32. A breath of fresh air

If you’re building an SLC and you’ve opted to go with the race tail one of the more popular modifications is to add a fresh air scoop at the back of the car to feed your air filter.  Placing the air filter right below a center-mounted scoop is especially popular for those running LS based motors as the throttle body is just an almost-straight tube away from the filter.  Unfortunately the required 4″ tube will most definitely not clear the race tail and a hole must be cut to accommodate the tube and air filter.

I purchased a scoop for use on my project; unfortunately there was some asymmetry in the part that was driving my OCD completely bonkers.

Looking “into” the scoop; the drivers side (right side of this pic) is “drooping” relative to the passenger side.

To bring this side back up I added several layers of fiberglass mat and added bondo to smooth it out.  By the time I was finished I think I’d added about 1/4″ of material – a bit more than I’d expected when I first started recontouring the scoop.  I decided it’s time to learn how to make a mold and make my own part!

Pattern held onto the body using clay; I wanted to design the mold so it could eventually be used for vacuum bagging as it’s a skill I’d like to learn.  The body serves as a great flange pattern for a future bag and sealing surface.
Contour is now symmetric left to right; I have a compulsive need for symmetry and I just couldn’t let this one go.  I found I also needed to put a fair bit of effort into balancing the arc of the opening (when viewed from above).
I’m sure there’s a more efficient way to build this up, but I used clay to fill in the inlet.
Green coating is PVA release film; I had already applied 5 coats of release wax before applying the PVA.  I’ve got some kind of issue with the PVA, it’s too thick and running on me which leaves these areas of high buildup on the part.  Of course this is going to transfer to the mold.  I tried lightly hitting it with sand paper but it just tore the film; I may not have waited long enough before trying the sandpaper.  If anyone knows how to fix this I’d be much appreciative of your advice!  I had originally tried to apply the PVA using a Preval sprayer but it was not coming out smoothly – so I followed up with brush application.  Maybe some way of diluting the PVA would help …
2 coats of gelcoat.  This stuff goes on thick and levels fairly well, I’m really enjoying using this stuff!  It’s pricey but leaves a much nicer surface you can finish.
6 layers of fiberglass mat; this is the biggest layup I’ve done to date with a TON of learning (aka mistakes).
Letting the fiberglass cure overnight was tough, I really wanted to pull the mold to see what it looked like!
Well …  it’s not completely ruined … Pattern separated from the mold.  The pattern came out surprisingly easily with very little damage to the PVA film.  In fact, I removed it from the mold almost fully intact.
… and here’s the issue … bubbles … LOTS of bubbles … While using the Preval sprayer to apply the PVA, it had formed a TON of tiny bubbles – which then created these cavities on my mold.  No biggie, I’ll just lay a thin coat of resin down to repair …
Here’s what it looked like before I Frenched the $h!t out of this mold.  Ooops … bad idea to put a skim coat of resin on the mold.  What ended up happening was I made the surface even more irregular.  My once-relatively smooth pock-marked surface was now not so smooth.  I whipped out the sander and went to town … eventually getting the mold back into decent shape but leaving a good number of scratches in the process.  So I traded a whole lotta cavities for a whole lotta scratches … a good lesson to take here!
Trimmed and refinished mold sitting in the trash can as mold release is curing.  This was about how I was feeling by the time I got to this step.  I was really kicking myself for having tried the resin repair method.  Oh well, gotta break a few eggs to learn how to make an omelette right?
Laying up my actual part; 2 layers of glass mat sandwiched between two layers of glass cloth and all sitting on a healthy coat of gelcoat.

Once again, the part was relatively easy to remove from the mold – phew!  Just a few stabs with a wedge and that characteristic POP! of release.  Nice … I was pretty relieved to see that many flaws from the mold didn’t actually transfer over to my part.  Here again, the PVA filled a lot of the minor imperfections while making new ones (yay).  The net result was a pretty smooth intake scoop.  The gelcoat layer will gave me sufficient material to knock down most defects to get it smooth enough to quiet down that OCD that was screaming at me before.

Scoop contour is symmetric left to right – SUCCESS!
It started raining as I was pulling my part from the mold so I didn’t have enough time to clean up the PVA before snapping this pic.

Even after all that work to even out the intake, I found I needed to add just a smidge more bondo to my new scoop to get it symmetric left to right.  Next, I had to come up with a way to secure the scoop to the bodywork.  I had originally considered bonding the scoop directly to the gelcoat but decided I’d rather have it removable.  It’d be a lot easier to replace if it somehow got damaged in the future.

For as many build threads as I’ve read through, and as much internet sleuthing as I could do, this isn’t an area that’s really been discussed.  On builds where the scoop wasn’t directly bonded to the bodywork, it seems many builders run fasteners through the top of the scoop and secure via nuts.  I didn’t like how the fasteners were visible so I thought about bonding studs or bolts to the underside of the scoop.  Not having a lot of experience bonding fasteners to fiberglass, I wasn’t sure what the best way would be to ensure proper alignment and holding strength.  More internet searching on fastener bonding and I still came up empty.

So I went way basic and just glued nuts to the underside of the scoop.  I picked up a few adhesive mounted nuts from McMaster, PN 98007A029.  These stainless steel nuts are welded to a perforated flange.  You can bond the nuts onto a flat surface by laying down a thick layer of glue and pressing the nut into place.  My strategy had to be a bit difference since I want to float these nuts in space and bond then onto a surface that’s off at an angle.  Bonding them directly to the scoop meant my bolts would have to be driven at a very extreme off-plane angle relative to the bodywork – doesn’t sound very stable to me.

I placed 3 nut assemblies on stacks of washers on the rear clam and verified there was sufficient clearance for the assembly under the scoop (there’s not much room as you approach the edges!).  Once located, I marked each position and drilled 1/4″ holes.  I installed bolts from the underside of the rear clam and secured each washer stack/nut assembly in place.

Nut assemblies secured to bodywork.
I used 1 washer between the nut and bodywork at the rear, 3 at each forward location.

I placed washers between each nut assembly and the bodywork so I could create a gap, and orient the nut so it stayed parallel to the plane through which each bolt will pass.  The gap will be used so I can provide preload to the intake scoop – I’ll adjust the thickness of the washer stack to adjust the amount of preload at each location.  This will help get the scoop to sit down snuggly against the bodywork.

Next, I mixed up a batch of reinforced resin peanut butter and slathered a good bit around each nut assembly, making sure I squeezed resin through the perforations means it’ll hold the nuts in place and provide some anti-rotation strength as well.  I then placed the scoop on the bodywork and added a few pounds to keep the scoop in place while the resin cured.

A few pounds to keep the scoop in place.
Before applying the resin I had laid down several strips of packing tape to help release the cured resin.  It was a cinch to remove the scoop – and it seems the fasteners are firmly fixed to the scoop!

Once the scoop was free I went back in with more resin to beef up the mounting points; I also threw down a strip of glass cloth at each location – belt and suspenders!

Some popsicles to use as a form to prevent resin from spilling toward the interior.
Ugly but should work well to keep the nuts in place.  Wouldn’t want this flying off while driving down the freeway!

OK, mounting nuts bonded and secured!  I then needed to go back and get a better fit between the scoop and the contours of the bodywork.  I had roughed the edges and they were pretty good, but if you poke your head down and look into the scoop it’s clear you can see gaps between the scoop and bodywork.  There’s some amount of air that’s going to spill through these cracks – and I don’t think that’s a big deal – but I thought I could do a better job to get a really flush fit.

I thought I had done a pretty good job at getting it flush via sanding – but looking down at the gap and seeing just how much daylight snuck through was a good wake-up call; my sanding job wasn’t very good!

I applied a fresh length of packing tape all along the areas where the scoop makes contact with the body.  I then mixed up another batch of reinforced peanut butter and applied a thin line of it all along the mating edges on the scoop.  Next, I turned the scoop over and attached it to the body using my newly anchored nuts and let the resin cure.

A little bit of sanding and voila – a perfectly flush fitting scoop with invisible fasteners!

I had to do this in 2 steps, the small bead of resin I added to get it all flushed wasn’t quite enough and I had to follow up with some body filler.
Test fit; intake scoop is symmetric and sits flush against the bodywork – finally!
Upper curve of scoop looks good as well.  Some minor asymmetry left to right but I’ll live with it!
  • Intake scoop symmetric? – CHECK
  • Intake scoop mounting complete? – CHECK

Now to make it pretty!  I found myself staring off into the distance and my jaw went slack … it took me a moment to realize I was mumbling “car-bone …” over and over.

Car-bone … Seems it’s time for more carbon work!
Scoop painted and ready for some skinning.

Having learned from my prior mistakes with aligning this particular weave of carbon, I decided to try a new approach to laying down the cloth.  I transferred a straight line down the center of the intake using a sharpie pen.  With plenty of light on hand, I then rolled the carbon cloth down onto the scoop making sure to set the center strands down along my sharpie line as the cloth touched down.  Since the scoop is now shaped and sanded to size, I wanted all surfaces and edges to be as perfect as possible.  I grabbed the carbon cloth and tightly folded around each edge and taped it along the inside.  This will address any bunching or pull up along the finished edges and allow me to apply resin all the way up to the visible edges.  Getting a really nice and consistent wet out coat along the leading edge was going to be difficult so I figured this would be the most critical area to get a good wet out coat so I don’t sand back into the fibers.

In lieu of my trusty laser I opted to use a sharpie’d line to use for aligning the carbon cloth.
Tacked carbon.  I did much better this time keeping the centerline straight.  Seems the sharpie method was more effective this time around.
Excess carbon cloth trimmed then taped into place.  I want the carbon to end along the visible edge of the scoop so anything on the underside will be removed.  As I folded the cloth over and under I tried to keep the weave fairly taut to ensure it’s tight against the scoop.
Carbon skinned and ready for some epoxy resin.

To fixture the scoop for coating, I set it on a stack of 2x4s to set the orientation of the scoop so the majority of the surface is horizontal.  This will help keep the resin from flowing off the part before it can gel.  The more vertical contours will be thinner, but this can be addressed after the final coats of epoxy are applied.  One of the benefits of skinning carbon is the carbon sits below the surface; so any light or reflections coming off the piece will be largely determined by the epoxy contour, not the cloth contour.  The upshot is minor surface irregularities of the part itself or minor variations in epoxy thickness will be obscured.

Weave variations however …

This lighter colored section near the back end of the scoop is due to a shift in the weave.
It seems I may have pulled the fabric a bit too taut as I rolled it over the back.  It resulted in narrowing of some of the carbon thread bundles.  This variation in bundle thickness caused the lighter pattern to form in the photo above.

It’s absolutely INCREDIBLE how your eyes can pick up even the tiniest flaws in the weave.  Our eyes are so well developed at picking out changes in patterns that just the slightest change in how light reflects off the carbon draws our attention.  The good thing is – once the resin goes on the gloss and color that develops totally overwhelms a good majority of these flaws and they become unnoticeable.

Awwww yeah … !
Centerline is … centered!
Wet out coats down, time to let it cure before coming at it with the sander.  That messed up area near the back end of the scoop is no longer obviously visible.
After curing; I took it out into the sunlight to see what it would look like.  It’s pretty amazing to me how much depth there is to the weave.  I’m really digging this whole carbon skinning thing!
Colors are another interesting thing.  I’m mostly used to seeing dry carbon fiber which is basically black/gray.  With the wet look you get a ton more color – reflections from everything around and there’s almost a rainbow effect of light coming off the individual threads.  Once this fully cures and I can polish it, it’s going to look like it’s a sheet of carbon under a thin piece of glass – I hope!

Boy, talk about lessons learned.  I thought I had done an awesome job of taping everything back and securing the weave.  It seems the resin flows really well and a good bit of resin got in under my tape via capillary action.  When it came time to grind back and remove the excess cloth I had a pretty difficult time differentiating between the parent material, the carbon cloth, and the epoxy resin.  It made for a pretty tricky clean-up of the wet out coats.  I was eventually able to get it sanded back so the intake sits flush against the bodywork once again but it took way more effort than it should have – lots to learn to this skinning thing.  Those YouTube videos I watched make everything look so much easier …

After letting the resin cure for a week I hit it with 220, 320, then 400 before applying about 5 coats of clear.  I’ll let it cure for another week before sanding it to remove any orange peel then a polish to get it mirror smooth.


In summary – this intake scoop project has been a HUGE learning process for me.  I’ve gained a ton more insight into working and making fiberglass molds, using gel coat, wet layups, and skinning carbon fiber.  One of the things I really set out to do with the SLC was to push the bounds of my comfort level and learn new things (while buying new tools of course).  Composites has always been a black art for me; while I don’t claim to be an expert – or have much ability beyond novice level – I’ve definitely pulled back the curtain quite a bit and peeked into a really cool field/hobby.  I plan to pursue more of this kind of stuff in my spare time.  The SLC is always in need of more car-bone!



Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s