I posted this in the C-3 tech section also - but i know theres a bunch of brain power here that can really help us out!

Many of you know we are running my brothers C-3 out on the Bonneville Salt Flats - last week - before the rain turned it into a gigantic lake - we accomplished 2 things
1. the car is now approved for exceeding 200 mph (passed safety inspection - parachute, 2 fire systems,6 point cage, etc)

2. Driver licenced to 175 mph

We also became painfully aware of our shortcomings - we knew we didn't really have the HP to make 200 - but we didn't realize our gearing and tire size would be such an issue at that high of speed -

essentially we get to about 165 - 170 - and the wind, wrong rear end, and fat tires etc wouldn't let us go any better!

So we need to do several things to get ready for the World Finals next month!
1. we will change out the rear end today - we were running a 2.73 - we will up it to the 3.08. we have a 3.73 available - but the math says - no way -
I tried running the numbers with a 3.33 and that didn't look very promising either!

2. we will be switching out the tire arrangement to tall 28.5 - 29.0 inch diameter much skinnier tire. The problem here becomes finding a rim that will hold a very skinny (4 - 5 inch ) tire with appropriate backspace - and be approved by scta.. I tried running the numbers with a smaller diameter tire - 24.5 - 25 inch - but that doesn't look to promising!

What happened with the 2.73 rear end is that the car could not maintain it's speed in 5th gear - to much wind resistance!
So we are hoping - with the 3.08 - more RPM per speed unit - maybe a better chance of maintaining and even slowly building speed!

What do we run -
ZZ4 with a hotcam
I have the HP and torque curves and numbers - but no scanner available. the engine peaks out at 394 HP at 5750 RPM. Torque maximizes at 417 ft lbs at 3500 RPM.

we do have a custom built carb maximized for the engine and application - think of a Holley 950 - maximized!

We need to figure out the best shift point between 4th gear (1.00 : 1) and 5th (0.68:1).

We also have the added advantage of the bubbled out rear window - that saves us quite a few ponys as compared to the turbulent drag created by the earlier flat window styles!

anyone with any help - physics, application, experience, ideas!

carl "i feel the need for speed" johansson

 

I think it awesome you are turning the MPH numbers you are with that motor , My "saltflat" friends tell me you will need at least 100 LBS more torque out of your engine to overcome the wiind drag. to approach 200 MPH....
Desert

 

How about a nice shot of nitrous... If you need another 30 mph on the top end at that speed, you're going to need more hp if you are anywhere close to your top end now. I'd think you need to do the math to figure your top speed at your hp peak and select the gears to put you there. Just need the hp to get you there. I don't have any ideas for the skinny rims, but I'd bet they would help considerably. Good luck!

 

Start contacting the wheel mfg's about suitable wheels; Fikse, HRE, Kinesis, Centerline, etc.

You may have to start considering a shorter tire and a longer gear.

Larry
code5coupe

 

I suggest you buy some books on streamlining if you have not done that already. Detroit has been doing that for a number of years to help meet their CAFE standards. Little things like dissappearing windshield wipers, no rain drip rails, no chrome strips around windshield (very smooth transitions), ground effect to lower under car air turbulence, rounded shapes instead of angular. Can you dump the rear view mirrors, any protruding bumps like door handles?

I have read the perfect streamlined shape is a raindrop which is actually pretty blunt in front trailing off to a point in the back (makes sense doesn't it?). Also look at the space shuttle. Very similar. The key is the air traveling over the sides and windows does not become turbulent by separating from the surfaces but "sticks" to the sides as in wing design. Turbulent air along the car body and the vortec at the abrupt back is the big killer to high MPH. It literally becomes a suction that gobs more of HP has very little effect.

You might have to pay for a custom wheel to fit the narrow width and still maintain the proper back spacing. I have looked closely at funny car bodies and most have little raised edges about a 1/2" around the trailing edges of open wheel wells and on their gigantic horizantal rear wings. So to top fuelers (on their horizantal wings). Aerodynamics is wild stuff

 

retard your cam 4-6*....that will raise the rpm at which max torque and hp occur...prolly get a couple miles per

 

Thanks for the input guys - interesting - this thread sat for weeks with nothing happening - now some action.

First of all, The world finals got cancelled - the salt flats are under water.

to address some of your comments
- streamlining is not an option - in our class GT - the body has to be stock - you can't add or change anything. (they do however make an exception for Hood scoops - and several teams have really stretched the meaning of the word - funny after they add a 4 foot long 2 foot high streamlined box on top of the hood - they claim to be the "fastest stockbodied corvette" in the world!

regardless - we cannot streamline. nitrous is an option - but we have some sort of moral committment to no nitrous - besides it puts you in a different class - and theres a good chance you'll blow up the motor eventually!

We did play with the timing - did nothing but slow us down 7 or eight miles per hour- so we probably have the timing set at max for this application.

We now have 4 inch wide tires all around - we will be able to choose between 26" 28" or 29.5 inch tires for ther back - so that gives us ac little flexability.

Guess we have to wait for next year - hope for better weather!
(thats OK we have another secret project underway!)

carl Johansson

 

[QUOTE=Carl Johansson
............essentially we get to about 165 - 170 - and the wind, wrong rear end, and fat tires etc wouldn't let us go any better!.......


.........I have the HP and torque curves and numbers - but no scanner available. the engine peaks out at 394 HP at 5750 RPM. Torque maximizes at 417 ft lbs at 3500 RPM.......

.......We need to figure out the best shift point between 4th gear (1.00 : 1) and 5th (0.68:1)......
[/QUOTE]

If 200mph is your objective, your best shot at getting there would be to select gears & tires such that you are spinning 5750 RPM at 200mph.

You didn't say what tire size you used when you got to the 165mph runs, but with a 2.73 in 5th gear, I estimate you were around 4000 rpm (past your peak torque) and would have only been around 5000 rpm at 200 mph had you made it. So your overall gearing was way too high.

This is also interesting in that if you had been in 4th gear at 165 you would have been somewhere around 5900 rpm....if my guesses are correct you might have done a tad better if you could have stayed in 4th gear a little longer.

Anyhow, I don't know anything about top speed runs, but it seems one problem is the relatively wide ratio between 4th and 5th.......don't know what it would take, but another approach would be to change the gears in the transmission so 5th gear was maybe 0.8 instead of 0.68.....I have no idea if this is feasible or not....Making 200mph in 4th doesn't look realistic either....

BTW, changing to the 0.8 is primarily to keep the engine at it peak power point, the old school close ratio concept. Looks like this might have been part of your problem in not getting past 165..

Just some thoughts from someone without a clue.......but good luck

 

not that I know anything about going that fast, but the big drop between 4th and 5th sure could be an issue...

What about using the tko600rr? It's got a .8 high gear?

 

You need more WAX!



You could cut the butt end off that 66 Riveria and glue it to the C3's nose for a hood scoop.

 

I played a little with this calculator...nice easy tool to work with
http://www.f-body.org/gears/

1)3.33 gears with 25.6" tire puts you right at 200mph @5950 in 5th gear.

2)2.59 gears with 29" tire puts you right at 200mph @6000rpm in 4th gear.

3) If you have the torque/horsepower dyno curves.....you can plot these against mph for various gear combos.....would show more clearly what happens when you shift gears & the effects of various tire/rear end combos. Incidentally, from you previous run you should be able to determine the horsepower required at 165mph.

4) To restate my previous comment, you have truly optimized your cars performance when the top speed and max horsepower rpm coincide.

Good luck..

 

Not all of this is entirely true. Laminar flow isn't always as good as it sounds. First off, no bones about it, a car traveling down the highway has zero chance at laminar airflow. Which isn't necessarily bad...

In a true laminar flow situation, take a perfectly round ball and place it in a unidirectional laminar flow field, you actually create MORE wake and MORE resistance than a sphere than has slightly roughed edges. The induced turbulence actually reduces the wake field and thus the size of the vorticies which, as you correctly stated, create drag. This is why golf ***** have dimples and why there are "vortex generators" on the wings of all commercial airplanes (those little fins that stick up towards the front of the wing!! keeps the flow from separating and thus makes the surface controls working at lower speeds!).

What you are fighting here is the drag induced by the wake verses the drag induced by the object itself

 

All I know is, I just watched "World's Fastest Indian" and I'm actually interested in all of this! Wish I could go watch it all!

 

[QUOTE=LT4BUD]4) To restate my previous comment, you have truly optimized your cars performance when the top speed and max horsepower rpm coincide.
QUOTE]

Another way of looking at this

1) If you got to a speed with your rpm greater than horsepower peak...you are geared to low and you could go faster.

2) If you got a max speed with you rpm less than your horsepower peak...you are geared to high and you could go faster.

 

This was my first thought also. I'd focus on aerodynamics which is the greatest horsepower user at those speeds.

1) Tape over all body seams
2) Clean up air flow around and under the car
a) recess the gas tank flush with the floor
b) add a full or partial belly pan to clean up flow under rear end
c) Close all possible places for air to leak out of radiator shroud
d) Reduce size of radiator opening to minimum
e) do some homework on C3 aerodynamics. I undersatand front end
lift on C3's at speed is a problem.
f) Add deep inner lips to fender panel to minimize drag from wheel
wells.
g) use Moon caps on wheels
h) wax car thoroughly

Just some ideas. Good luck.

Larry

 

[QUOTE=Vetracr]This was my first thought also. I'd focus on aerodynamics which is the greatest horsepower user at those speeds.

1) Tape over all body seams
2) Clean up air flow around and under the car
a) recess the gas tank flush with the floor
b) add a full or partial belly pan to clean up flow under rear end
c) Close all possible places for air to leak out of radiator shroud
d) Reduce size of radiator opening to minimum
e) do some homework on C3 aerodynamics. I undersatand front end
lift on C3's at speed is a problem.
f) Add deep inner lips to fender panel to minimize drag from wheel
wells.
g) use Moon caps on wheels
h) wax car thoroughly


great ideas ....but.....no can do at bonneville.....stock class means stock........C,F and H will work though...........have a friend with a built 71 and he won't take it over 130 mph because of front end lift

 

Man,
I don't know where this idea about front end lift came from - i suspect it flows from the idea that C-2 body styles had a problem with front end lift. regardless
I am here to tell you - that there is no front end lift on a C-3 - at least till 170 mph - we see no evidence of a front end even a little light. there is no front end lift!

someone actually did a calculation on this - turns out you may be generating about 200 lbs of lift - how much does the front end of that car weigh? and how much is air pushing down on the car?

One more time - NO FRONT END LIFT in the C-3 body style. As for the guy whos friend wont go over 130 because of "front end lift" perhaps larger heuvos would have enough weight to conteract his problem!

I will play around with ratios and tire sizes - thanks for the help - and particularly giving me the goal - we need to be at 200 mph at 5750 RPM - that helps alot! I think we can come up with the gear tire rearend combo to make that number!

Sorry I can't respond to specific posters - if I try to look to see who wrote what this reply get trashed.

BTW Steve - I'm counting on you to apply the wax - I can't get that concept down!!!

Carl "wax on - wax off - huh!" Johansson