since I sold the car probably three or four years ago, it is as far away as you are from me these days.

I will tell you what I do recall. I definitely preloaded the bars. Probably about turn and a half to two turns on one side and about a half turn less on the other side. I think that the Ranchos were set right in the middle setting. It was never my intention to drive the car, just sell it.


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Old 12-06-2005, 20:49   #7 (permalink)
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Re: What diff gears for my combo?

I find it useful to be able to calculate the revs you'll be pulling using different combinations, this way you can match everything to suit the cam's power band.

Hope this tool helps . . .

FREE Calculator/Converter : Version 2 is now available
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Old 12-07-2005, 05:08   #8 (permalink)
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Re: What diff gears for my combo?

Quote:
Originally Posted by davis
The best thing for your combination would be a 1.60 sixty-foot time. You need to improve traction first and foremost if you're looking to improve your ET. Add a pair of Caltracs or SOMETHING to get it under control. You'll be really, really glad that you did. You can easily drop that ET down to the mid to lower 12s with improved traction.


Be sure to make your safety checks along the way to ensure that you're safe, eg: driveshaft loop, SFI-rated axles...and then borrow a set of slicks from a mate and have a blast down the track.

:davis:
Some interesting comments.
I dont think calctracs are street legal? (in oz)
How many people here run drive shaft loops?

Quote:
Originally Posted by xdclevo
Are you going to only run street tyres, or step up to slicks?
What rpm/mph do you cross the line at now. With that height tyre and crossing at 107mph, i would guess you are crossing the line at 5800rpm??Which means 3.9 would be a good idea, but especially with some sticky tyres which would be heigher height than you have now.
26 inch slicks, with 3.9 gears would be a good start, and still good to drive in street trim.

I prefer to get some street legal tires, as street trim is what im interested in!
Thou it would be fun to try some slicks.

I have no idea what rpm I cross the line at! But I wouldnt have thought is would be as high as 5800?

I dont know about living with 3.9 gears on the street! What is your revs @ 100kph?

Quote:
Originally Posted by brenx

[edit]what rpm do you stop making power?
about 6100-6200
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Old 12-07-2005, 05:22   #9 (permalink)
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Re: What diff gears for my combo?

Caltracs = not legal in Oz for street use due to solid mounts.

I run a driveshaft loop as required by ANDRA. It's to be no more than 6 inches back from the front uni. Mines tucked away quite nicely. I don't think it's visable when driving behind me.

3.9/4.11's is an optimal diff ratio for crossing the line @ max rpm/power. As you say though you wouldn't personally want them on the street. I run 4.11's on the street. You do 3350rpm @ 100kph on my 16" rims with 255/50/16" tyres. It's and extra 50-100rpm rpm with 15" rims and tyres. For me it's not that bad as I have a fair amount of insulation/sound deadening in the car. I just cruise at my own speed. I'm not in any great hurry when driving the XB. I just enjoy the ride.
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Old 12-07-2005, 05:46   #10 (permalink)
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Re: What diff gears for my combo?

Quote:
Originally Posted by brenx
I run a driveshaft loop as required by ANDRA.
I wouldn't want to drive hard without one, street or strip, and din't fit for ANDRA, but mainly street safety purposes. There ain't no way i am getting a shaft through the floor or do any endos. Its a pure safety item for me.

northiam, i have the same street tyres as you, but with 3.9 and at 100km/h mine revs at around 3100rpm.
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