a69vette

Question. What would happen if I installed a 3:08 rear end with a M21 transmission? The car is a '69 350/350.

Thanks,
Rick

ED DINAPOLI

Rick,
Your low gear acceleration will suffer. You will need more clutch slippage getting started in !st gear. 4 gear will be the same.
Ed

Rowdy Rat

Rick,

Ed's description is right on the money.

I owned a 1974 LS-4 convertible for several years that was ordered with the combination that you mention - M-21/3.08. The car was an absolute slug off the line... Even with the torque of the big block. Lots of fun once you got it moving though and a great high speed cruiser as you would expect. If the latter is all you are interested in, then the reduction you'll see in acceleration may not be a problem.

My personal opinion is that this is NOT the best combination to use, particularly with a small block.

Regards,

a69vette

I currently have a 4:11 rear end. Great for off the line, but no good for going down the hwy. My biggest concern is that I would be over taxing the clutch for take off (not worring about any hole shot), and causing frequent clutch work.

Thanks,
Rick

rainman69

My car is quite happy with a 3.55...but she screams down the highway at 3500 and 78 MPH...She's also kindy doggy off the line on the streets, but I think that is because the engine was built up for HP and not TQ...I wish I knew what was inside the block... :smash:


If you want to compare rear ends at speed: http://www.smokemup.com has some great calculators. Also...an LS1 powered 240Z...

PRNDL

Why not compromise and put in a 3.36. I'm just guessing.... but my guess is M21/3.08 would feel a lot like starting out in 2nd gear for most of us. hmmm... could go crunch the #s I suppose... MJ


ok, I cant help myself :rolleyes:

3.08 * 2.20 1st gear (M21) = 6.776

3.36 * 1.88 2nd gear (M20) = 6.3168. Pretty close. MJ


[Modified by MNJack, 1:42 PM 2/14/2002]

ED DINAPOLI

Rick,
Incidentally, the highest gear ratio that Chev. offered in 69 with your engine and a m21 was a 3.70 rear. A 3.08 rear was offered with a m21 but only in a big block in 69.
Ed

71vettman

Ive never driven another corvette so I cant compare it to any others. I have modified 454 with an m20 and 3.08's. If I try to accelerate from a stop like a regular person I have to ride the clutch more than I would like to. When I really want to go though Its not a problem at all and I can run fast on the highway at resonable rpm's. :yesnod:
Kevin


[Modified by 71vettman, 7:44 PM 2/14/2002]

a69vette

A couple of things:
First: I've ran the numbers on the rear end and it is a 1AP12269. This tells me it is a 4:11 made on Jan. 22, 69. This is an all orignal car so I know that the rear end was not changed. I went to the '69 registry site, and they show that with the 350/350, the M21 had an option of the 4:11 rear end.
Second: I have a '68 BB 3:08, so to make some changes would cost very little to achive what I am hoping for. I would be willing to make some kind of a trade with some one who would like a BB 3:08 for a SB 3:36.
I do thank everyone who was commented on this subject. It helps me out a lot.

Thanks,
Rick


[Modified by a69vette, 9:44 PM 2/14/2002]

joevette57

Sorry Ed,
I have to disagree with you. I know Rick's car. It is VERY original and the rear end has never been touched. The 4:11 in his car is matching and original. However, you are correct that a 3:08 and M21 were only matched up in Big Block cars.

jkmhb

I have been trying to decide what to do in this area also, and I've decided to go with a 3.08 and a Richmond 5-speed. 5Th is a 1:1 with a 3.27 1st. This should scream off the line and be decent cruising on the freeway. I have most of the stuff together, if anyone is interested I'll post an update when it's done.

Paul Borowski

I have this current set-up in my '68. Took out the 3.70s two years ago and had Bair's install a 3.08 gear. It is OK, not the best with the M-21. I'm not interested in acceleration performance(I'd rather cruise at 80mph)and the 3.08s are super for that. I do have to let the clutch out feathering it a little with the taller M-21 first gear. I probably would have been better off going with the 3.36(which is what my '75 L-82/automatic has), but I do like to cruise at 65mph doing 2750rpms. I don't regret what it has now, I just drive it a little differently than what I'm used to. My '80 also has a 3.08 rear, but it has the wide ratio Warner with the 2.88 first gear-seems I can almost "dump" the clutch idling and it won't stall!!! :lol:

ED DINAPOLI

joevette57,
I believe you did not understand my statement. I said" the highest gear ratio availbe with a m21 was a 3.70 rear" I believe a 4.11 is a lower ratio than a 3.70. It is numericaly higher but it is not a lower ratio gear.
Ed

mrvette

IMO, there are no good compromises in rear gear ratios, the spread of gears in a typical 4 speed stick, Muncie, etc....are too close to keep the engine in the ballpark for both cruise and acceleration in traffic....spinning 4 grand at 80 typical Florida freeway speeds just don't get it, now having said that....
I lived for a couple of years up NE way where 80 will land you in jail...
much less run out of room after a mile or so at 80...traffic....
so the O/drive need was not so great....and a 3.36 rear was fine compromise...
now that I live in Florida...I needed o/drive...cheapest was to go for that some 4.5 years ago was a 700r4......today, good affordable 5-6 speed trannies are available on aftermarket....I"d do one of them, and leave the rear in the mid 3's .....even 4.11 IF I had a .50-.60 overdrive final gear...

GENE

a69vette

....spinning 4 grand at 80 typical Florida freeway speeds just don't get it,

Gene, you're exactly right! Van Steel is supposted to be done with the t-arms on tuesday, so I'll start putting things back together. I have the 4:11 out of the car, and I'll probably change the yokes into the BB 3:08. Red light to red light is over, but going to Orlando will have a new thrill!!
Thanks to everyone with the replies. It helps me be a little more knowledge about my passion.

Thanks,
Rick

joevette57

Ed,
Your absolutely right, I was thinking numericaly. Sorry


:cheers:

ED DINAPOLI

Joevette57,
This can be very confusing and is easy to mix up.
Regards
Ed