Jerry Feather

There are basically two ways to chop a top, particularly with a car such as just about all modern cars, with sloping posts. One is to cut the material out for the chop and then bend the posts, top and bottom so that they will come together again at different angles. I think that that would be the process necessary to accomplish what the photoshop pictures seem to show.

The other way to do it is to cut the top in sections, 4 in our case, and then cut the material out of the posts enough to lower it 3 inches, then move the top sections down the posts and weld them back together. That results in creating gaps in the roof between the sections that are moved down because they also move out in opposing directions, and either forward or aft, depending, when they move down. There are two ways to deal with the top gaps. One is to fill them in with raw material cut and shaped to fit, or to fill the gaps with sections cut out of an extra roof, or another is to use the spare top and cut opposing sections out of it wide enough to fill the gaps. I was going to do the latter, one corner from the original car and another from the spare roof, but because of the curves at the apexes (apexi?) on the sides, front and back there will tend to be a very slight flaw at the joints, kind of like the inverse point at the top of a valentine heart; not that exaggerated, but still something to have to deal with. I think now that the best approach will be to fill the gaps from the spare roof, even though that will involve more welding.

In the case of our project It is looking like the top sections will each move outboard about 2 1/4 inches and the front will go forward about 5 inches and the back ones back about 2 1/4 inches when they move down the front edge of the B-posts. Then the real complexity with the chop is the C-posts which is the top of the rear quarter windows and the sides of the hatch.

When that part moves down 3 inches and it creates a gap about 9 inches long before the post can be reconnected and keep the original curvature and slope. That is where some real interesting design and fabrication work comes into play. I'll describe that later, but I pretty much have it figured out.

Jerry Feather

Not too much in the way of other customization since my "model" as I described above, is to have it look pretty much stock but with the double take effect of seeing it then looking again to see that it is a long way from stock.

I will use the rear bumper cover that I developed with the narrow license plate recess. I'll be incorporating the Boxster side mirrors that fit in the front corners of the door windows. The hatch is going to be pretty much different and I am thinking that it is going to have a small spoiler along the front edge of the window. Incidentally, the hatch window is not going to change, just the hatch itself.

I am going to need to design a lower induction for the engine to clear the hood which is coming down 2 inches. I'm going to keep the rear WW opening the same; just move it up in the body, and the front ones will loose only 1 inch of height.

The car is red now, so I plan to leave it red. I have bought 5 true red hides and one a very very light tan to do the interior with. I have been working on a redesign of the seat layout with the separate cushions as the 928 has, but putting them longways instead of cross ways. They will be in the contrasting lighter leather. I am going to fabricate the headliner in two sections of formed plastic that will incorporate an overhead center console where I will put a special compartment for the radar detector and I'll put the window switches up there along with one or two speakers, I think. I'll do a different version of the flush center console, but it will end up being much more than flush. More about that later when we get there.

The S4 I am starting with is an automatic, but I really hope to be able to afford to convert it to a five speed. Either of the other S4s I might use to finish this up are autos, so that's I why I'll need to whole conversion "kit."

RennPartsDirect

Is that a real car or a Photoshopped picture?

Jerry Feather

Photoshop!

M. Requin

This is very interesting and fun besides. I am particularly interested in how you will modify the top to accommodate the wider effective spacing of the roof pillars. My guess is you will fabricate a jig/dolly to hold the 4 pieces of the top while you weld in new material to compensate for the spacing. That's probably not very clear, but it is in my mind's eye. I'm very interested also in your welding approach - I am pre-novice in MIG, and even less skilled in TIG, so it will be fun to watch what you do. Fun approach to 2016- thanks Jerry!

M. Requin

Also, go rad! I think those wheel well arches (as many have noted before) are way too dated...

Jerry Feather

The biggest impediment to doing the 3 inch chop when I first gave it some thought was in how to deal with the c-post next to the hatch. As mentioned above, when the top comes down 3 inches, if it were coming straight down it would require that the bottom end of it at the rear will need to move forward about 9 inches in order to come back together. Here is how that 9 inches is going to be dealt with:

First, when the rear quarters of the roof move down the B-post along its front edge they also are going to move aft about 2 1/4 inches. That takes up that much of the 9 inch gap at the rear.

Then, when the B-post is put together at its front edge, since it is kind of trapazoidal in shape the top and bottom of the chop cuts do not match and they are going to be different by and an inch and a quarter. That means that that much of the bottom of the B-post needs to be moved forward. In order for this to also translate to help in closing the rear gap the whole rear of the car, trans, drive train and all need to move forward. That means cutting the frame just forward of the rear cross member mounting point and across the inside of the car in the area of the rear seats (which are going to be eliminated anyway). However, since we want to kind of retain the proportions of the B-post, it appears that since we have to move the rear of the car anyway we will move it a full 2 inches thereby making the B-post a bit narrower, and then moving the top rear corner of the B-post and quarter window forward to match. That will give us another two inches of closure of the rear 9 inch gap. So, we now have about 4 3/4" more to take care of.

When we look at the rear of the quarter windows we see that they are actually at an angle to the car, about 40 or 45 degrees. When the top of the quarter windows moves down and outward (the C-post) that means that the rear corner of the windows must also be moved out. When they are cut free to move out, because they are at an angle they also move forward. This movement forward is about 2 inches. That 2 inches forward takes up another 2 inches of the original 9 inch rear gap.

The only thing left of the 9 inch gap is about 2 3/4 inches. When we look inside the rear hatch area at the rear floor we see a fairly broad flat panel there that is about 3 1/2 inches wide from front to back. We are going to take our last 2 3/4 inches out of that panel and then move the entire outer tail of the car, aft of the wheel wells, forward that amount. The object here is to be able to retain the spare tire well and the area for the fuel tank.

This is going to end up with the quarter windows significantly smaller than original, but I think they are going to be nicely proportioned to the overall new look of the car. It is also going to reduce the rear overhang by nearly 3 inches and that, coupled with the 2 inch sectioning is going to significantly reduce the big butt the 928 seems to be partly noted for.

There you have it. A lot of work for just a 3 inch chop.

Randy V

Here's some inspiration for ya, Jer. Hollywood is onboard to make the 928 Summer movie!

Jerry Feather

Thanks Randy. I am sad to see that your taste is only in your mouth. (Just kidding, because I know you are too.)

Jerry Feather

You are pretty nearly correct, Martin.
What I did when I did my 356 chop was to clamp a system of posts inside the cabin where the top of the posts were just exactly 3 inches below the inside of the roof. Then when the quarters were cut out and off and the top of the posts cut and dressed the gaps were closed with clamps and bar stock to keep them lined up and the posts were tack welded back together with the body of each quarter section resting on the top of the post structure. Then each of the gaps can be filled with material from the spare roof which are also tack welded in and later finish welded.

Jim Devine

If you chop the top, you can get the headroom back by dropping the floor pan. Rick Barry, the former pro basketball player had a Pantera that had dropped floor pans- it fit him well.

bran3b

I cant wait to see the results. I plan to make either my green 79 or brown 78 my custom, but nothing as ambitious as this, mine will be more rat-rod styled.

MjRocket

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Jerry Feather

One of the things that I learned when I chopped my 356, or actually two things, were first that after I had hammer welded the sections and filler segments into place I could go back over the welds with the hammer and dolly and the torch and work the joints carefully so that the roof was almost completely smooth and as nearly true as an egg shell. Where I found that it would oil-can I could either shrink it with the torch and hammer and dolly or I could stretch it just with the hammer and dolly.

The other thing I learned is that my ability to do that was limited to how far under the roof I could reach with the dolly in my left hand and still work the welds. I was left with a substantial amount of the weld joints on the more central area of the roof that I could not reach; and they are still not done.

The lesson I learned from that is that I should have devised a way to keep all of the welds out around the perimeter of the roof where I could work the welds with both hands. That will lend itself well to the 928 roof with the sunroof since that pretty much will require a whole new section of roof that I will graft into the middle area; and I'll arrange it to be far enough outboard all around to be able to reach the welds. The replacement section of roof for the center will likely come from some donor car at the junk yard when I find one with a roof, or maybe even a trunk lid, that has the right curvature both ways.

Jerry Feather

Thanks Jim for your input. I actually already mentioned that possibility somewhere above in this much too wordy thread.

I probably should have waited until I get the space in my shop cleared out and I could block the car up and start cutting before I started this thread. Then, much of what I have been trying to describe could better be shown in pictures.

Right now I am recuperating from having slipped on some ice and fallen on my back breaking two or three ribs last tuesday evening. I find myself surfing my computer with no ability to do just about anything else right now. However, after my ribs getting worse for three days, today I feel some improvement and hope it will continue. In fact tomorrow I fully expect to be able to form some Spare tire covers and put together the final pieces of one of my GTS Wheel Well Liner kits to ship.

Then I will get onto the process of cleanup and reorganizing some space for this project.