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Thread: Calculating theoretical top speed

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    Guest slider200's Avatar
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    Calculating theoretical top speed

    Hi all.

    Is it possible to work out the possible gearing for top speed of a project a friend is working on.
    Now I'm no expert on these things but he wants to try and crack 200mph. I know these depends on a lot of factors with aero etc but he wants to figure out if the gearing would theoretically do it.

    So he'll be running a 1jz but may change to a 2jz eventually.
    It will be almost 500 hp per tonne until he swaps to a 2jz.
    330d gearbox prop with a m5 rear subframe Inc differential but he has some different ratio different he could use.
    It's reported a m5 v10 de restricted will hit just over 200mph but with a 330 5 speed box he may need different diff ratio or even change to a 6 speed box and gear 6th longer.
    So I ask you guys. With a 1jz 500hp what sort of gearbox and diff ratio could hit the 200mph figure?

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    South West Rep Evilchap's Avatar
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    It should be easy enough to work out.

    You'd need tyre size, diff ratio, 5th or 6th gear ratio and rev limit.

    There's lots of online calculators you plug that into and you gear top speed in each gear.

    Whether he can do that speed is likely to be down to aero and drag as much as power. I think it would be not too difficult in a 300ZX Twin turbo or MKIV Supra, something like that.

    In something like an MX5 I expect you'd take to the sky before getting close... A big square fronted saloon would also likely take some serious shoving to hit massive speed - it'll be down to drag a lot.

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    Guest ANDY black s13's Avatar
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    skyshack has all the stats on the bonnevile salt flats s13 super speedy thing
    and the calculations knowledge
    is it a 13 or 14 being used?

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    http://www.enormouslysideways.net/calc.html

    Hitting 200 mph will be mostly about the distance you have to do it in and the down force to keep the car stable, if the final drive will allow that speed. More power just gets you to 200 mph quicker

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    Gearing for top speed.

    Km/h = engine peak power rpm x tyre circumference [meters] x 60 / (1000 x gearbox ratio x diff ratio)
    MPH = engine peak power rpm x tyre circumference [feet] x 60 / (5280 x gearbox ratio x diff ratio)

    So for a RS13 with CA18DET peak power at 6400rpm, top gear 0.821, diff ratio 3.916 and 195/60R15 tyre.
    Tyre circumference = pi x (15 x 0.0254 + 2 x 0.195 x 60/100) = pi 0.615 = 1.93m
    Km/h = 6400 x 1.93 x 60 / 1000 x 0.821 x 3.916 = 230 Km/h

    MPH = Km/h / 1.609 = 143 mph

    It comes up a bit short of that as it's about 7 bhp and 102rpm over geared ...

    141mph = 227 Km/h
    rpm = 227 x 1000 x 0.821 x 3.916 / (1.93 x 60) = 6298 rpm

    ++++++++++++++

    Diff ratio = Peak power rpm x tyre circumference[m] x 60 / (1000x Target speed[km/h] x gearbox ratio)
    Diff ratio = Peak power rpm x tyre circumference[ft] x 60 / (5280 x Target speed[mph] x gearbox ratio)

    ++++++++++++++

    Gearbox ratio = Peak power rpm x tyre circumference[m] x 60 / (1000 x Target speed[km/h] x diff ratio)
    Gearbox ratio = Peak power rpm x tyre circumference[ft] x 60 / (5280 x Target speed[mph] x diff ratio)

    ++++++++++++++

    How much power you need is a bit more complex.
    http://www.thecartech.com/subjects/a...Road_loads.htm
    Last edited by skyshack; 03-02-2016 at 08:52.

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    Guest slider200's Avatar
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    Quote Originally Posted by Evilchap View Post
    It should be easy enough to work out.

    You'd need tyre size, diff ratio, 5th or 6th gear ratio and rev limit.

    There's lots of online calculators you plug that into and you gear top speed in each gear.

    Whether he can do that speed is likely to be down to aero and drag as much as power. I think it would be not too difficult in a 300ZX Twin turbo or MKIV Supra, something like that.

    In something like an MX5 I expect you'd take to the sky before getting close... A big square fronted saloon would also likely take some serious shoving to hit massive speed - it'll be down to drag a lot.
    Aero may be the issue as it's a e type

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    Guest slider200's Avatar
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    Thank you skyshack

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    Quote Originally Posted by slider200 View Post
    Aero may be the issue as it's a e type
    A pessimistic approximation for power required to obtain a higher top speed.

    Required power = OEM power * (V target / V oem)³ (cubed)

    It's pessimistic as the rolling resistance is linear but is included as if cubed like aero drag.

    V oem = 146 mph
    bhp oem = 255 bhp

    power for 200mph = 255 * (200 / 145)³ = 670 bhp.
    700 bhp to be reasonably sure.

    Top speed with 500 bhp = 145 * qube root(500/255) = 181 mph

    (Hint, nice as E-type is if he wants to go proper fast on just 500bhp he's got the wrong car RS13 200mph = 495bhp, say 530-550 bhp to nail it. And messing with a classic that has survived 40-50 years to now is just wrong unless it's way too rotten.)

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    Guest ANDY black s13's Avatar
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    Quote Originally Posted by slider200 View Post
    Aero may be the issue as it's a e type
    is he limited to the stock e type body shape? or can he add some better aerodynamics to the car?
    we needs some pics btw,reads as a very cool project

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    Just what I've been thinking.
    Cd is about 0.4. Frontal area ????
    Required power = OEM power * (CDx / CDx oem) * (V target / V oem)³ (cubed)

    To do 200 mph on 500 bhp he needs to reduce the CDx (drag coefficient x front area) to 3/4 of what it is stock.

    MG EX225 should be a good template.



    Lose the wind screen. (This should be good for well over 50% of the needed reduction in CDx)
    Lose the mirrors.
    Smooth flat floor with rear diffuser.
    Sheet in the spare seat flush and smooth.
    Hunker down and if he can't put a teardrop cockpit lid on a vestigial fly screen is the way to go.
    E-type has wheels well inset, sheet in the wheel arches. Limit the steering lock and do the fronts as well. LSR cars have skinny tyres. If he can't sheet them in put pizza dish Bonneville wheel trims on.
    Reduce the rad intake area, it's too big as the road car needs cooling at idle in traffic.
    A more boxy rear with recessed transom = Camm tail. The rounded stock rear is a high drag feature, I have no idea why Porsche have hung on to it for 911 for all these years.
    Pump up the tyres (yes even on salt).

    This MG EXF may be nearer to a first go. Pizza dish wheel covers.


    I'll just leave this guy in his swimming trunks here.
    Last edited by skyshack; 06-02-2016 at 10:10. Reason: flat flooor!

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    Guest slider200's Avatar
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    What about weight as he's aiming for around 1 tone.
    The e type was a complete wreck and has gone thru a lot of changes.
    Check out huxlys insta for pics

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    Guest ANDY black s13's Avatar
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    if he can get it too a tonne /1,000 kgs with a 2jz etc in there will be an advantage,might take some doing to get it that light
    stock e type is about 1250 kgs iirc and the 2jz/g'box/turbo/m3 diff,subframe etc will be adding weight

    I think make it as slippery/smooth as possible aerodynamically and add some stability aides,
    I think the e type body shape is lacking there in stock form
    less weight and better aero would mean less power needed and distance traveled to get to 200 in theory

  13. #13
    Guest zeppelin101's Avatar
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    Weight has nothing to do with top speed.

    Standard engines are quite heavy from memory, it may come out in the wash putting a JZ in.

  14. #14
    Guest ANDY black s13's Avatar
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    Agreed I was going on less weight less distance traveled to hit 200 not it would help it
    ,speed is from aero and power to pull the gearing,

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    There have been 200mph Sierra Sapphire Cosworths without any aero for well over a decade now. Anything is possible.

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    The difference between going FAST and QUICK is some thing many drag racers don't understand, they keep telling people they are fast when what they really mean is quick.

    As already said, weight doesn't affect how fast a car will go.

    Power to Weight ratio affects how quickly a car will accelerate and weight (or lack of it) on driven wheels limits acceleration. So weight will limit how fast you can go in the distance you have.

    Bonneville cars are ballasted for traction on the dusty loose surface of the hopefully dry and hard salt. Some of the really fast cars weigh 5 tons. Best coefficient of traction is about 1/2 of that of tarmac road and can be 1/4.

    Dagger GT made a huge brag (reported on Top Gear News Series 16, Episode 1, 23 January 2011) that they are going to crack 300mph. 5 years on and they are still learning. The 1st lesson was if you want to go fast on salt then an open top super car is not the sharpest knife in the box, so they have had to put a lid on it. It started with a ignorant brag but they have gone from none, to 2 and now have 3 people that have experience giving advise. They are to use a Nelson 3000 bhp "race" motor for a shade over 300 mph. 1000 lbs of "front end ballast".

    Drag and outright top speed are 2 very different things. Drag only run 1/4 mile, under 6 secs at a time. Standing mile, just a days drag racing in one hit. Top speed is 10 miles, that's a whole season without a strip down.

    Quote Originally Posted by Dagger GT
    Dagger GT-LS:

    This supercar will be our land-speed package with our 3000 hp engine, 500 mph rated M/T landspeed tires, 1000 pounds of low front-end ballast, full bellypan with twin Venturi tunnels, land-speed wheel covers, Landspeed parachute, backup chute, Bonneville rollcage and a range of other options to help our buyers set new landspeed records in excess of 315 mph/500kph. Options will include landspeed instrumentation/avionics packages, base radio, windshield options and other possibilities.

    Dagger GT-D:

    This supercar will be our drag racing package with our 3000 hp engine, Goodyear Pro Stock Slicks, or DOT ET Drag tires, hydraulic-launched double NHRA drag chutes, drag wing, NHRA rollcage, drag racing transaxle, a drag racing rear suspension that will emulate a Pro Stock Dragster and enable 260+ mph in the quarter mile for our friends who love to drag race. We are also shooting for the 1900 hp Nissan Project Zed car’s 1.2 seconds in the zero-to-sixty mph. We also want to hit 300 mph to set a new IMRA Standing Mile record, (almost a 100mph faster than the Bugatti Veyron at 204 mph in the Standing Mile.)
    http://www.gtr-xs.com/models.html

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