Question about Dynamometer

[Smokie]

In what gear are our cars being dyno? 2nd or 3rd.
If the car has stock tires and gears, 6200 Rpm's in 2nd gear is 96 mph, in 3rd gear the speed is 148 mph.
If the run is done in 3rd what keeps our limited driveshaft from damaging the car?
What keeps the car from downshifting when dyno operator stomps on the gas? I would like to know the details of a correct dyno run from someone who knows.:)

[BillyGman]

I never thought of the driveshaft issue while at the dyno. Avery good point because my car was Dynoed in 3rd gear, and I think that's probably standard procedure. I have the Dynotech shaft, but because of the 4.56 gears in the rear it will vibrate at 120MPH. That's okay on the street since I don't take it past 120MPH and w/the stock shaft and the 4.56's it started to vibrate at 90MPH. But now that you mention it, the Dyno run is another issue.

Perhaps it's no biggie since the Dyno blast itself only lasts 10 seconds at the most, but it's still not a comforting thought. As far as the downshifting, I think that this is the reason why they began my Dyno run at 3800 RPM. I don't think the car will downshift at WOT after it goes past that 3800 RPM when it's on the Dyno machine, but I'm guessing on that.

Mac is one of the guys who knows a lot about this stuff, so hopefully he will chime in.

[Effster]

i cracked the tailshaft housing on the dyno from the driveshaft issue.i believe that i saw 130 mph registered on the dyno sheets.yes it is a 3rd gear od off run when they start to record data

[FordNut]

So somebody tell me again, why is it that the dyno runs aren't done in 2nd gear so it won't shift? Seems like you could get a wider range of rpm vs hp/torque data that way.

edit: Like from idle to redline!

[Glenn]

The cars are dynoed in 3rd gear with the OD and TC turned-off. The operator will normally take the car to its red line or close to it. My MM was run up to 140 mph and then killed. It is only for about 10 seconds, but what a thrill. You would bet your house they would let go, but they hold together. I saw a new built Mustang 6 cylinder with a SC on a dyno and did it ever scream. Even the operators covered their ears the noise hurt so much.

Glenn

[BillyGman]

Originally posted by Effster
i cracked the tailshaft housing on the dyno from the driveshaft issue.i believe that i saw 130 mph registered on the dyno sheets.yes it is a 3rd gear od off run when they start to record data

WOW!!!! Well I guess I won't be taking my car to the Dyno anymore!!! No wonder they make you sign a waiver saying that they're NOT responsible for any mechanical damage done to the car.:(

Thanks for letting us know that Effster........

[SergntMac]

My MM has had dozens of dyno tests, and I've watched hundreds more. I'm not an expert, I'm JAFO.

You can have a dyno "pull" in any flavor you desire, and each flavor will produce 411. In all flavors, a graph will be produced and that's helpful. But, the important 411 is hidden in a column/row report with data recorded at every 100 RPMs.

A pull from idle to redline will show you what you may expect to see in your full 1/4 mile run. However, the spikes from gear changes will skew performance numbers, and your RWHP and RWTQ will come in high and provide "proof" of unrealistic power numbers. My first dyno test on a bone stock MM produced a graph that show 385 RWHP and 1345 RWTQ! Great for winning beers, eh?

Second gear only pulls are good for watching low RPM performance, but again, your final numbers will be lower because the pull is limited to the spectrum of this one set of gears.

Second gear top end may calculate to 96 MPH with a 3:55 rear end ratio, but I've never seen that MPH on a dyno or on the street. Sounds unlikely to me, I'll keep it in mind next time I'm testing stuff.

Third gear pulls give you the most valuable 411. Once the car gets locked into 3rd gear, the pull will show the details of your power band. Bone stock cars usually can begin at 2500 RPM and run through 6000 for an accurate report of the full power spectrum. But, modified cars can be difficult to manage in low RPMs, it will want to down shift into second, and that will skew the report.

The dyno operator is the key here, one who wants to give you the best test he can will be patient enough to search for where he can start the pull. A dyno operator who doesn't care, will just start at a higher RPM, 3800 to 4000. The number will be accurate, but not inclusive of the lower RPM range.

I do know this for certain. With a 4:10 gear, third gear tops out at 120 MPH. This is 6000 engine RPMs, and 6000 RPMs for the driveshaft.

Dyno tests are not risky. They are actually eaiser on the car in terms of load and resistance, and a dyno is a much safer way to run your car at speed. If something gives up on a dyno, it was a ticking time bomb anyway, and it's better that it failed on a dyno than on the highway or a track.

A good dyno test samples everything, including wideband AFR. Industry prices are fairly uniform at 125 bucks for three pulls, or an hour's time, which ever comes first. Additional pulls are usually 15-20 bucks. If you get cheaper quotes, make sure you have checked all the options, the cheaper pricing may be so just to get you strapped in.

Dynos are like doctors, no two will agree, which is why racing authorities have selected a standard for themselves. DynoJet machines and S.A.E. balanced agreements, and despite this, there are sill a few "tricks " a good dyno operator can employ to refine your final RWHP and RWTQ to be what you want it to be.

[BillyGman]

went on the Richmond gear website, used their calculator, and did some math w/my hand held calculator using the MPH increments that the stock driveshaft was vibrating at before I replaced it, as well as the RPM's at which the new Dynotech driveshaft begins to vibrate at now that I have the 4.56's. Here's what I found:

Keep in mind that the wheel speed isn't relavent here, but it's the actual driveshaft RPM's that are.......

The stock driveshaft on my Marauder would take only 4900 RPM's before it would begin to vibrate. And at 5000 RPM's it was very apparent and would just get worse the faster I went.

The new Dynotech driveshaft will take 6500 RPM before it begins to vibrate.......

again, the RPM's listed above are NOT the engine RPM's but rather the Driveshaft RPM's. That's what's important here.

What gear ratio you have in the rear end is also irrelavent, because even though the rear end gear ratio determines how fast the wheels will turn at any given RPM, they will NOT determine how fast the driveshaft turns at any RPM of the engine. It's the gear ratio on the transmission that will determine driveshaft speed.

I hope I'm not confusing anyone, but the bottom line here is that third gear (O/D off) is 1:1 ratio. So the driveshaft spins at the exact same speed as the engine is in third gear. At the dyno the engine is taken to redline. The stock rev limiter is set at about 6100 RPM. Therefore IF the Dyno operator leaves the O/D off, then the driveshaft will be required to turn at a maximum RPM of 6100-6200 RPM provided you haven't raised the rev limiter.

Now that's fine for the Dynotech driveshaft since I know for a fact that it will NOT vibrate until over 6500 RPM's. However, since the factory stock MM shaft will only take 4900 RPM w/out vibrating, then guess what? That thing is vibrating ALL OVER THE PLACE during your dyno run if your car is in third gear.

Now IMO even if you have the Dynotech driveshaft Like I do, you better make sure that the guy performing the Dyno test on your Marauder has that O/D button in the "OFF" position, because if he takes the engine up to 6100 RPM w/the O/D in the "ON" position, your driveshaft will be turning at 8500 RPM by the time your engine redlines!!!!!!

"How can that be" you ask? Because while the transmission gear ratio in third gear is 1:1, in O/D it is .71:1. So do the math......

6100(redline) divided by .71=8591.5(Driveshaft RPM).

If you want to check your answer, then that would be....

8591.5 x .71=6100(engine RPM)

So for me personally, I don't mean to debate w/MAC here, cuz I've learned a lot from him, and the man knows a lot about Dynos and cars. But my opinion is simply that I'm not going to suggest to anyone to take their MM to the dyno IF they still have that lousy factory stock driveshaft. I will admit, that one or two 10 second runs probably will not do any serious damage, but in light of the math, and what happened to Effster's car, I just will NOT be quick to recommend it. Just my 4 cents.:)

[Smokie]

Sarge thanks for the info. I never had a car dyno or witnessed a dyno run and I wanted to know if it was worth my money and potential risk to drivetrain. If the car goes to 6000 rpm's in 3rd gear all stock components; it is doing 140 mph briefly. I have not done anything very significant to my car, the reflash is it, so I might just save my money for the driveshaft. Thanks for info.:)

[SergntMac]

This drive shaft problem should be considered when deciding whether to dyno or not dyno. Hanging in the balance, is knowing what's up with your AFR at WOT.

You might think "I'm not racing my MM, I don't need a dyno" but you love those 1/8th mile on-ramp runs. Anytime you're going WOT, the AFR is critical. Run too lean too often, and you could cause serious damage to your engine.

Dynos run your car up to 6000 RPM, and the drive shaft could vibrate at that speed. However you're only up there briefly, the dyno is set by the operator to shut down or "roll over" as they say at a predetermined point. I've watched dozens of MM dyno this past year, and not one showed any problems, or broke anything.

Yes, the car is being tested for it's full strength and anything can happen. But, the odds are in your favor and the value of the test is important too. It's not just bragging about power, it's a scientific analysis of what your power is doing for you.

[bugsys03]

3.55 gears/my car slams into 3rd at 93 mph/6100 rpm. been there done it. the above post is accurate mac.

Dave

[Marauderer]

I've had a dozen or so pulls on my car in 2nd and 3rd gear and yep you do hit 140 MPH in 3rd :) and my pulls are to 6,300 RPM. BTW, I've never noticed any driveline vibration on the dyno or on the highway, even at those speeds.

Sarge, I've never seen a shutoff point other than "a foot" and sometimes "my foot" on the accelerator :), especially on a dynojet. There isn't any mechanical control of the throttle involved, you just sit in the car and go, and all the operator does is hit a button to start and stop data recording.

Guys a dyno is definitely less load than actual driving due to a lack of wind and road resistance and it is safer than you driving 140 on the road, but they are not for everyone. You do not need a dyno if you have a stock car or purely mechanical mods like gears as they will not affect A/F, unless you plan on doing more and want a baseline or you are just curious.

However, if you buy a chip from someone, it is always nice to get a dyno pull and send the chip burner the results and your chip back so that they can tweak it even further. You have to think, your spark tables and fuel tables have to be generic with a mail order chip to account for differences in weather and gas quality and there will always be a little more room for improvement. They only way to get the burner this info is with a dyno pull or your own wideband O2 like the LM-1. The important thing here is A/f info and not HP info. You could actually save yourself a ton of money by purchasing a wideband O2 and collecting, MAF V, RPM, and A/F data. Along with an autotap you can provide a tuner with everything they need to milk every ounce of HP out of your car at the drop of a hat. By this same token, it is a good idea to double check any chip you get as you never know when something could be off, especially with our flaky MAF’s. As stated earlier, if your car is running too lean, you will burn it up. You can also destroy your motor by running too rich as well. If your car is running too rich, it causes cylinder wash, where the oil film is actually washed off the cylinder walls and causes; ooops, big problems.

While dyno pulls are not necessary for everyone, you most definitely want to schedule dyno time if you are performing mods that can affect A/F. Intake mods (S/C), MAF mods, chips, heads, fuel system mods, and maybe even exhaust mods just to be safe.

You can also benefit from data that can help optimize your shift points, but only if you are performing non standard mods. Most of the generic type mods are so well known you will not shed any additional light on the subject.

If you are going to be a dyno junky, find one that can take advantage of a sensor bung welded into your exhaust. You can purchase a sensor bung for about 8.00 and have it welded in. Make sure to have your mechanic weld the sensor in between the exhaust manifold and the first cat. Mine is on the drivers side, facing forward in the most vertical part of the pipe.

Lastly, make sure to shop around. There should be at least a couple of dynos in any large town. Here in Minneapolis, I have paid as little as 50.00 for 2 pulls and have called places that charge 125.00 just for setup, then 40.00 a pull!

[HookedOnCV]

I have had my car on the dyno a few times, and from the research I've found on some Mustang websites, and from some very knowledgable people, that the true measure of RWHP is in 3rd gear only. That is the only gear where there is a 1:1 ratio.

I had some dyno pulls in 2nd gear because it didn't downshift like it does in 3rd gear. I was able to look at a larger RPM range in 2nd gear, but I was told that the HP and TQ readings are about 5% low. It is possible to have a wide range (RPM) dyno pull in 3rd, but only if you have a chip programmed or have access to program the EEC to not downshift into 2nd gear once the speed is above 20-30 MPH. This is programmed for the dyno testing only, not for driving on the street.

If you are taking your car in from the showroom floor, or even with a normal program or chip in your car, the downshift 3-2 speed when going WOT might be 80 MPH or more. This forces the dyno operators to go up to somewhere around 85-90 MPH in 3rd gear (OD locked out) where the computer wont downshift into second. At that speed your RPM is going to be close to 4000 and you don't get the full range graph.

Todd

[BillyGman]

I think this had been an excellent thread and some great info has been shared by everyone.;)

This board just gets better and better.........

[Smokie]

I agree with Billy this information is priceless; it allows those like me and others to make a decision about a dyno pull such as pricing, type of dyno, S.A.E correction what gear produces the most accurate results and concerns about the stock driveshaft at a dyno pull. Also I have seen dyno graphs posted on this forum that have information starting at 2500 rpm's; can those folks explain how this was done in 3rd gear? I know bone stock MM's have been dyno and the info started at 2500 rpm's. How ???:confused:

[Marauderer]

I had read a couple of articles a while back that talked about how 2nd or 3rd gear did not matter and why as a dynojet compensated, which I intially thought was counterintuitive as the 1:1 argument makes perfect sense. Today I went back to find those articles and they are no where to be found! I did find this however:


The Dynojet chassis dynamometer is an inertia type chassis dynamometer. This means that it calculates horsepower and torque based on how quickly a given inertia - in this case, a set of rollers of a given mass and dimension - is accelerated. The length of time it takes to accelerate from one rpm level to the next is the sweep time and the rate that you accelerate from one rpm to the next is the sweep speed or sweep rate. Because it takes more power to accelerate the mass faster, you will see lower horsepower figures when a car is tested in a lower gear (1st gear for example) then when it is tested in a higher gear (3rd gear for example). This is because at higher speeds and higher gears, it take longer to accelerate from one rpm level to another (for example, 2000 rpm to 6000 rpm). This remains true until the gains in horsepower from increased sweep time are offset by the increased frictional losses of the transmission, differential and tires. As speeds increase, the frictional losses in the transmission, differential and tires increase. The higher the horsepower of the car, the faster it will accelerate the roller and the higher the speed (and therefore the higher the gear) will need to be to get the sweep time long enough to give an accurate reading. Because the Dynojet chassis dynamometer is an inertia type chassis dynamometer it does not allow you to perform fixed rpm or step type horsepower tests - you can not hold the vehicle at a given rpm or speed and check the horsepower level. Despite this limitation, the inertia type dynamometers give you a very accurate measurement of what the vehicle see's in real world situations. An inertia type dynamometer will show the effects of reduced driveline inertia (such as lightweight flywheels, driveshafts and wheels) while a steady state test does not show these improvements.

Because the Dynojet chassis dynamometer uses one large diameter roller per wheel, overheating the tires and the tendency of cars to try and jump off the rollers is not a concern. This design also means that the cars do not need to be loaded down against the rollers - further reducing tire heat build up and increased frictional losses through the tires. This means that the risk of tires exploding from too much heat and load is virtually nonexistent.

[SergntMac]

-30-

[Smokie]

Easy does it friends. :) Sarge please don't cut my strings just yet.:) Now I realize that knowing my A/F ratio is far more important in my case than HP numbers, I have a Dennis reflash, so I believe that it would by in my best interest to know my A/F at WOT. So please bear with me, during that test is the sensing device placed in both tail pipes?. If not, is there a preferrred side to test? Like maybe the infamous #7 side. What side is #7 located at?

Gentleman any answer in good faith is welcome!!!
No wars please, just help those of us that lack experience in this area.:D

[Marauderer]

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