Rezi1987TA

I have a 87 TA with borg warner 9 bolt 3.45 gears. I was thinking of putting higher gears. Which would you recommend? Would 4.11's be too much? It's not for drag racing. It's for street use mostly. Now my fifth gear is useless. All I care about is performance :-)
Need advises about which to buy: 3.7, 3.9 or 4.11
And would the 5th gear become usable and what would the top speed
be?

thanks

SR-71

To get your top speed, take your present top speed and multiply:

• For 3.73 gears, multiply by 0.925.
• For 3.95 gears, multiply by 0.873.
• For 4.11 gears, multiply by 0.839

So, if your present top speed is 150 mph (240 kph), with 4.11 gears your top speed will be:
150 x 0.839 = 126 mph (202 kph)

When your 5th gear becomes usable has everything to do with how your engine is set up. My 'Bird has 3.08 gears, AND the 0.61 5th gear option (normal is 0.74). But mine cruises very comfortably at 55 mph (88 kph), with the engine turning only 1400 rpm! At 75 mph (120 kph) on the interstate, it's turning only 1950 rpm. My engine is tuned for low-end torque with a very mild cam for its displacement.

Mine was originally like yours, with a 305. But in stock tune it ran very nicely at 55 mph in 5th gear.

If your engine is set up for high-end horsepower, with a big cam, then it depends on how "big" the cam is.

With your speed limit of 80 kph, if your engine tune is stock, then your 5th gear should not be "useless." It's not intended to be a performance gear. It's for highway cruising at steady speeds.

Rezi1987TA

Thanks for the reply.

My enginge is modified.
It has a Crane Cams 104224 Camshaft (http://www.summitracing.com/parts/CRN-104224/)

and that's about it. When I shift into 5th it just drops the RPM too much and it doesn't pull that hard.
I hate that 5th gear :-(

SR-71

The camshaft is the issue. The duration of 214/220 is actually quite radical for a street 305. Smaller engines like the 305 are affected much more by camshaft changes than larger engines.

In my original 305, I had a CompCams cam with 212/218 duration, very similar to yours. With that cam, the torque would not come on until about 3500 rpm, then it would pull strong. I got about 360 horsepower from it, but low-end torque was useless. 5th gear was not good until I got up to about 100 kph.

With that cam in a 305, and with your low speed limits, I would recommend 4.11 gears. That should make your 5th gear usable on the highway, since the cruising rpm at your speed limit will be about 2000 rpm with 4.11's.

And even then, 5th gear won't pull very hard. It's for highway cruising, not acceleration.

Rezi1987TA

And in that case what would the 5th gear top out at? let's say at 5000rpm? And would overall acceleration be a lot better or slightly better?
So what I'm asking is whether it's worth doing or not...

SR-71

To see the affects of gear changes, there is a very good rpm/gear/speed calculator at
http://www.wallaceracing.com/gear-speed.php

Figuring you have the stock size tires with 26 inch diameter, 0.74 5th gear ratio, and 4.11 rear, your speed at 5000 rpm would be about 127 mph, or 204 kph.

You would get a good improvement in acceleration. 4.11 gears should shave off about half a second from your quarter-mile ET.

Whether it's worth doing or not depends on what you want from the car.

jmd

iTrader: (4)

With the stupid TPI intake, you're gonna choke up high.

My L69 with a cam, edelbrock egr intake, stock exh./ hollow cat, .63 5th gear and. 3.73 gears would pull up to 140mph. I would shift a touch past 6k at the highest. What was nice about it was the usable mid to high rpm pull w/ the q-jet.

I have TPI on it now and can make that first statement.

Keep reading on what makes for an optimal engine powerband and cam selection, toss the tpi intake for a 4bbl w/ injector bosses and build an effective combination.

If you want a nice top speed, 3.7 gears are as far as you need to go. 5th is easy to change the ratio of, too

Rezi1987TA

:-( 200kph is VERY SLOW lol
I go over that pretty often :P
I need AT LEAST 240kph :-| I guess I'll stick with my 3.45s for now and later get the 6 speed
Thanks for your replies :-)))

SR-71

You need the 5th gear to make your top speed goal, so you'll just have to be happy with cruising around in 4th gear for normal speeds. With a T-56, you might be able to drive in 5th, since the T-56 5th gear isn't as tall. But you'll be even more frustrated with the 6th gear, because it is taller yet.

I also think, though, that with 3.45 gears, you'll be hard-pressed to make 240 kph with a 305. It's very rare to be able to turn 5000 rpm or faster at top speed. You'll more likely make that speed with 3.23 gears, and/or an upgrade to a 350 or bigger engine. At least you have a Firebird instead of a Camaro, so you have less wind resistance. But your Firebird is also a TransAm, meaning it has a bunch of extra weight and wind resistance from all those air dams and skirts.

My top speed at just under 300 kph comes from 3.08 rear gears and that 5th gear, along with gobs of raw torque. Plus mine is a Formula, which is the F-body model with the least-ever wind resistance. Yet about 4500 rpm is all the engine can do in top gear, even though it's making over 450 ft-lbs at that speed.

Good luck with your project. And above all, have fun with it.