TH350 to 700R4 swap
Melting Slicks
Joined: Jun 2011
Posts: 3,118
Likes: 599
if you are looking for 20+ you will need a 3.08 rear and a 5 or 6 speed.
I get 27.8 MPG (24.3 US) with a 3.08 and 6 speed.
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,615
Likes: 7,063
From: New Or-leens Loo-z-anna
Click on the link below....lots of info.......
https://www.corvetteforum.com/forums...ighlight=700r4
https://www.corvetteforum.com/forums...ighlight=700r4
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Not necessarily. You will get a 30% lower cruising rpm but in order to get better gas mileage the overall ratio must be matched to the cam's torque producing characteristics. You don't move the cross member but rather make an extension (I believe 4-1/2" hole centers) to extend from the transmission's rear mount pad to the cross member; the extension then sitting on the top of the mount.
Burning Brakes
Joined: Feb 2008
Posts: 1,168
Likes: 17
From: Iowa
Bowtie overdrives has a nice C3 conversion cross member for 700R4 swaps. It also relocates the E-brake pulley. It is made so that true duels pass under it instead of through the cross member. I can drop my whole exhaust out of the way in five minutes. That doesn't sound like a big deal but when you can painlessly get more room to work you'll do it more often on these cars than you think. Dave
Also, I had to have the drive shaft shortened exactly 2" for my swap. I like having the overdrive and am now running 3:55 gears but I have a roller cam motor that doesn't mind 1500 rpms with the TC locked up in OD.
Also, I had to have the drive shaft shortened exactly 2" for my swap. I like having the overdrive and am now running 3:55 gears but I have a roller cam motor that doesn't mind 1500 rpms with the TC locked up in OD.
Last edited by tixeon; Nov 14, 2015 at 05:06 PM. Reason: added thought
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
YES..installing a 700R4 will improve gas mileage..and YES...the rear differential ratio will also need to possibly be changed.
Look at Bowtie Overdrives and they have a kit that will allow this conversion. The only part of the kit that I an not happy with is the dipstick tube. I used one out of a 700R4 for a 1986 Corvette.
Highway driving is going to be where you see the biggest saving...and it is not so much that...but if the correct axle ratio is also used...the RPM's on the engine are dropped...which doesn't hurt either.
DUB
Look at Bowtie Overdrives and they have a kit that will allow this conversion. The only part of the kit that I an not happy with is the dipstick tube. I used one out of a 700R4 for a 1986 Corvette.
Highway driving is going to be where you see the biggest saving...and it is not so much that...but if the correct axle ratio is also used...the RPM's on the engine are dropped...which doesn't hurt either.
DUB
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
I just did the 700r4 swap in my 78. Very easy. The drive shaft is shortened 3" and the toung mount needs the mount surface moved back. I think that was 1.5". Easy to moc it up. No need for exspensive cross member. I retained the stock one. The parking brake pulley bracket needs to be removed and welded back on 3/4" to left (driver side) keeping it to the same angle as the cable will get into the u joint if you don't due the the relocation 3" back. I used the TCI lock up kit. Took keyed power from the cruise switch at the brake peddle. This allows disengagement when brake is applied. The TCI has a vacume switch that will need a vac source. I also used the shifter mod kit from ecklers. I did feel cooling lines where a challenge as the 700 takes up more tunnel space. I ran -6 AN fittings with steel braided hose. From the case ports with -6 strait fittings to 45 degree female/male expanded to the straight fittings on the hose. Plenty of clearance. Tighten the lines with the tail of the trans down. Not sure how factory lines are for this. Don't have a figure on mpg yet. I have a 3:70 gear and with the 1:1 third it ran 3500 rpm at 70 mph. With the OD it runs at 1900 rpm at 70 mph.
Last edited by Aggitated Monkey; Nov 14, 2015 at 10:20 PM.
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
I did the 700r4 conversion to my 81 back in 08.. with 525HP , I first went to a 3.31 ratio.. a mistake. did not like it .. Swapped in 3.73 and it was JUST RIGHT! Neck snapping launches and I m turning 2550 rpm at 80 mph on the Interstate . and getting 18-19 mpg hiway.. Its a pretty simple swap... get the shifter conversion kit from Shiftworks, and a TV Cable conversion kit.. Be sure TV Cable is properly adjusted BEFORE you drive the car.. You can modify your X-Member easily , Shorten your Driveshaft 2-3 inches depending on which Trans you have. I had a new shaft made up. Double thick tube, HD Solid U-Joints .. You will love the car with the 700r4 if you get it matched right
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Burning Brakes
Joined: Feb 2008
Posts: 1,168
Likes: 17
From: Iowa
I just did the 700r4 swap in my 78. Very easy. The drive shaft is shortened 3" and the toung mount needs the mount surface moved back. I think that was 1.5". Easy to moc it up. No need for exspensive cross member. I retained the stock one. The parking brake pulley bracket needs to be removed and welded back on 3/4" to left (driver side) keeping it to the same angle as the cable will get into the u joint if you don't due the the relocation 3" back.
Burning Brakes
Joined: Feb 2008
Posts: 1,168
Likes: 17
From: Iowa
I know you were talking about gas mileage. Something most don't consider is motor miles. Before OD transmissions were common car motors were worn out at a much lower odometer reading. There are other factors as well that contribute to modern car motors lasting longer, but fewer RPM's per mile is very significant. Dave
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
I know you were talking about gas mileage. Something most don't consider is motor miles. Before OD transmissions were common car motors were worn out at a much lower odometer reading. There are other factors as well that contribute to modern car motors lasting longer, but fewer RPM's per mile is very significant. Dave
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
I like fabricating and modding and there is certainly a good deal of pleasure involved with doing it your self. That said the Bowtie cross member is not expensive. It's only $225 dollars for a very well made piece of the puzzle that solved about three problems on my 80. In modding my car I have made many brackets and done both machining and welding my self so that was not the issue. On a large project if I can buy a true ready made bolt on piece for a reasonable price I'll spend my time elsewhere. Dave
Burning Brakes
Joined: Feb 2008
Posts: 1,168
Likes: 17
From: Iowa
Is that all you got from my input to the OP? An opportunity to defend how you purchased a cross member. My post is not criticizing purchase over mod. I just told the OP how I did mine. But now that you bring it up $225 is a waste of money for the litte time the mod took. I'd rather send it on a box of cigars.
Last edited by tixeon; Nov 15, 2015 at 08:36 PM.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
A TH700R4 will drop your cruising rpm 30% but that doesn't mean you would get a 30% increase in gas mileage. When the TH700R4's were first installed in the '82 Corvettes they were used with the CrossFire Injection that had very small cross section runners to give a high velocity at the low rpm that the TH700R4 gave. So the transmission was well matched to the intake system. When a TH700R4 is installed in an earlier C3 with a 4 barrel manifold and tall gears it'll often give a 1-2 mpg boost but that's all because of the slower runner velocity.
In my '71 with a 454" and 3.08's my gas mileage went from 8 to 9 mpg which wasn't an impressive gain. I installed a set of 3.70's and with the higher engine rpm and faster runner velocity my mileage jumped to 10-1/2 mpg. But when I converted my hood to a true ram air/cold air my gas mileage jumped an astonishing 3-1/2 mpg; now 14 mpg from the original 8 mpg. So it takes a lot more than a simple drop in cruising rpm to see a significant increase.
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
My MPG is right around 15-17 with the 700R4 and 3.54 gears. I don't use OD that often unless I am going to cruise at 45+ for more than a mile. I don't use in stop-and-go traffic.
I had 3.07's with the TH350C before. and, I've boosted HP/TQ across the board. So, fair swap all things considered.
I didn't get the 700R4 for the mileage but more to take the toll off the engine and let it loaf along in OD when I would otherwise see 2500rpm at 55mph. My TH350C was tired and slipping and I didn't relish rebuilding/reinstalling. Wanted something stronger and 'trick'.
I had 3.07's with the TH350C before. and, I've boosted HP/TQ across the board. So, fair swap all things considered.
I didn't get the 700R4 for the mileage but more to take the toll off the engine and let it loaf along in OD when I would otherwise see 2500rpm at 55mph. My TH350C was tired and slipping and I didn't relish rebuilding/reinstalling. Wanted something stronger and 'trick'.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Yeah, I am still baffled over the huge increase in my fuel mileage. I have a gut feeling its the slightly pressurized air that is causing it rather than just the cool air. But, it could be a combination of both. I have a 8:00 A.M. doctor's appointment 40 miles away tomorrow morning so I'll have the chance to see how well it does sucking in 35-40 degree air for half the trip. I also discovered my T/C clutch wasn't locking because of a blown fuse so that may increase it even further. I'll find out tomorrow.
