It seems like everyone likes to quote HP numbers that look good. Sometimes they have dyno slips to prove it, but usually they have "guesstimates". Others don't even really guess. The most common guesses I see are "stock it's 125hp, header adds 20hp, chip adds 15hp, underdrive adds 5hp, intake mods add 10hp, 160deg thermostat adds 3hp . . ." Let's see, that means this 125hp motor now makes . . . 178hp - 42% (53hp) more than stock with just bolt-ons??? I doubt it. There's one trick speed shops can use in advertising is publishing the biggest gap in the HP curve. Because HP gains with a given displacement generally means spinning the motor higher, the RPM where peak HP is is also higher. Take a look at the graph below :

This chart shows us two things : 1) Peak HP goes from 125hp@6000rpm to 142hp@6500. That's a gain of 17 hp, right? But advertisers can claim that the modifications add 29hp, because if you look at the chart, at 7000rpm there IS a 29hp difference. So who's right? They BOTH are, it's just that you need to be careful how you read the claims. So you can say you have 29 more HP than stock - at 7000 rpm. You don't have 29 more peak hp (154hp). This is important, because when you try and guess how much quicker your car will be, your times won't necessarily reflect the higher number. Also top speed depends on peak HP, so your top speed would reflect only the 17hp gain, not the 29hp gain.

There are a few ways to check your estimate. The first, of course, is to actually put your engine on a dynamometer. The second is to put your car on a chassis dyno, which measures your HP at the drive wheel, which I feel is the best method, as it's power at the wheel that matters more than what's at the crank, as different cars have different amount of losses though the drivetrain. What this means is two cars can have 200hp at the crank, but one can have 170hp at the drive wheel, and the other 160hp at the wheel.

Another fairly effective method is to run your car at the dragstrip. While this can be hard on your car, your Elapsed Time (ET) and trap speed (in MPH) will tell you a lot - especially if you know the weight of your car. Armed with these numbers, you can use various software programs (or websites that have these calculators), and estimate it from there. Or use car magazines (Car & Driver has the closest times to vehicles I've seen at the dragstrip - usually off only a tenth or two), and find cars with similar times (let's say 15.0@93mph). Use their power/weight ratio, and plug in your weight to some up w/ estimated power. EXAMPLE : A Mustang GT runs a 15.0@93mph. Same as your Hot-Rod SE-R. GT Weight is 3300lbs, and HP is 215. this comes to 15.35lbs/hp. Your SE-R weighs 2500lbs, so divide 2500 by 15.35. That comes to 162.9hp - in this case, at the crank. This works for crank or wheel HP. If the car with known hp has 180 at the wheel, then you will also get a drive wheel number. This won't be exact, but you can certainly see that if you think your SE-R has 200hp at the crank, it sure better run better than 15.0 at the dragstrip.

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