Dyno Testing Procedures
12-20-2002, 10:34 PM
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Dyno Testing Procedures
Hello all, I own a dyno shop in Southern Cal and a few of my recent customers have been 350Z owners. Each one expressed some concern about how their cars would perform on the dyno and referenced other owners experiences with getting what appeared to be subpar dyno results.
Now, I'm no Z expert, having only dyno'd two of them so far (and another scheduled to come in), but I do know a bit about dyno testing, so I thought I'd share a little of my experience with you and offer a tip or two.
- What should I know about dyno testing my car?
The first thing you should always remember is that dyno testing is a tool that is best suited to determining the effect of changes on the vehicle. It is not the best indicator of whether or not your vehicle is making its rated crank hp. Some dynos claim to be able to determine drivetrain losses through a coastdown test, but while this is better than a back of the envelope calculation of drivetrain losses, it still isn't a completely accurate indicator.
The second thing to remember is that comparing numbers between different types of dynos is a difficult thing to do. Comparing numbers between the same type of dyno (Dynojet 248 being the most common - the 224 does seem to read differently) is quite a bit easier, but there can still be variances. Comparing numbers on the same dyno should be very relevant, provided certain conditions are met (more on that later).
- Which brand of dyno is best?
The short answer to this question is that any dyno will work just fine for 99% of the people out there - provided that it is set up and used properly. I, having chosen to purchase the Dynapack dyno, prefer that model over others for its variable load capability, elimination of wheel/tire/alignment variables, and several other reasons. But for simply testing stock cars, or testing the results of modifications, any dyno will work just fine for you.
- How should my car be tested?
The key to accurate and repeatable testing is the setup.
For roller dynos, the car should always be tested on the same set of wheels and tires. Even switching from old to new tires can change results somewhat on a sensitive dyno (as the old tires are lighter with the weight removed from the best place - the outer edge) so be aware. Tire pressure should always be set at the same level and should be checked periodically if multiple runs are being made (roller dynos can put a fair amount of heat into the tires). You must also take care to make sure the car is aligned properly on the dyno (straight ahead). Excessive changes in vehicle alignment can also cause issues as large camber and toe settings create more tire drag than a 0/0 setup. Some people also advocate measuring strap down tension as well, although I don't know how much of an effect that has.
Once you've got the car on the dyno, you've got to make sure conditions are consistent. There are two basic areas we've got to be concerned with:
1. Atmospheric conditions - while we can't control the weather (unless your dyno facility has a climate controlled garage, which would be rare and expensive), we can apply correction factors. SAE correction factors are widely accepted and very effective - although I don't advocate comparing runs made on 30F and 110F days - that's a pretty large difference. The responsibility of the dyno operator is to ensure that the conditions are being measured accurately. With the exception of some very old dyno models, most modern units incorporate their own weather stations so that the process is automated. Thus, all the operator has to do is put the sensors in an appropriate place. I usually advocate keeping the sensor package as close to the car as possible without getting in the radiator wash. On my dyno, the weather sensors are located on the driver's side of the car, about 1-2 ft from the side of the vehicle and ahead of the front wheels. The temp sensor should never be placed near any source of heat - this includes direct sunlight, computer cooling fans, etc. While most dyno operators are quite reputable, there are a few that have been known to let a temp sensor hang in direct sunlight while the car is in the shade. This will result in an unusually high correction factor that will exaggerate the results. Whatever happens, the location of the sensor package should not be varied or else the correction factors will be useless. You're completely within your rights to ask the dyno operator to show you where the conditions are being measured.
2. Vehicle conditions - Here is where OBDII cars have an advantage. While they can be quite sensitive to changes in coolant and intake temps, they also give us the ability to accurately measure those conditions. I use a simple OBDII scantool that allows me to monitor these variables. In particular coolant temp draws my attention. So we always try and start each run at the same coolant temp - usually around 185F, with a variance of no more than 3-4 degrees in either direction. Large variations in temp can drastically affect power - by up to 5% in my experience. Additionally, you've got to make sure that oil, transmission and differential temps are up to spec (warm oil takes less hp to pump). Since it can be difficult to monitor these sorts of things, I like to make at least 1 or 2 "throw away" passes at the beginning of a session till dyno readings stabilize. This also allows engine temps to warm up to spec.
What gear should I test in?
This seems to be a common question on this board. The answer depends on the car and the dyno. I have noticed many people advocating testing in the gear ratio that is closest to 1:1. There is a theoretical reason for this. A 1:1 gear ratio theoretically has the best mechanical efficiency in transferring power. However, the actual change in efficiency between a 1:1 gear and a 1.5:1 or a 0.75:1 gear is quite small - on the order of 1% or less.
The real thing I'm concerned about when choosing a gear is load time. On a Dynapack we can set this to whatever we want, so gear ratio really isn't important (at least to hp readings, but we do have to watch how much torque multiplication we apply to the dyno). However, on a roller dyno like the Dynojet, the gear chosen will affect the load time. A taller gear means a longer pull. This has a couple of possible effects. First of all, the longer a pull, the higher the hp reading should be. What? Yes, you see the slower the engine accelerates through its rev range, the less energy required to accelerate the crank, flywheel, etc. That means more energy is available to accelerate the dyno, resulting in a higher hp reading. In experiments on the Dynapack, we've found that varying the load time by 20-30% in either direction can result in changes in measured power of about 2-3%. I usually shoot for a load time of 10 seconds or so on most cars (depending on rpm range and other factors, but 10 is the standard starting point). The downside of longer load times is that the car has more time to heat up and heat soak. A lot of load is being placed on the engine while dyno testing and its virtually impossible for your corner dyno shop to duplicate the cooling airflow available at triple digit speeds on the road. On a dynojet, I like to shoot for whichever gear gets me a test time of about 10-12 seconds. On lower powered cars this is usually 3rd gear, higher power cars it can be 4th or 5th (turbo cars like higher gears because it gives them more load to build boost with). Frankly, 5th gear on the Z is a little tall for testing IMO. Its good for 143 mph, which is a lot of wheel speed and time.
Those are some of the basics. If you remember any one thing, its the coolant temps - do what you can to make sure they're consistent run to run. Hopes this helps people in their quest to improve their cars as more modifications become available.
Rgds,
Shawn
Now, I'm no Z expert, having only dyno'd two of them so far (and another scheduled to come in), but I do know a bit about dyno testing, so I thought I'd share a little of my experience with you and offer a tip or two.
- What should I know about dyno testing my car?
The first thing you should always remember is that dyno testing is a tool that is best suited to determining the effect of changes on the vehicle. It is not the best indicator of whether or not your vehicle is making its rated crank hp. Some dynos claim to be able to determine drivetrain losses through a coastdown test, but while this is better than a back of the envelope calculation of drivetrain losses, it still isn't a completely accurate indicator.
The second thing to remember is that comparing numbers between different types of dynos is a difficult thing to do. Comparing numbers between the same type of dyno (Dynojet 248 being the most common - the 224 does seem to read differently) is quite a bit easier, but there can still be variances. Comparing numbers on the same dyno should be very relevant, provided certain conditions are met (more on that later).
- Which brand of dyno is best?
The short answer to this question is that any dyno will work just fine for 99% of the people out there - provided that it is set up and used properly. I, having chosen to purchase the Dynapack dyno, prefer that model over others for its variable load capability, elimination of wheel/tire/alignment variables, and several other reasons. But for simply testing stock cars, or testing the results of modifications, any dyno will work just fine for you.
- How should my car be tested?
The key to accurate and repeatable testing is the setup.
For roller dynos, the car should always be tested on the same set of wheels and tires. Even switching from old to new tires can change results somewhat on a sensitive dyno (as the old tires are lighter with the weight removed from the best place - the outer edge) so be aware. Tire pressure should always be set at the same level and should be checked periodically if multiple runs are being made (roller dynos can put a fair amount of heat into the tires). You must also take care to make sure the car is aligned properly on the dyno (straight ahead). Excessive changes in vehicle alignment can also cause issues as large camber and toe settings create more tire drag than a 0/0 setup. Some people also advocate measuring strap down tension as well, although I don't know how much of an effect that has.
Once you've got the car on the dyno, you've got to make sure conditions are consistent. There are two basic areas we've got to be concerned with:
1. Atmospheric conditions - while we can't control the weather (unless your dyno facility has a climate controlled garage, which would be rare and expensive), we can apply correction factors. SAE correction factors are widely accepted and very effective - although I don't advocate comparing runs made on 30F and 110F days - that's a pretty large difference. The responsibility of the dyno operator is to ensure that the conditions are being measured accurately. With the exception of some very old dyno models, most modern units incorporate their own weather stations so that the process is automated. Thus, all the operator has to do is put the sensors in an appropriate place. I usually advocate keeping the sensor package as close to the car as possible without getting in the radiator wash. On my dyno, the weather sensors are located on the driver's side of the car, about 1-2 ft from the side of the vehicle and ahead of the front wheels. The temp sensor should never be placed near any source of heat - this includes direct sunlight, computer cooling fans, etc. While most dyno operators are quite reputable, there are a few that have been known to let a temp sensor hang in direct sunlight while the car is in the shade. This will result in an unusually high correction factor that will exaggerate the results. Whatever happens, the location of the sensor package should not be varied or else the correction factors will be useless. You're completely within your rights to ask the dyno operator to show you where the conditions are being measured.
2. Vehicle conditions - Here is where OBDII cars have an advantage. While they can be quite sensitive to changes in coolant and intake temps, they also give us the ability to accurately measure those conditions. I use a simple OBDII scantool that allows me to monitor these variables. In particular coolant temp draws my attention. So we always try and start each run at the same coolant temp - usually around 185F, with a variance of no more than 3-4 degrees in either direction. Large variations in temp can drastically affect power - by up to 5% in my experience. Additionally, you've got to make sure that oil, transmission and differential temps are up to spec (warm oil takes less hp to pump). Since it can be difficult to monitor these sorts of things, I like to make at least 1 or 2 "throw away" passes at the beginning of a session till dyno readings stabilize. This also allows engine temps to warm up to spec.
What gear should I test in?
This seems to be a common question on this board. The answer depends on the car and the dyno. I have noticed many people advocating testing in the gear ratio that is closest to 1:1. There is a theoretical reason for this. A 1:1 gear ratio theoretically has the best mechanical efficiency in transferring power. However, the actual change in efficiency between a 1:1 gear and a 1.5:1 or a 0.75:1 gear is quite small - on the order of 1% or less.
The real thing I'm concerned about when choosing a gear is load time. On a Dynapack we can set this to whatever we want, so gear ratio really isn't important (at least to hp readings, but we do have to watch how much torque multiplication we apply to the dyno). However, on a roller dyno like the Dynojet, the gear chosen will affect the load time. A taller gear means a longer pull. This has a couple of possible effects. First of all, the longer a pull, the higher the hp reading should be. What? Yes, you see the slower the engine accelerates through its rev range, the less energy required to accelerate the crank, flywheel, etc. That means more energy is available to accelerate the dyno, resulting in a higher hp reading. In experiments on the Dynapack, we've found that varying the load time by 20-30% in either direction can result in changes in measured power of about 2-3%. I usually shoot for a load time of 10 seconds or so on most cars (depending on rpm range and other factors, but 10 is the standard starting point). The downside of longer load times is that the car has more time to heat up and heat soak. A lot of load is being placed on the engine while dyno testing and its virtually impossible for your corner dyno shop to duplicate the cooling airflow available at triple digit speeds on the road. On a dynojet, I like to shoot for whichever gear gets me a test time of about 10-12 seconds. On lower powered cars this is usually 3rd gear, higher power cars it can be 4th or 5th (turbo cars like higher gears because it gives them more load to build boost with). Frankly, 5th gear on the Z is a little tall for testing IMO. Its good for 143 mph, which is a lot of wheel speed and time.
Those are some of the basics. If you remember any one thing, its the coolant temps - do what you can to make sure they're consistent run to run. Hopes this helps people in their quest to improve their cars as more modifications become available.
Rgds,
Shawn
12-21-2002, 06:01 PM
#4
Registered User
Excellent intro to Dynoing Shawn 
.
So, in your opinion, what's the practical gear the Z should be dynoed in?
Of the 3 clients that have or will dyno their cars, do you think that Nissan's VQ engine is overrated? What gear did they dyno in?
.So, in your opinion, what's the practical gear the Z should be dynoed in?
Of the 3 clients that have or will dyno their cars, do you think that Nissan's VQ engine is overrated? What gear did they dyno in?
12-21-2002, 06:28 PM
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Frankly, if you're dynoing on a roller dyno like the dynojet, I'd recommend 4th gear. See how long it takes to make the pass. If its less than 10 seconds (starting at say, 1500 rpm and going to the limiter), consider 5th.
And no, I don't think the Z motor is overrated. I think its very accurately rated. The typical front engine RWD car seems to lose about 30 hp, plus or minus, to the rear axles (not wheels, since we remove those). Dynoing in the mid 250's on average would put the Z right at the rated hp. If the Z is overrated, then so is the S2000 as it typically dynos about 40 hp less than the Z, as one would expect.
SC
And no, I don't think the Z motor is overrated. I think its very accurately rated. The typical front engine RWD car seems to lose about 30 hp, plus or minus, to the rear axles (not wheels, since we remove those). Dynoing in the mid 250's on average would put the Z right at the rated hp. If the Z is overrated, then so is the S2000 as it typically dynos about 40 hp less than the Z, as one would expect.
SC
12-22-2002, 07:50 AM
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I don't know if I'd call them accurate, but used properly they can be repeatable, and that's more important.
The key things to using those measurement tools is to find a good, level stretch of road, develop a consistent driving technique and then test on days with similar weather conditions. All 3 can be difficult depending on where you live.
Shawn
The key things to using those measurement tools is to find a good, level stretch of road, develop a consistent driving technique and then test on days with similar weather conditions. All 3 can be difficult depending on where you live.
Shawn
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