horsepower calculation

[twist]

is there a way to calculate horsepower without a dyno? I am sure that there is now way to know exactly what the engine output is without the dyno test but would like to know a ball park estimate. Besides, I have been having trouble finding a reliable source for testing where I am, (San Francisco). I have done some modifications and would like to know what effect they have had on my motor.

[Duane Ausherman]

A calculation won't tell you much of anything. Even back in my day there were a few places with a motorcycle dyno.

[Deleted User 72]

A calculation won't tell you much of anything. Even back in my day there were a few places with a motorcycle dyno.

Back in your day I thought they just teamed up horses and had a tug o' war.

Back to the question... I'm pretty sure if you're unable to redline in top gear - top speed would be an indicator. I have no idea nor data on what the ratio would be. Torque in the right range is more fun, IMO.

[melville]

The musclecar guys can roughly calculate HP from trap speed at the end of a standing quarter mile, but the calculation usually used assumes a boxy car weighing 3000 to 4000 lbs. It's not accurate for a hot VW, for one, and they weigh 1500 to 2000 lbs.

[dwerbil]

A calculation won't tell you much of anything. Even back in my day there were a few places with a motorcycle dyno.

Duane, did you try this place?

[Major Softie]

What you can do is measure your current acceleration, and compare it to changes you make. While this won't tell you specific amounts of HP, it will tell you what changes are resulting in a genuine increase. Obvious things to measure are 1/4 mile and 40-60 mph and 60-80 mph roll-on. Those will give you much more useful information about usable power increases than top speed, and the roll-ons are really easy to find safe places to do. You'll want to do all the tests in exactly the same spot each time, and the wind conditions have to be the same.

[Garnet]

There has been a program called "Desktop Dyno" that has been around for quite awhile in the automotive hotrod market. It has a database of all the popular car engines and aftermarket cams etc built into it.

You select your engine and then fill in the blanks including oversize of pistons or brand, CR, cam specs, valve sizes and manifolds etc and it prints out a proposed dyno chart. It is imortant to remeber that that program is only usefull to compare changes between variables rather than actual imperical output of the engine.

I'm not aware of anything like that for our old engines. But I live under a rock.

[Rob Frankham]

The figures obtained from shop dynos are seldom any use except in before/after scenarios or direct vehicle to vehicle comparisons using the same dyno.

Dynos by their nature, need to be calibrated on installation and re-calibrated on a regular basis. They also need to be run in controlled conditions. Very few shop dynos are. Not going to even mention that some are deliberately mis calibrated by some 'tuners' to make it seem that their efforts have been more productive than they actually are... Oh I just did!

It is not unusual to get variations of up to 20% from two different dynos on the same engine.

Before and after readings on the same dyno are useful. Direct comparisons between engines on the same dyno may be useful. Direct HP readings, unless you can be sure that the dyno has been recently calibrated are simply mind candy. Comparisons using different dynos at different times aren't worth the paper they're printed on.

Rob

[Seth]

there are calculator apps that work with the g-sensor in the iPhone.

http://itunes.apple.com/us/app/horsepow ... 55060?mt=8

[Dave Backmarker]

+1 on Major Softie's post. Improvements in acceleration by measuring the change in speed from normal speeds (e.g. 30 to 60) will be the most valuable as it will be an apples to apples comparison and will factor in varying engine speed, drag, rolling friction, etc. An improvement in bhp as measured on a dyno might only happen within a rpm band that you don't like to use. Comparing improvements in speeds you drive at will bring out the benefits of changes you make to the bike.

DaveM
1975 R90S
2000 R1100RT