Hey guys,

I'm trying to figure out the rpm's associated with a dyno that is displayed in mph. SPeed is directly tied to rpm so it shouldnt be too hard.

The gear chart is
Final Drive 3.54
1st 3.794
2nd 2.324
3rd 1.624
4th 1.271
5th 1.000
6th 0.794

Tire size is 275/40/18

BUT, When I back calc rpm from the speed, the graph doesnt make sense really. Either WAY to fast or too slow spool.

Can someone double check what like 93 mph is and 127 mph is in rpm?

Not sure what gear, but I think the rpms would make sense if properly calculated. Just an example dyno to practice on that I cant figure out.

Thanks!

 

http://www.f-body.org/gears/

Did you take into consideration the wheel/tire diameter?
And you remember load is always greater in the higher gears and you spool sooner?
Edit
i saw you posted tire diameter etc.
How exactly did you calculate the rpm? what mathss did you use?

 

ok from that web page it looks like 4th gear makes sense, which is what you;d figure.

not sure how my calc was figuring rpms then

 

Yeah its probably just some funky math problem in your calc.
I personally hate dyno sheets with mph instead of rpm... The only reason I can see them being used is to make things look less laggy than they really are.

 

Or to clean up the curve when there is wheel slip

But Str8 Terry has an awesome excel file with this calculation, you can see what changed to each gear ratio does and also FD changes. Based on his sheet, it would be 4th gear and 93mph = 5300rpm and 127mph = 7200rpm, and that is with tire size considered

 

It is much easier to show speed on the bottom from a dyno operator's standpoint, because it is simply roller speed and no hookup/setup is required. Tacho pickups require messing with the clamp and calibrating, which can be difficult for some.

str8dum1, you can back calc, but I wouldn't rely on this as an exact measure due to tire pressure, strapping, actual height variances, slip, etc. Good for getting an idea though.

 

Thanks Hal,

rough idea was fine for this purpose.

 

Rich,
Mine was same way on SP's dyno (power v speed)...when I calculated RPM it wasn't exact (but was close enough)

tom

 

First of all, +1 on Hal's comment but if you want to ballpark it, multiply:
(mile/hr)•(1rev/pi•d)•(63360in/mile)•(1hr/60min)

In this case, d=18 + 2(0.4)(275)(1in/25.4mm) = 26.66"
So, pi•d= 83.76"

Testing 93mph, I get 3684rpm WHEEL angular velocity. Use the gear ratio as applicable to convert to engine rpm.
I cranked this out pretty quick, so if someone finds a mistake, please advise and I'll edit it as soon as I can.

 

That's fair, but it seems a tad bit unprofessional, especially because it's 'easier' (dyno operator is a professional getting paid to do the 'work'), because it is easier to hide lag, and is harder to read. I mean I got the mph memorized in my gears pretty well but cmon now... When I work on a song or something I intend on selling, I ensure its as buttoned up as I can get it. Although if it's only for one or two pulls, that's a bit more understandable...
Good to know though. Never had a chance to dyno any vehicle... lol all the cars around here are street tuned :O