I found this on a website today about different axle ratios:

"Another subject that often bothers people. The fact of the matter is you must use what works for you, although I have an opinion of my own. I don't know how many times I've had people tell me that they need 3.73 gears for their TPI car. I grow increasingly tired of this scenario. In fact I put 3.73's in my own car, a 350 TPI, it ran a best of 13.634, I switched in the factory 3.23's and with no other changes ran 13.496. I also gained .78 MPH in trap speed, some speculate that it was because I spun less, but my 60 ft. times are very close 1.937 (w/3.73) vs. 1.912 (w/3.23). Facts are facts TPIS will tell you the same, TPI motors like to be loaded hard - they respond better to mild gearing. I later put in a set of 3.42's and again went slower. Trail and error, isn't that how we all learn? I have had 4 TPI cars over the last 5 years, I basically know what works and what doesn't. I have learned that 305 TPI's are best with 3.08's and 350 TPI's are best with 3.23's. A more heavily modified 305 could use 3.23's also. The 7.5 rears in these cars are also a subject of debate. Properly built they can handle lots of power, a car posted on Car Pro Motorsports web site has a 383 and runs 11.70's with a 7.5 rear. The advantage to running a 7.5 is the light weight and the less HP it takes to turn compared to larger rears i.e.: Ford 9 inch and GM 12 bolt rears. Because of the pinion angle of the 9 inch rears, every car I have rode in has a vibration at certain speeds and the angle is not as efficient, costing HP."

What are your opinions on this? I know that gears have a "higher number is better" reputation, but could there be some truth to what this guy says? I'd like to know before I decide what ratio I want.

 

unless the weather and all other conditions were the same i'd say it's pretty meaningless.

 

What website was that posted on?

Miles

 

http://www.3gc.net/articles/drivetra...al_gears.shtml

 

Yeah, pretty meaningless... among the many logical mistakes that person made was to assume that because his TPI car reacted that way, it must be a general truth.

It's true that TPI cars are very easy to over-gear, because the restrictive and anti-RPM potential nature of TPI makes the engine's torque curve nose over at such a low RPM. About the only situation you can generalize that result to is that whenever the engine's induction system won't support the RPMs that the gear forces the engine to run at, the car will go slower. Since TPI's torque begins to vanish not too far above 4000 RPM, you can easily make the car slow down by gearing that forces the engine to run much more than 4000 RPM.

Build an engine to produce max power within a given RPM range; gear the car so that the engine spends as much time as possible within that range.

"It must be true, I saw it on the Net"

 

I generally take everything with some grain of salt. In this fella's case it is no different. For example, i've cut better 60's in my tank with only a stock 305 and w/ 3.23 gears to boot. Sure i have a 3k stall converter, but according to this guy's logic, i should be running 3.08s and something much tighter in the converter.

But what he says is generally true though. If you're trying for max et on a budget with a stockish long runner TPI car then 3.23s are going to be just fine and your money is better spent elsewhere (like the converter) 3.73s are overkill on a long runner set-up unless you have a pretty light car that can get up and move that fast. Then you might make enough et on the bottom to make up for that lack of breath on the big end of the track.

As for the 7.5" rear. Yeah they take less power to turn. And yeah they will hold up into the 12s/11s if not abused. But they are a time bomb regardless, no matter how well built at that power level. Just a couple years ago a few of the big hitter LS1 cars were keeping stock rears in 'em just for the extra power they saved. They were max effort type deals and had trailers for if (when) they broke too. If you can accomodate catastrophic parts failure on your car, then sure, use 10 bolts till they pop and put it back together and keep going. If you actually cruise and drive your car for more than just 1/4 mile at a time, especially if it's your primary driver, a 10 bolt will keep you cautious, and might leave you stranded on an impromptu 'let's see how fast this sumn bitch can go' run.

Oh yeah, forgot to add, the above stuff about 10 bolts applies to automatic tranny cars. Manual cars will pop 10 bolts a lot easier, especially if you can drive at all. The only question is, which pops first the T5 or the 7.5. They are pretty equally weak when it comes down to it.

 

Hey, my 7.5 made it almost one year longer than my T-5. But, I added a solid spacer to the rear.