cool vid! id love to know what the crank and oiling passages look like inside your engine... if you would like to customise the exterior of the holbert racer further with the use of vauxhall parts - i can advise

oh yeah, almost forgot to say that i think it would be worth smoothing the airflow under the car further not neccesarily for more downforce but for reduced drag. any increase in downforce might be a byproduct, but reducing the drag will help with acceleration as well as fuel economy and increasing top speed. honestly, if the top of the car looked like the bottom of the car aerodymanicaly we would all have put sheet alloy over it a long time ago, its just not as accesable.

 

Since this thread popped up again...

Mark, don't you know how to respond to a post yet?? You did such a crappy job putting everything into a big 'quote' I wasn't going to bother to respond, but there are were many erroneous statements made.

No it is not, unless you want a high-pressure zone under the car. Now combine that with the low-pressure zone at the transition from the nose to the hood and what do you get?

Which wind-tunnel tests are you referring to; did you rent time in a wind-tunnel?

Yes, that's ONE aspect of it, reduced drag. How does that reduced drag happen? By not letting the air stagnate against the headers and whatever else is under the car causing flow disruptions. If that air is not stagnant, then that means the air is moving, and the faster that air moves relative to ambient, the lower the pressure-drop.

The belly-pan helps keep the air-speed up relative to ambient, and likewise it's flow laminar, which creates a low-pressure zone.

If you have ambient under the car, then you have high-pressure.

No it is not. The air is accelerated over the hood due to the nose pushing the air over it. Add a splitter, and even more air is pushed over the hood. Again, what does faster moving air-flow over a surface relative to ambient create? A low-pressure zone, or lift.

That is part of the trade-off of using a splitter in that although it creates more lift over the nose-to-hood transition, it is offset by the high-pressure zone pressing down at the junction where the splitter's base meets the nose/body of the car, as well as the low-pressure zone created under the splitter.

How does it do that? By the air it accelerates air underneath it and above the ground.

Yes, this is true and why airplanes fly and rear-wings work by creating negative-lift, or downforce.

Yes, this is true too, but see answer above for a more detailed response.

If a properly designed splitter is on the car, then the highest pressure-zone is the junction where the splitter meets the body of the car.

Yes, that's part of it's job; see answer above.

This is simply not true; see explanation above.

Mark, where did you come up with this erroneous idea?

Says who??

I'll go along with that they're not worth the effort fitting them to the car, but to say the cars don't go fast enough to benefit is utter nonsense!

Is that considered a good time at that track? What was your lap-times prior?

 

Who loves Thunderhill 928 videos more than me? Nobody!

Nice job on the 2:01. I wanted to see you get a clean run through T8 to see the downforce in full effect . . . I don't think SCCA will let you guys shortcut 10 like that during a regional (Courtney's was more egregious). The run you got on the last lap through T1 to T2 was monstrous. That's the one time where I felt the wing working.

Is your LSD is getting a little tired?

 

True. Thats what i have been talking about. Plus i think many over emphasize the drag costs. sure, its important, but not as critical for the speeds we run as many would think. Look no farther than my race with the ferarri 360 challenge car.

the biggest drag as an isolated item would be the new wing, but as i stated before, its costing less than 1-2 ft-lbs of torque at 100mph, so thats kind of a rounding error anyway. BUT the increased speed and angle i can come off a high speed turn is much better giving 1-2 mph higher straight line speeds off those turns, and thats the idea of the wing. huge gains for minimal losses.

the 3 things i did (wing neutraled setting, cut vent hole in hood and finished splitter) made the car a dream to drive. it was REAL fun! cant wait to get a clean lap out there and push it in the turns that will be most effected instead of dodging traffic like the video showed!

Mk

 

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There were no "erroneous statements" made.

Lets just simplify it one more time. its actually pretty straight forward.

how a splitter works:

the moving car hits the air infront and builds up a pressure zone at the front and around the nose. This carries through the radiator and builds around the engine (at the cost of a small pressure drop). The air not entering the radiator and intake area, will normally just roll off the lower lip and go under the car, making that air slightly more dense. a splitter, will divert the "rolling off the nose " air , to the sides of the car. the less traveling under the car, the lower the pressure will be. (compared to the pressure on top of the car).

in the engine compartment, if there is a high pressure zone, which there is and i have confirmed this by measuring it , it will vent to a lower pressure zone. This is becuase air moves to differential pressure. Since the lowest pressure on top of the car is in the middle of the hood, if you vent it there, much of air that can move through the opening will be diverted through it, rather than going under the engine, thus addiing additional pressure to under the car. All this creates downforce.

The pans under the car, just smooth the flow, but do not create any downforce. a car is almost like a wing. in the early days, the bottom of the wing could even be left wide open! covering it happened later, and did produce slightly better flow charateristics but not critical but for low speed lift , the drag is not that bad. keep in mind, the drag coefficient of our cars are near .32 Cd. a perfectly smooth bottom may only change the drag coefficient by less than .02. we could use this number to see the total drag effects at any speed. its not going to be much. ( Now, if we use underbody panels for venturi and deffusers, thats a different story! ) what we try to do with or cars for racing, is reduce the amount of lift that the car inherently has by splitters and wings. the more air we can divert around or over the car, the more downforce we will achieve. it doesnt take too much of a change of pressure to create an effective amount of change.

comments?

Bottomline, the Holbert car has greatly different handling characteristics, for the bettter. times before at thunderhill on my best "used" tires was 2:01.9, but in traffic, was closer to 2:03. now with a 2:01.7 in some pretty substantial traffic, equates to about 1 second faster a lap, strictly due to this mod.

You are so far off base below, i dont even know where to start. think of the pressure under the car as being ambient, because the air is not doing much. the car is going over the air, but the air on top of the car is moving faster becasue it has a greater distance to travel. Splitters do not force more air over the car, because that WOULD cause lift, and this is not what happens.
The air doesnt stagnate around the headers, it moves to a pressure under the car that is ambient, and that raises the pressure under the car, causing lift. if you can vent this air flow up out the hood, like amost ALL modern high tech race cars do now, it adds to the downforce.

Mk



Here is the "crappy " insert message that i wrote:

Originally Posted by SwayBar
Mark, I think you have it backwards.

Having a bellypan on the car forces the air to go between it and the ground, causing that air to accelerate relative to 'ambient'; as a result, a low-pressure zone is created.
>>>>>only if the opening of the air entering the front of the car is larger than the area under the engine, which is it constant. the goal is to have the air going under the car to be at ambient. from the wind tunnel tests, you can see that the under car air is at ambient. the belly pans and all the underbody pans are for drag reduction under the car. at the speeds i race at, im not too worried about it. in fact, when racing a 360challenge car, with extensive belly and underbody pannels and diffusers, i had no problem keeping up!

Assuming the pressure under the hood is higher, the higher-pressure air under the hood it will then 'vent' into the lower pressure area under the bellypan via the bellypan's holes, thus allowing more flow through the radiator.
>>>>>>yes, and this is what happens and is the reason for the belly pan vents. exactly right

So a bellypan helps create a low-pressure area under the car, which is good as it will help 'suck' the car downwards. Now all we need is a high-pressure zone above to help push the car down into that low-pressure zone created by the bellypan.
>>>>> the belly pan doesnt create a lower pressure zone under the car.
we are talking about relative pressure here. ambient is under the car, on top of the hood is accelerated car due to the distance difference between under the car vs the air traveling over the car. the faster the air has to go to travel the same distance, the lower the pressure. The belly pan does create a smoother air flow under the car that reduces drag somewhat and keeps things much cleaner under there.

Which brings us to a splitter, which performs the same task as a bellypan by creating a low-pressure zone between it's lip and the ground, sucking the car down, as well as creating a high-pressure zone above the splitter's lip where the air stagnates above the lip and against the nose of the car, pushing it down into the low-pressure zone under the lip.
>>>>>>>>No, the highest pressure zone is the nose. the air pressure at the nose starts to roll off to go under the car, where it meets the splitter the splitter channels the air to the sides where it is vented. winglets on cars can even force this directed air to the sides of the car, where that air will meet the wider-than-body rear wings for added rear downforce as well. Lift is more than differential pressure, its change of momentum as well. there is no change of momentum due to the splitter like the rear wing. think of the splitter as a channel for the higher pressure zone to vent to the sides rather than travelling underneath the car creating more molecules to be forced under the car, increasing lift under the car. (or better said raising the ambient pressure under the car, and changing the differential pressure ).



You are correct that the air above the hood is at a lower pressure. But that lower pressure is due to the air accelerating over the hood's curved surface which has the same affect as the bellypan: it creates a low-pressure zone which causes the car to 'suck' upwards into the low pressure zone, exactly the opposite of what one wants on a high-speed track.
>>>>>actually the reason for the hood lower pressure zone is the distance the air has to travel vs the air underneath the car.

I'm thinking that if you're venting more air over the hood of the car and it's coming out of the vent as strongly as you say, then that extra accelerated air will cause a greater pressure drop on the top of the car, again causing the car's front-end to suck upwards more-so than before the vent in the hood was made.
>>>>>>>just the opposite. you are confusing the effect of diffusers. what is happening is that the ambient air pressure under the car and in the engine compartment will move to differential pressure. if the pressure is lower on top of the car than the bottom, it will try and vent in that direction. the less air molecules that go under the car vs on to of the car, will create downforce. the engine air fed from the radiator inlet, which is a good sized area, enters the engine compartement. it has to go somewhere. underneath the car increases the under the car pressure. if it can go over the car it helps two ways. the pressure isnt raised under the car (creating lift) and the air goes over the car lowering the already lower pressure on top of the wing, raising the pressure in the low pressure zone on the hood, creating downforce!


On the other hand, if that air coming out of the hood-vent is relatively slow-moving with-respect-to the air already moving over the top of the hood which started at the front of the nose, then, the turbulence created by the slower-moving air-column colliding with the faster-moving air-column will cause the air to slow down, thus creating a higer pressure on top of the hood than before. This in-turn will provide more weight for the contact-patch of the front-tires, thus providing more grip in the corners and the ability to carry more speed through a corner: if one has the ***** to do-so of course.
>>>>>>this is what is happening. this is also the reason for front diffusers. if i was able to make channels for the air entering the car's nose and slow it down. have a big channel feeding an even LARGER channel at the exit on the hood, the air would slow down and that would create even higher pressure changes on top of the hood. this same technique is used in rear channel diffusers.

This is very interesting; I wonder which way it's working?


Mark, where did you come up with this erroneous idea?


Says who??

I'll go along with that they're not worth the effort fitting them to the car, but to say the cars don't go fast enough to benefit is utter nonsense!


Is that considered a good time at that track? What was your lap-times prior?[/QUOTE]

 

think you meant

thus reducing pressure under the car ?

 

That was an awkward sentence. yes.

venting to the hood raises the low pressure on the hood and would lower the pressure under the car, because there would be less air traveling under the car at the same speed as without the hood vent.

Mk

 

MK
Nice run.......I do think with less traffic slowing you down you could pull a sub 2:00.......there were several spots where traffic or the M3 were slowing you down!

 

Thanks! I was pretty surpised to hear /see the time. Especially since ive been driving there for years and only knocking off .2 -.4 seconds with either better driving, track changes or a performance change. (ie lighter weight, better tires, etc)
The car sure handled a lot better and gave much better control in the fast turns. im a believer now! (in aero aids!)

MK

 

28PSI! that a bit of boost.
I hope you show him some tail lights.

 

Hi Mark,
What was the calculation on how big to make the hole in the hood, its placement, and its shape? I imagine the size and/or shape could be optimized for even better air extraction.

How did you go about measuring downforce?

And what's next for the aero plan? What kind of limits do the rules impose? (I can just imagine a giant inverted wing sticking up above the center of your car on the next race...) More seriously, it seems that there's lots of room to play with that front splitter for sure.

Sounds like you're hooked!!

Rick

 

I was placed in the low pressure zone from the aero charts that were passed around a while ago. (rim position on the hood) size was as large as possible without looking too funny. larger would be better.

Sure, i also reviewed most all race cars with hood vents, especially those that particularly those that had ducts from the nose to the hood. (for more efficient flow) because the 928 is not a tubed frame car, ducting would not be possible, and the other vent styles, which may work for increasing efficiency, probably not to a note worthy degree.

The key thing is to get enough downforce in the front , so that the rear can be used to tune the handling. I think im there now , based on the recent test day.

However, with more power, i many have to expand the front splittler to that of a size near anderson. right now, my splitter is near 5" all the way around the nose of the car. the only real disadvantage is that it is 4" high vs andersons 3" high splitter.

Look forward to testing it out at laguna at the next race again!

Mk

 

Hi Mark

Methinks as your hood vent is in the big low pressure zone and hence most likely not seeing laminar flow over the hood the only thing that will affect the amount the amount of air "sucked out" is the size of the hole. I suspect Naca ducts, venturis, leading edge flaps, wing fences - er can't think of any more aerodynamic gizmos would make very little difference. Hence your solution looks about right to me - and if it looks right it probably is.

that must have been exciting - somthing like wing walking - I can just see the next series of Jackass - """Special guest, 928 racer Mark Kibort goes hood walking""" Just kidding.

 

Reveived this from the UK list. Wish there was one for an S4.