I'm new to these kinds of cars, but I know they don't have an overdrive transmission, so I guess my question is this:

If I want to get on the highway and run 70-80 mph, is that completely okay to do so even with high RPMs? At 70 mph I get 3k RPM.

my transmission is a 3 speed TH350, and my engine is an L48.

 

I'm not new to these cars.Far from it.
Many times I have run mine all day long at freeway speeds. At RPM's well higher than 3K RPM.
Do you really think we all drove slowly back in the 70's??
My car has travelled across the great plains in the states running 80 -100 MPH. ALL DAY LONG.
And I've run from Brisbane to Cairns in Australia and back again. Stopping only for fuel and to eat. My Car still goes hard!

 

how do you think they drove them when new?

 

Well i don't doubt they drove them fast back then, but the thing is that the car is over 40 years old now. Plus rpms that high create more wear on the engine and tranny, so I'm not sure i could or should.

 

With a good tune up you should be alright running at those RPMs on the freeway, even for an extended period of time, but when your RPMs are lower you’ll get better fuel economy, less noise, and decreased engine wear.

If you want to lower the RPMs at higher speeds there are a number of options for overdrive transmissions. A 2004R will fit nicely and bolt right in, a 700R4 will be about the same, although requiring driveshaft modifications, or a TKX or a number of other 5/6 speeds if you’re interested in converting to a manual.

 

Your being silly. Yes these cars do spin higher RPM's than a modern car. They are NOT a modern car! Like I said, I've run mine hard for almost 50 years now. Still going strong!
Don't purchase a vintage car and expect it to be just like a 2022 model!

 

Overdrive is great to reduce engine wear and fuel consumption, heat generation both of engine and trans to reduce friction by turning more slowly.

As vehicles become more modern, the number of gears is increasing, as economy and friction improvements become more and more emphasized.

You can put any transmission behind a small block chevy, it takes energy and resources.

The factory 3-speed automatics for small block chevrolets can be replaced with overdrive transmissions, and many people have made this swap, but its not so simple that I would recommend it to a first timer who does not work on vehicles already for a decade+. If you have only a 3-speed, just take it easy don't try to do 80mph all the time. Friction will influence oil temps, careful monitoring is recommended when the engine is pushed say 3,500+rpm for extended duration. And there is a gray area where the rpm is higher than it needs to be but still not that high yet, when the engine vacuum goes way up during a cruise 16"Hg+ that it wants overdrive. The additional intake system vacuum is creating high pumping loss on intake stroke and acting like a syringe pulling up oil to the cylinder especially on a deceleration. If the valve seals are worn it will suck oil down the valve stems too. Thus overdrive goes a bit beyond simple friction reduction it also raises manifold pressure, loading the engine and putting more emphasis on drawing more air per intake stroke at higher pressure and making a larger combustion event with better brake specific fuel consumption and better compression acting to seal the rings, overdrive has a net affect that goes deeper than simply oil heating and engine friction over time, it lends efficiency to the chamber and reduction of oil consumption properties to the various pressure ratios around each seal.

 

I simply don't agree with that guy above. Keep it under 80 ???
Why? The car was designed to run well over 80 for hours on end!
They did race these cars in 24hour endurance races back in the day ya know!
Yes you can re-enginer the car. But it ran just fine before you bought it!
This very thing comes up on this forum every so often. Young guys worried about spinning an engine that was designed to spin!
Just drive it!

 

OP,
I understand why you are asking and I agree... We all drive our daily drivers which mostly have over drive trans of some sort and tach low RPMs at highway speed. So being new to these cars it's only natural to ask about 3K @ 70, my modern truck tachs 1800 at that speed. Is it good for the engine, no-not as good as 2K would be, can they take it-sure but not as long as an engine run at 2K. Engines are designed with MTBF in mind, mean-time-before-failure. An engine typically run at 2K will have a higher MTBF than an engine that is run at 3K, the rotating assembly on the 2K spins 1/3 less times per hour than an engine running at 3K so the parts will last a proportionally longer time on the 2K engine. Drive your car at whatever speed makes you happy, within reason (LOL), and I too would keep the speeds down, I have no need to go far over the speed limits these days..

A couple thoughts from the other side of the fence, I doubt they took a stock L48 and raced it- plus racers are not too worried how their engine will run in a few years, just have the crew put a new one in it,,, As far as re-engineering the car, then why put fuel injection on it, carbs have been running fine for years and properly tuned they perform excellent-mine does. I guess the young guys can't be bothered to learn how to tune a carb and just slap Fuel injection on---some older guys to...

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Exactly. Modern engines can go 200,000 miles, or far beyond, before any major engine work is needed while cars back in the 60s and 70s usually hit that mark by 80,000-100,000. It isn't all due to overdrive transmissions, but they've certainly and significantly extended engine life.

 

the introduction of modern fuel injection and better machining tolerances with better designs are the biggest reason why newer cars can go well over 200-300k miles.

 

The 82 C-3 crossfire came with the 700r4, came as standard equipment.

 

all you're losing is your fuel economy. I drove up to the beach an hour each way,every chance I could in whatever I had and they were all 4 speeds and non overdrive autos. we tried to stay at 70 to 75 mph if the traffic let us, if a Statey (cop) had someone pulled over we could do 80mph without worrying about another cop all the way to 95....never had an issue other than needing gas to come home if I didnt fill up when I left the house. Normally we could get there in 45 minutes...1/2 hour if we could stay above 80mph all the way.

 

An L48 and a TH350 would both be doing you a huge favor by failing spectacularly (but safely), and forcing you to upgrade.

In the meantime, just go drive the car! When something breaks, fix it! If you still want an overdrive transmission, make sure it also comes with a clutch pedal. That will make the car about a million times more fun to drive, even if you keep the L48.

 

That's all you have to say...

Compared to modern cars it's easy to think that old 350 is going to blow when you look down at that tach.
I've been told my '21 Camry SE turns 2300 rpm at 90 mph....

 

The only worries-good clean oil with pressure and don't ever overheat. With these in mind ,these engines will run for a very,very long time at speed.My '78 has spent most of it's life with me at 70 mph on interstate.But, it is thirsty.

 

Perhaps, but an engine running at 4000 rpm is going to have at least twice the wear as one running at 2000.

 

while I understand your logic here.
It's just really not necessarily true. How a engine is balanced, it's sweet spot so to speak could easily be at 4K RPM. And thus, the engine will be happiest running right there. And a engine that is "Lugging" won't necessarily last longer than one that is spinning in it's happy spot.
It has been pointed out that people really didn't take L48's out for endurance racing, fair point. But we did, all of us, hit the highway and run em at 80 MPH. Sometimes all day long on a longer trip. And, they just keep going!
And while it is true most of us got around 100K Miles on these engines before they needed freshining up. And more modern designs often run longer. I believe there is a lot more to that than just RPM.

 

 

Simple answer is yes you can drive 70-80 mph, they certainly were driven there when new.

Will your engine last as long as a new car, probably not.

Will you get 25+ mpg, absolutely not. Anything over 15 with an old, stock vette was a plus. 11-13 is where my '72 base 350, TH400, 308 was.

Looks like you have a 79. For most of the C3 run, automatics had a 308 gear with optional 355's. The '79 may have dropped the 308 and gone to the 336 for both trans. I'd have to look it up but really it doesn't make much difference between a 308 & 336.

The car was nearing the end of the run but still a car I liked. I recall looking at them new in showrooms. Probably about $9k new, give or take on the car. A lot of 79's were made, a lot had some issues. I knew a couple of people who bought them new and had a lot of warranty work done.

That was 46 years ago. Today, it is hard to find one that hasn't been worked on and in great condition. Yes, some are still out there but since you're new to these cars I would just drive it and see if you even like it. Those guys I knew who bought them, all sold or traded them in by 1980. If you do keep it and are not one to work on it, it can be a money pit. Even doing the work, they are costly. You either love them or come to hate them. Everyone is different and has a different perspective on how the car should be.