Dyno Question

[JFitzgerald]

Dyno Questions

At Scheid's Diesel Extravaganza, this past Friday, I had my truck dynoed. Results were 196 RWHP/575 TQ. These numbers were about what I expected for my minimal mods and stock injectors based on what I've read. The question is, during the run, I saw higher numbers displayed than the final results I was given. Can someone explain the details to this "dyno newbie"? Not complaining at all, I just want to understand the details. Also, the operator told me to add 20% for flywheel HP, but torque you don't add to for drivetrain loss.



By the way, there was a first gen there (not a TDR member) that also dynoed, and his results were 240 RWHP/815 TQ! I asked what his mods were, and he said his fuel screw was turned in all the way/backed off 1/4 turn(!), 16 cm housing, and Lucas injectors. He hadn't even messed with timing yet. He grosses 20-26K towing heavy equipment regularly. I was giving him a hard time for not pulling, but he said he tried it once and his clutch did not care for it.

 

[Dieselman359]

Missed Ya

Sorry I missed ya, J Fitzgereld, I'm like a kid at Christmas when I go to these truck rallys, looking a trucks with dump beds, stacks, lifts, etc... . Anyway, about your numbers, not bad. I don't remember seeing numbers higher than what they said when I had mine done. Everything happens so fast on that dyno.

Man there were a lot of high-powered trucks there!:--)

 

[JFitzgerald]

Dieselman359, I'm not sure what your truck looks like, but I was checking out any 1st gen I came across. I was only able to be there Friday, sorry I missed ya. It was like Xmas for me too! Picked up some good deals.



I saw 204/606 on the screen during the run, but one of the Sceid guys there at the dyno was trying to explain how they were trying to standardize the runs by running at rated rpm (2,500 for 1st gens). I'm not sure how they know the exact rpm, since I asked if they needed to know the gear ratio, etc. , and they said no. The guys were helpful, but busy, so I didn't want to hang around bothering them with more questions. Without gear ratio & tire size going in as data, I don't understand how they can know rpm. The chart they gave doesn't show rpm either. My truck has no tach, and it has larger tires than stock. The numbers are fine with me, I just want to understand the process. It was cool getting it on the dyno, and I now have baseline numbers, and can BOMB from there.

 

[Old Smoky]

Where are you guys getting your torque? Injectors? I ran 203hp and 421 ft-lbs at BD dyno day(a few mods sense then). Looks like I need to upgrade.



Mike

 

[JFitzgerald]

Originally posted by Old Smoky

Where are you guys getting your torque? Injectors? I ran 203hp and 421 ft-lbs at BD dyno day(a few mods sense then). Looks like I need to upgrade.



Mike

Not sure, Mike. It is strange how we can be close on HP, but not torque, and it looks like we have similar mods. Maybe it's my polished 5" stainless tip. Mine has stock injectors as far as I know. The previous owner said they were replaced at 68K for some reason. I would guess this was under warranty?? Other than the 16 cm housing, I've just tweaked the pump screws & diaphragm, and rotated the pump about 3/16" from the factory timing mark.



Power Wagon / others, can you give us any ideas?



Oh, I just noticed I typed in the torque number wrong before. It was 572, not 575. Didn't mean to inflate it. :-laf

 

[hdm48]

Dyno's are a funny thing. The last time I dyno,d I was putting down 258hp. What is strange is I do the quarter in 15. 80 at 84 mph. If you do the math, that works out to 325 or something like that. I think useing the dyno for tuning and not trying to use it to prove real time true horsepower is the key.

I am using the dyno numbers to prove/ disprove the gains when I do my twin turbo set-up.



--dave--

 

[Power Wagon]

I got more HP than I expected on the dyno, at 217 HP.



From other people's descriptions, I'd really expected about 190 to perhaps 200.



I did not use an rpm pickup, so I got no torque numbers. They make little sense on an automatic anyway.



In order to get a torque number, you have to use an RPM pickup on the dampner on the front of the engine. Probably what you saw was the rpm / horsepower numbers above 2500 engine rpm. I have no idea why they didn't give you those numbers. RPM changes are valid mods and horsepower changes.



Apparently, they cut it off at your published governed speed.



Doing that would have cut my HP numbers a lot, since the engine has to get past 2500 rpm just to get the converter to transfer all the HP.

 

[Old Smoky]

It couldn't be the 5" SS tip; I've got one too. Part of it could be that I have larger tires than stock. This would reduce torque multplication. I would like to dyno with my current setup, then dyno with stock sized tires. I just cant justify the $ to do that though.



I really don't know how a dyno can be accurate. Most I have seen do not ask about rear end gears, and don't take tire size into consideration. Because of this, RPM would be off, affecting the power output reading. Hp = (torque X RPM) / 5252. It seems that numbers vary between dynos as well. Like hdm48 said, dynos are good for tuning, but not good for 100% accurate numbers. I wouldn't think that tire size would make that much difference between JFitzgerald, and myself. I don't know what else it could be.



Mike

 

[BushWakr - RIP]

My numbers were wacky too. . (in my very limited opinion/experience)

I ran on BD's dyno and got 225. 2 HP@2200 and 737. 5 Tq@1480

They did not use an RPM sensor or whatever, but they did 'synchronize' at 2000 RPM using my tach.

I have no idea why they stopped at 2200 since governed for me is about 2750-2800, and 1480 RPM ?????? what's up with that???

On an auto too... go figure.



bob

 

[Philip]

Let me kick in my $. 02 worth.



I have many hours on chassis dyno's as an operator. I have run a Sun 1320 model and a Clayton 7100 model. These are older models by todays standards. Both were rated for 200 rwhp.



But the same principals apply. There sould be a standard base line applied to all. Some of you are saying they use your tach's for controlling your rpm. Others that don't have tach's it sounds like they are using roller rpm. But if they use roller rpm. They sould be pluging in rear gear and tire size into the equation. By not using rear gear and tire size it could throw off the readings quite a bit.



Dyno's are very good tuning aids. There will be some difference between brands and models. But the difference souldn't be over 5% on the readings. When we switched from the Sun to the Clayton my service truck lost 5HP. Corrected for temp and alt my truck dynoed at 130 rwhp on the Sun. On the clayton it was 125 rwhp. This was on a 68 C10 with a 250 C. I. six cylinder. It started with 85 rwhp.



USAF maint rule #1. No one can out run the service truck.

 

[JFitzgerald]

Thanks for the replies

Well, it sounds like it's all "relative". I think Sheid's was trying to give everyone a fair reading relative to the other trucks, which is fine, but I'm not in competition with any other truck, I just want to know what my truck can do on paper. No big deal.



The question still keeps coming back in mind though - how can you figure HP, which requires rpm going into the equation, without rpm input to the dyno? They must be using roller rpm as Philip mentioned.



Mike, I believe our tires are the same dia. If you have 285/75-16, and I have 33x12. 5x16. 5. Pretty much the same diameter, ~32. 8". I too thought they would need to compensate for the tire size.



PW, would you mind going into the consequences (if any) of turning the fuel screw all the way in, minus 1/4 turn?



Bob, gotta like that torque number!

 

[Old Smoky]

I didn't see that you had 33's, JFitzgerald. I guess our tires are about the same size. So what's with the torque variations? Don't get me wrong, I am happy with the way the old rig runs, I just like to know the details. I just might have to get a set of PW's and see what happens!



Mike

 

[JFitzgerald]

My guess would be that if both trucks were put on the same dyno, same dyno settings, same day, same place, etc. , the results would be very similar. Who knows?

 

[Philip]

The bad thing about going with roller rpm for a base setting for our use. It doesn't take into effect tire and gear ratio's. Setting the truck for 60 mph on the dyno. Then pull a full load is fine if everyone has the same gear/tires. But if someone has a 3:54 gear. His engine will be at a higher rpm than my 3:08 geared truck. So this means mine will show a little lower HP. The torque reading would be close to the same. Torque reading is always at a lower rpm than max HP. So the torque reading wouldn't be affected to much.



If I have a job by next year when Muncie comes around. I plan on being there to get my truck dyno'd. I'll ask some questions.



Make sure when you compare HP and torque spec's that everyone has the same rear gears. The way they are doing it. If you compare against someone with another gear. It won't be at the same rpm as you tested at. List you tire height also. The 235/16's I have on my truck are 2" taller than the 205/16's I took off. This made my speedo 5mph faster. Plus it droped my rpm also. I don't have a tach so I can't say how much it droped by.



Remember taller tires kill the rear gear. None of us put on shorter tires that I know of.