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Gearing for torquey cruiser

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68Monaro400

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Discussion starter · #1 ·
Putting together a cruiser. While it will mostly cruise around town it will do some 2000 mile drives to car shows.

68 RHD Impala
TH400 2200 stall, GV overdrive (.788) 275/60/15 rear tyres (28")
On the open road I will drive between 60 and the max legal speed of 80mph depending on how it handles. Open roads here are STRAIGHT.
When finished it will run 533 BBC rec port heads. 238/244@112 cam 10.2/1 L72 (1969) intake (looking for factory look) with throttle body efi long tube 2" headers. I expect it will make 650ftb & 600BHP@ 5500RPM

Presently has a 3.55 12bolt open wheeler. I was going to run this but it will be easier to pull the diff, rebuild suspension and brakes and stick a LSD centre with the right gearing in one hit.

I was thinking 3.08/1 rear but maybe 2.78/1 (plus overdrive) might give better open road mileage.
 
#2 ·
With that much TQ/HP either one would be a good choice. The 3.08 would give you better snap off the line, your effective final drive ratio would be 2.42. With 28" tires, you'll be turning a leisurely ~1740 rpm @ 60mph.
 
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#3 ·
Cubic .inches . will pull tight gearing .
Any idea of idle vacuum ?? This will give some idea of how the cam reacts with cu.inches .

All at 3rd gear ratio 1:1
70mph at 3.55 at 3125 rpm
70mph at 3.08 at 2700rpm
45mph at 3.08 at 1760 rpm [ u might suffer from a lack of responsiveness ,unless the th400 kicks down to 2nd if gearing is tighter ].

Considering tight convertor 2200stall going from 3.55 to 3.08 will give a big performance change . It will be definitely in the right direction without going overboard .
Depends on what you like there is also 2.6 gears !! LOL

The convertor might run hot if u drive under stall rpm for a long time
Convertor slip is measured into calculations at 5 %

Considering the 4th gear o/drive of .788
70mph at 3.55 at 2470rpm
70mph at 3.08 at 2150rpm
60mph at 3.08 at 1850rpm
45mph at 3.08 do not use O/drive [3rd = 1760rpm ]
 
#5 ·
68Monaro400 said:
Putting together a cruiser. While it will mostly cruise around town it will do some 2000 mile drives to car shows.

68 RHD Impala
TH400 2200 stall, GV overdrive (.788) 275/60/15 rear tyres (28")
On the open road I will drive between 60 and the max legal speed of 80mph depending on how it handles. Open roads here are STRAIGHT.
When finished it will run 533 BBC rec port heads. 238/244@112 cam 10.2/1 L72 (1969) intake (looking for factory look) with throttle body efi long tube 2" headers. I expect it will make 650ftb & 600BHP@ 5500RPM

Presently has a 3.55 12bolt open wheeler. I was going to run this but it will be easier to pull the diff, rebuild suspension and brakes and stick a LSD centre with the right gearing in one hit.

I was thinking 3.08/1 rear but maybe 2.78/1 (plus overdrive) might give better open road mileage.
Click to expand...
What tires? If sticky, probably want to upgrade axles too.
If not, Jack up the rear and slide in a Yukon Spartan locker. Based on my experience, With an open rear and that much power, your car must be borderline undrivable from a dig.
 
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#6 ·
Just to give you a comparison my 80E has a .75 OD and locks up. Locked up with 3.73's at 75 mph is doing 2500.
IMO best of both worlds.
Yours I would assume would be higher as the converter doesn't lock.
I'd leave what you have. Going lower "could" make the current converter act looser losing gearing in the rear.
Especially in OD you'll be taking more away.
I could be wrong though.
 
#7 ·
Is the rear end currently in working condition? if it is I'd leave it alone until you can drive the car to see what you think of the 3.55, seems like a good gear for around town driving
 
#8 ·
I agree with leaving the 3.55's in there for now.
With your .788 OD Gear Vendors this works out to a 2.794 final drive ratio.
If you go any taller than this you may encounter "bucking" or surging when driving on the highway.
Big blocks and big cams do not make for good low rpm cruising.
My 513 has a bit bigger cam than what you are using,and it does not like to be lugged down below 2200 rpm.
Guy
 
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#9 ·
GuysMonteSS said:
I agree with leaving the 3.55's in there for now.
With your .788 OD Gear Vendors this works out to a 2.794 final drive ratio.
If you go any taller than this you may encounter "bucking" or surging when driving on the highway.
Big blocks and big cams do not make for good low rpm cruising.
My 513 has a bit bigger cam than what you are using,and it does not like to be lugged down below 2200 rpm.
Guy
Click to expand...
Guy, He would/could get bucking with a converter that doesn't lock?
I was thinking he would lose mech coupling in converter in od.
 
#10 ·
I have a 540 with a 3.73 and GV also and I think for me a 3.42 would be good for cruising or a 2.60 final drive. That being said you have a smaller more streetable cam / set up and a 3.08 would be great for you.
 
#12 ·
My 461 is mated to a T56 Magnum 6 speed (2.97 1st, 0.63 OD) that spins 27" diameter tires via 3.77 gears. It accelerates "now" (11.2 inital drive) and cruises the freeway 70mph @ 2056 rpm (2.32 final drive). Best of both worlds.;)
 
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#13 ·
you all need to step up your game a bit more. 0.50 OD, 3.73 rear gearing, 28 tall tire.... 80mph @ 1700rpm. car runs an easy 10 degrees cooler now, oil pressure up 3 psi, and ALOT less road noise.


You got big displacement, run those rpms down. Not only will your engine thank you for the lesser workout, so will your wallet with todays gas prices.
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#14 · (Edited)
3.55, 3.31 or 3.08 should all work great. I have 3.31 behind my 600hp/600tq 489 without od but less cui and more duration 242/248@050.
 
#15 · (Edited)
On busy accordion freeways I often slow to 45 mph (1322 rpm) and can smoothly range back up to 70 mph (2056 rpm) without shifting out of 6th. No lugging or spitting. 33 lb steel flywheel helps :cool:
 
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#16 ·
Yo GOFAST, I aint no engineer but I would NOT run a 2200 rpm stall converter at 1750rpm for hours myself. Right?

Swampy should set his min cruise rpm to that stall. Its not a lock up. With the GV on, its effectively pulling a 2.80 final drive with 3.55. If Swampy could have run the 4th OD cruise RPM # for 3.23:1 final drive , it may have been SPOT ON.

So i;m with the keep the 3:55s for now, to see how they feel. If good, THEN upgrade axles+ posit unit. If not then swap out axle.
 
#17 ·
I'm with keeping the 3.55's as well until you figure out how the engine behaves while cruising at speed. That's a pretty good size cam and lugging the engine below it's cruise RPM sweet spot can cause bucking/surging issues. You got some cubes, so it's gonna swallow a lot of that cam, it'll be trial and error figuring out the cruise sweet spot.
 
#18 ·
427L88 said:
Yo GOFAST, I aint no engineer but I would NOT run a 2200 rpm stall converter at 1750rpm for hours myself. Right?
Click to expand...
So if you're cruising you are only at part throttle. A 2,200 is only going to stall that high at full throttle. At part throttle cruise you will get very little slippage. Not near enough to create a heat problem.

Myself and others have run down the freeway for long periods of time with converters that stall at 5,000-6,000 without overheating. Here's a video from Rocky Mountain Race Week 2020. This is like drag week where you drive the car 200-300 miles each day as you drive from track to track and race the cars.

 
owns 1969 Chevrolet El Camino SS396
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#19 ·
Whether or not a transmission will slip enough to overheat when run below the stall speed is influenced by the interaction of many variables. Some of the important ones include the weight of the vehicle, the engine's power-band, the rear end type and ratio, whether the car is running on flat terrain or traversing long steep grades, whether the engine is running in a steady and narrow rpm range or not, vehicle speed, transmission cooler (yes/no/fan cooled)?, the degree to which some of these variables change and interplay over the time and distance traveled, etc. IMHO the video above oversimplifies what mechanical engineers would term a complex dynamic system. I'm not arguing against the fact that transmissions can and do operate in this zone but warning against simply accepting claim that vehicles can run below stall speed without any detrimental effect on the transmission. Like many things in life, there's a lot more to it when closely attended.
 
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#20 · (Edited)
Rattin said:
Whether or not a transmission will slip enough to overheat when run below the stall speed is influenced by the interaction of many variables. Some of the important ones include the weight of the vehicle, the engine's power-band, the rear end type and ratio, whether the car is running on flat terrain or traversing long steep grades, whether the engine is running in a steady and narrow rpm range or not, vehicle speed, transmission cooler (yes/no/fan cooled)?, the degree to which some of these variables change and interplay over the time and distance traveled, etc. IMHO the video above oversimplifies what mechanical engineers would term a complex dynamic system. I'm not arguing against the fact that transmissions can and do operate in this zone but warning against simply accepting claim that vehicles can run below stall speed without any detrimental effect on the transmission. Like many things in life, there's a lot more to it when closely attended.
Click to expand...
So 20+ years ago the converter I ran flashed to 4,500. I had my transmission rebuilt in 2005 and went to a converter that flashed to 5k+. Since then my converters have flashed to 5-6K and I drive all over on the street. I'm still running the same transmission fluid because it looks brand new. I'll be adding a temp sensor to my transmission pan so I can keep track of trans temps with my Holley EFI. Since you didn't care for the video, which was from a well known transmission builder. What data would you like to see and under what conditions? Stop and go, highway, how long. I'm pretty sure that aggressive stop and go driving adds more heat than cruising down the highway. I'll post the results in a separate thread.

So I added this chart from TCI. There are many others on the internet. According to most, transmission life is affected when you get close to 200 degrees. And this is for 100,000 miles. For reference, the temperatures in the video were below 160 and again this is in a 2K+HP car with a 5K+ converter running through the mountains of Colorado on the highway.

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Rattin
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