What the tuner doesn't want you to know (speed dyno)
Are you looking at a dyno graph that's plotted against speed instead of RPM? Are you thinking...
A dyno graph measured in speed barely tells you anything of value except that max power number. The purpose of a dyno graph is to show you at what RPM the car spools the turbo and at what RPM it makes some usable power. For that, you'll want to see its RPM powerband and maybe figure out how the car might actually be like to drive? It could also be used to compare different setups.
Now, thanks to math, you can get a pretty good idea by easily converting SPEED into RPM with the table below, not just "wow it makes 1329.6kw @ 218km/h".
How To Use The SPEED VS RPM Table to Read A Speed Dyno Graph
Step 1 - Figure out what gear it was done in.
Most runs will be done in 4th gear, some will be done in 3rd. How do you know which gear? Look at the max speed on the dyno graph. If it ends at say 230km/h then look for 230km/h on the below table. Now trace it across to the right - It can't be 3rd gear unless you're revving to the moon, and it can't be 5th gear 'cause it'll only be around 5759rpm, so that means this car was ran in 4th gear up to around 7587rpm.
Step 2 - Figure out when "full boost" comes in.
Just check the speed when "full boost" (whatever that might be) is, say it's 150km/h, then simple run your eyeballs to down to 150km/h and across to 4th gear. The answer somewhere around 4948rpm.
Step 3 - Figure out when it makes max power/starts to spool/drops boost/etc.
Step 4 - Figure out when it maxes out.
Easy as pie since you've already worked out max RPM in step 1.
Step 5 - Is this a car/combo you'd like to own?
You can now decide :)
Speed VS RPM Table
I hope this sheds some light on speed dyno graphs. Remember that this is a generalised table and nothing beats a well configured dynometer, but when you don't have that then you have to make do.
Tech Numbers We Used To Make The Table Above
- Gear ratios: SR20DET 5 speed gearbox with the following ratios: 3.321, 1.902, 1.308, 1.000, 0.759
- Most Nissan's will be running a 5 speed box and they are almost the same between the S, R, and Z chassis.
- Diff / final drive ratio: 4.0
- The most common is 3.7, 3.9, 4.1 and 4.3. So let's just go with 4.0 to be smack bang in the middle to cover as many bases as we can
- Tyre size: 235/45/17
- A very common size
- Max RPM: 9000rpm
- Just to be able to work out what gear the dyno pull was done in
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