Would TALL Tires & 5th gear hurt Dyno #'s

[HEMI®Dart]

Last December I Dynoed the truck on a DynoJet. I got 379 Hp. No torque figures then.





These are the things I changed before todays Dyno Day:



1. ) I put on my "Summer Tires". These are LT305/85R16's. 36" tall. The changes the 3. 54 rear axle ratio to an effective rear axle ratio of 2. 94. I ran the OEM size tires at last December's Dyno meet. (30" tall)



2. ) Changed the #5 plate (that was slid all the way forward ) to Joe D's #0 Plate. (slid . 005 forward from stock).





3. ) Added an AFE Mega Cannon air filter.





In December I tried running the truck in 4th gear (direct). Got low HP #'s. Tried 5th gear (overdrive) This posted the decent HP #'s.







Todays 3 Dyno runs averaged 376. 3 HP :{ I did hit a high of 903 Ft. Lbs. torque. all runs were in 5th gear.



I didn't try 4th gear. I forgot. . EGT's were under control I couldn't get it hotter than 1200°. Max Boost went down 2 lbs. smoke was minimal.





The $64,000 question:



Do we lose alot of torque & HP with a much taller tire, especially in 5th gear?



Is it safe to slide the #0 plate forward some? I'm afraid the pin will be stoping the rack travel if it go too far forward.



Any help would be much appreciated.

 

[HVAC]

My experience is the trucks like more gear. You will see a difference from direct to OD. The taller gear allows more turbo spooling to burn the fuel.



Think how cool your truck feels when you hit the bottom of a hill and the turbo starts to whistle. It's like "I gotta go to work now"

 

[HEMI®Dart]

That's what I thought too, HVAC. The taller effective gear ratio will load the engine and make it work. This is where the Cummins puts out the power & torque.

 

[LSMITH]

Hemi, it sounds like you had a fuel delivery problem of some sort to me. The engine and the dyno could care less what gear you have provided it is tall enough to spool your turbo, you should have more than plenty of gear. As long as the dyno setup was spot on you should have seen the correct #'s. EGT seems low for the mods you list too.



I would bet money that tire size and gearing would have no effect on HP and torque as read by the dyno if the setup was good.

 

[Don M]

Bill,



The tall tires should help you some, IMO.

Timing?

Fuel pressure at idle and WOT?

Torque plate is where? 5 thou forward from stock. ?

I cant see your sig when posting so I will ask what gov kit you have in there?



Don~

 

[HEMI®Dart]

Thanks for the tips, Guys



Dee R sent me a P/M saying he had the same problem when he first put his #0 plate in. In the stock position he lost hp. He used Prussion Blue to check for rocker contact. At . 035 forward, no contact. At . 032 forward, the rocker just kisses the plate.



Don,



Timing @ 14. 7

Idle Fuel pressure: 20 psi

WOT 33 psi

#0 Plate approx. . 005 forward from stock

Governor Springs: 3200 rpm



I was baffled because I went from a #5 Plate to a #0, and lost 3 hp.

 

[Don M]

Bill,



More timing

More fuel pressure

More gov springs

Try less rack travel

Crank AFC all the way forward

Slide AFC full forward

Wastegate to 40-44 psi



Don~

 

[Joseph Donnelly]

Full rack travel may not be best. When Sickly ran 510, power went from 503-510-504-488 moving the plate back 1 mm at a time from 20. 5 mm.



3 hp is within experimental error especially over months. . . many things can get weaker over time, injectors, valve job, etc.



If you are about . 035" from full rack, from the above you see more rack may not help.



I saw no difference from gears, tires, etc. Parasitic drag balanced off with the extra load for no difference on our short dyno runs on the Dynojet.

 

[Hammer]

Ok,,Now why, if I may ask, would some of those here think tire diameter would have no effect on dyno numbers??,,IF that would be the case then ANY motorsport competitor can save a ton of money by only carrying one set of tires,,I disagree with that comment just on my experiences alone,,Think of it this way Bill,I'll use this example... You've got a 70 440/6 Challenger SS/FA car that weighs in at a modest weight of 3720lbs With it having a set of 4. 56's in the rear,,Which tire combo would et better and which would mph better,,Equal rim widths 15x10,,A set of 12. 5x31 or 14x32,,Any idea's??,,Shorter tire will et better but lack in top end mph and the taller tire will mph better but lack in et and short times,,Reason,shorter tire will move the power application faster and the car quicker,but,nose over sooner where the taller tire will be slower to apply its power and move it out the rpm range better,but,has more mph on the large end because she won't nose over as quick,,Its easy to say in my mind that the tire change has hurt the truck in the power area,but,the only way to tell for sure is to dyno with the tuneup you have now using the smaller tire,,On back to back pulls I'd be fairly certain you would see the taller tire has hurt the numbers some in the short run,but,will keep the truck in the power range longer(just may be a little slower getting to it),,Old racers tale. . "Quicker et vechicles will win over a mph vechicle in a short run,quicker is better than faster",,Just my . 02 worth. . Andy

P. S. -This is one of the class racers favorite tricks for those heads up races where the thousandth may equal a win and on to the next round... ...

 

[HEMI®Dart]

A guess:



On the Race Track, tire diameter would play a big role ( gas & diesel). On the Dyno, with a Cummins, I don't think it has much effect.



On the Dyno:



I think tire diameter would effect gassers more than Diesels. With the insane amount of torque the Cummins puts out, tire diameter wouldn't effect things as much as a lower torque gasser.

 

[Strick-9]

On a dynojet, we are overcoming inertia with power. Any reduction in rotational inertia will show up as more power.



That's the beauty of a dynojet over a load dyno for racing applications. If you swap to aluminum rims, the truck WILL be quicker and it WILL show up as more power on the dyno.



But I honestly don't have the experience with our motors to put numbers to it. It doesn't seem like it would be much, but I've been wrong before.



I believe Joe Donnelly recently swapped to lighter Alcoa rims. That should have showed up as more dynojet HP. Maybe he can enlighten us on how much he gained.



-Chris

 

[Hammer]

Chris,,One of the formulas we use in our race applications is very simple for all to undrstand on the weight deal... ... ...

1 LB rotating weight reduced= 7 HP added to ground

Most of the guys seeking higher hp and power to rear wheels are advocates of this,but,sometimes lighter is not always better,,Many guys a few years ago for example tried the "infamous" plastic rear wheels that were being made at the time for sprint cars,,Sure they were light and worked well on a dry slick(blue groove) track but when tried on heavy track(like Terre Haute) or a fast high bank(Eldora) the stress was way to great and caused many a upside down rides,,Driveshaft technology is one we can refer to here,stock is very heavy and the use of lighter would help on a dyno roll(as would lighter wheels and tires),but,to everyday drive them or sledpull there would be a problem,,Its a fact that larger tires/heavier wheels will hurt dyno numbers,not a myth... ... ... . Andy

 

[Don M]

The answer was given here from Joe in this statement :



"Parasitic drag balanced off with the extra load for no difference on our short dyno runs on the Dynojet. "







Don~

 

[Deezul1]

My truck made more HP and Torque in 5th gear than 4th gear, Larger tires would probably mirror that result. I believe turbo spool up and and time for the boost to build is why this happens.