Which OD swap for my 66? - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 17 (permalink) Old Jul 11th, 20, 9:28 AM Thread Starter
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Which OD swap for my 66?

What’s the current thinking on swapping a OD transmission for my current TH350. This would be going in a convertible with a SB 350 engine with a 350-400 HP an TQ range. Street use only. 3.36 or 3.08 gears in a 10 bolt posi, with tires in the 26.5”-27” diameter range. Which offers easy installation? Currently have 3.55 with 28” diameter tires. I would like to use an old 1966 holley 3140 carb that was designed for the 66 PG. What are the options, problems, cost, and recommendations?


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post #2 of 17 (permalink) Old Jul 11th, 20, 10:03 AM
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Jeff
 
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Re: Which OD swap for my 66?

I'd keep the turbo 350 and swap in the 3:08 cog.. You would want the 350 to make good torque right off idle..
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post #3 of 17 (permalink) Old Jul 11th, 20, 10:47 AM
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Greg.
 
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Re: Which OD swap for my 66?

TH 2004R so you can use the same Driveshaft with the 3.55 Rear Gears and a Lock Up Converter

Carb = a 625 cfm or 750 cfm both will give great Gas Mileage with none of the old Qjet problems

https://www.jegs.com/i/Demon-Carbure.../1901/10002/-1

https://www.jegs.com/i/Demon-Carbure.../1904/10002/-1

67 Chevelle Malibu SS Cdn
Born 283 / PG / A51
468/TH400 2,500 stall/ S60 3.70 Gs
Prev Best on M/T ET S/S #3454 street Tires/Dress 2018/05/20
60' = 1.664
1/8 = 7.532 et / 91 mph
1/4 = 11.814 et / 115 mph
NEW Best with 3,500+ Stall TC 2019/09/08
60' = 1.547
1/8 = 7.311 et / 91.76 mph
1/4 = 11.595 et / 114.28 mph


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post #4 of 17 (permalink) Old Jul 11th, 20, 2:55 PM
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Steve
 
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Re: Which OD swap for my 66?

I agree with both the above posts. Either swap in the 3.08 and keep the TH350, or keep the 3.55 and swap in a 2004r transmission.

If you go for a 2004r and you cannot find a local builder and have to buy one and have it shipped, I highly recommend CPT Transmission (Art Carr 2004R Performance Transmission and Torque Converters) - they have excellent customer service (or at least did for me!) and build great transmissions.

1972 Chevelle, MJ 467, 200-4r non-lockup, 12 bolt 3.73
1981 Camaro Z28, 355, TH350, 3.73
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post #5 of 17 (permalink) Old Jul 11th, 20, 4:42 PM
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Re: Which OD swap for my 66?

Having done the 2004r myself, I say stick with the TH350. There's a learning curve with the 2004r and most transmission shops today haven't the knowledge of them. I know all about TV cable geometry now but it was an expensive learning process.
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post #6 of 17 (permalink) Old Jul 11th, 20, 6:57 PM
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Re: Which OD swap for my 66?

Does a difference of only .47 between a 3.55 and a 3.08 make enough of a difference to go through all that trouble ???
I think you should go for the 200R4 but only if you already have the trans.Today there is a gizmo that takes the place of the TV cable adjustment but it costs $.

John
71 El Camino SS
406 SBC M21 3.31 12 bolt
"Quality is always remembered,long after the price is forgotten"
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post #7 of 17 (permalink) Old Jul 11th, 20, 7:55 PM Thread Starter
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Re: Which OD swap for my 66?

Changing to the 200r4 drops the highway rpm by about 30%. Changing to 3.08 and keeping the th350 only drops them by about 14%. Then if you go to a smaller diameter tire like about 26.5 from 28 you add about 7%. I changed my ss gears from 3,55 to 3.07 and I am still up around 2400 rpm at 60mph; but that car has its original muncie and original 14" rims. I really would like to be able to get under 2000 rpm @ 65 mph on the highway. Thinking a 200R4 would still be adequate and less work than a 700R/


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post #8 of 17 (permalink) Old Jul 11th, 20, 9:16 PM
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[QUOTE=71350SS;11337758]Does a difference of only .47 between a 3.55 and a 3.08 make enough of a difference to go through all that trouble ???

If you have a few bucks to spare for someone to do the swap I’d say yes, I went from 3.42 to 3.55 and like the 3.55 better even at highway. I’m running with that for now. Good all around. Stop lights & highway on an m20.... l
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post #9 of 17 (permalink) Old Jul 11th, 20, 10:13 PM
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Re: Which OD swap for my 66?

Do you have a 200 that's been built or going to do it yourself?

Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
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post #10 of 17 (permalink) Old Jul 12th, 20, 9:13 AM
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Bill
 
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Re: Which OD swap for my 66?

My Chevelle is a 350/4 speed, with a 3:08 rear. I have 29 inch tall tires. It still runs a little higher than I'd like on the highway.
I have a 56 210 with 350/700R4 and a 3:42 rear. It runs the same tire. It runs about 2200 rpm at 75 mph. With the A/C blowin' and cruise control set, it'll do 20 mpg on the flat Interstate highways out here.
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post #11 of 17 (permalink) Old Jul 12th, 20, 1:58 PM Thread Starter
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Re: Which OD swap for my 66?

rpm = (mph)(gear ratio)(12)(5280)/(60)(3.14)(d)

So currently at 65 mph with 3.55 and 28" tall tires I am turning approximately 2763 rpm.

with the OD ratio of .67, I would be turning only 1851 rpm.

with only a switch to 3.08 gears I would still be turning 2397 rpm

If I also change the tires size to 26.5 then I jump up to up to 2540 rpm.

with 3.36 gears and 26.5 tires I am at 2771 rpm.

With OD and 3.36 gears I am turning 1956 rpm.

So what do I want, the current 2763, or 1851, or 239,7 or 2540, or 2771, or 1963?


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post #12 of 17 (permalink) Old Jul 12th, 20, 2:02 PM Thread Starter
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Re: Which OD swap for my 66?

Quote:
Originally Posted by Dansk View Post
Having done the 2004r myself, I say stick with the TH350. There's a learning curve with the 2004r and most transmission shops today haven't the knowledge of them. I know all about TV cable geometry now but it was an expensive learning process.
Could you elaborate a bit. I know they say you have to buy some conversion kit; but expensive could be $100 to $2000 depending upon who you talk to.


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post #13 of 17 (permalink) Old Jul 12th, 20, 3:12 PM
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Re: Which OD swap for my 66?

>So what do I want, the current 2763, or 1851, or 239,7 or 2540, or 2771, or 1963? <

What kind of camshaft do you have ? I should have said how big of a cam do you have because at that kind of RPM (1800-2000 or thereabout's) people complain about their engine surging with a bigger cam.You need to take that into consideration

John
71 El Camino SS
406 SBC M21 3.31 12 bolt
"Quality is always remembered,long after the price is forgotten"
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post #14 of 17 (permalink) Old Jul 12th, 20, 3:31 PM
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Steve
 
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Re: Which OD swap for my 66?

TV cable adjustment is actually not too difficult, you just have to get the details right and be precise. Here is what I did. Geometry is critical.

I have a Holley carb. I picked up a throttle bracket that also has a TV cable bracket.

I picked up a TV cable from Rockauto for a 1988 Monte Carlo SS.

I picked up the TV cable adapter for my Holley carb.

Adjust the TV cable housing so it is all the way back towards the firewall. Connect cable to throttle bracket adapter. Slowly open throttle to full throttle. You will hear the TV cable click as it adjusts. Now let the throttle go back to idle.

At full throttle the cable should not be able to be pulled any further without it moving the cable housing. That is the full pull. At idle, there should be some tension on the cable - it should not be slack. With the cable disconnected and dangling, you will probably be pulling it from slack to around 3/8” to 1/2” to connect it to the bracket adapter at idle. This is expected and creates the initial tension and starting point of the TV cable.

Another thing to check - with the cable connected, measure the distance the cable moves from idle to full throttle as you move the throttle. It should move between 1.377” and 1.416”. This is key. If it does not move that far you have the wrong geometry on the throttle bracket and must fix it. Moving less or more is bad. Moving less is bad, but not as bad as moving more, because that will leave it slack at idle which means no pressure rise which can contribute to premature transmission failure.

Another way to think about it is this - once you have pulled the cable out as far as it will go, that is the full throttle point and it should always return back between 1.377” and 1.416” as you go back to idle.

You can additionally get a pressure gauge kit and ensure that at idle, as soon as you hit the throttle you see the pressures on the transmission start to rise. That tells you that you have the throttle start point setup correctly.

Again, none of this is difficult, it’s just precise.

(BTW - this easy to measure number for some reason is never really explained in many places. You see lots of the pictures showing the geometry, but this number is always left out. Basically, it is just some geometry math. The centerline of the throttle to the pivot point of the TV cable end is designed to be between 1.093” and 1.125”. It is supposed to travel in a 78* arc from idle to full throttle. This makes an isosceles triangle between the 3 points (throttle centerline, idle start, idle finish.) You also know the length of the two legs of the triangle and the angle between them. The third leg is the distance the cable moves. Doing the math tells you that the travel distance of the cable is between 1.377” and 1.416”, and that is very simple to measure.)

Quote:
Originally Posted by Jim_66SS View Post
Could you elaborate a bit. I know they say you have to buy some conversion kit; but expensive could be $100 to $2000 depending upon who you talk to.
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1972 Chevelle, MJ 467, 200-4r non-lockup, 12 bolt 3.73
1981 Camaro Z28, 355, TH350, 3.73
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post #15 of 17 (permalink) Old Jul 12th, 20, 3:53 PM
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Jim
 
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Re: Which OD swap for my 66?

About 2/3 of the way down, Jake tell how to do it too.

Transmission Tech Info – Jake's Performance
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Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
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