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PostPosted: August 12, 2017, 2:31 am 
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Joined: November 11, 2013, 4:47 am
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Location: No. Nevada
Looking to lighten gears for an Alfa Romeo trans rebuild.
They suffer from weak, Porsche designed, expen$ive synchro's.
The traditional method of lightening has fallen out of favor as it is thought that simply drilling holes leads to oil froth and excess drag.
Back-cutting is now the preferred method but does not remove as much weight.
I'm thinking of cutting a set of profiles into the remaining web but not sure what would be the best shape.
Perhaps something like the old Lotus wobbly wheel unless someone can point me to a better solution?. :D
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Last edited by RichardSIA on August 12, 2017, 12:42 pm, edited 1 time in total.

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PostPosted: August 12, 2017, 7:58 am 
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Joined: February 28, 2009, 11:09 pm
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Location: Connersville, Indiana
Any hope of lightening the clutch disc?

Bill


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PostPosted: August 12, 2017, 10:03 am 
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Location: Cornholio OR "Where the magic happens"
You are down to splitting hairs!
Have you already lightened the bigger gain items?
Or are you trying to improve shifting?

I don't buy the oil froth theory, drill away.
The gears are only immersed when stationary, when it's running most of the oil is on the case.
Holes in the gears will not aerate oil any more than gear teeth....

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PostPosted: August 12, 2017, 11:17 am 
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Trying to improve shifting and lessen wear on the synchro's.
Clutch disc will also be lightened.
Flywheel too, but that has less direct benefit to the trans.

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PostPosted: August 12, 2017, 11:41 am 
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Flywheel has zero impact, unless your not using the clutch. Also, keep in mind that lightening the main shaft gears will have no impact. Just things on the input side. At least on the few trannys I've looked at.

Bill


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PostPosted: August 12, 2017, 12:18 pm 
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Joined: February 28, 2009, 11:09 pm
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Location: Connersville, Indiana
Wait a minute, is this a constant mesh tranny? Doesn't that mean that during shifts, the gears do not change speed, only the layshaft and the things firmly attached to it?

Bill
(Edit) or maybe not. My brain is stating to hurt.


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PostPosted: August 12, 2017, 12:55 pm 
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Location: No. Nevada
Gear rotation speeds are changed by the synchro as each is selected.
Less mass = less inertia so faster changes and less wear to the synchro.
Alfa gear changes are their weak link, doing all I can to improve mine.

Here is a link to the PDF for how they have been done in the past.
As said, this is not the favored method now.
Looking to improve on this.
http://www.alfabb.com/bb/forums/attachm ... gspecs.pdf

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PostPosted: August 12, 2017, 2:27 pm 
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Joined: December 7, 2012, 8:28 am
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Location: Sarasota
Not sure what you use the car for but I removed 5th and 6th gears from my transmission. I saved nearly 1/3rd of the rotating mass within the transmission, also removing a lot of the transmission losses as the synchros and gears constantly spin on opposite directions.

My car is a dedicated autocross car so I typically start in 1st shift to 2nd an leave it. I was tempted to remove 3rd and 4th also but wanted the practicality of higher speed for dyno, track day etc. My car will still do about 150mph in 4th as it has a taller rear end. If it were to be street driven though I would lose highway mpg and have to be ok driving at 4-5Krpm at highway speed.

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PostPosted: August 12, 2017, 4:53 pm 
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Eventually doing more than one trans but all will be street driven.
Curious how much difference you see with such significantly reduced mass.
A lot, or not much with modern synchros?

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PostPosted: August 12, 2017, 6:33 pm 
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I have only run the car twice and it is so fast that the butt dyno is difficult to tell the difference. An actual dyno will not be much use either as I de-stroked my motor from 2.2 to 2.0 at the same time, which enables me to run in a lighter class.

Looking at it on a really basic level lets say you have 15% draivetrain loss. Maybe 5-8% of that is from the transmission, mainly as mentioned before that the synchros spin the opposite direction to the gears which leads to a lot of opposing forces in the transmission and power loss. I removed 2 of 7 gears (including reverse) Maybe reducing drivetrain losses by 2-3% total.

Lets say the car makes 200Wheel HP which would be 230 Crank HP the losses are now 12-13% rather than 15% which would result in 4-6 additional Wheel HP. Obviously generic numbers but you get the idea.

I will be getting on the dyno again soon to dial in the 2.0 so I will see how the power changed. Previously I made 249WHP and 193 Tq. I reduced the displacement by 10% but reduced the transmission losses and also saved 3lbs off the clutch pressure plate and clutch.

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