top speed?? big math involved maybe.

[The Riot Hero]

ok so me and a buddy are trying to figure out how fast i was going tonight but it makes my brain hurt.

245/40/17 tire
7980 rpm
4.36 nismo final drive
KA 5th gear.

wtf is it? i think KA 5th is 786:1 ??????????????


yeps, do some math kids itll help

[Nikeboy355]

Mach one.....

[breakindrifts]

Not that fast that it's worth trying to figure out. ~125mph

[S13Leprechaun]

yeah defin not fast enough to figure out... nor post a thread about... around 140 i predict... cause at about 6.5k i was doing like 130 in my old ka.. some shit like that.

[GhostFace]

here you go

http://www.grimmjeeper.com/gears.html

[240trooper]

its gotta be faster than 125, i toped out my sohc in 4th gear at 115 and when i droped into 5th i still had 3,500 rpms togo,plus u cant hit 7900 in 5th lol

[S13Leprechaun]

if youd like to blow your motor by the time it ever gets there haha.

[240trooper]

i seen a vid on youtube a guy hittin 8700 in an sr in 4th doin 170mph , just look up t66 in youtube and ull see it

[sxse240]

That would be 177 mph for Riot.

An SR at 8700 rpm in 4th with a 4.08 rear end would be 157 mph, but a 3.7 ring and pinion would get him to 173.

[shinrekka]

theres alot more variables then just gearing. At 120 my s13 felt a little floaty, and i would not take it any higher then that.

[DR NISSAN]

Next time you try that top speed get a complete video from the inside :P

[BoostinGST]

173.1mph... if the 5th gear was 0.786
136.1mph... at 1.00 gear(i do believe thats 4th)

[drifter_for_life06]

for the record rhombus had ~26.2" tires which plays as much in speed as gears

[jspaeth]

This is so absurd......


This "math" is like 2nd grade shit.....take your car out, put it in 5th gear....

If you claim you were at 7980 rpms, just go out for a drive in 5th gear, get it up to 2000 rpms, look at your speed, then multiply that by 4

wow

[Gertid1]

hmm okay how about my once cam that hits 8114rpm in 1st and 7xxx after its an auto how fast should i be goin after 4 gears lol

[jspaeth]

^ What??? Didn't understand that at all

[Gertid1]

my car is an auto

Gears RPM
1st: 8114
2nd: 7k
3rd: 7k
4th: 7k

since you guys talk about the speed jw wondering how fast mine can be going

[jspaeth]

It doesnt matter.....get in your car, go on the highway and get it into 4th gear (go like 60 or faster with almost no throttle) and get the RPMS at like 2000 RPM

Then if your redline is 7000, multiply your speed at 2000 RPM by 3.5 to get your top speed.

If you have an auto KA, you probably don't have enough horsepower to push the car past 120-130 (just a ballpark guess).

The RPMS tell you how fast your car can theoretially go, whereas horsepower tells you how fast your car can actually go (since you have to overcome drag, which goes as the square of your velocity).

bang bang

[jskateborders]

It doesnt matter.....get in your car, go on the highway and get it into 4th gear (go like 60 or faster with almost no throttle) and get the RPMS at like 2000 RPM

Then if your redline is 7000, multiply your speed at 2000 RPM by 3.5 to get your top speed.

If you have an auto KA, you probably don't have enough horsepower to push the car past 120-130 (just a ballpark guess).

The RPMS tell you how fast your car can theoretially go, whereas horsepower tells you how fast your car can actually go (since you have to overcome drag, which goes as the square of your velocity).

bang bang
So theoretically, My sohc should hit 160 at 7k rpm, asuming it has the power to get there??? Is that what your math is suggesting. I lol'd

[johngriff]

Dont trust your speedo after 127mph. The Vehicle Speed Sensor Signal is garbage after this speed, because of the design of the sensor.

If you are THAT into top speed runs, get a road speed sensor, or do the gear math, but you have to add in drag... so have fun w/ that.

[jskateborders]

haha, 240 speedos dont even go that high...

[jspaeth]

Dont trust your speedo after 127mph. The Vehicle Speed Sensor Signal is garbage after this speed, because of the design of the sensor.

If you are THAT into top speed runs, get a road speed sensor, or do the gear math, but you have to add in drag... so have fun w/ that.



You have to add in drag????????? I disagree 101%

Assuming that the speedo is wrong at HIGH speeds, but that the tach is correct, you can just do what I said and multiply....

I mean it's quite simple....if your car does 55 at 2000 RPM in 5th, and your redline is 7000 RPM, then you are redline-linited to 182.5

Obviously in a KA you aren't gonna get there because of drag, but the drag has nothing to do with the relationship between RPM and speed.

[jspaeth]

Or you can do gears....

Example:

Lets say you have a stock S14 with KA and 205/55/16s

Final drive is 4.083:1, 5th gear is 0.759:1

Wheel circumfrence = pi*(16 inches+2 *0.55*205 mm(1inch/25.4mm) ) = 78.156 inches

At 6500 RPM, can calculate speed as follows:


6500 RPM* (1 driveshaft turn/0.759 crank turns)*(1axle turn/4.083 driveshaft turns)*(78.156 inches/axle turn)

=164,000 inches/minute

=155.24 mph


So then as a check, if it's 155.24 at 6500, then it should be roughly 3250 RPMS to do 77 mph

That's pretty much exactly what I rev at for 77 mph

[bigOdom1]

at speeds higher than 150drag is a huge variable the rear gets loose and things start to float without great aerodynamics

[jspaeth]

So you are saying at high speeds, your wheels either skid or slip?

I can kinda see that....and at that point there is NO physical way to do the calculation...

You have to assume that the runout of the wheel is constant, so that you go a constant distance per rev...


This IS true if the wheels stick to the ground perfectly, but I guess if the conditions are right, there might be some slippage or skiddage.

For all intents and purposes, doing the math is pretty accurate



Answer:

For the poster of this thread, assuming no slipping between the wheel and the ground, which actually should hold if you are maintaining constant throttle, you were traveling 175.32 mph


So yes, DRAG can effect whether or not you really travel as far per rotation as the tire runout, but there is no way to explicitly calculate this.

[johngriff]

Hence the LOL.

Wheel speed sensors ftl, road speed, radar based, gps etc.

But i am talking "serious business", not

"I tach'd my ka out homie"

[jspaeth]

hahah true...but even if the speed sensor isn't good, isn't the tachometer accurate though???

I don't know the electronics ins and outs, I am an engineer and physicist so I just know the math, fluid mechanics, blah blah blah


love the "I tach'd my KA out homie....shit was wild yo, shit was wild...." haha

[johngriff]

yeah, the tach is accurate, has to be, if not, your in big time trouble.

The VSS is a magnet with opposing polarized pickups (+/-). After 127 ish it seems to create too much "noise". While data logging it, it just jumps all over the place. I think the nissan speedo has a noise filter in it to smooth this out, but after that it has to be high inaccurate.

GPS is probably the only real way to get real feedback on a high speed run.

[jspaeth]

I think this is dumb, don't you?

I mean if the speedo is accurate, it is extremely simple to build software into the ECU that takes the speedo signal and converts it to a speed based on stock tranny, rear end, and tire runout.

I guess then you are effed if you switch trannys, rear diff, or tire size tho

Whatever, who cares.....people should be able to figure this shit out if the tach is right....its not rocket science....just ratios

[KA-T_240]

I need to figure out how to record top speed on my TomTom. I was messing around, the speed on the GPS kept rising after i was coasting for a while cause it wasnt keeping up :(

I am wanting to find a differant speedo. I have a LS Auto 180mph one right now. But I got taller tires, probably going to taller gears, and a 300zx tranny.