Team Chevelle banner
  • Hey everyone! Enter your ride HERE to be a part JUNE's Ride of the Month Challenge!

Status
Not open for further replies.
1 - 13 of 13 Posts

·
Registered
Joined
·
75 Posts
Discussion Starter · #1 ·
I'm rebuilding a 70 SS Convertible that originally had an LS5 454 and 4 speed. This car was stripped of most SS parts including the engine and drive train and sold over the years. I plan on using the GM 454-425hp crate motor. I have bought an M21 4 speed and a 12 bolt posi rear-end which both came out of a 68 SS. The rear appears to be geared in 3.07 base upon an unscientific turn and count. I want to run 15" radials

After speaking to Richmond Gears and with several "cruise night experts" I am considering going with 3.73 gears. I will be using the car on weekends in good weather with 70% around town and 30% highway trips of up to 2 hours each way. I want a driveable car. With the M21, 15" tires and the proposed 3.73 gears RIchmond Gears told me I can expect about 3000rpms at 65 mph. They want me to consider the nash 5 Speed tranny but that's too pricey for me now.

Will this ratio and the M21 be driveable, without killing the clutch and shaking the passengers out of the car on start off?

While keeping the above components is the 3.73 gears the best choice?

Your opinions and thoughts are welcomed.

--Andy--
 

·
Registered
Joined
·
75 Posts
Discussion Starter · #2 ·
I forgot to mention that this is a factory A/C car and I plan to keep it, though with a convertible I won't use it around town, but occasionaly on longer trips :)

--Andy--
 

·
Registered
Joined
·
1,525 Posts
Andy, I have the M21 with 3.55's and 28" tall tires. My engine probably has a higher rpm band then the 425 HP crate, but even with the stock 396 I just replaced, I would recommend nothing lower than 3 70's with an M21 ( just like GM did originally). My first gear is too tall, even with a BBC. Upon DZ's advice I'll probably switch to an M20 first. If I had to switch my gearset, I'd go with 3.90's. But I don't cruise the highway all that much. I kinda like cruising at 3000-3100 and doing 65-70 now though.

If you do allot of cruising at 70-75 stick with the 3.73's/ If not consider the 3.90's ( unless original gear ratios are critical in your resto).

BTW the Nash 5 speed is the cat's ass. If you can wheel your M21 for a good buck, I would highly recommend you consider it. But I couldn't afford it either. $1800 for the box, $375 for the shifter. OUCH. As an offset, your 3.08s would be perfect.This would save you around $450 parts and labor on the gear switch. Do the math, if you're within $300-400, DO THE NASH! You'll save it in gas the first year or so depending on how much you drive. Again, you've got the right gears already.

Don't run the stock sifter whatever you do, IMHO. And I used a Centerforce dual friction clutch. I wouldn't ever consider another make/model of clutch than that. The pedal feel betrays what a sonuvabitch clutch it really is.

Hope this helps n good luck with the resto!


[This message has been edited by Gene Chas (edited 09-28-99).]
 

·
Premium Member
Joined
·
17,564 Posts
The M21 is a close ratio that would have come with a car with 3.73s. I think the cut off is about 3.55s. Anything higher (3.07s, 3.31s, etc would have included an M20 wide ratio with the lower gears (1st-3rd). So the M21 is the right choice for the 3.73s. The M20 and M21 have the same 4th ratio (1:1) so there will be no difference in cruise RPM on the highway.
Those Richmond 5 speeds, I don't think they make an overdrive 5 speed. Therefore with 3.73s, cruising would still be the same. The Richmond does have a lower 1st gear, which would be ideal if you want to keep the 3.07s. The money you would save on changing the rearend ratio could be spent on the Richmond trans. Then you would have better driveability all around...
I am thinking of going to the richmond 5 speed myself. They do have a nice 6 speed with overdrive, but that is quite a bit more $$$.
 

·
Registered
Joined
·
75 Posts
Discussion Starter · #5 ·
Gene Thanks!

Am I correct in assuming that you believe that the 3:73 rear will make the gearing in 1st manageable on start off?

With a wife and 3 kids, I can't be shattering their teeth and shaking them out of the car -- medical bills are the one thing more expensive than the car hobby!

--Andy--
 

·
Registered
Joined
·
1,525 Posts
Just an added insight on the Richmond. They recommend 3.50 or less, actually 3.30 or less. IT IS NOT AN OVERDRIVE. So your final gear ratio is what you end up with. The key to it is the much steeper 1,2 gears. For example, a 3.27 first with 3.08 final is like running 4.56's with an M21. Whoa! That's a holeshot baby!

Again, if it's not an orignal resto, and you're within $400 or so, DO THE NASH. Plus I think it's considered more bulletproof than the Muncie. Run your 3.08 with the tallest tires you can fit in your wells, you'll be very plaeased. 4.56 at launch, 3.08 at cruise.

Hey check out Wes Vanns tech reference site on gearing for a good overview. You shgould read it before you make a final choice anyway.

Seeya.

Just saw your reply. Yeah Andy, my 3.55 with tall tires just ain't enough. Plus the Centerforce grabs quick. It make for very unsmooth starts, or slipping it too much. I'd say 3.73 MINIMUM with a M21.

I know what you mean, I slip it good with my wife and 2 yr old with me! Smooooth as I can.

[This message has been edited by Gene Chas (edited 09-28-99).]
 

·
Registered
Joined
·
246 Posts
Hey Andy,

I have a '71 SS convertible with a 427, Doug Nash 5-speed, and a 2.73 posi 12 bolt. I was looking for both highway drivablility (45 work commute) and low speed torque multiplication. There really is no other way to get it. I had 3.73's and an M22 in it but cruising at 70-75 MPH gets old fairly quickly.

Don't worry about killing the passengers at start off worry about them while cruising as you'll be doing more like 3500 at 65 MPH.

You can decide for yourself by driving your current car in "normal drive" (which probably doesn't have 3.73 gears and a 454) and note the RPM and engine noise.
 

·
Registered
Joined
·
67 Posts
Andy: I would suggest going to tech ref., page #24, titled gearing formulas. I have also done some figuring based on the formulas on this site. Presently I am running an m/20 and 3:31 rear ratio with 28.0 inch tall tires. At 65 mph on freeway I tach approx 2550 to 2600 rpm and based on pacing with my wifes car. Using the formula and finding the circumference of the tire, with a 28 inch tall tire and at 65 mph. with 4:11 gears for example, I would be tacking 3207 rpm. with the m/20. The circumference is pie (3.1416 x dia., 28.0) Consequently with a taller tire, the lower gear you can go with and keep rpms low also. I am in the same situation as Gene Chas. I too want a stronger launch but with the m/20 I am contemplating a 4:11 rear gear ratio. Since I do not drive more than 15 to 20 miles max. per trip and cruising around town I think I can live with 65 mph. and taching around 3200 rpm. for 10 to 15 minutes.
Just my thoughts.
 

·
Registered
Joined
·
425 Posts
Based on what you have said I think you want a good cruiser. Since you already have the tranny and rear, try it and then decide.

I have an M21 and to be honest I wish it was an M20. My car originally a 70SS396 came from the factory with the M21 and 331 gears. For a street car, it's never been on the track, it was fine. You can still light up the tires at a light or buzz down the highway. In the early 70's when the 396 died ( too much highway running at 75-85 mph) I replaced it with a 350. The 350,M21, 331's ran good also.

Now I have the 454/425 crate motor, M21 and 373's. Taching 3K at 60. I plan to switch back to 331's. I feel like I'm in 3rd gear while doing 55. Neck snapping power @60 in 4th
What I would really love to have is the Nash 5 speed and 2.73 or 3.07 rear. Would cruise all day.

I also have a 65 vette, 350/350, M20, 3.08 rear.
It is a great cruiser... and 18mpg

So like most topics, we all have our opinions. I fully agree with Kevin's post. Only you can decide.

Good luck,
OH YEA, you will love your 454/425

Bill
 

·
Registered
Joined
·
49 Posts
I would go with the D Nash and the 3.07's out back. I have 3.31 in my 72 with a LS5. I took out the M22 and switched to a B/W Supper T10. That droped my fist gear from 2.2 to 1 to 2.64 to 1 and widened up the ratio between gears. I just finished the car and haven't driven it to much yet but even with the 3.31's I feel like I could use another gear on the highway. The big blocks have so much torque that they feels like they could pull down a tree at 1500 rpm. I would like to cruise at the low 2000's on the expressway's. Mines not a race car either but I would like to sell the Supper T10 and the M22 and put the money into a 5 or a 6 Speed. Has anyone done the switch? Is it a straight bolt in or do you have to start cutting?

[This message has been edited by Don Boc (edited 09-28-99).]

[This message has been edited by Don Boc (edited 09-28-99).]
 

·
Registered
Joined
·
246 Posts
Dan,

Check out my post. I have just performed the swap - the tranny bolted right in. The only details I'm working on now is how to do the tranny hump as I can no longer use the console unit and the non-console unit would need to be modified. The problem is that the Hurst shifter mounts a little higher than on an M22 and the handle comes straight up and not with the 2 3/4" offset back to the tranny centerline. Still working (read: avoiding) on it.
 

·
Registered
Joined
·
49 Posts
Kevin:
Did you have to cut the floor hump at all? Are you just talking about not useing the console and the center shifter? Don
 

·
Registered
Joined
·
246 Posts
Hey Don,

The shifter comes up through the same hole but neither hump will work without modification. This is due to the shifter sitting so high on the trans that the humps can't sit in their stock location. The non-console hump is close but I since the shifter mount has to go thru the hump "hole" it won't mount in the center of the car.
 
1 - 13 of 13 Posts
Status
Not open for further replies.
Top