Just a quick few words of caution. The chaps at APT are really good guys but do not under any circumstances view the numbers that come of their rollers with any significance or use them for comparison with anything else.
Why? Well they use dyno-dynamics rollers which don't do a coast-down measurement so cannot compensate for losses through tyres or drivetrain, therefore the only numbers that are valid on it are those taken on a single car, in a single gear in a single session and even then as your tyres and transmission warms up some inaccuracy creeps in.
OK, so you're happy that the only numbers that you can trust are "at the wheels" figures, well what is wrong with that? Gears, that's what. Rolling roads can only measure torque and from that calculate bhp, your gearbox is a torque multiplier so do a run in 4th gear, then again in 3rd and you'll suddenly have gained 25% extra power. This is a common trick in the US at disreputable performance shops, they test you in 4th, convince you to give their new air filter or whatever a go and then do a run in 3rd to get you an "unbelievable" power gain.
On rollers that have a coastdown compensation the transmission and tyre losses are factored out so irrespective of tyre pressure and gear selection you can accurately compare results. APT have in the past claimed to do some "magic" compensation that they can't explain to ensure that gear selection isn't an issue. When people rely on "magic" I get deeply suspicious.
Also consider that in shoot-out mode they pump your tyres up to 50psi. This alone can cause severe mis-readings as differing tyre compositions and in particular different side-wall stiffnesses which cannot be taken into account. Given that compressing tyres against a roller at 50psi and >100mph can rob you of 10-20% of your power it's a big margin for error! Please make sure that both you and your tyres are going to be happy doing a full bore run whilst strapped down and pumped up to 50psi and make doubly sure that you let your tyres back down to the correct values afterwards.
Pro's & Cons of Dyno's
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Christian and Beccy
- Posts: 344
- Joined: Thu Oct 05, 2006 10:52 pm
- Location: Norwich
cng1 - It's certainly not the place to discuss political matters such as your own personal opinion on a Dyno on thread such as this. We have invested heavily in our Dyno set-up not only to be able to measure power, but also for its extensive mapping abilities.
The 'Old-school' have trouble grasping a new way to look at things and tend struggle to move on from the age-old idea of how things used to be done. When buying the dyno, on top of much research we invested a certain amount of trust in Dyno Dynamics who are a global company who have many many dynos out there operating successfully and largely honestly across the world. As such, as with all dynos, the figures are often scrutineered and there has yet to be proven a conclusive argument that the figures achieved on the DD dyno differed from those measured on an engine dyno (DD make these also).
Dyno Dynamics have also researched tyre pressures and as an organisation with more ability in this area than me or you, if they say that a tyre holds a more uniform shape that is less affected by heat at 50psi, then I believe them.
A dyno that measures coast-down is far more succeptible to operator influence and inaccuracies of such variables. Tyre pressure as one of many many factors could just as easily affect outputted power.
If we accept that a dyno is not an absolutely fail-safe way of measuring engine power, but more that it should be seen as a way of fairly accurately estimating such, we have been very shocked at the predictability of the results we have seen. For example, standard cars making quoted power where they ought to (or not in the case of certain manufactures who are know for producing optimistic figures) and modifications not always making a positive impact on the resultant power.
Either way, as stated earlier, this isn't the place for this debate. Dyno Dynamics have many many satisfied customers all over the world, how many other dynos have such a global following? If Stuart wishes to clean this thread up by moving these replies to a thread in their own right (or not), I'm OK with that.
The 'Old-school' have trouble grasping a new way to look at things and tend struggle to move on from the age-old idea of how things used to be done. When buying the dyno, on top of much research we invested a certain amount of trust in Dyno Dynamics who are a global company who have many many dynos out there operating successfully and largely honestly across the world. As such, as with all dynos, the figures are often scrutineered and there has yet to be proven a conclusive argument that the figures achieved on the DD dyno differed from those measured on an engine dyno (DD make these also).
Dyno Dynamics have also researched tyre pressures and as an organisation with more ability in this area than me or you, if they say that a tyre holds a more uniform shape that is less affected by heat at 50psi, then I believe them.
A dyno that measures coast-down is far more succeptible to operator influence and inaccuracies of such variables. Tyre pressure as one of many many factors could just as easily affect outputted power.
If we accept that a dyno is not an absolutely fail-safe way of measuring engine power, but more that it should be seen as a way of fairly accurately estimating such, we have been very shocked at the predictability of the results we have seen. For example, standard cars making quoted power where they ought to (or not in the case of certain manufactures who are know for producing optimistic figures) and modifications not always making a positive impact on the resultant power.
Either way, as stated earlier, this isn't the place for this debate. Dyno Dynamics have many many satisfied customers all over the world, how many other dynos have such a global following? If Stuart wishes to clean this thread up by moving these replies to a thread in their own right (or not), I'm OK with that.
Point taken.stuart wrote: You have raised some valid points but not in the right place. We have a dyno forum for opinions on the dos and don't of dyno hype. Search and you shall find
No, you absolutely cannot do that as their rollers already include a large fudge factor to account for rolling losses. As they say in the investment game "... can go down as well as up". For the purposes of the pub if everybody revises up by 20% or down by 20% to suit their own purposes then you should be fineAre you suggesting that any figures that we get from Garys rollers can be inflated by 10-20% when we get down the pub later? Bring it on! :thumb:
Christian. I believe that we've spoken about this in the past face to face and agree to disagree on this matter. You are more than welcome to your opinions and indeed to take what a vendor says at face value but I choose not to.
Unless you make an attempt to measure the losses then there is absolutely no way at all to generate a "flywheel" figure or indeed to make adjustments for the gearing to ensure that all "at the wheels" figures are comparable. If you input the gear ratios, the gearbox type, the tooth pattern, the gear fluid type and differential ratio you can make a gear ratio compensation but that does nothing to address the other elements that affect your rolling losses.
You've made a significant investment in your rollers and they are great at many things as you well know, but doing comparisons is a huge weak point on the Dyno dynamics system.
Just to pull you up on another point, you say that a coastdown system is susceptable to tyre pressure. That's not really true. As long as you do a per-run coastdown you can change your tyre pressure add and remove air from your tyres between runs at will and it just get compensated out.
Unless you make an attempt to measure the losses then there is absolutely no way at all to generate a "flywheel" figure or indeed to make adjustments for the gearing to ensure that all "at the wheels" figures are comparable. If you input the gear ratios, the gearbox type, the tooth pattern, the gear fluid type and differential ratio you can make a gear ratio compensation but that does nothing to address the other elements that affect your rolling losses.
You've made a significant investment in your rollers and they are great at many things as you well know, but doing comparisons is a huge weak point on the Dyno dynamics system.
Just to pull you up on another point, you say that a coastdown system is susceptable to tyre pressure. That's not really true. As long as you do a per-run coastdown you can change your tyre pressure add and remove air from your tyres between runs at will and it just get compensated out.
Also, are you havig a pop at DD rollers in general or APT in particular?
From what I hear of DD, you're thoughts are in a minority.
Can't speak for APT as I don't know them or their work, but there's plenty on here that do. So again, I imagine you'll be in a minority. IMO
From what I hear of DD, you're thoughts are in a minority.
Can't speak for APT as I don't know them or their work, but there's plenty on here that do. So again, I imagine you'll be in a minority. IMO
Handle every stressful situation like a dog. If you can't eat it or hump it, piss on it and walk away!
Gear ratios are not required when you have two known variable - RPM taken from a direct input from the engine and a known circumference of a roller.
Spin the roller against a set load for a known time and hey presto - wheel power torque :thumb:
Apply simple maths to work out the whp and the jobs a good un.
Bringing it up to a flywheel figure is largely guesswork even on a dyno that measures coast down losses as these losses are not measured under load and therefore open to debate again.
Wheel figures are what makes the car go forward - the rest is hot air. (which incidentally is what this thread is looking like it is filled with )
Spin the roller against a set load for a known time and hey presto - wheel power torque :thumb:
Apply simple maths to work out the whp and the jobs a good un.
Bringing it up to a flywheel figure is largely guesswork even on a dyno that measures coast down losses as these losses are not measured under load and therefore open to debate again.
Wheel figures are what makes the car go forward - the rest is hot air. (which incidentally is what this thread is looking like it is filled with
)I am in no way having a pop at APT.
Duncan, I see that you're local to me, happy to meet up and discuss rollers over a beer sometime. The problem that you're missing is that transmission losses differ in each gear. Adjusting RPM by gear ration gets you part way there but 4th is typically a straight though 1:1 gear so has substantially lower losses than 5th which is typically overdriven to say 0.8:1 - in 5th your transmission losses will be different but so will your tyre losses because your wheels are now spinning faster and thus are compressing a greater amount per engine revolution.
The trick to coast-downs is to balance the clamping load, on a double roller setup this is comparatively easy as you can tie the car such that you compress agains the rearmost roller on coastdown and against the foward one as it tries to climb out under acceleration. Yes there is a still a certain scope to get it wrong but it's not hard to get runs repeatable within 1% irrespective of which gear you run in or what you tweak the tyre pressures to be.
Duncan, I see that you're local to me, happy to meet up and discuss rollers over a beer sometime. The problem that you're missing is that transmission losses differ in each gear. Adjusting RPM by gear ration gets you part way there but 4th is typically a straight though 1:1 gear so has substantially lower losses than 5th which is typically overdriven to say 0.8:1 - in 5th your transmission losses will be different but so will your tyre losses because your wheels are now spinning faster and thus are compressing a greater amount per engine revolution.
The trick to coast-downs is to balance the clamping load, on a double roller setup this is comparatively easy as you can tie the car such that you compress agains the rearmost roller on coastdown and against the foward one as it tries to climb out under acceleration. Yes there is a still a certain scope to get it wrong but it's not hard to get runs repeatable within 1% irrespective of which gear you run in or what you tweak the tyre pressures to be.
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Christian and Beccy
- Posts: 344
- Joined: Thu Oct 05, 2006 10:52 pm
- Location: Norwich
Sorry, but 'at the wheels' is just that? At the wheels. How do any other factors hold any relevance?cng1 wrote:indeed to make adjustments for the gearing to ensure that all "at the wheels" figures are comparable.
You may or may not realise this, but prior to buying the DD dyno at APT, Gary and I were vastly experienced using one of the more recent SUN dyno's that does use the coastdown method for measuring losses. Having the benefit of experience in both, we know which we prefer, remembering that it is highly beneficial to us (with our high level of repeat custom) to be able to produce dyno results with some sort of weight to them.
I'm guessing that you have a 'home' dyno that uses the coastdown method, which is understandably why you fly the flag for it. God knows we spent quite some time looking at DD's methods from a constructively critical point-of-view, but in our hands-on experience, the results speak for themselves.
pablo wrote:Cng1 from what perspective are you making the above statement. It does seam to be fairly critical of what is a widely respected company. What is your background in this field and your personnel experience of APT.
I have no beef with APT at all, they are really nice chaps with a great facility who do superb work.
I am just pointing out for those who are perhaps less familiar with how differnt roller types operate (inertial/water driven/eddy current then there is single small roller, single huge roller, double roller - enough to fill a short book on) the points to be aware of with the particular roller setup that they use and what you need to keep in mind when considering and comparing any numbers that you get out of a shoot-out day.
I'm an instrumentation and datalogging obsessive and record everything, with the benefit of modern data acquisition tuning is pretty easy. If you know the injector flow, pressure and duration at a given air pressure, temperature, rpm and air:fuel ratio then you can make a pretty good calculation of power, if that calculation agrees with what you expect and what the rolling road is telling you then you can have a very high degree of confidence in what they are telling you In my case on a variety of coastdown corrected rollers it works to a tee, the theory and physics correlates within 5% of reality. On DD rollers it doesn't always work out so.