Well, call my mind blown then. I thought that the increased low pressure area would have meant increased drag, since that downforce's energy has to come from SOMEwhere. In hindsight, it might reduce drag by giving the high pressure in front of the radiator a place to go to instead of just stalling it.
I dunno. I know enough about aero to know that it makes my head hurt 
Knurled wrote: Well, call my mind blown then. I thought that the increased low pressure area would have meant increased drag, since that downforce's energy has to come from SOMEwhere. In hindsight, it might reduce drag by giving the high pressure in front of the radiator a place to go to instead of just stalling it. I dunno. I know enough about aero to know that it makes my head hurt
I have a degree in it, and it still makes my head hurt. Although, part of that is that cars are much different shape and because of proximity to the ground... have different interactions.
So, yes, you are right... that low pressure is creating suction that is creating drag. It's just that that drag is much less than the drag the air would create by bouncing around "random bits of car hanging down from the bottom"... and that's just based on my "first pass thinking about how this works".
It's probably about time I pulled out some books and did some re-reading 
So you can't add an undertray. Are you allowed to fabricate a custom oil pan? ;-)
In reply to AngryCorvair:
So a custom oil pan with a 'heat sink' on the bottom that just happens to be rather large and flat on the bottom?
T.J. wrote: In reply to AngryCorvair: So a custom oil pan with a 'heat sink' on the bottom that just happens to be rather large and flat on the bottom?
Exactly! I'd even argue that it was a performance disadvantage because it's so inefficient at cooling my oil. ;-)
The car is turning 8000 Rpms in 4th 1:1 power peak is/was 7900 (I've made some slight changes so it could be 7800 or 8000) so I need to get it back on the dyno.
I can add an oil pan shield but as said in an earlier post it cannot travel further back than the front of the fender opening.
The rules can be found at TABS West (Trans Am 2.5 B-sedan) I'm currently running VARA GTL (Vintage Auto Racing Association GT Lite 2.0 liter or smaller) but I'd like to start running B-sedan.
And yes pushing the air around the sides of the car would be better than it tumbling around under the car. There are specific air dam rules for B-sedan that don't apply to GTL but would stick with B-sedan so I'm only doing this once.
My very rough very non scientific estimate is it might be possible to pick up the equivalent of 4-5hp possible a tick more.
I still run a steel hood (30lbs) versus fiberglass (9lbs) and a glass rear window (8lbs) versus polycarbonate (4lbs) that would take the weight out of the top of the car.
The bottom line is the faster cars in the B-sedan group is 9-11 seconds a lap faster than me on a 3.4 mile course but the 4-5th place cars are "only" 5 seconds faster. 6-8th are only 1-2 seconds ahead. When I'm in the small bore group I perennially finish 5th the fastest car in this group is is a little over 2.5 seconds faster and the cars just ahead of me about 1-1.5 seconds
Hoosier has a new version of the tire I run which is likely going to be 1-1.5 seconds a lap faster. I'm expecting the motor tweaks to bring 2 seconds perhaps a bit more (the switch from the 1.2 liter 73 whp motor to the 1.5 105whp motor netted 7 seconds). So end of the day I'm hoping that I can pull another 6-8 tenths with some aero tweaks. The goal is top 5 in the B sedan group or overall in the small bore group.
In the last 7 years I've gone from 3:09 to 2:54s so the car is hugely faster while still being pathetic. The fast Spec Miata's run 2:43-2:44 (same as the B-sedan cars) A friend manages 2:39 in his Super Miata. I've run 2:40s with a friend's stock motored Exocet.
In reply to Tom1200: Not having a "real" race motor is probably not the handicap you imagine.. "race motors" trade bottom and mid rev torque for high RPM horsepower. Yes you need more horsepower to shove air aside at high speed but you benefit from more low and mid range torque which might give you and advantage coming out of corners.
If you could add lightness to your car by removing stuff you could gain positions without increasing the tendency for the motor to go Bang! Hole saw with panel dimplier, replace glass with polycarbonate, steel replaced with aluminum. Carbon Fiber fenders, (not all that hard to do) Acid dip the roof. (used battery acid and an old plastic wading pool)?
frenchyd wrote: In reply to Tom1200: Not having a "real" race motor is probably not the handicap you imagine. If you could add lightness to your car by removing stuff you could gain positions without increasing the tendency for the motor to go Bang! Hole saw with panel dimplier, replace glass with polycarbonate, steel replaced with aluminum. Carbon Fiber fenders, (not all that hard to do) Acid dip the roof. (used battery acid and an old plastic wading pool)?
Definitely good suggestions, although the rules Tom1200 is adhering to is (imho) heavily focused on maintaining the originality of the cars racing and only allowing 'period correct' modifications. Thus, no carbon fiber, and only Fiberglass hood and trunk replacements. Which would still be helpful weight reduction.
I'm a little fuzzy, how long are the races, and could you go to a "battery drain" setup without the alternator/generator? (there doesn't seem to currently be a restriction on battery chemistry).
I suggest buying the book "competition car aerodynamics" by Simon mcbeath. It will give you answers to pretty much everything regarding theory (not specifics in your application because that needs tested).
Your biggest enemies I see are the air flow and drag/lift under the car and on the tires. Your car is being picked up off the ground so you can't put as much power down or brake as hard. The gap under your front bumper is massive, so the easiest go to solution is airdamn and under tray going back as far as the rules let you.
Forget about the vents and drop trays and whatever that stuff is minimal in comparison. The mirrors are good since you have them. Or can you get away with mirrors inside the car only?
Can you do a hood vent then exhaust the radiator out the vent? Boxing in the radiator and dumping it out the hood will keep it from dumping under the car like it is in the photos.
Look for a local source of corrugated plastic and Google search hypermilers for some good application ideas. Not sure what the rules would say about it but it's good for trying aero stuff out cheaply. Its tough, cheap, and lightweight.
sleepyhead wrote:frenchyd wrote: In reply to Tom1200: Not having a "real" race motor is probably not the handicap you imagine. If you could add lightness to your car by removing stuff you could gain positions without increasing the tendency for the motor to go Bang! Hole saw with panel dimplier, replace glass with polycarbonate, steel replaced with aluminum. Carbon Fiber fenders, (not all that hard to do) Acid dip the roof. (used battery acid and an old plastic wading pool)?Definitely good suggestions, although the rules Tom1200 is adhering to is (imho) heavily focused on maintaining the originality of the cars racing and only allowing 'period correct' modifications. Thus, no carbon fiber, and only Fiberglass hood and trunk replacements. Which would still be helpful weight reduction. I'm a little fuzzy, how long are the races, and could you go to a "battery drain" setup without the alternator/generator? (there doesn't seem to currently be a restriction on battery chemistry).
The question is are we really racing or just play acting at racing?? If we really are racing then it would benefit from exploiting all the grey areas of the rule book.. Fiberglass? Hmm does that mean matt or cloth? Chopper gun or hand layup? A can of spray paint can make Carbon fiber look like fiberglass. Considering how easy it is to do fiberglass or carbon fiber that is at least worthy of a cold winter day discussion..
I suppose too it depends on you.. Are you the sort who ask permission or feel it's easier to beg forgiveness? I know in the decades I vintage raced there were always people who pushed the rule book to the breaking point and beyond.. The only ones who were called on the carpet were the few who were both unpopular and winners. If you are perceived as a nice guy nothing is said even if you win..
I've always made sure my cars were legal. I know,some people,who bend the rules but for me any accomplishments gained with an illegal car mean nothing. There are some exceptions like my autocross car is underweight but there are no other F500s running so it's not a huge deal. If there were other cars I'd simply add the weight or bump up in class.
The big key for any of the classes I can run is the coach work has to remain relatively stock.
GT5 / GT-Lite is the class I run at SCCA regionals, the car was a glorified ITC car with a JDM motor. When I switched to vintage the car was mostly C-Sedan legal but VARA's C-sedan class was a spec B-sedan class rather than the original B-sedan class. Because of that I ran VARAs GT-Lite class, in the meantime I went from the 1171cc to the 1508cc engine which is legal in GTL.
I keep the motors for this car at around 80% of a race engine so they will go several seasons. On the subject of torque and corner exit it's a non factor; I clobber people coming off corners, it's 150-200 feet off the corner where cars with 40-80 more horsepower start rapidly reeling me back in.
The rules have since changed to allow the 1500 engine in B-sedan. Right now there are two things that make the car illegal for B-sedan; I have 39mm Keihin flat slide carbs (no motorcycle carbs) and the lower control arms are non stock (280ZX), were modified and have spherical bearings in them (the rules say no rod ends or shoerjiral bearings).
With all that said the rules maker would very likely make an exception for the mods outside the current rules as I'm running a 1.5 liter car against mostly 2.0 liter cars.
Back to the original topic; while ducting the air out of the engine compartment would help but that won't fly with the rules. I can duct the air behind the grill into the radiator. Also if the bolt on flares are done properly they will duct the air around the leading edge of the tires and meet up with the air dam.
Tom1200 wrote: I've always made sure my cars were legal. I know,some people,who bend the rules but for me any accomplishments gained with an illegal car mean nothing. There are some exceptions like my autocross car is underweight but there are no other F500s running so it's not a huge deal. If there were other cars I'd simply add the weight or bump up in class. The big key for any of the classes I can run is the coach work has to remain relatively stock. GT5 / GT-Lite is the class I run at SCCA regionals, the car was a glorified ITC car with a JDM motor. When I switched to vintage the car was mostly C-Sedan legal but VARA's C-sedan class was a spec B-sedan class rather than the original B-sedan class. Because of that I ran VARAs GT-Lite class, in the meantime I went from the 1171cc to the 1508cc engine which is legal in GTL. I keep the motors for this car at around 80% of a race engine so they will go several seasons. On the subject of torque and corner exit it's a non factor; I clobber people coming off corners, it's 150-200 feet off the corner where cars with 40-80 more horsepower start rapidly reeling me back in. The rules have since changed to allow the 1500 engine in B-sedan. Right now there are two things that make the car illegal for B-sedan; I have 39mm Keihin flat slide carbs (no motorcycle carbs) and the lower control arms are non stock (280ZX), were modified and have spherical bearings in them (the rules say no rod ends or shoerjiral bearings). With all that said the rules maker would very likely make an exception for the mods outside the current rules as I'm running a 1.5 liter car against mostly 2.0 liter cars. Back to the original topic; while ducting the air out of the engine compartment would help but that won't fly with the rules. I can duct the air behind the grill into the radiator. Also if the bolt on flares are done properly they will duct the air around the leading edge of the tires and meet up with the air dam.
I completely agree with your quandary. a lot of the rules of vintage racing are either gray areas or treated like gray areas by many in vintage racing..
The problem is unless the car has a well documented history, (the 1956 LeMans winner or a Chaparral Mk1 for example) exact details are mainly a foggy issue. My own Blackjack special for example.. I insisted it have the proper (for the first year it was made) 3.8 Jaguar even though it ran most of it's life with a small block Ford V8. Yes I could have run the Ford and had a lot more power but I wanted the car that way..
I'm sure you feel the same way so please don't ever change your car beyond what you want just to go faster.. Get/buy/build a whole new car for the added speed you want.. I sat down and did a budget on what it would cost to be competitive with the fast guys in my class. Instead of vintage modified of the 50's I moved up to B production with a Jaguar XK-E V12 roadster. The money was the same and I wouldn't have to lose the Blackjack.
I do wonder if some couldn't question that the aero mods aren't beyond the car's original nature? Is it a case of following one rule while ducking another? More to the point, the big aero issue is frontal area, and while maybe some modest gains can be achieved are you sure the changes will be justified by the costs and time involved?
I'm looking to stick with one rule set; as mentioned it's easy to mix and match but I won't.
The side mirrors, belly pan and air dam are totally legal. The 1972 SCCA GCR specifically allows for any type ducting air into the radiator behind the grill.
I do have the choice of going to the F500 and that is likely to be 20 seconds a lap faster than the 1200. The issue there is that with the 1200 I have loads of people to race with where as in the F500 I likely would not.
The goofy thing about vintage is the cars if For instance I run a 4.2 liter Jag motor in a moderate state of tune it will be slightly slower than the all out 3.8 but will be much much cheaper. Naturally folks will say that motor wasn't run back in the day, while neither was 3.8 in the state of tune with the type of internals some folks run in the current vintage cars.
I often think that vintage cars should be put on a dyno; if you have the same motor type ( BMC A series, Datsun L Series, Volvo B series etc.) and it doesn't exceed the output of back in the day then it shouldn't matter.
Back to the aero; while the frontal area is very difficult to reduce it is possible to get a combo of slight frontal area reduction along with a lower CD to get a little bit more out of the car. My buddy was over with his work van and it has a small air dam that looks like it wood do the trick as far as drag reduction.
Tom
So you can't add an undertray.
You need to be very careful about doing this to an older car.
My Z4M has a factory undertray made up of various bits of sheet metal that you have to remove before doing any service under there. Bit of a PITA, but it woks aerodynamically and causes no cooling issues as it was designed for the car.
When you install an undertray on an older car, you can get some pretty severe overheating issues, for instance cooking the oil in a transmission or diff. It needs some temp sensitive stickers on various bits under the car before and after installation of the under tray to see if you are causing more issues than you are solving.
Are you allowed hood vents?
@wspoon on my car the "tray" will be an optimized oil pan shield, the car had a stock shield which could be improved on. I took the shield off years ago which in retrospect I should have left it on. No clue what I did with it but it's long gone. The tray cannot extend into the fender wells nor back past the crossmember. This is the reason I'm looking at an air dam to keep air out from under the car.
@LanEvo not sure on the hood vents, I think louvres would be allowed but ducting the radiator airflow out the top of the hood would be a no go.
I've been thinking about your original question more, specifically about the cooling vents in front of the windshield. Are your driver/passenger windows required to be in a specific position? (Isn't stated as far as I can tell in the rules)
I assume that hoods/trunk lids are required to be "latched", is there any variance for them being spaced open ~1" ?
The door windows are supposed to be down and or removed (mine are removed already) I'd have to look again on the exact wording of the rules.
Now the rear side windows can be stock or Lexan / Polycarbonate; they're is a wrinkle on this, the standard 1200 rear windows have latches that allow them to be opened, specifically the trailing edge can be pushed out 2 inches.
On the hood I seem to recall something listed that appeared to be written to prevent you from raising the back of the hood.
Tom1200 wrote: The door windows are supposed to be down and or removed (mine are removed already) I'd have to look again on the exact wording of the rules. Now the rear side windows can be stock or Lexan / Polycarbonate; they're is a wrinkle on this, the standard 1200 rear windows have latches that allow them to be opened, specifically the trailing edge can be pushed out 2 inches. On the hood I seem to recall something listed that appeared to be written to prevent you from raising the back of the hood.
if the windows have to be open, then it might not be a big deal about having the vents... they might be relieving some high pressure from the base of the windshield. You could use some tape to test if blocking them off is a benefit.
I'd leave the rear windows closed. There's a slim chance that sucking air from the front vents and pumping it back to the rear hatch, which is propped up a little bit, could make a benefit... but I'm sure it'd raise some eyebrows... and there's a chance that the power required to run the fan would outweigh the reduction in drag.
Alright, so it's taken me a little while to organize some time to drop some of these into a similar spreadsheet as I used for wrangling some numbers on Mazdeuce's former Insight (this sheet assumes sea level, and “1976 atmosphere standard day conditions”). I’ve gleaned some Cd improvement based on the airdam ‘study’ you can find here: https://robrobinette.com/S2000Aerodynamics.htm
I’m going to provide the improvements as “top speed improvement”. I'm using a "as raced weight" of 1911 lbs (based on 1676lb listed before, an assumed 175lb driver, and 10gallons of gas), 17.5 sqft of area and .42 Cd, and a rolling friction of .027.
At 111mph I get 231.52 lbf of drag, 51lbs of friction drag, and a total required horsepower of 104.75hp. So, I’m going to base all my improvements compared to 111mph and 104.75hp (i.e. I’m going to raise the speed until I get 104.75+/-.05 hp).
The mirrors (based only on the .3sqft of area saved) will increase your top speed about 0.6mph. If they also happen to provide a form improvement of 5 counts (i.e. 0.005 to .415) {this is a WAG}, then it could improve your top speed 1mph.
It’s a little difficult to draw out the benefit of an “air dam alone” from the Hancha Group miata study, since all of the airdams are study with a 4” drop suspension. They show an improvement of 0.4 and 0.5 over the “stock ride height drag”. Theoretically a “stock ride height” car should be able to realize the same drag reduction as long as the airdam provides the same 2” gap between the airdam and the ground.
I don’t know if you can make an airdam as nicely as in the study (one that wraps around the same way), nor if you can get a 2” gap (maybe less?). So, I’m going to conservatively guess a form drag improvement of 0.3 (i.e. from 0.42 to 0.39). This should net an improved top speed of 2.6mph, and could get up to about a 3mph improvement with the mirror change. (another way to look at this is, at 111mph this mirror/dam would mean a 7.5hp reduction in required power).
If you can realize the full 0.5 improvement from the miata study, then you’d see a top speed improvement of 4.4mph, which might get up to a 5mph improvement with the mirror change. Although, something to keep in mind, that at 116mph, you’ll be turning 8300rpm. What’s your redline again?
*Note: These are all ‘ballpark numbers’, and not guarantees of performance… and they might have unforeseen knock-on effects that I’m not accounting for here. Buyer beware, etc etc.
A couple of things; I weigh 140lbs and the car only has a 5 gallon fuel cell.
The main motor will rev to 8600 it just isn't making peak power at that point, it won't actually float the valves to 9K. I could also shift to 5th (overdrive).
Your ball park figure is kind of what I was thinking as well.
I was at a track day yesterday and one particular moment summed up where the car is at. Coming through the last section of the course behind a ZR1, I was glued to the back of it, even on to the front stretch, 50-100ft later it's like the Vette shot out of a cannon.
Basically the places I get past at vintage races are 150-200ft after the corner exit and the braking zone. I've lead half the opening lap for a couple of times now; them that pesky back stretch shows up and I get shuffled 3-4 spots, I'll claw back a spot or two but then straight shows up again and those cars get back past. By the third lap the gap is to big to recover.
However wishful it may be I'm hoping a bit of aero improvement will also help me stay close enough to pick up the draft from the other cars.
.
Tom1200 wrote:A couple of things; I weigh 140lbs and the car only has a 5 gallon fuel cell.
The main motor will rev to 8600 it just isn't making peak power at that point, it won't actually float the valves to 9K. I could also shift to 5th (overdrive).
Your ball park figure is kind of what I was thinking as well.
I was at a track day yesterday and one particular moment summed up where the car is at. Coming through the last section of the course behind a ZR1, I was glued to the back of it, even on to the front stretch, 50-100ft later it's like the Vette shot out of a cannon.
Basically the places I get past at vintage races are 150-200ft after the corner exit and the braking zone. I've lead half the opening lap for a couple of times now; them that pesky back stretch shows up and I get shuffled 3-4 spots, I'll claw back a spot or two but then straight shows up again and those cars get back past. By the third lap the gap is to big to recover.
However wishful it may be I'm hoping a bit of aero improvement will also help me stay close enough to pick up the draft from the other cars.
I don't think it's wishful thinking... I think you're on the right track. I'm just trying to give some caveats so you understand the ballpark I'm trying to confine things in. Sometimes I get things right, but sometimes (like on page 1 of this thread) I get things off just a little bit. Hopefully my numbers, and they seem to be, are more on the right side. 
Re:weight & gas Thanks for that, although... currently that doesn't change my model much. I basically tweak the rolling friction up to match the "known condition" of 105hp, 17.5sqft, .42 Cd, and 111mph so that it all lines up. (So, with your new numbers I'm using a friction coefficient of 0.0285).
Theoretically, this friction should change w.r.t. speed... but, in the regime we're looking at, and with the realization this is all 'upper level napkin sketching', holding it constant should 'suffice'.
.
Tom1200 wrote:Back to the original topic; while ducting the air out of the engine compartment would help but that won't fly with the rules. I can duct the air behind the grill into the radiator. Also if the bolt on flares are done properly they will duct the air around the leading edge of the tires and meet up with the air dam.
So, some background reading on grill ducts and/or blocks, you might noodle around the ecomodder forum, for a start anyways.
As you work on this, make sure your temp gauge is in working order, and easily viewable. And, the idea about temperature stickers on the some of the components in the engine bay seems like cheap insurance for being ahead of something failing because it got too hot.
If you're still running the front bumper, I'd consider taking it off, and then try blocking off the two openings below the bumper and between the marker lights. If things get sketchy, you can always drill holes back into the "cover panels" to open up some extra flow into the engine bay.
re:wheel flares so the rules state something along the lines of "small period correct fender flares are allowed, if not in Carbon Fiber".
In case it hasn't already been made clear: Tires exposed to the air are really high drag.
It looks like you're familiar with loosecannon's scca mgb, and specifically these aerodynamic developments. (if not, go skim through the pictures there)
Potentially we can use some fender flares to do the same thing... although, I don't exactly know what "period correct" is/means. One of the benefits your car has (by dint of the picture on the front page) is that the wheels are tucked inside from the top, although they are "seeing" some 'freestream' flow from the front. However, as seen with the MGB, we actually want the backside of the wheel to be open, especially in the front, to evacuate air from the engine bay / brakes / etc.
Potentially, you could take a set of flares and (if they are the type that taper from "top" to "bottom") you could cut about 40deg off the "front" of the flare and clock that around so that the "tall" part that sticks out is actually blocking the front of the tire from seeing high-speed air... and then that means that the backside is left clear/open. You could probably do the same thing with a "constant width/height" flare, and just either cut off the back portion... or drill holes / cut louvers into it.
If you do this, I'd worry about the front before bothering around with the rears.
Dealers choice about whether you try and do this before/after the airdam.
Good note on tapering the back half of the fender flares as well as blocking off as much as possible on the valance (brake cooling may limit how much)
I checked out the linked MGB aero work, very interesting.
Summer is our off season so I've got three months to get it done.
Tom1200 wrote: I was at a track day yesterday and one particular moment summed up where the car is at. Coming through the last section of the course behind a ZR1, I was glued to the back of it, even on to the front stretch, 50-100ft later it's like the Vette shot out of a cannon.
Tom1200 wrote: I keep the motors for this car at around 80% of a race engine so they will go several seasons. On the subject of torque and corner exit it's a non factor; I clobber people coming off corners, it's 150-200 feet off the corner where cars with 40-80 more horsepower start rapidly reeling me back in.
I'm going to address the 800# gorilla....... you need more power. Maybe it is time to bump the engine closer to 90%? Maybe another builder can make more power from the parts you currently use?
You'll need to log in to post.
