how to increase maximum speed?

[arsenicpants]

My car tops out at 110/120 km/h, and it always has
from when it was an NA diesel with a 7A, to a 1.6TD with a 7A, to now with a 1.6TD and an FN
from stock size wheels, to 15" wheels with 50 series tires, to now 13's again with 50 series tires
every time i do these upgrades i hope to God it will go faster than 110 km/h but it never does, and it's driving me absolutely crazy
so what the heck is wrong? what can i do to increase the top speed? i don't mean like "YEAH, I WANT TO GO FASTER!" i mean "Yeah, i'd like to be able to keep pace with traffic and not feel like the engine is about to explode"

can anyone help me out?

[rabbitman]

How much boost are you running? Does it smoke when you're trying to go faster? How high is it revving?

I've had my NA rabbit with a FN tranny up to 80-85mph with the revs around 3500rpm.

Is your governor kicking in?

[arsenicpants]

Either you're driving a 1.6TD school bus or you've got issues.

that's exactly what i mean; what could the issues be?

How much boost are you running? Does it smoke when you're trying to go faster? How high is it revving?
Is your governor kicking in?

it's running about 15lbs of boost with a k14. it doesn't smoke, no grey, no white, no black, nothing. it's revving really damn high at the top of 5th, although i don't know exactly since it's a mk1 and finding or making a tach for it isn't exactly easy
it has not been de-governored as far as i know, or can tell, although having never driven a car that i 'know' has been de-governed i'm not sure. the previous built the motor and i'm waiting on conformation from him

can extra power increase maximum speed?
i was under the impression that more power just got you to that speed faster?

the motor has relatively low mileage (rebuilt less than 50,000 km ago) and runs strong

[Vincent Waldon]

can extra power increase maximum speed?

The simple answer is: yes.

The simple explanation is that a vehicle's speed is the balancing point between the energy the engine puts out vs. the energy the vehicle consumes.  The vehicle "consumes" energy thru forces like friction but most notably (in terms of max speed) wind resistance... a resistance that increases with speed.

More engine power -> more energy to push thru wind resistance -> higher top speed.

You can also see that decreasing friction is another way to gain top speed... dragging brakes?? Tight wheel bearing(s)?  Poor wheel alignment?  Beat tires / low tire pressure ?

[mystery3]

There is a point where extra power will not overcome the aerodynamics of a given vehicle but you are nowhere near that point and I don't think a 1.6td could ever be tuned to get you there in a mkI.

Use this to calculate rpm for a given speed: http://www.scirocco.org/gears/

[arsenicpants]

thanks
so i guess i've got a lot of tuning to do, and doing the governor mod if it hasn't been done
i'm almost positive it hasn't, it really doesn't feel like it from what i've read about it

I've used the gears table a bunch of times to help me calculate, but it seems pretty inaccurate
i mean, if more power can increase top speed, then wouldn't the RPM at any given speed change depending on the amount of power you're making?
according to the table, if i'm at 4500 RPM at 110 km/h now, i'd have to be running 7 or 8000 RPM before i'd get to anything close to normal

also, has anyone noticed that if you you put 'Kph' instead of 'kph' (first letter capitalized) you get a completely different RPM reading?

[745 turbogreasel]

Unless you have a van pump, you should be able to rev to 5200.
I had an NA Fox up to 100 MPH, but it took awhile to get there.
Turbo it sholdn't be  a problem, gear limited speed in my caddy+T was 126 (200 KMH)
I never got it going that fast, but criusing at 75-80MPH was no issue.

is your fuel filter 10 years old?

[carrizog60]

you have to know your rpm.
if you are toping out your rpm the only soluction is a longer geared gearbox.
no matter how much power you have if you dont do more rpm or change gearbox ratios, you will never have more spped.

110 is really low!
try to get a 4s or 4t gearbox, they will do around 180/200km/h.

i had that problem on my passat,it would top at 180.changed to a tdi gerabox and no more high rpm/low speed problems.

[theman53]

If the gov mod hasn't been done then you are not reving too much. It will stop you when it hits it. Since you don't have a tach and don't know what it is reving then I would say you probably aren't reving as much as you think. It is basically more like a gas car in terms of revs. It sounds pretty bad unless you have seen one rev too much then you realize what these can handle.
I would ask what the PO timed the engine to and check out how to make your 1.6td a faster car. The main fuel screw is your friend.

The FN gearbox is pretty tall. .71 5th and the 3.89 final...the only way to get more top end would be with a 3.67 final. It isn't your gearbox.

[8v-of-fury]

I have a 325,000km n/a bolted to a FN in my 84. I did 155km/h on the .... er.... closed course of course

pic for proof,


With my FN and tires 185/60/14's, I was spinning about 3950. Also I have done 130 in 4th, which i spin 4300.. OR 115 in 3rd! 5250!

[rabbitman]

has anyone noticed that if you you put 'Kph' instead of 'kph' (first letter capitalized) you get a completely different RPM reading?

I just messed with that and if you put anything but kph the calculator defaults to mph.

What size tires do you have? I used 175/70R13 and a FN tranny (my setup) in the calculator and @ 4500rpm = 176kph. It appears as if your max rpm in 5th is 2802rpm.

if more power can increase top speed, then wouldn't the RPM at any given speed change depending on the amount of power you're making?
according to the table, if i'm at 4500 RPM at 110 km/h now, i'd have to be running 7 or 8000 RPM before i'd get to anything close to normal

Doubling the rpm in a given gear WILL double your speed, 8K rpm will get you up to 314kph or 195mph.

[maxfax]

What is your exact tire size??   Roughly figuring, with at 7A at 110kph (68 mph) you should only be turning the engine a tad under 2800 rpm.. Even without a governor mod you got more to go... 

You say it was NA and now is TD, is this with the same pump???  I'm not saying you can;t use a NA pump, on a TD, but if it's the same pump, maybe there is a pump issue..  Or else you got some other variety of fuel starvation issue such as an air leak, plugged screen in the tank, 10 year old fuel filter..  If brakes were dragging that bad you'd have smelled that by now..  What's the timing set at??  How's the clutch??? 

[zagarus]

i've had my old stock 1.6td jetta at 190km/h once. Down hill...A really long hill.  Rev's were maxed out with a 4S tranny.

[arsenicpants]

What is your exact tire size??   Roughly figuring, with at 7A at 110kph (68 mph) you should only be turning the engine a tad under 2800 rpm.. Even without a governor mod you got more to go... 

You say it was NA and now is TD, is this with the same pump???  I'm not saying you can;t use a NA pump, on a TD, but if it's the same pump, maybe there is a pump issue..  Or else you got some other variety of fuel starvation issue such as an air leak, plugged screen in the tank, 10 year old fuel filter..  If brakes were dragging that bad you'd have smelled that by now..  What's the timing set at??  How's the clutch??? 

The motor and pump came as one, and it's a proper TD pump, it doesn't leak or anything
I replaced the filter when I did the swap
Actually, I updated it to the mk2 style filter at the same time
Looking at the fuel lines when it's running I can't see any air bubbles, well maybe 1 or 2 small ones, but nothing huge
No idea what the timing is set at, I'll have to figure out how to tell
It could very well be something in the tank, is there a way I can test to see or run something through to possibly break it up? I'm guessing I'll have to crack open the tank and have a look

Thanks for the suggestions, I'll make a list and see if I can track down the issue, or issues

[maxfax]

From a safety standpoint this isn't a good idea for driving down the road at 120 kph, but If you could run it off a container of fuel instead of the tank that would work to rule it out.. Does the engine seem to rev up to a certain point then poop out at the speeds mentioned, or can you push it any further(even though it sounds like it's screaming already at that point)


Since it has been 2 different pumps, I kinda *suspect* that isn't your problem. 2 difference engines can sort of rule out timing too, but it can't hurt to check..  Timing retarded would make it start like crap too..

Are you absolutely sure you have a 7a trans??  Another thought, although incredibly rare and odd, how's the exhaust look?  Does the muffler have anything rattling around inside?