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demon452

Posi unit cost

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I found a local  that has 2 posi units for sale. He has a 323 and 342 for sale. Both units are supposed to be install ready. I have a 307 so I am thinking about upgrading. 342 may be more rpms on the expressway than what I want. What is a fair cost for the units? 

 

Also what are your guys thoughts about the 342 for some expressway driving good/bad etc? I have a 64 riviera.

 

Thanks in advance

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1 hour ago, demon452 said:

I found a local  that has 2 posi units for sale. He has a 323 and 342 for sale. Both units are supposed to be install ready. I have a 307 so I am thinking about upgrading. 342 may be more rpms on the expressway than what I want. What is a fair cost for the units? 

 

Also what are your guys thoughts about the 342 for some expressway driving good/bad etc? I have a 64 riviera.

 

Thanks in advance

The Gran Sport guys are the ones to tell you what 3.42's are like on the highway.  I had a 3.91 in my 63 for a number of years.  Other than more frequent trips to get gas, it was totally streetable.  RPM's were a little higher but nothing unreasonable.  There are 'calculators' online that take your tire size, rear gear ratio, etc. and will tell you what your RPM's will be at different speeds.

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In good shape, maybe $600-700 for the 323, $750-850 for the 342. Both are fine for the street. I prefer the 323 for the highway and the 342 for stop and go.

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Many say a 3.07 posi is more ideal. We decided to put the 3.23 in our Grandfathers 64 in attempts to get the best of both worlds. 

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14 hours ago, demon452 said:

Also what are your guys thoughts about the 342 for some expressway driving good/bad etc? I have a 64 riviera.

 

Hey Demon - I think you're on the right track going with the 3.23 ratio.

 

I have a Gran Sport with the 3.42 ratio.  As mentioned this is an ideal ratio for around town use.  Very responsive. 

 

However, on long drives on the freeway it isn't ideal.  First of all the revs are getting up there at any speed above 70mph.  The engine certainly has enough power to propel the car at much faster rates, but engine noise becomes more noticeable.  On long multi-hour trips I wish the car had overdrive.

 

Another factor is gas consumption.  The 3.42's run the engine at higher rpm's, regardless of road speed.  This means lower gas mileage, especially at cruising highway speeds.  Not that gas mileage is big factor to most of us who use these cars occasionally.  I drove mine daily for about 10 years back in the day and SHE BE THIRSTY! 

 

Best mileage I ever got was 16 mpg driving moderately (i.e. no jackrabbit starts, no embarrassing pony cars off the lights, etc.) 

 

I think the 3.23 ratio will give you the best compromise between acceleration and overall flexibility.

 

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The gentleman ended up selling the unit. So I am just going to wait until I find another 323 posi. Have a couple mths anyways with the weather.

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The gentleman still had the 342 but not my preference. Found another local that said he has a 307 posi. Just don't want to regret not getting what I want or buying another unit at a later date because I settled. The guy with the 307 posi swears that is ideal but again he is the 1 selling it. Haha

 

I have a 307 open seems ok but want a little more pep. 

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Having made a similar swap in the past on my '56 Chevy (3.55 to 3.36) and honestly it was hard to discern any real difference in performance (aside from cruising RPMs).  If you really want to feel more 'pep', I think that you should reconsider the 3.42 option...

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If all of my schemes work out, I'll be running a 3.91 in my '64 but between the engine and the rear end will be  TH200-4R transmission.  That will give me all the zip I need but with a .67 overdrive I'll have a 2.61 cruising gear for the highway.  One other nice thing about the later TH200-4R is that it has a lock-up torque converter.  Time will tell. 

A 3.07 rear gear with an ST400 transmission and a 28/3" tall tire (225/75R15) will run down the highway at 70 mph with the engine turning 2550 RPM's. 

A 3.91 rear gear with a TH200-4R transmission and a 27"tall tire (same as above) will run down the highway at 70 mph with the engine turning 2175 RPM's.

 

I'm thinking engine wear and fuel savings.  

 

My '94 Roadmaster has a 2.79 rear gear with an LT1 engine and a 460LE transmission.  That trans has a .70 overdrive feature.  With the same 28.3" tire  it only turns 1700 RPM's at 70 mph. With all of the new O/D transmissions, it's no wonder that cars are getting the fuel economy that they do now. But you need a lot of torque to move a car that fast with such low RPM's.  The ol' Nailhead is just such an animal.

Edited by RivNut (see edit history)

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I think it comes down to what RPM the engine is at peak efficiency. But I have no idea how to determine that. I had a old mustang with a somewhat mild HO 302, and 4.11 gears. It rung the motor out to do 70, but jumped like a bear at lights. I wanted better mileage from it, so I paid a 4x4 shop to put 3.55's in it. They somehow put even lower gears in it (no doubt used, at new prices). I swear they were 4.60, something in that range. Absolutely killed the performance.  Acceleration was slower at all speeds. After some arguing, I got them to put the 3.55's I requested in it. Just the slightest loss of off the line acceleration, but the car felt so much faster at all speeds. I think it got the motor down into the most usable power range. I believe it was a 700 or 800 rpm drop at 70.

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3.07 has the higher end speed and best RPM performance and many wise Riviera people I talk to prefer the 3.07.

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Did a rpm calculator 3.07 @70 is 2696, 323 @ 70 is 2837, 342 @ 70 is 3003. Not to much of a difference. I know its just an estimate. Now my biggest thing is I want more "getty up" from a light. I don't even know if changing to a 323 is worth it at this point. Kinda want the best of both worlds .

 

Now chimera could you actually feel the difference from a 307 to 323? Just with around town driving. Maybe I am just starting to second guess myself. Multiple people have told me as well just to go with a 307 or go with a 307 posi.

 

 

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A few years ago, whilst cruising in my ‘69,  I inexplicably blew out my 3.07 (hot shrapnel caught the dry grass on fire on both sides of the highway). I replaced it with a 3.42. Very nice off the line but I missed my highway gear. So, I added a hardened 2004R from an ‘87 Buick Grand National. Now I have the best of both worlds; fast acceleration from a standstill, and only 2400 rpm in 4th gear at cruising speed.

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On 1/20/2019 at 7:38 PM, jsgun said:

I think it comes down to what RPM the engine is at peak efficiency.

Good question.  The 401 and 425 both develop their peak torque at 2,800 RPM's.  Would that be something you'd take into consideration.  

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On 1/21/2019 at 7:01 PM, Rivdrivn said:

A few years ago, whilst cruising in my ‘69,  I inexplicably blew out my 3.07 (hot shrapnel caught the dry grass on fire on both sides of the highway). I replaced it with a 3.42. Very nice off the line but I missed my highway gear. So, I added a hardened 2004R from an ‘87 Buick Grand National. Now I have the best of both worlds; fast acceleration from a standstill, and only 2400 rpm in 4th gear at cruising speed.

 

I'm sure that's a very nice combo!  I considered swapping in a 2004R for the 2-speed Powerglide in my '67 Corvette, which has a 3.08 rear.  Instead I went with a built TH350.  This was a way less expensive swap vs. using the 2004R, was a near bolt-in, and gives me way better launch from standstill with modest RPM's on the highway.

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27 minutes ago, 65VerdeGS said:

 

I'm sure that's a very nice combo!  I considered swapping in a 2004R for the 2-speed Powerglide in my '67 Corvette, which has a 3.08 rear.  Instead I went with a built TH350.  This was a way less expensive swap vs. using the 2004R, was a near bolt-in, and gives me way better launch from standstill with modest RPM's on the highway.

First gear in a 200-4R is 2.74; first gear in a 700-R4 is 3.06; the Powerglide is 1.82.  ST400 is 2.48. 

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3 minutes ago, RivNut said:

First gear in a 200-4R is 2.74; first gear in a 700-R4 is 3.06; the Powerglide is 1.82.  ST400 is 2.48. 

 

Yes, and the TH-350 first gear is 2.52.  That's why the acceleration is noticeably better than with a Powerglide.

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The lower the gear ratio, the quicker off the line.  With the same transmission, a 3.42 rear gear is quicker than a 3.07 gear.  On the other end, a 3.42 gear has a lower top speed than the 3.07.  Same goes for transmission gears.  If you have a ton of torque there's really no need for a transmission.  The less horse power and torque you have, the more you need steep gears.

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