no_name

I am building a 12a stockport atuocross/daily car and thinking about messing around with the rear end. I am thinking about using 4.44 or 4.77 rear end gears. Now the car will be bone stock except for suspentin, and the rear end.

My questinion is will I be out matched/ bumped up a class to high in my 12 a, with suspention and rear end mod, with stock exhaust?

I am also wondering which gear ratio would work well with that lay out. From what I have researched it seems that 4.77 is to big for daily driving. I do a fair bit of highway driving, and I am wondering what the actual top speed of a 12a stockport, with 4.77 gears, and 4.44 gears would be, with stock exhaust?

Thanks Ted.

aussiesmg

iTrader: (1)

4.77 will increase your RPM by approximately 20% in each gear, so if you were running 3000 rpm at 60 mph now it will be about 3600, this is not too high for an FB. The acceleration difference will be marked.

no_name

intresting... Will top speed be affected? I figure it should be unchanged or maybe raised, because these cars produce more power at high rpms. Before the car mazes out at let's say 4000 rpm in 5th now it will max out at almost 5 000rpm. At this rpm the car will produce like 7 more hp??

How much better is teh acceleration, will it shave lets say 2 seconds of the 0-60mph time??

Thanks

onslaught_81

bump for this thread also very interested in this info....

trochoid

iTrader: (6)

Shorter gears will reduce top speed, usually getting you there quicker. Taller gears will increase top speed, to a point.

no_name

intrsting... How much aof a difference will there be. A stock 1985 GSL tops off at 110. With this mod where would it top off at??

And thanks for the bumb

trochoid

iTrader: (6)

Maybe your GSL tops out at 110 mph, but one in good running conditon will top out higher. Though about 100-105 they start to float with the stock front body panels.

The 'to a point" in my last post relates to hp and aerodynamics. At some point the hp cannot overcome the drag that the air creates on the body. Theoretically, if you put very tall gears in, you could top out at XXX mph, but that theory only works in a vacuum.

I can't answer your question because you provided no info on gears, and I won't answer it because it is simple ratio math. Divide your current gear by the new gear and multiply by your speed, That will give you your actual speed, at the same rpms. This only works if the same rear tires are used in the test. With my 245/50/15 rear tires, I need a 4.44 gear to just to have an accurate speedo.

no_name

Well I would bve using stock trany, and I am jsut wondering how much the top speed will be affected. I calculated that the same rpms as traveling 110 mph i would achive 90mph. I am wondering if this is teh new top speed. I figure i should be able to still hit 110mph or atl least 100mph, because now i am closer to peak power out put, thus i would get a bit more power then before no???

jgrewe

iTrader: (1)

With a 4.87 gear one of my FBs made it to 131 mph at Pocono. This was figured by rpm and gear info not speedo. Stock ports, SCCA ITA car.

DriveFast7

iTrader: (1)

4.444 is the de facto go fast gearset for street cars. Yes, it will limit top speed but you really should not be driving around at 150 MPH anyways.............................

The motor really revs up faster with those gears. Very worth it.

no_name

When i say top speed I don't mean overal top speed. I mean the top speed of a stock port GSL with this mod and stock exhaust.... just thought it might clear things up. Drom what I read the top speed of a stock 1985 GSL was 120mph, which is very close to my 110mph. I am wondering if I do the 4.77 rear end mod my top speed will become something like 90mph.

I would concider 4.444 rear end gears but they are harder to find from a junk yard, and I am not sure how much faster the car will accelerate. I know the 4.77 rear end will shave around 2 seconds of teh quarter mile. How much would 4.44's take off the quarter mile, and what vihicles can i take them out of from a scrap yard?

Thanks

Pele

iTrader: (28)

There is another thread with that info. Look for Kia Sportage.

EDIT: Continue discussion here: https://www.rx7club.com/1st-generation-specific-1979-1985-18/pics-kia-sportage-gear-pinion-506638/

no_name

Thanks I am keeping an eye at that one too lol. I am a bit confused if the Kia pumpkin works with LSD or not, if any one could help with that I would apritiate it...

jgrewe, did your car really make it to 130mph with a stock port engin?? What mods did you do it?

no_name

bump

gsl-se addict

iTrader: (3)

As trochoid said, your top speed is a balance between power and drag. You will top out where the drag on the car matches the power the engine can make. This is called a drag limited case. You can also have an RPM limited top speed. In this case, if you have very low gears and/or lots of power, you will hit the redline in 5th gear before the aerodynamic drag matches power.

The gears won't drop your top speed from 110 to 90, per se. This would only occur in the RPM limited case. The top speed should be similar as before, provided you don't run out of RPMs up top. It really comes down to engine characteristics, driveline loss, and aerodynamic drag when determining a drag limited top speed.

no_name

intresting.... I get it now thanks all.