Topspeed vs. RPM do not make sense

Gentlemen, hello.

I am new to this brand and this forum, but hope someone can answer what I could not find in literature.
(I have read the threads on rev limiters, hence there should be one kicking in at some point. I have also read the thread on the possibility of different gear ratios on the T depending on region, but still could not find what I am looking for)

The car: 1991 built Mondial T, US-delivered.

The question:
These cars are said to have top speed of 255km/h (=159mph). (OK, that is EU-literature but still).
They are said to have a max rpm of 7500.
Driving my car now, at above 7000rpm (in 5th of course) I barely drove 125mph
Is there such a hughe difference between US and EU gear ratios ?

Could not believe it, so I had to know where it maxed out on, but NO rev limiter came in, not even well above 8.000rpm (probably 8.200) reading on the tachometer, speed however on GPS read only 137mph whilst the speedo touched the 140mph.

It does not make sense to me, unless I have a 17% shorter gear.
There was still a bit of air underneath the accelerator, but given the high rpm reading I did not try any further for max speed. Still it did not seem to me it would reach the promised 159mph.
The car does not seem to be modified in any way, so might the tach be inacurate?

Thank you for reading

 

Your first mistake is to rely on the cars instruments. Quite common for them to be wrong. Magazines always use their own instrumentation for performance testing no matter the make and Ferrari is quite well known for inaccurate instrumentation.

 

Some speedometers read high, never low. I guess that's to help prevent speeding tickets. The spec I'm seeing on top speed is 155. Top speed depends a lot on air resistance. So if you have a head wind, that will slow you down. This is why some speed tests require you to take runs in both directions. The throttle should be open all the way. The motor should be tuned up, and have good compression. Use synthetic oil, and a good quality fuel.

 

Thats just not true. I have sent many out for repair reading low. We are not talking about any designed in inaccuracy here.

 

Some regions such as the EU have regulations on speedometers reading low:

https://www.fleetco.co.za/index.php/information/speedo-accuracy.html

It's only logical that this would be affecting the design.

 

I guess you just have not considered the fact that they can break. Well they do and I deal with it regularly.

In a word, Ferrari instruments of the period are crap.

 

Yeah, but they look nice !

 

This test shows 156 MPH @ 6,367 RPM

http://alexachua.com/autocar_1981.pdf

 

Thank you for all these replies. Indeed it seems the tach reading had to be wrong, but still it would mean a 30% off at high revs.
cruising at 65mph reads aprox. 2.800 revs and that would correlate with the values of this test.
So at some point my tach just starts double counting the impulses it gets.
Is there a simple way of checking or even fixing this?

 

That's very accurate then - within 2% of true speed and on the correct side of the legal tolerance. No reason to complain here...

 

Amen


Sent from my iPhone using Tapatalk

 

If you consider a weight incl. driver of 1'590kg, 221kW of power, a driveline efficiency of 84 % (gearbox + final drive + 4 cv joints), a frontal area of 1.92 sq.m. and you want to achieve 255 km/h (the design speed at the max. power rpm of 7200), the drag coefficient would have to be 0.33. That's most unlikely...
However, according to Paul's test, the car did achieve 246km/h. With the above data but a Cd of 0.38, slightly lower than the QV/3.2's 0.39, it works out and the engine did indeed deliver the claimed 221kW.

 

1981 euro mondial 8;
factory transmission

clocked at 60 mph at 3300 rpm in 5th...
and, I got BIG 60 series rally type rear tires.
?????

125 max????

 

Edwardo, according to the Euro homologation papers, the Mondial 8 will reach 132mph with its gearing @ 6'600rpm=max power.

If you consider a weight incl. driver of 1'582kg, 157kW of power, a driveline efficiency of 81 % (transmission + gearbox + final drive + 4 cv joints), a frontal area of 1.91 sq.m. and a claimed Cd of 0.39, you can achieve 134mph. That's consistent with the above gearing.

On your car, I would test/adjust if necessary both the speedo and the tach. Obviously, something isn't consistent. At 3'300, you should be traveling at 66mph, not 60.

All of this only holds true with a standard rear tire at standard pressure, so that the effective radius is correct.

 

Oversize rear tires will make the speedo read low. Try the GPS.

 

...and reduce your top speed because you can't reach the top of the power curve due to the slower revving engine at the same drag.

 

Thank you "Afterburner" for that data of 3.300 @ 66mph.

So my speedometer is fine (after all GPS said 137 whilst speedo said 140) and within the margins, but the tach is completely off track.

Has the model T (Bosch Motronic 2.7) any OBD socket one could plug in a digital rev-o-meter to get some true RPM reading?
This is my first prancing horse and I don't want to fry up the engine for something as stupid as having wrong information as to the revs I am making.

Secondly: Are the instruments by any chance standard / bulk of FIAT only with nicer dials?
In that case I could just get a similar one and flip the faces.....

............ face off .......

 

Wait - that was Edwardo's 8. On the Mondial t, at 66mph you should be getting 2'980rpm.
Your speedo is indeed very accurate.
The t has 2 diagnostic sockets, one for each ignition unit, in line with a button to cycle through the error memory.
But I think your issue is with the tach itself since the engine seems to run fine.
I wouldn't worry too much about over revving the engine as it has a built-in rev-limiter.
The instruments to my knowledge aren't standard FIAT items with a different faces.

 

Our Mondial t has a limiter set at just under 8k. In fifth I can double the tach/100 and pretty much get get miles per hour. Your test data of 7500=159 is close to that. If you can find a timing light with a tach you could easily test it.

 

You can get a cheap digital tach to check against:

https://www.amazon.com/RacingPowerSports-Backlit-Digital-Tachometer-Meter/dp/B07931P2P1/ref=asc_df_B07931P2P1/?tag=hyprod-20&linkCode=df0&hvadid=241970175828&hvpos=1o2&hvnetw=g&hvrand=10874969874254126812&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032807&hvtargid=aud-801090373433la-629833188418&psc=1

I believe the tach signal comes from one of the ignition boxes on that one. You could try swapping boxes to test.

 

Thank you Spicedriver. I 'll check it out. Actually some of those devices look as an excellent replacement for the silly $5,-- digital clock that spoils the center-console experience....

 

If you have the Waze app, it uses the satellites to track your location and speed. When active, a little speed limit sign shows up with the correct speed from above. Just remember, most speedos are a bit optimistic.

Regards,
Jerry

 

Hi all,, lil tardy, was busy chasing parts and tail.

Thanks for all the speedo and Cd info,,,, I’ve been looking to model it.
My Mondial gets enough Highway Patrol attention anyway,,
Don’t want to be 6 mph over
:-/
Sorry oscifer,, it’s a rental Ferrari.
:-/
Eduardo

 

You can buy a stand alone tach that clips on to a plug wire. They are cheap and accurate.

 

Not sure about the t tach, but on the QV, there is a pot in the backside of the tach which allows adjustment.
I had a failing tach a few years ago and moving that pot left and right removed corrosion and the tach works perfectly since.

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