kevink0000

The butt dyno. Much maligned and disparaged.

Does it exist?

Is it useless?

Is there even a modicum of accuracy?

My thinking is that I sit in my car many hours a day, putting over 25k miles per year on it.

I can tell when the plugs are fluffy, and need changing. I use a mileage threshold, but it is based on my perception of how the engine is running. I am proven right, as the engine responds immediately better with fresh plugs.

I can also feel increase/decrease in elevation, since my drives can go from 2000ft to over 6000ft above sea level.

Less than 1000 feet or so of elevation change I must admit I have trouble detecting. But I can tell when I have descended or ascended a 1k ft increment.

That is about a 3-4% difference in power in an NA car, according to some sources.

Has anyone else had the same perception?

I know this is an opportunity for flames, but lets actually have a meaningful discussion.

TeamRX8

iTrader: (25)

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spectre6000

That depends on your definition of a butt dyno....

If you're talking about the bench racing variety of "my cone filter adds 50 horsepower!", which is what's often referred to and maligned, then... IFF (logical "if and only if") you have EXTENSIVE and INSTRUMENTED experience with a particular platform, a butt dyno can absolutely be as accurate as your typical dyno where there's such and such variation from one make/model/setup to the next. That doesn't mean one single vehicle necessarily, but hands on tuning many of the same.

What it sounds like you're talking about though would more accurately be called mechanical sympathy. That is 100% real and attainable. A strong mechanical sympathy can be developed across a variety of platforms, and is a very powerful tool. I don't know that anyone calls this a "butt dyno" though.

Side note re: altitude differences. I live in the mountains above Denver and traverse thousands of feet of altitude, often multiple times daily. Your 3-4% change in power "according to some" only requires that modifier due to a misunderstanding/conflation. There is a 3% reduction in air density per 1000' increase in elevation. There are things that cause this to vary, but for tuning purposes, that's your number. With that reduction in air density comes a reduction in gross oxygen available for combustion (air is the pretty much same recipe wherever you go, so if you have less air in a given volume, you have commensurately less oxygen). In a computerized fuel delivery paradigm with everything functional, you're not going to notice that really, because the trim is adjusted on the fly. With carburetors or certain fixed mixture injection systems, it's a totally different story, and you can absolutely feel the air density change even in those (relatively) small increments. That's due to a combination of that air density change with the accompanying change in mixture for a multiplying effect. It's also more noticeable with low hp engines. This is true for NA; forced induction erases the density change.

BigCajun

iTrader: (1)

Not sure exactly what you're going for, but I've always been able to notice when my 8 runs better after the good tuneup with plugs and cleaning the sensors.
I haven't had to get any coils since I got the SakeBomb IGN1A Kit right after they came out.

I definitely notice the difference with my new engine.
I had gotten used to the gradual, almost imperceptible decline of the old engine as it was dying.

Reminds me of how I fell in love with it when I first test drove it 11 years ago.