Rear end gear choice??

I'm planning on having the rear end rebuilt this spring and I'm thinking about doing away with the original 3:38 gears.  Here are the particulars:
 
1969 350/350 M21 4spd. Convertible (Convertible=Heavy)
 
Rebuilt Original Engine:
Still has 11:1 compression
LT-1 intake
Holley 650 carb
Cam is slightly bigger than '70 LT-1 with solid lifters
MSD ignition and distributor
Headers
2.5" Dual exhaust with H pipe
Trans is original
 
Needless to say its built for the higher end and not for low end torque.  It's a dog out of the hole but really screams over 3500rpm.  I've found that I have to run at least 25% 110 octane fuel to keep it running right which I have no problem with since we don't drive these cars for economic reasons.
 
Any thoughts on gear ratios?  I'm thinking about 3:55's or 3:73's.  I'm not concerned about fuel economy so much but I'd like to be able to drive 70 mph without feeling like the engine is screaming.  We live in the city and we don't take it on long trips since there aren't a lot of gas stations to buy race gas at anyway. 
 
Also, as a sidebar topic, how do the trans gear ratios of an automatic (350hydromatic?) compare to a manual?
 
Any input from those that have experimented with gear ratios would be appreciated.
 
Thanks!
Rick.
 

If you go to the 3:73 gear you will not enjoy highway cruising any more. I had 3:73's in my '79 and the RPM's were just too high and the car was too noisy. I changed over to a 3:24 ratio ( I know it's an oddball #) and am much happier with the lower highway RPM's. I do have a manual trans so I can't tell you about the automatic gearing. My choice has always been a stick - just for the pure enjoyment. My opinion - keep your rear gear ratio the way it is.
 

   It would seem to me that you wouldn't want  small gears as you say that your engine doesn't have alot of low end torque, so why waste the engine and "bog" it around town.  My opinion is; go with an overdrive transmission.  With that extra gear, you have the best of both worlds.  The more I read about the reworked 2004R for a small block, the more I think that it's the one to go with.  Just gears won't give you everything you want.  Either you have a good hole shot (w/ bigger gears) or a big top end (w/smaller gears).  I know first hand that it sucks turning over 3000 rpm on the highway for any period of time.

3.08's, 3sp manual stick w/od ( still waitin' for rebuild ), 400+ horse 454, 75 mph @ 2300 rpm.
 
( correction, it's a 3.07 )

quote:

ORIGINAL: ricknhis69
  It's a dog out of the hole but really screams over 3500rpm. 
Any input from those that have experimented with gear ratios would be appreciated.

Thanks!
Rick.

 
Experiment with slipping the clutch a bit more comimg off the line at higher RPM's, this should give you a better shot off the line and still maintain your cruising RPM's. I would think that with 11:1 compression you should have plenty of grunt!
 
Why are you considering a rear end rebuild, is yours going bad?
 

I agree w/ DV8.  I've got 3.55s and w/ 255-60/15s, get 22mph per 1,000rpm.  No problem on launch cuz I've got the M20 w/ a 2.52 1st vs your's w/ a 2.20 1st.  Just another option for you.

And for automatics 350 =2.52, 400 = 2.48. PG.

Plus on top of that you get the inherent torque multiplication of the convertor at launch. 

At this point I guess I don't NEED to have the rear end rebuilt.  However, to my knowledge it's never been opened up or cleaned out.  I have no idea of the condition of the lube inside or if there is anything inside that shouldn't be there.  The bushings where it mounts to the frame are disintegrating quickly as are all of suspension rod bushings.  Since I'll already be paying for the labor (this is one job I think I'll skip doing by laying on the garage floor) to get far enough into the rear end to replace the mounts it makes sense to, at the very least, open up the rear end, inspect it, clean it, and replace the fluid.  At that point, if I can gain some out of the hole performance by changing the gears I think its worth doing, provided of course the sacrifices aren't too great.
 
73shark, you mentioned that you have the 3.55's and you are turning 1000rpm's for every 22mph.  So is 66mph at 3,000rpm's accurate?  (that doesn't seem bad) Also, I'm assuming that 4th is the same (direct) for both the rock crusher and the M21.
 
Last but not least, changing the trans is not an option.

Yes 4th gear is 1 to 1 on both transmissions, heres the #'s
M20   2.52                       M21    2.2
        1.88                                 1.64
        1.47                                 1.27
        1.                                    1.
 
Heres a ball park of rear ratios:
gear ratio          mph per 1,000 rpm          rpm at 70 mph
4.11                 18.                               3800
3.70                 21.                               3200
3.55                 22.                               3100
3.08                 25.                               2500
 
There's a lot of labor involved to get to the rear end so while your at it you might consider having the trailing arm bearings and bushings replaced, that way everything will be new and good for many years. PG.

But shark, that only 66mph at 3 grand.  You can't take a fast highway cruise for too long a time like that.  Ever try 75 mph for 2 or 3 hours?  I have and it sucks.  Not knocking you, just my opinion.
 
Sorry Fellas', I thought that you were looking to come out of the hole well and have a good friendly cruising rpm at real highway speeds. 

M22 is the "rock crusher" and has same ratios as the M21.  As far as the rear end goes, just drain it and refill with new lube and a bottle of GM's posi additive.

"But shark, that only 66mph at 3 grand.  You can't take a fast highway cruise for too long a time like that.  Ever try 75 mph for 2 or 3 hours?"
 
That's why I mentioned that we don't take this one on any long cruises.  We haven't had it on the freeway over 65mph for more than 25 minutes since we got it.  It's just not a travelin' car between the race gas and the tendency to heat up even with a big block radiator (which oddly enough came with the car from the factory even though it is a number matching 350/350 small block.  I think it has something to do with it being an A/C car ordered in Texas).  And yes, I know what you're thinking.  But, It's a fresh 3 core rebuilld with all the proper seals and shrouds.  I know an Aluminum radiator will cure the overheating but it's really not an issue since I don't drive it long distances at high speeds.
 
So that being said.  Has anyone had any experience switching from a 3.38 (or 3.36, I can't read my tank sticker all that well) to a 3.55 rear end?  Will I notice a difference?  Most of the comments seem to be related to 3.08 rear end to 3.55 or 3.73 which is a more substantial jump.
 
And yes, A/C cars did come from the factory with other than 3.08 gears.
 
Thanks again for all of your comments.
 

Hi there,
I have  your same problem.
I still have to check my rear end gear ratio , but it seems 3.73 or 4.1 . I'll know this later today , when I'll lift the car at my mech and check underside.
 
Anyway , I think that such a type of ratio is closer to my driving attitudes.
Just to give you an idea of the roads here in south Italy , please have a look at these youtube clips. No drag strips here but , just 10 miles from here , there's the "Fasano Selva race" .
Anyway all the roads here are about like those in the clips. We have flat , straight roads too , closer to the sea ( 5 miles from here: my town is at the top of a hill , about 100 m above sea level)
http://www.youtube.com/watch?v=cByaKOxT-Ps
http://www.youtube.com/watch?v=t9q39fX5ElA
 
I look forward to have my Corvette with new tires and new suspensions to test these roads 
 
I attach a pic of my car , just to remember to you all who I am
Aldo
 


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quote:

ORIGINAL: ricknhis69

"But shark, that only 66mph at 3 grand.  You can't take a fast highway cruise for too long a time like that.  Ever try 75 mph for 2 or 3 hours?"

That's why I mentioned that we don't take this one on any long cruises.  We haven't had it on the freeway over 65mph for more than 25 minutes since we got it.  It's just not a travelin' car between the race gas and the tendency to heat up even with a big block radiator (which oddly enough came with the car from the factory even though it is a number matching 350/350 small block.  I think it has something to do with it being an A/C car ordered in Texas).  And yes, I know what you're thinking.  But, It's a fresh 3 core rebuilld with all the proper seals and shrouds.  I know an Aluminum radiator will cure the overheating but it's really not an issue since I don't drive it long distances at high speeds.

So that being said.  Has anyone had any experience switching from a 3.38 (or 3.36, I can't read my tank sticker all that well) to a 3.55 rear end?  Will I notice a difference?  Most of the comments seem to be related to 3.08 rear end to 3.55 or 3.73 which is a more substantial jump.

And yes, A/C cars did come from the factory with other than 3.08 gears.

Thanks again for all of your comments.

 
I struggled with overheating issues for a long time - had a fresh 4 row copper/brass radiator in it too. Long story short - I replaced everything and finally broke down and got a universal fit aluminum radiator (2 row) for under $200.00. Now the temps never go up past 200*.
 
If you plan on keeping your car I would strongly suggest an aluminum radiator. It will prolong the life of your engine and also help keep you more comfortable in the cockpit. The universal radiator was almost a direct fit, all I did was modify the upper radiator bracket on the drivers side ( I ground it down a little on a grinding wheel) and substitute a stainless steel flex hose (eye candy) for the lower radiator hose.
 
As far as gearing choices? Lower RPM's extend engine life and keep the heat down too, as well as increase fuel economy.
 
What else do you know about this engine? 350 engine with 11:1 compression ratio - what else? this doggy performance off the line and engine pinging bother me, something's not right and I don't think it's the gearing. Inquiring minds want to know....
 

 
 
 
 

hey rick,
  You did open this topic by saying that you didn't want to go 70mph and feel that the motor was SCREAMING.  Seems like you've contradicted yourself and/or asked for two contradicting happenings.  I suggested the best of both worlds and then bowed out as what you wanted seemed to change.  My comment about 3 grand at a chump-ass highway speed of 66mph was, like I said, what you said that you didn't want, but, you say that you don't go that far to worry about that...WTF...and you quote me like I'm not understanding what you're asking for...Well, you're right.
 
Oh, or is 3500 rpm @ 77 mph a comfortable cruising combination for you?  That's right, you don't make those kind of trips...Yes, I read the whole topic again.

Jim,
 
Like I said, I am thinking about changing the gear ratios, thus I'm weighing my options.  If improving out of the hole performance isn't possible (at least a noticeable improvement over what I have with 3.38's) without the engine screaming at 60mph then I know that changing the rear end ratio is not going to accomplish what I want it to, and thus isn't worth doing.  In regards to the comments on this thread about living with higher gear ratio on trips that involve hours at 70-75mph, there is a big difference between living with an engine/trans combination for hours at a time versus 20 minutes on the freeway.  What is not tolerable for 2 hours may be acceptable for 20 minutes.  What I'm trying to avoid is turning the kind of rpms that make me think I'm driving on the freeway in 2nd gear.
 
Your advice about changing to a 5 or 6 speed is on the money but for a lot of us that isn't what its all about.  In a lot of ways these cars deserve to be preserved for what they are.  And if the driving experience is rougher than modern cars then that is part of the vintage corvette experience.  Bottom line, if your focus is performance than I recommend selling your C3 and buying a C6 since the average Altima will smoke most of these cars at a stop light and burn half the gas doing it.
 
I don't mean to be short but don't tell me I'm contradicting myself.  To me its a contradiction to put a modern 6 speed in a 30 year old chasis with a carbuerated motor, I just didn't feel the need to say WTF when the thread strayed outside of the original question.
 
Thanks to everyone that weighed in with their experiences.
 
Rick.