I've always wanted to install one of these "butt dynos" in my shop. The Jessica Simpson butt dyno is the one I want!

Seriously, there's tons of references to people's "butt dynos" but my experience has been, when comparing actual chassis dyno testing to "butt dyno" testing is there has to be a pretty big improvement in performance for the average "butt dyno" to detect anything and most butt dynos have a huge error factor involved in the data because the brain part of the butt dyno seldom is objective about the modifications. I mean, like...who wants to pay 100 bucks for a set of plugs and then say you couldn't feel much difference?

As for hard numbers, typically the average person starts to feel a difference when the power increase gets to 15-20 horsepower. A very sensitive person who's done a lot of butt dyno'ing and is doing their A-B test on a back-to-back basis with the same road and weather conditions might be able to feel 10hp.

Bottom line butt dynos, unless they are the butts of very sensitive persons, with a lot of butt dyno experience and an objective viewpoint on the modification being tested, provide mostly bull**** information.

Chassis dyno give you good numbers. Also, timed acceleration from 5 mph- 60 mph can be valuable as well.

 

 

This depends on what you are referring to. Once a car is moving, I agree, especially as a passanger, that it is hard to feel minor changes. Personally, I have driven many fast cars in my day-big block Camaros and Mopars, very worked small blocks in AMX, Camaro, Datsun Z, turbo Z's and Mazdas, etc. But in the right seat, it is hard to tell just how quick a car is when you are not in charge of the gas pedal. A buddy took me for a ride in his 700 HP Roadrunner (has run low 10"s) and it didnt take my breath away.

That said, when you are used to driving a car everyday, and you are in 'tune' with the car, then you can notice many things. I can feel the difference when I bump up the air pressure in my tires. As for performance mods, I can tell by how much throttle it takes to accelerate, as well as how easily the car breaks traction. When I installed the Autolite plugs, for example, the car would do an easier and longer burnout than before. I did not have to put my foot in it as much to spin the tires. I can not speak about the HP gain or torque improvement-only that the car felt quicker.

And, the truth of the matter is that nobody really cares about the numbers except for bragging rights. The only thing that matters is how the car feels to the driver. If he thinks it is fast and running good, then that is what is important.

 

All I can say is that I've been building cars and testing them on chassis dynos for 20 or so years and my experience has been that people's "butt dynos"...when it comes to quantifying performance increases...are pretty darn inaccurate.

Example:
One time I was working with a guy who had a 68 Vette with the 3x2bbl "Tri-Power" carb set up. He kept telling me what a thrill it was when those end carburetors really "kicked-in". Thing is, when that sysetem was working properly, you'll feel a surge in acceleation as the two end carbs open but it's not a huge surge or a jerk.

Well, on the dyno we found that what he was feeling was actually a momentary deceleration just as the end carbs opened. It turns out there was a problem with the carb system causing a "flat spot" just as the end carbs opened. As he stepped on the gas, he was pushed back as the engine began to move the car. As he stepped into the throttle more and got to the point that the end carbs "kicked-in" he was actually "thrown" forward for a split second as the acceleration abruptly stopped and the car slow during the "flat-spot". A few hundredths of a second later, the engine began to pull, even stronger now with all three carbs open. So he went from acceleration, to decel. for a few hundredths while the engine ran through the flat spot, and then back to acceleration that was higher than before. The owner thought that was the thrill of increasing acceleration. The human body can't decide the vector of the acceleration when it changes so rapidly and lasts for such a short time.

I bet if I evaluated those Autolite spark plugs in a proper A-B test, the improvement would be way too small to measure with a "butt dyno" and almost too small to measure.

It is true that in the end, one measure of performance is driver satisfaction but where "butt dynos" are inaccurate to the point of misleading is when you start talking about how much performance increase the same mod you perform on your own truck will offer to other trucks.

 

Do not buy these plugs for the hhr. I tested a set. I think it would have been 80 bucks for 4. One burnt out and caused a misfire. They are not iridium fisrt of all, so low mileage. The central electrode burned out completely on one . The other 3 were well worn. This was 3000 miles roughly when it happened. They were pulstar brand. I put back in the stock plugs which had 15000 or so on them. There was no increase over stock in mpg or butt dyno. It threw codes at first when I changed them. Resseting didn't help. Eventually the code went off in a day or so. At any rate I would have been pissed if I paid for them. I did not so it was just an inconvienience that it went out.

 

Hib has some really great points
Let me say this,a spark plug is not gonna give you 8 tengths in a 1/4 mile.
If you can light the mixture with stock stuff the rest is out your control.
I have a stock 2.4 HHR and it runs fine.
My Harley on the other hand is my baby and it gets tuned on,hopefully i get some dyno time today.
Plugs wont mean doodly unless you have a nother problem and then they are just a bandaid.
Yes i have NGK Iridium plugs in the Harley with an aftermarket ignition control that allows me to change the timing curve and rev limits,i can also look at the last 30 minutes of engine operation with my laptop.
Now the last time the Harley was dynoed the fuel curve looked rich so i changed the main jet down 10 and it feels better down stairs but doesnt feel as good upstairs.
Really what im saying is Butt Dynos dont work and plugs dont mean squat.
Having rattled on about that i will rattle some more.
Hot bike magazine did an article about engine oil and filters and claimed like 9 hp and 10 ft lbs tq on a 96 inch big twin,so i buy into this nonsense and call the MFG of this killer filter which costs 200 bucks (no they dont make an HHR filter yet) well i call the guy and talk for about 12 hrs and get him so pissed he wont even sell me the filter but he does agree to give me 5 gallons of his Blended engine oil so the next day i have the oil on my porch.
So today if the weather permits the Harley goes back on the dyno to see if the results agree

 

Dual plugs.......install a 2nd set on the exhaust side of the cylinder and watch the results.
delay the spark on that side by a few mili's and you'll gain 50 rpm at idle with no other changes.What this does is form 2 flame fronts( controlled )thats ythe only way you'll gain HP from a spark plug.............