Rear End Ratio Blues
02-02-2014, 04:42 PM
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Rear End Ratio Blues I have a 74 Corvette that I'm planning an LQ9/4L60e swap. I've planned for the LQ9/4L60e swap by converting the tachometer from mechanical to electronic. I've converted the automatic shifter from 3 speed auto to a 4 speed auto using the Shiftworks conversion kit in preparation for the 4L60e. The other night, I crawled under the car, grabbed some numbers off the rear end and confirmed is was a 3:08 ratio. Rather disappointed, hoping for a little lower gear, I began researching to find out whether the 3:08's will provide a good match with the 4L60e, and here is where there seems to be many differing opinions. First, let me lay out my plans. I figured right out of the gate I'd end up needing a stall torque converter, somewhere in the 2800-3200 rpm range and figured at least a 3:50 rear end ratio leaning toward 3:73's. Not particularly wanting to spend the money on a gear swap, I ran into a few ideas from others I'd like to pass on for your input. Keep in mind, I'm an old school smallblock kind of guy who knows their way around this engine quite well, however the LS1 is a different animal. So is the 4L60e with it's oh so cool overdrive. Others have suggested is to leave the 3:08's intact, go with the stall converter and have the shiftpoints of the 4L60e matched with the 450 or so HP and 3:08's for optimum shifting by changing parameters in the ECM.. I also understand the 4L60e has a very low 1st gear which is great, but what about 2nd? it almost sounds like there needs to be a gear between the two, and that is why I question the 3:08's. Several have said a good stall torque convertor of the 3000 rpm flavor would correct the issue. Others have said it will work fine, just like it is. My goal for this project is to have around 450 hp of dependable engine, good fuel mileage, and be able to take it to the 1/4 mile strip with my boys and run a good 12 second time OR hop in it and simply go. I live not far from the Kiamichi Mountains, and I can visualize a Fall ride through them with my top down, living the dream I have carried for over 40 years... Look forward to hearing some input!
02-02-2014, 05:11 PM
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I think you will have a nice setup without changing the rear end. I'm running a 3.73 rearend on a carb. 350 engine and a 350 trans with similar stall that I am very pleased with. Given that the 4L60E has a lower 1st gear you should be able to get a similar final drive to what I have and also be able to get the shift points dialed in how you like it. I would run it for awhile and see how it performs and if you are still not satisfied then you can always do the rear end swap later.
02-02-2014, 05:33 PM
#3
Just a couple thoughts - looking at numbers on the outside doesn't actually confirm what is inside, you need to open it up to really be sure. But that's easy enough to do so no big deal.
Also, it's a torque converter, not a stall converter; the stall rpm range is just the point at which you're spinning as fast as you can before you start moving (assuming it's in D). Not trying to be a dick here, just clarifying since people often toss these terms around without considering what they mean.
But changing the gears will be just as easy (or easier, since the car will move under its own power) once the engine is in, so there isn't much harm in trying it like it is. Might as well give it a shot, at worst you get an idea of where you are starting from and how much (if anything) you need to change. There isn't any real reason why what you have shouldn't work fine, but good gas mileage and good 1/4 times are sort of opposite ends of the gearing spectrum.
Also, it's a torque converter, not a stall converter; the stall rpm range is just the point at which you're spinning as fast as you can before you start moving (assuming it's in D). Not trying to be a dick here, just clarifying since people often toss these terms around without considering what they mean.
But changing the gears will be just as easy (or easier, since the car will move under its own power) once the engine is in, so there isn't much harm in trying it like it is. Might as well give it a shot, at worst you get an idea of where you are starting from and how much (if anything) you need to change. There isn't any real reason why what you have shouldn't work fine, but good gas mileage and good 1/4 times are sort of opposite ends of the gearing spectrum.
02-02-2014, 05:35 PM
#4
IMHO that gear ratio is on the tall side for your combo. No knowing what ratios are available for your rear I would think something in the 3:40 to 3:50 range would be a good compromise. Low enough for the quarter mile, but still have good crusing rpm in overdrive for decent gas milage. By the way what diameter tires are you running?
02-02-2014, 06:16 PM
#5
IMHO that gear ratio is on the tall side for your combo. No knowing what ratios are available for your rear I would think something in the 3:40 to 3:50 range would be a good compromise. Low enough for the quarter mile, but still have good crusing rpm in overdrive for decent gas milage. By the way what diameter tires are you running?
02-03-2014, 08:22 AM
#6
Keep in mind the 98-02 f body cars ran a 5.7/4L60 with 2.73 rear. The 3.08 rear will be fine with the 6.0/4L60 unless you go really big cam, carbed, etc.
It may be a bit less than ideal for the track, but just look at the 1/4 mile times of the LS1 f body cars. They ran low, mid 13 stock. You'll have more torque, less weight, a little better gear, so high 12s is possible as is. Good milage, drivability too
Try it with your existing rear. Then swap out if needed.
Btw, I love my 68 corvette convertible, LQ9, Richmond 6 speed, 3.08 rear combo.
It may be a bit less than ideal for the track, but just look at the 1/4 mile times of the LS1 f body cars. They ran low, mid 13 stock. You'll have more torque, less weight, a little better gear, so high 12s is possible as is. Good milage, drivability too
Try it with your existing rear. Then swap out if needed.
Btw, I love my 68 corvette convertible, LQ9, Richmond 6 speed, 3.08 rear combo.
02-03-2014, 04:04 PM
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Keep in mind the 98-02 f body cars ran a 5.7/4L60 with 2.73 rear. The 3.08 rear will be fine with the 6.0/4L60 unless you go really big cam, carbed, etc.
It may be a bit less than ideal for the track, but just look at the 1/4 mile times of the LS1 f body cars. They ran low, mid 13 stock. You'll have more torque, less weight, a little better gear, so high 12s is possible as is. Good milage, drivability too
Try it with your existing rear. Then swap out if needed.
Btw, I love my 68 corvette convertible, LQ9, Richmond 6 speed, 3.08 rear combo.
It may be a bit less than ideal for the track, but just look at the 1/4 mile times of the LS1 f body cars. They ran low, mid 13 stock. You'll have more torque, less weight, a little better gear, so high 12s is possible as is. Good milage, drivability too
Try it with your existing rear. Then swap out if needed.
Btw, I love my 68 corvette convertible, LQ9, Richmond 6 speed, 3.08 rear combo.
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02-03-2014, 05:01 PM
#8
A 2000 Camaro SS had a tire diameter of somewhere around 25.6". His 255/60-15 is somewhere around 27" in diameter. That alone will drop the rpms. I would say for now run the 3.08 gears and see how you like them. Down the road maybe change the gear ratio.
02-03-2014, 05:55 PM
#9
Check over at CF, like I said I'm running a manual trans.
The LQ9 is somewhat under rated, it's about 400 hp with headers and simple tune. 500 with L92 heads, cam, intake is doable, but most will be above 5000rpm. so I just stuck with the stock LQ9.
http://www.hotrod.com/techarticles/e...#ixzz21ehbAsre
LS motors don't follow all the rules of a 60s SBC, ie, no need for big cams, lots of overlap. Try the 3.08. Vette rears are expensive to swap/rebuild. If you need a better 1/4 mile time, swap it out. But for the street, fun driving, the 3.08s will be fine.
Oh yeah, the C3 IRS is not ideal for 1/4 mile times, so adding hp or better gears might not gain you as much on a stock suspension as it would on other cars.
The LQ9 is somewhat under rated, it's about 400 hp with headers and simple tune. 500 with L92 heads, cam, intake is doable, but most will be above 5000rpm. so I just stuck with the stock LQ9.
http://www.hotrod.com/techarticles/e...#ixzz21ehbAsre
LS motors don't follow all the rules of a 60s SBC, ie, no need for big cams, lots of overlap. Try the 3.08. Vette rears are expensive to swap/rebuild. If you need a better 1/4 mile time, swap it out. But for the street, fun driving, the 3.08s will be fine.
Oh yeah, the C3 IRS is not ideal for 1/4 mile times, so adding hp or better gears might not gain you as much on a stock suspension as it would on other cars.
Thanks for chiming in Gary! I guess I'm having a hard time with old school thinking. I've never had a rod with an overdrive, much less an LSx engine other than my Silverado. My son has a 2000 TransAm that we are equipping with twin turbos (sschoeffler). Without his knowledge and expertise I never would have entertained taking on an LS swap. Now that I'm knee deep in it, it's starting to get exciting! It's also hard to get a grasp that I can take an LQ engine, swap heads and cam only and make very respectable hp numbers....and still be docile enough to be street-able with good fuel mileage numbers. Have you known others running a high stall converter, and if so, what rpm stall is it?
Last edited by garys 68; 02-03-2014 at 06:10 PM.
02-04-2014, 09:38 AM
#11
Launching!
I run 3.73 with a 26 inch tall tire and a 700r4 in a 68 Nova and see approx. 2400 rpms at 70 mph. Mileage is 18.5 with 333 cid small block with port fuel injection.