In the middle of doing the swap to the 700R4 in a 1980 (not completed). What I am curious from those who have done the swap is whether when normal driving around town when shifting into OD does your engine lug? Do you see 1200-1400 rpm's after it shifts from 3rd into 4th? If so, would that not be lugging the engine?

(I believe the rear gears are stock 3.07's? )

 

I just got my '79 back from the tranny shop this afternoon I had mine rebuilt. I have 3.55 gears. It lugs a little. I have seen some guys post that they put a toggle switch inline with the power to the trans so they can keep it from locking up for around town driving. I may have to do the same thing. My old 700r4 was so screwed up that there was nothing I could do with it.

 

I have 3.36 gears and low throttle O.D. is close to lugging...so I'm sure that it would be a problem with 3.07 gears. In 'suburban' type driving, you would be best to put the shifter in 3rd range and just leave it (IMO). Your overdrive will only be useful on roads posted at 45 MPH and above.

You could install a shut-off switch for power to the torque converter so that the lockup function will not be creating that 'lugging' situation when you get into 4th gear.

 

This is why I installed a manual switch for the lock up in my trans...

 

A 0.70 od 700R4 tranny locked up feeding a 3.08 axle will be about 2000 rpm at 70 mph. Hard to say at what speed you'll be lugging, but as mentioned, maybe just keep it third cruising around town, or at least keep the converter from locking up. For comparison, C5 & C6 manual 6 speeds with 3.42 axle and 6th 0.50 od are 1500 rpm at 70 mph. Have no problem running my LS7 in 5th 0.70 od at 35 mph on hilly lake roads. 1980 4th .070 od and 3.08 axle = 2.20 total. C6 5th 0.70 od and 3.42 axle = 2.40 total. Not trying to make any direct comparisons, just showing some different gearing examples. Recall my brother having an 81' full size Buick wagon work vehicle. 307 engine with 2.27 axle and 0.70 od 700R4 for milage. How's that one grab ya'. Thing was way over geared for the power, and struggled to see 80 mph in top gear.

 

5.5 you need to do the math and look a tranny calc online program. OD trannies just suck with about any gearing less than a 3.73 rear end. I put a 4.10 in my 98 Vette My friend here on the forum had a 700R4 for 20 years with his C-3 and it also had a 4.11 rear. He also drove it over 200 mph with a 4.11 rear end

 

Interesting... considering I have a 78 vette with 3.55s, owned an 87 with 3.07s, and have a truck with a 4L60E and 3.42s.

My 78 does fine, with an L-82 like cam. The cars with L-48 cams should do just fine. 2000 RPM is a lot for a 350 (or larger) V8, that's not lugging at all. Cruise range on an L48 cam is like 1200 rpm, I don't see the 3.07s being an issue at all.

All the 6-speed cars had cars cruising on the freeway at 1500 rpm.

 

Don't need any math corrections, as mostly quoted the way GM built them from the factory, including the 81' Buick wagon 700R4 with 2.27 axle. One more. My brother has an 89' Camaro 305 5 speed tranny with 0.74 od and 3.08 axle. Had an LS1 Vette with 6 speed manual 0.50 6th od and 3.42 axle. Now an LS7 wth the same tranny and axle ratios. Both cars are 1500 rpm at 70 mph, and over 25 mpg.

 

'71 with 700R4 and 3.36 gears: 21 mpg on the highway...

In my book, that doesn't 'suck'.

 

My 98 was so hopelessly over geared in sixth that it would actually go faster in 5th gear.

I do not believe tha a 3.08 with a 700R4 is doing 2000 rpm at 70 mph.

Good MPG is achieve by engine efficiency and correct gearing. Any engine has its own perfect gearing. over geared and it takes addition throttle to maintain highway speed and the opposite is under geared and the motor is doing additional rpm for no reason.

Our big V-8s probably need a final drive ratio some where in between 2.50 and 2.80. C-3's push alot of air compared to modern cars.

As I modded my 98 Vette the less it liked 6th gear. The .50 OD was much happier with 4.10 I even considered 4.56 or some rear ratio like that.

 

3.36 gear w/700R4 does 2050 rpm at 70mph; 3.07 gear w/700R4 would be around 1800 rpm at 70 mph.

The car and I are very 'happy' to run at 2050 rpm @ 70 mph. There is no lugging on slope increases...just a little added throttle. I also have an [aftermarket] cruise installed on the car. Very stable with no surging in "lockup" mode at speeds of 45 mph and higher.

 

Pretty much the same with my 2004R and a 3:36 rear...

 

The T56 6th gear was designed for fuel economy, not performance, and it's certainly not comparable to 4th gear in the 700R4 lineage. Most cars that have a 700R4 can still accelerate decently, regardless of gearing. I too owned an 02 Coupe with T56, and I know that 6th was only good for mild speed adjustments. That doesn't mean it sucked. I could get 33 mpg on a flat freeway trip, and if I wanted to pass someone I still had a minimum of 3 more gears to choose from. My Z06 with the same gear ratios can use 6th and accelerate quite a bit.

I never said that a 3.08 rear with a 700R4 would be cruising at 2000 rpm, I said 2000 RPM is not a big deal for most V8 engines. My truck spends most of its time on the freeway at under 2000 and that's a smaller motor in 2000+ lbs more vehicle.

The 3.08/700R4 combo is going to be taching roughly 1700-1900 RPM at freeway speeds (60-70mph). That's well above the engine's sweet spot and they're not pushing so much air at this speed that they're unmovable.

I think any of the 350s offered in C3s, with possible exception to the LT1, would be comfortable cruising with a 3.08/700R4 combo, as would most of the big blocks. I choose not to comment on the LT1 because I'm not familiar with the motor's torque curve, but do know that it's most likely a higher-rpm powerband than the others.

 

I had a 3.55 rear end when I first installed my 700R4. It did not have much zing in OD. the tranny had been modded to drive at WOT in 4th gear and not kick down. You had to pull the shifter for a manual down shift.

I spent days on the calculator figuring out tire diameter, rpm, and MPH.


It was a close toss up between 4.11 and 3.90. I chose 4.11 for the highest possible MPH. It takes max HP to peak at the intended top speed.

I now have a bigger motor and 4.11 was good for 5th gear with a .64 OD, but the first 4 gears were kind of low. I went to 3.55 and now it is to low of rpm in 5th, but the first 4 are much better.

 

Mine 3.73 rear gear 700R4 mild build @ road speed just over 18 mpg in the mountains pleased with it. Interstate should do a little better.

 

The 700R4 with the 3.08 rear end was the staple (standard) set-up for the coupe for most C4s and works well there. But C4s have their torque curve a bit lower. If your engine is "torquey," you should have no problems.

My '91 (same trans/rear) cruising the freeway at ~70-75 gets 25-26 mpg. That engine never lugs.

 

Yes, I agree with gkull that you're not going to have much "oomph" in 4th gear with a tall rear end ratio, but with the 700R4 you have to really compromise one or the other.

The 3.36-3.73 rear ends remain a compromise as always, not as fuel efficient as tall gears and not as quick as short gears.

The problem you run into going into 4.11s and shorter is that first gear becomes more and more unusable; even in my 3.07 powered L98 I could easily light up the tires in first. Going to a 4.11 would have made the gear completely worthless.

The OPs question was "will it lug", not "will I be able to light the tires off in 4th".

The will it lug depends on how you try to drive it, what kind of engine you have, etc. I could put 4.56 rear gears and a manual valve body on my 78 and I can guarantee you that 4th gear will lug at 20mph.

You could probably also "lug" an L88 by trying to cruise in 4th with 3.08 gears at 70mph, again with the manual valve body and a low RPM stall converter.

 

Picked it up today, 20 mile highway test drive. Highway rpm's are: 60 mph 1400 rpm, 65 mph 1600 rpm, 70 mph 1700 rpm. I believe 75 mph was 1800 rpm? It is a stock 350 with true dual 2.5 exhaust, no converter.

I was under the impression it would be around 2000-2150 at 70mph? Was not aware it would end up being 1400 at 60mph, which seems too low to me.

Around my area, 60-65mph would be typical highway speeds, so it will see more of the 1400-1600 rpm cruise time.

It launches fine, shifts are solid, quick.

I am wondering if this was the right choice with these low rpm's? Am now wondering if 3:36's or 3:55's would make this a better combination? Will need to drive it some more to see how it drives.

 

This may help:
http://vexer.com/automotive-tools/speed-rpm-calculator

 

I have a 700r4 behind a 632 and with 3.73's its a blast and will get 20mph if you stay out of it