Hey John, I think he was trying to answer all the questions people have posted/asked about it over the last little bit... That's where the trunk stuff came from. 
Homemade complete rear undertray / diffuser
- Thread starter greenberet
- Start date
Hey John, I think he was trying to answer all the questions people have posted/asked about it over the last little bit... That's where the trunk stuff came from.
Oh, I see. I thought maybe his trunk monkey may have been going ape back there. :tongue:
My core interest is top speed and that’s why I chose a 5° diffuser angle (see post #1) and have since dropped it to 4.4°. Based on the attached chart, that’s where I figure drag is minimized given the length of the final upsweep compared to the total length of the underbody. If others are more interested in downforce as has been discussed – use a larger angle.
I’m also starting to get the feeling that squeezing more top speed out of my NSX is like squeezing water from a rock. Buying some 225/55 16 rear tires and mounting them on the OEM 1991 rims in my basement would lengthen my effective gearing and mounting some 195/50 15 tires in the front would reduce my frontal area and lower the front a touch. But that just doesn’t sound very tempting. There are modern sports cars with higher top speeds than an NSX and I would need some serious modifications if I wanted to catch up with things coming off the showroom floor nowadays.
Yes, 100°C is nothing. That was my typical oil temperature when doddling around without the rear undertray. With the undertray, the doddling around temperature has remained 100°C – which is good news. Since my oil temperature climbed to 140°C (284°F) on the Autobahn without the undertray before I backed off, I disagree that I have no need for supplemental cooling. With the NACA ducts, the oil still gets that hot before I back off, but it doesn’t seem to get there any quicker than without the undertray.
As theman1990 mentioned, some people do care. There have been threads about how hot the trunk gets. With the rear undertray, it doesn’t get any hotter than without it. I care about that myself because when I drive to Italy on vacation, it’s nice to be able to keep things my trunk without them spoiling or melting. Now if I no longer had a trunk, I might see it more from your point of view.
Agreed. And the next time I jack the car up I’m going to check carefully for any signs of stress. Getting down on my hands and knees and looking at all of the attachment points with the car on the ground, it looks like everything is still perfectly OK.
Thanks! My car is a naturally-aspirated 1991 3.0 liter 5-speed with OEM 2002-2005 rims and tires.
Horsepower modifications:
Comptech airbox
Uni foam air filter
Extrudehoned intake manifold
Comptech ported and polished cylinder heads, milled for increased compression ratio (~10.6 :1)
Comptech high-lift camshafts
Cantrell headers with ceramic coating
HKS muffler
Autothority engine management chip custom programmed with car on dyno
Aerodynamic modifications:
Homemade complete front and rear undertrays
Homemade air dams in front of all four wheels
Dali 2002-2005 trunk lip spoiler replica
Car lowered with Bilstein shocks set to lower perch
Procar front blinker covers (only mounted temporarily with duct tape)
Styrofoam plugs for air conditioner openings in nose of car (temporary)
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What is your ride height?
Power helps.
140-145C is perfectly normal under sustained use. It will do that all day long.
Just install some aluminized heat shielding \ thermal mat if it is a concern.
My ride height is about 11.5 cm.
Yeah, power helps and my 18 year old naturally-aspirated 3.0 engine is a bit down in the overall power stakes.
140-145°C oil temperatures may be fine and since my dealer says the engine is still in like-new condition, those temperatures are probably nothing to worry about. The oil temperature gauges in modern Ferraris and Audis redline at 150°C. But such high temperatures make ME uncomfortable so I back off the throttle. I don't know how high the temperature would go if I kept it floored longer but over the course of 135 seconds the oil goes from 100°C to 140°C. I'm thinking about installing a supplemental oil cooler so that I can keep the throttle floored longer without finding out what temperature the oil would otherwise climb to. In your experience, does the oil temperature peak at 140-145°C with the stock oil cooler and then not get any higher? That would be great.
I already installed some Thermo-Tec aluminized heat shielding around my trunk and to the upper catalytic converter and that's why I can transport chocolate back there without it melting. The heat shielding weighs next to nothing and it really worked.
Yeah, power helps and my 18 year old naturally-aspirated 3.0 engine is a bit down in the overall power stakes.
140-145°C oil temperatures may be fine and since my dealer says the engine is still in like-new condition, those temperatures are probably nothing to worry about. The oil temperature gauges in modern Ferraris and Audis redline at 150°C. But such high temperatures make ME uncomfortable so I back off the throttle. I don't know how high the temperature would go if I kept it floored longer but over the course of 135 seconds the oil goes from 100°C to 140°C. I'm thinking about installing a supplemental oil cooler so that I can keep the throttle floored longer without finding out what temperature the oil would otherwise climb to. In your experience, does the oil temperature peak at 140-145°C with the stock oil cooler and then not get any higher? That would be great.
I already installed some Thermo-Tec aluminized heat shielding around my trunk and to the upper catalytic converter and that's why I can transport chocolate back there without it melting. The heat shielding weighs next to nothing and it really worked.
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cool, how much power is it making now?
I don’t know how many horsepower my car really has. I’ve had it on two dynos and they came up with different results.
In 1995 it was on Autothority’s Clayton Hydrokinetic chassis dyno in Virginia when the chip was being programmed. According to Autothority, the engine put out 34.1% more power than a stock NSX. They measured a stock NA1 as putting 293 hp to the wheels so if you correct that down to 235 rwhp, that would give mine about 315 rwhp.
In 1997 I had it on Bosch’s FLA-203 dyno in Vienna which came up with a corrected 201.6 kW to the wheels, or about 270 rwhp, but there was no baseline.
I had the engine modifications done in early 1995 after reading the article about Comptech’s NSX in the May 1994 issue of Car and Driver. In that article, Comptech stated an NA1 with all the engine mods they sold at the time put out 358 crank hp, or about 312 rwhp. I bought all the engine mods they sold.
In any case, in 1999 my car was able to pull to an indicated 8000 rpm in fifth gear in one direction on the Autobahn and an indicated 8050 rpm in the other (avatar pic) running 245/40 17 rear tires. With no aerodynamic modifications other than plugging the a/c openings in the nose of the car.
So I don’t know how many horsepower my car really has, but it’s enough to get to 300 km/h.
That is like 4.5 inches!! If you want to have real impact on lowering your drag then I suggest you first lower the vehicle more and install an air dam with minimal ground clearance. That would be huge. Many of the super-cars you guys eye oogle with active suspension technology do the same thing at speed.
When the vehicle is in motion at high speeds it disrupts air as it punches a hole through it. The frontal area is highly relevant to drag, but also the height of the air buffer under the vehicle becomes part of the equation as well. I would minimize that first before worrying about close-out under panels.
Power would be the next step and make the biggest difference. Plan big.
That should be where you sit. I would be cautious if you start going beyond that for any significant amount of time.
That should be fine. Don't want the Chocolate Covered Cherries to melt half way down the autobahn. :smile:
John, how low is your car?
Where are you guys measuring your ride height from?
Dosn't oil start to break down at 300*F ? I know a race engine motor builder not liking to see oil temps over 300*F... He was very happy at 250*F
Where are you guys measuring your ride height from?
Dosn't oil start to break down at 300*F ? I know a race engine motor builder not liking to see oil temps over 300*F... He was very happy at 250*F
Well, to be sportsman like I ran the "IT required ride height" last year. Measured from... now where did we measure that from?
:biggrin:I am running the ST ride height this year which is 3 inches.
Measured from the jacking tabs on the frame rails. Apparently that is good enough for the scrutineer at impound.
You are correct. It's not the technical ideal. With maintenance it will do it thou year after year.
I agree that lowering the car even further would reduce drag. Regardless how smooth you make the bottom, you’re always going to have wheels jutting directly into the airflow, spinning, and creating turbulence. You’re never going to be able to get the bottom of the car as smooth as the top and sides. So the less air you have flowing under the car and more you have flowing over the top and sides, the lower the drag coefficient will be. But I drive my car on the street, into driveways, into parking lots, etc. and I don't want to lower it even further for the sake of a higher top speed. It’s already as low as I can live with and lower than most people think is practical. By the way, I measured the ride height from the front undertray to the ground.
I think that using a deep front air dam to lower drag is no longer current thinking if you have a relatively smooth underbody. The car’s overall drag is its drag coefficient multiplied by its frontal area and a deeper front air dam will increase the car’s frontal area. If you have a rough underbody with the engine, exhaust, spare tire, etc. hanging down into the airflow, the gain you get in terms of drag coefficient will outweigh the penalty a deep front air dam will give you in terms of frontal area so overall drag will decrease. If you have a relatively smooth underbody, you won’t get that much of a benefit in terms of drag coefficient so the penalty in terms of frontal area may outweigh it and actually increase overall drag
In case you haven’t seen it, there’s a good article on the drag reduction of cars here. Towards the end they touch on the aerodynamic development of the Opel Calibra, which was the lowest-drag car in production when it was introduced. The depth and shape of the front air dam were extensively tested and the engineers found that making it less deep in the center decreased the car's overall drag.
In the case of the NSX, a deeper front air dam won’t make more air go over the smooth top of the car. It will reduce the amount of air flowing under the car but that air will be forced into the radiator opening instead, through the radiator, into the spare tire area, and then it will exhaust under the car or into the wheel wells. If you have underbody cladding, I expect the air would face less resistance just flowing under the car rather than taking that tortuous route, so I highly doubt that a deeper front air dam would reduce an NSXs overall drag.
But then what the hell do I know? The aerodynamic modifications I made to my car haven’t brought much of a measurable benefit so maybe I should forget theory, slap on an asphalt-scraping front air dam, and see what happens.
Their will be a point where the air pressure in front of the radiator is limited by the efficiency of the radiator fins, so with excess speed, air will go around the car and up over the top. Adding an air dam or lowering the car won't make additional air go through your radiator at those speeds. Lowering should reduce drag as will a front air dam.
Interesting point! I never thought about the air reaching a terminal velocity past the radiator fins and above that speed no more air can flow through. I do know the new BMW 7-series has louvers in front of the radiator that close to reduce aerodynamic drag. But maybe those only work at low and medium speeds and at top speed they’re irrelevant because the additional air couldn’t flow past the fins anyhow. Don't know.
Since Opel decreased the depth of the Calibra’s front air dam (in the center – in front of the wheels they made it as deep as they could) to minimize drag, my hunch would still be that a deep air dam will increase drag if you have a smooth underbody. But I guess you’d have to test it on an NSX to be sure.
Since Opel decreased the depth of the Calibra’s front air dam (in the center – in front of the wheels they made it as deep as they could) to minimize drag, my hunch would still be that a deep air dam will increase drag if you have a smooth underbody. But I guess you’d have to test it on an NSX to be sure.
Just for comparison, here’s a car that was designed to minimize drag, not maximize downforce. BMW built it to set speed records for hydrogen-powered vehicles.
Some noteworthy design features:
- low car with low ground clearance
- undertrays front to rear
- underbody shaped to prevent air from hitting the wheels
- rear diffuser with gentle upslope
- no front air dam
Some noteworthy design features:
- low car with low ground clearance
- undertrays front to rear
- underbody shaped to prevent air from hitting the wheels
- rear diffuser with gentle upslope
- no front air dam
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You forgot: Small frontal area - another reason to close big gaps in the front of the car (radiator inlets).
Yes, front tires are obviously a significant source of aerodynamic drag. In a pro race context it is often a compromise between front turning/bite and aerodynamic drag induced by the wider section width for the engineers (I get excellent IMSA coverage on the speed channel) :biggrin:
You know what this means? It is time to buy a stream liner before you go mod crazy on your NSX. I've seen several good deals lately and I am sure we could get one over to you.
I think you are dwelling on theory which will only get you so far. Vehicles like the Hydrogen BMW probably had 1000 hours in a wind tunnel. NASCAR cup cars don't have a smooth underbody per regulations and manage to go plenty fast in a straight line. All you really need is a front air dam and like 800 hp.
Again, another ubber optimized vehicle with countless hours of CFD design mock-ups on super computers. Like tires, aerodynamics is often very over-simplified. Their are often interactions between 5-6 components at play in a single area which change by speed that need be compromised between.
My caution here is that if you are not careful with what you are doing you can easily generate unwanted lift or screw up your front/rear aerodynamic balance and do more harm than good.
Hmmph. All interesting design elements that seem to be strangely lacking in this BMI video with Gan San... :biggrin:
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Yeah, building a streamliner or a strapping a rocket to my car would make it go faster, but you’re right, I want to keep my NSX stock-looking and everyday useable and I don’t want to go mod crazy on it. Lowering it further, putting fairings over the rear wheel arches, bolting on some aluminum discs instead of wheels, getting rid of the rearview mirrors, building a tail extension, etc. don’t sound all too appealing. Neither does rebuilding the engine to run on an oxygenated 117 octane fuel only available in Greece and then no longer being able to fill her up at gas stations.
Right now the car and even the engine compartment still look stock except for the rear undertray peeking out beneath the muffler. I wish I could hide that somehow. With the stock front air dam, stock rear spoiler, and a flat underbody, I’m not worried about the aerodynamic balance and subjectively, it felt perfectly stable at top speed. Looking at the video, you can see the car wasn’t weaving around.
But about that deeper front air dam you’re recommending to decrease drag, dude … since NASCARs are not allowed to have smooth underbodies, it makes sense to keep air away from those underbodies at all costs, i.e. to use a deep front air dam. If you have a nice, smooth underbody, keep air away from the wheels but don’t go overboard increasing your frontal area just to keep air away from the smooth centerline of the car. Not only will that probably increase drag, it will shift downforce to the front axle. Playing Devil’s advocate, unless your deep front air dam modification is balanced out by a bigger rear wing (which will increase drag further), you’re really going to start changing your high-speed aero balance. Without thousands of hours in a wind tunnel, slapping on a deep front air dam may cost you your life if you get snap oversteer in a high speed corner. But then again, since you’ll be going slower, maybe it will be OK. Seriously, though, I think adding a front air dam with minimal ground clearance to a car that has a smooth underbody will increase drag and may screw up the front/rear aerodynamic balance. Plus it won’t look stock. Plus you’ll scrape more on driveways.
Edit: I just read Wolf-Heinrich Hucho’s book “Aerodynamik des Automobils” (fifth edition from 2005), the standard aerodynamics textbook for automotive engineers in Germany. It makes for great reading on the beach and discusses, amongst other things, front air dams.
According to the book (pages 236 and 521) the wind resistance of cars with rough underbodies goes down if you increase the depth of the front air dam until the point that the air dam is almost the lowest point of the underbody. If you make the air dam deeper than that, overall wind resistance starts increasing again but front lift will keep going down. If the car has a smooth underbody, a deeper front air dam will increase drag, period.
Applied to an NSX, that implies that if you want to maximize your straight-line speed, stick with the OEM front spoiler.
Right now the car and even the engine compartment still look stock except for the rear undertray peeking out beneath the muffler. I wish I could hide that somehow. With the stock front air dam, stock rear spoiler, and a flat underbody, I’m not worried about the aerodynamic balance and subjectively, it felt perfectly stable at top speed. Looking at the video, you can see the car wasn’t weaving around.
But about that deeper front air dam you’re recommending to decrease drag, dude … since NASCARs are not allowed to have smooth underbodies, it makes sense to keep air away from those underbodies at all costs, i.e. to use a deep front air dam. If you have a nice, smooth underbody, keep air away from the wheels but don’t go overboard increasing your frontal area just to keep air away from the smooth centerline of the car. Not only will that probably increase drag, it will shift downforce to the front axle. Playing Devil’s advocate, unless your deep front air dam modification is balanced out by a bigger rear wing (which will increase drag further), you’re really going to start changing your high-speed aero balance. Without thousands of hours in a wind tunnel, slapping on a deep front air dam may cost you your life if you get snap oversteer in a high speed corner. But then again, since you’ll be going slower, maybe it will be OK. Seriously, though, I think adding a front air dam with minimal ground clearance to a car that has a smooth underbody will increase drag and may screw up the front/rear aerodynamic balance. Plus it won’t look stock. Plus you’ll scrape more on driveways.
Edit: I just read Wolf-Heinrich Hucho’s book “Aerodynamik des Automobils” (fifth edition from 2005), the standard aerodynamics textbook for automotive engineers in Germany. It makes for great reading on the beach and discusses, amongst other things, front air dams.
According to the book (pages 236 and 521) the wind resistance of cars with rough underbodies goes down if you increase the depth of the front air dam until the point that the air dam is almost the lowest point of the underbody. If you make the air dam deeper than that, overall wind resistance starts increasing again but front lift will keep going down. If the car has a smooth underbody, a deeper front air dam will increase drag, period.
Applied to an NSX, that implies that if you want to maximize your straight-line speed, stick with the OEM front spoiler.
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Since I just needed to remove my undertray, I noticed that dirt has been deposited in such a way that you can see how the air flows beneath it. In case anyone else wants to build their own undertray, that might be useful.
Two things struck me. First of all, the diffuser angle of 4.4° doesn’t seem to be causing any flow separation. You can see the streamlines in the middle of the diffuser extend smoothly all the way to the trailing edge. They don’t tear off and become turbulent where the diffuser reaches its steepest angle. I don’t know at what angle the flow would separate, but at 4.4° it doesn’t.
Secondly, the air does not flow in a straight line from the nose to the tail. It curves. The tires divide and push aside the airflow, the air curves around them and then rushes in to fill the vacuum behind the rolling tires. The flow patterns on my diffuser look similar to the ones in the drawing below, which is from an aerodynamics textbook. The straight “fins” I have sticking down into the airflow are therefore at an angle to the actual streamlines, which is not ideal. If you build fins or strakes or whatever that stick down into the airflow, making them a bit curved (kind of like what Ferrari did on their 360) would probably be better than making them simply straight.
Two things struck me. First of all, the diffuser angle of 4.4° doesn’t seem to be causing any flow separation. You can see the streamlines in the middle of the diffuser extend smoothly all the way to the trailing edge. They don’t tear off and become turbulent where the diffuser reaches its steepest angle. I don’t know at what angle the flow would separate, but at 4.4° it doesn’t.
Secondly, the air does not flow in a straight line from the nose to the tail. It curves. The tires divide and push aside the airflow, the air curves around them and then rushes in to fill the vacuum behind the rolling tires. The flow patterns on my diffuser look similar to the ones in the drawing below, which is from an aerodynamics textbook. The straight “fins” I have sticking down into the airflow are therefore at an angle to the actual streamlines, which is not ideal. If you build fins or strakes or whatever that stick down into the airflow, making them a bit curved (kind of like what Ferrari did on their 360) would probably be better than making them simply straight.
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i think it would be very neat if you could install some kind of device that would simulate the flowing dirt on command, such as a constant drip thick fluid that would smear on the undertray at speed. that would simulate the aerodynamic streamlines almost on command (a la wind tunnel)
for vehicle lowering, have you looked into member 'kane' ravi's (umbrella auto design) VRH (variable ride height) kit that installs on any nsx with almost any suspension? it would allow you to lower your vehicle and raise your vehicle in whatever situation you find yourself in.
also, you may want to look into sizing different brake ducts/shields for the front and the rear. the air flowing under the vehicle is disturbed by the rotating tires, but also by the rotating wheels. if you can minimize the air being disturbed (very close to the underbody) by the wheels (larger inside brake covers) you may be able to optimize the streamlines under the vehicle more. I guess you could achieve the same effect by covering the outside of your wheels, but that eliminates the functionality of a stock looking vehicle.
also, replacing the screws/bolts used on your undertray to something with a lower profile, such as pop rivets may reduce some drag from the undertray. it will also be lighter! i doubt this will actually change the aerodynamics at the speed you are running at, but in theory, it would help.
for vehicle lowering, have you looked into member 'kane' ravi's (umbrella auto design) VRH (variable ride height) kit that installs on any nsx with almost any suspension? it would allow you to lower your vehicle and raise your vehicle in whatever situation you find yourself in.
also, you may want to look into sizing different brake ducts/shields for the front and the rear. the air flowing under the vehicle is disturbed by the rotating tires, but also by the rotating wheels. if you can minimize the air being disturbed (very close to the underbody) by the wheels (larger inside brake covers) you may be able to optimize the streamlines under the vehicle more. I guess you could achieve the same effect by covering the outside of your wheels, but that eliminates the functionality of a stock looking vehicle.
also, replacing the screws/bolts used on your undertray to something with a lower profile, such as pop rivets may reduce some drag from the undertray. it will also be lighter! i doubt this will actually change the aerodynamics at the speed you are running at, but in theory, it would help.
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F1 uses a viscous paint that they apply to aero parts during track testing.
A “dirt on command” system or viscous paint would be neat, but driving fast enough in the rain is probably more appropriate to my level of sophistication!
I like the idea of aluminum pop rivets since they’d cause less drag than the semi-spherical allen key screws I’m currently using, they’d be lighter, plus there would be no possibility of contact corrosion. But I like tinkering around and being able to unscrew things makes that a lot easier.
I also think that larger diameter splash guards behind the brake discs would reduce the turbulence created by the wheels, but I’m afraid of the effect that would have on brake cooling. I actually removed my front splash guards years ago to improve cooling. Also, when the car is on the ground, the rims are almost completely above the plane of the undertray. I suspect that most of the underbody turbulence in front of the rear wheels is actually coming from the front wheels.
Funny you should mention Ravi’s variable ride height system. I’ve been looking at that and the guys at Cargraphic told me they could do a custom installation for NSXs as well (http://www.cargraphic.de/sources/downloads/Airlift.pdf). Having the car low on smooth roads and higher to clear driveways, speed bumps, etc. would be really practical. To be honest, so would ultrasonic parking sensors, but that may be going a bit too far.
By the way, since I took the pictures below, I painted the top of the undertray and the outer air fences black. Now you can barely see the rear undertray / diffuser when mounted.
I like the idea of aluminum pop rivets since they’d cause less drag than the semi-spherical allen key screws I’m currently using, they’d be lighter, plus there would be no possibility of contact corrosion. But I like tinkering around and being able to unscrew things makes that a lot easier.
I also think that larger diameter splash guards behind the brake discs would reduce the turbulence created by the wheels, but I’m afraid of the effect that would have on brake cooling. I actually removed my front splash guards years ago to improve cooling. Also, when the car is on the ground, the rims are almost completely above the plane of the undertray. I suspect that most of the underbody turbulence in front of the rear wheels is actually coming from the front wheels.
Funny you should mention Ravi’s variable ride height system. I’ve been looking at that and the guys at Cargraphic told me they could do a custom installation for NSXs as well (http://www.cargraphic.de/sources/downloads/Airlift.pdf). Having the car low on smooth roads and higher to clear driveways, speed bumps, etc. would be really practical. To be honest, so would ultrasonic parking sensors, but that may be going a bit too far.
By the way, since I took the pictures below, I painted the top of the undertray and the outer air fences black. Now you can barely see the rear undertray / diffuser when mounted.
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a solution to achieving the low profile of pop rivets while maintaining the functionality of removability is to use press-in studs on the lower aerodyanmic appendages (gates and panel joining). you can get them from companies such as PEM and press them in (i would suggest finding a vendor that is not PEM as they only deal in bluk as they are the supplier), using washers and locking nuts on the inside of the panel. the underside of PEM studs are virtually zero-profile.
again, this in theory would benefit aerodynamics.
what is the weight of your car? also, have you looked into getting any accelerometer-type thing to use in your car? such as the g-tech? it would provide more details to gauge your car's performance as your experience with dynomometers hasn't been that consistent.
again, this in theory would benefit aerodynamics.
what is the weight of your car? also, have you looked into getting any accelerometer-type thing to use in your car? such as the g-tech? it would provide more details to gauge your car's performance as your experience with dynomometers hasn't been that consistent.
just to update this thread, andreas just sent me a PM regarding accelerometers and i thought it was a good idea to add it to this thread.
Dear Shiann,
Sorry I didn’t get back to you earlier regarding whether I have an accelerometer-based dyno, but I didn’t when you asked and I do now. A few weeks ago I got an iPhone and one of the first apps I bought was Dynolicious.
Last weekend I got my NSX out of winter storage, did two acceleration runs with it, and put it back into storage. First run: 242 whp. Second run: 234 whp. I’m really hoping that isn’t right. I calibrated the accelerometers before testing my BMW and Mercedes a few weeks ago but forgot to calibrate them before testing the NSX. Maybe there was a program update that erased the calibration settings in the meantime. Also, since I disconnected the NSXs battery when storing it, the ECU probably forgot all the short and long-term fuel and ignition trims, so horsepower will probably increase when the battery is connected again for a while and the car goes through a few tanks of gas. Also, there’s a gasoline stabilizer in the tank right now and maybe that cost some horsepower as well. I hope at least one of those excuses holds.
If my car really only puts out 238 rwhp with all the engine mods it has, it must have lost a lot of horsepower. When my avatar picture was taken, the car had no aerodynamic modifications other than being lowered a bit with Eibach springs and closing off the a/c openings in the nose. Like that, the car pulled to an indicated 8000 rpm in one direction on the Autobahn and 8050 in the other. 8025 rpm should have been 303 km/h if there was no tire slip, the tires were actually made to spec, weren’t worn down, the tachometer was accurate, etc. Given that those caveats probably don’t hold, the real top speed was probably closer to 295 km/h. With stock aerodynamics, an NA1 would need 31% more whp to increase the top speed from 270 km/h to 295 km/h, or about 307 whp at 8000 rpm. If Eibach springs and closing off the a/c openings reduce the Cd by .02 (just a guess), the car would still need 291 whp to get to 295 km/h. Given that my car still doesn’t consume any oil and my dealer told me this year that the compression still looks good on all cylinders, I hope my car hasn’t lost 18% of its horsepower since my avatar pic was taken.
However, if my car really only has 238 whp now, not only did it lose a lot of power, the undertrays must have cut wind resistance a lot. If the car needed 291 whp to get to 295 km/h back in 1999 and got to 300 km/h with only 238 whp in 2009, the undertrays must have lopped an additional .085 off the Cd. Which I find hard to believe.
So now I have dyno measurements ranging from 238 whp to 408 whp for the same engine in the same configuration. Nice. In the spring, I’m going to do some more Dynolicious runs after the ECU has had time to learn the right fuel and ignition trims and when the fuel stabilizer is out of the tank. And I’m planning on getting a leak-down test done as well to better diagnose the health of the engine, if I can find someone in the area to do it. If the engine really is worn out and only has 238 whp left, shipping the car across the ocean to California sounds interesting. If my car really only needs 238 whp to get to 300 km/h, it would only need 294 whp to get to 200 mph. Which a naturally-aspirated Comptech 3.5 liter engine would surely provide. But I can’t believe my engine really is that worn out and that that calculation holds.
The best way to measure horsepower would probably be to find a stock NSX, take that to a dyno along with mine, measure both, assume a stock NA1 really has 235 rwhp regardless what the dyno says (and a stock NA2 has 250), and calculate the difference.
Best regards,
Andreas
I think you are correct in your questioning of the results. The dynolicious software on the iphone has proven to be pretty accurate as long as it is used correctly. I think you are right on what has possibly robbed your car of 'power'. In the spring, when the car is back up and blasting down the autobaun, i think your reading will change. I think the control factor of a stock nsx, or tryign it out in different vehicles is a good idea as a baseline.
your experimentation has provided a wealth of information for the forum. thanks very much for your contribution.
i never got to do the CFD simulations on the nsx profile. i could probably have gotten more accurate numbers for coefficient of drag and such on lowered vehicles and such, but never got around to it and have since, graduated and no longer have access to the tools. it's too bad.
i hope to do the same in terms of underbody trays on my current car (integra) in a quest to increase aerodynamic performance as i do quite a bit of highway driving (~500 miles per week not counting weekend excursions)
thanks again! hope to hear back more experimental results in the spring!
Dear Shiann,
Sorry I didn’t get back to you earlier regarding whether I have an accelerometer-based dyno, but I didn’t when you asked and I do now. A few weeks ago I got an iPhone and one of the first apps I bought was Dynolicious.
Last weekend I got my NSX out of winter storage, did two acceleration runs with it, and put it back into storage. First run: 242 whp. Second run: 234 whp. I’m really hoping that isn’t right. I calibrated the accelerometers before testing my BMW and Mercedes a few weeks ago but forgot to calibrate them before testing the NSX. Maybe there was a program update that erased the calibration settings in the meantime. Also, since I disconnected the NSXs battery when storing it, the ECU probably forgot all the short and long-term fuel and ignition trims, so horsepower will probably increase when the battery is connected again for a while and the car goes through a few tanks of gas. Also, there’s a gasoline stabilizer in the tank right now and maybe that cost some horsepower as well. I hope at least one of those excuses holds.
If my car really only puts out 238 rwhp with all the engine mods it has, it must have lost a lot of horsepower. When my avatar picture was taken, the car had no aerodynamic modifications other than being lowered a bit with Eibach springs and closing off the a/c openings in the nose. Like that, the car pulled to an indicated 8000 rpm in one direction on the Autobahn and 8050 in the other. 8025 rpm should have been 303 km/h if there was no tire slip, the tires were actually made to spec, weren’t worn down, the tachometer was accurate, etc. Given that those caveats probably don’t hold, the real top speed was probably closer to 295 km/h. With stock aerodynamics, an NA1 would need 31% more whp to increase the top speed from 270 km/h to 295 km/h, or about 307 whp at 8000 rpm. If Eibach springs and closing off the a/c openings reduce the Cd by .02 (just a guess), the car would still need 291 whp to get to 295 km/h. Given that my car still doesn’t consume any oil and my dealer told me this year that the compression still looks good on all cylinders, I hope my car hasn’t lost 18% of its horsepower since my avatar pic was taken.
However, if my car really only has 238 whp now, not only did it lose a lot of power, the undertrays must have cut wind resistance a lot. If the car needed 291 whp to get to 295 km/h back in 1999 and got to 300 km/h with only 238 whp in 2009, the undertrays must have lopped an additional .085 off the Cd. Which I find hard to believe.
So now I have dyno measurements ranging from 238 whp to 408 whp for the same engine in the same configuration. Nice. In the spring, I’m going to do some more Dynolicious runs after the ECU has had time to learn the right fuel and ignition trims and when the fuel stabilizer is out of the tank. And I’m planning on getting a leak-down test done as well to better diagnose the health of the engine, if I can find someone in the area to do it. If the engine really is worn out and only has 238 whp left, shipping the car across the ocean to California sounds interesting. If my car really only needs 238 whp to get to 300 km/h, it would only need 294 whp to get to 200 mph. Which a naturally-aspirated Comptech 3.5 liter engine would surely provide. But I can’t believe my engine really is that worn out and that that calculation holds.
The best way to measure horsepower would probably be to find a stock NSX, take that to a dyno along with mine, measure both, assume a stock NA1 really has 235 rwhp regardless what the dyno says (and a stock NA2 has 250), and calculate the difference.
Best regards,
Andreas
I think you are correct in your questioning of the results. The dynolicious software on the iphone has proven to be pretty accurate as long as it is used correctly. I think you are right on what has possibly robbed your car of 'power'. In the spring, when the car is back up and blasting down the autobaun, i think your reading will change. I think the control factor of a stock nsx, or tryign it out in different vehicles is a good idea as a baseline.
your experimentation has provided a wealth of information for the forum. thanks very much for your contribution.
i never got to do the CFD simulations on the nsx profile. i could probably have gotten more accurate numbers for coefficient of drag and such on lowered vehicles and such, but never got around to it and have since, graduated and no longer have access to the tools. it's too bad.
i hope to do the same in terms of underbody trays on my current car (integra) in a quest to increase aerodynamic performance as i do quite a bit of highway driving (~500 miles per week not counting weekend excursions)
thanks again! hope to hear back more experimental results in the spring!
shawn110975
Suspended
Build me one and i'll buy it from you!!
great thread!!
