Rear gear recommendations???

[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.

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.