700R4 thoughts for 2:73 12 bolt rear?

[Z-11Guy]

Hey, guys, I'm scratching my head whether to stick to the stock turbo 350 trans or go with a 700r4 for my '70 Impala powered by a stock '72 400 sbc. The '70 has 2:73's now though have been told it's not wise to use the
700r4 with highway gears and stick to the t350. I know with my '86 Caprice with a 9C1 350 and 700R4 I got in the 20's MPG on road trips. Kind of want some kind of decent highway mileage with the '70 as well, thanks

 

[1961BelAir427]

If the 400 is bone stock then a 700r4 wouldn't be a bad combo with the 2.73 gears. Any camshaft change unless so small that it wouldn't be worthwhile will shift the rpm range up enough to hurt mileage and make it a slug in overdrive.
In my experience one of the best things you can do is to improve the exhaust side. Small tube headers (longtubes preferred but even shorties would help) and a decent dual exhaust system will help maximize efficiency. It's the one hot rod modification that not only adds power but helps economy too.

Also, take some time to consider the total cost of the overdrive swap. How many highway miles will you be putting on the car? Highway miles are the only time the OD will help you. The cost of the swap will pay for a good bit of gas. I'm not saying it's a bad idea.....just that in my opinion a lot of people jump on the OD trans swap bandwagon when they are going to save less $$ in fuel than they spent on the conversion.

 

[Don Jacks]

A lot of cars left the GM factories with and overdrive transmission and 2.75 gears,both the 2004r,and the 700r4 were used,and they worked quite well.As Jason said,A decent exhaust with small tube,long tube headers,and a dual exhaust will make a tremendous improvement.One other improvement would be to ditch the factory 2 bbl,in favor of a nicely tuned Quadrajet 4 bbl.A set of Vortec heads,Edebrock performer intake,Quadrajet,and small headers with the stock cam would make a really sweet combo for that 70 Impala.

 

[1961BelAir427]

I was going to recommend a Q-Jet too. Left that part out. Can't beat them for gas mileage!

 

[Fathead Racing]

I agree with Don, the 400 sb should have enough ass to pull with those gears. If it doesn't or downshifts to much on the thruway you can fine tune it with tire height.

 

[Don Jacks]

One other thing on these 72 engines.In order to reduce NOX emissions to meet federal smog regulations,Chevy retarded the cam timing by using a different crank sprocket Since you're freashing up the engine anyway,or even if you're not but the engine has 80,ooo miles on it[at that point the factory chain will have quite a bit of stretch/slop in it],use one of the "performance" double roller chain and gear set ups in place of the stock stuff.The end result will be an increase in cylinder pressure at the low and and midrange rpm band increasing torque and mileage potential.The cams themselves
weren't changed from the eariler 70 engines since it was cheaper to change the chain than to engineer and grind a new cam profile.

 

[MRHP]

Well Don, I would like to disagree with you, but I can't. That's what I would do.

 

[Don Jacks]

Why me,what'd I do?

 

[Mearl]

My 93 Corvette has 2.59 gears and a 4L60, it pulls just fine on the highway and still does a nice burn out when I accidentally hit the traction control and my foot slips on the gas.

 

[ken smith]

I "just ran the numbers" for your 1970 Impala with 2.73 rear gear , 700R4 trans , G78 x15 tire ( 215-75 x15)
Looks like 1390 RPM at 60 mph....seems like low rpm but good for gas mileage...1853 rpm @80 mph!

 

[Z-11Guy]

One other thing on these 72 engines.In order to reduce NOX emissions to meet federal smog regulations,Chevy retarded the cam timing by using a different crank sprocket Since you're freashing up the engine anyway,or even if you're not but the engine has 80,ooo miles on it[at that point the factory chain will have quite a bit of stretch/slop in it],use one of the "performance" double roller chain and gear set ups in place of the stock stuff.The end result will be an increase in cylinder pressure at the low and and midrange rpm band increasing torque and mileage potential.The cams themselves
weren't changed from the eariler 70 engines since it was cheaper to change the chain than to engineer and grind a new cam profile.

Hey thanks for the tip! I plan on going through the 400, and really keeping it stock as much as possible. Just going to install a isky 262 mild RV cam ,true roller chain, and switching the intake from a 2 barrel to a quadrajet with stock 1970 4 barrel L48 manifold and sticking to stock ram horns. I'm just a stock type of guy when it comes to driver's. boring I know.
700R4 is the way to go, just got to figure out how to wire the lock up converter.

 

[Don Jacks]

The best and easiest way to do that is go junk yard hunting for an 80-85 full size pick up,van,or 82-85 S-10 pick up.The tcc harness is a seperate ,stand alone piece.Get the harness,and the little vacuum operated fenderwell mounted switch,as well as the brake light switch.Problem solved.If you're going to change cams,Comp cams,and Lunati both have a couple of small cams that will make better torque and mileage than that Isky ever will.I can supply you with numbers if you wish.

 

[HemiChallenger71]

Just my .02 here. I have a couple vehicles (winter beaters) that are "under geared." First is a 2001 buick with the 3.8. It has 2.73's. Gets better mileage at 80 than 70. Here its hard to drive 80 all the time with traffic. The other is the silverado drivetrain donor for my '59. It has a 4.8 w/the nv3500 5 spd. 3.42 rear gears with a 30" tall (factory size) tire. Same thing gets better mileage at 80 than 70. At 70 its lugging to stay at speed because the rpms are so low. If it had a 3.73 or 4.10 it overall would get better mileage, and be easier to drive. I can barley get out of 3rd gear city driving and have to use 1st to start and below 5mph. W/3.73 or 4.10 I could use 2nd like it should be.

I also had a dodge dakota r/t with 3.90's and a 27" tire. I was concerned with all the RPM's especially freeway driving. It got just fine mileage for a 360 just because it was working so easily. Guys with a cummins diesels and 4.10's have similar situations. Freeway rpm's are high but they still get mileage because the engine is barley being loaded.

Everything is a compromise. No matter what it takes X amount of fuel to sustain a certain speed and load. The 700 would work better for you with some 3.08's or 3.23's but it cant hurt to try it first with 2.73's.

 

[Z-11Guy]

Thanks for the info, my '86 Caprice I installed a 1991 9C1 350 with a RV cam and used the original 700R4 trans. I only pop it on O/D on the freeway at 55 MPH and above and never in regular drive in town an get in the 20's for mpg on the freeway.

Don, what cam Lunati or Comp cam do you recommend, I like torque though want to have mileage, thanks.

 

[Don Jacks]

Either the Comp Extreme energy 256/268,or the Lunati Voodoo 256/262 would be a nice choice in this application.I think that in this case,I'd lean more toward the Lunati.The reason being that the Lunati is ground on a 112/108 set up,where as the Comp is on a 110/106.

 

[Phil Reed]

Why me,what'd I do?

ask Phil!!!!!!!!!!!!

 

[Don Jacks]

ask Phil!!!!!!!!!!!!

Oh yeah,like then I'd get a straight answer!!!

 

[Z-11Guy]

I "just ran the numbers" for your 1970 Impala with 2.73 rear gear , 700R4 trans , G78 x15 tire ( 215-75 x15)
Looks like 1390 RPM at 60 mph....seems like low rpm but good for gas mileage...1853 rpm @80 mph!

Thank Ken, will get started this week and order the 700r4 crossmember and TV cable from shift works. Also need to cut the driveline down to 56.5 inches as well.

 

[303Radar]

Here's a calculator I like to use:
http://www.markwilliams.com/calculators.aspx

 

[boxerdog]

My '74 Nova currently has a 2.73 with a BTO 700r4 behind a mild 383 with a Q-jet. The combo works fine, gets great mileage on the freeway at 80 or so, and causes no problems at all. The one exception might be the one-legger rear end, so I have a posi and a set of 3.23 gears waiting for some spare time.