Chevy Corvette Forum banner

1 - 14 of 14 Posts

·
Registered
Joined
·
9,695 Posts
Discussion Starter #1
I just got back from a 525 mile round trip and I got 19.1 mpg on the way there and 18.9 mpg on the way back. As my previous best with the TH400 was 13 mpg I increased it by 6 mpg. A whopping 45% increase. As I live out in the middle of nowhere I am always driving at 55 mph or better. So I can really use the 30% overdrive. But if you just drive your car around town and seldom get it up to sustained 55 mph or better speeds a TH700R4 won't be any added benefit to you. In my case it was a great idea especially as I have the torque monster 454" ahead of it.
 

·
Premium Member
Joined
·
9,048 Posts
Steve, thats amazing! I know you and I both sing the praises of the TH700r4 swap, but from 13mpg to 19.1mpg , thats just outstanding results.. I ve had C3 s with 427 s, 454 s and the best they ever saw was about 10-12 mpg.. iT WILL PAY FOR ITSELF IN NO TIME.
 

·
Registered
Joined
·
9,695 Posts
Discussion Starter #3 (Edited)
Steve, that's amazing! I know you and I both sing praises for the TH700r4 swap, but from 13mpg to 19.1mpg? That's outstanding results. I've had C3's with 427's, 454's and the best they ever saw was about 10-12 mpg. IT WILL PAY FOR ITSELF IN NO TIME.

As I was driving along yesterday I was thinking about my recent recommendations to other C3 owners about swapping over to a TH700R4. I realized it wouldn't be of any benefit to them if they didn't drive their cars at sustained speeds of 55 mph or better most every day. Such as daily drivers commuting 25 miles or more 5 days a week to their jobs. If the C3 owner had to hire the work done they would likely end up paying $2500 to $3000 for the swap so each owner would have to decide if it would be worth it. Fish, you and I are very skilled mechanics so a swap like this is easy for us. It cost me close to $1700 for the swap but about $400 of that was for heavy duty parts to beef it up to withstand the torque output of my 454" engine. P.A.T.C sells TH700R4's that only cost $1300 outright (no core charge) and they are plenty strong for the average 350" C3. And then they sell two other models that cost more. Mine has all the performance upgrades to handle everything my 454" can dish out.

In my case the nearest town is 25 miles away which means I'm in overdrive for the bulk of the trip and am running anywhere between 55 and 65 mph. So I really needed the TH700R4 for the increased gas mileage. In fact it was the only logical thing to do since gas prices are so stinking high. As I previously had a TH400 in it that made things a bit more difficult as I had to have a rear end yoke welded to my drive shaft tube (an additional $100). And then the yoke added additional problems as it's outside diameter was considerably larger then a standard TH350 yoke. Fortunately I was able to buy a new tail housing that had a mount on it near the rear (about $75) so I only had to slot my crossmember about 3/4". And for my T/C lock-up I added a Hobbs (Stewart Warner) 15 psi pressure switch ($50) to the 4th gear servo pressure tap to ground my solenoid. That worked out great as my T/C lock-up happens about 1 second after it goes into 4th and unlocks the moment it downshifts into 3rd. All automatic without me having to operate an under-dash switch.
 

·
Premium Member
Joined
·
9,048 Posts
it is a little work to do a conversion, but the end results speak for themselves.. 13mpg increased to 19mpg! and thats not even mentioning the performance increase or the longer life of the engine. I ve had mine in for couple years now,and I love it.
 

·
Registered
Joined
·
584 Posts
its a good increase 13 to 19 but it would be true you have to drive 15,000-18,000 miles on the overdrive to start saving $$$ ( about a year ago mine was doing 8 MPG ( in stop and go traffic)but the carb was dirty so I rebuilt it haven't done the math but it feels better)
 

·
Registered
Joined
·
9,695 Posts
Discussion Starter #6
its a good increase 13 to 19 but it would be true you have to drive 15,000-18,000 miles on the overdrive to start saving $$$ ( about a year ago mine was doing 8 MPG ( in stop and go traffic)but the carb was dirty so I rebuilt it haven't done the math but it feels better)

Yeah, I figured it would take 2-3 years to pay for itself. But as it's my daily driver and I'm in overdrive most of the time it was the right thing for me to do. With 87 octane now over $4.00 per gallon here I couldn't afford to not make the swap. And I also did it for the "cool" factor as very few C3's have one. Since the swap I have noticed there's far less engine vibration at 60 mph as the engine is only turning 1600 rpm. About 800 rpm slower than it was. And as the engine is turning slower the interior noise is a lot less.
 

·
Registered
Joined
·
584 Posts
well gas is $3.29 at the time, but I do plan to keep the car for years
My dad suggested just to change the rear end ratio, but I like my take off
 

·
Registered
Joined
·
9,695 Posts
Discussion Starter #8
well gas is $3.29 at the time, but I do plan to keep the car for years
My dad suggested just to change the rear end ratio, but I like my take off

Like I said, if you don't drive your C3 very far every day the swap wouldn't be worth it. I drive my '71 about 100 miles each week and the bulk of it is at 60-65 mph.
 

·
Premium Member
Joined
·
9,048 Posts
David, your car likely has a 3.36 gear now, changing that to 3.08 would net very little in mpg..and cost $1000. No, the biggest bang for your buck in mpg s is the reduction Overdrive transmission.
 

·
Registered
Joined
·
9,695 Posts
Discussion Starter #11
Chills Down my Spine

Every time my '71's TH700R4 shifts into overdrive I get chills down my spine. Knowing I'm saving myself about 35-40% in gasoline costs. My '71 454" is now getting within 1 mpg of what my computer controlled '82 350" gives me. I ordered a Mr. Gasket distributor mechanical advance spring kit for it yesterday so I can probably hit 20 mpg on the open road in the near future. I'm presently running the stock 8 degree initial timing so I'll start by bumping that up to 12-14 degrees and then get my mechanical advance to come in sooner. As my engine only has an 8.6 to 1 compression ratio I can dial in a lot more advance before any pinging starts.
 

·
Registered
Joined
·
93 Posts
I just got back from a 525 mile round trip and I got 19.1 mpg on the way there and 18.9 mpg on the way back. As my previous best with the TH400 was 13 mpg I increased it by 6 mpg. A whopping 45% increase. As I live out in the middle of nowhere I am always driving at 55 mph or better. So I can really use the 30% overdrive. But if you just drive your car around town and seldom get it up to sustained 55 mph or better speeds a TH700R4 won't be any added benefit to you. In my case it was a great idea especially as I have the torque monster 454" ahead of it.
525 mile trip???? Sounds like you went over to the crossfire meet in Arizona that I have been reading about.
 

·
Registered
Joined
·
231 Posts
The other benefit to the 700 r4 is the low first gear, giving the car more grunt off the line & between stop lights. the overdrive would be nice for fuel savings & extending the life of the motor @ crusing speeds. lets not forget that the top end speeds are increased, but up here the are very few places were i can reach the top speed of the 350 turbo. It's in my cars future to do the same, just not sure when.
 

·
Registered
Joined
·
9,695 Posts
Discussion Starter #14
Overhauling a TH700R4

The other benefit to the 700 r4 is the low first gear, giving the car more grunt off the line & between stop lights. the overdrive would be nice for fuel savings & extending the life of the motor @ crusing speeds. lets not forget that the top end speeds are increased, but up here the are very few places were i can reach the top speed of the 350 turbo. It's in my cars future to do the same, just not sure when.

Buy a Haynes automatic transmission manual and overhaul one yourself. The Haynes manual gives excellent step-by-step instructions and pictures. I overhauled a two-speed Powerglide many years ago and I was astonished over how easy it was. Next I overhauled a 727 Torqueflite and that was very easy too. Then a few TH350's and finally a TH700R4. A Powerglide only has about 10 moving parts but a TH700R4 has over 250. But it's not harder to overhaul. It just takes longer. On the average it takes me about 4 hours to go through one IF the pump doesn't need reconditioning. And then before I put it back in I drill and tap the torque converter and oil pan for an 1/8" NPT drain plug.
 
1 - 14 of 14 Posts
Top