QwkTrip

iTrader: (22)

Anybody have any good reading to help me choose a rear gear ratio for my car? I have a manual trans and it seems everything I have found is concerned only about reaching peak horsepower at the top of 4th gear across the traps (1/4 mile racing).


Surely it must be more complex than that?

articwhiteZ

iTrader: (56)

its a combo of parts. not just the tranny..

MoJoe

iTrader: (5)

What criteria makes it "best" for you? Lowest ET? Street manners? Fuel economy? All of the above.

For 1/4 mile drag, it does seem as simple as what you wrote. If you have to shift into 5th, then the gear is too low (Ex. 4:56s) for your power. If you don't wind out 4th, the gear is too high (Ex. 3:23s). If you are planning to add more HP, then the latter gives you room to grow into the high rear gear.

jmd

iTrader: (4)

Depends on what engine powerband and gear ratios your trans. has.

If you're concerned with basically what late 4th owners had in their cars, might wanna read some posts on tech.

AlkyIROC

That's as simple as it gets.

For drag racing, you want to cross the finish line at or just after you hit your highest shift point. Technically, your shift light should come on a few hundred feet before the finish line. What gear ratio is best depends on tire height and with an automatic transmission, converter stall speed. It also depends on the engines rpm range. An engine that peaks out at 6000 rpm needs a different gear than one that peaks out at 7500 rpm. Same goes for a car with 26" tall tires compared to one with 28" tall tires. What gear ratio works well with one car may not work well with another.

My car has 32" tall tires and 4.86 gears. The converter stalls around 6000 rpm. I shift and go through the traps at 7400. One weekend I tried 4.57 gears and the converter stall wasn't correct. The car ran slower. Stall speed should have been lower for better performance. I put the 4.86 gears back in and performance picked right back up. I'd love to try something like 5.13 gears but feel they may be a little too much.

For road course racing where full top speed is never reached, a very deep gear for torque multiplication to get you out of the corners quicker.

For long highway driving, a shallow gear in the mid to low 3.xx range will give good performance and reasonable mileage.

For high speed racing such as the Silver State challenge, Bonneville etc, a very shallow gear like 2.50 or worse will give poor acceleration but very high top speeds.

That is one of the advantages of the 700R4 transmission and cars with the crappy 2.73 gears. The transmission has a very low first gear to help get the car moving quickly when the diff gears can't do it. Combined with the overdrive and 2.73 gears, the rpms are very low during highway driving for good fuel economy.

QwkTrip

iTrader: (22)

I'm not changing transmission or tire diameter so those things are fixed. The only thing I would do is change rear gear ratio. (T56 manual so there is no torque converter)

My cam has a large lobe separation angle with a VERY WIDE torque curve. Cam has not peaked yet at 6800 rpm (rev limiter). Seems to me the car would work better with more gear. I went to the drag strip and learned that I am at 6100 rpm across the traps with 3.73 gears.

If I change to a 4.10 gear (12 bolt axle) then it would put me right near the rev limiter thru the traps. Also, I could have +25 RWHP going into each gear if I raised the rev limiter another 500 rpm, but I'm not sure the T56 trans can handle shifts at that speed. The alternative is that I advance my cam to bring in the power a little earlier.

You can tell that I'm weighing my options to make the car faster....

MoJoe

iTrader: (5)

I bet a shot of nitrous would bring the RPM from 6100 up to 6800!

Is it the "speed" of the engine/shifts/whatever that would kill a manual trans? Or just the total power/torque applied and traction/resistance that applies the force to the guts?

QwkTrip

iTrader: (22)

The T56 synchros don't work at that speed. It literally will not shift until the speed drops low enough, and it ruins the trans in a hurry. Need to faceplate the gears if you want to run a T56 that high.

AlkyIROC

He could always short shift the lower gears but would still need a deeper diff gear like 4.10 for better performance. I wouldn't recommend 4.56 unless you are running at least 28" tall tires.

MoJoe

iTrader: (5)

Thanks for the clarification

92RSZ

iTrader: (6)

The t56 absolutely will shift at 6800-7000 rpm. I have one in my car and it's been shifting fine at those rpms with 500+ horsepower for 10,000 miles. Make sure you have a good clutch and master cylinder and you're good to go. 3.73 gears if you're boosted. 4.10s if youre n/a.

QwkTrip

iTrader: (22)

I'm talking about lifting rev limiter to 7300 - 7500 rpm. I already shift at 6800 rpm today.

JoeMonte

he already has an incredibly good clutch, if I remember correctly.

jmd

iTrader: (4)

Time to ditch the standard T56 and go with a C5 hybrid build or a TR6060 based unit so you have triple and double cone synchros.

Orr89RocZ

iTrader: (20)

Thats seems strange i have not heard that before

AlkyIROC

Go with a Lenco or a Jerico

skinny z

You may want to consider working on your 60' time.
A tenth in the 60 is worth two tenths at the top. At least that's a general sort of thing for this power level. That might drag your MPH along with it. And with that so goes the rear gear.
Now it gets complex. Add more gear, increase the traction issue. Work on traction improvement, re-evaluate the gear. And so forth.

jmd

iTrader: (4)

He's talking > 7,000. Borg Warner developed the triple / double cone T56 for the Vette for GM. They knew the tested LS1 would spin (and be tracked) in ways to warrant it. Holden, Caddy, and Pontiac benefitted from the usual GM trickle down program. Then, Tremec got away from the umpteen combinations of parts begat by the GM program by going triple/ double across the board in the 6060.

92RSZ

iTrader: (6)

If you're talking about shifting a stock fbody t56 at 7000 rpm and beyond, of course it's not going to last long as it wasn't designed for that kind of rpm. I have the fbody spec t56 (not stock) with the stock type syncros. I've ran it up to 7100 a few times and it shifted fine. If you're engine doesn't peak til 6800, you need a 4.10 rear gear and shift it at 7300 or so. Definitely need a built transmission for your application.