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roverman
Art Gertz
Winchester, CA.
(2975 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Air drag and gear ratios
Posted by: roverman
Date: March 29, 2013 11:07AM

Double the speed and hp requirement is squared, right ? I suspect gear ratios are important to getting there, on time. Not just drags, but in longer events, the higher numerical ratio should help counter, that ever increasing aero drag. Since speed quest, almost aways include distance, optimized ratios help to make it happen. Case in point, I just bought a used 4.11 r&p from OZ. Not cheap, but it will render 19% faster accelleration, overall. It seems fruitless to ponder, a similar amount spent, to equal the performance gain. Discussion ? Oops, should be in "Drivelines", roverman.



Edited 1 time(s). Last edit at 03/29/2013 11:08AM by roverman.


flitner
John Fenner
Miami Fl
(168 posts)

Registered:
03/11/2010 10:58AM

Main British Car:
1972 MGB 350 CHEVY

Re: Air drag and gear ratios
Posted by: flitner
Date: March 29, 2013 04:16PM

Good point, my BIL used to complain about the Fox bodied Mustangs wouldn't keep accelerating once shifted into 5th gear when they had the 3.08:1 gears, but if they had the taller set they'd keep climbing, this clown got to "test drive " many of them till their ring got busted.


Todd McCreary
Todd McCreary

(201 posts)

Registered:
03/16/2012 10:57PM

Main British Car:


Re: Air drag and gear ratios
Posted by: Todd McCreary
Date: March 29, 2013 07:12PM

Double the speed and hp requirement is squared, right ?


That's a good rule of thumb, yes.




I suspect gear ratios are important to getting there, on time. Not just drags, but in longer events, the higher numerical ratio should help counter, that ever increasing aero drag.



True, to a point.

You have to be careful NOT to gear the car too tall or you will actually kill top end speed because you'll be too low on the torque curve when you shift up and the car won't be able to maintain accelleration in top gear.

Gear too short ( especially with an old 3 or 4 speed trans ) and you'll get great accell but you won't be able to triple the car because you'll be bouncing off of the rev limiter at ~90mph.

It's a balancing act. Typically a drag car will be biased towards a shorter rear gear because the amount of time spent at low mph is so detrimental to overall time in the 1/4 ( top attainable speed is really irrelevant ) whereas a land speed car is going to be critically tuned between it's torque curve, transmission, Cd and projected top speed. You would want the projected top speed to occur just at or beyond the rpm torque peak. Once drag force grows past the available torque force at the contact patch you're just not going to go any faster.

Under 60mph aero drag effects are mostly irrelevant. Over 100mph they become critical.



It seems fruitless to ponder, a similar amount spent, to equal the performance gain. Discussion ?

So far as "bang for the buck" goes, you're simply not going to be able to beat streamlining. Because streamlining, especially for a home shop built project, is going to be almost all elbow grease.

Drip rail around the window? Take it off. Side mirrors? Don't need 'em. Door handles? Shave them bad boys off and say you did it for the looks. Fit and finish of all sheet metal will be critical. A truly max speed effort would probably weld all the body panels together so that there were no exposed seams. Wax the car. There are no high speed runs in the rain, YOU DON'T NEED WINDSHIELD WIPERS.

Definitely DO install an air dam / lip on the front of the car and think about side skirts. Get the lip and skirts as close to the ground as you can. This will also greatly aid in keeping the car stable at high speeds.

Look into building an airbox between the radiator and front grill opening AND close off as much of the front grill as you can get away with.

Look at the bottom of your car. Don't look very aerodynamic, do it? Cover as much of the bottom of the car ( not including the engine bay, you have to get the radiator airflow out of the engine compartment ) in straight sheet metal as you can.



my BIL used to complain about the Fox bodied Mustangs wouldn't keep accelerating once shifted into 5th gear when they had the 3.08:1 gears, but if they had the taller set they'd keep climbing


years ago, Car and Driver did one of their semi-annual Mustang/Camaro faceoffs. They challenged the GM and Ford special projects groups to present the *best* car that they could deliver for the competition.

Ford brought an SVT ( maybe even back in the SVO era ) test mule.

GM brought a Camaro built by an independent tuner company. *facepalm* C&D was suitably unimpressed but they had 'x' number of magazine pages they had to fill with story so they ran with it.

While doing high speed testing, C&D was aero bogging the Mustang in 5th gear. The Ford engineers told C&D to put the 'Stang in 4th and wind it up because it would go faster. Unfortunately, when they did this the test mule ( with thousands of miles of track time thrashing on the engine) puked it's guts.

With the Mustang unable to finish the competition, C&D awarded the win to the (non) factory built Camaro.

Moral of the story: Don't thrash your equipment showing off when you've already got the victory sown up.

What were we talking about again?


flyinlow
Kevin .
Elko NV
(84 posts)

Registered:
01/25/2011 04:52PM

Main British Car:
1964 Spitfire Ford 5.0

Re: Air drag and gear ratios
Posted by: flyinlow
Date: March 30, 2013 02:28PM

top speed and maximum acceleration usually require two different sets of compromises
My last open road race car was a Mazda RX7 with a 1996 Corvette LT1 engine and TransAm T56 transmission, the first year I had the stock diff. with 4.10 gears, it would make it to my tech speed of 168 mph without problem but using tall overdrive gears in the trans creates allot of heat and after 85 miles my shift lever was really hot to the touch so I can only imagine how hot the transmission was.
I switched out to a dana 36 differential center section with 2.59 ratio adapted to the stock axles and was able to break my tech speed in 4th gear which has a direct 1 to 1 ratio and never had any more transmission heat problems and as a bonus at 80mph on the highway it got 35mpg


roverman
Art Gertz
Winchester, CA.
(2975 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Air drag and gear ratios
Posted by: roverman
Date: March 30, 2013 03:37PM

Ok, a little more info is due, now. "Track Day" car, so accelleration off the turns is important. Close ratio T56, so .80 in 5th and .62 in sixth. With 25" dia tire, 4.11/1 final, this calcs. to 190mph @ 6,500,(probable red line). Granted, this calcs to approx. 118mph, in 4th., but for this application, it won't be spending much, if any,time in 6th. Kevin, what was your 6th gear ratio ? What lube ? Trans cooler ? Heavy car will likey weigh 3,400-3,600lbs , with fuel and driver. Cheers, roverman.


Dan Jones
Dan Jones
St. Louis, Missouri
(268 posts)

Registered:
07/21/2008 03:32PM

Main British Car:
1980 Triumph TR8 3.5L Rover V8

Re: Air drag and gear ratios
Posted by: Dan Jones
Date: March 30, 2013 06:46PM

> Double the speed and hp requirement is squared, right ?

No. While aerodynamic drag is proportional to velocity squared,
the power required to overcome drag is proportional to the cube
of velocity. That's because power has an implicit rate term.

> I suspect gear ratios are important to getting there, on time.

Yes, for maximum acceleration, you want to select gear ratios that
provide maximum average torque at the wheels between shift points.
The ratios that do this will be the ones that keep the engine in the
band that provides highest average power between shifts. Ideally,
you'd end up at top speed at the engine power peak in top gear.
The ratios are, of course, overall gearing which is product of the
transmission gear ratio, final drive ratio and tire diameter.

> Since speed quest, almost always include distance, optimized ratios help to make it happen.

Yes. If you know your engine's power versus RPM and can estimate the
vehicle drag (cross sectional area, drag coefficient, and rolling
resistance), it would be easy to write a simulation to optimize gearing
for top speed and/or acceleration (with or without a distance restriction).

> my BIL used to complain about the Fox bodied Mustangs wouldn't keep
> accelerating once shifted into 5th gear when they had the 3.08:1 gears,
> but if they had the taller set they'd keep climbing

Yes. I still have the 1987 Mustang GT I purchased new with the 3.08:1
optional final drive ratio. With the T-5 5 speed, the standard final
ratio was 2.73:1. Peak HP was 225 SAE net at only 4400 RPM. Ignoring
tire growth, with the stock 225/60/15 tires and optional 3.08:1 final
drive ratio, the observed 148.1 MPH in fourth would have been at 5982 RPM.
With the 0.68:1 5th gear, RPM would be 4068 at the same speed. 4400 RPM
in 5th would be just over 160 MPH and the stock engine just didn't make
enough power to go that fast. With the standard 2.73:1 final drive ratio,
148 would be around 5300 RPM so the engine would still have some useable
RPM left. I have the drag coefficient and frontal area for the Mustang GT
and wrote a little program to calculate the top speed. Depending on what
you assume for driveline losses, it predicts a top speed of around 152 MPH.

> years ago, Car and Driver did one of their semi-annual Mustang/Camaro faceoffs.
> They challenged the GM and Ford special projects groups to present the *best*
> car that they could deliver for the competition. Ford brought an SVT ( maybe
> even back in the SVO era ) test mule. GM brought a Camaro built by an independent
> tuner company.

Perhaps you are recalling the October 1986 issue of Road & Track where they did
a series of tests, including top speed, between the new 1987 Ford Mustang GT and
1987 Chevrolet Camaro IROC-Z? The Mustang GT was a production test mule:

"As it happened, our prototype Mustang GT (one of only two in existence) was manned
by Product Development Engineer Arch Cothran when it arrived at TRC. After the 148-mph
lap, Cothran calmy said, "It'll go faster in 4th." Now here was a man with confidence
in his product. The message was duly relayed to our driver, who has something of a
penchant for the big numbers himself. RPM was climbing in 4th, he said from the far
side of the track, hitting 5800, then more. The needle was somewhere very close to
the 5900 RPM redline, he said, when the funny noises began. That was the end of that
engine that had been through many hard miles of magazine and Ford testing. It died
100 yards short of the groundhog. A tie, almost."

The IROC-Z did 149 MPH so it was quite close.

Dan Jones


roverman
Art Gertz
Winchester, CA.
(2975 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Air drag and gear ratios
Posted by: roverman
Date: March 30, 2013 09:04PM

Ok, Anyone have total aero drag for an 88' Z28 ? Car hasn't been on a chassis dyno yet, but theoretical dyno indicates approx. 450hp, near 6,500 rpm. My judgement call, for final drive of 3.45-1, I felt too tall for road course work, especially considering the 5th and 6th ratios. I don't understand the excessive heat build-up, that "Kevin" was getting, with his T 56 ? Perhaps it was .50 in 6th ? I plan to use Mobil 1 atf., as it has a good reputation, for this trans. Onward, roverman.



flyinlow
Kevin .
Elko NV
(84 posts)

Registered:
01/25/2011 04:52PM

Main British Car:
1964 Spitfire Ford 5.0

Re: Air drag and gear ratios
Posted by: flyinlow
Date: March 31, 2013 12:29PM

My t56 had .74 5th and .50 6th and I used synthetic 0w oil. the pony express first leg was approx. 85 miles and all at higher speeds than you will likely see at a road course but only had a 130 target speed in that race so used 5th and rarely 6th,
once I changed to the 2.59 gears I still had equal acceleration just in different gears except from a start I had to slip the clutch a little to keep from either spinning the tires or lugging the engine but once rolling there was no gear ratio induced problem.
any time you force a tranny to spin the driveshaft faster than the engine you will create heat but in most cases you wont be working it hard enough or long enough to matter.
I have always built my engines to have a fairly broad torque band rather than a narrow band at high rpm so that so yours may have completely different results.
I think the 3.45 should be just about perfect for road course work for your car but of course it is yours and your opinion is the most important.
an 88 Z28 CD is about .36 if it doesn't have any extra spoilers, add on spoilers can increase the CD significantly and kill your top speed but may be needed for down force


roverman
Art Gertz
Winchester, CA.
(2975 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Air drag and gear ratios
Posted by: roverman
Date: April 01, 2013 11:24AM

Thanks Kevin, When I do the math on your final drive ratios, going from 4.1/1 to 2.59/1, should give you 37% slower accelleration, through all the gears ? Onward, roverman.


Dan Jones
Dan Jones
St. Louis, Missouri
(268 posts)

Registered:
07/21/2008 03:32PM

Main British Car:
1980 Triumph TR8 3.5L Rover V8

Re: Air drag and gear ratios
Posted by: Dan Jones
Date: April 01, 2013 05:33PM

Input/Output from my quickie top speed program:

Enter drag coeficient:
0.34
Enter frontal area in square feet:
21.23
Enter rear wheel horsepower:
450
Enter vehicle weight (including driver and fluid weights):
3600
Cx = 0.3400000000000000
Frontal Area (square feet) = 21.23000000000000
RWHP = 450.0000000000000
Aero Drag (pounds) = 375.1376645443401
Rolling Drag (pounds) = 161.5688126148613
Drag Limited Top Speed (MPH) = 209.1175210900184

Dan Jones


roverman
Art Gertz
Winchester, CA.
(2975 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Air drag and gear ratios
Posted by: roverman
Date: April 01, 2013 07:47PM

Dan and clan, Ooooweee, I'm gonna need a bigger seat belt ! Thanks Dan, roverman.


flyinlow
Kevin .
Elko NV
(84 posts)

Registered:
01/25/2011 04:52PM

Main British Car:
1964 Spitfire Ford 5.0

Re: Air drag and gear ratios
Posted by: flyinlow
Date: April 01, 2013 09:21PM

Yes Art you are technically correct but the only time it really has a real world effect is at very slow speeds.
At all other times you just use a lower gear to accelerate
6th gear in the T56 trans was never engineered for extended extreme torque use but as a highway cruising Gear
You can add an external pump and cooler which the S.S. and unlimited class open road racers do but they still use extremely tall rear end gears. To avoid using overdrive in the tranny
Of course it is your car and you should do what you feel is best and I am just relaying my experience ( I have been a drive train tech since 1979) and the experience of other open road racers that I have allot of respect for.


Todd McCreary
Todd McCreary

(201 posts)

Registered:
03/16/2012 10:57PM

Main British Car:


Re: Air drag and gear ratios
Posted by: Todd McCreary
Date: April 06, 2013 04:26PM

Perhaps you are recalling the October 1986 issue of Road & Track



Holy crap.

How on earth did you find that, Dan?

I'm pretty sure it was an SVO 'Stang vs an aftermarket Camaro ... but in the absence of any other evidence, I'll concede the point to you. I'm sure not going to go looking through the library stacks for it.

It may be that gearing down to 4th for top speed runs was a common trick for the Ford development guys and it happened more than once? I do have to admit, that story quote looks awfully familiar.


Dan Jones
Dan Jones
St. Louis, Missouri
(268 posts)

Registered:
07/21/2008 03:32PM

Main British Car:
1980 Triumph TR8 3.5L Rover V8

Re: Air drag and gear ratios
Posted by: Dan Jones
Date: April 06, 2013 07:22PM

> How on earth did you find that, Dan?

I've got the article around here some place, along with a couple others and the sales brochure
and paper work from when I ordered my 1987 GT. I remembered it was a Road & Track test between
the 1987 Mustang GT and Camaro IROC-Z and that the Mustang went 148.1 MPH so I googled it up
and found this:

[www.prophetsofmadness.com]

> I'm pretty sure it was an SVO 'Stang vs an aftermarket Camaro

1986 was the last year for the SVO Mustang which was powered by a turbocharged 2.3L Ford.
While it also used a T5 5 speed, the gear ratios were shorter, as was the final drive ratio
so the math doesn't work out for fourth gear being faster than fifth. Top speed was also
lower than the (1987) GT. I test drove a 1986 SVO Mustang when I graduated college but held
off for a year to buy the 1987 GT.

Dan Jones


Todd McCreary
Todd McCreary

(201 posts)

Registered:
03/16/2012 10:57PM

Main British Car:


Re: Air drag and gear ratios
Posted by: Todd McCreary
Date: April 07, 2013 01:08AM

1986 was the last year for the SVO Mustang which was powered by a turbocharged 2.3L Ford.



Poor phrasing on my part. I didn't mean the SVO badged, for retail sales 4cyl. I meant a v8 'Stang prepped by the SVO division.



roverman
Art Gertz
Winchester, CA.
(2975 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Air drag and gear ratios
Posted by: roverman
Date: April 08, 2013 04:10PM

Many sanctioning bodies, do not allow certain streamlining modifications, for certain classes. My point being, changing a R&P for 19% faster accelleration, through the gears, is likely cheaper faster, than comparable other mod's,like streamlining. Time is money, how much work for how much gain ? Onward, roverman.


302GT
Larry Shimp

(189 posts)

Registered:
11/17/2007 01:13PM

Main British Car:
1968 MGB GT Ford 302 crate engine

Re: Air drag and gear ratios
Posted by: 302GT
Date: April 09, 2013 01:52PM

I know that Mercedes 5 speed manual transmissions (almost unheard of in the US) use a 1:1 top gear and low ratio rear axles (around 3:1) and higher ratios in the lower transmission gears. This gives efficient high speed cruising without sacrificing low end acceleration. I wonder why this practice is not more common?


roverman
Art Gertz
Winchester, CA.
(2975 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Air drag and gear ratios
Posted by: roverman
Date: April 09, 2013 02:11PM

Larry and clan, As in Doug Nash 5 speed, from approx. 20 years ago ? The 5 speed Getrag, as used in 75'-up, Jensen Healeys, were direct in fifth. Never really caught-on. Evidently, too difficult to re-train the public perception, overdrive = economy. I suspect, really low first gear ratios, require more centerline distance, for adequate gear strength. This requires the trans to be larger in girth, heavier,with more ratio drop/spread, making it more difficult for the syncro's. Onward, roverman.


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