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If we keep talking about exhausting air out of the (sealed) engine compartment (you know, the one with carb throats ingesting air and a giant fan sucking air to blow out the bottom), I think I might blow a gasket.

The problem is getting enough air IN the engine compartment. That's the secret sauce - getting air in.

Perhaps the reason the Spyder louvers point backward is that the Spyder engine compartment isn't sealed, and the rear wheelwell is positively pressurized? No idea -just a guess.

I'm 99% sure the Trans Am hood scoop was sealed from the factory. You had to remove the blank-off plate to get it to do anything. I think the general sealed it up to quiet the car down. Removing that plate was boy-racer modification number 1 back in the 70s.

Last edited by Stan Galat

99% of the replicas have an unsealed engine compartment in a Spyder.

But all the factory OE cars had an underpan. It's debatable that they were sealed, there were a lot of louvers, brushes, exhaust, and moving suspension parts going on under there.

I do know that the louvers in my car allow the intake noise to carry right to your ear canals...how air flows whichever way: who even knows?

I wouldn't be surprised if the rear wheel wells had positive air pressure at speed. I'd push the easy button and one put the inner section of that louver set in the sidewalls of the engine compartment through to the wheel wells facing "backwards". I'd not use the louvers at all and avoid all that bodywork. Baffling for rain intrusion would be pretty easy as discussed earlier.

Airflow, especially at the rear half of any car, is hard to predict.  Measurements taken at 3" from the surfce won't tell you much because it's outside the boundary layer of airflow that the louvers live in (unless the boundary layer has separated that much from the body, oh-oh! Now it's turbulent flow, see below).

As much as we would like to have a laminar flow over the entire car, by the time the air gets to the rear fender arches its probably already a turbulent flow. Even if it were laminar the louvers would create local turbulent flow. That's important because it would reduce flow through the louvers. That's another consideration, what is the area of the openings in the louvers? You can't ignore that if you're gonna be concerned about how much air flows through the inlet on a turbo hat. Whatever that area is, the turbulent flow will reduce its effective area considerably. We'd need a wind tunnel and a room full of sliderules to figure it out exactly, or a super accurate solid model of a 356, a big computer, and someone good with fluid dynamics software.

To make matters more complicated we are trying to duct air into an area of lower pressure. The question is how much lower? We have the airpump of the engine and the air pump that is the fan removing air from the engine compartment and moving it into the air flow at the rear of the car via sled tins and exhaust, so we can reasonably assume that the air pressure in the engine compartment is lower than outside of it when the car is at a standstill, and it will remain so until the speed is high enough to create a larger low pressure zone at the rear of the car. At what speed does that happen? I don't know, but I'll bet it's fast enough to get your attention. I don't think Porsche could afford a wind tunnel in the 50's, but they did have hole saws, sheet metal shears, and inmagination. They did the best they could.

The question of these louvers is, to me, mainly one of aesthetics. You like the look or you don't. I don't, but it ain't my car and for all I know they might help.

You could put on yarn tufts. You'd need to film from an another car far enough away that it didn't influence the airflow. If the flow is laminar the tufts will lay flat in the direction of the laminar flow. If the tufts lift and dance that's turbulent flow, and how high they lift will give one an idea how thick the turbulent flow is in that area and how much boundary layer separation has occurred. Also, they will do different things at different speeds.

It would be interesting, but we're talking about such marginal gains that I don't think it would matter much.  There's a reason they stuck F1 snorkles way up in the air in the 70's.

I do things a bit different on Speedster builds, I leave a 1" - 1-1/4" air gap at the base of the engine firewall and assume doing that the engine fan pulls clean cool and also gets additional airflow.  With the functional side louvers, the firewall gap and deck lid grill there has to be a lot of air movement and hopefully enough to keep the Outlaw project's 2165 cool ( also have a 72 row fan cooler in the wheel well)   

Last edited by Alan Merklin
@imperial posted:

Who is the Aircraft engineer in the room ?

what do we need to check airflow ?  Is it a change in air pressure or ?

might be interesting to know , and also airflow from 911 Porsche style shrouds etc….

You could put on yarn tufts. You'd need to film from an another car far enough away that it didn't influence the airflow. If the flow is laminar the tufts will lay flat in the direction of the laminar flow. If the tufts lift and dance that's turbulent flow, and how high they lift will give one an idea how thick the turbulent flow is in that area and how much boundary layer separation has occurred. Also, they will do different things at different speeds.

It would be interesting, but we're talking about such marginal gains that I don't think it would matter much.  There's a reason they stuck F1 snorkles way up in the air in the 70's.

@aircooled Bruce did a whole series of tests using his Manometer that measured positive/negative airflow across various parts of the Speedster body. He published those results on the site many years ago.

I tried to search for "manometer" and "man-o-meter" but didn't find the thread. I'm sure I'm spelling it wrong but maybe he has the link handy.

It is Manometer Robert.  The two posts were on 8-3-2015 and 7-24-2015.  on th 7-24-2015 post I noticed that I mdid mention that I tested air pressure right on the paint surface of my car in that area of where the vents might be with our 3" or 6" stand-off's. The readings were: 35 mph was +.25 ", At 50mph it was +.75", At 60mph it was +1" and at 75mph it was +1.50". (all in H2O). You can read what they were at 3" and 6" in that post as well.

Some mentioned the wheel well and the void area in front of the firewall.  I measured in these places too.  The right wheel well (inboard of the tire) read:  +.75"@50mph, +1"@ 65mph, +1.5"@75mph

The forward firewall area measured: +1.25"@50mph, +1.5"@65mph and +2"@75mph.

The only place I got negative readings was in the engine compartment of -1 to -1.5" both driving at speeds listed and at 3000rpm floor idle.  My engine compartment was meticulously sealed except for the bonnet grill. I believe that the "rain guard" under the grill is what causes this negative reading. If you just open the bonnet, that restriction disappears. you can watch it on the Manometer appear and disappear when opening and closing the bonnet while the engine is at 3000 RPM.

I never, ever had an overheating problem in Rhonda.  That was with all 4 cyl heads monitored and the oil in four places........Bruce

P.S.  Don't ask me about all the other stuff I F----d up though !

That's easy.  Nobody has.

^That.

I keep saying the same thing over and over. Getting air into the engine compartment is the secret sauce. Erwin Komenda never got it. The little louvers on a GT were a cute nod to needing to do something. To my knowledge, nobody (including the luminaries in our world - Rod Emory, et al) has given it more than a cursory "common sense" effort. We're all just giving educated guesses - even with a manometer, which are devices famously influenced by a lot of different things (including orientation of the pitot tube, turbulence, etc.).

The root problem is that the engine and all attendant cooling apparatus sits in a giant low pressure zone. A 12" pipe running front to back (through the passenger compartment, as in the picture @ALB will hopefully post) would help a lot, as would putting the engine in the front of the car, or maybe racing around in reverse.

Since it's in such a bad place, anything we can do to help... should help, but none of us know more than Erwin Komenda did. I'm a giant fan of these side louver panels, just as I am of cutting out the rain tray and the space behind the license plate, and any other modification that can be made to get outside air FROM THE TOP SIDE OF THE CAR (not underneath in the turbulent mess that is the underside of the car (you know, the place we're dumping all the hot air from the cooling system) to the engine. But, I can't tell you if it works. Nobody without the engineering horsepower of a manufacturer could tell you for sure.

I'm not sure what the answer is, but I know it'd make probably a 10- 20+ hp difference to get it figured out,

Last edited by Stan Galat

Hi guys.  This is a little bit off topic from the side louvers, but if I'm getting my VMC speedster with the larger 2332cc engine, is the optional rear louvered decklid advisable to provide more air flow or is not necessary?

I like the cleaner look without the extra louvers, but if the 2332cc engine runs hot and they're needed, I'd reconsider them.

Let me know what you guys think as I'm a newbie learning about the Speedster replica platforms.

Thanks

I don't know if Greg totally seals up the engine compartment and if he still puts the 4" hole in the firewall.  My guess is that he does but you should ask him to be sure.  Kirk, the former owner of Vintage Speedster that was in the same location as Gregs place is today, did not adequately seal it.

In any case DO make sure it is Tightly sealed up Completely so that the Only air that gets in there is thru the bonnet grill.  What Robert and Stan said above if Good info and advise.......Bruce

@Stan Galat posted:

^That.

I keep saying the same thing over and over. Getting air into the engine compartment is the secret sauce. Erwin Komenda never got it. The little louvers on a GT were a cute nod to needing to do something. To my knowledge, nobody (including the luminaries in our world - Rod Emory, et al) has given it more than a cursory "common sense" effort. We're all just giving educated guesses - even with a manometer, which are devices famously influenced by a lot of different things (including orientation of the pitot tube, turbulence, etc.).

The root problem is that the engine and all attendant cooling apparatus sits in a giant low pressure zone. A 12" pipe running front to back (through the passenger compartment, as in the picture @ALB will hopefully post) would help a lot, as would putting the engine in the front of the car, or maybe racing around in reverse.

Since it's in such a bad place, anything we can do to help... should help, but none of us know more than Erwin Komenda did. I'm a giant fan of these side louver panels, just as I am of cutting out the rain tray and the space behind the license plate, and any other modification that can be made to get outside air FROM THE TOP SIDE OF THE CAR (not underneath in the turbulent mess that is the underside of the car (you know, the place we're dumping all the hot air from the cooling system) to the engine. But, I can't tell you if it works. Nobody without the engineering horsepower of a manufacturer could tell you for sure.

I'm not sure what the answer is, but I know it'd make probably a 10- 20+ hp difference to get it figured out,

What Stan  says here (and I agree that more air IN is the answer) leads me to the questions:

1. is this why they went to the twin grill on later 356?  bigger opening so less of a restriction... kind of like cracking the lid 2 inches?

2. would NACA ducting work to "pressurize" the engine compartment? only works at speed, but then that's when you need more air in because that's when more air is being consumed by the ICE "pump" and fan..   (although these are not really in the period style)

3. do the direction of the louvers impact air flow?  It does with NACA ducts..

In the late 90's RAM air intakes, ducting and sealed air boxes on motorcycles were commonplace..so I saw a lot of what that did for horsepower and the different ways manufacturers went about solutions..  BUT there's a lot of Engineering Horsepower required to figure all this out WELL (While I get the theory, and basics, I understand enough to know there's a whole lot I don't know).   Location of the inlet, shape & size. The flow path duct length-shape-size, the Airbox size and turbulence in the airbox, all variables that are interdependent and relative to speed...  I've done a little computational fluid dynamics analysis on ducts (of the HVAC variety) and some other fluid flow analysis..  but nothing with this many variables..the intellect gets a little dizzy from here but it's a great conversation:  "Never go in against a Sicilian when death is on the line."    https://www.youtube.com/watch?v=rMz7JBRbmNo

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