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 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

 

Can you post a few pics so we can see your aero mods? You probably know that rake, front spoiler, ground effects, wheel well turbulence control, underside smoothing, etc. are all important.
I have a professionally rebuilt 3:08 available if you need one.

 

You'll actually lose speed going from a 2.73 to a 3.08 rear gear. You'll gain acceleration, but you'll lose top end.

I don't know if the extra bit of torque is going to help with the wind resistance, it sounds like you're going to need more horsepower.

 

well in our class C GT - No aeromods are allowed - we cannot mess with the body panels - we do have a significant nose down attitude - like every body else running in that class. We plan on smoothing air flow under the car with some sheet metal work this week - but you cannot "pan" the car! essentially more than 50% 0F THE BOTTOM OF THE CAR HAS TO BE "EXPOSED"

you can "radius" the wheel wells - I'm not sure how to do that - we will be running Moonie wheel covers to smooth the flow!

thanks for the offer of the 3:08 - we fortunately already have a good sound one - we use it for some of the faster Road Racing courses - so it is pretty beefy - at least so far!

Carl Johansson

 

You have no idea how much wind you are pushing - this turns out to be a common problem for people out there - you can do the math all you want - hell - we have the gearing combinations to make 300 mph at 5000 rpm without any problem - but that little thing called wind resistance really bites ya. for instance - on this car - at 130 mph, wind resistance is pushing on the car at about 800 lbs of pressure - if you just up your speed to 150 mph - now you are pushing 2000 lbs of air pressure - a 2.5 X increase for a measly 20 mph. I'm trying to find the guy who has the computer program to calculate the wind resistance - so far no luck ! essentially what you look for is the sweet spot in the engine where RPM Torque and HP are all working together to maximize your ability to push through the wind!

Regardless - you are right we would need more HP - but right now - there is nothing bolt on about that - we have maxed out the engine and carb we have - to get more HP - entails new motor - or at least all new internals.

So right now wwe are just interested in getting the max speed we can get out of this set-up!

Carl Johansson

 

How much horse power do you have to work with? I remember reading that the C4's need about 500 to 550 HP to go 200 MPH. Your power needs will be higher than that. What RPM are you turning at speed in 5'th gear?

The trick of top speed running is to have your gearing and tire diameter put the engine speed right at its power peak as you hit your target speed.

I have a program called Car Test 2000 that can model top speed. If you would like I could run some tests for you.

http://www.cartestsoftware.com/cartest2000/index.html

BigBlockk

Later.....

 

I think you may be just a little high on what a c-4 needs - c-4's have a frontal area of .32 (I think) while a C-3 has a frontal area of about .42 if i recall. Thats a big difference!
I think the lowest estimate we got for a C-3 was 500 HP - but that doesn't take into account the bubbled rear window etc.

as for Putting engine speed at Peak power - would that be HP in that case we want 5750 RPM - but if we are looking at Torque - well then you are looking at about 3500 - thats quite a big gap!

One of the problems we are having is the big drop from 4th (1:00 : 1) and 5th (0.68 : 1)

So we are thinking that our focus needs to be on the shift point between 4th and 5th.

I would love to see ya run some simulations - what info do you need?

Carl Johansson

 

With a 27 inch tire for example and if you wanted ( and had the power ) then a 3.73 rear gear would be optimal for hitting 200MPH at approximatley 6000 RPM in 5th

CHeck out the speed analyzer on the Keisler site, lets you fool around with tire size, rear end etc. According to the site my car will go 260MPH @ 6500 RPM

 

Yes, You want to be at your peak HP 5750rpm at 200mph. If you can get there.

I have doubts too that you can get there with that much hp.

 

What rpm were you topping out at with the 2.73 gear (in 4th I assume)? It looks like about 5300 rpm from your numbers, but it isn't clear what size tire you were using. If that's about right, you're not going to be able to go much faster by changing gears.

And you certainly wouldn't want to shift into 5th with the current setup. To determine shift points, replot the power curve from your dyno with the x-axis being the car's speed. There'll be a curve for 4th gear and another one for 5th. Shift when the two cross if you can. But in this case, that will probably be beyond your redline.

 

I will need:

1) Engine size.

2) Peak horse power and torque if you have them. (I can model something on my dyno if need be).

3) Engine redline.

4) The number of forward transmission gears, their ratios and the rear end ratio.

5) Tire circumference.

6) Coefficient of drag.

7) Frontal area. For a Corvette this is something like 22 Sq/Ft.

8) Overall height.

9) Overall width.

10) Ground clearance.

That should do it.

I decided to go ahead and run some of the numbers you posted. All of these simulations were run with 28.5" tall tires and a coefficient of drag of .40 (this is probably a little better than your car).

The first run is with your 2.73 rear gear using only 4 gears. The simulation came up with 167 MPH at 5300 engine RPM.

The second run is with a 3.08 rear gear using only 4 gears. The numbers were 167 MPH at 5950 engine RPM.

The third run is with a 3.55 rear gear using 5 gears. Your car slowed down to 163 MPH at 4570 engine RPM.

The forth run is with a 3.73 rear gear using 5 gears. It did a little better at 165 MPH at 4850 engine RPM.

The fifth run is with a 4.11 rear gear using 5 gears. You're back to 167 MPH at 5400 engine RPM.

The sixth run is with a 4.30 rear gear using 5 gears. Your car is holding at 167 at 5660 engine RPM.

As you can see from the data the engine RPM at the target speed must be close to the peak horse power. In your cast that would be 5750 RPM. That overdrive transmission is doing you no good. Also 167 maybe 168 is about all you can do with 394 HP.

Just to see what would happen, I built a 565 HP engine on my dyno and ran it in the simulation with no changes except raising the redline to 7500 RPM. The best I could get out of it was 191 MPH at 6800 RPM. Pushing the combination to 650 HP finally got me to 201 MPH.

You have your work cut out for you.

BigBlockk

Later.....

 

A couple of thoughts:

- I assume you are running a Tremec 5 sp with the .68 OD. There is a roadrace version with something on the order of a .83 OD that would reduce your RPM drop going from 4th to 5th. I assume you don't want to do a new tranny right now.

- Normally transmissions are most efficient (less power loss) at a 1:1 ratio. I've always heard that you lose 3-5% in OD versus 1:1. Might want to consider running it in fourth.

- I have no experience running topspeed events, but I would assume that you really want to be at your peak Horsepower RPM at top speed. Horsepower is a measure of work. Theoretically you want max work (engine) to occur at max speed (vehicle) to have your setup optimized.

- I did a quick calculation. With a 27" tall tire running in 4th (1:1) with your 2.73 rear you should be at 169 MPH at your HP peak rpm (5750). A 28" tall tire will give you 175 MPH at the same RPM. So you might want to consider just running your current setup in 4th (with the appropriate tire).

Anyway, hope this helps.
Regards.
-

 

Carl i tried to call you yesterday. I was able to do 174 on the speed gun with 4.11 .070 OD with 28 inch tires @6500 rpm with one of my old 355 ci.

Back then I had a stock nose extended chin spoiler 25.4 inch front tires with the 550# one inch shorter springs. My 79 had the nose down to create front rake.

 

All I can say to this is that your CD figure is higher than it really is. Several C-3 vettes are exceeding 200 with less hp. I saw 217 mph out of a relatively stock looking chrome bumper car. i couldn't find the right picture, but here are a couple more.

 

The only data that I have been able to find on coefficient of drag for the C3 has the 1980 improving to .443 from the previous .503. I believe he stated that he is not allowed any body modifications. The cars in the pictures you posted may have a few body modifications.

BigBlockk

Later.....

 

George is going to have to introduce us the next time your in Reno!


IMO - From my times out for speed week. I'd get the nose on the ground and the rear up. I'd also do some real calculations on rpm VS speed.

That blue "Sundowner" Vette every year was way over 200 mph and they would let 8-10 different people hop in behind the wheel for passes. I think that it went 240 mph or something 15 years ago with a TT motor.

 

You want the attitude of your car as level as possible. Hopefully you can get it completely level and not have any front end lift. The more nose low rake you have on the car the more frontal area you are exposing to the wind. Less frontal area is less drag. I assume that you have the mirrors and all protrusions removed from the body. Do you have some photos of the car in race trim? Those would be useful to see. Smooth all abrupt edges and fill all gaps. Does the sanctioning body allow taping the gaps and seams? Do it. If they don't then figure out a way to fill them such as making foam strips that can be wedged in so the gaps are smoothed. Chamfer the windshield trim edges with tape or silicon sealer so that the edge is smooth. Adjust the headlight doors so they are as flush as possible and fill the gaps. Make a small fairing to smooth the air as it goes by the vertical edge at the back of the door glass.

Make a gap filler for the front edge of the T-top so air flows over the top of the car better. You may also want to experiment with blocking off the front grills to see how that helps with front down force, the less air under the car the better.

 

The lower pink primer car is considered un modded as far as earodynamic class. They all slam the front end down and very the degree of rake to get the most stable at the highest speed. They often add 1200 or more pounds to car to get high speed traction.


If you look close on the rear window You can see "AA" So it might have some 700+ ci mountian motor


AA 501 cid and over 8.21 liters and over
A 440 thru 500 7.21 to 8.19
B 373 thru 439 6.11 to 7.19
C 306 thru 372 5.01 to 6.10
D 261 thru 305 4.27 to 5.00
E 184 thru 260 3.01 to 4.26
F 123 thru 183 2.01 to 3.00
G 93 thru 122 1.51 to 2.00
H 62 thru 92 1.01 to 1.50
I 46 thru 61 0.76 to 1.00
J 31 thru 45 0.51 to 0.75
K 30 cid and under

This pink car is a "D" class. So it has to be 305 ci or less and in GT N/A gasoline. It was a 9000 some RPM screamer injected

The top orange car would probably be right on the A 500 ci limit and A/GMS the G stands for Gas M for modified class S is for partialy streamlined

 

Of the cars you have pictured, how many have run over 200 MPH? Would you, by chance, know how much horse power their engines produce? Just by your brief description of these cars it seams that they all have over 394 horse power.

Back to Carl's car, it seams to me that his combination is doing all that it can do. My simulation says so and apparently his car does to. With the data that I have so far, his engine is not strong enough to move the car through the air at more than about 168 MPH. His track time mirrors the simulation. The car is having trouble getting past the high 160's, no?

If someone can get more accurate coefficient of drag numbers I would be happy to re do the simulation.

Pushing a stock bodied car over 200 MPH has never been easy and it never will be. There is a reason for that.

BigBlockk

Later.....

 

http://www.nelsonracingengines.com/motors.html

this site will help!!!!!!!!!!!!