Homemade complete rear undertray / diffuser

[john@johnhudakracing]

I'm not sure I follow how the skid block would be affected by the holes in the undertray. And since that swiss-cheese portion of the undertray is at a slight angle, it seems to me that fluids wouldn't collect there anyhow even without holes (except under acceleration, I guess). They'd tend to flow forward and collect underneath the fuel tank, where the underbody looks to be completely flat.

Maybe picture the vehicle after it has flipped 10 times and on fire.

But maybe the JGTC rules simply required that the under-tray beneath the engine have holes, period.

Their is no need to speculate on the rules. They are posted online. Originally the series was managed by the JAF and recognized by the FIA. All vehicles had to comply with JAF requirements. Later, the series transitioned to FIA management, and this opened up the series to FIA homologated entrants which allowed for wider participation.

Historically, the underside of the touring cars were not flat. It shows that quite clearly in the pictures of the early cars. The teams were free to tailor the underside so long as it fit within a fairly loose set of rules. From what I have come to understand, they then later mandated the flat bottom between wheel centerlines to reign things in and bridge the disparity between the factory teams that were doing millions in extensive wind tunnel testing at dome up to that point, and other competitors without quite the same degree of preparation.

From an aerodynamics standpoint, since the steep portion of the diffuser starts right behind the rear wheel centerline, it seems to me that the JGTC car should generate a good deal of its down-force just ahead of the rear wheel centerline - and that those holes will tend to bleed turbulent air, reducing the efficiency of the underbody aerodynamics.

The air pressure drops as the air increases in velocity moving front to rear. On a flat bottom production race car the bulk of the downforce will be just fore the rear center line, in front of the leading edge of the diffuser. The holes would have a relatively minimal effect on overall aerodynamic efficiency. They are allowed to, and regularly do have peripheral holes for air jacks, ducts, maintenance, and such anyway.

 

[CB169]

nice job, now that would be worth the money over the over priced stock type R under trays

 

[greenberet]

Their is no need to speculate on the rules. They are posted online.

I just looked online for the JGTC rules concerning the location of holes in GT500 underbodies before 2003, when the FIA-like flat bottom was introduced, but couldn't find them. The only thing I've found so far is the article linked to in post #23, which was translated from the Japanese Honda website. To quote it, "In the factory NSX, the engine's oil pan, transmission, rear suspension are all exposed. The GT version hides those parts with a panel with holes on it. The panel is installed as a means to protect the engine compartment, and is allowed by GT rules. However, in order to control ground effects performance gained, GT rules mandate that there be regulation-size holes in the panel in order to disturb airflow." If you've found the 1999 JGTC regulations, let's have a look and end the speculation whether the Honda website is right or wrong.

I ran across a great research article on the aerodynamic effect of drilling relatively large holes into a flat plate over which air flows but unfortunately I didn't save it and haven't been able to find it again. The closest thing I've found is this from NASA, where they measure the drag caused by drilling small holes into a flat plate. Basically, drilling holes with a diameter of 1-2mm into a flat plate increases drag and if you drill enough holes, you'll double the drag coefficient. The article on large holes measured similar (but if I remember correctly greater) effects - holes have awful aerodynamics. After reading it I figured that's it, I'm not going to use round holes for ventilation, I'll fabricate NACA ducts instead.

On a flat bottom production race car the bulk of the downforce will be just fore the rear center line, in front of the leading edge of the diffuser.

Which is right where the JGTC NSXs were required to have holes to "disturb airflow". Instead of the suction peak at that point pulling down on the car and generating downforce, the suction peak will be pulling air out of the engine compartment, generating turbulence.

The holes would have a relatively minimal effect on overall aerodynamic efficiency. They are allowed to, and regularly do have peripheral holes for air jacks, ducts, maintenance, and such anyway.

Putting holes for a jack into the wake of the wheels is probably not nearly as bad as putting them in the clean air towards the car's centerline. The aerodynamic penalty is probably more than offset by the increased speed of the pit stops.

Maybe picture the vehicle after it has flipped 10 times and on fire.

Did that burning Ferrari have holes under the engine (like the JGTC NSXs) instead of under the fuel cell (where you suggested they should be for safety reasons and where my undertray allows for drainage). I can't tell from the picture.

Be that as it may, I like the rear undertray you built for your car.

nice job, now that would be worth the money over the over priced stock type R under trays

Thanks and I fully agree. The aluminum for the undertray cost something like EUR 80 ($100).

 

[CK_SB]

Guys, correct me if I am wrong, but don't the NACA ducts accelerate air from under the car just like the diffuser does, but with a reduced effect obviously? Forgetting the conversation about the turbulence created by the NACA ducts as part of this discussion, if we accelerate the air out from under the car with NACA ducts and a diffuser don't we have the best of all worlds for both cooling the engine and creating more down-force? Anyone who knows the physics please chime in.

Just an aside: To the best of my knowledge Ferrari uses no holes in the underpanels of their modern mid-engine cars. They have engineered the air flow to the engine from the quarters much more effectively then our own cars. This would tell me that no holes or NACA ducts in the underpanels is ideal, but they are necessity for our cars due to the lack of airflow to the engine.

 

[john@johnhudakracing]

I just looked online for the JGTC rules concerning the location of holes in GT500 underbodies before 2003, when the FIA-like flat bottom was introduced, but couldn't find them. The only thing I've found so far is the article linked to in post #23, which was translated from the Japanese Honda website. To quote it, "In the factory NSX, the engine's oil pan, transmission, rear suspension are all exposed. The GT version hides those parts with a panel with holes on it. The panel is installed as a means to protect the engine compartment, and is allowed by GT rules. However, in order to control ground effects performance gained, GT rules mandate that there be regulation-size holes in the panel in order to disturb airflow."

It is my belief that that the article you reference was put together to simply high-light some of the key design differences, in laymans terms. It is obvious that a multi-million dollar GT500 NSX factory race car has about as much in common with a road going NSX as a Corvette C6 has with the Pratt & Miller C6R at Le Mans. i.e. maybe the tail lights.

If you would like definitive clarification on that one historic technical rule in JGTC, then I feel confident that someone in Japan, that was at one point directly involved with the series (perhaps a driver or technical official) could clarify that for you definitively.

In the absence of speaking Japanese, I can only apply my knowledge from comparable GT racing series, in which case aside from the rules- they primarily use weight and throttle plate restrictors as a tool to equalize and guarantee competitiveness. While I would not completely discount it as a possibility, and it could well have been a supplemental requirement in JGTC as a way to control aerodynamic over-development during that period... my only point was that generally speaking I have not heard of officials checking hole sizes in under panels to disturb air flow before.

I have heard of concerns where fuel cells can rupture, spilling 32 gallons of petrol wide across an under-pan making it extremely difficult to access and extinguish; so it would not surprise me if perhaps foremost their was a safety requirement. These vehicles are blindingly fast and thus prepared/held to the highest level of safety standard. I know in F1 they apparently use holes in the skid blocks to establish the conformity. I can only speculate as to the significant issues that came and went over the years in the All-Japan Grand Touring Series.

If you've found the 1999 JGTC regulations, let's have a look and end the speculation whether the Honda website is right or wrong.

As the series is now managed by the FIA, you can download a current PDF of the 2009 GT regulations on the FIA's web site.

I ran across a great research article on the aerodynamic effect of drilling relatively large holes into a flat plate over which air flows but unfortunately I didn't save it and haven't been able to find it again. The closest thing I've found is this from NASA, where they measure the drag caused by drilling small holes into a flat plate. Basically, drilling holes with a diameter of 1-2mm into a flat plate increases drag and if you drill enough holes, you'll double the drag coefficient. The article on large holes measured similar (but if I remember correctly greater) effects - holes have awful aerodynamics. After reading it I figured that's it, I'm not going to use round holes for ventilation, I'll fabricate NACA ducts instead.

I'm not clear what bearing that article has on motorsports. NASA's concern with drag starts at about mach .6 and extends to 28,000 km/h

At these speeds the effects of drag are completely uncomparable.

Be that as it may, I like the rear undertray you built for your car.

Thanks.

 

[greenberet]

I'm not clear what bearing that article has on motorsports. NASA's concern with drag starts at about mach .6 and extends to 28,000 km/h

At these speeds the effects of drag are completely uncomparable.

I agree that drag changes at transsonic, supersonic, hypersonic, etc. speeds, but in that article NASA only measured drag at subsonic speeds down to 200 mph, which are achieved in motorsports. The report on the subsonic drag of a flat plate with larger holes would be more relevant, though.

Forgetting the conversation about the turbulence created by the NACA ducts as part of this discussion, if we accelerate the air out from under the car with NACA ducts and a diffuser don't we have the best of all worlds for both cooling the engine and creating more down-force?

Good question. I've never seen any research into the downforce created by underbody NACA ducts. I've only seen tests of how much flow they capture and how much drag they create. Since they do suck air up into the car, they must create some downforce, too.

 

[Hugh]

This would tell me that no holes or NACA ducts in the underpanels is ideal, but they are necessity for our cars due to the lack of airflow to the engine.

The engine gets air forced into the compartment from the passenger side scoop. It's further cooled by the radiator in the front of the car. I doubt that an undertray with no holes in it (except for the Fumoto valve) would have an adverse effect.

 

[john@johnhudakracing]

Good question. I've never seen any research into the downforce created by underbody NACA ducts. I've only seen tests of how much flow they capture and how much drag they create. Since they do suck air up into the car, they must create some downforce, too.

The NACA ducts on the under tray have nothing to do with creating more downforce. Zip. Zero. Nothing.

Their primary purpose is simple. When you seal up the underside of the power train their is no longer any airflow getting to the components, as was factory design intent. The little fins on the bottom of your transmission case for example become useless to efficiently dissipate heat. The air stagnates around the oil pan and your entire power train literally bakes under sustained use on-course.

On a mid or rear engine race car, adding a couple of NACA ducts to your under tray will siphon off airflow with minimal drag. This allows positive pressure to develop, which combined with the negative pressure on the backside of the vehicle pulls/scavenges air and allows for a steady air current to develop under the power train helping to dissipate heat. When used in tandum with oil and transmission coolers, this can be one of many additionals tools to effectively control temps on a race track.

 

[swerve2007]

john is correct, you need to use minimum of material around the engine, just to keep majority of the airflow from going turbulent in the area but allowing air to be pulled out of the engine compartment. naca ducts will work but not at lower speeds.

 

[greenberet]

The NACA ducts on the under tray have nothing to do with creating more downforce. Zip. Zero. Nothing.

I’ve never heard of someone installing NACA ducts to gain downforce or even read about what gains or losses they can bring in that regard. However, thinking about it, if you siphon off a portion of the air between the underbody and the ground, you will tend to reduce the pressure of the remaining air, giving you downforce. If the siphoning off creates turbulence and drag, then the air under the car may slow down, increase in pressure, and create lift. Which of the two effects is larger will depend on the efficiency of the intake. My gut feeling is that the net effect will be small, but NACA ducts will generate some force perpendicular to the airstream, not NADA. Maybe so little as to be effectively zip, but the force should be measurable and if no one has researched it before, it would be a great topic for a paper.


Regarding engine cooling: I only have the stock engine oil “cooler” and with that, I am unable to drive at full throttle continuously. My engine and transmission oil temperatures climb to 140° C even without any undertrays and then I back off. Based on that, I’d expect the temperatures to climb even faster if the rear undertray had no openings for the reasons John@Microsoft and swerve mentioned.

 

[CK_SB]

I can tell you all with certainty that with the engine panel in place I am experiencing a 20F rise in engine temps while on the track. I am running an engine oil cooler with a 12 quart oil system, as well as a remote transmission cooler, and a massive Ron Davis radiator. I also have my trunk wall removed and a vented trunk lid to pull air out from under the engine hatch. I believe what we need for sufficient cooling is a single, very wide, upward sweeping radius'd opening in front of the oil pan, wide enough to get air to the transmission. This would remove a substantial amount of air from under the car and possibly give us two points of additional downforce with the first substantially ahead of the wheel centerline. The venting through my diffuser and my trunk opening should eliminate uplift for me, but I am not sure how a street car would address the additional needed venting...perhaps the diffuser venting is enough.

 

[syam]

would anybody be interested if i did a CFD simulation like 'greenberet' posted in post #25 with the outline of an nsx? this simulation would be a stationary nsx outline with moving air and a moving 'ground'...

i am not exactly sure how accurate the results would be then, since i would only be running a 2-D simulation without rotating wheels (apparently, this attribute, which is very difficult to implement, is very important). it can, however, give a better idea of the high pressure and low pressure areas above and under the car at a given speed...

let me know. it should only take a few hours, when i get into school.

 

[CK_SB]

"would anybody be interested if i did a CFD simulation like 'greenberet' posted in post #25 with the outline of an nsx? this simulation would be a stationary nsx outline with moving air and a moving 'ground'..."

Yes. That would be great if you can get to it. Thanks for offering.

 

[swerve2007]

I believe what we need for sufficient cooling is a single, very wide, upward sweeping radius'd opening in front of the oil pan, wide enough to get air to the transmission. This would remove a substantial amount of air from under the car and possibly give us two points of additional downforce with the first substantially ahead of the wheel centerline. The venting through my diffuser and my trunk opening should eliminate uplift for me, but I am not sure how a street car would address the additional needed venting...perhaps the diffuser venting is enough.

what you have described would actually conflict with the way air is circulated in the engine bay. all the airflow is generated by the movement of air under the car and its vacuum sucks the air in through the side vents and window garnish. the more shields you place around the bottom of the engine, the less airflow you will get. additionally if you remove airflow under the car you will lose downforce as it is directly proportional to airspeed.

 

[CK_SB]

Just so we're on the same page...I am, in fact, proposing a change in the way the air flows through the engine area, but in a way that increases down force without sacrificing cooling. My concept rams air into the engine bay from beneath the car and that air is in turn wicked out the back of the car through the diffuser, trunk porting and rear vented deck lid lip. The rear venting that I am using is a direct copy of the Realtime NSX of years ago. Realtime told me that it produced an additional 100lbs of down force. What was good enough for them is good enough for me.

 

[john@johnhudakracing]

However, thinking about it, if you siphon off a portion of the air between the underbody and the ground, you will tend to reduce the pressure of the remaining air, giving you downforce.

Adding a duct would have an insignificant effect. BTW, what you are proposing has already long since been done. It is was called a Brabham BT46B fan car, circa 1978. Before it was banned they experimented with using power from the engine to power a fan to literally suck the remaining air out from under the back of the vehicle to create more down-force.

<EMBED src=http://www.youtube.com/v/1d7iR72UVh4&hl=en&fs=1 width=425 height=344 type=application/x-shockwave-flash allowfullscreen="true" allowscriptaccess="always">

Regarding engine cooling: I only have the stock engine oil “cooler” and with that, I am unable to drive at full throttle continuously. My engine and transmission oil temperatures climb to 140° C even without any undertrays and then I back off. Based on that, I’d expect the temperatures to climb even faster if the rear undertray had no openings for the reasons John@Microsoft and swerve mentioned.

I can tell you all with certainty that with the engine panel in place I am experiencing a 20F rise in engine temps while on the track.

My experience has been that 140-145C is normal in the higher revving Honda's stock. You don't want to compound the problem. If you seal that up without making a few necessary changes you are going to eventually have problems if you drive for any sustained amount of time.

I believe what we need for sufficient cooling is a single, very wide, upward sweeping radius'd opening in front of the oil pan, wide enough to get air to the transmission.

You just need two NACA ducts. I also have one in front of the oil pan, and one in front of my transmission case blowing air right at it. Their are also several other little changes that made a difference for me. As you would know, managing heat is important in a race car.

The rear venting that I am using is a direct copy of the Realtime NSX of years ago. Realtime told me that it produced an additional 100lbs of down force.

I was not aware that the team ever purchased tunnel time. Without this, I am not sure how it is that they came to quantify that.



</EMBED>

 

[greenberet]

I just had dinner with an aeronautical engineer who designs airplane wings for a living. Since he doesn’t work on automotive aerodynamics, he could only give me his gut feelings to my NSX questions, but those were that:

• the downforce created by the inlet ramps of underbody NACA ducts will probably not be noticeable
• the amount of drag created by holes in the undertray under the engine will probably not be noticeable
• holes in the undertray under the engine will noticeably reduce downforce if you would have otherwise generated downforce there. The pressure differential above and below the undertray will equalize due to the holes and since the engine compartment also vents to the top of the car, the pressure above and below the car will equalize, resulting in neither downforce nor lift at that point. Air will flow into the engine compartment, heat up, the heated air will flow over the oil pan and gearbox and then vent out the bottom - if the air is flowing fast there and would otherwise generate downforce under the engine.

He also said there are so many interactions that you would need to do computational fluid dynamics (CFD) or wind tunnel analyses to be sure. When doing the CFD analyses, you need to simplify the model of the car otherwise no supercomputer in the world will be able to solve all the Navier-Stokes equations. What details you decide to leave out of your model and what details you decide to include will impact the computed results, so you better be sure you included all the important details and only left out unimportant details. Knowing beforehand which are which is something of a black art that comes with experience.

John - how big are the inlet openings of your NACA ducts? Mine are 19*77 mm and I would have made them bigger if I could have.

 

[john@johnhudakracing]

I just had dinner with an aeronautical engineer who designs airplane wings for a living. Since he doesn’t work on automotive aerodynamics, he could only give me his gut feelings to my NSX questions, but those were that:

• the downforce created by the inlet ramps of underbody NACA ducts will probably not be noticeable
• the amount of drag created by holes in the undertray under the engine will probably not be noticeable

That's pretty much what I said. While I never suggested anyone to swiss cheese it, I wouldn't worry about losing downforce to having a few holes where it makes sense to do so.

Their is no reason to fuss about infinitesimal improvements for what most of us are doing.

He also said there are so many interactions that you would need to do computational fluid dynamics (CFD) or wind tunnel analyses to be sure. When doing the CFD analyses, you need to simplify the model of the car otherwise no supercomputer in the world will be able to solve all the Navier-Stokes equations.

Oh, well in that case maybe I could just call my friend and leverage the resources at the San Diego Center of Supercomputing to help me win the aerodynamic battle at my next regional race.

... or I could simply drive faster.

Decisions, Decisions.... :biggrin:

John - how big are the inlet openings of your NACA ducts? Mine are 19*77 mm and I would have made them bigger if I could have.

Standard 11" X 7" I pop riveted on. Combined with a few other tweeks, I found them to be a cumulative improvement.

 

[CK_SB]

Thanks for all the good input gentlemen. Since I will be redesigning my panel with NACA ducts that I will fabricate I decided to look around for a tool to help with the sizing and I found this: http://sports.racer.net/tech_info/aero/naca_profile_calculator.xls

Below is a video that is a decent demo of NACA ducts flowing air. It would have been better if it were configured exactly like we are discussing here, but it might be of some value nonetheless.

http://www.youtube.com/watch?v=j0q-tzuoD-8

Cheers,

Chris

 

[greenberet]

Since I will be redesigning my panel with NACA ducts that I will fabricate I decided to look around for a tool to help with the sizing and I found this: http://sports.racer.net/tech_info/aero/naca_profile_calculator.xls

That calculator gives you just about exactly the shape that is recommended in the original NACA design documents. According to NACA, a 7° ramp angle is about the most efficient, so if you enter the following into cell B3 in the spreadsheet, it'll automatically calculate the correct depth based on the length: =SIN(7*PI()/180)*B4. Also, NACA found that setting the width to about 4x the depth is best, so cell B2 should be 4*(B3 minus the thickness of sheet metal). Basically, you can then enter the length you want into cell B4 and the spreadsheet will spit out all the other dimensions of an optimal NACA duct. And of course, when you fabricate the ducts make sure you keep the edges of the side walls nice and sharp (don't round them off) because it's the sharp edges that create the vortices that get sucked into the inlet opening and make the ducts so efficient.

John, I assume 11" by 7" is the size of the duct, but how big is the inlet opening at the end of the duct? I hope I didn't make mine too small.

 

[john@johnhudakracing]

John, I assume 11" by 7" is the size of the duct, but how big is the inlet opening at the end of the duct? I hope I didn't make mine too small.

 

[greenberet]

Thanks – so you have 91 square centimeters of inlet opening (14 square inches) and I have 73 square centimeters. That’s in the same ballpark so hopefully I’ll be OK.

 

[greenberet]

This morning there was no precipitation and very little traffic so I figured I'd drive to Germany for a top speed run.

Engine temperature

The engine oil temperature didn't seem to climb any quicker with the rear undertray in place than without it. With the cruise control set to 138 km/h (86 mph) for three hours, the oil temperature was about 100°C - the same as it would have been without the rear undertray. When flooring the throttle for extended periods in Germany, the oil temperature climbed, but it didn't seem to climb any faster than before. At least at high speeds, the NACA-ducted undertray doesn't seem to cause thermal problems for the engine.

Temperature of trunk

I tossed a remote temperature sensor into the trunk to measure how hot it gets in there. The temperature was 13°C when I started the car in the morning. After three hours on the highway, it had climbed to 19°C. After the top speed runs, it had gotten to 20°C. I insulated the trunk after reading about what Malibu Rapper and D’Ecosse did in this thread. Since then I've had a cool trunk and the rear undertray doesn't seem to have impacted it.

Stability of undertray

Since the rear undertray didn't fall off above 300 km/h (186 mph) it seems to be stable enough.

Top speed

Unfortunately, it didn't increase from my last run without the rear undertray.

Fortunately, I was able to download the video from the memory card this time without a problem (see video here). Above 230 km/h the engine noise got so loud that the camera automatically turned down the gain on the microphone. It's too bad there's no user adjustment for that because a naturally aspirated NSX is a beautiful thing to hear. I guess that's what you get for using a point and shoot camera set to video mode instead of using a real video camera. You can hear some rattling sound in the video that came from the tripod and some kind of Darth Vader inhaling sound that must have come from the camera itself. (Please note before you flame me: driving at these speeds is perfectly legal in Germany and there are only half as many deaths per person-mile traveled on German Autobahns as there are on US highways.)

292 km/h heading into the wind: I ran into the speed limiter at 282 km/h because I forgot to turn off the traction control. Then I got off the throttle and disabled the traction control before flooring it again and accelerating beyond 282 km/h. The time spent coasting may have cost me a km/h or two at the end of the straight. Maybe. I was running low on gas and didn't want to risk additional runs. The weather report says there was a 14 km/h headwind with gusts up to 22 km/h at 7:00 am, when the video was shot.

303 km/h running with the wind: The car may have hit the rev limiter. Last year, it hit the rev limiter at 305 km/h going downhill and then stuttered along at 303 km/h (and after that I needed to replace the oxygen sensors). With a bit more wear on the tires, 302/303 may be the maximum continuous speed I can get with my current effective gearing.

Overall, the results aren't as good as my last top speed run, when I went out on a day almost totally without wind and got up to 299 km/h in one direction and 301 km/h in the other - without a rear undertray. Possible excuses:

• maybe my rear undertray doesn't reduce drag as expected due to some obscure interactions
• maybe the wind was not completely parallel to the Autobahn so the car had to travel slightly sideways through the wind and from the side, it's not as aerodynamic as from the front
• maybe the cleaned and flow tested fuel injectors I installed over the winter are not working better than the dirty ones that were in there before
• maybe the engine management computer hasn't yet learned the ideal long-term fuel trim for the new injectors
• etc.

Since the car ran 292 km/h into a headwind and may have run into the rev limiter with the wind, the top speed might have actually increased with the new rear undertray. Without testing it on a still day or with longer gearing, I can't say.

In any case, the undertray didn't fall off and the engine didn't overheat but the gains (if any) are not yet obvious.

 

[john@johnhudakracing]

This morning there was no precipitation and very little traffic so I figured I'd drive to Germany for a top speed run.

If top speed is your core interest, then you should be focusing on stream lining the vehicle and lowering drag. Frankly, their are better places to start like the door mirrors.

A flat bottom or wing-in ground effect design with NACA ducts as has been discussed would not be the preferential design to accommodate this scenario. To have significant impact on your total CFD- ideally, you would want a low drag body design, sealed with a tapered rear end. Frankly, difficult to implement well on an NSX without significant changes.

The engine oil temperature didn't seem to climb any quicker with the rear undertray in place than without it. With the cruise control set to 138 km/h (86 mph) for three hours, the oil temperature was about 100°C - the same as it would have been without the rear undertray. When flooring the throttle for extended periods in Germany, the oil temperature climbed, but it didn't seem to climb any faster than before. At least at high speeds, the NACA-ducted undertray doesn't seem to cause thermal problems for the engine.

100C is nothing. That's 212F. Idling and doddling around temp.

Your on the street. It just really doesn't matter. Even on the autobahn their is no genuine need for NACA ducts. You are not going to be getting the car hot enough for long enough to require supplemental cooling as is the case on a road course.

I tossed a remote temperature sensor into the trunk to measure how hot it gets in there. The temperature was 13°C when I started the car in the morning. After three hours on the highway, it had climbed to 19°C. After the top speed runs, it had gotten to 20°C. I insulated the trunk after reading about what Malibu Rapper and D’Ecosse did in this thread. Since then I've had a cool trunk and the rear undertray doesn't seem to have impacted it.

I am confused on this point. Who cares what the temp of the trunk is? Is their an elf living back there?

Since the rear undertray didn't fall off above 300 km/h (186 mph) it seems to be stable enough.

Good to hear. Still, I would suggest checking it regularly to ensure it is positively attached. Pre-flight checks are always sound advice at those speeds.

 

[stuntman]

Greenberet: What year is your car/engine modifications have you done?

300+kph is awesome!