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4.875 gears on my GSL-SE

Old 12-02-05, 01:56 AM
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4.875 gears on my GSL-SE

I've got an '85 GSL-SE that I am slowly molding into a track car (track = the kind that involve turns lol). In the meantime however, it is still being used as a daily driver.

I was looking through the available list of gearsets available from Mazdatrix, and saw that between the 4.44, 4.875, and 5.12, the 4.875 were cheaper. How would the 4.875 set work on my car? I do mostly city driving, but some freeway driving. However, I like to obey the speed limit nowadays (speeding tickets + youth = high insurance). I'll usually cruise on the freeway at around 70mph, which equates to about 3000rpm.

As for power, my car is currently running the stock ports with the aux ports wired open. I have a header, and a collector into a 2.25" single exhaust all the way back, so I'm not making too much power.

I am, however building an engine to go along with an IDA throttle setup, and hoping to make over 200rwhp.

How would the 4.875 gearset compare with the different amount of power I have now vs. in the near future?

As for the tracks I will be getting to: most of the ones in Southern California. The fastest one we have down here is WSIR. I was there last year in my dad's '95 M3 and was getting up to ~125mph on the back straights. However, the car is a full ~3200lbs, has shitty tires and doesn't have much power (only mods are intake, chip, and cat-back). If I was able to exit turn 6 & 7 faster, I'm sure I could have squeezed another 5+mph down that straight.

I know it's hard to say, but might I be able to reach those speeds with the 4.875 gears on my current car at the stock 7000rpm redline?

Thanks in advance for your time

*edit: BTW, I will be running either 50 or 60 series tires on a 14" wheel.

Last edited by Infini IV; 12-02-05 at 02:00 AM.
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Old 12-02-05, 04:46 AM
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We use a 5.12 in our ITS 2nd gen. The difference between the stock 4.10 and the 5.12 isn't something you feel as a night and day difference in power to the ground or even pop out of a slow corner. The big advantage of the 5.12 over the 4.10 is that it requires you use 3rd, 4th, and 5th gears versus 2nd, 3rd, and 4th. What this does is tighten up the gear spacing and does away with the long 2-3 shift that dips the engine out of the meat of the powerband.

I'd do a 4.88 in a street car but don't expect it to feel like you've bolted on another 50hp.
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Old 12-02-05, 04:49 AM
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4.87 gears are good for coming out of the turns but how long are the straits and how fast would you be goin out of the turns. but i think the 4.44 would be better, because with that high of a ratio that to me sounds more like an autox type of gearing such as the 5.12 which would be good for drag. now if it is a long track with a good size strait i would go with 4.44 but if it is a short track then i would go with 4.87. this is the track where i live http://putnampark.com/images/track.gif it is 1.766 miles long for this track the i would use 4.44 gears
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Old 12-02-05, 06:16 AM
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Hmm, interesting although both of you guys have FC's.

Well yes, I wanted the higher gears because I figured I might as well use all the gears to my advantage since I'm not exactly making so much power. I was wondering if when I have finished building the more powerful engine, will the 4.875 gears limit me? By limit, I'm encompassing the idea that the rear suspension on the 1st Gen is limited to a point. If I were to make 50% more power than the stock engine put out, would I be able to use it all with such high gears?

As for the fastest track I will be headed to (WSIR), http://www.willowspringsraceway.com/...adCourseLg.htm

Many faster street cars on street tires are able to get up to and over 140mph and take turn 8 at over 120mph. The course is 2.5 miles long, with a 0.5 mile long front straight and a "back straight" which is slightly longer. Turns are pretty much all high speed except for turns 3 and 4.

I'm using this course as a basis because I figure I'll only want to buy 1 set of gears, and I may as well buy a set which will allow me to reach as high a speed as possible within rev limits on the particular course. A compromise indeed.
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Old 12-02-05, 01:39 PM
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Originally Posted by rx7b13
4.87 gears are good for coming out of the turns but how long are the straits and how fast would you be goin out of the turns. but i think the 4.44 would be better, because with that high of a ratio that to me sounds more like an autox type of gearing such as the 5.12 which would be good for drag. now if it is a long track with a good size strait i would go with 4.44 but if it is a short track then i would go with 4.87. this is the track where i live http://putnampark.com/images/track.gif it is 1.766 miles long for this track the i would use 4.44 gears

If the car is setup stricly for road racing then 5.12 is the gear. It is good for a bit over 140mph with the GTUs .756 OD gear and 225/45-15 tires considering we'll rev the engine to 8500+. You won't see that at too many tracks. There are only two tracks where IT prepped cars might run out of gear that come to mind and that's Pocono and Daytona. At those tracks the 4.88 seems to be the way to go from what I've heard. The 5.12 is not an autox gear. Frankly it's too short to autox with. Puts most courses between 2nd and 3rd.

FWIW, our car puts 175+ to the wheels. The front straight at IRP is 5/8 of a mile long with a third gear corner leading on to the front straight. You enter the straight at around 50mph. We see a bit over 120mph at the end of the straight which leads into a 5th gear right hander. We still have plenty of gear left with the 5.12. With any kind of NA powerplant, regardless of porting, you are not going to run out of gear with the 4.88. I've driven a GT35R equipped 3rd gen on the same track with Tein suspension, etc, etc. Though cornering speeds were off some compared to my car the 3rd gen put about twice the power to the wheels compared to my car. With the twice the power we'd top out at around 130mph on the same straight.

Want to know what your top speed limit will be. Use this calculator.

http://www.race-technology.com/WebPa...lculation.html

Last edited by C. Ludwig; 12-02-05 at 01:44 PM.
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Old 12-02-05, 02:35 PM
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Chris,
I don't know about Pocono, but Daytona is no problem with the 5.12.

Infini,
I think you need some IT7 guys to chime in, instead of us FC ITS guys. Some of them run the 4.88 and some the 5.12. One guy down here jsut swap out his 4.88 for a 5.12. to be quite honest, I didn't see a difference in his lap times. IF anything, he went backwards. With the shorter tires, the 4.88 might be the way to go. I'll try to call him and see what he thought after the swap.

BTW, I've driven a street ported S5 on the street with a 4.88 and ISC road race exhaust. It wasnt' that bad, just a little loud.

Jim
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Old 12-02-05, 03:09 PM
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Originally Posted by its66
Chris,
I don't know about Pocono, but Daytona is no problem with the 5.12.

Jim

I hope to find out someday.
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Old 12-02-05, 03:47 PM
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Come on down. It's still over 8000 in 5th gear at the stripe. You owe it to yourself to do the trip at least once. Just keep that haffmaster guy away...he is too fast...LOL
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Old 12-02-05, 06:04 PM
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I run the 4.88 with an FC also, just because it was cheaper. If i had the money, I'd would have went with the 5.12. Everyone I talk to says to run the 5.12. I doubt you will run out of gear at Willow (I am assuming Big Track). Use the gear calculator, but you should be approx 4000rpm at 70 mph in 5th depending on tires. I would think running it on the street, you'll be going slower to attract less attention, especially in California where they like to crack down on modified cars now.

-Jack
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Old 12-02-05, 08:20 PM
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Thanks guys! The gear calculator was just what I was looking for!

Actually, it's good to know how the FC's do with different gears as well. There are many more NA FC tranny's I can find than good condition 1st Gen tranny's.
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