abarth squadra storica forum
abarth cars - community => 1000 tc , radiale , replika => Topic started by: Pantdino on November 13, 2007, 06:28:12 am
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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
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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¤t=MVI_5285.flv (http://s45.photobucket.com/albums/f79/kikolemus/Abarth%20Record%20Monza/?action=view¤t=MVI_5285.flv)
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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¤t=MVI_5285.flv (http://s45.photobucket.com/albums/f79/kikolemus/Abarth%20Record%20Monza/?action=view¤t=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?
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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
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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.
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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
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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
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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
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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
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Gil,
thanks so much for posting that table--its' very helpful.
Jim
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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.
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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
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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
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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???
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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.
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I hooked up my electronic tach that is part of a diagnostic machine I have and compared what it said to what the car tach said.
The results were hard to make sense of until I compared the photos of 8000 and 10,000 rpm Abarth tachs in Tony Berni's catalog.
If you assume someone put at 10000 rpm face on a 8000 tach then the numbers I got match very well-- so I think that's what happened.
I KNOW the speedometer has that problem-- it has a 200kph face on a 160kph speedo, and you can even see the raised shapes of the original numbers beneath the new paint if you look with the light reflecting just right. So it reads 130kph when you're really going 100-110. That makes it feel like you're going fast but is no help in avoiding tickets from the police.
Can replacing the speedometer face be done yourself, or do you have to take it to a speedometer repair company?
Jim
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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
These numbers correspond to those in Tony Berni's catalog, which says they are with 135-13 tires with circumference of 1.708m.
The tire size for good speed and acceleration will depend on how much power your engine makes and how much acceleration you feel is enough
Jim
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Hello Jim,
The "standard" differential ratios for Fiat 600D was 4.88 (8/39). The standard 4th gear is .896.
The top speed of an automobile, in any gear, is dependent on the rolling circumference of the tires, the differential ratio and the ratio of the gear that you are in.
Here are some speeds in MPH (Kph), at 4000 and 6000 RPM in fourth gear, using a 185/60X13 tire, based on the various differential ratios available.
Diff ratio 4000 6000
8/43 (5.38) 53.69 (86.40) 80.53 (129.60)
8/39 (4.88) 59.19 (95.26) 88.78 (142.87)
9/41 (4.55) 63.48 (102.16) 95.23 (153.26)
9/39 (4.33) 66.71 (107.36) 100.06 (161.03)
As you can see the various differential ratios make a big difference in top speed. One problem however is that this assumes you have enough horsepower to pull that amount of gear. Lets face it, to do 160 Kph you need some serious horsepower to push a Fiat 600 along that fast. The Fiat 600 is not what you would call aerodynamically svelte. Likewise, the standard gearbox has a HUGE hole between 3rd and 4th. Add to this that 4th gear is an "overdrive gear", then you can easily see there is a further "mechanical disadvantage" working against you, in attaining anywhere near that top speed.
If we change the gearset in the transmission to a close ratio one, say with a 1.04 fourth gear, what happens to our top speeds?
8/43 46.26 (74.45) 69.38 (111.65)
8/39 50.99 (82.06) 76.49 (120.10)
9/41 54.69 (88.02) 82.04 (132.03)
9/39 57.40 (92.38) 86.21 (138.74)
As you might have guessed, the top speeds have dropped up to 14 miles per hour. More significantly with the close ratio gearset, the shift points are now spaced closer together, to where there is only a 1200 RPM drop from 3rd to 4th. If we assume that the motor delivers peak torque at approx 5000 RPM, then if we use 6500 as a shift point in third, then the engine will end up at around peak torque in 4th gear, giving us a good chance of using the available horsepower.
Assuming that we have 70+ horsepower to work with (a standard 1050 motor) then with a 9/39 differential gear and a close ratio gearset, then you will be at 5500 RPM for a 70 MPH (112 KPH) freeway cruising speed. I consider this to be a fairly good combination of components. BTW - It would not be unusual for a 1050cc motor to turn 7500 RPM or more. I would consider that your car could easily do that. At Scuderia Topolino we dyno street motors to 7500 RPM and race motor to 8500 RPM.
BTW the ratios of the 4 speed close ratio gearset are 3.38, 1.75, 1.20, 1.04 As you can see the gap between gears gets smaller the higher the gear ratio, as the amount of transferable torque at the wheels decreases due to less mechanical advantage in the gear ratio.
Yes Jim, I would say that you have an "optimistic" speedometer. You can solve this easily with an in-line conversion gearbox. You might want to contact Gails Speedometer Service 187 E 16th St, Costa Mesa, CA 92627 Phone: (949) 646-9120 . He has done work for me in the past and can organize the bits. He can also measure how far out the speedo is. Alternatively, one of the best speedometer repair/renovation shops is Palo Alto Speedometer, 718 Emerson St. Palo Alto, CA 94301-2410 Tel: 650-323-0243
I hope this helps.
Regards
Paul Vanderheijden
(http://scuderiatopolino.com/only the best label.jpg)
www.scuderiatopolino.com