Topic: how many RPM at 100 kph in your car?

[Pantdino]

Either the tachometer in my car is wrong or the car has non-stock ratios in the transaxle.  When I drive with other cars (Toyotas) to see what speed I am really going, my car shows 4862 rpm at 100 kph.
That seems like way too many.

What do your tachometers say when you are going 100 kph?

Thanks,

Jim

[kikolemus]

Jim,

I looked at the video of my old car and it looks like 4000 rpm for 100kph.
It had a 9/41 ring & pinion and 12" wheels.

Mike

http://s45.photobucket.com/albums/f79/kikolemus/Abarth%20Record%20Monza/?action=view&current=MVI_5285.flv [nofollow]

[alapimba]

Jim,

I looked at the video of my old car and it looks like 4000 rpm for 100kph.
It had a 9/41 ring & pinion.

Mike

http://s45.photobucket.com/albums/f79/kikolemus/Abarth%20Record%20Monza/?action=view&current=MVI_5285.flv

sorry for the offtopic but... the 9/41 was worth compared with the original or you ended up wanted to convert to 9/39?

[Pantdino]

Alapimba,

You're probably right--I wish I had bought that from you, but I wasn't aware of this problem then.  The other problem is that I do not know of anyone in my area who could install the new gears if I bought them, so I'd have to ship it to Scuderia Topolino or something. That would end up being expensive.

But first I need to check my tachometer against another one, since with 185/60-13 tires I'd have to have an overall ratio in 4th of 5.0 (rather than the stock 600D ratio of 4.37) to have that many revs at 100kph. From the information I have Fiat did not make a car with that ratio-- they were either higher or lower, so something is goofy here.

What engine revs do people with standard 600D transxles have at 100kph?

Jim

[viotti600]

The other problem is that I do not know of anyone in my area who could install the new gears if I bought them, so I'd have to ship it to Scuderia Topolino or something. That would end up being expensive.

  Changing the gears is relatively easy, it's properly setting the backlash that's tricky. Try PBS Engineering http://pbseng.com/ or Costa Mesa R&D (John Edwards) http://costamesardauto.spaces.live.com or http://www.engine-machining.com , both in the So. Cal. area.

But first I need to check my tachometer against another one, since with 185/60-13 tires I'd have to have an overall ratio in 4th of 5.0 (rather than the stock 600D ratio of 4.37) to have that many revs at 100kph. From the information I have Fiat did not make a car with that ratio-- they were either higher or lower, so something is goofy here.

  The 600D ratio was not 4.37, it was 4.88 (8/39). 600D Multipla had 5.38 (8/43). You may have either of thoese. The 13" wheels/tires would throw the final drive ratio calculations off a bit, since the 600D originally used 12" instead.

-JS.

[Pantdino]

It seems to me that matters is the overall ratio,   final drive 4.875 x 4th gear ratio  0.896 = 4.37.
Since I know the circumference of my tires (I measured it myself with string) and the ratios the car should have (4.37 overall), I can calculate that it should be turning 4110 rpm at 60 mph. The fact that the tach says 4700 is either 1) tach error, 2) speedo error in the Toyota, or a combination of both.   I'm sure the Toyota is not off by more than a few percent (the other Toyota I compared to read even lower), most of the error has to be the tach.

This theory is given strength by the fact that the previous owner said he suffered bearing failure while racing at 8500 rpm. I suspect that at that rpm the crankshaft or a rod would break.  So I suspect he wasn't turning as many revs as he thought.

Hopefully I'll have time tomorrow to hook up my other tach to see what it says.

Jim

[ChrisD]

I am using 165-55-13 tyres in my 600 with stock gearbox setup (8/39).
I compared my speed with a GPS not another car. It is much more reliable than another cars speedometer. Seems that my car only has a top speed of 120Km/h at full revs. The bad thing is that I still have no tach installed in my car to compare with you. I know that the low top speed of mine is because of the small tyres, but I like the grip they have in cornering so I prefare them.

Does anybody know the optimal setup of tyres and crown-pinion to get the lowest rpm possible at 100km/h I hear of 9/41 and 9/39 witch are better than the stock 8/39 but what is the diference between the 9/41 and 9/39??

Chris

[zippyfiat]

Here is a table from Rich Motor's booklet  "Tuning and Modifying the Fiat 600 Engine" from the early 1960s.  This might help?  The speeds shown are for 5.50-13" size tires.  You would need to know that tire overall diameter, which I don't, as that is obsolete designation.  There is also another table for a modified 4th gear ratio of 30/26 (.867) if anyone is interested.

BTW, I have had the John Rich booklet (a photocopy) for years, but have never read it until a few days ago.  It has some interesting info. in it!

Gil

[Pantdino]

I am using 165-55-13 tyres in my 600 with stock gearbox setup (8/39).
I compared my speed with a GPS not another car. It is much more reliable than another cars speedometer. Seems that my car only has a top speed of 120Km/h at full revs. The bad thing is that I still have no tach installed in my car to compare with you. I know that the low top speed of mine is because of the small tyres, but I like the grip they have in cornering so I prefare them.

Does anybody know the optimal setup of tyres and crown-pinion to get the lowest rpm possible at 100km/h I hear of 9/41 and 9/39 witch are better than the stock 8/39 but what is the diference between the 9/41 and 9/39??

Chris

Chris,

If you don't have a tachometer, how do you know when you are at full revs?  Do you judge by the sound of the engine?

You calculate the ratio by dividing the top number into the bottom one, so 9/41 is 4.555 and  9/39 is 4.33.  Multiplying these times the 4th gear ratio (0.896 for stock 600D) gives you the overall ratios,  4.07 and 3.88.  This is the number of times the engine has to turn over for each time the tires rotate, so a lower number gives you lower revs at a certain speed. 

So the 9/39 gears would give you the quietest cruising, but also remember that as you lower the ratio your accleration gets less. so if you come to a hill the car will tend to slow down you'll have to downshift.

Jim

[Pantdino]

Gil,

thanks so much for posting that table--its' very helpful.

Jim

[viotti600]

It seems to me that matters is the overall ratio,   final drive 4.875 x 4th gear ratio  0.896 = 4.37.
Since I know the circumference of my tires (I measured it myself with string) and the ratios the car should have (4.37 overall), I can calculate that it should be turning 4110 rpm at 60 mph. The fact that the tach says 4700 is either 1) tach error, 2) speedo error in the Toyota, or a combination of both.

Try using the 5.38 final drive in your calcs, just for kicks. The calculated rpm's may be a little closer to your shown 4700. (5.38 x 0.896 = 4.82 absolute ratio)

This theory is given strength by the fact that the previous owner said he suffered bearing failure while racing at 8500 rpm. I suspect that at that rpm the crankshaft or a rod would break.  So I suspect he wasn't turning as many revs as he thought.

Or...he had gearing giving high rpm per mph, allowing a high-rpm-powerband engine to do it's "work" more efficiently per certain desired speeds (race courses). 8500rpm is not unheard of on well-tuned race engines.

-JS.

[Pantdino]

Try using the 5.38 final drive in your calcs, just for kicks. The calculated rpm's may be a little closer to your shown 4700. (5.38 x 0.896 = 4.82 absolute ratio)

Working backward gives 5.01 as the overall ratio. Fiat didn't make any gears that would combine to make that overall ratio.  Now that I have my transaxle filled (with Red Line MTL) I can run the engine and hook up my other tach.

Jim

[ChrisD]

Chris,

If you don't have a tachometer, how do you know when you are at full revs?  Do you judge by the sound of the engine?

You calculate the ratio by dividing the top number into the bottom one, so 9/41 is 4.555 and  9/39 is 4.33.  Multiplying these times the 4th gear ratio (0.896 for stock 600D) gives you the overall ratios,  4.07 and 3.88.  This is the number of times the engine has to turn over for each time the tires rotate, so a lower number gives you lower revs at a certain speed. 

So the 9/39 gears would give you the quietest cruising, but also remember that as you lower the ratio your accleration gets less. so if you come to a hill the car will tend to slow down you'll have to downshift.

Jim

Just like you told...with sound, plus I can really feel that the engine will not rev up anymore. I will have one installed as soon as I build a custom fiberglass housing for it. Then I will know evrything.
I knew the basicsa about the 9/41 and 9/39. What I didn't knew was the diference at top speed witch is shown in the table Gil posted. Thanks Gil!!! It really helps.
Does it mention anywhere what is the tyre size so that we can compare that too? I once had taller and wider tyres and I noticed the top speed was bigger. What I don't know is what is the proper tyre size to allow both a good top speed and good acceleration when using a 9/41 or 9/39 diferential

[viotti600]

Working backward gives 5.01 as the overall ratio. Fiat didn't make any gears that would combine to make that overall ratio.

The 600 Multipla ring & pinion set = 8/43 = 5.38. Using the "4th gear = .0896" info you gave earlier, then final drive 5.38 x 4th gear 0.896 = 4.82 overall.

Where did you get the "5.01" from???

[Pantdino]

Working backward gives 5.01 as the overall ratio. Fiat didn't make any gears that would combine to make that overall ratio.

The 600 Multipla ring & pinion set = 8/43 = 5.38. Using the "4th gear = .0896" info you gave earlier, then final drive 5.38 x 4th gear 0.896 = 4.82 overall.

Where did you get the "5.01" from???

I got 5.01 from the measured circumference of my tires and calculating how many times they would go around in 1 mile. Since a car goes one mile per minute at 60mph, at that speed that number is also the RPM.  So if you know the other data you can figure out what the overall ratio must be.