Moparts
Highway RPMs
Posted By: MoparJ
Highway RPMs - 03/06/15 09:05 PM
My Duster with 3.91s runs about 3500-3600 rpm at 68-70 mph. Usually I will run around 3300-3400 rpm on the highway, but sometimes you need to run a little quicker to keep up. Eventually a 4 speed will be going in. Tires are 255/60/15s. Motor is a warmed over stock compression 318 (with XE268 cam) with 45K since the last rebuild. Doesn’t use a drop of 15w40 Delo oil between changes. The engine buzzing doesn’t bother my ears, but with regular oil changes (every 1500 miles), am I flirting with danger at the revs mentioned? I don’t think so, but just wanted to hear some opinions!
Posted By: stumpy
Re: Highway RPMs - 03/06/15 09:22 PM
Nothing wrong with that. Cars did that for years before OverDrive became a standard item.
Posted By: D_C
Re: Highway RPMs - 03/06/15 09:35 PM
If you plan to swap in a Standard 4-Speed Transmission, fourth gear is Direct, as in no different than Third Gear in a 3-Speed manual transmission, or Drive/Third Gear in a 727 or 904 Automatic Torqueflite.
You would need a an Overdrive Transmission to bring the RPM down at freeway speeds. Higher continuous RPM does accelerate engine wear, though you may be able to live with that.
Swapping to Taller tires works similar to swapping in, say, 3.54 or 3.23 rear axle gear ratio, but off-the-line performance would suffer a bit as well, and there are practical limits on how tall a tire you can fit in your wheel wells.
There are lots of great Gear Ratio calculator applications available, wherein you input rear-axle ratio, transmission ratio (1-to-1 in high gear for a Standard, non-overdrive transmission) along with Tire Diameter which will calculate Engine RPM based on Speed in MPH.
You can run the numbers to determine what combination would produce your desired results, though it is always a compromise.
Posted By: MoparJ
Re: Highway RPMs - 03/06/15 09:38 PM
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If it is a Standard 4-Speed Transmission, fourth gear is Direct, no different than Third Gear in a 3-Speed manual transmission, or Drive, Third Gear in a 727 or 904 Torqueflite.
You would need a an Overdrive Transmission to bring the RPM down at freeway speeds. Higher continuous RPM does accelerate engine wear, though you may be able to live with that.
Swapping to Taller tires works similar to swapping in, say, 3.54 or 3.23 rear axle gear ratio, but off-the-line performance would suffer a bit as well.
There are lots of great Gear Ratio calculator applications available, wherein you input rear-axle ratio, transmission ratio (1-to-1 in high gear for a Standard, non-overdrive transmission) along with Tire Diameter which will calculate Engine RPM based on Speed in MPH.
You can run the numbers to determine what combination would produce your desired results, though it is always a compromise.
904, with TF-2 kit, 2800 stall.
Moved away from 3.23s a while ago, so the 91's are here to stay.
Posted By: D_C
Re: Highway RPMs - 03/06/15 09:45 PM
Always a choice/compromise. I have 4:10 gears and a 727 in my Charger. Engine screams on the freeway, but I don't drive it like that all that often.
Say, driving from 40-miles East of Los Angeles to Woodley Park in Van Nuys (Spring Fling) for an hour-and-a-half on the freeway, wouldn't be healthy for all involved on a regular basis.
Posted By: John_Kunkel
Re: Highway RPMs - 03/06/15 09:46 PM
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If you plan to swap in a Standard 4-Speed Transmission, fourth gear is Direct, as in no different than Third Gear in a 3-Speed manual transmission, or Drive/Third Gear in a 727 or 904 Automatic Torqueflite.
Except for the converter slippage.
Posted By: BSB67
Re: Highway RPMs - 03/06/15 09:50 PM
Quote:
If you plan to swap in a Standard 4-Speed Transmission, fourth gear is Direct, as in no different than Third Gear in a 3-Speed manual transmission, or Drive/Third Gear in a 727 or 904 Automatic Torqueflite.
You would need a an Overdrive Transmission to bring the RPM down at freeway speeds. Higher continuous RPM does accelerate engine wear, though you may be able to live with that.
Swapping to Taller tires works similar to swapping in, say, 3.54 or 3.23 rear axle gear ratio, but off-the-line performance would suffer a bit as well, and there are practical limits on how tall a tire you can fit in your wheel wells.
There are lots of great Gear Ratio calculator applications available, wherein you input rear-axle ratio, transmission ratio (1-to-1 in high gear for a Standard, non-overdrive transmission) along with Tire Diameter which will calculate Engine RPM based on Speed in MPH.
You can run the numbers to determine what combination would produce your desired results, though it is always a compromise.
The manual transmission will lower by 200 rpm his highway cruise rpm.
Posted By: D_C
Re: Highway RPMs - 03/06/15 09:54 PM
Yes. Some of the gear-ratio calculator programs/apps allow you to factor in torque-converter slippage as well.
Even with slippage, 200 RPM helps, but is it enough to solve the problem?
Posted By: MoparJ
Re: Highway RPMs - 03/06/15 10:06 PM
The A-833 with OD swap will take place sometime before the year is out. I drive one or two days a week, with maybe 30-40 of the 110 or so mile I may drive being at those engine speeds. All variables such as coolant temp and oil pressure have not changed in the past year. I am also in Southern California.
Posted By: YO7_A66
Re: Highway RPMs - 03/06/15 10:34 PM
My 340 Challenger runs 3500rpms at 70mph (255/60/15's & 3:91's) and I never sweat it!
Same here the 340 runs 3500-4000 depending on the 4.10s or the 4.56 with short tires Actually I find it blows out the putt putt crap from the engine. On my 360 with 4.30s I used to hold 4000 constantly for 2 hours plus and it never seemed to bother it
Posted By: BDW
Re: Highway RPMs - 03/06/15 11:25 PM
Couldn't take my 340 screaming as everyone passéd me on the highway.
Car is much funner to drive with the 518 w/LU.
3.73 gears, 26.7 inch tires.
Posted By: DaytonaTurbo
Re: Highway RPMs - 03/07/15 02:58 AM
I wouldn't worry about it. I don't think you'll ever notice increased engine wear. I doubt you'll every put enough milei on it to ever get to that point. I'm talking 150-200k miles. Ive run my civic for hours at 3600rpm at 78 mph. Still knocked down some great mpg! Only reason for the Od trans is noise, peace of mind and some mpg gains. Im doing the od in mine too, mostly because I can't stand to see the tach sit that high!
Posted By: 383man
Re: Highway RPMs - 03/07/15 04:21 AM
I drive the 90 mile run to Carlise at 60 mph in my 63 with 4.30's. I use a 30" tall tire and it runs about 3200 at 60 mph. Its no problem for me to run the 90 miles at 3200 rpm as it usually takes me about an hr and a half. Ron
Posted By: TJP
Re: Highway RPMs - 03/07/15 05:18 AM
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Nothing wrong with that. Cars did that for years before OverDrive became a standard item.
Posted By: Grizzly
Re: Highway RPMs - 03/07/15 06:56 AM
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The A-833 with OD swap will take place sometime before the year is out.
Best thing you will ever do. 
As far as will high rpm on the highway hurt it? Eventually, yes: there's a reason why new vehicles with overdrive are hitting 300,000 miles and the old girls barely made 100,000.
Did a head gasket on a '98 350 700R4 half-ton and at 170,000 miles the cylinders still had cross hatch in them. 
Posted By: Von
Re: Highway RPMs - 03/07/15 07:14 AM
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As far as will high rpm on the highway hurt it? Eventually, yes: there's a reason why new vehicles with overdrive are hitting 300,000 miles and the old girls barely made 100,000.
No offense, but considerably more involved than just final drive ratio....
Posted By: DaveRS23
Re: Highway RPMs - 03/07/15 02:28 PM
3.91s have been our gear of choice forever. So hiway RPMs has always been something we keep an eye on. 3,000 has been the dividing line so to speak. Staying there abouts or under is preferable and the farther over that you get and the longer you stay there, then the more pounding you are doing to the engine.
Posted By: BSB67
Re: Highway RPMs - 03/07/15 08:51 PM
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3.91s have been our gear of choice forever. So hiway RPMs has always been something we keep an eye on. 3,000 has been the dividing line so to speak. Staying there abouts or under is preferable and the farther over that you get and the longer you stay there, then the more pounding you are doing to the engine.
I've always used 3,000 rpm as the dividing line as well. No testing, data, or science, just feels about right.
Posted By: 383man
Re: Highway RPMs - 03/07/15 09:36 PM
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3.91s have been our gear of choice forever. So hiway RPMs has always been something we keep an eye on. 3,000 has been the dividing line so to speak. Staying there abouts or under is preferable and the farther over that you get and the longer you stay there, then the more pounding you are doing to the engine.
I've always used 3,000 rpm as the didviding line as well. No testing, data, or science, just feels about right.
Thats not a bad idea. But I will say with the solid flat tappet cam I use my eng sounds real nice singing along about 3200 RPM with my cam. Ron
Posted By: rftroy
Re: Highway RPMs - 03/07/15 09:42 PM
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The A-833 with OD swap will take place sometime before the year is out.
You may not like the OD4 behind that engine. It will cruise nicely, yes, but the ratios are such that it is no fun getting going.
The ratios are 3.09, 1.67, 1.00, 0.73.
1st to 2nd is getting very close to a 2 to 1 ratio change.
I had a 318 with 360 heads, thermoquad, and a DC hyd. with specs right between the stock 318 and 340 cams, and an OD4.
It was miserable to drive because of the torque curve. It winds out fast due to the low first, but then 2nd gear puts you way down on the torque curve, and you just lug for a while. If you shift at 4500 from 1st, you start 2nd at 2432.
It was especially no fun in stop and go traffic, and you know what that's like here in SoCal.
I ended up pulling the OD4 after just a few months and put the std 4 speed back in.
I have the OD4 behind my slant 6, and it runs great. But that's when you find that the stock slant 6 has a really wide torque curve, and it can pull comfortably at low rpm. A warmed over 318 is going to be more "peaky", and that's where the wide gear spread works against you.
Not saying it won't be right for you, but keep in mind that acceleration runs aren't going to be as much fun.
Robert
Posted By: Aspen7695
Re: Highway RPMs - 03/07/15 10:40 PM
I had a '69 Dart Swinger 340 with 3.23 gears that would cruse at 2700 rpm's at 65 mph with stock tires. I went to 3.91 gears and the rpm's went to 3200. I took many 400 mile trips from the bay area to LA with no problems. Mileage was not great, but high test gas was only $.25 a gallon back then.
Raul
Posted By: dogdays
Re: Highway RPMs - 03/10/15 01:32 AM
Don't forget that Jamie Passon has his own set of OD gears that have smaller drops between the first three gears, although the gearsets are pricey.
In reality it shouldn't hurt the engine, but I'd like a nice set of earplugs to make it easier on me!
R.
Posted By: ahy
Re: Highway RPMs - 03/10/15 01:43 AM
The engine can take some spinning... but over 3,500 RPM to keep up with normal 75 MPH traffic would be a bit much for me. If you plan any meaningful highway miles, suggest 3.23's or the OD transmission or even both. A healthy moderate 318 should handle 3.23's easily. Better range on a tank of fuel would be another benefit.
Posted By: Supercuda
Re: Highway RPMs - 03/10/15 02:37 AM
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Did a head gasket on a '98 350 700R4 half-ton and at 170,000 miles the cylinders still had cross hatch in them.
My 87 318 with more miles and no OD still shows the cross hatch. It's called moly rings.
Posted By: superbeedave
Re: Highway RPMs - 03/10/15 02:43 AM
Posted By: d-150
Re: Highway RPMs - 03/10/15 03:03 AM
cool site
Sure, it "can" spin that high, but I am definitely tired of 3.91s...even with 28.4" tall tires.
My 0.62 6th should put me at a 2200rpm cruise at 80. Now that's more like it! [1900 at 65mph]
Posted By: dogdays
Re: Highway RPMs - 03/10/15 06:17 PM
Yep, that was my point, too. It's hard on the driver, the engine should be able to run at that speed indefinitely.
R.
Posted By: GTX MATT
Re: Highway RPMs - 03/10/15 08:32 PM
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As far as will high rpm on the highway hurt it? Eventually, yes: there's a reason why new vehicles with overdrive are hitting 300,000 miles and the old girls barely made 100,000.
No offense, but considerably more involved than just final drive ratio....
The gear ratio helps but is probably one of the least important factors. Harder bores, moly rings, oil technology, and fuel injection have helped to make big strides.
Also theres lots of 4 banger cars that turn 3000-3500 @ 70 in overdrive.
Posted By: DaveRS23
Re: Highway RPMs - 03/11/15 01:11 AM
I think there is a huge difference between a 4 banger and a big block spinning well over 3 grand for hours at a time.
Posted By: Magnum
Re: Highway RPMs - 03/11/15 02:52 AM
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The A-833 with OD swap will take place sometime before the year is out.
You may not like the OD4 behind that engine.
Completely agree. Many do the calculation and think they have struck gold with the desired rpm but in the real world rpm drop sucks.
The answer after 100 years of building cars. Just like the OEM's do it. More gears. rftroy, dogdays and goody get it. A rear ratio will not solve it, nor will putting up with 4000rpm on the highway in the right lane will everything passing.
Overdrive is the only answer.
Posted By: Grizzly
Re: Highway RPMs - 03/11/15 03:33 AM
A Member who I have not seen on here for quite some time, had an 833od in an A-body with a 360. I asked him about it and he said he never noticed any drastic difference: the 360 pulled the wide-ratios with ease and still did low 13's in the 1/4. 340Shorty had the same set-up in his truck and he liked his as well.
I just got one in an '84 Ram with a 318 2bbl. With 2.94 gears, 30" tires, and at 4000 pounds that 833od is fun as heck to drive. I got back into one of my close-ratio ratio cars and it felt like you had to shift all the time. It seemed like it was wasting power.
The 1st to 2nd is not an issue at all, 2nd to 3rd is a little much but nothing you can't get used to.
Posted By: HotRodDave
Re: Highway RPMs - 03/11/15 05:16 AM
Grizzly is dead on.
Posted By: Magnum
Re: Highway RPMs - 03/16/15 03:39 AM
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I got back into one of my close-ratio ratio cars and it felt like you had to shift all the time. :
Good point. As a daily driver, too many gears is not neccessary but for racing they are there. Close ratios are what you need to keep an engine in it's power band. If you are not competative, then it's not a big deal.
During city commuting. Go with skipping gears. In my 5 spd Mustang I often go 1,3,5 and even GM had skip shift on their close ratio T56's 1 to 4rth.
Posted By: ademon
Re: Highway RPMs - 03/16/15 05:24 AM
It's not the engine rpm, it's the engine noise with the headers and hi flow mufflers, wind noise/open windows, no a/c. We are just use to driving modern cars. My wifes CRV turns the same rpm as my demon.
Posted By: jose jones
Re: Highway RPMs - 03/16/15 06:15 AM
Put a set of 3.23 gears in it
I was in California this week everyone goes about 80 on the freeways!! There is no need to run your RPM's that high on a street car
Posted By: 70Cuda383
Re: Highway RPMs - 03/16/15 06:55 AM
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I think there is a huge difference between a 4 banger and a big block spinning well over 3 grand for hours at a time.
Yes and no
Smaller engines usually have shorter strokes which is conducive to high rpm cruising. But, 3000 rpm is 3000 rpm... The piston sweeps the bore just as many times regardless of the bore/stroke, and rings/pistons sweeping up and down is what adds to engine wear.
Posted By: MuuMuu101
Re: Highway RPMs - 03/16/15 08:04 AM
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I think there is a huge difference between a 4 banger and a big block spinning well over 3 grand for hours at a time.
Yes and no
Smaller engines usually have shorter strokes which is conducive to high rpm cruising. But, 3000 rpm is 3000 rpm... The piston sweeps the bore just as many times regardless of the bore/stroke, and rings/pistons sweeping up and down is what adds to engine wear.
Again, yes and no. With the smaller bores and stroke, there's less moving mass that doesn't move as far and that allows the engine to have a higher natural frequency (otherwise known as red line). But I don't really see a difference from a wear standpoint. Also, 4 bangers need to spin at a higher rpm at higher speeds in order to keep the car moving. It's actually a little wasteful for a v8 to be cruising at 3000 rpms (all depending on the vehicle's weight and aerodynamics).
Posted By: DaveRS23
Re: Highway RPMs - 03/16/15 03:47 PM
I was thinking of the weight stopping and starting with each stroke in the 8, especially big blocks. Our stuff is usually bigger and heavier inside and really wasn't designed to spin that high that long. Which is at least part of the reason why anything but a performance car got 2.76s or so. Of course, newer, lighter reciprocating assemblies lessen the impact.
And doesn't the load increase exponentially with RPM?
I just think that after a certain point, the higher the RPM and the longer it is held there, the more impact it has on our engines. So I don't cruise my big blocks over 3,500 for any length of time and usually try to keep it closer to 3,000.
Too old school?
Posted By: GTX MATT
Re: Highway RPMs - 03/16/15 04:20 PM
The point was that wear is essentially the same, and plenty of late model stuff buzzes down the highway around 3K even with OD.
Of course the connecting rods and pistons will see more stress on a bigger engine, that's obvious.
I personally try to keep it to 3K or less as well.
Posted By: DaveRS23
Re: Highway RPMs - 03/16/15 04:56 PM
I don't think I have ever worn out a muscle car engine.
But I have managed to damage or destroy a good number. So wear wasn't at the top of my list of considerations.
Posted By: dogdays
Re: Highway RPMs - 03/16/15 07:01 PM
Actually.............
1. If properly designed, the parts of a V8 DO NOT see more stress than those of a properly designed 4. Stress is defined as force / area. Sure the forces are higher, but so is the area. Mechanical things are designed either to a maximum stress or a factor of safety.
2. If the 4-cylinder is more than 2 liters, chances are it has a pretty long stroke. Strokes of 3.7 to 3.9 are not uncommon in the 2.4 - 2.5 liter range.
3. Running the "big V8" at 3500 rpm for hours isn't outside what the designers built into these engines. The same basic engines were used for industrial purposes and many ran at 3000 or 3600 rpm for their entire existence. Trucks also run at that speed, at least they used to. If your engine comes apart after 100 hours of 3500rpm speeds you did something wrong.
4. No, the engine's natural frequency is not the same as the redline. The redline is the maximum safe speed for the engine based on the design stresses. I don't believe there is an identifiable natural frequency for the complete engine. There are natural frequencies for crankshaft, camshaft, rods, pushrods and valvesprings. There is a natural frequency for cylinders, and as they aren't all the same configuration, every cylinder could have its own. But as an assembly it would be very hard to identify a natural frequency for the engine.
5. There are two problems with running high rpm: wear and noise. Running the engine more rpm per mph (gear ratio) increases wear on the moving parts in the engine. This cannot be ignored. It is up to the owner to decide how much to tolerate. Higher driveshaft rpm with lower gears mean the driveshaft is closer to its natural frequency and stresses the universal joints more.
The same is true of noise. Noise increases with rpm, and noise causes fatigue in the driver. It doesn't matter so much to the passengers, they can take a nap. But the driver needs to stay alert.
I changed from 3.91s to 3.23s in my '64Dog and surprisingly saw no difference in gas mileage. But, after a 600 mile trip I was not so worn out with the 3.23s. I did notice a considerable dropoff in acceleration ability around town, and so that part of driving became less fun. I also did not want to invest thousands in an overdrive, although that would have solved both problems.
R.
Posted By: Ramtough
Re: Highway RPMs - 03/19/15 02:49 AM
High rpms on a Mopar not a problem in my option. The main reason I run them is when I was young dad had a 66 Chrysler 361 cid, most driving at 100 mph plus, rpms at 3500 plus. We put 660,000 miles on this car over 20 years like this, oil and filter changers every 2,000, the only work on it the was a couple of timing chain and gear sets, and at 380,000 miles new rod bearings rings and oil pump, no re-bore or other parts, oil would be down 1/2 quart at oil change when car was parked, pulled engine and put in my 70 Coronet for another 35,000 miles still ran good when I parked that.
74 - 318 truck drove the same for 230.000 miles no problems there either till the timing chain chain broke and bent valves( at the track with engine near red line ready to shift from 2ed to drive ), could have fixer easy most likely but gave a good reason to install a 440 in the truck.
