Guest Anonymous Posted November 15, 2000 Share Posted November 15, 2000 I just got done blasting up and down an access road to MSFC with a manometer...... The leading 1/2 of the hood had a fairly constant pressure profile - 1.5-2 inches of water at around 100mph over the whole surface. In the foot or so at the middle of the hood the pressure dropped rapidly to roughly 0 at any speed up to 100mph. From there back the pressure was constant at 0 all the way back to the trailing edge of the hood. Pressure in front of the radiator under the hood was very high, around 4-5 inches at 100mph. All readings were gage pressure, gage being inside the cabin of the car with windows up and heater on full blast. In layman's terms, this means the best place to vent underhood air out is in the rear 1/2 of the hood, and a hood scoop would be best placed in the leading 1/2 of the hood to take the best advantage of the dynamic pressure. But that depends on underhood pressures as well, perhaps next time I'll extend the other end of the manometer up to the engine bay to really get a good idea. String bit pictures coming soon! ------------------ Morgan http://z31.com/~morgan/s30 http://carfiche.com Quote Link to comment Share on other sites More sharing options...
pparaska Posted November 15, 2000 Share Posted November 15, 2000 Cool! I want that data on the underhood pressures. Can't wait to see pictures of the hairy Z . Quote Link to comment Share on other sites More sharing options...
Drax240z Posted November 15, 2000 Share Posted November 15, 2000 Woo hoo! Excellent work Morgan. I also do look forwards to seeing the hairy Z. Are you planning on doing a bit of testing on the rear end as well? Also, do you have any plans to experiment with a G nose, headlight covers or other front end mods? It'd would be cool if we could test all this stuff out and actually get some concrete numbers... ------------------ "Gimme Fuel, Gimme Fire, Gimme that which I desire" -Metallica Drax240z 1972 240z - L28TURBO transplant on the way! http://members.xoom.com/r_lewis/datsun.html Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted November 15, 2000 Share Posted November 15, 2000 i body got a wind tunnell?i havent done this kind of testing but i have driven mine to 135 mph-it feels a little light.maybe if car is equipped with coil overs you could lower front real low and raise rear.any body want to try black rock dry lake for a weekend? Quote Link to comment Share on other sites More sharing options...
Modern Motorsports Ltd Posted November 15, 2000 Share Posted November 15, 2000 quote: Originally posted by Morgan: The leading 1/2 of the hood had a fairly constant pressure profile - 1.5-2 inches of water at around 100mph over the whole surface. In the foot or so at the middle of the hood the pressure dropped rapidly to roughly 0 at any speed up to 100mph. From there back the pressure was constant at 0 all the way back to the trailing edge of the hood. In layman's terms, this means the best place to vent underhood air out is in the rear 1/2 of the hood, and a hood scoop would be best placed in the leading 1/2 of the hood to take the best advantage of the dynamic pressure. I'd caution against assuming it's best to vent from rear half of the hood. To move air or anything you either need a push or pull in 'laymans terms'. The higher pressure restrained (quite confined by hood as a boundary surface) by the hood will continue along the hood surface and if the NACA duct is designed right could serve to pull the air out. Refineries routinely use injection of this nature on pipelines etc, 30 degree to the approach/feed site of the pipe and the mainstream sucks the 'injection' along with it. Same as any soap/chemical additive on your garden hose. It's taken along for the ride, the mainstream doesn't revert into the lower pressure feed as it has an easier outlet ahead in it's direction of momentum/inertia. I think that air running by would 'drag it out' (picture the air as water flowing by, it does have mass/fricton after all as does your exhaust). Of course the underhood data would be great to know as well:-) Their are many cars ducting on front 1/3 of the hood in a rearward NACA duct for race purposes and OEM design. Others have a forward Naca duct so this air is sucked in. At the base of our windshield is another matter as it's not full laminar flow so cowl's work when not even optimally setup:-) Cowl's also work best when they're a sealed unit (ie. sealed from underhood pressures/rad air etc) so it has a vaccuum from the engine ingestion and will definitely suck it on in:-) ------------------ Of all the things I've lost, I miss my mind the most. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted November 15, 2000 Share Posted November 15, 2000 YIKES!!!!! It was late, I goofed up that message. The front 1/2 of the hood is around 2 inches VACUM relative to the cabin, not pressure. ------------------ Morgan http://z31.com/~morgan/s30 http://carfiche.com Quote Link to comment Share on other sites More sharing options...
Modern Motorsports Ltd Posted November 15, 2000 Share Posted November 15, 2000 quote: Originally posted by Morgan: YIKES!!!!! It was late, I goofed up that message. The front 1/2 of the hood is around 2 inches VACUM relative to the cabin, not pressure. Hmm, I didn't catch the relative to the cabin part before. I guess they'll all be relative to each other at least but not neccessarily to atmospheric as your cabin will have some difference right? Now with a vaccuum their as you say and the positive pressure (I just have to assume this, we force a lot of air in and it's not easy for it to get out) inside the engine bay makes more sense on the ducts existing on some race cars etc. ------------------ Of all the things I've lost, I miss my mind the most. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted November 15, 2000 Share Posted November 15, 2000 I can't imagine the front 1/2 of the hood having a vacum over the surface at speed, I'm sure it's because the cabin is somehow getting pressurized at speed, making the hood have a vacum according to the manometer. I'm gonna re-do my manometer and have 2 hoses off of it so I can measure pressure difference anywhere(behind radiator and in front of radiator, in wheel well and on inner fender, bottom of hood and top of hood, etc, etc). Nothing fancy, just a meter stick with clear tubing wired to it filled with colored water. ------------------ Morgan http://z31.com/~morgan/s30 http://carfiche.com Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted November 16, 2000 Share Posted November 16, 2000 Might be a silly question, but it wouldn't be the first I've asked... What about the side of the car? Behind the front wheels (not unlike some Ferrari front engined cars)? Any chance of venting there? Or are pressures similar, it would seem they'd be more constant as they are running on that surface for quite some distance, but perhaps the rotating wheel is screwing that up? Of course the route to venting at point would'nt be a piece of cake, but with all the sharp minds here not insurmountable. Regards, Lone Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted November 16, 2000 Share Posted November 16, 2000 Picky picky picky! I'll do the side of the car next time around. I'm trying to figure out a way to get a static gage pressure. I don't think that's gonna happen though, so I'll just use underhood pressure for reference. ------------------ Morgan http://z31.com/~morgan/s30 http://carfiche.com Quote Link to comment Share on other sites More sharing options...
Guest pauli Posted November 16, 2000 Share Posted November 16, 2000 you can't just use a sealed, rigid container for constant pressure? Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted November 16, 2000 Share Posted November 16, 2000 Sure, but like what? It would have to be rigid and have a large volume to minimize pressure changes as the manometer fluid moves up and down. ------------------ Morgan http://z31.com/~morgan/s30 http://carfiche.com Quote Link to comment Share on other sites More sharing options...
Guest pauli Posted November 16, 2000 Share Posted November 16, 2000 milk jug? one/two/three liter bottle? a large urn? Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted November 16, 2000 Share Posted November 16, 2000 Uh, a milk jug isn't very rigid! ------------------ Morgan http://z31.com/~morgan/s30 http://carfiche.com Quote Link to comment Share on other sites More sharing options...
Guest pauli Posted November 16, 2000 Share Posted November 16, 2000 yeah, i thought of that... oh well. supersoaker! snag the biggest supersoaker you can. some of the tanks are fairly big, 2+liters. is that big enough? Quote Link to comment Share on other sites More sharing options...
Modern Motorsports Ltd Posted November 17, 2000 Share Posted November 17, 2000 quote: Originally posted by Morgan: Sure, but like what? It would have to be rigid and have a large volume to minimize pressure changes as the manometer fluid moves up and down. Morgan, how about plumbing your manometer hose to a 5 or 10 gallon compressed air tank like all us motorsports junkies have. Your manometer fluid isn't moving that much to create a significant pressure change by displacement given compressibility of air IMO. If differences are quite small accuracy can be gained by tilting the manometer as far over as is convenient. (ie. a 2" change on a pure vertical measurement is 2", tilt that to a 4H:1V slope (ie. 12" run for 3" rise)and you're measuring the same 2" vertical change but over a 8-1/4" range (slope of 8h:2v) and increased precision of measurement by ~4x's. Useful if you do find differences to be quite small that would be immeasureable on a pure vertical. (used a manometer once or twice ) PS thanks a mint for all your efforts to date and to come, we can all talk till we're blue but working from real Z data is a nice progression:-) ------------------ Of all the things I've lost, I miss my mind the most. Quote Link to comment Share on other sites More sharing options...
Guest pauli Posted November 17, 2000 Share Posted November 17, 2000 quotewe can all talk till we're blue but working from real Z data is a nice progression:-) now if only datsun had done this in the first place... Quote Link to comment Share on other sites More sharing options...
Michael Posted November 20, 2000 Share Posted November 20, 2000 Just some brief comments.... When taking pressure data, it is critical that we make note of dynamic vs. static pressure. If the mouth of the tube which acts as the pressure probe is flush with the surface of the hood (i.e the hood has a hole drilled in it, and the tube is inside that hole), it measures static pressure. If the tube opening faces the exact direction of the oncoming flow, and the other end of the manometer is connected to a static port near the same location, the measurement is the dynamic pressure. Any in-between orientation of the tube opening will give some mixed reading. In Morgan's experiment, the difference between what the tube end senses, and the cabin static pressure, would of course also depend on how the tube end was oriented. But regardless of the technicalities, there are some important issues raised by Morgan's data. Here's my attempt to reason things through.... If there were no flow separations anywhere on the car, the total pressure (static + dynamic) would be everywhere constant. Where the flow was faster, the dynamic pressure would be higher, and the static pressure lower. But, whenever there is flow separation, there is loss of energy, and thus a loss of total pressure (pressure is mechanical energy per unit volume, in a manner of speaking). I would expect the total pressure in the front of the hood to be slightly lower than at its trailing edge, because of the separation "bubble" near the hood lip. More important for purposes of venting and intake is the static pressure. Static pressure over the front of the hood should also be lower than towards its trailing edge, because the flow accelerates in going over the hood lip. Separation reduces total pressure, but the generally accelerating flow increases dynamic pressure, so static pressure falls yet further. Static pressure at the hood trailing edge/windshield junction is higher, because here the flow slows down, also with a possible separated roller along the windshield lower edge. Quote Link to comment Share on other sites More sharing options...
Mikelly Posted November 22, 2000 Share Posted November 22, 2000 Man this is good stuff... Keep the data "Flowing" (pardon the pun) on this topic. I for one will be printing all of this out. I also need to make a manometer and do some testing on my fiberglass when we start building... Mike ------------------ "I will not be a spectator in the sport of life!" mjk Quote Link to comment Share on other sites More sharing options...
John Scott Posted December 3, 2000 Share Posted December 3, 2000 colored water, tubes, milk jugs and supersoakers all wired to the Z?? I'd like to see the look on the arresting officers face with 150mph displaying on his speed stalker. Morgan, thanks for the info and nerve to do it! Keep it coming. JS Quote Link to comment Share on other sites More sharing options...
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