This came up in my previous thread and didn't want to side track my thread so thought I'd start a new one. Why are some of you so adament about running 3rd gear on the dyno??

I had told Kieth (owner of Force Fed Performance) that I should dyno in 3rd and not to WOT until 3100 RPM so it wouldn't down shift. Here is what Keith said to me:

"We use second gear to limit speed on the dyno. In addition, you're wanting to check your A/F's to see that they are safe. If I run 3rd and WOT at 3100 RPM, you have no idea of what your A/F's are doing between 1500 to 3100 RPM. With the second gear pull, we can data log everything and show you HP, TQ, and A/F in 100 RMP increments (smaller if you choose)."

That all made perfect sense to me. My dyno sheet actually broke down by MPH because he didn't know where to hook up on the L to get an accurate tach signal.

Just made me think of something else. Does anyone have a picture or drawing of where to hook in to get an accurate tach signal?

TIA for your responses bro's.

 

You want to dyno in what's called Direct Drive. Direct Drive is when you engine rpms match your driveshaft speed. On the Lightning, 3rd gear is a 1:1 ratio. So in 3rd gear, WITH the converter locked, you should have a true direct drive and your driveshaft speed should match engine rpms. You don't want to dyno in a lower gear, because there is less load. So if you get tuned in 2nd, your A/F will be different in 3rd going down the track. You need to start the sampling at such a late rpms because under 3k, the truck will downshift back to second. If your dyno tuner has in house software, he can disable the 3-2 downshift, and start from an earlier rpm. In all honestly, there is really no reason to do anything with A/F at such a low rpm. You will be launching somewhere in that low rpm range, but the truck is there for only a split second in first. And once it hits second gear, you're over 3k for the rest of the pass. Hope this helps.

 

Can 2nd really handle the torque this motor produces?

 

Thanks for the info Sal. Can you by chance clue me in on where to pick up a good tach signal? Thanks

 

My dyno just put an optical sensor on the lower pulley. Worked like a charm.

Don't think you can use a listener...

Coldie

 

Yup, you want a 1:1 drive ratio. I have actually been dynoed in 2nd and 3rd gear in the same session. Not sure why, but my numbers were lower by 25-30 in second gear

 

Sal,

Using the W.B.C. while I'm at the track I notice that the truck gets progresively "Fatter" the higher the gear.

In other words if I'm at 11.5:1 in 1st by the time I get to 3rd I'm low 10's. Would it be Ok to run 11.5:1 in 3rd even though in 1st the truck is running 12.7:1.

Thanks,
Mike

 

Ummm...the fact that he couldn't find a tach signal and wanted to dyno in 2nd gear should say something.

 

One of the wires leading from any of the coil packs will give you an rpm reading. I think it's the gray wire with a green stripe. If it's not one, then it's the other. Check 'em both.

 

Because (it's my understanding) that rear wheel dynos measure how fast a given vehicle can spool up the drums (rollers). The more HP you make, the faster rate you will spool them up. Second gear, while having more leverage due to the gearing, wouldn't spool up the rollers as fast.

That's how I have heard it. If I'm wrong, perhaps somebody will chime in.

 

Maybe a dumb question.

Overdrive??

ON or OFF?

Thanks
Rat Trap

 

Off.

 

 

Doesn't matter. The shifter is in the 2nd gear position so it doesn't engage OD. At least I think that's correct.

 

With the gear shifter in 2nd gear posistion, the transmission would never reach 3rd gear. The overdrive is the 4th gear in our 4 speed transmission. As such a Dyno test would be run with the gear selector in Drive but with the overdrive set in the OFF position.

Gear Ratio Maximum Speed In Gear
1st 2.71 43 mph
2nd 1.53 76 mph
3rd 1.00 119 mph
4th 0.71 142 mph (Overdrive)