QOTW #89 – Special episode with Aston1936

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Mike: Hello, welcome to Bamford Rose, and another question of the week. This week the question is posed by Steve over at Aston 1936. If you haven’t already checked out Steve’s content, there’s some great videos and great subjects he covers over on his YouTube channel and website, aston1930 6.com. So, over to you then, Steve, what is this week’s question of the week?

Steve: Hi, Mike. I’m… you know, I’ve been trying to do some performance, bolt on performance upgrades to my car, things like the secondary cat delete. And as part of that process, I thought I would dyno test my car and so I can see what the change would be. And I dyno tested my car.

I got, you know, 376 horsepower, and 368-foot pounds of torque. But it left me kind of scratching my head. Because the engine, if you look in the manual, says it’s rated for 450 horsepower, and 420-foot pounds of torque. So, can you tell me what’s going on there?

Mike: Yeah, dyno results, they’re the staple of any good forum. When I was working as a performance development engineer at Aston Martin, I was actually responsible for power homologation sign off with vehicle certification agencies around the globe. So, your DB9 there at the flywheel does actually punch out 450 bhp on the nose. The 4.3 V8 Vantage punches out 380 bhp on the nose.

And to his credit, this was a directive of Dr. Bez. He acted… he wanted every single driver to experience the exact horsepower that the engine produced. And what the engine produced needed to be 100% true for reflection. You know, you can have many allowances on the test day, which will give you more bhp on paper. But, in reality versus the car, you know, the car’s got a lot less.

And when I signed off the DB9 performance and V8, the legal and certification people that came to witness the test, you know, I’ve done lots of these with different manufacturers before, they all follow a set process, it’s all quite formal, they’ll sit outside the dyno cell and witness the power curve. They’ll then walk into the cell and they’ll inspect your ancillaries. You’re allowed to not run an ancillary on the front, because that would represent too much of a loss on a dyno compared to the car.

Steve: Right.

Mike: And you’re allowed to run within a testbed exhaust system, not a vehicle exhaust system. So, you can sort of setback pressure in your favor to give you more bhp. And always remember on DB9 and VA that we’d run the test and achieved 450 horsepower on the nose. And the guy from the certification agency walks into the cell, and he sees an engine that’s got an alternator, a power steering pump, full ancillary pack, and pretty much a vehicle exhaust system in the dyno cell. And he was almost taken aback. And he looked at me and he said, “Well, you’d never said that you wanted to claim any losses before I witnessed the test.” And he almost said, “Right. Well, you know, I’ll give you this horsepower for the alternator, this horsepower for the water pump, this horsepower for the exhaust system.” No, no, no, no, no, no. Dr. Bez wants every single owner to have 450 horsepower. And it’s a straight up reflection of the test.

Steve: And so, it was benched… the engine was bench tested with all the ancillaries attached…

Mike: Yeah, yeah.

Steve: … in the… pretty close to the legit exhaust system.

Mike: Yeah, it was very difficult to replicate any further the engine actually as it was in the car. And the guy looked at and he said, “Well, you know, that’s very commendable,” he said, “I’ve never come across that before,” you know?

Steve: Well, that’s good.

Mike: So, yeah, it was really cool. And that followed through to DB9.2, where the facelift 470 bhp and 510 DBS and V12 Vantage, and 4.7 up to 420, that… that was all following Dr. Bez’s directive of every owner must have the performance. So, you got at the… did you get a wheel figure or a flywheel figure?

Steve: Wheel figure, yeah, at the rear wheel.

Mike: Okay, and what was your wheel figure?

Steve: 376 brake horsepower and 368 torque.

Mike: Yeah, okay. Well, that’s obviously not quite right. So, we know that every DB9 out the flywheel is 450 bhp. But this is measured on an engine test cell with a dynamometer attached to the engine. It hasn’t got the vehicle. So, on your car, there you have the transmission, the prop shaft, the gearbox, connected to the road wheels, which is spinning a dynamometer. So, your chassis dyno testing setup is very different from the engine testbed setup. So, from the wheels to the flywheel, there is going to be what’s called a transmission loss.

So, when you’re dyno testing on the chassis in the aftermarket, you’d have to record a wheel figure, then you would know a transmission loss to calculate back to a flywheel figure. And then you’d have to look at your wheel figure that you obtained. And all of the OEM data is done at standard reference ambient conditions. So, reference ambient air temperature, air pressure, and relative humidity. And when all of these inputs are at the standard, there is no correction applied, because the test was done at the standard.

If you increase your air temperature, the air is less dense, the engine will be achieving less power, then the correction factor that is applied would actually add torque to your at-the-wheel figure recorded, and then it would apply the transmission loss on top of that. So, there are many, many inputs that need to be correct at the wheel figure to then make sure that figure is corrected for ambient conditions, to then back calculate to a flywheel figure. And in that whole chain of events are many errors that can creep in.

Steve: Yeah. And, you know, that was sort of my experience. And the dyno tuner guy that was working with me was, you know, making that really clear to me that, if it’s a hot day, you know, my car is not going to generate the same numbers at the rear wheel as if we tested, you know, at 6 o’clock in the morning when it was, you know, nice and cool. So, yeah, it was so variable. So, what kind of losses are normal? I always hear people talk about, you know, it’s 15%, you know, and that’s how… that’s as easy it is to calculate. Is that really the truth? Or how would I, you know, what should I expect to see is a loss or a difference with the rear wheel? How do you work it out?

Mike: Well, the ZF Auto box on your DB9 there or the Graziano 6 speed in the V8 Vantage, if you got those gearboxes on a test rig out of the car and got their oil temperature up to normal operating temperature and measured the friction torque, what it took to turn those units over, then that is the transmission loss, you know, that is what the transmission is consuming to turn over.

So, if you have say 450 bhp being produced by the engine or you have 460 bhp being produced by the engine, if you increase your horsepower by 10 bhp and then you said, “Oh, my transmission losses 15%,” then the constant of the friction it took to turn that transmission over is being incorrectly calculated now. So, on the DB9, if that said ZF Autobox is consuming about 65 bhp at peak power speed, okay? So, we know that we’ve got 450 bhp at the flywheel on a standard ambient day, take away 65 bhp from that, and we’re in the order of 385 at the wheels.

Steve: And I got 376. So, that’s more in the ballpark. You know, for my bone stock DB9.

Mike: Yeah. So, then the remainder of your loss is probably in the inputs that the Dyno was recording versus what actually the engine was using. And this is also a common area for mistakes to creep in.

Steve: Yeah. So, I was 9 horsepower less than that 385 number at peak then. So, yeah, takeaway 65 for the transmission loss, and I’m still 9 under that. But so that’s when other factors influence the test result. So, what are those, you know, other factors? Obviously, you talked about temperature, are there other things that influence the test result?

Mike: Yep. So, the biggest one is barometric pressure. And for the correction factor, 1013.25 is standard ambient pressure. The correction factor is 0, doing nothing at 250 inlet air temperature, and we’ve got relative humidity, which is going to be roughly 55%. So, with a relative humidity of 55%, engine air, combustion air temperature at 25, and 1013.25 barometric pressure, there will be no correction factor.

So, this is… those are the conditions that we want to see on the dyno, because really, we don’t want to have a correction factor manipulating our results. So, when we go and plump the car on the dyno, what we want to do is measure that the intake air temperature is 250. We’ve got no control over barometric pressure, but you know, look at the weather forecast and don’t go testing on a high-barrow day or low-barrow day. And normally, relative humidity follows the barrel.

So, if we’ve got a correction factor doing nothing, so the correction factor is 1.0, it’s not 1.25 adding on performance for a hot day, or a low barometric pressure day, or it’s not 0.98, taking away performance from the observed result, which you would have occur if the air was too cold, or the ambient pressure was too high. So, what we can do is, I’ve got a spreadsheet which I’ve used for all my sort of development testing on our exhaust systems on DB9 and V8. Obviously, we’ve, across the years, dynoed many, many cars, standard modified. So, I have a spreadsheet, which when I go to a dyno, I always ask them for their observed figures at the wheel. I don’t want them to do any correction to it, because I can’t trust that they’re meticulous enough to measure it properly, or correctly calculate it properly. And I don’t want them to motor the engine after the power curve to calculate transmission loss, because that’s very inaccurate. Because I’ve done lots of rig testing, I know what the transmission loss is. So, I will apply my own transmission loss.

So, I’ll go dyno testing on standard atmospheric condition days. I’ll ask them only for observed wheel data. I’ll then run that through my own spreadsheet, because I’ll be recording air temperature and pressure. And I’ll work out my correction factor if I need one. And then I’ve got a very reliable wheel data, bhp at the wheels. And what I’ll then do is, for my friction curve for the transmission, I’ll apply my known transmission loss, and then I know I’ve got back to a very reliable OEM flywheel result.

Steve: Can you share an example of that with us on your spreadsheet? Is that something you can show me?

Mike: Yeah, because now we should be able to swing a few inputs in the air temperature, barometric pressure, and explain why your result might have been inaccurate. So, just before I show you the spreadsheet then, let’s talk about the sensitivities of this and what happens. So, you know, it’s quite common that people will put a car on a dyno and maybe dyno operators do 3 runs, and the first run is always the best run. You know, this is because the car hasn’t been on the dyno for too long and it hasn’t heat soaked the loaded temperature into the engine, into the inlet manifold.

So, a common problem at dyno tests. And what happens is they put their temperature measurement for combustion air at the air pickup pipe, which is in front of the grill on the DB9, and that might be nice and cold at 250 on the outside. But they’ve been running the corner dyno for a long time, the engine’s heated up, it’s heated the inlet manifold up. And although the air in the pickup is 250, the air in the engine could be up at 500, meaning that the engine is actually consuming 500 air, but to work out a power correction figure, they measured 25. Therefore, in reality, the engine needs power adding to it because it was running too hot compared to the standard. But because they’re measuring in the wrong place, basically, they’re not doing any power correction. So, you know, if you do run 3 and it’s just down on power, and that’s probably a reason why, because heat soak temperature has not been accounted for.

Steve: That’s exactly what happened on each of my dyno visits. Almost always, my first pull was the best one. And as the car got hotter and hotter after each run in the dyno cell, it seemed like it’d be just a little off that peak bigger. You know, I’d be back a couple more horsepower, back a couple more horsepower.

Mike: Yeah. And what the dyno operator does is, I know because I’ve had this happen to me, they know that the car is heat soaking on the dyno. So, they might run it for a minute at say 3000 RPM at quite high throttle just to drag air through. But if you measure temperature in the inlet manifold, once it’s heat soaked up to that 500, it’s very, very difficult to bring it down. And it probably takes 5 minutes of running to actually get the air temperature back down.

So, let’s go to the spreadsheet. And we’ll just work a few numbers. And now, you’ll be able to see the variability that you can get.

Steve: Alright, spreadsheets.

Mike: Okay. So, on DB9, we’ve got peak power at 5750 rpm. And this is some data that I took from one of my cars that I was doing some dyno testing on, and I got an observed wheel figure of 559 Nm. So, I’ve got 55% relative humidity. I’ve got 101.4 kPa barometric pressure. And I’ve got an inlet air temperature of 250. And I’m pretty much at those inputs running a neutral correction factor of 0.992. So, from that I calculate 446 bhp. And this is all at the flywheel because I’ve already applied my transmission loss.

So, now if we put the air temperature from 250 up to 500, so this is going to be that condition where the dyno operator thought the outside temperature, what he’s measuring to correct for what… what he’s measuring to calculate the power from, he thinks it’s 25, but the engine is actually running 500. So, it’s really easy to get 500 inlet air temperature in the DB9.

So, we’ve got 446 bhp at 250. We’re going to alter it for 50. So, now the correction factor has jumped up to 1.109, because the correction factor knows it needs to add power on because the engine is achieving less-dense air. And I’m up at 499 bhp. So, you can see there, that 250. inaccuracy results in a 50 bhp difference in results.

So, you know, 500 in the air temperature is super-hot. But let’s just say that it was in the middle of there. It was something like 370. You know, that’s going to be a 25 bhp error.

Steve: So, it’s super sensitive to air temperature. So, what are the other big factors that really make a difference?

Mike: Well, barometric pressure, so you know, if you’re testing altitude, you need to be adding on performance because you’re… you’ve got significantly less-dense air. So, let’s just simulate that here. So, we’ve got 101.4 kPa for 446 bhp. So, if they inaccurately measured the barometric pressure, and we were, say on a 98 kPa day, we’ve gone 446 to 465. So, you know, 20 bhp for 3 kPa. So, if you dyno test and you’re a little bit lower than standard atmospheric pressure, they didn’t really record that too accurately, and they were recording, you know, standard day, but it was only a couple of kPa down, then, you know, you’ve inaccurately calculated a result that could easily be 20 bhp off for what is a very small change in barometric pressure.

Steve: So, that’s really a big difference that the barometric pressure made as well, because the shop I was getting dyno tested out was probably 1000 feet of elevation, compared to where I live normally was up in the hills. Makes me wonder, well, did we complete… Did we correct that properly? Which makes me wonder even more, well, what’s the… what’s the purpose of doing a dyno test other than it feels great to have a big number stuck on my forehead, “Oh, I have a 450-horsepower car,”? You know, what would an owner be trying to achieve? I know I went into it trying to just test relative change from stock to a performance upgrade I was adding. So, I wasn’t so interested in the final number. I just wanted to know, is the car better or worse than it was before? You know, do you have any thoughts on that?

Mike: You know, as we said earlier, you’re guaranteed that your flywheel is 450 bhp if we’re talking about a DB9. And if you’ve made no changes to your car at all, then you know, what’s the point in dyno testing? Obviously, as the car ages, it’s going to lose a little bit of power. But you know, not a lot. And you’re not really interested in day-to-day what you’re bhp figures. I think owners like to dyno test to validate if they’ve made some performance changes, that the performance has increased by what it should have done. But really, the onus to prove that should be on the seller of that equipment. And that’s the reason why I’ve done lots and lots of dyno testing. Because, you know, back in the early days, 10 years ago, when we were developing our equipment, we needed to do lots of dyno testing. But I’ve not been on a dyno for what literally must be 8 years because I don’t need to, because a dyno is not the best tool to validate performance.

Steve: What would you recommend that we… if a dyno isn’t the best thing, what sort of solutions do you think we should be looking for?

Mike: Well, there’s… there’s 2. What I look for as a measurable ultimately is going to be the same as what an owner should. You know, I’m a big advocate of getting to know your car really well, you know, finding a quiet road somewhere, get it in third gear, get it down to about 1500 RPM, the lowest that the engine management system on auto will let you do, and then give it full throttle and just run it through to the redline. And get in tune with how your shoulders are being pushed back on the seat, or the sort of forces on your head and neck for certain performance, and how the engine is revving in terms of responsiveness and time, especially from say 4, 5, 6, 7000 RPM. Because if we’re producing more power, then your engine should be punching its way to the redline much quicker. If it’s not accelerating, from say, 4000 to 7000 RPM much quicker, well, you’ve not made any more power.

What I do is, I know from dyno testing what the air consumption through the mass airflow meter is. So, I take a car at standard, I perform that sort of power curve out on the road and I measure mass air flow. We then put our exhaust system on high-lift inlet cams and then run that car down the road again. And because I know certain imperial units, for those Europeans that are watching, because the engine management system is Ford. So, it measures pounds per stroke of air consumption.

So, I know that an increase of say 5 pounds per stroke is X bhp. So, when I’ve modified a car or run it down the road, I know that I should have achieved 50 bhp. I know that that must mean an airflow increase of X, and I check that I achieve X, you know? But I drive so many of these cars that my seat-of-the-pants dyno is well tuned. So, as soon as I pull out from… from the car park, I want to feel that I’m pushed back in the seat in a certain way. I don’t want to feel that the engine has got like… almost like a power band to it as it goes from 4, 5, 6000 RPM, it just punches its way to the red line where previously it was a bit wheezy. So, you know, I’m… I’m a big fan of just avoiding dynos because they are so inaccurate. And owners getting more in tune with… with their car through the seat of the pants.

Steve: Well, I think that was my own personal experience too. As I went through 3 or 4 different dyno-pull sessions, I started to realize it’s not so much about the number, it’s about my driving experience. And what am I really trying to achieve? I’m trying to improve my driving experience, that seat-of-the-pants feel that you’re talking about. And I got some advice during that… that… that process I was going through, where I stopped worrying about the dyno number, and started trying to measure the performance that… that pushed you’re feeling.

Well, a device I discovered along the way while I was doing that, Mike, was this little thing called a Draggy, and it was really from the… the guys that are into the quarter mile type stuff were pointing me at this. And what this is, is an ultra-precise timing system, you know, what’s your time through a quarter mile? But if you have more power in your car, and you’re in third gear, you should be able to get through the RPM range faster, right? So, you can basically use this to say, how long does it take me in third gear with my foot flat to the floor on that country road to go from 30 to 105 miles an hour, which is about 1500 RPM to redline?

And so, I started to use my Draggy for recording it ultra-precise. I got really good results with this. Like, I could… if I’m just interested, is my car faster or slower? This was doing the trick. And of course, I had to sort of go out in the same weather conditions, right? Because if I went on 1000 California day, my car is going to be down on power. But I just always go out in the morning when it is about 600 Fahrenheit and test my car. And I found this was a really inexpensive way, instead of pouring tons of money into the dyno system, kind of backed up my seat-of-the-pants experience.

Enjoy the ride along.

Well, that’s better than coffee in the morning to start your day, isn’t it? Well, after the run, you get a performance report on your app on your phone where you can see your speed and acceleration. If you look at the acceleration graph, that’s the brown line, you can even see the shift points. Along the bottom, you can see the distance I did the 0 to 61, did it in 4.78 seconds. There was no slope. I was on a level or road next to the airport. Along the top, you can even see some of the climatic information is 340 a cool California morning, and was at 16 feet of actual elevation.

Mike: But well, exactly. Because what we… what you’ve just shown there in that device is putting some objective data to the subjective seat-of-the-pants feel. So, you know I drive many cars at, you know, 380 for the V8, 420, 450 DB9, 47, 510, 565. You know, my subjective seat-of-the-pants dynos just so calibrated that I can jump in a car and just more or less feel what power it has. I can watch someone’s YouTube video where they put a dash cam on their dashboard, and I can just see versus time how the engine is revving up and responding. And from a YouTube clip or dashcam clip, I’ll tell you your bhp. I’ve gotten quite confident about that. But that bit of kit that you are showing now is perfect, because people who are less confident in their subjective seat-of-the-pants dyno, they can actually put some data to it.

Yeah, rather than, you know, a dyno test and a number on a piece of paper, the Draggy there is your ultimate acid test, because you’re measuring if the car was going any quicker or not on the tarmac. It’s… it’s perfect. So, that piece of data is really good. And I just talking about that they reminded myself of my sort of second pet hate of dynos, in that, if they’re not set up properly to cool the car, you’re actually stressing the car out on the dyno. You know, I’ve taken a classic Vanquish for a dyno test before, those cars have got undersurface trays from front to back, and cooling on those is critical.

And, you know, we overheated this car. And, you know, the smoke coming out of corners was not good. You know, it’s dyno test is the… you know, I wouldn’t want my car to sit on the dyno not… perhaps not being cooled properly, and being put under… under stress just to get some number on a piece of paper, which is probably of questionable quality. Whereas your Draggy there, you could do a few runs before you modify, put the modifications on, either if you bought them and the garage installed them when you got the car back, run those tests, and instantly, you get all in gears, everything. That… that’s… that would be my message, you know, avoid dyno testing. Leave that for people developing the kit.

Steve: Alright, Mike. Well, thanks for answering my question. That was really helpful.

Mike: It’s a pleasure, Steve. I hope you liked that Bamford Rose question of the week. As always, it really helps us if you can like, comment, and subscribe to our channel. And if you haven’t done so already, please pop over to Steve’s channel Aston 1936 and follow what Steve does there too. We’ll see you on the next question of the week.

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