Changing the final drive ratio...

I am looking at the pictures of the gearbox and differential on the 456M and the 550. They both have different gear ratios and final drive ratios. But by multiplying these to ratios together, the number is pretty much the same, but still, looking at my pictures from my two cars it appears that the 550 have slightly higher rpms in 6th gear. Now to the question:

Will the pinion and crownwheel from a 456M fit into the 550 gearbox? I mean the ratio in the final drive gears are 11/40 in the 456M (and probably 456 non-M) while the 550 has 11/43, which means that the axle goes slower around on the 550. What I am looking for is to lower the rpms. on the highway. If I could change the final drive ratio, all six gears would be slightly higher and the engine revs would drop. But when looking at the pictures, it looks like the biggest toothed wheel is facing the opposite way. Is this just the way the parts catalogue is done, or is it mounted differently? This would of course affect the rotation of the axle too. What sm I missing here?

And the speedometer... where are the signal to that taken from? Would the speedometer read differently if I manage to put the 456 gears in my 550 gearbox or would the speed be sensed from the rear axle/driveshaft so the speedometer will show correctly anyway...?

Any idea how much difference there would be in rpms.? Will it be minor or more significant and noticable? I might have to change one synchronizer anyway and then the box must come out anyway snd it would be the perfect time to make this modification. I would assume the box is identical except for the internal gear drives.


Thanks
Erik
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Thank you!!

Is it right to calculate the difference like this:

3,91 - 3,64 = 0,27

0,27 / 3,91 = 0,069 = 7% lower rpm.... that is not very much.

My wish would be to get 5th gear to match today’s 6th gear for top speed, and then have 6th gear as the high cruising speed ”överdrive” for more comfort. Possible? Only down side is that the 6th gear is the direct drive and that means that 5th and the other gears steal power and generate more heat. But probably not a problem if the top speed is only done for a short period of time which is actually what you do when going that fast.

 

Ok Taz. Thank you. Yes your must be right about the 1:1 direct drive of course. I just read it somewhere that 6th was a direct, but that is probably wrong then.

I have also been in contact with Colotti, a company in Modena who was a pioneer in the development of many gearboxes and differentials used for racing. They can custom fabricate the parts in any ratio I want...! Haven’t got a price yet though, but probably an arm and a leg.

See www.colotti.com for more interesting reading about their history.
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If we say that the combined ratio in a 550/575 is the 6th gear ratio 0,76 x the final drive 3,91 = 2,97. Then if 5th is now going to match today’s 6th, then
2,97 / 0,94 (which is the 5th gear ratio) = 3,16 would be the new final drive ratio if I am right here?

6th gear ratio of 0,76 x new differential ratio of 3,16 would make a new 6th gear total ratio of 2,40.

Old 6th gear ratio 2,97 - 2,40 = 0,57
Ratio difference of 0,57 / old one of 2,97 = 0,1919 = 19,1 % reduction in revs.

Cruising speed rpms. would now be 19 % less in all gears and at all previous rpm.

Today the revs are 2.300 at 100 kph.

2.300 * 0,81 = 1.863. Would this be too low? I don’t think so. The car will not be a sprinter when each gear will take the car to higher speeds, but the cruising rpms. would make the car much more comfortable.

50 kph. today is 1.150 rpm.
100 kph today is 2.300 rpm.
150 kph. today is 3.450 rpm.
200 kph. today is 4.600 rpm. I feel this is too high.
Even if these engines likes revs., i feel that the almost 5000 rpm. is too much for continious operation. The car use exactly 20 liters per every 100 kms. at this speed. The thing is that the car is super nice to drive at 200. I can cruise at that speed all day long. And in Germany, the modern sedans and station wagons with 6 cylinder turbo diesels like BMW, Audi and even cheaper cars like Skoda, Ford etc. has the ability to cruise effortlessly and comfortable at these speeds and they actually do it all the time. With a consumption of about 10 liter every 100 kms. In reality, these cars can travel through Europe just as fast as my Maranello costing 5x more and they do it for half the price and can go much longer between each full up. The thing is that to drive fast is very bad economy and you need to stop all the time, because there is some places that have a very long distance to next gas station, so even if you have a 110 liter tank, you should refuel when the light comes on and that means when you have used 80-90 liters. That means maximum every 4th houre. This us quite irritating, when the new cars blow by you. With a longer ratio, the revs go down, the noise go down so you can cruise faster in more comfort and listen to music etc., the houres between each fill up goes up, and the average speed goes alot up through the continent because if all this. My dad has a 2011 Mercedes E200 CDI. That car had the same Oslo to Nice speed as my Maranello, because it has low rpm, not too much noise and good economy and that makes you go much further before you need to stop and relax.

The drag from the air is exponential. And since the original ratio is perfectly set up for the power of the engine’s torque and hp. curve at that speed, with an enormous drag/resistance, the horsepower to drag ratio is perfect. But since the drag is much less at half the speed for example, then that would also mean that the power (if we say that in theory is kundar with the revs) is much higher than the drag would require for power... Excessive power per drag would give good passing and accelleration at this speed, but not really neccesary. And at normal speeds in the 110-150 kph. range where most cars are cruising in Skandinavia, the drag is alot less. And I feel the 5,5 liter engine has sufficient power even with 19% lower rpms.

What do you guys think? Would be really interesting to try it!!

Erik

 

I agree Taz. Why would some want to reverse even faster than they go forward in first?! It is very seldom though that I actually reverse the car uphill. I usually park the car in underground garages away from sun and other cars. But I get your point. I a low reverse gear would be awesome. About the torque... with 270 Newton Meters from idle, i doubt that this will ever be a problem. And concearning the accelleration, when the final drive ratio is changed, but the other gears remain the same ratios relative to each other, wouldn’t that mean that you actually will have the same drop in revs everytime you gear up to next gear? That means that you will still have the same torque/rpm. when you put in next gear. It just that each gear take a longer time to empty before redline. Quite interestingly, I noticed that when driving a six speed manual 355, that because this is a very high revving engine with five valves per cylinder and 8.500 rpm red line, that car is also geared for the right rpm at top speed. This car was so low geared that you could well drive the car and go from 1st to 3rd to 5th or even from 2nd to 4th to 6th. I felt these tight ratios actually made the car slower when going through all six gears. I sometimes go from 1st to 2nd to 4th to 6th on my Maranello because there is so much torque availible that it is not really neccesary to go through all the gears. Now I am not talking about a real sprint from 0-60 but normal everyday driving. A Diablo on the other hand is the opposite. It has just five gears and can do 100 kph. in 1st! They kept the five speed gearbox because they said i was just as fast with five instead of six gears. That lead me to wonder if those five gears make the car let the torque to be taken out of the engine in a better way...? I have had a 1993 Maserati Ghibli 2,8 V6 twin turbo with five speed box. Later I got a Ghibli 2,0 with about 20 more horses, but with less torque, and these came only with six speed transmissions. The strange thing is that even if the 2,0 was more fun and reving to 7.300 rpm., the earlier 2,8 with only five speed felt like it was faster because it had longer legs and could to 100 kph. in 2nd instead of the 2,0 which had a 3rd change at about 90 kph. before you reached 100! That is why I believe the 550 can be just fast while driving normally. What do you think? I never ever rev the engine and take off with a real wheel spin and do a 0-60 way of take off. With a longer final drive, the torque to the wheels would decrease and the sprint in first could make more speed in 1st with less wheel spin if I want a quick take off.

 

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Diablo five speed. The engine spec is almost identical! Three horses less, and Max power at 7.000 rpm. Slightly higher top speed probably because the car is lower and has less drag.
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Very interesting! Yes and the later model Diablos like SV and GTR etc. could be delivered with different ratios as many were driving fast, but not near the top speed of 320-340 kph. So it is more what kind of use you do with your car. My use is normal driving and some times fast highway through Europe, but never ever track use.

 

Thank to the both of you for great input. Well, I feel that the gears are quite close to each other now and for normal driving, they could well be longer. I have sent an email to HCF now and await their reply. Same to Colotti in Italy!

 

Just an update: I just got an e-mail from Marco Colotti who very kindly provided me with speed/rpm. calculations with the ratios of a 550 gearbox (different from a 456 box) with the crown wheel and pinion of a 456 manual. (The picture shows «456 GTA», but 456 GT is more correct).

With my engine revving to 8000+ rpm., the theoretic top speed is phenomenal, but I doubt it would make that speed. Maybe with Fabio’s exhaust and some other tricks......

To change to 456 crown wheel and pinion is probably the easiest way to lower the revs, but then it will not reach top speed in 5th, and I doubt it would be as fast in 6th as it is now with the original ratio. I would actually prefer even lower rpms. with a custom crown wheel so 5th will be the same as the original 6th in regards to the total ratio with the gear and the final drive ratio.

Have anyone replaced the crown wheel and pinion? How difficult is it?

Thanks,
Erik

 

On the other hand, it seems like it will do 320 kph. at about 8.300 rpm. Which is much higher rpm. than today’s 6th, so who knows, maybe it will be faster up to this speed and maybe do some more with the new 6th ratio if it only have enough power to continued to accellerate. My car want more at this speed, but then it is already on top of the rev scale.

 

Yes James. My 456M had a better cruising comfort with lower rpm. and you could go faster with the same noise level. I do think also that 456M had cushioned shock absorber mounts, while the 550 have non and the general road noise is quite high in the 550.... I want the car also to burn less fuel at high speed because it means less stops and better average speed. At 180-220 kph. which is quite normal in Germany with little traffic, it will do 600-650 kms per tank of gas, and that means that the tank is soon empty very 3–4 houre. Some times when driving at night at dark, the gas stations are far from the Autobahn, and the distance between them is also very long, so sometimes when you see that «this rest fuel is probably not going to last to the next stop», you take off the Autobahn and spend 30 minutes before you are back on the Autobahn. Sometimes, a station is closed, or the card reader is not working etc.... This can be very frustrating when you are in the middle of bowlere, without hotel and fuel etc. That’s why I prefer to boost up the range without needing to go down on the average speed. If the car could go for example 800 kms at a lower speed and with more comfort, then I could do more distance in comfort without having to stop all the time. And choose the more easy to get to fuel stations instead. Because (and this sounds a bit strange), but when all cars with economic engines pass you when you stop to fill up, you have to go at a MUCH higher average to keep up with them and that raises the consumption significantly. I really hate to not being able to keep up with a modern diesel car worth 10.000 euro!!

 

Yes I thought about that, but the ratios are shorter in the 456 box, so the combined ratio of the gears and final drive (differential) is quite the same as the 550, just slightly higher. I want much higher. So the car goes fast up in speed with less gear changes, and that 6th give a really low rpm for fast cruising!!

 

Thank you. Yes, probably the best advice. I guess the pinion and crown wheel is very special on this Graziano transaxle, but maybe two gear sets are less expencive and require less work to be carried out... i have no sound coming from the gearbox except in 3rd and 4th if memory serves me right... Who would be able to supply a set of gears or two here in Europe, I definately want a modification so that the can can achieve its top speed. Also with added horsepower with different exhaust etc. Then 5th must be the gear for that speed. And with the 456 pinion and crown wheel into the 550 gearbox, this seen to be too light. Must have a longer ratio. The way the car is today, it will do 240 kph. in 4th, 280 kph. in 5th and with enough power 340 kph. In 6th.

This means that after going to 240 kph. in 4th, there will be a quite big drop in revs to the new 5th if 5th is now going to have the same ratio as today’s 6th... but I kind of like that idea. To have a gear set that is perfect for top speed. And 6th as the overdrive. It is far to short now for continious fast Autobahn cruising. The new cars of today will do 250 easily on the limiter, even without being the AMG 65 V12 biturbo. Even my old 2002 XK8 is electronic limites to 250 kph. All this means that the traffic is going seriously fast by a lot of cars. Even Audi A3 / S3 are in the 200 kph. cruising speed class now. They all have long ratios to do this with some level of comfort.

I have a fellow Ferrari 550 owner here in Norway who said he had done 340 kph. in his 550 with sport exhaust and spirt air filters. I do wonder if he said it also had a trim chip in it. One for each bank probably. That is pretty impressive. You need a F12 to go faster than that. And they do 350 kph.!!