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I put a 4:10 in my 95 Z28 t56 and absolutely loved it, never noticed mileage change because it never tried to lug again. I'm going from 3:7 in my stock 76 L82 to a 4:10 with the 4 speed, I'll deal with it, maybe gear vendors od.
Use one of the online ratio calculators-- figure out where your peak torque with the cam is going to be (RPM), then figure the speed you will be running most of the time and gear for that.
I ended up building a 3.55 rear to go with my 4 speed and 496. The original 4.10 was just nuts. 4k plus at 70..
Put it simply, torque ain't an issue.
Interesting thread for me - I currently have 3.36 gears with my tko600 and a similar sbc stroker to the op. Having recently got the car back on the road after some work, the gearing really is tall. The engine has no trouble pulling the gear, but I can easily top 80mph in 2nd gear... I have a 3.7 center to rebuild and swap out, but am thinking I might actually prefer 3.9 or 4.1. Now having said that, for pure street use the 3.36 are fine/great - I can turn street tyres with ease in each gear, and it's just idling along on the highway. But I think for my track use the extra gearing might be an advantage - just need to try it and see.
I'm looking for input from folks who may have done a similar build.
72 car, originally 350 w/auto, now 383 around 450-500hp with a 6 speed t56 w/2.66 first gear.
Car will be mostly a cruiser with occasional spirited driving. No track or strip.
I'm not sure what it has in it now but I'm assuming it's a 3.08 or 3.30. Either way, not what I'm looking for. Considering 3.70 or 4.11.
Thoughts?
My 1978 is a Chevy crate ZZ383 with a T56 Magnum 6 speed 2.66 first gear. The rear is a 3.08 ratio. Car is used for track days / cruiser. There is no problem running in any gear. 1st - 6th.
Now if you what to spin the tires (make smoke) then you need a lower gear to make this easier. If you like a 2-4 sec pedal to the metal runs then lower the gear. Other wise leave the diff @ 3.08 or 3.36 and give it a try.
6th gear @ 74 MPH = 1800 RPM which is for me ~ 23- 24 MPG.
But I think for my track use the extra gearing might be an advantage - just need to try it and see.
Try it on a simulator first and then you will start to see lower gears at the track are usually faster than higher gears.
eg. My local track I made on a simulator and tested different rear ratio's. The 3.08 was usually the fastest with the T56 magnum ratio's.
So for track days what I look for is reaching the max torque / hp RPM near the end of the straights.
eg. my zz383 rpm ~ 5500 in third gear = 109 MPH 3.08 with 3.73 rear = 90 MPH
so on a straight, if the car needs a extra shift to 4th because I reach a red line and lap times are slower. To goal is to keep the engine RPM in the max torque range and long as possible. High gears shorten the time in the max torque RPM ranges. Unless you have a very flat torque / hp curve which then again shifting costs lap times.
I'm looking for input from folks who may have done a similar build.
72 car, originally 350 w/auto, now 383 around 450-500hp with a 6 speed t56 w/2.66 first gear.
Car will be mostly a cruiser with occasional spirited driving. No track or strip.
I'm not sure what it has in it now but I'm assuming it's a 3.08 or 3.30. Either way, not what I'm looking for. Considering 3.70 or 4.11.
Thoughts?
I bought a close ratio T56 Magnum which I believe is the same as yours. My 74 L82 4 speed had a 3.55 rear end. The body is still off my car but I think I would have been content with a 3.55 or even the 3.30 ratios. However, I went with a 3.73 for two reasons. On the low end, when I looked at the 1-2 shift points I figured that my LS3 (and your 383) have so much low end torque that I would probably end up using 2nd gear for casual take offs regardless and 1st gear would be a tire roasting monster. On the top end, there was only 150 or so RPM difference at 70 and since I have a convertible my car will be heavily muffled so no issues at highway speeds either.
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It really depends on what you want. I had 3.55's with a M21 and that was a 2.20 first gear. A real dog off the line but would find 110mph pretty easily. 3500 rpm at 85. I have since switched to 3.70's and a M20 with a 2.52 first. A real neck snapper now. For you, you should decide what you want , better acceleration from a crawl or stop. Or stupid high top end, its all in how you intend to have fun. Theres a lot of online gear calculators that will give you top speed in each gear or E.T.'s. I would look for a gear ratio that gives you a high 9:1 or 10:1 launch gear if you want to accelerate quickly and with a 6 speed the top end still wont be limited
Also, I totally forgot to mention that I am running an 18lb flywheel so my concern is that the tall gearing which is stock might cause bogging off the line. Also since the cam is happy 1900+ I was figuring on 75mph near there. I don't know why I completely blanked out about the online gearing calcs so thanks for the suggestion. I am off to play with those for a bit and see what I can find that will work for me.
**EDIT**
Some really interesting stuff here, thanks again to all who suggested the online gearing calculator.
So, given that I have absolutely no need to be doing 100mph at 1900rpms, the 3.08 is out
Going shorter down the list, even the 4.1's barely put me into the cams operating range @ 75mph so it looks like that will be the way that I go.
Last edited by PainfullySlow; May 7, 2019 at 08:13 AM.
Also, I totally forgot to mention that I am running an 18lb flywheel so my concern is that the tall gearing which is stock might cause bogging off the line. Also since the cam is happy 1900+ I was figuring on 75mph near there. I don't know why I completely blanked out about the online gearing calcs so thanks for the suggestion. I am off to play with those for a bit and see what I can find that will work for me.
**EDIT**
Some really interesting stuff here, thanks again to all who suggested the online gearing calculator.
So, given that I have absolutely no need to be doing 100mph at 1900rpms, the 3.08 is out
Going shorter down the list, even the 4.1's barely put me into the cams operating range @ 75mph so it looks like that will be the way that I go.
One of the forum members installed 4.30 rear gearing with his 6 speed because of the .5 OD to make 6th gear usable on the freeways. He actually considered a 4.56 rear end. I have had 4.11 rear end in my 79 Vette since the late 1980's because of OD trannies. It really makes a car fast and fun to drive when you close up the gearing.
I have a 11.5 pound flywheel with a 2.89 first gear in a tko600 and with my 650 hp 427 was really nice even for full powered take offs or just minimal clutch slipping just above idle.
Launch ratio. ResDog mentioned it. 1st gear times final drive gearing. Street: low side of 10. 9, 9.2 optimal. Drag strip: around 11. But you don't shift into second at 6500 on the street...
Last edited by derekderek; May 7, 2019 at 09:25 PM.
You generally want an overall ratio of 10-11:1 in first gear. That 2.66 first gear gives you 9.92:1 with the 3.70 and 10.9 with the 4.11's. Why not split the difference and install 3.90's?
Mike,
I would say 95% of the diff's I built for an OD trans were 373's, some 411, few 355. You are wise to match the gear to your eng/trans.
Some notes
Toms has gears made for a 3 series case in ratios
308,336.355,373, 411, 433,456 These are USA made by US Gear. The 370 ratio is not offered as the 373 is stronger. A real 370 gear can be purchased but it will be from another company and imported. 273's are long gone. Richmond had 390s same as the GM performance parts catalog from 40 years ago. I stick with US Gear for good reason.
In addition to ratio you need to address other areas within the diff at your power level. If you are coming to the June 15th seminar I can show you everything you would want to know about these diffs.
You generally want an overall ratio of 10-11:1 in first gear. That 2.66 first gear gives you 9.92:1 with the 3.70 and 10.9 with the 4.11's. Why not split the difference and install 3.90's?
if you are going to the track, yes. for pulling out at traffic lights, 11 will have you hitting second in 1 car length. 9.0 is best driver ratio. 6 speeds are really overkill. they are more for LS engines with 50 year newer computerized engine management to make the engine happy at 1900 rpm's under load.
I am running a T56 from an 06 GTO with a 3.73 rear gear ratio. 6th gear is .57 and it rarely gets used unless you are on the highway. Around town 5th is good with a .84 OD gear. I am running an LS3 for power. Cam is a mild upgrade from the stock. Pushing around 500HP.
I was not aware that 3.90 gears are no longer available. If I had to choose between the 3.70 and 4.11's I would probably go with the 3.70's. I have had cars where first gear was too deep, and cars where first was too tall. I would prefer to drive too tall. If first gear is too deep, its almost useless. You are shifting before you get halfway across the intersection. With that said, 4.11's may not be too tall for your application. I have an 18 lb. flywheel behind my 427 small block, and even with that torque you have to be careful not to bog the motor off the line.
What diff gear ratio?
, I'd go with a 2.73.
