I had my truck chassis dynoed about a month ago by a portable Mustang Dyno that was setup at a car show. Only cost 30 bucks for 3 runs so i said what the heck. Well, my speedo was off by like 20 mph too fast and i couldn't get it straight so i was unable to take a peak hp reading because my truck thought it was hitting 125 when it was only hittin 92 mph by the dyno, which was only 4500 rpm.

The guy had only had the setup for 3 months and apparently was having a hard time getting everything ready, and he didnt even have a fan to put infront of the radiator. Added to the fact that the dumbass wanted me to make a pull in 2nd gear "A4" which sure as hell isnt 1:1. I told him that and he argued for a second and talked to one of his buddies and then told me to run in 3rd.

So by now were on my 3rd and final run, and my top speed limiter kicks it off at 4500 rpm because of my speedo. I've already spent like 30 mins on the dyno trying to fix my speedo, and he's wanting me to get off so he can grab some more money for some of the other people waiting to use it. He prints out my #'s and im like WTF?!?! It says i made 175.2 hp @ 4500 and only 217.5 ft.lbs @ 3750 rpm. My hp was still climbing when it shut off, but unless it spiked like hell after 4500 it doesnt look like it was even going to make it to 200 hp. They guy said that with normal sized tires i would get about 40 more figure's on both numbers because of the rotational weight of the wheels and tires "roughly 102 lbs each" I dont really think that the tires would cause me to loose THAT MUCH hp, but to me the numbers seem kinda low even if i only have bolt ons "see sig". So if anybody wants to comment on this i would be more than happy to listen, is they guy right or is he just really new to the machine and has something screwed up somewhere. Because to me, those numbers seem REALLY REALLY low.

 

Tires that heavy eat up a lot of power. They are almost twice as heavy as stock ones. For every pound on the wheels equates to about 4 pounds I beleive. Could be wrong on that but , I'm sure someone will chime in. So extra 100 lbs on the wheels is like 400 lbs. You don't have much as far as mods either. Stock rwhp is in the 220-230 range.

 

Okay, i'm being logical and saying i might have gained an overall 15 rwhp? with the mods i have. But even with that, and the slight possibility that i would have pulled 190 rwhp on the dyno, adding his claim that my rotational weight is eating up 40 hp, that would only put me at 230 rwhp....basically stock. I can see what your saying about transferring rotational weight into static weight, but static weight would only come into play through 1/4 mile times and wouldn't affect actual hp output.

On top of that, aren't the stock tq numbers close to 300 on the 5.3L? or do i have my engines crossed? I was running my Predator at the same time as the dyno, and i had a 92 degree IAT, 197 degree engine temp, and no KR. I'm just trying to figure out if there was something screwy with his dyno, or is there something wrong with my truck. Hell my truck can hang with a hemi dodge ext. cab until about 60-70 mph and then he slowly edge's out infront of me. And with numbers that low there's no way in hell i would even be able to get the jump on him off the line the way i do.

Im just trying to get a rough estimate of my actual rwhp with the 35's, i've thought about going and getting another chassis dyno now that my speedo is setup right. But i doubt i'll see that guy again until next year and dyno time at a shop is pretty high for me to just know exactly where i stand now.

 

I beleive they are rated at 330 tq at the flywheel. Mustang dynos usually give lower #s. You still had about 1000 rpm left. If you were able to run the rpms higher you may have been in the 200 rwhp range. Which would be closer to 240 if you were on a dynojet. If I'm not mistaken hemis put about 245-250 to the wheels. They are heavy too. This would make you pretty evenly matched. He may be edging you out in hp but he's also heavier.

 

Okay, its starting to make a little more sense to me. I guess technically i should be making around 200-210 rwhp with the 35's. Now i don't know exactly on what ratio rotational weight eats hp, but if that was the case then the 40 hp loss the guy commented on would make a little better sense. Although the torque # is really crappy seeing how i was able to get a peak reading, something has got to be off there or my tires are really killing it. I guess i'll just have to settle for that now, i really want to wait until i get some more mods for another dyno run. I'm thinkin my next mod is going to be a STS turbo after i tend to the transmission. Thanks for your help man.