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MerrilGordon
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Gear Ratios & Final Drives

Post by MerrilGordon »

Been thinking about gear ratios and final drives.

A=Alfetta, G=GTV6, M=Milano/75

1st Gear: G=3.5, A=3.3, M=2.88
2nd Gear: A=2.01, G=1.96, M=1.72
3rd Gear: A=1.38, G=1.25, M=1.23
4th Gear: A=1.04, G=0.95, M=0.95
5th Gear: A=0.83, G=0.76, M=0.76

As most people on this forum know, the “Platinum” (Mijlano/75) transaxle with limited slip is pretty much the best choice available for a close ratio gearbox.

Why ? Besides having the LSD, the big change with “Milano/75" transmission is that 1st and 2nd were moved closer to the upper gears. I don’t know why Alfa Romeo chose such a wide ratio spread for the GTV6. I mean 1st is more of a tractor gear than that chosen for the Alfetta! Moving 1st and 2nd closer to 3rd substantially reduces the very wide gap that is way too apparent in my transmission.

Well, I’d say the Platinum transaxle is the best choice if you have a 2.5L engine, either stock or modified. With more displacement and torque I’m not so sure. The reason is 1st, 2nd, and 5th Platinum gears. I think 1st is so low as to be pretty useless. Then 5th gear with the 4.10 maybe useful on the racetrack but numbing on the highway with the engine spinning at something like 3300 rpm at 70mph.

Note that my 82 GTV6 with the 3.42 R&P and 3.5 1st gear produces a total ratio of 11.97. The Platinum gearbox with its 4.10 R&P and 2.88 1st gives nearly the same ratio of 11.81. My experience is that you’re out of 1st a little after the tires are rolling. Kind of a wasted gear with that stump puller of a ratio.

Most any engine can pull 2nd once your speed is up to about 15mph. Might as well have a taller 1st to take advantage of the engine’s torque band and keep the ratio gaps close as possible. So I like the choice of 3.55 R&P for the Milano gearbox ratios. That makes 1st and 2nd quite useful. I’ve driven a 3.0L Verde model and thought 1st & 2nd to be fully useable with total ratios of 10.22 & 6.11 respectively.

For those who want a strong dual purpose car (like me), I’d like my Milano’s 3rd and 4th gears to be shorter than stock, but keep the tall 5th for highway cruise. I’m not sure how he does it, but Richard Jemison (on the Yahoo alfaracers news group) swaps in Alfetta ratios to get closer to what he wants in a transmission. Of note, he’s put in Alfetta 3rd & 4th gears into Milano boxes. That’s pretty cool. The Alfetta 3rd is shorter and moves closer to 2nd, and 4th gets shorter as well. In fact, those two ratios (with the 3.55R&P) get pretty close the Platinum boxes total ratios. Milano/Alfetta 3rd=4.90, Platinum 3rd=5.04, Milano/Alfetta 4th=3.73, Platinum 4th=3.9. A strong 3.0L would pull out of a corner like a freight train with those gears.

Maybe the gap between 2nd and 3rd would be closer than necessary but with these two gears you could choose either 2nd or 3rd that best suits exiting a track corner in the fat part of the torque band. Honestly, 3rd is a little closer than I’d like. Sure would like a ratio of 1.30 for 3rd. That would space the gears more evenly between the Milano 2nd and Alfetta 4th. I don’t think that’s available though.

So I guess my idea of an ideal transmission is a Milano Verde unit with short 3rd & 4th gears. May have to try and get there one day. Any thoughts from you guys on this forum ?

Merril
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junglejustice
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Post by junglejustice »

Andrew Garcia is building me an LSD box from a non LSD Silver/Gold box....

Wondering like you Merril, what would happen if we put the 3.55 final drive ring and pinion from a Verde box in that Silver/Gold 4.10 (now essentially a Platinum-converted) box that he is building there with the shorter ratios that are in there...?

Do we essentially come up with something magic like a 3.94 equiv. with the current 5th of the silver/gold/platinum box spaced a bit further (revving a bit lower but still higher than the 3.55 Verde box...?)

Do we gain a more usable 1st (rather than the useless stump-puller that only gets you to about 15 mph...) from the Silver Gold, but not quite as long as the Verde's 50mph 1st and 72 mph 2nd...???? Do we gain a lower revving (longer) 5th than what the standard 4.1 has (but with a higher revving 5th (shorter 5th) than what the Verde box had...? (Only now with the better 2-3-4 spacing of that Silver-Gold-Platinum tranny...?)

I always liked my 5th in the Verde (but could use it shorter for the track) and I wanted a shorter 1st and with 2nd then moved down with it (AND moved closer to 1st to begin with), 3rd moved down a bit and 4th about where the 4.1 box's 5th is now and then the Verde 5th or a 5th somewhere in-between the Verde 5th and the Silver/Gold/Platinum 5th...

Asking too much?
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Post by MerrilGordon »

Hey JJ,

That was a great convention. Had a lot of fun at the track. Too bad you had some gremlins you were chasing.

Regarding the Milano era transaxles, weren't they all the same except for the ring and pinion ratios ?

What car is the Andrew Garcia built box going into, your standard Verde or the 3.7L Monster ? The standard Verde could use a shorter R&P for the track, but maybe that would be pointless once you have the 3.7L car performing track duties. Even at that the 3.55 maybe ok.

I say so because at the Convention track day I noticed I stayed in 3rd gear from just after turn 3b through turn 8. RPM's were in the 4000 to 5000 range except leading into 8 where I hit 6000. (My car was limited by suspension and traction to travel any faster between those turns) Through turn 8 I was reving approximately 3400 rpm, maybe low for good power when exiting the corner, but someone else with good sticky track tires could corner 10 to 15mph faster and thus have the revs up into the 4500 rpm range. Right into the fat portion of the torque band with plenty of upper rev room before having to take the time to shift into 4th. A good argument for staying with a 3.55R&P. A 4.10 box at that speed in 3rd gear might be reving needlessly high in mid corner with nothing left on the exit, where 4th may drop the revs too low to be most effective.

Of course this scenario is just at one track and not for others we all hope to drive at some point.

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Post by junglejustice »

Yes, it is my understanding that all of the US Milanos had the same gear-spacing... (I was refferring to the feel in each gear...

It is all in the R&P final-drive ratios I guess.... (Sure feels like the Verde 3.55:1 has a longer first and second than say the Silver/Gold/Platinum diffs with 4.1:1 final drives, but any way...) Good way to mix and match would be to combine Alfetta, GTV6 and Milano gears in one box with some combination of final drive.

Yeah, valid point on the 3.55 on the track with more power... I have been wondering if the 3.55 wouldn't be better with the new 3.7 on the track. It is hopeless with the stock 12 valver 3.0 for me though. Up the hill at Laguna Seca and also the hill after the turn 6 at PR I am just lugging in 3rd at about 68-69rpm or I am screaming in second at 70-72 over-revving....

With the 4.1 you'll love it (come down on the 18th and we'll switch...) MUCH better!

With the 3.7 I not only have more power but also a higher rev-range so it will make both boxes more useable...
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gearbox math

Post by mark_atterbury »

Very interresting. I have been doing some math and think I understand why the second gear sincros don't live long. Look at the difference between first and second gear ratios for the GTV6 for instance. It is 1.54, while the difference between the 2-3 ratios is .71 and it keeps going down as you move up through the gears.

Now think back to physics. The difference between the ratios is essentially an indication of the acceleration (change in velocity) that needs to occur as you change from one gear to another. This change in velocity is accomplished through the syncronizers. Therefore the larger the difference in gear ratio, the more acceleration (change in velocity) needs to occur (given a fixed shift time). The energy that the syncros impart from the first gear to the next is what causes the acceleration. Energy in this case can be looked at as force over time. F=MA. Les energy transfer = less wear. Therfore to make the syncros live you can either reduce the mass that is accelerating (lightened gears or clutch disks) or reduce the acceleration (reduce the ratio difference).

So although a Milano box with Alfetta 3 and 4 gears has a consistant "close" ratio for 2-3, 3-4, and 4-5, it still has a 1-2 ratio difference of 1.16. If you substitute the GTV6 2nd gear into this box, the ratio difference from 1-2 drops to .92. This is a 40% drop in ratio difference. Of course you trade off some in the 2-3 ratio difference, but with the Alfetta 3rd you would still have an 18% decrease in ratio difference from an original GTV6 2-3.

My thoughts are that the "best" box may be:

1st - Milano
2nd - GTV6
3rd Alfetta
4th Alfetta
5th Milano/GTV6

You would then chose the final drive ratio that best suits your needs. I think I would take the 3.55 Verde.

I don't know if all of these components can be assembled together or not, but if I can find enough cheap gearboxes this winter I will give it a try.

Any thoughts?
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DaveH
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Re: gearbox math

Post by DaveH »

mark_atterbury wrote:Energy in this case can be looked at as force over time. F=MA. Les energy transfer = less wear. Therfore to make the syncros live you can either reduce the mass that is accelerating (lightened gears or clutch disks) or reduce the acceleration (reduce the ratio difference).
I guess you could also reduce A (dv/dt) by shifting 1st to 2nd a lot slower :wink: Also, with my '6, if I'm pointing downhill when starting off, I'll always start in 2nd. Although I suppose my 1st gear is lowest of all years/models, since my diff is 4.11. It reminds me of a Massey-Ferguson I once drove...
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Post by Mats »

I think its the downshifts that kills the syncros (unless you know how to shift properly).
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Post by Luis »

MerrilGordon
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Post by MerrilGordon »

Hi Mark,
Yea, when I was thinking mostly about ideal gear ratios and wasn't thinking about stress on the synchros. Mostly focused on being at the right rpm when exiting most any corner. I figure one doesn't want to exit near max power as they'll have to shift right a way, but if the engine is near max torque, then you can stay on the gas longer. I know I've certainly found racetrack corners where I'm either lugging in second or reved out in 3rd. Besides shifting slower, I've heard that reducing the rotating mass by lightening both flywheels helps with synchro life.

JJ,
You must have had a type-o there.
"Up the hill at Laguna Seca and also the hill after the turn 6 at PR I am just lugging in 3rd at about 68-69rpm or I am screaming in second at 70-72 over-revving.... "
Lugging at 6800rpm ?

Louis,
Nice little calulator there.

Merril
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Post by mark_atterbury »

Lightening both flywheels probably won't help the syncros much (they are disconnected from the accelerating mass during shifting). However that will allow the engine to rev faster, potentially allowing you to double clutch faster (I never could get this technique down to where I like it and just rev match as best I can while braking).

Lightening the clutch disk and the gears will certainly help as their mass is accelerated during shifting. This is worthwhile, but potentially costly.??

If you are setting up a box for a particular track, I understand the ratio selection for proper engine RPM at the corner exit speed - sounds wonderful. But you ideally would have a box for each track. Way to expensive for me. I am trying to find a general use - low buck solution.

Thaks for the wonderful discussion though.

A couple more questions:

Has anyone ever investigated the LSD from the 115 spiders. Can that be fitted to the Alfetta final drive with minimal machining? They are plentiful and cheap compared to LSD transaxles.

Does anyone know a source for a light weight clutch disk? I would prefer to go this direction (with lightened flywheel) than just buy an assembly (once again expensive).

Thanks for the link to the ratio spread sheet Luis. Looks like there are several other final ratios out there (3.8 ? :) ) I don't know how available they would be in the states though.

Thanks,
Mark
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Post by TS_turbo »

hm lighter clutch disc will do almost nothing comapred to presure plate .... try spec clutches or clutchnet.com :)
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Post by Giuliettaevo2 »

What about using a front mount box from a BMW 535i? I'm not really into the gearingnumbers, i just want an unbreakable box that shifts nice and firm... :oops:

For the gearing-minded people :) :
BMW 535i E34:

1st 3.83
2nd 2.20
3rd 1.40
4th 1.00
5th 0.79

Together with a Alfa Sei diff, which looks absolutely indistructable :shock: and has a lsd.. :wink:

2.0 4.27
2.5 auto 3.5
2.5 man. 3.9

I happen to have such a gearbox and a Sei 2.0 diff so i'm planning on using that. The 2.0 diff is a bit short but it can be used while searching a 3.5 or 3.9 diff. We are not sure about the clutch we are going to use, but it must be fitted to a 164 type flywheel. Probably we'll use a BMW 535i clutch.
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MD
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Gearbox

Post by MD »

All that sounds interesting and likely to function as you expect but aren't you missing the essence of what this site is about and more to the point what these cars are all about. Sure they have idiosycracies but that is all part of their character.

Instead of drastically changing it and turning it into just another bit of transport why not do some brain sweating to make the existing set up better than it is but keeping the configuration as originally conceived?

I don't see too many MGB enthusiasts running around turning their beloved British piece os sh*t into a wanna be Mazda RX something for the exact same reason even though the Jap thing is a more superior thing five times over.

What's the re sale value gonna be???

Think it over... :?
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Post by Giuliettaevo2 »

MD:

resale value is not a problem for me, the car will never be sold! :)

The biggest reason for me to change to the front mount box is the planned output for the v6 TT engine... The standard Alfa gearbox would not be very happy with this and it would cost me too much money to make it work properly. A porsche transaxle was considered but this is too much work. now i just use a bimmer gearbox and an Alfa diff.

I know it kills one off the main characteristics off the car :( , but it has to be that way :oops: ... The weightdistibution won't actually change very much since the Sei diff is a very heavy piece(i don't think it weighs less then a gearbox).

In the end it is a very strong gearbox that shifts really good and costs little money. Also i can upgrade to a 6 speed if i want to, or to a dog-leg 5 speed!

Hope you people can forgive me the bimmer thing! :wink:
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