Rear gear recommendations???

[DWG]

I have a 68 Camaro being built and I bought it with a Currie 9 inch already installed w/ 3.89 gears. Motor/tranny is as follows:

Procharged 406 making over 650 rwhp
TKO 600 5 speed
typical setup - 17/18 inch wheels on 255/285 tires

Built for 98% street driving and short weekend trips once in a while maybe. I know highway driving is not an issue either way with the 5th gear, and the low gears will be rendered useless
with the power and street tires. But I am thinking 3.50 or 3.70 would be ideal with the blown motor.

Thoughts?

[chicane67]

First question... which TKO 600 ??

The general rule of thumb is to keep your first/final gear ratio between 10 and 11:1. If you go too deep one way, youll end up having to slip the clutch a little more and the other way... first gear becomes too short and almost usless. So, anything that fall in between 10 and 11:1 is considered good.

TKO 600
2.87 1st. 1.89 2nd. 1.28 3rd. with a .64 or .82 5th.

W/ 3.89's = 11.16:1
W/ 3.73's = 10.70:1
W/ 3.50's = 10.11:1

.82 w/ 3.89's = 3.18
.82 w/ 3.73's = 2.98
.82 w/ 3.50's = 2.87

.64 w/ 3.89's = 2.48
.64 w/ 3.73's = 2.38
.64 w/ 3.50's = 2.24

Im sayin 3.50's are a go... best of both worlds... especially with having that much rwhp.

[DWG]

Sorry, meant to say that I was planning on going with the .64 overdrive. But haven't put the order in yet so it could change if needed.

Okay, thanks for the calcs - didn't know about that rule of thumb. Originally planned for an NA motor with about 600-650 crank hp so figured the 3.89s would be acceptable.

Thanks for the guidance.

[chicane67]

I myself have chosen to run a 3.90 with a .80 5th / .62 6th... N/A, 650hp crank. Its really because of the first gear ratio of 2.66 that I went this route... and the double OD helped out with freeway cruzing speeds.

[DWG]

chicane67 - with that much power aren't you going to basically have the same problem? Isn't first gear going to be worthless for you?

[rockdogz]

3.50

[JChristian1835]

I'd have to say 3.50 or lower.. any more and 1st gear would be practically useless...

[6'9"Witha69]

3.50.

[chicane67]

chicane67 - with that much power aren't you going to basically have the same problem? Isn't first gear going to be worthless for you?

No, not really. My first gear ratio is 10.37... so Im on the lower end of the scale.

That, and my power production is in mid to upper rpm... unlike having everything off idle like that of a supercharged build.

[Kal]

I don't understand how you come up with the numbers.

Doesn't your tire size effect your end ratio?

I'd like to know how to "do the math"

Thanks

[chicane67]

Gear ratio X final ratio = ... 2.66 X 3.90 = 10.374

Does it (the tire size) effect the ratio.?.?. yes, it does... but we are talking mechanical gear ratios, not final driven ratios. Its like talking about horsepower... you have flywheel and you have rearwheel. They are the same, but one deals with variables.

FWIW, most everyone here is running a wheel and tire package that is probably within 2%... and somewhere around 25.5-26 inches. So when I made the statement of the first/final gear ratio being "good" or acceptable when its within a 10 - 11:1 ratio... it is made in reference to the final mechanical ratio and a 25.5-26" tire size.

Well... that, and being street driven without having first be useless from being too short or too deep.

[osdmike]

Hi !
This web site could be helpful.

http://www.chevyhiperformance.com/techarticles/148_0208_gear_ratio_calculating/index.html

Also scroll down to "Sidebar articles"


Osdmike

[Turbo Hen]

I would go with a much lower gear......in fact i used to run a 3.23 gear in the camaro with the twin turbo powerplant. I bought a 3.00 gear for my '69 Firebird w/ 9" Phord

If i were you, it would be a 3.50 MINIMUM but i'd shoot for something in the 3.27 range......it will really help calm the tires down from spinning.

I had a 3.73 in the camaro & when i switched to the 3.23 it didnt CURE the problem, but it cured 80% of it.

-Carm

[CarlC]

Tom,

Do the numbers hold true for lightweight flywheels, or just shoot for the top of the range?

[John S]

I would go with a much lower gear......in fact i used to run a 3.23 gear in the camaro with the twin turbo powerplant. I bought a 3.00 gear for my '69 Firebird w/ 9" Phord

If i were you, it would be a 3.50 MINIMUM but i'd shoot for something in the 3.27 range......it will really help calm the tires down from spinning.

I had a 3.73 in the camaro & when i switched to the 3.23 it didnt CURE the problem, but it cured 80% of it.

-Carm
ok, I'm just getting into the "math" of the gears but I have a question. I always thought that lower gears meant a higher numerical number such as 4:11 was a lower gear then a 3:73.
But you speak about running a lower gear at 3:23. Wouldnt that be a taller (higher) gear?
If this is true and I am going to shoot for between a 10 -11:1 ratio I would need a 3:73 -4:10 rear gear ratio with the T56 and 2066 1st gear. Is this correct?

[chicane67]

Tom,

Do the numbers hold true for lightweight flywheels, or just shoot for the top of the range?

Well... kinda sorta. Now we are looking at an answer that is influenced by actual chassis weight. Lets say in your case, with a fully interriored and cozy cockpit... I might lean more towards the higher end but would still be a little warry of near 11:1 and over.

Now on the other hand, like in my case with a weight conscious chassis... somewhat compensating for my fat@ss, I can go lighter and/or numerically lower. I have even run a 6.006:1 with a 30lb wheel without beating up the clutch too much... but it really depends on what you can tolerate as a driver.

I like the lower side of 10... and that is a personal choice. But remember... I am a 'whole picture' thinker when it comes down to specific considerations. I really think it comes down to the chassis and driver when making the final decision.

[chicane67]

ok, I'm just getting into the "math" of the gears but I have a question. I always thought that lower gears meant a higher numerical number such as 4:11 was a lower gear then a 3:73.

It can go either way... it could mean going numerically lower... or... refered to as a lower gear, which is typically a statement when discussing transmissions. First would be considered 'low' and 4-5 high.

I too would consider the 3.50 ratio as a 'maximum' numerical rear end ratio. It also falls into the first/final ratio guideline. But, I dont agree with it being a controlling factor in wheel spin. The theory of "chassis to wheel speed" when braking traction, proves that differently.

[John S]

It can go either way... it could mean going numerically lower... or... refered to as a lower gear, which is typically a statement when discussing transmissions. First would be considered 'low' and 4-5 high.

I too would consider the 3.50 ratio as a 'maximum' numerical rear end ratio. It also falls into the first/final ratio guideline. But, I dont agree with it being a controlling factor in wheel spin. The theory of "chassis to wheel speed" when braking traction, proves that differently.

I never would have considered 3:50 rear gears. It just seems that these would be too tall for a weekend driver with a T56 and approx 600hp. I'm sure top end would be great, as well as highway cruising but wouldnt around town drivability suffer not to mention low end torque?
Again, i am a bit ignorant about gearing and trying to learn and understand the relationships.

[chicane67]

With a T56 and its 2.66 first, a 3.73 would be on the low end of the scale... a 3.90 is near the middle and a 4.10 would be on the high end of the scale. Light chassis - 3.7... heavy chassis - 4.1. I went 3.90 due to chassis weight and where the engine makes its real power.

The biggest comtributing factor in this thread dealing with the TKO and his combination is... a blown 406. Its gonna make torque anywhere... and everywhere.

Around town you are using 1 thru 4 more than anything else. 5th is pretty much a freeway gear, so I guess the idea is not to buzz it too hard when freeway/highspeed cruzing. With the .82 OD and 3.50's he'll be spinning about 2200rpm at 60mph. With the .64 OD and 3.50's he'll be spinning about 1550rpm at 60... and have the same first gear ratio of 10.1:1. Considering that its a blown 406, I dont think he will have any problems turning the wheels with the amount of torque at lower rpm's.

With cubes and/or lost of torque being produced, you can run in the lower end of the scale much eaiser.

[Flash68]

The biggest comtributing factor in this thread dealing with the TKO and his combination is... a blown 406. Its gonna make torque anywhere... and everywhere.

chicane67- I meant to get back to your thread a while back but forgot, and now I just saw it again.

See the attached chassis dyno sheet of my motor on a very similar 69 Camaro with a T56 and 3.73 geared 12 bolt. The torque curve is very late in the rpm range it seems... looks like it is making very little power down low - 2500-4000 rpm. I also think this was with the 14 lb pulley and I will be starting out running the 10-11 lb pulley.

Maybe the 3.89s will work with such low torque numbers down low??

[chicane67]

Wow. That engine is flat until 3k... ??

The 3.90's would still kill the first gear ratio. 11:1 is pushing it. Afterall you are only looking at a 250 RPM difference from 3.73 to 3.90... but mechanically, its 4/10ths in first and almost 1/10th in final.

Coin toss...

[Flash68]

Wow. That engine is flat until 3k... ??

The 3.90's would still kill the first gear ratio. 11:1 is pushing it. Afterall you are only looking at a 250 RPM difference from 3.73 to 3.90... but mechanically, its 4/10ths in first and almost 1/10th in final.

Coin toss...


Yeah that dyno graph just doesn't seem right compared to what I have seen. The cam in it is not huge (244/252 dur @ .050, .620 lift, 114 lsa) and the heads are well-ported Victor Jr 215cc heads. Any idea why the lack of low end power?? The smaller/more boost pulley having too much lag in the powerband?

Anyway.. so the 3.89 would render 1st useless. Would this lack of low end power my motor seems to have help offset that at all? Or is just a matter of needing to go from 1st to 2nd right away because it just runs out of room, so to speak?

Also, FYI I have my tires and they are 26.65" tall.

[Rick Giles]

Hope it's ok to reply to this thread but I have a naturally aspirated 408 BB in a 67 Camaro with a TKO 600 with a .64 OD and a 3:73 rear end and it is an absolute blast to drive. Drivability is fantastic. I also have 255/45x18 rear tires. I couldn't be happier. I would think if you have either a 3:55 or a 3:90 gear with 600hp on the street you are going to blow the rear tires off the back anyway.

[Flash68]

Hope it's ok to reply to this thread but I have a naturally aspirated 408 BB in a 67 Camaro with a TKO 600 with a .64 OD and a 3:73 rear end and it is an absolute blast to drive. Drivability is fantastic. I also have 255/45x18 rear tires. I couldn't be happier. I would think if you have either a 3:55 or a 3:90 gear with 600hp on the street you are going to blow the rear tires off the back anyway.

It's absolutely okay! Thanks for your comments. How much power is your 408 putting out? How usable and short is your 1st gear? Don't you have to shift into 2nd pretty quickly upon acceleration?
I kinda wish I had the 1st gear ratio of the T56 compared to the TKO.

[Rick Giles]

The H.P. is probably around 425 at the flywheel. This car is VERY quick on the street. This motor went 12.20's in a 70 Elcamino with 3:31 gear on street tires. I just put a 6600 rpm chip in the MSD box and she pulls hard up to that rpm.
I also thought that I would have to shift out of 1st real soon but it is nothing like I thought. Its easy pulling off from a light and the clutch is very smooth. I love it!
On the highway at 75 mph it is turning around 2300-2400 rpms. (I think I have that right) I may have to double check that.

[mac71ss]

The general rule of thumb is to keep your first/final gear ratio between 10 and 11:1. If you go too deep one way, youll end up having to slip the clutch a little more and the other way... first gear becomes too short and almost usless. So, anything that fall in between 10 and 11:1 is considered good.

I would have to run atleast a 4.56 with my m21 to hit that.:eek:

so i am going to asume my rule of thumb is not so high

[chicane67]

so the 3.89 would render 1st useless. Would this lack of low end power my motor seems to have help offset that at all?
It would not really render it useless... but it would make it really short.

And... yes. A little more gear would make up for the flatness... but only a little. It still makes it short.

In most cases, a 9-10:1 will work just fine. Its when you got too far past 10:1 that first gears is really short.


Or is just a matter of needing to go from 1st to 2nd right away because it just runs out of room, so to speak?

Yup. You nailed that one.




so i am going to asume my rule of thumb is not so high
I hear you on that. It depends on the chassis and even more to do with the driver. I ran an M22 (2.20) with a 2.73 ring gear... which is 6.000:1 and it worked... but I had to sort of slip the clutch a little. It definately too some finesse to drive that set-up. Actually, with a four gear and a 2.20 first... I like to run 3.42's if the chassis is light enough.

[Flash68]

Chicane.... thank you for all your insight. Appreciate it much.

[beach70]

what rear end ratio would you recommend for a 70 chevelle, fuel injected bigblock, and t56 with 0.5 6th gear and 275/40/17 tires

I'm guessing either 3.55 or 3.73

thanks in advance

[TnBlkC230WZ]

I'd go 3.42 or 3.27 if you use the .82 OD transmission. That much power on the street will be fine with 9 - 10:1 gearing.