Ive been searching for about 40 min now, and cannot find a clear final answer about the inner working of the dynojet. Ive read arguments from both sides of this in other forums and there is no clear post that would settle this.
I will call dynojet tomorow and make them email me a response I can post.
It will depend on exactly which model of dynojet dyno. Some are inertia dynos, others are load bearing dynos.Originally Posted by JeepLab
Simple inertia dynos are only capable of acceleration sweep tests, and the load is simpy the mass of the dyno drum. This type of dyno is 100% guaranteed to have the problems I'm trying to describe with significantly different results with a turbocharged vehicle with different transmission and/or axle ratios. Rotational inertia of the drivetrain/tires/etc also affects the readings on these dynos, 100% guaranteed.
Load bearing dynos are capable of performing steady state dyno pulls, where it holds the engine steady at full throttle and records the amount of load (torque) was required to hold the engine at a steady rpm (rinse and repeat for many different engine speeds, and you get data points to form a cull rpm range dyno chart). This would give consistent results no matter what transmission gear, axle ratio, or tire size was used, because there is no torque lost to accelerating those components (so therefore no differences in the amount of torque lost to accelerating different configurations of those components), and the turbo lag would not be an issue either because the turbo would be allow to spool up at each rpm point and reach equilibrium before the measurement is taken.
Load bearing dynos can also perform acceleration sweep tests, but can be configured to perform the sweep in different ways. This is even more complicated than a sweep test on an inertia dyno, because at least an inertia dyno can only operate in one mode. The results of a sweep test on a load dyno can be affected by a combination of gear ratios, tire size/weight, AND how the dyno operator configured the dyno to operate (slower sweep with more load? faster sweep with less load? constant load? varying load to achieve constant rate of acceleration?).
Yes, it's a big mess
I didn't misquote you (that wan't a quote where I said "overall gear ratio"). I was trying to bring attention to the fact that it is the overall gear ratio that caused the difference in Jessees results. Transmission ratio and axle ratio both contribute to overall gear ratio.
It doesn't matter whether the difference in gear ratio is in the transmission or in the axle. A difference in gear ratio is a difference in gear ratio. If one matters, then the other matters. There's no way around it. The overall gear ratio is the transmission gear ratio multiplied by the axle gear ratio. One is not not inherently more significant than the other just because of its physical location. If you agree that transmission ratio is a factor, then you are also agreeing that axle ratio is a factor.
4th gear is a 1.25:1 ratio.
5th gear is a 1:1 ratio.
How is 4th gear "closest to 1 to 1 ratio" if 5th gear is EXACTLY 1 to 1?
The difference between 4th and 5th gear is 25%. The difference between 3.21 axle and 4.88 axle is 52%. The difference between 3.73 axle and 4.88 axle is 30%. The difference between Jessee's axle ratio and stock axle ratios is MORE significant than the difference between 4th and 5th gear!
AGAIN: This is is a special situation to turbos only, because it's related to the amount of load on the engine and the duration of the dyno pull allowing the turbo to spool up. This is not about the typical factor of drivetrain rotational inertia. I agree that the axle ratio (and transmission gear) would have minimal effect on a naturally aspirated or supercharged engine, because they do not depend on engine load to produce power.
The turbo needs engine load and time to spool up. The amount of load on the engine on a simple inertia dyno is primarily directly proportional to the overall gear ratio from the engine to the dyno drum (including transmission gear ratio, axle gear ratio, AND tire size). It's the overall ratio between engine speed and "road speed" that matters here.
This also all only matters on an acceleration sweep dyno pull (which is the only type that an inertia dyno can perform). If we could get steady state results on a load bearing dyno, then we wouldn't have to worry about any of this (tire size/weight, gear ratios, etc).
We already have a test that shows that gear ratio matters hugely on the dyno that Jessee used. His dyno results clearly fit the my explanation of the turbo spooling up more slowly with respect to engine speed when using gearing that creates less load on the engine.
All the people that have all told you that axle ratio doesn't matter... was it clearly being discussed in the context of a turbocharged vehicle on an inertia dyno (or sweep test on a load dyno)? it's not a matter of either me being right or them being right. Context is key. Their statements are true in many contexts. Their statements are false in this specific context of a turbocharged vehicle on an inertia dyno (or in a sweep test on a load dyno).
The Dynojet response is at the top, my question is at the bottom.
Ross,
The reading (as you call it) will most likely be different because the work performed by the Jeep will different with different rear gears.
Even without changing the rear gear, you can make a run in 3rd and another in 4th and so on and they will show different HP.
But think about what is going on here---
the computer is monitoring the acceleration of the dyno drum, it has no idea what type of vehicle is on it or what gear it is in.
Measuring Rear Wheel HP where the tire touches the drum means any change made before the drum has the potential to change the RWHP .
Above is some theory to ponder.
Please don’t believe everything you read in forums.
Thanks,
Bill
From: ross@jeeplab.com [mailto:ross@jeeplab.com]
Sent: Monday, August 11, 2014 7:59 PM
To: dynotech@dynojet.com
Subject: Dynamometer Tech. Inquiry
Entered on: http://www.dynojet.com/Contact/Dynojet-sales.aspx
Submitted On: 8/11/2014 10:58:33 PM
-----------------------------------------------------------------------------------
Regarding: Model 224x
-----------------------------------------------------------------------------------
Name: ross
Email: ross@jeeplab.com
Country: United States
Comments: Im from JeepLab.com and we are dyno testing a new turbo Jeep Wrangler with a dynojet. The argument in the forum right now is weather or not the axle gear ratio will alter the dynojet's reading. Will 3.21 gearing give a different dyno reading than 4.88 gearing assuming all other variables are the same? Thank you!
Soooooooooooooooo where do we go from here?
NOLA, Did you ever get the instructions?
We will need the stats on the NOLA jeep to compare to JG for a point of reference?
Nope did not get the instruction manual. I'm out of town until Sunday.. I dropped my rig off Tuesday morning without the instruction manual, and emailed Dan at prodigy .. Told him I had dropped off the jeep and the shop was waiting for his instructions.. Haven't heard from the shop or prodigy. So I am assuming everything is going well. I will update whenever I get additional info.
We have a wide array of pictures of the install. I'm not sure of the utility of each one. If your installers run into a challenge - post it here and I'll try to post some pictures to illustrate how it was solved in our installation. Worst case - I can get some picture of how its set up in JG and post those.
Thanks! I haven't heard from the shop.. So I'm assuming everything went well. They said they would call me if they ran into any complications. I'm sure Dan contacted them and instructed them. Thanks again though.
NOLA did you dyno the truck before the turbo?
Did NOLA say his gear ratio?
and if pickles has 3.21s, shouldnt he make a lot more power?
If he is putting stage 2 on, and he has the far higher gear ratio, and all the same parts as JG, if he would dyno, we would have a clear answer.
We could also get to the bottom of what the actual value of the axle gearing is.
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