Homemade fuel tank undercover

[greenberet]

Since I had some polycarbonate sheeting left over after making a front undertray, I decided to make an undertray for the gas tank as well.

I wanted the gas tank undertray to:
1) mount to existing mounting points so that no holes would have to be drilled into the bottom of my NSX
2) have its leading edge not sticking down into the airflow so that it doesn’t catch air
3) be mounted stably enough that it doesn’t fall off at high speeds

Looking at the gas tank area, there are unfortunately no existing mounting points that you can just screw an undertray to (it’s a lot easier mounting a front undertray). Downforce offers a fuel tank undercover, but it requires you to drill holes into your NSX and at least parts of the leading edge do stick out into the airflow, as far as I can tell.

The most practical method I could think of to mount an undertray was to make some little cross members in the gas tank area and then make some tabs to attach the cross members to existing mounting points. As you can see from the pictures, I have two cross members holding the leading edge of the undertray and one cross member at the rear. There are two tabs holding each cross member to existing mounting points and the rear cross member is additionally attached to the chassis directly at the two ends.

To make the cross members, I bought some L-shaped pieces of anodized aluminum (25mm x 25mm x 1.7mm) at a local hardware store and cut notches into them wherever they would have otherwise hit something when mounted. I just held them into place, marked them with a marker, and made the cuts. The tabs are also made of anodized aluminum (20mm x 2mm). To mount the undertray to the cross members, I drilled holes through both and then fastened them together with some OEM NSX push-clips. In addition, I used two cable ties in the middle of the leading edge where there’s no cross member, between the chassis’ “center rods”.

In the end, the gas tank undertray ended up being 125cm x 47.5cm x 2mm. It nestles in behind the hump in the floor just in front of the gas tank and stops before hitting the header for the front bank of cylinders. It ended up costing very little. The 205cm x 125cm x 2mm polycarbonate sheet I bought for about EUR 71 was big enough to make both a complete front undertray as well as a gas tank undertray. The aluminum for the cross members and tabs cost an additional EUR 20, if I remember correctly.

I’ve had the gas tank undertray on for a few weeks now, including some high speed driving, and it hasn’t cracked, popped off, or melted. I’m not sure how much it reduces wind resistance, but theoretically, it should help at least a little bit.

 

[greenberet]

some more pictures...

 

[PhiAlpha44]

That looks really good man. Good job.

 

[goldNSX]

Thanks for sharing.

Just wondering: Does the front cylinder bank get enough cooling air?

 

[ChopsJazz]

Great project. What are the plastic down-tabs in front of the rear wheels for?

 

[greenberet]

Thanks!

I'm not sure how much cooling air needs to circulate around the front bank of cylinders and I'm not sure how much air usually gets in there past the front engine crossmember and the bulkhead. My headers probably radiate less heat than most because they are ceramic coated. In any case, you can see how much space there is between the fuel tank undercover and the headers in the attached picture. If I would have mounted the undercover to the rearward bolt in front of the wheels, it really would have been almost touching the header (the Downforce undertray mounts to the rearward bolts). Since that was too close for comfort for me, I made the undertray a bit less deep and mounted it to the forward bolt in front of the wheels.

Now only the air dams in front of the rear wheels are mounted to the rearward screws. Air dams in front of wheels supposedly channel air around the wheels, which create turbulence as they rotate. By having less air get turbulent, aerodynamic drag is reduced. At least a little bit. Hopefully. Modern Porsches are very aerodynamic and they have air dams in front of all four wheels. My BMW has air dams in front of all four wheels. They can't be all bad so I put some on my NSX as well.

 

[MvM]

Thanks you for sharing.
I have been thinking about making the same thing as the available stuff is a bit big on $$$, but never got around to it. Never could figure out where to mount a plate like that.

Thanks also for the small airdams in front of the wheels. That should be an easy thing to do. And as you say, all little bits help.

The only thing you now have left to do is make a kind of sheet cover and diffuser to cover the part of the rear end from the most rearward suspension arm to the rear valence :smile:

 

[greenberet]

An undertray that goes from the rear suspension arms to the rear valence would definitely be a nice thing. Aerodynamically, a rear undertray that is functional would be great.

It would trap heat from the exhaust system, but making the undertray out of aluminum should allow it to withstand the heat. When my car was on a dyno, the exhaust pipes leading from the catalytic converters into the muffler glowed red hot. Covering that up with an undertray would make the trunk at least a little warmer when driving on the Autobahn. Also, I'm not sure how efficient a rear undertray would be if the leading edge were just standing free in the wind between the rear wheels. If it were in the airflow and caught air, maybe it actually would work like a parachute, increasing the car's drag coefficient instead of decreasing it.

To find out if it is really functional, you could mount one and then compare the pressure of the air above the undertray with the ambient air pressure. If the pressure is higher above the undertray, it is probably swallowing air. It would be great if it actually worked, though. Hmmm...

Regarding how close my fuel tank undertray is to the header, you can see in the attached picture how close it would have been if I had mounted it to the rearward bolt in front of the rear wheels. I held it in that position and it would have fit, but with only a few mm of clearance to the header. Considering how much heat the engine produces, it didn't seem like a good idea to mount it that close. The forward bolt seemed close enough. Also, I positioned the vertical portion of the cross member so that it faces the header, acting as a bit of a heat shield for the push-clips and the polycarbonate.

 

[Rutnsx]

I think the NSX-R has a complete undercover but not by the engine.

 

[MvM]

Hi Andreas,
I understand what you mean.

I was thinking about an undertray like the BIG Downforce diffuser with the long edge. I think they mount it to the most rear suspension arm, so it doesn't cover up the engine. It does of course cover up most of the exhaust, but you.
The JGTC cars have the underside covered with plates. The rear plate which also acts like a diffuser has round holes in it. I think this is to let the heat out and still have they effect of improving downforce.

Update: Here is a picture of the DF big diffuser. It's NOT mounted on the rear lower suspension arm but just before it.

 

[greenberet]

In this thread, the 2002 NSX-R show car rear diffuser was discussed, which was a real diffuser - not the atrophied little stump of a diffuser that made it onto the production version (the picture below is of the production version). According to Detlef, Honda didn't mount the large rear diffuser because of heat problems. He also said that Procar will produce a real rear diffuser for the NSX if they can get the problems worked out. I look forward to seeing if Procar produces one and if they do, what it looks like!

2002 NSX-R underbody:

 

[Jeff@Carbon6]

nice job. I'm interested to see how it effects the street drivability (heat etc.). I have a very similar project myself.

Big props' for doing it yourself.

 

[Mac Attack]

I like it!

If worried about cooling near the headers and oil pan, you can always add a NACA duct in that area :smile:

But, I'm guessing that you probably won't have a problem. It looks like there is still plenty of space for air to circulate. Find a small digital temperature gauge (for your home, etc.) that will record the maximum temperature, and has a high enough range (you can get an idea of gross temperatures using an infrared gun on the lower firewall). There should be plenty of room on the lower firewall where you can mount it securely (and not worry about it melting) and go for a drive. Readings before and after the install on a similar test circuit should give you conclusive results.

 

[greenberet]

I also think there should be enough space between the fuel tank undercover and the header as mounted on my car. I've done some (very) fast driving with the fuel tank undercover in place and it doesn't show any signs of discoloration, melting, etc.

I'd be more worried about heat with a large diffuser that covered up the catalytic converters and muffler, though!

 

[1albagtp]

Very nice and tidy job!