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RPM-MPH chart for W58 & stock wheels/tires

67K views 120 replies 63 participants last post by  daiphucka 
#1 · (Edited)
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#71 ·
i've gotten my 86 p-type with an aftermarket magnaflow exhaust and a k&n cone filter up to 145 i believe, the county sheriffs around here put up these speed reminders on the side of the freeway at times and it just flashed a red 45 at me. i realize that going that fast in the first place is pretty dumb now, but forgive me for i was an idiot teenager back then :)
 
#5 ·
Tanya, I have run nearly 6000 rpms in 5th gear in my turbo '82 with a 3.73. Probably around 140 mph. Won't be re-visiting those speeds until I get the new brakes on.
 
#70 ·
Been there, Done that and Aint doing it again. Not saying that the MK2 Aerodynamics arent the best but its a bit unstable at those Speeds. Im running the 3.73 Gears in my car at the moment and I had the Speedo needle to the point that it wounldnt go any farther. Ofcourse Im running a 7M so that I make a Difference. Funny thing is that I hadnt hit redline yet but with this turbo Im running, That might be a good reason for me getting that fast pretty Quick
 
#6 ·
whew, I've pinned speedo @ 130 @ about 6000rpms w/ 4.30 gears which appears to be accurate comparing to the chart... I would like to see someone w/ a stock n/a mk2 "officially" top their car out @ redline and see what mph that would be...just can't figure out a way to get a speed check unless you have a cop friend who'll clock you, heh
or maybe an aftermarket speedo that does over 130? I dunno
I think Norbie has said he's done 160mph but that's definitly NOT stock...
all mag articles I read has the mk2 topped at 130mph, so is the car not supposed to pick up even a little more speed if it still has a few more rpms to go?
 
#7 ·
well, i topped my speedo out sat on the way to daves house. this late 90's firebird wanted to play. so i downshifted to 3rd and nailed it and left him easily. the second time was quite a bit later. i didnt notice him get on it, and when i finally saw him he was passing me. i down shifted to 3rd again, then shifted to 4th while next to him and past him then shifted to 5th and was still pulling a little,not a whole lot. looked at the speedo and it was pegged. i was ~5500rpm's. i let off the gas and started braking. he didnt slow down, just kept on going. guess he didnt want a round 3. i know the speedo gears cant be right for the 4.30 gear, also i have 235/60/14's on it rigt now and am running an r154 so have no idea how fast i was going, just know it didnt take very long to get there.
william
 
#9 ·
well, that and the stock clutch will be slipping well before then. btdt
william
 
#10 ·
ive topped out my 120mph speedo a few times, with the only mods 245/50 bfgs, and a 2.25 cat free exhaust.
 
#73 ·
is it even possible to go that fast with those tires? :ugh:
 
#12 ·
hmmm, i cant really claim a verified 5th gear max speed, however when i maxed out 4th in my 4.30 rear p-type the state trooper said i was going 110 mph.

that is going totally from him though since my speedo starts swinging back and forth pretty wildly after about 90 mph.
 
#14 ·
Ross said:
120 is a lot short of 160 though
sure is, i was simply trying to make the point that 120 is easy, and i should have elaborated on that.

120 mph isnt hard, with a mostly stock, unknown milage low compression 82.

the problem with cars is, once you reach a certain speed, thats it, your aerodynamically limited, drag is equal to the forward motion provided by your drivetrain.
as you get faster, it takes considerably more power to go any faster.
look at modern cars and compare the gearing, power, weight, friction, and drag.
then look at their top speed.

usually, if you can find a car with everything else the same, except drag, your gonna notice a huge difference in top speed.

basically what my long rambling confused post is trying to say is that i dont think a mkII is possible of an honest 160mph without insane power.

its easy to read a speedo and say my car does 120 or 150 or whatever, but we all know stock instruments are pretty inaccurate.
anyone wanna go to bonneville and bring a hot supra?? :D
 
#15 ·
Power goes as the speed cubed so if 161 hp gives u 127mph (car and driver top speed as i remember) then u need 161* (140/127)^3 or 216 hp
to get to 140 mph. prolly a little more hp required since wp losses prolly get horrific at 6500 rpm.
 
#16 ·
According to StanS formula, how much power would it take to do 160 MPH with a 3.7 diff?
Just wondering, and too lazy to work it out myself.
I think that formula may be a little simplistic, as it doesn't take into account aerodynamics and drag, as mentioned by dogstar.
(I should have mentioned the drag myself, which is why I said it would take an awful lot of power to hit 161)
 
#17 ·
OK I worked it out using StanS supplied formula = 328 hp
I still think it would require more than this though.
It's just a gut feeling, I don't have any proof, but I would expect you to need at least 350 and maybe 400 hp
 
#19 ·
as it doesn't take into account aerodynamics and drag,
It does take aero and drag into account since the same car is used in both cases. It doesn't take non-linear losses (such as wp losses) with rpm into
account. I'm only guessing that wp losses are not linear i.e. double the
rpm and get more than double the loss.
 
#20 ·
In fact the formula is solely based on aero drag. It predicts the power
required to move the car at any speed. Gearing has nothing to do with it. Gearing only keeps the motor operating in it's design rpm range. Gearing will require different motor Torque so the motor will produce the required hp predicted by the aero based formula.
 
#21 ·
StanS, I wasn't really trying to argue with you.
I've always agreed with your formulas, I've just never heard this one before. You admit it may be a little more than what the formula suggests.

I'm trying to understand this - I thought that if you were trying to push a car shaped like a brick through the air it would take more hp than a more streamlined one. This is cd isn't it?

But if you have any downforce, the faster you go, the harder it pushes you to the road, therefore requiring more hp.

So don't we have air resistance and rolling resistance?

Or is air the only important resistance, which is why engineers have wind tunnels?

And also, I think I read somewhere, that without gearing multiplication ie. 4th gear (1:1) or 5th (overdrive) you can't get any more revs than where your peak hp is developed. So if peak hp is developed at 6100rpm, you couldn't do 6500rpm in 4th or 5th.
Is this correct?
 
#22 ·
When car and Driver did top speed test a few years ago with tuner cars, they had a neat little horsepower formula from top speed and tire size gear ratio-basically, if you arent driving a brick, 400 hp=160mph-500=175-600=185 and you need 750 to do 200-Callaway sledgehammer had 880 or so and did 217 mph with slippery vette body-I would love to do Bonneville some day in any car-what a rush-back in the day, everybody had real problems crackin 200 at Bonneville-Air resistance/lift becomes a reaL BIG FACTOR at about 175
 
#23 ·
didn't take it as arguing or flaming. just trying to reply with info. medium distorts intent as we're aware.
I'm trying to understand this - I thought that if you were trying to push a car shaped like a brick through the air it would take more hp than a more streamlined one. This is cd isn't it?
yes

But if you have any downforce, the faster you go, the harder it pushes you to the road, therefore requiring more hp.
reaching the limits of my knowledge here but up/downforce is produced by the pressure pushing down on car top minus pressure pushing up on car bottom. since air path over car top is longer the air "moves" faster over car top and due to Venturi effect pressure on car top is lower than on car bottom resulting in more lift at hi speed. think that cd takes this into account.

So don't we have air resistance and rolling resistance?
yes, as i said formula only takes aero into account, but i believe that aero is by far the most dominant effect.

And also, I think I read somewhere, that without gearing multiplication ie. 4th gear (1:1) or 5th (overdrive) you can't get any more revs than where your peak hp is developed. So if peak hp is developed at 6100rpm, you couldn't do 6500rpm in 4th or 5th.
Is this correct?
Yes. Remember that Power = Torque x [Speed (or RPM in a gear)]. Torque (to ground) is what pushes car, not hp. At gear ratio of 1:1 (or higher) and peak hp, T must be falling with increasing speed (RPM) , by definition, since constant T with increaing RPM would yield increasing hp and we said we just passed peak hp. As RPM and speed go up, aero drag requires more T to push the car, but T is falling with increasing RPM past peak hp.
hope this makes sense.

Sorry, i didn't respond earlier but i lost topic notification.
 
#24 ·
OK It's making sense now.
Your maximum speed would be at the RPM that you're creating maximum torque. For a 7M-GTE that would be 3200 RPM, no just re-read you post again, it's where hp starts falling off, so for the above 7M-GTE that's 5600 rpm


Thanks StanS
 
#26 ·
Thanks Jamie
I was really just thinking about 4th at the moment, trying to decide what difference (if any) there would be changing from a 3.9 diff to a 3.7
I know it would give lower revs per mph, but would you also go faster?
 
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