What is the estimated top speed of a modified 65 Mustang SIX-Cylinder?

[Charlie Cheap]

Using on-line formulas I show my modified 65 Mustang SIX-Cylinder can reach 122 MPH...don't think so! I understand outside forces act on a moving object, requiring more force to act against that force, to increase speed. I am old and not an engineer, so please excuse me if I use wrong terms, but I am sure you will understand my question. With roughly 140 HP at 4300 RPM, (I think it could pull that in high gear given enough room...say Kansas), a vehicle weighing 2850 LBS, with 26.3" tall rear tires, and 2.83-1 rear gears, hauling an old corn-fed 210 pound guy, what do any of you think a top speed would be? The frontal area is roughly 5' x 2.75' plus the windshield at 5' x 1.5'. All undercarriage stuff is tucked well up under the chassis and the car has been lowered about 2". Don't ask how I know, but I do know it will go past the century mark on the speedometer. Some of the guys who ask me questions think their 6-cylinder can top 130 or even 140, (as if their Pony is a Shelby GT), and my explanation of HP needed to get from 0 to 100, is probably less than that needed to add another 40 MPH. I am not sure they understand what an "Exponential Curve" is.

 

[Mech_Engineer]

Since the drag-limited top speed is somewhat complicated to calculate accurately, you could start by estimating what the theoretical engine RPM-limited top speed is. Given your engine's redline, final drive ratio (top gear in transmission multiplied by differential ratio), and tire diameter you should be able to estimate what your top speed would be if we assumed the engine had more power than drag.

 

[Charlie Cheap]

Mech_Engineer, that 122 MPH is the speed I got off-line using drag, gears, tires, weight and frontal area. NO WAY will my Mustang SIX go 122 MPH. I am a long time engine builder and my old 1940 Ford coupe, one of the most aerodynamic cars made, would have trouble actually hitting that speed. It had a 289 Mustang V8, 2.79 gears, big-car flywheel, 29" tall rear tires, bored .040", 4-barrel, hot ignition, cast iron headers and more. "It" possible could, but I am sure my SIX can't. My math using a Cd of .40, Crr of .017, weight of 3050 counting driver, the listed tires/gears, 70 degrees F, Barometric Pressure of 30 HG, I come up with 122 MPH. Surely my math ability sucks! That is why I am here where people understand formulas and math better than me. The little Mustang is light and I have modified it considerably, but 122 MPH seems way to fast for 140 HP. Any ideas where I am going wrong? Thanks for any help.

 

[Charlie Cheap]

 

[Mech_Engineer]

Honestly there's nothing more to be done, simple equations for estimating air drag make a lot of assumptions. Keep in mind also your power output is not constant- your vehicle's 140 hp only happens at 4300 rpm and it might turn out you don't have the right gear ratios to reach that speed. Also it's very possible your engine isn't putting out 140 hp due to wear and/or carburetor tuning.

From a purely theoretical perspective, the drag equations you're trying to use (or ones similar to it) seem to indicate only about 118 hp is needed to overcome air drag and rolling resistance at 122 mph. If the car can never reach that speed, it has to do with how the power's being put to the ground, but remember theoretical calculations make a lot of assumptions and can really only be used as a guideline.

http://www.wallaceracing.com/Calculate HP For Speed.php

 

[Mech_Engineer]

One more thought- the limiting factor will be wheel horsepower, not crank horsepower. If your car puts out about 140 hp at the crank, after transmission efficiency loss it might be only 90-95 horsepower to the wheels (if you have 30-35% loss for transmission/drivetrain loss). This is why dyno guys are always bragging about wheel horsepower, because that's what actually makes it to the ground and pushes the car forward.

 

[Charlie Cheap]

Thank you very much Mech_Engineer. Having built and raced cars I just did a quick math check. A 140 HP motor at the flywheel, makes about 119 HP using a standard 15% loss, due to mechanical things happening before the rear wheels. All the drive-line is new or rebuilt and is tight and well set-up. If 118.27 HP is needed, maybe it will top-out at 122 MPH...but I will not bet on it. My reason for all this, is to show other 6-cylinder Mustang owners (with motors much less modified than mine), who think their new header and loud muffler just added 40 HP, it probably did not add that much. And achieving a very high top-speed requires lots of true on-the-ground HP. My 206 CI SIX is built for torque, and I feel sure it can pull 4 grand or close to it in top gear. I keep it tuned and have driven it very fast just to check out the modified suspension, disc brakes and motor...but just once. Once past 100 MPH the little car gets VERY light, even with an added rear sway-bar, gas shocks, poly bushings and GT springs. Nice to know the little motor can pull, and with a 287 CFM 2-barrel, it still had something left. I ran out of stuff, not the car! Thanks again.

 

[AZFIREBALL]

At 100 MPH air resistance (aerodynamics) become a big factor (if not the biggest) in attaining a high top speed. The question becomes weather or not your car will reach red-line in top gear and if the gearing and wheel size produces the most velocity at that very point.

If the gearing is too tall, you may never reach red-line in top gear; even after a long run. If the gearing is too low you may reach red-line before attaining the highest possible top speed.

Six cylinder cars can reach 120 mph top speed. I have a six cylinder that has seen 150 mph.
It does 75 mph at 3000 rpm. At 6000 rpm red-line it is running at 150 mph.
I have shifted from third to fourth at 120 mph.

Can you guess what it is?

There are lots of cars today that can exceed 150 mph.

 

[Charlie Cheap]

AZFIREBALL, I will guess your 150 MPH SIX is a late-model V6 or German in-line. Unless you are talking about a much modified Detroit in-line. And yes, I know several 6-cylinders that can honestly hit 150 or more. Mine is doing just over 2700 RPM at 75 so our gears/tires are a little different. I honestly believe my 65 with the original SIX modified the way I built it, will see 110 to 115 MPH honestly. An in-line 6 is perfectly balanced compared to 4's and V8's, but the V configuration gives somewhat of an counter to the imbalance by having a piston on the opposite bank help relieve the stress.Over the century mark most old speedometers are not accurate while later ones are darn close. I think I read where the Mustang Eco-boost 4 cylinder can top 140, and my wife's Escape has 170+ HP with a 1.6 liter 4. Times are definitely different in the automotive world.

 

[AZFIREBALL]

Charlie Cheap: Thanks for the guess. It is not ‘late model’ German, and it came from the factory capable of 150 mph.

 

[cjl]

It wouldn't surprise me if your car can do 122, though given the shape of a lot of older cars, they tend to make a lot of lift at high speeds, which can lead to a dangerous loss of control. It takes surprisingly little horsepower to go 120ish, though as you said, the power requirements ramp up pretty dramatically with speed. If you take a car that can do 120mph and give it double the power, it can do 150. Double it again and you'll do 190. Each doubling only gives you about 25% more speed.

As for 6 cylinder cars, my favorite car that I used to own had 6 cylinders, naturally aspirated, and could pull 174mph (though I never got it up that high).

 

[jrmichler]

You can get the actual drag and rolling resistance for your car by a coastdown test, then use those numbers to calculate power required for any realistic speed. Here's how I did a coastdown test for my truck: https://ecomodder.com/forum/showthread.php/coastdown-test-06-gmc-canyon-20405.html.

 

[Randy Beikmann]

One likely inaccuracy is your value for drag coefficient. I have a reference that gives the Cd for a 1960's Mustang notchback as 0.475, much to my chagrin (I had a 1969 Cougar that would have been about the same). For as nice as they looked, 1960's cars were, by and large, bricks being rammed through the air. I definitely agree with those who've said that aero drag is the major limiter to top speed.

Also, depending on where you got the HP number, it could be high. My 351 Windsor was rated at 250 HP (gross), but Hot Rod tested one on the dyno which put out 210 HP. At the time, engines did not have to drive cooling fans during dyno tests, and they often had less restrictive exhaust and intake systems than used in the actual car. If you looked up the 140 HP, it might actually produce 110 HP at the flywheel. Of course if you've had it tested, that's not an issue.

 

[Charlie Cheap]

AZFIREBALL, you running gas or Nitro...or maybe a shot of laughing gas? Just wondering. Maybe a V6 Japanese sporty car? Beikmann, you could be right, as I seem to recall a number similar to your .475 for CD. I think the Mustang was .46 or close. Anyway, that would slow the Pony down. The Mustang picture with my name is how it looked 2 years ago, before more suspension work. I balance the reciprocating mass while I have the engine apart, keep tolerances as close as possible, try to match cam to gears and tires, remove all the restrictive exhaust stuff I can and keep it tuned. I was amazed at the ease with which it hit 100, after the .060" overbore (not recommended but necessary), installing a 2-barrel carb, hot ignition, mild cam-lifter kit, and a few other mods. You are right about "stated HP" back in 65. That was gross and today it is net. Kinda like their MPG testing...with a stripped car, the narrowest/tallest tires available, 2.35 gears and a 90 pound driver wearing only his skivvies. At least now, after talking to you guys, I can tell my fellow M6A questioning youngsters, "HEY, don't question me, I have done this for over 50 years and got my answer from real for sure-nuff engineers. They too are sometimes wrong, but they do it with style, using formulas longer than a moon shot, lots of techno-talk while speaking proper English." Or is that "While properly speaking English?" Anyway, Thanks again.

 

[cjl]

If you cleaned up the undercarriage and lowered the car as you stated above, that could help the drag coefficient quite a bit. A surprising amount of drag is underbody drag, and that would definitely help your top speed. A coastdown test would be interesting, as jrmichler suggested.

 

[Charlie Cheap]

cjl, you are right about the undercarriage. A friend who ran Bonneville made a full "belly-pan" for his T-roadster. Those who run top-end understand drag-at-high-speed. When I construct an exhaust system, drive-line or any undercarriage parts, drag and looks are paramount.

 

[jack action]

Using your data and feeding my app for aerodynamics, I estimated your car has a $C_D$ of 0.461 and frontal area of 17.42 ft², for a $C_DA$ of 8.04 ft².

Then using my app for acceleration, adding a power of 140 hp, a mass of 3060 lb and a tire friction coefficient of 1.0 (street tires), you would pass the ¼-mile after 16.47 s @ 85 mph. Top speed is 124 mph. The average acceleration between 85 mph and 124 mph is 0.05 G, or about 1 mph/s. With quick calculations, this means you have to drive for almost a minute before reaching 124 mph from a stand still and you would travel about 1.4 mile during that time. Although theoretically possible, those last mph are hard to reach.

Also note that if you drive at 75 mph @ 2700 rpm, you should be at 119 mph @ 4300 rpm. So it probably won't go over that speed. Furthermore, if you have a small, say, 2% error on your speedometer, it might be anywhere between 117 and 121 mph in reality.

 

[Charlie Cheap]

jack action, THANKS very much for your trouble. My numbers are very close to what you came up with, but yours appear to be more accurate and complete formulas...more data with which to work a better answer. Back in the late 60's and early 70's when I ran a Hot Rod 1940 Ford coupe, I began looking into using engineering formulas to get "ball park" numbers for various modifications, before doing the work. Sometimes even a non-engineer can see the advantage of doing math BEFORE doing serious engine-chassis mods. I took what was called college-prep courses my senior year in high school, but did not get the baseball scholarship I hoped for, so my advanced education was from the flight deck of 3 aircraft carriers. I never lost my curiosity about all things mechanical, and when I got home I started building Hot Rods. Fortunately, a friend turned out to be an aeronautical engineer, another was a tool maker, and still another turned into a well known custom car builder. All 3 did things ON PAPER first. A good habit they passed on to me.

 

[etudiant]

Hi AZFIREBALL,
Guessing your six is a Jaguar XK-E, probably with the 4.2 litre engine. Splendid car, very beautiful, even Enzo Ferrari said so and a real performer.
Just wonder how it does in Arizona, its performance was cooling constrained.

 

[Charlie Cheap]

YES, the Jag could be the car, etudiant. I completely forgot it, but the old British in-line 6 was a great top-end runner. Is he right AZFIREBALL?