Before I get flamed, I did my homework and researched this but, I've got a question. With a pretty much stock 255, which produces 200 foot pounds of torque at 2500 RPM, 30" tall tires and a 3.00 rear axle, am I actually going to get the best highway performance at 70MPH?
The "RPM Calculator" says you will be turning 2352 RPM. I have a 255 Merc in my car, but it has reworked Edmunds heads, a Rochester 2G, a Mallory dual point, and Fenton headers. I have found that mild flatheads run best between 2200 and 2700 RPM. I have a 3.55 rear end and turn 2600 at 70 effortlessly. You should be fine unless your engine is not up to snuff or you have weight or aerodynamic problems. Mine is a '51 Ford coupe.
I don't want to be a smart arse but I think more information is needed to give you an answer because I think the answer will depend on what do you mean by "the best highway performance". By best highway performance do you mean best gas mileage / economy, enough acceleration so you can pass others at speed, engine in the sweet power spot, engine not to sluggish? I didn't do the sums and maybe I misread - are you going to be doing 70MPH at 2500 RPM? If so then whilst you will be a little slow off the mark from stop, if your goal is highway speed of 70MPH and you are not too worried about time to get there then I think you are good. If you want faster acceleration to get to 70 MPH then obviously you would need to change your ratios somewhere in the equation but then you would be doing higher revs at 70MPH Gas mileage is a whole different story
3.00:1 on a 30" tall tire with a stock flatty. I'm not feeling it. I see a lot of around town downshifting in your future. I'm 3.78s, 30.5 tire, calculator says I'm almost 2700 RPM at 65. BUT, I'm less worried about my engine RPM at those speeds than I am at braking and handling with 1939 tech, even tho I've made improvements. Lower, gas shocks, sway bar, but I'm still not winning any handling competitions. Hiway gymnastics at 70+? Lemme know how you make out...
Mine is not a flathead but a 250 inline six with 155hp and 235 Gross torque @ 1600 rpm ( advertised ). The truck has a 3.08 gear with 28" tires, 70mph is pretty easy to run, but generally I run 65. With only 155 hp it's no race truck, but it gets me there and milage is decent at 20 -21 mph. Mine ran 17.90 @ 79 mpg years ago in the 1/4 mile. Yours will do better just because of the aerodynamics of a coupe verse a truck.
My thinking is that I don't have to overwork the engine to maintain a highway speed of 70 if the engine is giving me maximum torque at 2500 rpm. Would I go racing with that set-up, no. I'm just looking for the most economical way to cruise.
You will use much more power accelerating than you will use to maintain your desired 70 mph once you have got her there. That's why the magic efficiency setup is somewhere in between cruising and all out acceleration. You may get a better overall result using a steeper rear end ratio. It is a real eye-opener to see how much fuel an engine uses during acceleration. If you have a modern car that has the capability to display real time fuel usage figures, give it a go and you will see what I mean.......
I get your point. We should really know what kind of car the O/P has before we can answer his question properly. I do know one thing. During the time I had this '51, I also had a '36 three window with a 276 Merc, a '39 transmission, and Ford Lockheed brakes. It was a good, solid car, but it was not comfortable at anything above 55. I think it was mainly the 4.11 gears, but it just didn't seem right going any faster. The '51 is perfectly comfortable at that speed. Ford seems to have made some real changes and improvements in the intervening 15 years. The only time I don't like it is when the traffic is heavy, and that doesn't have anything to do with the car. As a matter of fact, I bought the '36 to replace the '51. I kept the '36 five years and I still have the '51 after 36 years.
I would say rather than use max TQ as a target/measure, how much is available after peak? Since you're rolling, more than enough. You need the TQ output when load is variable like leaving a light, making a turn without having to stop, having to slow down and resume speed, that's where it's most important. Brain food....
I tried 3.00 gears in my old '32 5 window after a friend suggested that 3.55s seemed to be too much for the 255 flathead. After a test drive I changed back to the 3.55 gears, been that way for the last 18 years.
IMG_0152 by Travis Brown, on Flickr OK... 1951 Club Coupe 3220LBs Stock 239 with headers. Stock LOM distributor. Stock 8BA "94"2bbl. 670.15 BF Goodrich Silvertown (Coker) 27.42" diameter Stock 3 speed in 3rd. Stock Shoebox rear with the standard 3.73 rear gear ratio. Factory Engine specs.... Maximum HP 100@ 3600 RPM Maximum Torque 180 @ 2000 RPM LOM full advance WOT 4000 RPM RPM at the shown 60MPH (according to Summit's calculator) 2742 de-ja-vu RPM @70 MPH 3199 RPM @ 80MPH 3656 Ok the Ford's peak torque is around 2000 RPM. You feel this while rowing through the gears. 1 is quick, 2 is long and you drop into 3. You especially feel this with the 3.73 in the Ford. Peak HP is close to 3600 RPM. I can feel this above 50-60-70 MPH. Top speed and it matches the figures is roughly 83-90 MPH and this matches closely with the WOT of 4000 RPM. Above are numbers with just a little talk of drivability. Real world drivability........ The car is torquey and fun to drive. The combination allows a quick start, you know it's a V8. It's defiantly no slouch. While it's really unfair to compare a 1951 flathead car to say a 1961 390, 292, 283, 409 car, the little car is peppy to a point. 2nd Gear is a blast you can really stretch it out. High gear.....The car loves 55. Once you get past 65, it's hard to explain but the car wants to go. It wants to go! Strange thing....once you surpass 70 it wants to keep going. I guess here you getting to the peak HP and you can feel it. For the Ford with a standard transmission Ford had it figured out with the 3.73 Ratio. 3.73 is perfect for all around drivability in my opinion. The 255 was in a bigger car and I believe they had a slightly higher ratio. Still though, I'm like Highlander, 3.00 gears with 30" tires that's awful tall. With a Flathead it may be a slug off the line and sluggish going through the gears. With the 255 and 3.00 gear it may pick up past 70 but do you really want to go there? I might consider a lower gear. Play with the calculator some and compare Torque, HP and RPMs.
Minor changes in gearing don't return profound effects unless it's really radical. 3.78 to 3 is radical. 3.55 to 3 as well. The consensus among the flathead faithful, 3.55 or numerically higher is best. Unless you're really light and have a measure of excess power it's harder to pull tall gears in normal driving. Now I'll throw another worry in the mix, and some will always call bullshit but, excess load in the driveline will beat the shit out of the thrust surfaces in the crank bearings. Several in the classic car set were in the habit of not downshifting, figuring the massive TQ was enough to avoid the effort. A season or 2 later that cold thud could be heard, lessens when you press the clutch, but eventually it's always there until it's fixed. Those excess driveline forces have to go somewhere, sadly it all doesn't go to the pavement. Something to ponder.
I put 3.54 gears in my '42 two door with a fresh stock Mercury 255 engine. Lots of low end torque for such a small engine and though it sounds like it's killing it at 70 mph, that's when it really wants to go. Although it sounds like it's over revving the speed vs rpm charts don't confirm that. Perhaps I'm just too sensitive to noise. Maybe I'll hook a tach to it and see what it says. Quieter mufflers might ease my aching brain, too.
My avatar has pretty much a stock flathead with two carbs progressively activated, 5-speed w/.86 5th gear, 235/85-r16 tires @ just under 32” tall, and a banjo rear with 3:54 gear set. At 75mph I am running 2600 rpm and @ 85, and that’s as fast as I can go fully loaded, about 2850 rpm. Rpm is give or take a little. I am fairly low, chopped top, and if I hit a head wind I will lose a bit of rpm when running flat out at 85. My avatar really does not weigh that much so head winds will affect me. When I did all my calculations my goal was 75mph @2500 rpm. For my avatar that has worked well with me. I cannot say the same combo would have worked well in my old coupe - too much body height, weight, and wind resistance. Everything was the same for the coupe save for the rear tire - they were 235/75-r15 making me run 75 at higher rpm but I had enough rpm at speed. It too was a flathead, 255 cube French block, 4” crank, Elgin cam, 2-deuces running simultaneously. I do not know if I am right or wrong in my setups but with both cars I was very happy with the results. One last comparison - 32-5 window, 364 Nsilhead, 5-speed w/.72 fifth gear, 9” w/3:23 rear gear, 235/85-r16 rear tire. @ 75 I am running 1800 rpm, 85 @2K+ rpm. I am taller and a bit heavier than my roadster running a torquey nailhead and I can run around town in 4th from 25-50 mph and not go over 1600 rpm. On a steep hill I will have to downshift. If I had my druthers I would like a 3:30-35 rear gear set for the hills as the 3:50 rear gear does not make around town driving fun. But,as it is, I am satisfied with this combo as well. Just my experiences …..
Piston speed has a lot to do with the mechanical inputs. That 4" Merc crank is flopping those pistons farther than a 3.75 std crank. This to say my 239 would likely "feel" different at 2500 than a 255 might. My old Boss 302 felt the same at 3200 as my 350 Z28 felt at 25-2600. Who can chime in on that?
Don't forget that your best highway speed with a certain setup is also going to want to be within it's top gas mileage range when you are doing road trips. That torque curve chart usually shows at what rpm the engine is going to pull the most efficianty when pulling a load up a grade in a certain gear and you want to be in the gear that keeps the rpm in that range. It may not be the most efficiant cruisng rpm. That's going to be where the engine runs great but isn't at the point where it is burning extra gas to gain a few mph or rpm. On a road trip we did in the Ot car we had then we figured out that 73/74 mph was the point just before gas mileage started to drop drastically the speed limit was 80 but 80 cost 7 mpg over 73 and I wasn't in that much of a hurry. Looking at the torque band of my68 292 for my 48 it's right at 2800 rpm With the 3.55 rear end, 27.7 inch tall rear tires (215-75-15 ) and .7o overdrive I shold end up with 2110 rpm at 70. If I were to run at the max torque point I would be doing between 80 thatsetup at 2411 rpm.
Thanks for that . I'm pretty sure that 50 years ago, I knew or at least was aware of that information. It really sucks to get old .
We are used to High RPM engines these days. Pretty much, for a whole generation or more, that's all they know. The irony about these "modern" or more correctly "post modern" High RPM engines is that they are run at speed, just a little above idle. So, you have an engine that can tach to 9K but it runs at 1.5K at 70 MPH. Now a vintage engine like a flathead, a little over 4K is the redline. Yet it makes it's peak power at 3600 RPM. New High RPM capable engines are driven just a little above idle at Highway speeds. They are designed for that. Old Low RPM engines are driven at the high end of their power band at highway speeds. They are designed for that and that's where they make their power, at the high end of their RPM range. Many drivers these days are not used to a high revving engine. When you take away the high revving power from a vintage engine with overdrives and high gears, you create a slug. In a sense you take away the only thing it's good at, making it's power.
It may be that 4" bore 3" stroke "thing". I think I have heard that referred to a "square engine". That may be incorrect terminology. Anyway, it would be interesting to compare a 302 Chevy vs the 302 Ford in your scenario.
I think bigger bore is oversquare and close to equal is square, which makes the opposite undersquare. (erases the chalkboard) But what I was referring to is piston speeds effected by rod length. It's widely accepted that piston speed slows with a longer rod, so it would be interesting to "feel" a 302 SBC vs 302 SBF. This might be grossly overthinking the original question, but it would be fun to also "feel" a 239 59A at 2400 vs a 255 8BA (4" crank) at the same RPM. Which one would seem less strained from the driver's seat?
Rod length does not effect average piston speed, that is strictly a function of RPM and stroke. However you are correct, peak piston speed is decreased with a longer rod. Rod length has a more pronounced effect on piston acceleration and dwell time. Longer rods increase dwell time, so peak cylinder pressure increases as the fuel and air burn in a confined volume for a longer period of time, increasing torque output. Although, alot more goes into it. Olds 455's had very short rods (ratio of 1.585) and are generally considered to be "torque monsters". Compare that to the 283 SBC ratio of 1.9, a motor that I don't think anyone ever accused of being a "torque monster"
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