Autocross gearing - TCCoA Forums
 
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post #1 of 14 (permalink) Old 07-20-2010, 08:58 AM Thread Starter
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Question Autocross gearing

Hey guys,

Quick question I'd like to open up for discussion to those who have had more experience with these cars than I have (which is basically all of you!)

I'm gonna Autocross the 'Bird. Mine's a '94 LX, 4.6 car, 4R70W trans (obviously).

The engine will be in good health and stock (save for twice pipes, Flows, and maybe a tune if I can scratch the cheese together for an Xcal or SCT chip and dyno session).

The trans will be stock, except for a converter that won't balloon on me when I try to spin the motor, maybe a j-mod, and depending on the gear a forced-lube kit.

The diff will be what it be, probably a Torsen and whatever gears we determine will work best in a Mark 8 chunk.

I'm looking at running on Sport wheels and 245/50R16's eventually, A6's in 255/50R16 if they'll fit. For now I'll have fanblades with 235/60R15's, which are 26.1" dia.

Typical of autocross courses, there are plenty tight low-speed turns, and I'll want lots of torque multiplication to power the 3800-lb beast out of them. However, somewhat atypical of autocross courses, our club's course has one straightaway and a sweeper that we can carry some speed on (one guy has a Wankel powered DSR than can hit 70 through it, but a decent street car could probably hit 55 or 60). I hit 6800 RPM in Second in a borrowed Miata on that particular part of the course.

My desire is to have enough gear that I can stay in Second while negotiating turns and not have the car fall on its face when I pick up the gas on the way out, but not need the upshift to third for the fast sweeper. A considerable amount of time is lost in shifts, particularly in an automatic.

I would just throw a 5-speed in the car if it were just for me but two things hinder this: First, swapping the trans would throw me straight into the street prepared class, where the car would not have a prayer of being competitive. Second, my wife and stepdaughter also want to play in this car, and neither can drive a stick.

Any input you guys can throw my way would be greatly appreciated!
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post #2 of 14 (permalink) Old 07-20-2010, 10:16 AM Thread Starter
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Originally Posted by solo2dirtybird View Post

My desire is to have enough gear that I can stay in Second while negotiating turns and not have the car fall on its face when I pick up the gas on the way out, but not need the upshift to third for the fast sweeper. A considerable amount of time is lost in shifts, particularly in an automatic.
What about holding in First gear? Would that possibly work better with a higher final ratio?

Last edited by solo2dirtybird; 07-20-2010 at 10:23 AM.
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post #3 of 14 (permalink) Old 07-20-2010, 03:07 PM
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I can't speak for the converter, but if I was going to do gears - I'd go with 4.10's, definitely do a J-Mod, and a big trans cooler.

And I wouldn't hold first gear - I would go to 2nd and stay there. With 4.10's, and probably 3.73's, you'd be good in 2nd gear.

-Melon

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post #4 of 14 (permalink) Old 07-20-2010, 03:31 PM Thread Starter
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What kind of rev range does the 4.6 'Bird like in order to keep putting the torque down?
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post #5 of 14 (permalink) Old 07-20-2010, 04:15 PM
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3.90's

Hello

How about 3.90's?


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post #6 of 14 (permalink) Old 07-20-2010, 04:19 PM
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I can't speak for the SOHC 4.6, but from my experience, the DOHC 4.6 likes higher RPM's

If you look around at the mods other 4.6 owners have, most go with 3.73's or 4.10's.

-Melon

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post #7 of 14 (permalink) Old 07-20-2010, 06:24 PM
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Hello

How about 3.90's?

That would be great! About halfway between the 3.73 and 4.10.....

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post #8 of 14 (permalink) Old 07-21-2010, 09:24 AM Thread Starter
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Here's my train of thought on the idea, and maybe this would have been helpful to include in the OP.

First off, the car will be accelerating between 20 and 50-55 mph through various parts of the course. In order to go fast, I'll have to keep the revs up at those speeds at a level where the car doesn't fall on its face when I pick up the gas coming out of a corner.

We happen to know that Low gear in the 4R70W is 2.84:1 and Second is 1.55:1 What I'm not familiar with is what the NPI 4.6 likes in the way of revs that keep it in the meat of its torque curve.

The trans cooler goes without saying. I have a hookup on nice trans coolers. I will need to find a shop in my area (Evansville, IN) that I can trust to do the J-mod. The converter will need changed because I have a feeling that in order to keep the revs where they need to be I'll be ballooning the stock converter.

I found a neat tool online for calculating RPM based on tire size, speed, trans ratios, and third member ratios: http://hotrodstohell.net/rpm_tool/rpm.htm

According to it, in order for Second to be useful from 30-60 (2700-5500 RPM) I'd need a 4.56 gear. If I have to brake to 20 (which I will) the revs get killed down to 1800. Pushrod engines aren't especially effective at that speed so I doubt the SOHC motor will like it, either.

If I leave it in First, picking up the gas at 20 and accelerating to 50 has me revving from 2200 RPM to 5600 with the stock 3.08 gear. Maybe that is an option?

I don't intend to run anything lower than 4.30 as this car will not be towed and will see regular civilian street use.
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post #9 of 14 (permalink) Old 07-22-2010, 06:32 PM
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Quote:
Originally Posted by solo2dirtybird View Post
I found a neat tool online for calculating RPM based on tire size, speed, trans ratios, and third member ratios: http://hotrodstohell.net/rpm_tool/rpm.htm
Good find; thanks!

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post #10 of 14 (permalink) Old 07-23-2010, 08:18 AM Thread Starter
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Quote:
Originally Posted by tbirdtess View Post
Hello

How about 3.90's?

Hey tbirdtess:

I've got some friends that do the oval-track thing: Is the MN12 chassis well suited to that?

Also, I presume you run on pavement because that thing is too pretty to have run on dirt... what size oval do you run on?

Just curious...

Charlie
"We all know that small cars are good for us. But so is cod liver oil. And jogging." -Jeremy Clarkson

94 LX V8 - No longer slated for AutoX; I took the coward's way out and bought a Miata.
TODAY IN THE DIRTYBIRD SKUNKWORKS: Fixing drivability issues, rear swaybar endlinks, 2.5" duals w/ H-pipe, Flowmasters ,TCCOA spec alignment, 235/60R15s on fanblades
NEAR FUTURE: Chassis Bracing, ADDCO bars 1-3/8" F&R, hubswap, Mustang brakes, FR500's or similar BA-looking wheels.
LOOKING FOR: Mark VIII rear LCA's, differential housing and driveshaft, Marauder TC, Torsen Diff, Good Trans Guy, Good Dyno Guy
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post #11 of 14 (permalink) Old 07-23-2010, 09:27 AM
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You need a higher stall converter (3000+) to have any chance of pulling good out of the corners. 4.10's would also be nice. Locking the car in 2nd gear is your only option to avoid constant downshifting. That may be hard on the tranny because I believe that applies both 2nd and OD bands, and the drivetrain may "thump" every time get on and off the gas.

Thinking about autocrossing one of these cars makes my head hurt. That poor, poor, T-Bird doesnt deserve that kind of fate!

Al

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94 Supra TT Auto - street/strip car
04 CVPI- Brenda's car - 76k miles
Previous Fords:
95 T-Bird LX 4.6 - fully optioned, owned 15 years, 220k miles
96 Cougar XR-7 4.6- Brenda's car, owned 11 years, 187k miles
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Last edited by T6Rocket; 07-23-2010 at 11:47 AM.
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post #12 of 14 (permalink) Old 07-23-2010, 01:03 PM Thread Starter
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Originally Posted by T6Rocket View Post
You need a higher stall converter (3000+) to have any chance of pulling good out of the corners. 4.10's would also be nice. Locking the car in 2nd gear is your only option to avoid constant downshifting. That may be hard on the tranny because I believe that applies both 2nd and OD bands, and the drivetrain may "thump" every time get on and off the gas.
T6Rocket: Thanks for the heads up. I'm a little concerned about using too loose a converter and cooking the ATF. I'm currently planning on a Marauder converter unless someone suggests a more ideal solution. What's the "thump" from... driveshaft? I'm going Mark VIII d-shaft. Or is it trans mount? I'm going to put everything I can in poly or solid rubber just to firm it up.

Quote:
Originally Posted by T6Rocket View Post
Thinking about autocrossing one of these cars makes my head hurt. That poor, poor, T-Bird doesnt deserve that kind of fate!

Al
I'd gladly do a track day or a hillclimb instead , but our chapter doesn't have a venue open to us good for a hillclimb and the closest road course is inside of Nashville Superspeedway.

Please take solace in the fact that I'm not just stripping the car, making it a beater and then flogging the whee out of it. Quite the contrary.

My goal for this car is a very nice cruiser that's easy on the eyes with a motor that's properly tuned and as "unencumbered" as I can make it and still have it appear stock to the untrained eye. In addition it will be able to manhandle the twisties and just so happens to take full advantage of the rulebook and tech procedures for Stock class autocross

Charlie
"We all know that small cars are good for us. But so is cod liver oil. And jogging." -Jeremy Clarkson

94 LX V8 - No longer slated for AutoX; I took the coward's way out and bought a Miata.
TODAY IN THE DIRTYBIRD SKUNKWORKS: Fixing drivability issues, rear swaybar endlinks, 2.5" duals w/ H-pipe, Flowmasters ,TCCOA spec alignment, 235/60R15s on fanblades
NEAR FUTURE: Chassis Bracing, ADDCO bars 1-3/8" F&R, hubswap, Mustang brakes, FR500's or similar BA-looking wheels.
LOOKING FOR: Mark VIII rear LCA's, differential housing and driveshaft, Marauder TC, Torsen Diff, Good Trans Guy, Good Dyno Guy
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post #13 of 14 (permalink) Old 07-23-2010, 01:57 PM Thread Starter
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Quote:
Originally Posted by T6Rocket View Post
Locking the car in 2nd gear is your only option to avoid constant downshifting. That may be hard on the tranny because I believe that applies both 2nd and OD bands
Yeah, I double-checked Jerry's article and you're right, putting the shifter in "2" applies the OD band. Putting shifter in "1" applies the Reverse band and the forward clutch. Either way, it appears that I'm gonna be rough on a band. If my wife and stepdaughter could drive one I'd put a 5-speed in and be done with it, but sadly it's not the case. For now I'll put it in Low and take my chances I guess.

Charlie
"We all know that small cars are good for us. But so is cod liver oil. And jogging." -Jeremy Clarkson

94 LX V8 - No longer slated for AutoX; I took the coward's way out and bought a Miata.
TODAY IN THE DIRTYBIRD SKUNKWORKS: Fixing drivability issues, rear swaybar endlinks, 2.5" duals w/ H-pipe, Flowmasters ,TCCOA spec alignment, 235/60R15s on fanblades
NEAR FUTURE: Chassis Bracing, ADDCO bars 1-3/8" F&R, hubswap, Mustang brakes, FR500's or similar BA-looking wheels.
LOOKING FOR: Mark VIII rear LCA's, differential housing and driveshaft, Marauder TC, Torsen Diff, Good Trans Guy, Good Dyno Guy
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post #14 of 14 (permalink) Old 11-24-2010, 11:48 PM
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