Custom Gearbox Rebuild - Advice Needed

[gt'79]

Hey Gents,

Busy with an upgrade on the daily Mk1 1600 HM.

Currently the car is stock and running a AEN gearbox that recently started slipping.

busy fitting my stock bottom end 2L 8v (2e long blok), Stock U-flow head, 363DCD carb, 276 estas, lightened 210mm flywheel with new 210mm SACH clutch kit, branch and already have an existing 57mm exhaust.

By Jan next year, i will be fitting 32mm mikuni mechanical bike carbs on a ported and flowed X-flow head with 288/296 split cam.

with the above setup i'd like to build a gearbox that doesn't run out of ponies too quick.

So i have the following gearboxes: AEN, 2Y and 4S

Initial plan was to fit the 2Y with the 4th and 5th from the 4S gearbox seeing that the ring and pinion ratio is 3.667 on 2y compared to the 4S with ratio of 3.94.

Bare in mind i will be running a lighter flywheel.

4S - 3.45 1.94 1.29 0.91 0.75 3.94

2Y- 3.45 2.12 1.44 1.13 0.91 3.67

AEN - 3.45 2.12 1.44 1.13 0.89 3.67

[Donavan]

Stock 2Y will be more than capable for your needs

[gt'79]

Stock 2Y will be more than capable for your needs

Had stock 2y in my 2L mk3 with lightened fly and doing 120kph i was at About 4000rpm in 5th uphill. Compared to stock flywheel at 3000rpm.

Felt like engine was working overtime.

Jetta3 was much heavier if one takes power to weight into account with having a lightened fly.

Would like a longer 5th gear but shouldn't be too lazy when engaging from 4th,hence the reason for sticking to the lightened flywheel

[jovann46]

At 7000 rpm with the 2Y you will do 227 kmh on 195/50/15's in 5th so that gives you an indication

[jovann46]

PS go with the 282 Estas cam you will not regret it,or 298 Estas as in VW Motorsport applications,both very good options even in a daily drive! I had both in my Caddy for more then 3 years although that was a 2lt short block mp9 with flowed head and Go-Tech management

[gt'79]

At 7000 rpm with the 2Y you will do 227 kmh on 195/50/15's in 5th so that gives you an indication

that's decent, stock or lightened fly?

Im again running on 13" wheels 185/60/R13 tyres

PS go with the 282 Estas cam you will not regret it,or 298 Estas as in VW Motorsport applications,both very good options even in a daily drive! I had both in my Caddy for more then 3 years although that was a 2lt short block mp9 with flowed head and Go-Tech management

had the 282 which was nice and recently swapped it for the 276 to gain some power more low down. 298 will most likely be on the cards for a preferred cam on xflow head

[VAG Fan]

The 2Y only has a slightly longer 4th than the AEN, the rest of the gears are identical. So why not just keep the AEN?

[gt'79]

The 2Y only has a slightly longer 4th than the AEN, the rest of the gears are identical. So why not just keep the AEN?

actually only the 5th gears that differs between AEN and 2y:

2Y- 3.45 2.12 1.44 1.13 0.91

AEN - 3.45 2.12 1.44 1.13 0.89

Honestly, i haven't opened the AEN yet since i bought the car, 4th and 5th just feels very close and 5th doesn't feel like a cruising gear.

i will open all 3 boxes and check them out. would be nice if i can fit the 5th gear from the 4S. i'd be hoping for 5th to be at about 0.80 on the 3.67 ratio similar to that of the CHE gearbox (CHE - (3.67) 1st 3.45, 2nd 1.94, 3rd 1.29, 4th 0.97, 5th 0.80

[VAG Fan]

Yes, my apologies, I messed up. The last number is not the 5th gear, but the final drive ratio.

But even so, in my opinion the 2% difference between 0.89 and 0.91 is hardly worth doing the change. At 4000 rpm, that's less than 100 rpm.
0.80 on the other hand, is about 12% difference, which would reduce 4000 rpm to about 3500. That would be nice for cruising.

[chucker02]

A lightened flywheel means noting for gear ratios. It also will not affect where the car revs at a certain speed. You cant do 120km/h at 3000rpm with a stock flywheel and suddenly do 4000rpm with a lightened one

[PapaJo]

A lightened flywheel means noting for gear ratios. It also will not affect where the car revs at a certain speed. You cant do 120km/h at 3000rpm with a stock flywheel and suddenly do 4000rpm with a lightened one

My suggestion would be to use the 4s just as it is. The 4th and 5th from 4s in the other two boxes with 3.67 will drop too much between changing for 3rd to 4th and 4th to 5th on the 3.67 diff ratios. Should be ok for flat highway roads, but may need to gear down when you are reaching twisty roads that is slightly up hill.

[gt'79]

A lightened flywheel means noting for gear ratios. It also will not affect where the car revs at a certain speed. You cant do 120km/h at 3000rpm with a stock flywheel and suddenly do 4000rpm with a lightened one

Believe me it does. Its rev happy.
Had a lightened fly on my last 2 mk3 cli's running che and 2y box. When i fitted stock fly on car with 2y, instantly better torque and more on the bigend. It's as if the engine does less work than running like a vtec going nowhere slowly.

[gt'79]

A lightened flywheel means noting for gear ratios. It also will not affect where the car revs at a certain speed. You cant do 120km/h at 3000rpm with a stock flywheel and suddenly do 4000rpm with a lightened one

My suggestion would be to use the 4s just as it is. The 4th and 5th from 4s in the other two boxes with 3.67 will drop too much between changing for 3rd to 4th and 4th to 5th on the 3.67 diff ratios. Should be ok for flat highway roads, but may need to gear down when you are reaching twisty roads that is slightly up hill.

Will put my concoction to test and let you guys know how it goes.

[chucker02]

A lightened flywheel means noting for gear ratios. It also will not affect where the car revs at a certain speed. You cant do 120km/h at 3000rpm with a stock flywheel and suddenly do 4000rpm with a lightened one

Believe me it does. Its rev happy.
Had a lightened fly on my last 2 mk3 cli's running che and 2y box. When i fitted stock fly on car with 2y, instantly better torque and more on the bigend. It's as if the engine does less work than running like a vtec going nowhere slowly.

You are not understanding what I am saying, yes it does make a difference in response and revs quicker but it cannot and does not affect the revs when driving. The revs wont go up with 1000rpm in the same gear. A lightened flywheel has got nothing to do with the ratios in the gearbox

[gt'79]

On a flat earth it may not so much but it does when we're taking other factors into account like climbing a hill and weight of vehicle.

Chuker02 have you driven with a lightened fly in your car?

[chucker02]

On a flat earth it may not so much but it does when we're taking other factors into account like climbing a hill and weight of vehicle.

Chuker02 have you driven with a lightened fly in your car?

Ok, you are missing the point. Moving on, and yes I had a 2.0 citi with lightened flywheel and balanced bottom end.

[HendrikCTI]

Unfortunately I must fully agree with chucker02.
Flywheel does absolutely nothing with gear ratio. Really nothing.

Please read this to have a better understanding:
https://www.torquecars.com/tuning/flywh ... tening.php

[PapaJo]

Unfortunately I must fully agree with chucker02.
Flywheel does absolutely nothing with gear ratio. Really nothing.

Please read this to have a better understanding:
https://www.torquecars.com/tuning/flywh ... tening.php

2

[missioner]

Having had AEN, DGG, FFZ and finally an FF gearboxes in my car over the years I think I can add some objective input into the discussion.

Of the 3 boxes you have I would run the 4S. Its 5th gear is only marginally shorter than my FF, 0.71 vs 0.75.

Stock the box had a 3.89 final drive, until I broke it, and ended up with a 3.94 diff from an FFZ. It made almost no difference to speed or acceleration.

Rpm at 120kph in 5th was +/- 3000rpm. This compared to FFZ/DGG (same ratios exactly) 3800rpm. The 4S should be similar to my FF.

Dont be fooled by the long final drive in the AEN box it's meant for small displacement engines and the diff itself is quite weak due to a low tooth count on the gears. It cant handle very high load. DGG is the same just close ratio.

If there is anything I'll recommend it's a diff bolt kit. Oh and 100mm driveshafts.

[gt'79]

Unfortunately I must fully agree with chucker02.
Flywheel does absolutely nothing with gear ratio. Really nothing.

Please read this to have a better understanding:
https://www.torquecars.com/tuning/flywh ... tening.php

i guess myself and Chucker02 misunderstood each other.

im well aware of the flywheel having no change in the gear ratios but,

My argument was based on a heavier car, twice the weight of a mk1, with a lightened fly going uphill

He stated and i quote

You cant do 120km/h at 3000rpm with a stock flywheel and suddenly do 4000rpm with a lightened one

and interesting enough, the link to the article you sent me clearly states the following....

The big downside to a lighter flywheel is that engine momentum or inertial spin is reduced – most noticeably on a hill. The lighter the flywheel the faster engine revs will rise and fall but you will lose momentum on a hill more quickly. Whereas the momentum in the engine is maintained with a heavy flywheel the momentum is reduced and the hill has a much more direct effect on the engine output.

i dunno it that makes sense to you guys but as i said, i experienced it myself, and perhaps i should have mentioned it was a stock 2L in a mk3 jetta with 8J 16" steel rims on 205/40/R16 rubbers.

[VAG Fan]

In my considered opinion, that quote from the linked article is problematic. Here's a practical example.

The rotational kinetic energy stored in a flywheel is
0.5 * I * w²
... I = moment of inertia = 0.5 * mass * radius²
... w = angular velocity

For a flywheel of, say,
... m = 10 kg,
... r = 0.15 m,
... w = 4000 rpm (about 420 radians per second),
The stored kinetic energy comes to
0.5 * (0.5 * 10 * 0.15²) * 420² = about 10 kJ.

The kinetic energy stored in the moving car weighing, say, 1400 kg and travelling 100 km/h, is
0.5 * m * v² = 0.5 * 1400 kg * (about 28 m/s)² = about 540 kJ
So, in terms of energy, the flywheel contributes something like 2%, and the body of the car about 98%.

And if you look at momentum, the difference is even greater.
The angular momentum of the flywheel is
I * w = about 50 kg.m/s
While the linear momentum of the car is
m * v = about 39000 kg.m/s
So, in terms of momentum, the flywheel contributes way less than 1%, and the car's body way more than 99%.

There's really not much energy or momentum stored in an engine at 4000 rpm. Think about it. Rev an engine up to 4000, now release the accelerator, and at the same time pop the clutch - and see how far you come, purely with the help of the flywheel.

If the writers of the article can really notice and measure such a minute difference, kudos to them. But flywheel energy (or momentum) is not a significant contributor to what keeps your speed on an uphill. It's engine torque, which is independent of flywheel mass.

[chucker02]

Unfortunately I must fully agree with chucker02.
Flywheel does absolutely nothing with gear ratio. Really nothing.

Please read this to have a better understanding:
https://www.torquecars.com/tuning/flywh ... tening.php

i guess myself and Chucker02 misunderstood each other.

im well aware of the flywheel having no change in the gear ratios but,

My argument was based on a heavier car, twice the weight of a mk1, with a lightened fly going uphill

He stated and i quote

You cant do 120km/h at 3000rpm with a stock flywheel and suddenly do 4000rpm with a lightened one

and interesting enough, the link to the article you sent me clearly states the following....

The big downside to a lighter flywheel is that engine momentum or inertial spin is reduced – most noticeably on a hill. The lighter the flywheel the faster engine revs will rise and fall but you will lose momentum on a hill more quickly. Whereas the momentum in the engine is maintained with a heavy flywheel the momentum is reduced and the hill has a much more direct effect on the engine output.

i dunno it that makes sense to you guys but as i said, i experienced it myself, and perhaps i should have mentioned it was a stock 2L in a mk3 jetta with 8J 16" steel rims on 205/40/R16 rubbers.

Nope, I understand it completely.

[amstel18]

Only time that revs will increase by 1000rpm with the same ratio and output speed is if the clutch is slipping.

There is NO WAY that if you take a stock setup, and go 120kmh @ 3000rpm
Remove flywheel and lighten i and then install into the SAME SETUP, you will still do 120kmh but now at 4000rpm.

It's just not physically possible

[chucker02]

Only time that revs will increase by 1000rpm with the same ratio and output speed is if the clutch is slipping.

There is NO WAY that if you take a stock setup, and go 120kmh @ 3000rpm
Remove flywheel and lighten i and then install into the SAME SETUP, you will still do 120kmh but now at 4000rpm.

It's just not physically possible

Exactly, thank you

[Trishen94]

Only time that revs will increase by 1000rpm with the same ratio and output speed is if the clutch is slipping.

There is NO WAY that if you take a stock setup, and go 120kmh @ 3000rpm
Remove flywheel and lighten i and then install into the SAME SETUP, you will still do 120kmh but now at 4000rpm.

It's just not physically possible

Ive lightened a ton of fly wheels in many builds and even if u drove the car under the ocean u will still do 120km/h at 3000rpm no matter if the flywheel is lightened or not ... it is mechanically impossible for gear ratios to change unless u physically change them ...here is an eg if u on a 20 degree incline hill and u using 50% throttle at 120km/h at 3000rpm u then take the flywheel out skim/lighten put it back go back to the exact same hill and u will have to now use 85% INSTEAD OF 50% of ur throttle at 120km/h at 3000rpm or else if u still use 50% ur car will not have enough power/momentum to keep it going and it will drop in speed equal to revs thats the most simple explanation