Car buying - how should I compare engines?

So my car is totalled (not our fault, no one hurt) and I’m looking for a new one.* I’ve almost convinced myself to buy my first completely new car (I’ve always bought used).

It’s a commuter car, so I’m looking at small, high-MPG cars. I know that I’m not going to get great performance out of an economy car, but some are better than others. (I only drive manuals, so that will help some right there, right? ;))

I can look up specs and see the horsepower and torque and engine size (in litres). I’ve got brochures for some of 'em that also give compression and displacement and such.

How should I compare info to figure out which cars to test drive? (I’ve got a list of 30 right now and really don’t want to drive that many.)

For instance, a Civic has max hp 140 at 6300 rpm with max torque 128/4800 rpm from a 4 cylinder 1.8l engine. A Suzuki Aerio has max hp 155 at 5400 rpm with max torque 152/3000 rpm from a 4 cylinder 2.3l engine. What should I be looking at here?


*Couldn’t’a done this a few months ago, before high gas prices made my little tiny cars hard to find, no, that woulda been too easy. GRRRR!!!

You should be looking at test data, because weight and gearing and the final drive ratio could mean a more powerful engine turns out to be slower.

Additionally, ‘peak’ hp, is not reflective of the cars powerband. The Civic is a screamer, making it’s hp high on the revs…and not surprisingly, the 2.3 is more torquey, and will feel more friendly.

Also, when 6-speeds are available over 5, you get more flexibility between drivability, economy and performance.

By a Honda or Toyota, since they are known for maximizing the factors mentioned above, with refined engines and reliability.

As you’ve already determined, there are too many factors to make an easy comparison possible. Drivability will be influenced not only by engine-specific factors (hp, torque, what speed ranges these occur, etc. bloody etc.), but also by car-related factors (weight, gear ratios, ergonomics, etc.).

To first order, driveability will be determined by torque-to-weight ratio (which is almost like saying displacement-to-weight), max acceleration will be power-to-weight, and mileage will be 1/displacement - but those are only rough guides.

Since you already know what segment you are shopping (“small, high-MPG cars”), I would suggest trimming the field from your 30 candidates - cross out cars based on cost, country of origin, reliability, etc. Also, look at popular consumer publications and see how they rate the cars in your target segment - for instance, tends to be good, and they have a recent 2005 Economy Sedan Comparison Test that you might find helpful (note, however, that the 2006 Civic was not out for that test, so that might skew the results).

In any case, once you have it narrowed down to several likely candidates, go for test drives - see how you like their engines. I would think a list might look like Honda Civic, Toyota Corolla, Mazda3, and Hyundai Elantra - as you can see, in this segment particularly I am biased towards the Asian nameplates.

If you don’t plan on racing it, compare two things.
For performance, compare ft-lbs of torque.
If you’re comparing cars of disparate sizer, you may want to rate the cars by 'pounds of weight per ft-lb of peak torque.
If you’re like most American drivers, you will almost NEVER use RPMs higher than your car’s torque peak. That means your horsepower number is irrelevant.
For fuel economy, average the EPA city rating with the EPA highway rating for the car.

“disparate sizes”

I’d throw in the Volkswagen Jetta, Golf, or Beetle with the TDI diesel engine, especially with the manual transmission. Great torque, high mpg. Good for commuting.

Test drive them. Try to simulate most of YOUR driving situations, even rent a car you are seriously considering buying, if possiable.

OK, that makes sense. Yah, I’m looking primarily at Japanese and Korean cars with the Civic and Corolla high on the list.

And I’ve already got a chart that has hp, torque, weight, mileage and price for the different models (from I knew that was the important info but wasn’t sure which affected what and how to compensate for the differences (e.g. weight). Now I’ve got a better idea.

Thanks much!

I had those on my list until I saw the Consumer Reports reliability rating…I believe they were listed in the “worst” section. Too bad, really, they’re cute little cars and get great mileage. And diesel’s almost always cheaper than gas around here.

There aren’t many things that matter for high MPG in a conventional (non-hybrid) car.

Modern engines are all pretty close efficiency-wise - a 302ci V8 replacing a 121ci inline 4 in the same car will not change the gas mileage*. Therefore, the chassis matters a lot more than the engine.

The figure that matters most for mileage in the city is curb weight. A lighter car will almost always get better mileage than a heavier one. In the city, almost all of what the engine does gets undone by the brakes - and what isn’t braked off is lost in rolling resistance and driveline friction, both of which depend primarily on weight. KE= 1/2 mv^2 so the more mass the car has the more energy you have to use to accelerate it to the same speed and the more energy you lose when you stop. Pumping losses in an automatic transmission screw you over in the city; I’ve never seen an automatic car achieve its EPA city mileage rating in actual city use whereas the mileage estimate isn’t too far off on manuals.

Out on the highway, weight becomes slightly less important; aerodynamics and gearing play starring roles here. The slower the engine has to turn on the highway, the better. “Longer” gearing reduces the engine’s pumping and frictional losses of the engine. This is why manual-transmission Corvettes get more than 35 MPG just cruising down the highway; that big motor’s barely turning above idle. How much your mileage depends on speed generally depends on how aerodynamically slick it is. There are SUVs that burn twice as much gas going 75 as they do going 55, but my old CRX gets the same gas mileage doing 85 as it does going 55 **. The EPA numbers are effectively worthless as the EPA highway test never goes over 48 MPH - if you put a Corvette into 6th at 48 the engine would be shaking like a leaf as the computer tried to prevent it from stalling.

As for how energetically a car accelerates at normal speeds, there are exactly three things that matter: power-to-weight ratio, gearing, and driver ability.

By far the biggest factor is the power-to-weight ratio. It would be nearly impossible to get a car that has 10 lbs/horsepower to feel slow in everyday driving; it would be nearly impossible to get a car that has 30 lbs/hp to feel fast. Most cars are (thankfully) closer to the former than the latter nowadays.

Torque really doesn’t matter - you just need to rev the engine higher to get the same performance out of an engine with the same power output and less torque. This typically means being in a lower or shorter gear. A 240-horsepower, 2800 lb car will achieve the same performance regardless of whether it has 150 lbsft of torque (Honda S2000) or 300 lbsft of torque ('93 Ford Mustang Cobra) - the Honda’s engine just has to turn nearly twice as fast as the Ford’s.

Most cars are geared pretty well for both acceleration and economy nowadays. With the six-speed manual gearbox fast becoming the standard, you can have some very short first and second gears to shove you out of the blocks hard while still having a sixth gear long enough to turn 2500 RPM at 70.

Most drivers are wasting their money on extra power that they never use. If you never put the throttle to the floor and don’t rev the engine to near the redline every time you go to merge onto a highway or pass somebody, then you’re not using all the power you paid for and would’ve been better off going with a less-powerful version of the same car - a four instead of a six, the 120 horsepower 120ci four instead of the 160 horsepower version of the same engine. Don’t worry about wearing the engine out - just about anything that isn’t a Ford should be able to make it to 200,000 miles without an overhaul.

So, to sum up, just get the lightest car you can find, with a six-speed manual if you can find it and a five-speed if you can’t, and then spec out as much power as you’ll use - 95% of the time it won’t matter economy-wise and during the other 5% economy’s the last thing on your mind.

*This is based on observed mileage of Ford Focuses with 302 engine swaps versus previous mileage by the same driver in the same car. This repeats itself for swaps that don’t involve such major surgery - a hot B-series engine in a Civic will typically get better gas mileage than the original D-series motor despite (usually) additional displacement and up to twice the power.

**As an engineer, it distresses me somewhat to write this. There are a lot of logical reasons for a car’s mileage to drop off at higher speeds: the aerodynamic drag force goes up with the square of the speed, the frictional drag force goes up linearly with speed, and the engine has to turn faster so its pumping losses increase.
However, I like empirical data - it’s really what matters at the end of the day. There is one huge reason an aerodynamically slick car gets virtually identical real-world highway mileage over a wide range of speeds. You don’t have as wide a speed variation over hills at high speeds - on an uphill grade where you have to floor it to keep the car going 55 in high gear or downshift, you might not even have to open the throttle any more to climb the same hill at 85. Throttle and speed variations are even worse for mileage than just a steady fast pace.
In addition, automakers program injection computers to 1) ace EPA mileage and emissions tests and 2) keep the engine from being damaged during full-throttle abuse by car magazines. The throttle is never floored during an EPA test. As a consequence, there is absolutely no official penalty for a car whose computer tells the injectors to just dump fuel into the engine under full throttle in order to avoid detonation. When journalists fill the tank with the rattiest 87 octane fuel they can find, go out to a SoCal track on a 100-degree day, and pound on the car for hours on end, the last thing the automakers want to hear is that the magazine blew the car up. As a consequence, the mixture is made a lot richer at full throttle than it needs to be - extra fuel decreases combustion temperatures and evaporates on the intake stroke, taking away some more heat when it does.

Lousy advice. Asking what car performs best if driven badly, and allowing it to influence your buying decision, is like asking which Olympic sprinter is fastest when towing a 400-lb sled, and then using that information to determine who you want to bet on. The average American driver drives like an idiot anyway.

If you don’t expect great performance out of an economy car, you’re gonna be bowled over when you drive a Sentra or Corolla that’s as quick as a Mustang GT was 10 years ago, or a Civic that’ll do 145 miles per hour, or a Neon that’ll beat a hemi 'cuda in the quarter…

A few years ago, 17-second quarter-mile performance was considered zippy for any car and quite hot for an economy car. 17-second quarter-miles are dog slow now. 15 is the new 17, and a new 15-second car is as fast as any other 15-second car.

I’ll start out by saying that I’ve loved your posts in this thread, but the above quoted post of yours leaves me scratching my head.
I’ll grant you that the average American driver drives like an idiot.
I take from your response to my post that you believe American drivers should drive in such a fashion that they frequently advance beyond the torque peaks of their engines and take their cars near or up to their horsepower peaks.
The question that’s rolling around in my mind is how to do that safely.
Does your conclusion pre-suppose that we get rid of automatics?
Do you think we should change our speed limits?
Is driving around with an engine sized so that it can barely maintain 80 MPH part of your suggestion? I had a car like that once, and I remember running it to redline on half of its shifts.
Should we drive around in sticks and shift at 6000 RPM regardless of whether we’re running wide-open throttle or 15%?

I am asking sincerely. If I just came across as a snarky 4ssh4t, I apologize, as such was not my intention. I have a habit of brushing people the wrong way without meaning to. I am genuinely interested in your response.

No. I was just pointing out that if you want to drive fast, you should rev the engine up to where it’s most effective - otherwise you’re not getting any advantage over the guy who has the smaller, less-powerful engine and is driving it hard.

I want to seriously reduce the number of automatic transmissions on the road, but that’s for another thread. I also want the speed limits increased by a LOT outside of cities and population centers, but that’s for another thread.

You should shift near or at the redline if you’re driving flat out, and well below that in everyday driving. If you’re not revving it up when you’re passing, merging, etc. you’re wasting money on power; if you’re winding it up while just poking around town you’re wasting gas.

I think there’s more fun to be had on the road winding out every possible ounce of performance out of a tiny slow car than scaring yourself just cruising along in a bigger, faster one. I now drive a 90-horsepower CRX and it’s a blast on the open road - nailing the throttle on every straightaway, shifting up right before the 6500 RPM cutoff, trail-braking and using the mild lift-off oversteer to get into the corners then getting back on the power hard as the car exits the corners. Drive like that in a CRX and you get … 35 miles per gallon. Do that in a Corvette and you get … brown trousers.