Today I took my car to The Dyno Shop in San Diego because they had 5 different dynos in the same shop including a DD and Dynojet. I wanted to run on both dynos today to confirm what most folks say that the DD dyno is about 8-10% lower than the Dynojet.

First two runs on the Dynojet on averaged only 428rwhp/475rwtq. Dissapointing given the fact that Rich@DynoComp gave me a dyno sheet showing 450rwhp/480rwtq (91Oct map). I was ready to call Rich and ask him what's the deal?

So I was expecting 385rwhp/432rwtq based on 10% lower numbers on DD pulls coming in the next 10 minutes. To my surprise the first two pulls yielded nearly identical numbers at 478rwhp/495rwtq! I asked the operator what's up?

He then admitted that the DynoJet had recent problems and all the recent customers had complained about much lower numbers than at other dyno shops. Apparently the shop owner acknowledged that they knew the Dynojet was inaccurate and charged me for only 1 dyno instead of 2 so basically half price and another half price offer to come back next Sat after they fix the DynoJet and calibrate all the Dynos before calling me to confirm my appointment.

What I really wanted to share with this forum was the TRUE difference between a Dynodyamics vs. Dynojet with the same car under identical weather conditions. Was the DD really more or less than 10% less than a Dynojet. I can say this experiment failed miserably and I will try again next week. There aren't many dyno shops that have five different dyno types in the same shop.

The operator told me that load setting on the DD dyno was set to exactly match the Dynojet so there should not have been such an opposite relationship between the two.

The Dynojet's boost gauge was also broken and unvailable but measured perfect 11.5-10.5 AFR which was the same of the DD but the DD captured the peak boost at 11.5psi which was even higher than the 10psi measured in AZ.

So in conclusion, next week I will go back for my free Dynojet pulls after the owner calls me and confirms the repairs and calibration are completed on the DynoJet. I will put 100 octane next week and switch the tune to match the octane.

I can't imagine having to drive 260 miles round trip for this experiment and the Dynojet malfunctions. The true Dynodynamics vs. Dynojet difference will be determined next Sat for sure.

What if BOTH dyno machines are wrong!!! I should just get a full refund and go to another shop? This just goes to prove how unreliable dyno results are period!

Thank god I'm going to the drag strip this month and next at Famoso, Sac and Fontana to find out the ONLY truth about measuring power performance but we all know strip times also vary based on track prep, weather, DA, the slope of the strip and even potential malfunctions of the clock.

 

Trap speed?

 

While a neat experiment, here's the thing... chassis dynos can be setup and calibrated much differently, both brand-to-brand, and also shop-to-shop. I've seen DDs calibrated to read significantly higher than DJs, and vice-versa.

Subsequently, any real-world results you might record, are really only applicable to how that one dyno shop/operator has their machines calibrated. So... in the end... dynos are a useful tool (if you go back to the same shop, on their same dyno, using the same calibration/correction factors) for gauging pre- and post-mod efficacy - and that's pretty much all they're useful for. They're relatively useless for much else.

 

the most important thing is the delta. and even still, hp numbers are just that. bring it to the track and get a trap speed. that's real world. that's what counts.

 

Hello AMGSC (and forum). I was recently directed to your post regarding your concerns of unreliable HP measurement on a Dynojet Dyno. Upon reading your post above, I am left with a number of questions.

First, a little information on how a Dynojet inertia dyno works. Horsepower can be measured and calculated based on the formula that HP (force) = mass x acceleration. A Dynojet inertia dyno has a known, constant mass (the inertial weight of the drum) and we measure the acceleration (change in speed over time). Given these simple principles, the inertia dyno itself is quite repeatable and consistent.


So, there are only a few things that can corrupt the horsepower values. All are relatively easy to spot, so here are the basics of what to look for:

• Wrong configurations - this is the least likely culprit. Each Dynojet dyno is measured at the factory for its specific inertial mass. Every dyno is different and every dyno has its own configuration file. This MUST stay with the dyno. In the WinPEP 7 software, you can verify that the correct configuration file is in place by choosing TOOLS -> DRUM INFORMATION and verify that the serial number of the dyno in the software matches the actual dyno chassis. Again, this is the least likely scenario.
• Incorrect atmospheric measurment values, leading to incorrect correction factor values - the Dynojet DynoWare EX+ hardware stack that interfaces the dyno to the computer, has a weatherstation built into it. It automatically samples the temperature, humidity and absolute pressure and uses these values to compute the correction factor as chosen by the dyno operator. Here in the US, this should always be the SAE correction factor. These values are shown in the MAKE RUN CONFIGURATION screen before a run is made (where you enter notes, choose the run save directory, etc), sometimes they are shown on the MAKE RUN SCREEN, where the gauges are present, and they can also be shown on the graph and printout. Here, the operator and client can verify that the temperature, humidity and pressure match the actual conditions of the day.
• Incorrectly calibrated load cell - this only applies to the newer 224x (24" diameter drum) dynos, as this option was not available for the original 248 model dynos. If the dyno you tested on had our load control (eddy brake) option, it is possible for the operator to enter a "load testing" mode that records torque cell data as well as inertial acceleration. If the torque cell is improperly calibrated (or not calibrated) it can skew the values.
• Vehicle inconsistency - it's very common with todays' computer controlled cars, to see many variables interfere with power output. Computers today have modifiers based on engine coolant temp, intake air temp, knock activity, inferred catalyst temperature, etc. With our OBDII data logging unit, you can monitor and record the vehicles parameters into the dyno run for evaluation. This can often show us why the vehicle is inconsistent.

One of the other things can cause HP values to be shown as low is that some dyno centers do not dyno the car until it is consistent. Sometimes we see shops that advertise "3 dyno pulls for $XXX" - unfortunately, not every vehicle is up to temp and consistent in 3 dyno pulls. I have seen some cases where run after run, the vehicle will continue to increase in delivered power as the entire drivetrain comes up to temperature, and it take up to 10 dyno runs to achieve back-to-back repeatability.

You should always ask the operator to make back-to-back runs, one after another, until the vehicle puts up the same HP from one run to the next, or until the subsequent run shows lower HP than the previous. Obviously, all other engine and vehicle paramenters need to be monitored.

We may be able to point out a few things if you'd like to forward the dyno runs to us. These are the actual DYNORUN_0xx.DRF files that the dyno software creates. You can e-mail them to dynotech@dynojet.com for our analysis. We may be able to give you more information once we see those runs.

The Dynojet inertia dyno is VERY repeatable and consistent, and very consistent from dyno to dyno, assuming that all other things are equal. However, as with any tool in the field, it's only as good as the operator(s) and data it is given (atmospheric, data files, etc).

It would be good to get this sorted out prior to your return trip to the dyno next week. There is no reason their dyno should be showing "much lower numbers than at other dyno shops."

 

when you run on the dynod do it in shootout mode....

 

Is that a real setting on a DD? Would it be higher or lower than no correction factor.

 

I am going back tomorrow and will share the results. Their is another AMG that will be doing the same thing. Back to back DD vs DJ runs. Hopefully the results for both my car and the other AMG will be consistent.

 

May I ask what kind of price range are they charging you? Just want an idea for when I do some mods...

 

$90 for 3 pulls on the DJ but $140 for 3 pulls on both DD and DJ. Special cash only price for Saturdays by appointment.

 

That doesn't sound too bad. Thanks!

 

yes. All dynos with Shootout accreditation have been calibrated the same accross the board, and should not vary more than 1% when comparing between Dyno Dynamics dynos anywhere you would go to get it done.

It does work, I have done it many times.

 

Well, I went back to SD and the DD Dyno result with the same correction factor as last week was 498rwhp on 91 octane + a bottle of Octane booster from Autozone. The AFR was perfect with mid 11s across the entire run and boost was 11.5psi dropping to 8psi at the end. If a DJ is 8-10% higher then it could be as high as 548rwhp.

I will need to go to another shop to get my DJ dyno since they damaged my emergency brake by strapping over the brake line and kinked it. The left rear wheel was locked and they had to hoist the car up and unjamb the cable. I smelled some burning brake pad when suddenly the car came to a screaching halt. I was pretty pissed off but at least they released the cable so I could drive home. The cable will cost me $170 plus labor.

I called Rich@DynoComp on his cell Sat afternoon and he was very helpful. GREAT SERVICE as usual.

Instead of a DJ dyno, I might as well just go to the strip and see what trap speed and ET I can get later this year in cool weather at Sac.