Andrew, no number plates on that car? Im guessing it dosent see the street.. that front lip would get ripped off the first time you tried to turn into a driveway!

 

You are putting this through your insurance?

 

Maybe some sort of indicator light on the dash? You can use a small pressure switch under one of the pins to short it. Put a spring on the pin so that the hood won't trip the switch unless it is compressed by the cotter pin.

 

The only thing I like about the hood side vent concept is that early prototype 928s used side openings in the hood for air intake. Can't say it looked cool, but it was certainly different from what we know.

I suspect wind tunnel testing scrapped that location.

I like the 968 RS hood vent, which is similar to the 962 you shared... however the '68 postdates your build.

I've always thought it would be neat to graft a 911 hood onto a 928.

 

Yeess Dear..

My wife asked the same thing.

I'd never trust it anyway so probably not going to happen,
but nice idea.

I thought the same but the air collector for fresh air on the 911 hood would be flipped 180 to be the outlet for hot radiator air. The screen could be used too.

Well now i'm thoroughly confused.
I cannot believe i did this to my car Jim.

 

Now you're talkin'!

 

Matt,

I was out of town and missed your post originally. Sorry to hear of this! Thank God it wasn't worse. Being blinded suddenly at speed can be deadly.

I know you will get it sorted and it will be better than ever!

 

Had a couple of tech questions for you, so we may be if you pick up this weekend.

One can buy the 968 Turbo RS hood duct. I think it could be made to fit but it may be ugly or it may be really cool. hard to say..

Thanks Guy,

Your opinion on the below 968 Turbo Rs duct is appreciated.

(if used on the 928) I think i'd use the Porsche hood badge with it though.

I know it would be effective, i could duct much of the radiator air up and over the hood with secondary duct work under the hood because there is room in there.

But i have a hunch it may be a bit much to look at. Don't know..

 

I wasn't a big fan of the raised louvers in the hood. I do like the turbo duct better. It has a modestly more elegant look to it. Sorry to hear about your hood damage.

My Ferrari has louvers just behind the radiator to let heat escape. It works remarkably well. The only issue I see with the 968 Turbo duct is that it's not that big. But then, neither were the old louvers. If you look at the 917 heat exhaust, it's important to make it large enough to do some good. There is a significant amount of adiabatic heating as the air goes through the radiator. If the duct is too small, it may just make a heat riser pressure bubble behind the radiator which would defeat the purpose entirely. As it is now, the air leaving the engine bay at the bottom is drawn away by the air under the car, and there is an area of low pressure right behind the tires that draws the hot air down that way.

As I recall, this was a problem with the prototypes of the P-51 Mustang which uses a water jacket and radiator to cool the engine. It was re-designed and eventually there was a net thrust washout due to the exhausting of the hot air from the radiator back into the slip stream. Aerodynamics don't always work the way we think they do.

 

Can the damaged hood be bent flat enough and secured in place well enough that it could serve as the guinea pig for fitting whatever ducting you decide to try? It's already the right color...

 

I always re read your posts doc, thanks I agree.


doc is saying it could be good if big enough,
but you never really know what is going to happen from an aerodynamic
point of view.

Rob is saying test with the old hood.

That is what i will do. I'll find ways to see what is happening aerodynamically short of building a wind tunnel in the back yard.

Anyway, i taped off the correct location on the bent hood. It falls between the intake suction hoses, the fan shroud and motor.
It's perfect in terms of low pressure area on the hood. Almost crying to be done.
I'd duct down from the hood opening in sheet aluminum to try an catch the top half or more of the hot radiator air..
The rest should go out under the car. I would think it would be more stable at speed this way but less kid's finger friendly..

I actually think it could look really nice too.

As soon as i am sure the adjuster is not coming back i'll get started.

pics of mock up done with paint, (shape is what it is due to suction tubes shape) but i'd like a shaped opening
more like the 968 or simple like the 917 or possibly influenced by the OB 928 rear tail lights shape, which is awesome.

one pic shows the original vents in place with the 917 or 968 T RS inspired vent.

naca ducts over the suction tube end locations would be a bit much but i am very tempted. I'll mock that up too to see.

Thanks for the help doc and Rob, it is appreciated. -Matt

 

Spot on. I forgot about the pressure graph. Sometimes I get lucky.

 

When working out stuff like this on budget race cars, it's not uncommon for us to resort to tape and string. A row or two of taped thread will tell you quite a bit about how the air is moving around that section of the car. If you have a decent camera you can mount to film the action, you can even review the data later. While the "live observer" method works, I prefer the more detailed feedback a recording provides.

 

Yes, it's the perfect location. Doc since you're around right now..

What are your thoughts on the 928 intake plumbing design: the air must flow from the front of the car all the way across the engine compartment, thru the air filter, and then back forward again before it even gets to the engine.

Would a plenum or two next to the stock air box that pulled fresh air from above (through hood) in the high pressure area near the windscreen be a better performance design for a nice turbulent air supply to my motor.

It would be cooler air, the plenum could be designed to drain out any water that entered. Your thoughts?


I like the way you think eroshi, i agree 100%

 

Heh, man yur kinda putting me on the spot a little bit here! I mean, on the one hand we've got Porsche with decades of design and performance engineering, and actual wind tunnels, and baro-graphs, and then me out here in the real world with no data at all to back stuff up.

Having said that, you and me, and many others have seen the empirical evidence that the intake geometry is pretty convoluted, if I may use some four dollar words. Mooney aircraft designed their intake on the small planes so that at cruise they get back about .7" of manifold pressure due to ram air effect. Pretty smart there.

There's a lot of physics involved with the airflow in conduit or chambered systems. You have laminar flow, boundary layer flow, eddie currents, stalled air, pockets, etc. To put it mildly, it's darn complicated. If you've ever seen the intake of an S4 after 150k miles, the air flow patterns are weird.

I guess I will say without being too committal that there's improvements to be made. We had some folks locally make some spacer plates for the sides of the S4 plenum, and the difference in flow rates was negligible as I recall.

There are two areas to improve, the first is temp input. Using PV=NRT as our base, we can postulate that Manifold Pressure is fixed(ambient), and the Volume of the chamber is fixed(non-S4), so that any reduction in T will require an increase in N(number of molecules) to balance the equation. This is of course, all in front of the throttle plate. So, any reduction in Temp is a good thing, and that's why the ricers have cold air intakes. The 928 could really benefit here because all of the air going into the engine sits up top where it heat soaks before reaching the throttle. one thing that helps is to remove that insulator blanket on the hood, and leave it bare so that heat will exit the hood radiantly. Since we're not dealing with steady-state air, but air flowing air, we discuss that next.

In a perfect system, we would have a smooth, continuous column of air reaching the throttle plate all in laminar, continuous rate speed delivery. Of course, we are far from a perfect system. We have a ton of bends, and different sizes plenums and ports, and also differing surface finishes along the way. The old hot rodders of the 60s used a simple velocity stack to straighten the air coming into the carb, and that worked quite well. Since were not tootling along in the air at 150 knots, we are on the ground with all the dust and grit, we gotta have a filter, and since it's a Porsche with a V8, we gotta have some bends.

Without going into a flow analysis, there seems to be some improvements that can be made before the throttle plate. I recall another 928 DIY guy that made up a huge plenum on top with individual throttle bodies leading directly to the intake valve. That seemed to flow really well, but I can't recall who it was. I guess I could search for it, but I'm not that invested.

All I can say is to maybe drill and tap a sensor port a few cm up from the head, and put a gauge on it, then start testing. Try to reduce the number of bends involved, and the increase the radius where possible. Try to keep it as cool as possible, and where you can't keep it cool, insulate the chamber. I think I remember seeing some of the air duct intakes that have an insulator on them, and that can help as long as the air inside is always moving. I won't get into intake reversion due to valve operation, but that's another area where things get a little cranky. This kind of stuff is so hard to measure without a magnehelic gauge, some flow gauges, temp gauges, and a dyno. It's basically modify and test, which gets costly. I don't think this helps, but maybe gives you some ideas. The old carbed Porsche had downdraft IDFs with the velocity stacks and they ran really, really well for what you got. Of course, back then it was state of the art. Good luck if you try something, maybe search for the guy that did the intake plenum, and copy that. As I recall it worked out pretty keenly.