Simple Maths - or is it?

[BLOWN]

Alright, i didn't want to get into this elsewhere so i figured i'd put it here.

On my last pass in the Monaro i did 115.39mph but i have a problem with this

My diff ratio is 3.91
My tyres are 235/35/19 (25.48" diameter)
and i did the whole run in 3rd which is my 1 to 1 ratio
Rev limit set to 5800

The formula for MPH is as follows
MPH = TIRE RADIUS ÷ 168 x ENGINE RPM ÷ GEAR RATIO

You can check it here:
http://www.edelbrock.com/automotive/math.html

Now putting in my figures

MPH = 12.74 ÷ 168 x 5800 ÷ 3.91 = 112.48mph

How the hell do i get 115.39 when the numbers say 112.48 is the max

I know tachos can be out so im not going off the dash, when it was tuned the limiter was set at 5800 so that cant change
They are normal street tyres so they wont exactly be expanding enough to increase mph by any large amount
they cant be wrong with the diff gears as they my cruise speed would be totally different with 3.73's
Gearbox ratios dont change

Any ideas people?


I plan to try different diff gears which would result in the following maximum speed:

With 3.46 diff gears
MPH = 12.74 ÷ 168 x 5800 ÷ 3.46 = 127.12mph

With 3.73 diff gears
MPH = 12.74 ÷ 168 x 5800 ÷ 3.73 = 117.92mph

And if i then change to a set of Nittos (for example) in a 245/45/17 that would chage the tyre diameter to 25.68" will increase the tyre figure to 12.84

With 3.46 diff gears
MPH = 12.84 ÷ 168 x 5800 ÷ 3.46 = 128.12mph

With 3.73 diff gears
MPH = 12.84 ÷ 168 x 5800 ÷ 3.73 = 118.84mph


And if i then change to a set of Nittos (for example) in a 305/45/18 that would chage the tyre diameter to 28.80" will increase the tyre figure to 14.40
Although i dont know it i could fit this - although air bags help

With 3.46 diff gears
MPH = 14.40 ÷ 168 x 5800 ÷ 3.46 = 143.68mph

With 3.73 diff gears
MPH = 14.40 ÷ 168 x 5800 ÷ 3.73 = 133.28mph

Naturally this is just numbers and doesnt take into account power, weight etcd. It's just to show the maximum MPH possible with the tyre/diff combination

[pagey]

I know tachos can be out so im not going off the dash, when it was tuned the limiter was set at 5800 so that cant change



Without getting into the accuracy of the formula & any associated variables.. just be aware that while your limiter may very well be set @ 5800 there is still a chance it is actually reaching higher revs.

This has happened twice to me.. no matter what the limiter was set to (edit) the limiter ran to 6400/6500... (A4) - I am pretty sure there may have been a little bug in the trans that helped (hindered!) however I have had it happen to me again since in a different car (not the VY).

This may not be happening to you of course - & you may probably pick the difference between 5800 V's 6000 (or maybe not?) but it may be one part of puzzle....

[ROARZ]

Any ideas people?

I have two pieces on input on the 112 mph vs 115 mph.

1) Measure either the circumference of the actual tyre on your car, or the actual diameter. I found mine were not exactly the same as the rule of thumb for 235/40/R17, 245/45/R17, and 245/35/R19. The small differences between target and actual size might be enough to explain 3 mph difference. My calculations show that you'd need a radius of 13.07 to get to 115mph, but it would be interesting to know what your measured radius in. Obviously too hard to know what it is during the actual run, although you have already spoken about that in your post.

2) Check to see what the 5800 rpm rev limit actually does in the factory computer? I'm using the Motec M800, so course we can't assume it does things the same, but my rev limit is set to 6000, but it doesn't actually shut the engine down at 6000 rpm. I believe there is a staged approach, where timing and fuel is reduced and then there is a hard kill at 6100 (or something like that). I guess the theory is that when I hit 6000 rpm, fuel and timing is taken out, and that will drop the rpm down under 6000 rpm at which time things fire back up again (hence that sound you get when you "bounce" off the rev limiter in an EFI car). What my log files tell me is that I actually get to 6040 rpm or so during the exercise, and just the smallest extra RPM could be making that difference to the MPH you are seeing. Perhaps you getting a few extra RPM and hence the extra MPH. I did the math, and if you're getting to 5950 RPM, then your 115 MPH is dead on correct. Combine it with output from (1) above, and perhaps you only need to get to 5850 or 5900.

Perhaps you should go back to the stock auto diff for a couple of runs... 3.07's right? Run those through the numbers with some tall tyres... hee hee...

[ROARZ]

Oh, and your other point is taken... Which is that MPH is very much a factor of your ability to tractively spins the wheels fast enough with the gearing in the car, and that you have enough power to actually turn them that fast against the weight of the car and friction. Horsepower and weight are big pieces in the final equation.

[BLOWN]

And if i then change to a set of Nittos (for example) in a 245/45/17 that would chage the tyre diameter to 25.68" will increase the tyre figure to 12.84

With 3.46 diff gears
MPH = 12.84 ÷ 168 x 5800 ÷ 3.46 = 128.12mph
i will probably go for this option as the gears and tyres are available to me and it most represents what general people would run
I think it should, even with the weight and my busted ass driving style, be able to run around 120mph (matbe more) with the kit as it is in full EPA form.

[ROARZ]

Interestingly, I have 245/45/R17 Falkens that I've been using for my latest runs (3.46 diff), but I'm maxing at 127.5mph at 6000 rpm. Using the calculators, that would suggest that they measure up more like a 245/40/R17, because a 245/45/R17 at 6000 RPM should turn at 132.53 mph and a 245/40/R17 at 6000 RPM should turn at 127.58 mph... I'll have to go measure them...

Here's another good calculator resource:
http://www.speedworldmotorplex.com/calc.htm

[BLOWN]

running under your maximum mph would just indicated traction issues even if you are hitting the limiter on the line

I have talked to a number of racers years ago about this and a lot of them tend to hit red line prior to the line - although they will usually have the limiter set higher than their red line

[Malcolmsp]

Just to confuse you even more...the trap speed is the average of your speed over the last 66 feet according to the rule book anyway.

Although you would be accelerating slowly, you would still cross the line at a slightly faster speed.

cheers
Mal

[BLOWN]

Just to confuse you even more...the trap speed is the average of your speed over the last 66 feet according to the rule book anyway.

Although you would be accelerating slowly, you would still cross the line at a slightly faster speed.

cheers
Mal
Ok, now that really confuses the issue :confused:

[ROARZ]

i will probably go for this option as the gears and tyres are available to me and it most represents what general people would run
I think it should, even with the weight and my busted ass driving style, be able to run around 120mph (matbe more) with the kit as it is in full EPA form.

Heh, heh... Go on, put the stock GenIII auto 3.07's in... With your current tyre setup that comes to:

MPH = 12.74 ÷ 168 x 5800 ÷ 3.07 = 143.26 mph

:D Got Torque? :D

I concur with your selection of a 3.46. While the 3.9's and 3.7's are great for street use with stock output motors (as they get the engine up into the torque producing range more quickly), they really don't suit the higher HP setups. We found this out in the Corvette ZR-1 community a few years back, as everyone went for 4.11's on their stockers for the same reason, and then when the 383's and 405's stated being built the 4.11's were just wheel spin city... A lot of people were able to put their stock gears back in, and sell 4.11's on the second hand market. 3.46's are a nice balance for street cars with race use. You still get a good top speed on the strip and track, and you still get nice tractable take off on the street.

[BLOWN]

Heh, heh... Go on, put the stock GenIII auto 3.07's in... With your current tyre setup that comes to:

MPH = 12.74 ÷ 168 x 5800 ÷ 3.07 = 143.26 mph

:D Got Torque? :DWould be nice but there's not enough power for the weight - and the car weighs a lot too :)


I concur with your selection of a 3.46. While the 3.9's and 3.7's are great for street use with stock output motors (as they get the engine up into the torque producing range more quickly), they really don't suit the higher HP setups. We found this out in the Corvette ZR-1 community a few years back, as everyone went for 4.11's on their stockers for the same reason, and then when the 383's and 405's stated being built the 4.11's were just wheel spin city... A lot of people were able to put their stock gears back in, and sell 4.11's on the second hand market. 3.46's are a nice balance for street cars with race use. You still get a good top speed on the strip and track, and you still get nice tractable take off on the street.It sould make everything work harder too which will make the turbos produce more grunt i hope :)

[dean]

Just to confuse you even more...the trap speed is the average of your speed over the last 66 feet according to the rule book anyway.

Although you would be accelerating slowly, you would still cross the line at a slightly faster speed.

cheers
Mal

I agree with this scenario. Try doing 2 runs: 1 full throttle across the line and 1 back off at about 100' from the end. The diff in mph will be substantial, but the et may only be a few 10ths.

[DaveHAT]

How the hell do i get 115.39 when the numbers say 112.48 is the max
Any ideas people?



Blown,

Perhaps a bit left field in the thinking here, but your 115.39mp/h run, was it in the left or right lane?

Reason I ask is, the left lane @ WSID can have a higher mp/h reading than the right lane. :confused:
PM OZTRACK, he has many of timeslips from his runs at WSID that would seem to indicate a mp/h difference from the left lane to the right, with the left being higher.

Just a thought. ;)

[BLOWN]

Perhaps a bit left field in the thinking here, but your 115.39mp/h run, was it in the left or right lane? :) Already been told about that but i was in the left lane

[Mongy]

:) Already been told about that but i was in the left laneThen you just answered your own question :confused:

[DaveHAT]

:) Already been told about that but i was in the left lane

Exactly, left lane mp/h reads higher than the right. So mp/h in right lane probably was as you calculated 112.48, but left is higher, thus 115.39.

[BLOWN]

Then you just answered your own question :confused:Doh, sorry, i thought you said right
That should explain it then
Although i did get a 113.33 in the right

[Mongy]

Although i did get a 113.33 in the rightTyre growth would more than cover that.

[STATIE]

On my last pass in the Monaro i did 115.39mph but i have a problem with this


and i did the whole run in 3rd which is my 1 to 1 ratio



Are you fair dinkum or what?

You wonder why you've got a relatively (considering the power you've got) shitty MPH/ET and then you say you did the whole run in 3rd.
You got a full manual box? :confused:

Put some decent tyres on the thing and run a decent time.
In my limited experience with an A4 on street tyres even backing off for a split second in order to gain traction can lose up to 10mph and add .5 to .8 to your ET.

Realistcally you should be running 125mph plus.
Stuff the formula's and theory - get decent tyres and put it into practice.

:cheers:

[BLOWN]

Are you fair dinkum or what?

You wonder why you've got a relatively (considering the power you've got) shitty MPH/ET and then you say you did the whole run in 3rd.
You got a full manual box? :confused: Auto



Put some decent tyres on the thing and run a decent time.
In my limited experience with an A4 on street tyres even backing off for a split second in order to gain traction can lose up to 10mph and add .5 to .8 to your ET.Not interested in putting slicks on it
And it can't go 125mph with the gearing - impossible