Overdrive Dilema

At 60MPH RPM's are about 2250 with 225/70/15 radial tires. 225/75 or 235/70 rubs when turning. In the rear larger tires are not an option since the exhaust is so close to the tires to begin with. Even though the speed limit here in Ct. in most places is 55MPH you would be cruising in the 1st. lane or get run over. Most are doing 70+. At 70MPH RPM's are 2500. When I'm where the limit is 65MPH many are doing 75+ so that brings the RPM's to 2800. I would also like to see lower RPM's but have just not done anything about it. Think I would have by now since I've owned the car for 50+ yrs. The torque will pull it though at 70-75MPH @2000 RPM's or slightly less if in proper tune.
Again, just my thoughts.


Tom T.

 

I'd be happy with that! My 70 Vette does 3,500 @ 70 MPH with a 3.70 rear. The lowest gear for that rear is 3.08 and a 5 speed kit is $3,600.

 

I was considering an overdrive for my 71 Skylark 350 while the engine is out for a rebuild...

I gathered info here and decided to exercise the ease factor and have my TH350 rebuilt.

I don't do that much highway cruising...yet.

 

The 4L80e will fit without body mods. The only draw back to the TH400 w/GV combo that people mention is the fact that the Gen 1 & 2 49000 series (Riviera) frame is X shaped precluding a GV without significant mods. A 200 or 700 R4 will fit without banging on the car. Excluding the electronics, the drive shaft, frame mount, shift linkage, engine adaptor and speedo provisions will need changed. If I were to guess, as with 'A'bodies, the 200R for a Riviera could be the most cost effective. From a pure factual design point, a TH400 equivalent OD trans should replace a TH400 non-OD trans.

As another individual has pointed out, what is the rearend gear ratio? This also factors into the decision process.

For my situation, my reason was this, I don't like spinning my motor @ ~3000 RPM on the highway to keep with traffic having a 3.42 rear. I like the idea of saving engine wear and potentially fuel $.

For OD selection, I used very simple reasoning by asking the question, what is the closest OD trans to a TH400? Answer:4L80e. Because, all the aforementioned changes to the driveline had to occur regardless of trans selection and knowing the GV option was very costly to fit - 4L80e was my choice.

For the record, I have only saved an average of $50 in fuel cost after driving 1000 miles.

Having a stock 3.07 rear in your car (if you can confirm) almost negates the need for OD. But it still is a preference decision.

 

I wanted to add to this post regarding useable rpm ranges and gears.
A great example of a good cruise speed on a stock or mild engine was that at 2000rpms, there was enough torque at light throttle to sustain 70 mph. This is an incredibly important observation.

With the age of these and the reality of rebuilding and restoring, often with performance upgrades, there's a lot of misunderstanding of what really helps mileage.
The entire big picture regarding vehicle specs must be considered to have any meaningful experiences to share.

As mentioned, the cruise speed in one's geographical area must be considered when taking influence from the vast internet, rather than targeting a specific rpm or OD type or gear.
Many of the projects I've worked with fell into the "monkey-see-monkey-do" mindset, assuming OD's and low gears/rpms always get better mileage.

A good example of a different way to skin that cat would be when using a longer duration cam, headers, ported heads, and a little more gear. These are common upgrades to many rods.
Obviously this pushes the efficient rpm range of the engine up. This also might radically change the tuning requirements.
Now things like average piston speeds, stroke lengths and rod ratios come into play very differently when dealing with PART throttle use.

A short stroke engine like a Nailhead becomes very sensitive to large duration changes, requiring more avg. piston speed to maintain good draw on the carb and velocity for atomization.

Why is this long post hopefully insightful?
Because when increasing the torque of an engine by a healthy amount, healthy increases in mileage are there just waiting to be tuned. People tend to miss the part about tuning the part-throttle aspects of the engine.

My other point to this is just my opinion coming from my personal experiences.
I guess I'd rather put $2800 in parts into an engine and see a drastic increase in torque along with maybe a 10mpg bonus, rather than throwing an OD trans in to see modest, if any mileage increase.
No disrespect is intended to anyone's cars or experience, we all have fairly different specs in the builds anyways.
I've had no real issues being able to tune 400" engines to 20mpg, and an even easier time getting 425-500" pumps into the 25-28mpg range (and a few slightly better), because of the available torque at part throttle.
I don't have any better tuning skills than the next rodder, I just stick with it because I've seen them do it.
Have a nice day .

 

As I agree with yours, and everyone else's.

The point to my narrative was that it has to be looked at case by case.

My first one was an accidental benefit. After that, mileage was an important metric to the projects. (Not strip cars!)

As far as wear and tear, I used one that started as out as an abandoned re-ring engine that had 125k miles prior to the re-ring. Some intentional performance upgrades got it where I wanted it. I ended up using the car as a mfr's rep vehicle, putting 140k more miles on it. Cruise speed was between 2800 and 3000+ rpms, with many daily blasts to 6500. The abuse, sometimes for an hour or more per day brought my avg down to the low 20's, for the day. A normal cruise even with a few blips here and there would hold 28-30mpg's.
I got worse mileage 55 and lower, but recovered some by using 2nd gear below 55.
That was with a 454, and a .560" lift cam.
Around town only, like from light to light, it got maybe 10-12mpg's.
The maintenance items with the engine were flexplates (6) and valvesprings (3 sets), and lots of plugs, with a required head swap after breaking a bunch of v-springs, bending valves and denting pistons.
Swap to unported heads lost 4mpg, then recovered it by swapping back. Valid on that combo only.
It was definitely worn, and what killed it was an accidental cold rev w/ open headers.

Even though that one wasn't a Buick it's an extreme example to benefit from, even if I don't share all the details. Vaguely similar bore/stroke as a B455. The long stroke engines were much easier to get to there, just slightly lower rpm's.

What happens in every case is that the inherent "point of diminishing returns" rear up. They have their respective "curves" shaped differently depending on many factors, most notably aerodynamics. The above engine in a pickup did poorly. Trucks hit that aero wall much quicker. The above example was an f-body that had all the spoilers. I could maintain the mpg's for a much faster speed. My theory was that I was reducing running time considerably on long trips, as there were many indications of the engine running less efficient.

I've got at least 1/2 million road miles and nearly 30 years into this. I will say that the vehicle and it's use have waay more influence on mileage than the engine.

Just food for thought.

 

This subject goes on and on and there's a lot of physics involved. I'm sure a lot of automotive engineers lay awake at night thinking about this. The first guy who designs a decent car that gets 50MPH will rule the world. We can't come close to emulating the cars they design as systems to work together with microprocessors making decisions and tweaking things a million times a second.

I drove a 72 Chevy pick-up, 350/700R4 3.08 rear, for 7 years. The best MPG I ever got with it was 16 in town, 19.5 highway. I put roller rockers and a an Edlebrock manifold on it and lost 1 MPG. I put a newer Q-jet (the last one before computer control) and it made no difference. I experimented with jets, double and triple taper metering rods and springs and really saw no change. After I got rid of that truck I had and idea I wish I had tried: get rid of the metering rods and put fixed jets in the primary. I'm pretty sure jets from a 2 barrel Rochester would fit. Start out with #52 or #54 jets and tweak it from there. Use the primary for economy and the secondaries for power. If anyone tries this let me know how it works.

It would be helpful to know what the mixture is while using O/D under various vacuum conditions. They sell mixture "RICH/LEAN" gauge that runs off an O2 sensor. Most speed shops sell the nut the O2 sensor screws into, it needs to be welded onto the header as close to the engine as possible. Without this you're just guessing.

 

I'm assuming you mean 50mpg. The VW TDIs have been getting that for years. while its not a V8 rwd, its a nice car.

 

My 4.7L Tundra pickup truck cruises at about the same RPM (3.90 and OD on 16" tires) as my Buick at 70MPH (3.73 200-4R and 275/60-15) but my truck gets 19mpg on the highway but the mild 455 only gets 15 at the same speeds.

I've not spent any time on tuning the old motor since I am working on a newer, higher compression, TA212 cammed motor.

At some point, it takes a given amount of fuel to move a given amount of weight and a given displacement engine is going to burn more than a smaller motor if it is more than required to move a vehicle down the road. You can't cheat physics.

 

Sorry for the massive derailment of this thread.

Agreed.

Disagree, with conditions.
When the smaller engine is not up to the task...
Anyone care to do the math on this scenario? What's the actual displacement or cfm of a 350 running at 2500 rpm showing 10" manifold vacuum vs. a 455 at same rpm showing 20" vacuum? This would assume cam and valve action not part of the equation, mixture the same...ponder this aspect, what would the difference in velocity or vaporization be in either, considering the intake tracts equal in proportions? What would the torque output be in either? Wouldn't this affect the throttle position to avoid an increase in speed? What would the change in BSFC be based on the vaporization change in the two examples?

I've had a hard time bettering 24 mpg with 5.7 sized engines using short runner intakes in musclecars or 20mpg in trucks. No probs getting 7 liter's and bigger to prove 20-25+ in musclecars.

From what I've studied, I don't think OEM's place ultimate mileage that high on the list.
As stated before, I believe the vehicle ultimately sets the mileage limitations.
This should make for some interesting conversation...

 

Thanks you for pointing that out. Yes MPG. Speaking of TDIs, they sell one in Europe that gets 78 MPG but they can't sell it in the US because the EPA measures CO2 by the gallon consumed even thought it emits less per mile traveled. In Europe they measure it in KM (distance) traveled, so they are stuck selling the one that gets 53 MPG here in the US. The EPA does it again!!!

By the way, I said a DECENT CAR, not an cracker box. I mean a car that weighs 3,500 lbs not 2,500. Or better yet a full size pick-up.