|The $15 Dyno
Or How To Get A Cheap, Accurate, Repeatable HP Estimate
By Bill Watson
It's natural that with a group like Turbobricks, the talk commonly turns to
power output. Even though the intent is good, the talk is just that: talk. Sometimes
we all wish we could get free dyno time to see just what kind of power our cars
are really making.
I would suggest that the most accurate CHEAP method out there is a trip to
your local quarter mile dragstrip, which typically costs around $15 for a whole
night of racing. We can argue out the accuracy and repeatability of engine dynos
with rear wheel dynos with the dragway in another article.
The paragraphs below focus on the fact that you'll use speed instead of time
as your yardstick, and the point is... YOU DON'T HAVE TO ABUSE THE CAR TO GET
YOUR NUMBERS. That's the biggest excuse I hear for why someone doesn't go to
the strip. You DON'T have to buy slicks, or do a neutral drop, or burn your
clutch, or buy 4.10 gears, or even warm up the tires in the water box. IT'S
EASY, NOW QUIT MAKING EXCUSES AND JUST GO DO IT. (Whew!)
Like it or not, the trap speed at the end of the quarter is an excellent indicator
of your HP to weight. If you know weight (almost every track has a scale), then
you can calculate HP. It's that simple.
Note that I have not used the term "ET" (Elapsed Time) yet. I have
said TRAP SPEED, the speed of the vehicle by the end of the quarter mile. However,
almost everyone involved with drag racing will discuss the ET, the elapsed time
to cover that quarter mile. Yes, the two are related of course, but for all
intents and purposes, here's what I hope you learn from this article;
- Trap Speed will tell you about your HP to weight.
- ET will tell you more about traction and your launch.
Of course ET is important to true drag racers, because the winner is the one
that gets there first. However, we're not necessarily true drag racers in our
attempt to get a power estimate. Honestly, ask 10 guys at the track "What
kind of trap speed are you running?" and 8 out of 10 will answer with their
ET - to one or two decimal places even. When you say, "No, no, I meant
trap speed", they will fumble with a broad estimate with NO decimal places
and might even have to pull a time slip out of their pocket to check. Try this
question when you're at the track; it's almost funny.
THE DYNAMICS OF TRAP SPEED VS. ET
After running lots of quarter miles, it becomes clear that how well you do
in the first 100 feet of the track is KEY to a good time. The last half of the
track is KEY to a good speed.
Let's use an example of a stick-shift mini-pickup that on a perfect run, gets
a timeslip of 19.50 seconds at 70.00 mph in the quarter.
Imagine that the light turns green, the truck moves two feet and the engine
dies for three seconds. After restarting the engine, the driver proceeds to
then complete a perfect pass. His time slip would show 22.50 seconds at 69.97
mph. The ET was 3.00 seconds high but the speed was almost unaffected.. why??
It's because his racetrack was 1318 feet long instead of 1320, and in those
last two feet this truck usually gains an additional 0.03 mph. However, the
clocks recorded the long time. My point? Much of a great ET is made by a great
Now take this truck again, and the driver leaves right on the green light.
However, he misses the 3-4 shift when he's at 1250 feet. He coasts for the last
70 feet while trying to find fourth gear. Now instead of accelerating another
few mph in this final 70 feet of the track, he decelerates over this distance.
His timeslip; 19.51 at 67.83 mph. Note how the et is almost perfect (only off
by 0.01 second) but the trap speed is way off (over 2 mph slow)! On a good run,
traveling that last 70 feet at an average of 69 mph, would have taken .692 seconds.
At a 68 mph avg., that 70 feet takes .682 seconds. That's why his ET only varied
by .01 seconds, yet the trap speed was 'way off'. My point here: the end of
the track is critical to trap speed; shift rpm, missing a gear... these are
the big players.
Hopefully these examples are clear. Neither of these runs are 'perfect' runs,
it's just that one has an error at the start, one at the finish and the results
are obvious. The start of the track is a big player in the ET, but a small player
in the mph. The end of the track is a big player in the mph, but a small player
So for the casual T-Bricks member who wants to get a HP value, you don't have
to buy slicks, or wish you had a limited slip differential. You don't really
need to heat the tires in the waterbox, or launch with huge power braking. As
long as people get their shift rpm right and don't miss a gear, even a rookie
will get the appropriate trap speed for their vehicle.. but honing the perfect
ET. requires being rude to a clutch, buying steeper gears or slicks.... hey,
we're trying to make this recreational.
If your goal is to get a good trap speed, what are your options? More power,
of course - and less weight is obvious (but it will come out in the power calculations
as no increase in power). Shift rpm chosen (auto or manual) and the time it
takes you to shift (with a manual) are probably the most important tools you've
got. Try different shift points to maximize your trap speed. Reduce rolling
resistance by pumping up all tires to their rated pressure. Some people think
that running lower pressure might help the traction in the rear, though. Of
course more traction will help et, but with most street tires, running street
tires within 5 psi of rated pressure will provide you with maximum traction
in the first place.
The ET clocks don't start until you've actually moved around 8 inches (this
is called the rollout)... so don't worry about trying to leave right on the
green light. You could wait 5 seconds after the light turned green, and still
get a 19.50 timeslip in our truck example above. Your timeslip does show a separate
calculated time, the "Reaction Time", which in this case would be
5 seconds. That is the time from the light turning green until you rolled out
of the starting zone. It's not a big thing for our discussion here.
For the most part, a decrease in ET is accompanied by an increase in trap speed,
but don't go overboard on the launch in your zest to rule the world. Just try
to get smartly underway without spinning the tires much at all. Traction levels
usually drop a solid 0.10 g when the tires start spinning.
THE HP FORMULA
Here's the formula to use to calculate HP:
Net HP = Weight in pounds* (Speed in MPH/228.4)^3
As an example, Car & Driver tested the 744 Turbo in their June 1990 issue.
The car weighed 3,081 lb. without the driver.. the 'race weight' was 3,231 lb.
The car ran a 15.7 second quarter at 86 mph. Let's plug it in to the formula:
HP = 3231 * (86/228.4)^3
HP = 172 Net
Volvo rated this at 162 Net. We come out a little high. Or does Volvo underrate
a little? I'll say this - I've used this formula for years and that's how the
228.4 was honed - actual experience from cars that had actual power curves -
and when I use it on Volvos it tends to always come out a few percent higher
than the factory rating. This could simply be that Volvo underrates just a little.
Still, for such a simple formula and such a simple test, it's surprising how
accurate this can be. And the best thing is - there's no arguing the numbers
on a timeslip. There are always differences between a DynoJet and an Eddy Current
Dyno, or G-Tech numbers, but every setup is done by someone different and subject
to error. The quarter mile is arguably the best comparison a diversely located
group like Turbobricks will ever have. The only real difference to argue about
is the altitude of the track! You can compare ET and mph all day and have a
HANDY RULE OF THUMB
Once you have a baseline, you should probably use a rule of thumb that each
additional 6 HP will give you another mph. That's for a 3200 lb car that runs
88 mph. If you want the real formula for different weights or speeds, here it
HP for another mph above "X" speed: = Wt * (((X+1)^3-X^3) / (228.4^3))
For instance a 89 mph quarter vs. an 88 mph quarter for a 3200 lb car:
HP delta = 3200 * ((89^3-88^3) / 228.4^3))
HP delta = 6.3 HP
Once you're going 110 in the quarter, it would take an additional 10 HP to
go 111 mph in the 3200 lb car.
60 FOOT TIME
This is the standard measurement tool to evaluate your launch. It's the time
that it took you to travel the first 60 feet of the track. Naturally, patterns
emerge again after looking at lots of runs and of course these correlate best
to time, not mph. Typically, most everyone's 60' time will be between 14% and
16% of their quarter mile time. If it's under 13% or over 17%, this was not
your best pass.
1/8 MILE VS. 1/4 MILE
After monitoring tons of good passes, patterns emerge. Typically, the mph at
the quarter is around 1.26 times of the mph at the eighth, and the time at the
quarter is around 1.55 times the time at the eighth. You can use these values
if you only have a 1/8 mile track and get a real good idea of the theoretical
IS MY ET TO SPEED RATIO REASONABLE?
One fact of the quarter mile is; no matter how slow or fast your car is, the
mph multiplied by the ET will pretty much be the same number every time. Before
the NHRA changed the way that speed is measured in 1989, the product of speed
and time was around 1400. Let's calculate some easy examples of this. A 14.00
et usually resulted in a trap speed very near 100 mph. A 10.00 et meant around
140 mph. A 200 mph pass usually takes around 7.00 seconds. These are still good
rules of thumb to remember, but now the product is more like 1380 for us - The
example from Car and Driver above comes out at 1350. (The reason for this shift
is explained below). Remember, most everyone focuses on ET so much that they'll
even optimize a car for slower mph if it gets them a better ET. (Rear end gearing
is one way to do this). Those guys tend to have a product closer to 1300.
RESPECT MORE SPEED - A LOT. EVEN 3 MPH.
If you look at the formula again, you'll note how trap speed shows up as the
cube root of power to weight. That's critical to understanding how fast one
car is over another. Let's say your car does a 90 mph quarter and the guy who
raced you in the other lane ran 71 mph. After the race, he wanders over to you
to say the 'race was close'. Your reply: "I could have towed you and still
beat you". (This might not be the best way to make friends, but yes, it
is TRUE if the cars weigh the same.)
Do the math. (90/71) cubed is 2.04. Yes, the 90 mph car has 2.04 times the
power to weight of the slower car. It has 2.04 times the acceleration of the
slower car. It's just that the track is a fixed length, and in accelerating
to higher speeds, you use up the track quicker. You accelerated to 90 in about
20% less time than he had to accelerate to 71, right?
Bottom line; Down where most of us run, a 3 mph difference between two cars
is NOT a race. It was a clear win. There's a full 10% difference between these
SOME MAGAZINES SHOW THE CONSTANT AS 230.5 OR 234.0. WHERE DID YOU GET 228.4?
Some people try to correct to different things. Like Gross HP instead of Net.
But most commonly, these other constants that you'll see in magazines were originally
published before 1989 when the NHRA changed their lights, and the 'new' journalist
doesn't realize the formula should change accordingly. Here's what I mean; previous
to 1989, there were three timing lights at the end of the track; one AT the
end of the quarter mile, and one 66 feet before, and one 66 feet after. The
middle light was used to calculate the et of the run, and the time to travel
the 132 feet at the end of the track was used to calculate the trap speed. This
gave the average speed at the end of the track, but you can see what this lead
to. Most of the racers stayed on the gas for an additional 66 feet past the
quarter to get a consistent speed to evaluate their setup. The track's 'shut
down area' of course is a fixed length, but the pro racers were starting to
hit 300 mph plus by the end. In an attempt to get these guys off the gas 66
feet earlier and 'make' the cars appear slower, the NHRA stopped using the last
light around August of 1989. Today, the trap speed is calculated between the
light at the quarter mile and the one 66 feet before. So any timeslip after
1989 is really giving the average speed 33 feet from the finish, which is pretty
close to one percent slower than before. The old constant of 230.5 became 228.4
CORRECTING FOR ALTITUDE
If we were dealing with non-turbo cars, this would be easy and we'd publish
a formula. But with pressurized cars, the correction factor for altitude depends
on the boost you run.
For instance, Sea Level air pressure is 14.7 psi. If you go to a track in Boise,
Idaho (2850 feet above sea level) the air pressure is now around 13.25 psi.
That's 90.1% of sea level pressure. If the temperature doesn't change and you
have an normally aspirated car, your power output will now be 90.1% of what
it used to be, so I'd tell you to correct by multiplying your calculated HP
by an extra 10.9% (1/.901, or 1.109).
However, (and this is the beauty of turbo cars!!) Let's say you were running
10 psi of boost in the first place. So at sea level, your car was really getting
24.7 psi (14.7 + 10). Now you leave the wastegate at 10 psi and race at Boise.
Your manifold pressure is now 23.25 psi (13.25 + 10). Note that YOUR power isn't
down as much.. it's down to 94.1% of what it is at sea level. So you should
correct with an extra 6.2% (1/.941, or 1.062).
If you wish to calculate your own correction factor, here is a handy table
of elevation (feet above sea level) vs. standard day atmospheric pressure (psi):
Yes, the detail oriented will notice that I'm ignoring lots of small effects
of higher pressure ratios in the compressor, lower density air across the intercooler
and even the fact that there's less wind drag at higher altitudes, and they're
right. However, the overall concepts above still hold true.
There's lots of discussion of 300, 400, even 450 HP on the Tubrobricks list.
It would be great to see these power levels turn out to be true. Just keep in
mind that an honest 300 Net HP in a 3200 lb Volvo (includes driver) will go
just under 104 mph in the quarter. 400 HP would push it 114 mph, and 450 HP
should propel the car to a trap speed of nearly 119 mph at Sea Level!
I hope this helps further your understanding about the quarter mile and some
of its myths. Here's your chance to get some numbers of your own, so you'll
be more accurate when you estimate your car's true power.
- Bill Watson, Sr Engineer, Honeywell Jet Engines
- Database manager of over 400 Honeywell employees' 1/4 mile runs, has
extensively studied the relationship between variables in 1/4 mile