Ok, I am going to be looking at putting a 9 inch Ford rear end under the camaro here pretty soon and am having a tough time. I am pretty sure that I am going to run a Detroit locker because I feel that it is the most positive 'locking' differential that provides more equal power distribution to the wheels, while still offering some measure of driveability. This is a street car first and foremost but will get taken to the track and beat on frequently. Now, with that being said I have come up with 2 different gear ratio choices.. either 3.75 of 3.90. The 3.75 is the closest to the typical 3.73 ratio that is loved by all(well mostly all) as one of the best all around gears. I know that alot of stick guys are running the 4.10's because they have a little bit extra on the overdrive in 5th to make up for the rpm droop at highway speeds. The 3.90's are right in between the 3.73's and 4.10's. I am running a 500HP 383 stroker with a Workd Class T5. When the T5 goes south for the winter, I will more than likely throw a G Force unit in there. So what do you think? A 500HP 383 stroker 5 speed camaro street strip car should use a gear ratio of _________?(fill in the blank)

That's a 4% difference.... to put it in perspective, about 100 engine RPM at 65 mph, or less than 200 mph at the big end of the track.

That's pretty much in the class of "measure with micrometer, mark with chalk, cut with axe".

The ideal choice will depend on your engine's torque vs RPM curve; which just being a 383, doesn't allow us out here to estimate very well. Likewise, there are numerous ratios available in the T-5, depending on which model you have (except that 4th is always 1:1); just it being the 2nd design doesn't tell us that either.

3.75 would be 12 & 45, and has no prime numbers (I'd avoid it); 3.90 would be 10 & 39. 3.73 is 11 & 41. Among those, the 3.90 would be the strongest but tend to be noisiest; the 3.73 would probably be the quietest over the long term and wear the best because it's 2 prime numbers.

3.70, 3.82, and 3.89 are also available for the 9". Those are 10 & 37, 11 & 42, and 9 & 35.

Lots of choices.... lots of potential other factors to consider besides what you've mentioned.

Ask and you shall recieve....



My engine is the graph on the left. Pretty nice torque curve I think. So you are saying that the difference between the 3.75 and 3.90 is only 4%. That isn't too bad then. I have 3.42's in my factory 4th gen rear end now so the jump won't be too big hopefully. Anyway, what is with the prime numbers? Does it have something to do with vibration/resonant frequency of the gearset? I wasn't aware of the other gear ratios that were available for the 9 inch. When I called Currie about information on the rear end, they said they only had a few gear ratios that would fit with the Detroit Locker and I remember those 2 standing out. That doesn't mean that I am not willing to use another gear ratio, just wanted to make sure that it was right for me before I committed to it. As for the T5 ratio, I'm not really sure. I bought it as a newly rebuilt unit from a standard transmission shop. They claim that it came out of a 1991 Z28. It has the Timkin bearing so I know that it is a WC at least, but not sure on the ratio. With the 315/35/17's out back and a 3.42 in the pig I run about 2200 RPM in 5th gear at 60 MPH. When the transmission breaks(and I know it will) I will replace it with the G force unit and I will more than likely opt for the *economy* 5th gear ratio of .59. Not sure if any of the additional info helps but that's what I'm working with.

Is that graph measured and as built as opposed to somebody's ad copy?

reason I ask is, the differences you're chasing after are SO SLIGHT, that any difference in one build compared to another, even something as tiny as header tube length, might shift the balance in favor of one or the other.

The thing about prime numbers is, each pinion gear tooth will mesh with each ring gear tooth an equal number of times during its life, in the most "spread-out" possible pattern. In any non-prime situation, the combinations of gear teeth will repeat more often. In some REALLY BAD ratios (like Frod 3.00, 13 & 39 I think it is, for example), tooth #1 on the pinion will ALWAYS mesh only with teeth #1, #14, & #17 on the pinion. No equal distribution of wear at all. So clearly, combos with 2 prime numbers are best. 11 & 41 (3.73) is popular in part for that exact reason.

You can get virtually any gear ratio that exists in a Frod 9". To give you some idea, go here http://www.drivetrainspecialists.com...9/randp-1.html and just browse through the pages. And there are MULTIPLE manufacturers that aren't listed in there, that have other ratios besides. If you want one and whoever is building your rear doesn't offer it as a "catalog" option, I'd bet you could tell them what you want, and they can get it, for little or no extra charge.

Go with common sizes. Although software such as DD2000 will suggest a specific gear ratio, that ratio may be uncommon and expensive or impossible to produce a ratio. Common 9" ratios in that range are

3.70
3.73
3.86
3.89
4.11

The lower number ratios have a pinion gear with 7, 8, 9 or 10 teeth. The higher number ratios, 6.xx etc have 5 or 6 teeth on the pinion. How many teeth around the ring gear will determine the ratio available. The 9" typically has 34-37 teeth but not all combinations are available.

39-9 = 4.33
37-9 = 4.11
35-9 = 3.89

34-9 = 3.77 but that's uncommon as they would normally produce it as 41-11 to make 3.73

26" or 28" tire?

I like a much more basic approach...

I base rear gear ratio on what 1st gear is.

With my T-5, TH350 or 2004R transmissions, I like 3.90-ish ratios. 3.89, 3.90, or 3.91 depending on what axle it is, such as Dana 44, 60, Chevy 12 bolt, 10 bolt, or Ford (which you can get anything for, as stated above.)

I like 3.73's with the 700R4 (deep first gear, 3.08?)

I like 4.11-ish (4.09, 4.10, 4.11) gears with TH400 (2.46 first gear)

I run stock size tires and street cars only, no race-only stuff. That means 235/60-15 or 245/50-16. I might switch to 3.40-ish to 3.50-ish in one of my other cars with a 700R4 only if I am running smaller 14" tires...

Your car seems pretty well built so you may be in totally different tire size territory than me. The above recommendations have served me well to be both decent on extended cruises and still pull the left front if I've got between 400 and 425hp...

My vote for your car is 3.90 with a Ford 9-inch.

Good luck!

It is measured as an ad copy. There are actually 2 different engines that were built with 500HP and they were practically the same. They were both built by American Speed. Here is the page on AFR's site, you just have to scroll down a little bit to see all the spec

http://www.airflowresearch.com/chevy_dyno.php

I actually had made another post in the carb forum about this. I built the SAME exact engine with the same parts EXCEPT for the carb. I have an Edelbrock 750 CFM manual choke carb on my engine instead of the Holley 750 DP that were on the other test engines. I honestly don't think that right now I am actually getting all 500 horsies out of that carb.

Probably 26 inch tires. I would like to keep the same overall tire diameter as stock. No specific reason as to why but I just don't know if 28 inch tires will fit without modifications. Also, running a 28 inch tire at the track ONLY would make my gear ratio higher(numerically lower overall) right??

I was actually leaning more towards this gear ratio as well, but I am not very familiar with Ford rear ends. Not to mention that I am not a hardcore drag racer and the car is not a drag only car I needed to find a happy medium.

i'd do the 3.89 or whatever then.