bump on firewall - why

I am making a new firewall for the new build and see that my CMC has a semi-circular bump on the firewall protruding into the engine bay

My other car is Suby powered so I cannot see why this protrusion is there. Is there a valid purpose and why should I just not make a flat firewall

Anyone ever wish the firewall was relocated as now is the time

Build manual has you cut that 1/2 moon piece out to allow cooling air to T1 engine fan. Yeah big dead space between that firewall and rear seat. Toyed with using a boat access plate to make a glove box in rear seat area for camera/tools/beer or even gas heater.

https://www.speedsterowners.com/library/cmcmanual2/images/cmc43.jpg

I removed it outright. That's where my battery is now, along with my 6AL box and coil.

I elected to cut out the rear seat area so as to fit a dry sump, and do the wiring, and a few other things. Each of the cut outs will be fit with an aluminum panel held by Dzus fastners when the car is completed. I plan to place a fuel injection control module beside the power distribution panel as well as the electronic ignition. I notice some of the European makers of Speedster replicas do make this a storage area.....it is a lot of space to ignore.....

i cut mine as per the instructions, ran like that for a while, the realized it did nothing. sucking "cold" air from beside the exhaust it not a good idea.

so when i pulled my motor, i cut it off, fabbed up a wall and completely sealed the engine bay (just like the beetle). two huge fresh air vents in the decklid provide more than enough cold air.

i used the space between my firewall and back seat for battery / ignition.

Jim - LOL. I wondered what Cory did with his SawZAll when he deployed! Now I know. Sure makes changing the starter, adjusting the clutch, getting to that upper engine-trans bolt and draing the trans easy. Guess you aren't worried about sound deadening or water intrusion. Cory used some kind of flanging tool to put ridges on is aluminum panels - perhaps if you did same to make a lip you could add weather stripping. Plus the ridge would keep panel from drumming and stiffen it up some.

And I was kicking myself that I made my tunnel inspection plate 1/2" too big.

Wolfgang, the plan is to use the fiberglass cutouts as backing for .040" 6061 aluminum panels, with the rolled stiffeners as you mentioned, bonded together with rivets and epoxy, with weather striped edges.....a hinge will run along the backside of the "seat" as well as the large back cutout, and will all be retained by Dzus fastners. I am not real concerned about noise, the intended look will be faux racer or outlaw if you prefer. The engine I built a couple of years ago and used in my Spyder for 10,000+ miles. At about 220 hp and very little muffling I am not expecting quiet.... the panels should be pretty close to watertight, but as this is my 3rd Speedster (and one Spyder) I know better than drive in the wet.....just find a dry pub and wait it out....

Stellar work, Mr. DuBois, as always.

Wolfgang, the tool is a bead roller. There's nothing better to keep sheet metal from -- as you said -- drumming. But there is a special place in Hell for the cheapy Harbor Freight model with a missing tooth in the machine.
Roll-roll-roll-bump sucks when you're making aluminum panels. I learned on my car and experimented on a few race cars -- no money changed hands; it was a good learning experience -- and eventually got good enough to begin doing work on show-quality cars for friends.
It's an easy tool to use, but it's all about prepping the piece of metal first.

Robert, if you cut out that entire piece, you'll want to leave an inch or two of remnant fiberglass to support the outer body like frames in a boat hull. I left a little, but not enough in one specific spot. I have a small crack there now, and some filling to do.

Sucking hot cooling fan air through the firewall per CMC does not seem such a great idea.

Using the space between the rear seat and firewall is in the cards, and an ideal space to house the Megasquirt ECU/relay board, USB amp so there is no head unit hanging under the dash, and maybe a sub-woofer. Speakers can go in the sides of the rear seat area.

But how to do the firewall so that the engine can suck cooling air? Not only is there the exhaust, but an oil cooler shedding heat, and sucking cooling air from this area seems counter productive. The easy way is to seal the engine bay per a Beetle, but I do not know if the single engine cover grill will flow enough cooling and combustion air - I suspect not in 35 degree C weather if idling.

I have my own mould so making a dual grill engine cover is not out of the question if this will solve the (perceived) problem. I could also open up the inside of the cover to provide max open area.
Does the dual grill cover use 2 single grills mounted side by side?

I am welding in a cross brace today following the curve of the body 3.5 inches forward of the engine bay cover. This will act both as a body support, and top firewall mounting point.
Now where to suck the cooling air?

The exhaust is behind the firewall so I don't see how cutting a hole in the firewall would suck in hot air...with the exception of when idling. If a concern, a scoop could be fab'd that would catch air behind the rear seat and direct it right to the firewall opening.

If you don't want to cut a hole in the firewall, an option is to install ducts that pull air farther front and direct it right to the air cleaners.

My thoughts were like Bruce's (great east coast minds think like?) When you're moving the air comes off the flat bottom floors (following transaxel) thru that hole in center - and on sides cooling the exposed exhaust. The spent heated engine air goes out the engine lid vent. Plus there is a rubber splash skirt th partially seal off the sides (if you followed the build manual). Don't have wind tunnel and remote detecting thermometers (so where is Frazier when ya need him and his lab?) to prove. Remember old debate about leaving tail gate down on pickup trucks? Air flow seemed to create a vortex that negated any benefit. Seemed when I did it to blow out debris - it all ended up behind cab and I lost none. With oil cooler mounted to one side or in wheel well it wouldn't affect this flow. I recall 914s having some small platic scoop added in later years to catch air flow under car and pass it by oil cooler - don't recall it on my '72 though. I've thought of cutting out fiberglass behind rear plate and mounting plate on a hinge - at speed it would expose the vent hole - kind of James Bondish too as plate id would not be visible at speed.

On moving chassis support pieces - I wish that rear side to side cross brace behind the engine were back 2" so a flat T4 with OEM cooling would fit easier and have a rear brace.

I left a gap in the firewall about a half-inch by two feet at the midline of my fan. It's enough to keep cooling air coming in, and the DTM outlet is four inches lower than that. The DTM blows out on the driver's side, directly into the wheel well.
There's a metal shelf for the ignition system on the forward side of the firewall, with a Lexan partition in the aluminum separating the two sides.
It looks nice, it doesn't impede flow, and wasn't terribly expensive to make. The whole works is framed in three-quarter-inch box tubing.
The third picture is Tom DeWalt's coupe. We made his engine surround in four pieces out of galvanized tin and edged it in high-temp rubber. The picture is ony posted here to demonstrate how clean the install was -- and how simple the pieces were.
It was entirely done with hand tools and existing materials. Time spent was less than two hours.

So are CMCs only moulds with this "advanced engineered cooling feature"? Can't picture them doing any R&D or being smart enough back then. Would think their predecessor FiberFab would have and going back to first moulds - the early IMs. VS? Chamoix-Becks?

Is that a heater boost fan with all the duct work in back of Tom's. I know the galvanized duct work time and the painted trailer side sheeting is quick and cheap. But always scares me danger of a nasty slicing cut. Sheet 1/16 Aluminum is better, I think. Plus it can be polished, painted or tooled - or left as is. I bet it dissipates heat better too.

Another option:

This graft in option would be great IMHO,

Too bad Rustytubs is so difficult to deal with.
TR Speedster have a similar rear section, maybe they would sell you that part.


http://www.rustytubs.com/porsche-550-kit-car-top-convertible-550.html


Cheers, THOM