Ideal diff ratio
Thread Starter
Racer
Joined: Jan 2005
Posts: 370
Likes: 77
From: Perth Western Australia
Ideal diff ratio
I'm putting together a new driveline and would appreciate any input to what would be a suitable diff ratio.
Engine is 550hp sb with .562 .580 and about 250@.050 lift duration cam. Transmission is SS700 5 speed with a close ratio gearset (2.66, 1.78, 1.30, 1.0, 0.68) and 26.66" tire diameter.
Currently have 3.36 diff ratio but looking at 3.90.
I don't want to be riding the clutch in first so figure high 3's or low 4's final ratio, considering there is a 32% overdrive.
Brad
Engine is 550hp sb with .562 .580 and about 250@.050 lift duration cam. Transmission is SS700 5 speed with a close ratio gearset (2.66, 1.78, 1.30, 1.0, 0.68) and 26.66" tire diameter.
Currently have 3.36 diff ratio but looking at 3.90.
I don't want to be riding the clutch in first so figure high 3's or low 4's final ratio, considering there is a 32% overdrive.
Brad
Burning Brakes
Joined: Apr 2007
Posts: 1,039
Likes: 62
From: Kona Hawaii
Ideal start off ratio is said to be 10:1.Used this formula for my trans selection and it works very well.My rear gear 3.08 X the trans first gear ratio of 3.28 gives a 10.1 just about perfect.
Your ideal rear gear with a 2.66 1st gear would be 3.75 or close.The 3.90 should work well with that 1st gear and the .68 OD will keep those highway rpm's down and not lug the engine either.If you keep your 3.36 rear gears you'll slip your clutch more on start offs and your engine will lug at highway speeds.For optimal performance match the rear gear to your transmission. One of the reason's I went with a Richmond 6 spd trans was It matched up well with the existing 3.08 rear gear.Didn't want the added expense of replacing the rear gears.
Your ideal rear gear with a 2.66 1st gear would be 3.75 or close.The 3.90 should work well with that 1st gear and the .68 OD will keep those highway rpm's down and not lug the engine either.If you keep your 3.36 rear gears you'll slip your clutch more on start offs and your engine will lug at highway speeds.For optimal performance match the rear gear to your transmission. One of the reason's I went with a Richmond 6 spd trans was It matched up well with the existing 3.08 rear gear.Didn't want the added expense of replacing the rear gears.
Burning Brakes
Joined: Dec 2003
Posts: 1,004
Likes: 7
From: Adelaide South Australia
3.73 works well for me with a TKO600 RR ( 0.82 OD) behind a
383 with a 242/240@ 0.050 hyd roller ~475 hp / 495 lbft
Our highway speeds in Australia are limited to 100 to 110kph (60~70 mph) so my highway cruising is around 2400 to 2600 in 5th..... sits at a nice sweet spot just where torque begins to really climb.
You are running a bigger cam, so it's reasonable to assume your power band is shifted several hundred rpm higher than my setup.
With your 2.66 1st gear and 0.68 OD I would suggest 3.90 would be better.
383 with a 242/240@ 0.050 hyd roller ~475 hp / 495 lbft
Our highway speeds in Australia are limited to 100 to 110kph (60~70 mph) so my highway cruising is around 2400 to 2600 in 5th..... sits at a nice sweet spot just where torque begins to really climb.
You are running a bigger cam, so it's reasonable to assume your power band is shifted several hundred rpm higher than my setup.
With your 2.66 1st gear and 0.68 OD I would suggest 3.90 would be better.
Melting Slicks
Joined: Aug 2012
Posts: 2,548
Likes: 379
From: Woodinville WA
Consider what you're doing - with a lower gear, you'll be able to control your 1st gear launch a little better, but after you're moving, what the rear pumpkin contains doesn't make much (if any) difference. What do you think it'll be worth in your 60' time? maybe .02? That's maybe 1/10 on the top end.....
I had 3.91s in a 06 GTO with a 6 speed... 1st gear was useless after the swap (from 3.42s), and I honestly don't think swapping gears helped in any way. It did mean you had to change gears more often and shift at least one more time on your normal cruise.
In the end, I swapped back to a stock differential - but that was more because 1320gforce couldn't set up the gears so they didn't howl (3x).
I had 3.91s in a 06 GTO with a 6 speed... 1st gear was useless after the swap (from 3.42s), and I honestly don't think swapping gears helped in any way. It did mean you had to change gears more often and shift at least one more time on your normal cruise.
In the end, I swapped back to a stock differential - but that was more because 1320gforce couldn't set up the gears so they didn't howl (3x).
Thread Starter
Racer
Joined: Jan 2005
Posts: 370
Likes: 77
From: Perth Western Australia
Thanks everyone for the input
I think 3.90 might be the go.
I'm currently using a muncie with a 2.52 1st gear (3.36 diff) and a 383 with about 220@0.50 lift duration. I don't like the way it performs with this set-up and think the 3.73 would suit this better.
My new set-up is a 427 with a larger cam so would have to agree with OzzyTom and iokepakai on the 3.90 rear even with the new 2.66 1st gear.
A diff ratio calculator puts the revs at 2300 at 110 kph (about 70 mph) with a 3.90 diff, 0.68 overdrive and my diameter tyres. The revs drop down to 2200 with the 3.73 diff at the same speed. I think 2200 would be pulling the engine down to low.
I think 3.90 might be the go.
I'm currently using a muncie with a 2.52 1st gear (3.36 diff) and a 383 with about 220@0.50 lift duration. I don't like the way it performs with this set-up and think the 3.73 would suit this better.
My new set-up is a 427 with a larger cam so would have to agree with OzzyTom and iokepakai on the 3.90 rear even with the new 2.66 1st gear.
A diff ratio calculator puts the revs at 2300 at 110 kph (about 70 mph) with a 3.90 diff, 0.68 overdrive and my diameter tyres. The revs drop down to 2200 with the 3.73 diff at the same speed. I think 2200 would be pulling the engine down to low.
Last edited by wabco40; Apr 27, 2013 at 06:48 PM.
Pro
Joined: Oct 2009
Posts: 523
Likes: 4
From: Calgary Alberta
Thanks everyone for the input
I think 3.90 might be the go.
I'm currently using a muncie with a 2.52 1st gear (3.36 diff) and a 383 with about 220@0.50 lift duration. I don't like the way it performs with this set-up and think the 3.73 would suit this better.
My new set-up is a 427 with a larger cam so would have to agree with OzzyTom and iokepakai on the 3.90 rear even with the new 2.66 1st gear.
A diff ratio calculator puts the revs at 2300 at 110 kph (about 70 mph) with a 3.90 diff, 0.68 overdrive and my diameter tyres. The revs drop down to 2200 with the 3.73 diff at the same speed. I think 2200 would be pulling the engine down to low.
I think 3.90 might be the go.
I'm currently using a muncie with a 2.52 1st gear (3.36 diff) and a 383 with about 220@0.50 lift duration. I don't like the way it performs with this set-up and think the 3.73 would suit this better.
My new set-up is a 427 with a larger cam so would have to agree with OzzyTom and iokepakai on the 3.90 rear even with the new 2.66 1st gear.
A diff ratio calculator puts the revs at 2300 at 110 kph (about 70 mph) with a 3.90 diff, 0.68 overdrive and my diameter tyres. The revs drop down to 2200 with the 3.73 diff at the same speed. I think 2200 would be pulling the engine down to low.
Please keep us posted on how this goes for you. I currently also have a 427 SBC with .545/.565 intake/exhaust, and 241@.050 lift duration making about 550hp and 550 ft/lbs. Currently my car has a stock 3.70 diff and I want to install a SS700 Keisler trans to replace the BW ST-10 but would like to know if it is worth while replacing the diff gear with a 3.90 or even a 4.11 gear. My tires are 27 inches in diameter. Thanks for the info.
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Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Ideal start off ratio is said to be 10:1.Used this formula for my trans selection and it works very well.My rear gear 3.08 X the trans first gear ratio of 3.28 gives a 10.1 just about perfect.
Your ideal rear gear with a 2.66 1st gear would be 3.75 or close.The 3.90 should work well with that 1st gear and the .68 OD will keep those highway rpm's down and not lug the engine either.If you keep your 3.36 rear gears you'll slip your clutch more on start offs and your engine will lug at highway speeds.For optimal performance match the rear gear to your transmission. One of the reason's I went with a Richmond 6 spd trans was It matched up well with the existing 3.08 rear gear.Didn't want the added expense of replacing the rear gears.
Your ideal rear gear with a 2.66 1st gear would be 3.75 or close.The 3.90 should work well with that 1st gear and the .68 OD will keep those highway rpm's down and not lug the engine either.If you keep your 3.36 rear gears you'll slip your clutch more on start offs and your engine will lug at highway speeds.For optimal performance match the rear gear to your transmission. One of the reason's I went with a Richmond 6 spd trans was It matched up well with the existing 3.08 rear gear.Didn't want the added expense of replacing the rear gears.
10:1 in first gear, that's the old rule of thumb. That's why the 4.10 and 4.11 gear used to be so popular; combine it with a 2.52 or so first gear (which was about what most trannys used for first gear back then), and there's your 10:1 ratio. Much more than that, and the car starts to drive kinda like a tractor. That being said, you're running a pretty good sized cam there, and with the overdrive, I'd be tempted to go with the 4.10, which will give you 10.9:1 in first gear (2.66 x 4.10 = 10.906) and a final drive in top gear of about 2.79:1 (4.10 x .68 = 2.788)
Scott
Last edited by scottyp99; May 8, 2013 at 12:04 AM.
Drifting
Joined: Aug 2005
Posts: 1,443
Likes: 65
From: Wauconda IL
Depending on where your 500 or so ft/lb of torque comes in should also be factored in. The 10:1 number is more of a guideline than a rule.
If you have 350 ft/lb in at 2000 RPM then you probably don't need to have the magic 10:1-
On the other hand, if you can't muster 150 ft/lbs at 2000, then you may need 10:1 or more.
If you have 350 ft/lb in at 2000 RPM then you probably don't need to have the magic 10:1-
On the other hand, if you can't muster 150 ft/lbs at 2000, then you may need 10:1 or more.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,443
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Why incur the additional rear end cost.
Get the wide ratio 700 with the .80 OD.
I drove my 427/434 SBC with a 4.11 with a TKO 600. The .64 OD sucked even with a 4.11 because of the big gear jump 4-5. The first 4 gears were fun and then it was the road gear 5th.
I have over 11:1 compression with my 248/250 solid roller. It runs fine on 91 super unleaded.
I hope your motor plan included good heads
Get the wide ratio 700 with the .80 OD.
I drove my 427/434 SBC with a 4.11 with a TKO 600. The .64 OD sucked even with a 4.11 because of the big gear jump 4-5. The first 4 gears were fun and then it was the road gear 5th.
I have over 11:1 compression with my 248/250 solid roller. It runs fine on 91 super unleaded.
I hope your motor plan included good heads
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,443
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I was going to add that the TQ of a 427 ci is so much greater than the less than 400 ci motors the need for a closed ratio type gear box is gone.
As power goes up less time shifting is more desirable. You need to do some math. I changed to a 3.55 rear end for making my first 4 gears functional at going around the local road racing track. I never have to shift into 5th on the main straight because I can now go about 160 mph in 4th before hitting the rev limiter and the brakes before the first turn.
Another down side to 3.90/4.11 rear gears is drive shaft rpm and rear diff. heat. MY 4.11 really needed a rear end cooler
As power goes up less time shifting is more desirable. You need to do some math. I changed to a 3.55 rear end for making my first 4 gears functional at going around the local road racing track. I never have to shift into 5th on the main straight because I can now go about 160 mph in 4th before hitting the rev limiter and the brakes before the first turn.
Another down side to 3.90/4.11 rear gears is drive shaft rpm and rear diff. heat. MY 4.11 really needed a rear end cooler
Racer
Joined: Feb 2013
Posts: 386
Likes: 3
From: Clifton Virginia
I'm putting together a new driveline and would appreciate any input to what would be a suitable diff ratio.
Engine is 550hp sb with .562 .580 and about 250@.050 lift duration cam. Transmission is SS700 5 speed with a close ratio gearset (2.66, 1.78, 1.30, 1.0, 0.68) and 26.66" tire diameter.
Currently have 3.36 diff ratio but looking at 3.90.
I don't want to be riding the clutch in first so figure high 3's or low 4's final ratio, considering there is a 32% overdrive.
Brad
Engine is 550hp sb with .562 .580 and about 250@.050 lift duration cam. Transmission is SS700 5 speed with a close ratio gearset (2.66, 1.78, 1.30, 1.0, 0.68) and 26.66" tire diameter.
Currently have 3.36 diff ratio but looking at 3.90.
I don't want to be riding the clutch in first so figure high 3's or low 4's final ratio, considering there is a 32% overdrive.
Brad
Thread Starter
Racer
Joined: Jan 2005
Posts: 370
Likes: 77
From: Perth Western Australia
Hi wabco40,
Please keep us posted on how this goes for you. I currently also have a 427 SBC with .545/.565 intake/exhaust, and 241@.050 lift duration making about 550hp and 550 ft/lbs. Currently my car has a stock 3.70 diff and I want to install a SS700 Keisler trans to replace the BW ST-10 but would like to know if it is worth while replacing the diff gear with a 3.90 or even a 4.11 gear. My tires are 27 inches in diameter. Thanks for the info.
Please keep us posted on how this goes for you. I currently also have a 427 SBC with .545/.565 intake/exhaust, and 241@.050 lift duration making about 550hp and 550 ft/lbs. Currently my car has a stock 3.70 diff and I want to install a SS700 Keisler trans to replace the BW ST-10 but would like to know if it is worth while replacing the diff gear with a 3.90 or even a 4.11 gear. My tires are 27 inches in diameter. Thanks for the info.
Its still a few months away from happening. I am still waiting on Keisler to ship the trans and the engine is currently being built. The whole lot then needs to be shipped down to Australia.
I'm still thinking I'll start with the 3.9's
Thanks everyone for the input