Topic: Re: change differential ratio
in Forum: C3 Driveline Components
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dapperdan said: How tough was it to do the removal and install of the rear end?
Dan that's a loaded question. It all depends on how mechanical you are. Also if you have the tools. You really need to get a sevice manual.................. On a different note: Im sure that you miss your cat......he or she was beautiful.
corvette440hp
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Welcome Dan. My 71 L48 has 3:36 gears, 4spd trans. At 70 mph I'm right around 2900 R's. Now the good news besides the mileage I get, my punched out 383, mild cam, and some other toys thrown in I have absolutely no problems breaking em loose and roasting the back tires.
I agree with others that's my ratio of choice.
I agree with others that's my ratio of choice.
Former Member
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Rochester, NY - USA
Joined: 6/12/2012
Posts: 95
Vette(s): 1980 L- 82 fully restored and upgraded.
I would leave the rear alone , the gains in MPG will be little at best and the drive-ability around town will off set any highway gains in MPG you get . So leave the rear alone and swap the transmission because you then maintain all the power and driveablity BUT get positive and solid MPG gains you will notice at the pump.
|UPDATED|11/15/2012 7:47:04 AM (AZT)|/UPDATED|
I having an automatic just dropped in the 700R4 trans with a 3.55 rear it is wow .. You can hardly hear the motor on the expressway in OD.
Is the trans swap more ? yes however driving experience alone should make you feel like the money was very well spent .
|UPDATED|11/15/2012 7:47:04 AM (AZT)|/UPDATED|
1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI
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My '77 coupe is rare. It is one of 2060 out of about 50,000 that had the combination of an L82 engine and a 4-speed transmission. I don't want to change the trans.
I had a '76 L82 with a 4-speed many years ago and it had the 3.36 differential. It was not a screamer, like this '77 model.
The gas mileage is one thing, but the wear and tear on the engine is what concerns me. My '77 only has 31,000 original miles on it but the engine has been revving way too high throughout those miles. I want to calm it down some.
I have 2011 GS that I can drive for performance purposes, so I don't care about the '77 as a performance car. i just like its looks and the way it handles. It is not going to be driven hard by me.
Former Member
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Rochester, NY - USA
Joined: 6/12/2012
Posts: 95
Vette(s): 1980 L- 82 fully restored and upgraded.
dapperdan said:
My '77 coupe is rare. It is one of 2060 out of about 50,000 that had the combination of an L82 engine and a 4-speed transmission. I don't want to change the trans.
I had a '76 L82 with a 4-speed many years ago and it had the 3.36 differential. It was not a screamer, like this '77 model.
The gas mileage is one thing, but the wear and tear on the engine is what concerns me. My '77 only has 31,000 original miles on it but the engine has been revving way too high throughout those miles. I want to calm it down some.
I have 2011 GS that I can drive for performance purposes, so I don't care about the '77 as a performance car. i just like its looks and the way it handles. It is not going to be driven hard by me.
Well glad you cleared that up after several ideas of it having been mentioned to you.
As far as gears at 60mph you are not hitting 3500rpm , unless you installed short tiny tires but that cant be because it is an original 77 right ? Or you have a 4.56 rear gear set other wise ...... At 60mph your car is going to turn 2723 rpm .. Using a 225/70/15 tire .. If it has an optional 255/60/15 it will turn 2758 at 60mph . .If you gear change it you will save 250 rpm with a 3.36 .Nothing to write home about and nothing you will recover expense wise making the change and nothing that will extend engine life.
PS) you would have to run 75mph to hit 3400 .. if your tach is showing 3500 at 60mph you have an issue because you in no way shape or form could hit that rpm at 60 in a stock 1977 corvette at 60mph
1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI
I put a mark on the drive shaft and had my wife turn one of the rear wheels one turn. The drive shaft turned 3 & 3/4 turns, approximately. I agree with all of your numbers on the RPMs at different speeds. I know the formula for computing the RPMs. I have a new set of Radial T/As on the car: 225/R70/15. The tire measures about 27 inches in diameter. Thus, RPMs at 65 mph are calculated: (65 X 336 X 3.70) / 27 = 2993. Maybe, I indicated a higher RPM in a previous note, and maybe my Tach and/or speedometer are not quite right. I was just trying to indicate that the 3.70 rear end is a really low gear ratio for this car.
I realize that a 3.36 rear end would get me about a 10% reduction in RPMs at all speeds 336/370 = 0.908. But that would help reduce the general feel and vibration level of the car at highway speeds. The 350 V8 does not have to run that fast to accommodate the weight and drag of the car. Chevy put the 3.70 in the performance car to give the owner the feel that the car was responsive, I think???The same car in 1976 had a 3.36 as the standard differential. I don't think the '76 had an A.I.R. pump, so it had a little more rear wheel HP.
I got a quote from Keisler Engineering on a 5-speed manual transmission. It's a little shy of $3,000, but the tranny has a very low 1st gear (3.37) and 2nd is 1.99. So, it is not a close ratio setup. They had a better combination, but don't produce it any more. One of their engineers indicated that I wouldn't like the current combination. My current 1st gear is 2.64 and that is a really low starting ratio with the 3.70 rear end. My car is easily driven at 20 mph in 4th gear. The engine doesn't lug.
Hey, I thought this was supposed to be a friendly site?!? I just found out that my newly acquired car is a rare one. Don't think the previous owner realized that. He only drove the car 3000 miles in 24 years, so he probably wasn't concerned about drivability. I was just seeking advice from others that may have had the same questions and some possible solutions.
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Ok folks....before this gets any further, let's just all try to be
helpful without being sarcastic, or anything like that. It's hard to
tell the emotion behind printed(typed) words, and sometimes it looks
like someone is talking trash, or talking down to someone. I hope that
is not the case here. Most of our long time members do joke around, and
talk a little smack to each other every once and a while, but it is all
in fun. We want to keep it that way for everyone.
Dan, you can get the diff ratio code from the bottom of the diff. That would at least tell ya what it was originally built with, assuming it has not been changed....a lot of them have been over the years. If it does indeed have a 3.70 in it now, then I think going to the 3.36 would be just about perfect for what you are looking for, RPM-wise. My 3.36 & 225/70R15s runs just shy of 3000 rpms @70mph.
http://s77.beta.photobucket.com/user/aapple2/media/brakes/Differential/codes.jpg.html?sort=3&o=1
|UPDATED|11/16/2012 11:27:35 AM (AZT)|/UPDATED|
Dan, you can get the diff ratio code from the bottom of the diff. That would at least tell ya what it was originally built with, assuming it has not been changed....a lot of them have been over the years. If it does indeed have a 3.70 in it now, then I think going to the 3.36 would be just about perfect for what you are looking for, RPM-wise. My 3.36 & 225/70R15s runs just shy of 3000 rpms @70mph.
http://s77.beta.photobucket.com/user/aapple2/media/brakes/Differential/codes.jpg.html?sort=3&o=1
|UPDATED|11/16/2012 11:27:35 AM (AZT)|/UPDATED|
Joel Adams
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I have the same concern with my '76 (38 K miles) The engine feels like it labors so much in 4th. gear at 60-65 mph. I also am interested in driving this car a distance without feeling I need to shift into another gear...I know very little 'bout gear ratios but would like to get more answers ...Not certain what gear I am running currently. Happy to see someone with the same dilemma. Any suggestions are appreciated!!
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Rochester, NY - USA
Joined: 6/12/2012
Posts: 95
Vette(s): 1980 L- 82 fully restored and upgraded.
The gears of a std 4spd are 1.00 in 4th ,, same with all the automatics 1.00 in 3rd ..
I have the 1980 with a 3.55 rear and at 70 or so i was at 3200 plus or minus a little and that also was not in my opinion a good place to run the engine day in and day out , so I would drive 55 and be the corvette every one passed ha ha ha ha ha ..................
The solution for me was the 700R4 including a new cross member and most importantly a new shifter detent that not only gave correct access to all 4 forward gears but also gave me the correct lens readings so when you look down at it you see R N D 3 2 1. It looks so stock only us corvette nuts will notice and I bet ya people who see it will ask is this an 82 ?
Is it factory original ? NO , but the sad fact is unless it is a 0 mile or 50 mile original its value today and for the foreseeable future is low . ( c-3 low 5k high maybe 15k ) Low mileage heated storage never touched museum piece 25k to 35k and that is not going to apply to a 77 or 80 ..
With the 700R4 My first gear is now 3.06 when multiplied by rear ratio I am getting a 10.8 t/q multiplier that a good street car would want if it seeks decent performance, and I am now 30% less rpm to go 70 add the lock up converter and I drop 100 or so more rpm when on the highway. Making the car a dream to drive and I have to be careful because I am so use to the revving noise I tend to keep my foot in it and realize I am at 90 lol...
So it is all a compromise , even original numbers matching can be maintained by storing the th350 in a crate wrapped in plastic .. I long ago loving the body style decided power and good street manners trumped all else yet also felt the car should look correct to observers and even under my hood it should be less flash and more oem in how it looks.
430hp
430tq
3.55 rear
And to look at it and here it one would be hard pressed to suggest any serious changes have ever been made and I am proud of that .
1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI
You can check the differential's gear ratio by marking the drive shaft and have someone turn a back wheel one complete turn, while you count the number of times that the drive shaft turns. That is a very simple way of determining the gear ratio. Most of the members on this site have recommended the 3.36 differential for those with a 3.70.
I have talked to several different suppliers and they won't make a clear recommendation, understandably. However, one of these suppliers indicated that owners have even gone to the 3.08 to quiet down the RPMs. That would make the car much better at highway speeds, but would affect the "hole shot."
I am going to have a different differential put into my '77. I will keep the site posted on what I decide upon, but it is most probably going to be the 3.36. The speedometer gear has to be changed. That is a simple change and the correct gear can be acquired from Ecklers or several other suppliers.
in Forum: C3 Driveline Components
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