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Old 06-08-2012, 05:20 AM   #1
DaTT
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Default Help me understand, does weight play a HUGE role in top speed?

A guy told me that his 2000lbs Honda Civic with 450 FWHP would do 180 Mph. I don't believe him. I figure aerodynamics would play a much larger role than weight in top speed.
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Old 06-08-2012, 05:58 AM   #2
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It actually plays zero role in top speed, at least once you remove any impact on rolling resistance (which would increase as weight does). It's all about aerodynamics.

I don't know the specific math of what it would take an S2k to hit 180 mph but if his car really has that much horsepower it's probably not far off.

Stock car does about 149 according to a link I found with 237 horsepower, so doubling that I think 180 sounds pretty doable.
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Old 06-08-2012, 06:06 AM   #3
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Gear and Final Drive ratio, Engine RPMs, and Horsepower have a lot to do with top speed. Other things such as co-efficient of drag and surface area also affect top speed. Making a car lighter may have an effect if the Engine RPMs are able to overcome the weight difference but weight has more to do with quickness vs top speed.
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Old 06-08-2012, 06:10 AM   #4
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Quote:
Originally Posted by Doppel View Post
It actually plays zero role in top speed, at least once you remove any impact on rolling resistance (which would increase as weight does). It's all about aerodynamics.

I don't know the specific math of what it would take an S2k to hit 180 mph but if his car really has that much horsepower it's probably not far off.

Stock car does about 149 according to a link I found with 237 horsepower, so doubling that I think 180 sounds pretty doable.
Its a civic, not an s2k.

The limiting factor here is going to be aerodynamics, followed by gearing. All depends on if he put in longer ratios to even have the possibility of spinning the wheels to 180mph, which I kind of doubt. Most likely it probably does closer to 130, give or take some (I have no idea what a stock civic is geared to in 5th, pretty low from my experiences in Honda manuals so maybe more like 115).
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Old 06-08-2012, 06:48 AM   #5
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Back in the mid 80's, those Nascar cars were hitting 230 mph in testing at Talladega.

Tim Richmond took a Nascar Monte Carlo to over 240mph at a top speed comparo with Al Holbert's Porsche 962LMP, which could only manage 228 on the same track.

The main reason was that the Porsche had a lot of downforce, and the Monte Carlo had comparatively little. Aerodynamics.

The Porsche only hit 212 initially, then managed 228 with a lot of downforce removed, but it still had more df than the Monte Carlo.

Most people think the Nascar cars are "bricks", but in fact they really aren't. And they weren't, even back then.

Had the test been at Watkins Glen for lap times, the 962 would have cleaned the Monte's clock.
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Old 06-08-2012, 06:56 AM   #6
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If it has 450HP and a lower final drive on the transmission it may do 180MPH. The civic is a light car, my 2008 EX does close to a buck 130 stock, I believe the governor is set to 127MPH and I've seen cars without governors do close to 230 kilometers per hour. The Si's rev to 8500 RPMs. The RSX was a great example of this.
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Old 06-08-2012, 06:59 AM   #7
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Get the guy to prove it, talk is cheap
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Old 06-08-2012, 08:13 AM   #8
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Quote:
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It actually plays zero role in top speed,...
This thread should have been locked after the first 8 words.

Zero effect on top speed. It affects the time needed to get to top speed.
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Old 06-08-2012, 09:16 AM   #9
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zero is a really small number so I'd rather say not much
Here is a handy calculator that will give you the horsepower used by rolling resistance and aerodynamic resistance, all the way to 200 MPH!

http://ecomodder.com/forum/tool-aero...resistance.php
Drop some numbers in and you can see as that Aero drag rises exponentially, the rolling resistance drag just moseys along up a shallow slope.
At the end at 200 MPH with the example numbers, aero is 370 hp and rolling resistance is 7.81 hp

You can find numbers for your car on the web, take a guesstimate on crr. I use .011 for typical tires.
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Old 06-08-2012, 09:28 AM   #10
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Taking a known comparable:

MS3 = 263hp,155mph top speed (gear-limited, so this is conservative)

Power required for aero drag is quadratically related to speed: (180mph^2)/(155mph^2) = 135% power requirement to go 180mph

263hp*1.35 = 355hp

Should be doable with 450hp assuming the car is not gear-limited.
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Old 06-08-2012, 11:08 AM   #11
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Would that car even be stable enough to drive safely at that speed?
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Old 06-08-2012, 11:19 AM   #12
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Race cars generate a shit-ton of downforce for handling. That induces even more drag.
I'd say no, not safe in any stock aero form.
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Old 06-08-2012, 11:23 AM   #13
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A little bird told me it wouldn't be crash-worthy at 3x the design speed (8x the energy) of the crumple zones...
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Old 06-08-2012, 11:27 AM   #14
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Fatality!
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Old 06-08-2012, 12:42 PM   #15
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I had a hard time believing him because he also told me a 400hp civic can do low 8's. 1/4
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Old 06-08-2012, 12:48 PM   #16
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I had a hard time believing him because he also told me a 400hp civic can do low 8's. 1/4
lol!
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Old 06-08-2012, 02:17 PM   #17
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Weight is irrelevant but why wouldn't it do 180mph? 450hp is a lot of horsepower and Civics are very aerodynamic.

But I'm guessing his tires aren't rated for that speed, and the aerodynamics will make it dangerous. No, a wing on the trunk doesn't help!
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Old 06-08-2012, 05:58 PM   #18
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Would that car even be stable enough to drive safely at that speed?
Hard to say. Probably not. The problem is usually rear downforce. The front usually has a pretty decent amount just because of the hood slope. Most cars at that speed form a suction as the air flow lifts off the rear window (Bernoulli number, no longer laminar flow, etc...).

The '04 era Mustangs are limited to the speed they are exactly because of that reason.
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Old 06-08-2012, 05:58 PM   #19
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Weight is irrelevant but why wouldn't it do 180mph? 450hp is a lot of horsepower and Civics are very aerodynamic.

But I'm guessing his tires aren't rated for that speed, and the aerodynamics will make it dangerous. No, a wing on the trunk doesn't help!
Um... actually a wing on the trunk DOES help.
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Old 06-08-2012, 06:03 PM   #20
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Hard to say. Probably not. The problem is usually rear downforce. The front usually has a pretty decent amount just because of the hood slope. Most cars at that speed form a suction as the air flow lifts off the rear window (Bernoulli number, no longer laminar flow, etc...).

The '04 era Mustangs are limited to the speed they are exactly because of that reason.
I've owned 2 Mustangs and neither one of them felt safe at speeds over 130mph. They get really light in the rear.

Adding a rear wing will help but also creates drag that will further limit top speed.
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Old 06-08-2012, 06:29 PM   #21
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Quote:
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I had a hard time believing him because he also told me a 400hp civic can do low 8's. 1/4
V-TEC, anything is possible, crate motor from Japan [overnighted], double-clutching, beef the welds on the manifold etc. it might happen!

In truth can a FWD on the planet even do 8's? Obviously it would need literally magnitudes more than 400 horsepower, but I question whether it could get the traction. A 1000 HP bugatti veyron with AWD does 1/4 in 10 flat.
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Old 06-08-2012, 06:31 PM   #22
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beef the welds on the manifold
Gotta avoid that danger to manifold.
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Old 06-08-2012, 06:34 PM   #23
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Quote:
Originally Posted by Doppel View Post
V-TEC, anything is possible, crate motor from Japan [overnighted], double-clutching, beef the welds on the manifold etc. it might happen!

In truth can a FWD on the planet even do 8's? Obviously it would need literally magnitudes more than 400 horsepower, but I question whether it could get the traction. A 1000 HP bugatti veyron with AWD does 1/4 in 10 flat.
Apparently they can.

http://www.youtube.com/watch?v=ct36IUKaP5w
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Old 06-08-2012, 07:25 PM   #24
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Quote:
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Um... actually a wing on the trunk DOES help.
The trunk lid is not the place for a lot of weight.
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Old 06-08-2012, 07:26 PM   #25
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Quote:
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Um... actually a wing on the trunk DOES help.
Yes, it helps traction at the rear, but it will also lift the front
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