Torque needed to move a 3000 # car calculation needed 4

[TxGat]

I have a 1967 Mustang that weighs approximately 2950 #s, about the same as a Prius. What I've been trying to do is calculate how much torque it takes to move the car down the road at a given speed, say 40 mph and up. I've looked on the net for how to calculate the minimum torque needed and haven't been able to find anything. Is there either a formula I can plug numbers into or a website that has a calculator you guys know of?

My initial guess is anything in excess of 200 ftlbs from prior experience but I'd like to nail it down with a formula.

Thanks!

 

[patprimmer]

Torque alone cannot move a car. power is required.

Even at idle speed, 200 ft lb is much more than required.

Power consumption to maintain a steady speed varies greatly depending on large number of factors.

Regards
Pat
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[BrianPetersen]

If you know the temperature and the barometric pressure, you can calculate the density of the air.

If you know the drag coefficient and the frontal area and the density, you can calculate the aerodynamic drag force for a given velocity. The direction of the wind matters when calculating the force.

The above is in any fluid dynamics textbook.

The friction force will vary widely depending on what type of tires are used and what air pressure is in them, whether the brakes are dragging, etc., but it is not too inaccurate to assume that this does not vary with road speed, so you can measure it by pushing the car.

Whether you are going uphill or downhill will make a very substantial difference and this calculation is easy physics. "greenies" who accuse auto companies of oversizing their engines (they're not entirely innocent of this, but the "greenies" take it to extremes) often forget that you have to go up hills at more than a crawl speed.

 

[NormPeterson]

If you have some idea how much drag the car generates at whatever speed, you could work backwards through the tire dimensions and gearing. That'll only be the torque required to maintain that speed in that gear and at whatever rpm the gearing and tire size puts the engine at (any acceleration comes from having a 'surplus' of torque).

Try writing your own spreadsheet.


Not to go down the 'torque vs HP' road on yet another forum, Pat, but if you know torque and rpm you essentially do know power. For the sort of driving that knowing the torque required to sustain 60 mph suggests, torque being closer to "constant" than HP is arguably the more useful quantity. Far easier to calculate acceleration from, assuming that a surplus exists.


Norm

 

[patprimmer]

Norm

He never mentioned gearing or rpm.

Unless you have gearing or rpm you cannot calculate from torque.

I know a LOT of other data is missing, like almost everything required, but starting with power eliminates a few calculations and specifications.

Regards
Pat
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[black2003cobra]

TxGat - Bowling & Grippo have a road-load calculator on their web site. On the left of their main page, click on
"Aerodynamic and Rolling HP Loss Calculation". It doesn't include road inclination and such, (which you could find elsewhere), but that should get you started.

Link =>

http://www.bgsoflex.com/auto.html

 

[NormPeterson]

No Pat, he didn't. But minimum torque is apparently what he wants to know, for whatever reason that is as yet unknown to the rest of us. Yes, you can compute the power required with a little less math, though you still need to know the drag and the speed involved.


Norm

 

[TxGat]

Thanks guys. I'm just doing some math to try to figure it out. I have a 5 speed tranny I'm putting in that has a .59 5th gear. I want to try to use this gear as much as possible for daily driving and plan to use it as low as I can, def in the mid 40's and up. I've calculated that in 5th gear I will be going approx 80 mph at 2000 RPM's.

I have a 289 with Edelbrock Pro Flo EFI and the motor is performance built. On Comp Cam's website I found software, CamQuest 6.0, that allowed me to input all the variables on the car and motor and it recommends Comp Cams for it. There is only one but it should produce around 235 ftlbs of torque at 1000 rpm and max at 396 @ 5000 rpm, and 425 HP @ 6500 rpm. Not bad for a little street car. But it must be a daily driver also. This cam also improves fuel economy via the low rpm high torque numbers.

So, for fun I decided to try to perform an academic excercise and try to see what is the minimun torque I would need to push this car down the road. It will also allow me to see how low I can go and still use 5th gear, and from what I can compute around 1100 rpm it should be going about 41-2 mph and be producing, from the graph, about 250 ftlbs of torque.

The difference between this calculated minimum torque needed at a given speed and the excess torque produced at that rpm should give me an indication of how quickly it will accelerate when more gas is applied, shouldn't it?

 

[patprimmer]

I would not trust an after market cam grinders data re power output. They are very prone to exaggeration or to only quote best cases by cherry picking matching mods.

A relatively small V8 making high power at high rpm won't pull all that well at 1000 rpm. If you have the airflow and cam timing for top end you give away some bottom end. Multi point fuel injection does help low speed especially if it is sequential.

Regards
Pat
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[TxGat]

Pat - The cam specs are for mild performance gains with increased fuel economy and high torque. Where I pick up the HP is the Edelbrock RPM heads, RPM 2 Intake, flat top pistons (10:1+ CR), balancing the crank assembly, power pullies, full length headers (3" chamber/2.5" dual exhaust) and EFI with max flow of 1000 cfm. I've played with the software and changing variables the heads are a big difference. I found it interesting that the 1.90 intake valves actually increase HP on the 289 over the 2.02 valves (by about 10 hp).

My buddy has his own auto shop and is putting the motor together. He runs on the local speedway a 9 sec flat car on pump gas and has another that ran a 12.2 with (for him) a "mild street motor". He says mine should have "well in excess of 400 hp" and run the quarter in mid to high 11's so I tend to think the software isn't that far off. It will be interesting to see. The 289 stroke is what makes the high rpm HP top end. I'm more surprised at the torque numbers, the 289 was not known for high torque.

 

[BrianPetersen]

If you have (say) 250 lb.ft available at 2000 rpm that's 95 horsepower. Certainly this is more than the car will take to go at that speed on level ground. Don't even have to do any calculations to know that.

I've owned cars that are about the same size and weight (albeit with a better drag coefficient) that have less than 90 horsepower TOTAL, and they certainly had a top speed much higher than that. The car I drive right now has only slightly more horsepower than that TOTAL, and it will go considerably faster than that even when I have a trailer in tow, which completely ruins the drag coefficient.

 

[70btdc]

The easiest way to find out how many Nm's you need at any given speed is to work out your Running Resistance Force curve. Take you vehicle out to a nice flat road and coast down from as high a speed as you can in Neutral. Record time vs velocity (in 10km/h increments if you dont have any logging). Make a couple of runs in each direction. Average your 4 runs and plot a graph of velocity vs cumulative time. Add a polynomial trendline. The output will give you an equation for the amount of Torque (Nm) you need to overcome rolling and aero losses at any speed. Of course this equation will only work for THAT road, THAT tyre pressure etc etc...

You then just need to work backwards to determine which gear/rpms you need to be in in order to make that torque. This assumes your torque is measured at the wheels, and transmission losses should be deducted from engine torque to equate.

 

[NormPeterson]

I have a 289 with Edelbrock Pro Flo EFI and the motor is performance built. On Comp Cam's website I found software, CamQuest 6.0, that allowed me to input all the variables on the car and motor and it recommends Comp Cams for it. There is only one but it should produce around 235 ftlbs of torque at 1000 rpm and max at 396 @ 5000 rpm, and 425 HP @ 6500 rpm. Not bad for a little street car. But it must be a daily driver also. This cam also improves fuel economy via the low rpm high torque numbers.

I'd put more faith in the shape of the torque curve than in the absolute values it uses to plot it.

So, for fun I decided to try to perform an academic excercise and try to see what is the minimun torque I would need to push this car down the road. It will also allow me to see how low I can go and still use 5th gear, and from what I can compute around 1100 rpm it should be going about 41-2 mph and be producing, from the graph, about 250 ftlbs of torque.

Like Pat has said, acceleration with gearing that gives 40 mph/1000 engine rpm in a pushrod engine that pulls strongly to 6500 will be a good bit less than "sparkling" at normal road speeds. Having two V8/300+ HP 5 speed MT cars geared at 30 mph/1000 rpm and 35 mph/1000 rpm respectively, I can tell you that while you can make the cars operate down there, you won't want to for very long. With a MT, maybe 1500 rpm.

The difference between this calculated minimum torque needed at a given speed and the excess torque produced at that rpm should give me an indication of how quickly it will accelerate when more gas is applied, shouldn't it?

1000 cfm max flow is way more than you need, and will be overly sensitive to tip-in unless you do something to mechanically slow down the initial throttle plate motion. There is a difference between making some target amount of power/torque and making it driveable.


Norm

 

[TxGat]

Norm,

1000 cfm is way too much. It adds some HP gain, but very little over 700 cfm. That's just air flow, the injectors have a max flow of 29 psi, both of which can be dialed back via the mapping for daily driving and via the foot feed .

The O2 sensor and the internal chip for the EFI are key and the chip is the last thing you get from Edelbrock. They set it after you get your cam and they also will tell me what O2 sensor I need. Before buying the EFI I spoke with their tech guy and that's what he told me. I'm getting the chip now.

This cam makes low end torque by the valve overlap pattern being minimalized at low rpm. If it were all out performance it would have more overlap for high rpm applications. The cam is rated for using in the 1000-5200 rpm range due to this overlap. As long as it's making +200ftlbs of torque at 1000 rpms it should work well for me. I'm not looking to accelerate for performance in 5th gear, just be able to keep the rpms low and get down the road efficiently. The first 4 gears are for performance but it is still a street car and should be what I want, some decent performance in a daily driver.

 

[NormPeterson]

As long as it's making +200ftlbs of torque at 1000 rpms it should work well for me.

You're missing my point.

I think you'll have poorish resolution at small throttle openings - IOW it will be very easy for small pedal inputs to open up too much throttle area. The torque may be there, but your ability to smoothly control it may not. It's a driveability issue, there can be such a thing as a too-sensitive throttle, and I would not underestimate the importance of this in traffic or in slippery going when you're in the lower gears.

Use of an old-school carburetor throttle attachment to a pin on the throttle arm rather than a sector that the cable wraps around as in more recent non-DBW EFI setups is likely to exaggerate any difficulty.

I've been through what you're looking at doing, just with a 3450 lb '79 Chevy Malibu with a 355 SBC and an Accel/DFI SuperRam with calibration software and flash programming instead of an early Mustang, 289-ish CID, Edelbrock Pro-Flo and burned chips.


There are a few other things you'll have to address, maintaining suction at the EFI pump as the fuel level in the tank drops being just one. EFI does not well tolerate its pump sucking air.


Norm

 

[LionelHutz]

The required power to move at a certain speed really is not simple to calculate. It's mostly the aero drag that isn't easy to calculate. Best would be the coast tests that 70btdc recommends. What he's basically saying is that if you know the car mass, the deceleration rate is caused by the losses you have to overcome.

Edelbrock still uses chips in it's Pro-Flo EFI system? That's kind of sad this day and age. I thought they had redesigned that thing to use laptop software. I suggest you get familiar with Tunercat because that Edelbrock chip will likely need some fine tuning.

Personally, a 425HP+ 289 doesn't sound like it will be the smoothest running engine at 1000rpm under some load.

 

[TxGat]

I have the older Edelbrock. They just came out with the newer one when I got it & I didn't want to put another $500 down to get it. I bought it several years ago when I could afford to put money into it (it isn't cheap) and it's been waiting for the rest of my car to catch up. My mechanic buddy's wife got cancer while the car was up there in thousands of pieces and everything was put on hold for a bit. Such is life.....

As for idle, I guess I'll find out. Usually cams that have a range from 1000+ tend to have good idle. Where it get's choppy is when you're built for high rpms. A larger valve overlap on the cam is what makes choppy idle. Cams like that tend to have a range starting at 1500-1800 rpms.

I think I may have to wait until I get the car on the road and do some testing if I decide to go forward.

 

[patprimmer]

There is more to camshafts and idle than overlap.

If it really has over 400hp at that displacement, it needs to rev and make power up high. The cam duration required for that will detract from idle and low rpm power and will cause rough running and erratic response.



Regards
Pat
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