i'm trying to find out where my 1/4 mile mph and time would be if i put 3.73's in my car, what would be the best way to do this?

i'm not sure what my 1:1 gear is, how do i find this out?

i have a 350TPI, 700R4 1989 Firebird Formula btw... thanks!

 

i have 3.73s and havent had my speedo corrected and at 40 mph and up it is about 20 mph off

3rd gear is 1:1 in a 700r4

 

oops, sorry about that i meant 1/4 mile time/mph.

but what do they mean by 1:1, ok its in third gear, but what is it exactly

i was looking in the tech articles, and i found the article on gearing... let me see if i have this straight, all i have to do is:

1) get the peak hp of my car at a particular rpm by putting it on a dyno.
example: 250hp @ 5200 rpm

2) multiply 5.2(the rpm) by my mph at 1000rpm(based on my gear ratio)
example: 5.2 x 27.35(mph for 2.73 gears @1000 rpm)

3) that equals 142.22mph, but this is my peak mph?

i'm a bit lost at this point, heres the link if anyone wants to see what i'm talking about: https://www.thirdgen.org/gears

 

3rd gear in 4 speed autos including: 700r4 4L60 4T60 4T80 4T40 th2004r is direct 1:1 ratio

the engine crankshaft is virtually spinning the same speed as the driveshaft

4th gear in a 5 speed

4th gear in a 4 speed

3rd gear in all 3 speed autos including: th175 th200c th250 th350 th375 th375B th400 3L80 th475 3L80HD




get it now ?

good luck

 

ohhh ok then, so since they are spinning at the same rate, everything after that, power is declining?

i have a built rear already, but i want to know if there is a formula to see what my 1/4 mile time will go to if i put that rear in my car, with 3.73 gears from 2.73 gears. this is what i'm asking, is it possible?

 

Depends on the 5 speed. Talking about a T5, then yes. Even getting into the 6 speed trannys doesn't mean the same. A T56 has 4th gear as 1:1 with 2 OD gears. A Richmond 5 speed is a true 5 speed with 5th gear as 1:1 while the richmond 6 speed has an OD as the 6 gear.

There's even a few transmisisons available that don't have a 1:1 gear. They come close but jump over 1:1 directly into an OD gear.

1:1 is considered your high gear. OD gears are not very strong and are designed to lower engine rpms while highway cruising.

Gear ratio choices will depend on operating range of the engine and tire height. Your 2.73 gears are going to be very low for 1/4 mile performance. Lets say you shifted at 4000 rpm. At the end of the 1/4 mile, you may be at 4000 rpm in high gear.

Since engine build more power at high rpm, going to 3.73 gears may mean you need to shift at 5500 rpm now to cross the finish line at 5500 rpm in high gear.

Now take of the stock height 26" tall tires and install some 28" tall tires and you're back down into the 4000 rpm range.

All numbers used as examples.

Have you taken your car to the track yet to see what it does with the 2.73 gears? Knowing your shift points and engine rpm, speed and transmission gear you're in as you cross the finish line will help determine a better combination.

 

oh i see, well basically what i know is that its probably a mid-high 14 second car, from what i've learned from the g-tech i've used. i shift at 5000 or 5200 rpm. i have 305 heads on my 350 block so its making 10:1 compression, i have headers and a 3" exhaust on the TPI

 

But you're choking a 350 engine trying to feed it enough air through castings designed to feed a 305. You need some better flowing heads.

 

Not necessarily - depends which 305 heads he's using. Some 305 heads breathe better than most 350 heads.

 

Yes, some 305 heads outflow some 350 heads, but now we're talking junk vs junk. The ONLY decent stock 305 heads are the tpi center bolts and the 305 vortecs. They are decent for a 305, but will not provide enough air for even a mild performance 350.

Also, a 350tpi with the proper heads has a realistic peak power of around 4800. With a cam and modded(aftermarket) intake there's more to be had, but realistically not much over 5500.

I would guess to say you're going by the factory tach?

Back to the topic - gearing largely depends on the engines powerband. 3.73's would be better in the 1/4 than the 2.73 for a mostly stock tpi, but would probably be too much for the 305 heads. 3.73's are usually the lowest used with TPI intakes.

 

i gotcha, well i guess we will see how it goes once i throw that rear in, it also has a power-tracks differential in it(which works like a detroit locker)... i'm trying to put the rear in, BMR subframes, BMR adjustable torque arm, BMR rear lower adjustable control arms, and a BMR adjustable pan hard rod all this summer. right now i'm spending all my time at an internship mon-fri so i would have to get it done over the weekend, but i also want to paint the car and do some minor body work haha..... so much i want to do but NO time to do it..... ugh. anyway, thanks a lot for your input guys, it really helped, as always.

as for the motor, i have 113 casting aluminum 350 corvette heads in my garage which i'm planning to put on my motor along with a super-ram intake, and i'm not sure about the cam yet. i'm planning to send those heads to TPIS, this is the link has the flow numbers the heads should make after they are worked: http://www.tpis.com/index.php?module...ted+Cyl.+Heads

 

a locker in a 10-bolt is a great way to break parts.......and an easy way to crash a car with c-clip axles.....

 

my friend, i have c-clip eliminators also in that rear, and its not a FULL locker, it disengages around turns

 

Yeahhhh, they sort of disengage, but not always. Just a little advice from someone who's ran a powertrax before. Wait til you're going around a turn and it suddenly catches and locks back up for a second. It gets your attention real quick, and it sounds like you just snapped an axle in half. Not to mention it causes the car to jerk violently sideways, which is real fun if it happens to be raining. Just watch out for telephone poles! I'm sure you don't want to hear this since you already spent the money for one, but they're garbage. I took mine out and sold it on ebay after putting up with it for several years. Also, before you throw more of your hard earned money away, think twice about having TPIS do anything to your heads. The prices they charge for anything they offer is completely ludicrous. Ask some of the big TPI gurus on here what they think about TPIS, and you'll see what I mean.

 

haha ugh all my dreams have once again gone down the drain LOLOL so funny, well i guess i should still keep the super ram though??? is that a waste of money as well??!?!? haha.........

well, the thing about the rear is that i bought an entire 1989 formula for 1200 off my friend with all that BMR suspension i listed above plus the rear, and body panels i need for my car. there is only about 1,000 miles on all that stuff....

 

I hope the Super Ram isn't a waste of money. Otherwise, we're both in the same boat! I bought one off here about a year ago that I haven't installed yet. Since you got the rearend with a whole car, you really didn't waste any money there. I was just relating my personal experience with the Powertrax unit. I've since gone to a regular clutch style posi, and I love it because it's so much smoother in operation compared to the locker.

 

haha ok then, i'll keep my SR plan hey since you have one, heres a post i made a while ago with a lot of good info on them and peoples experiences with them. https://www.thirdgen.org/forums/alte...-superram.html