turbo flywheel and diff q for an m20

[JMAL]

M20 turbo q: I'm building with hx35, mspnp, arps, like to run 9-11psi. I have driveline questions. I'm aiming for a flat torque band, minimal lag, usable power. I have a stock flywheel and 13.6lb flywheel available also a 2.93 lsd and a 3.73 lsd. Is it worth using the lighter flywheel? Is there a combo of the parts that is ideal for what I'm looking for? I was thinking 13.6 and 2.93, but now I'm unsure. There is so many different takes on flywheel load and boost.

[bobross]

Neither the diff or flywheel will effect the torque band.
lighter flywheel will rev up a bit quicker in neutral. 2.93 will have longer gears.


Id go with the lighter flywheel and a diff somewhere in the middle of the 2 options.

[Sirotto]

Lighter flywheel and 2.93 diff. 13.6 is close or is stock euro flywheel from 323i. That is a very good combo with nice highway cruising. You can search and spend an extra $300 and look for a low three LSD diff but i don't think it is worth the money and effort over the 2.93.

[JMAL]

Neither the diff or flywheel will effect the torque band.
lighter flywheel will rev up a bit quicker in neutral. 2.93 will have longer gears.


Id go with the lighter flywheel and a diff somewhere in the middle of the 2 options.

If i can find a low 3 diff that sounds like good advice.

Lighter flywheel and 2.93 diff. 13.6 is close or is stock euro flywheel from 323i. That is a very good combo with nice highway cruising. You can search and spend an extra $300 and look for a low three LSD diff but i don't think it is worth the money and effort over the 2.93.

it is a euro 323i flywheel, nailed it. I was curious if anyone would pick up on that. Like bobross said, maybe i can swap someone for a low 3. If not, its not an all out performance build. Im learning forced induction and trying to make fun usable power.


im sure the 2.93 with boost will still be quicker than stock m20 with 3.73. And living in NYC i wont feel like im shifting every 3 seconds! Thanks for the advice

[Wanganstyle]

If i can find a low 3 diff that sounds like good advice.



it is a euro 323i flywheel, nailed it. I was curious if anyone would pick up on that. Like bobross said, maybe i can swap someone for a low 3. If not, its not an all out performance build. Im learning forced induction and trying to make fun usable power.


im sure the 2.93 with boost will still be quicker than stock m20 with 3.73. And living in NYC i wont feel like im shifting every 3 seconds! Thanks for the advice

Turbocharged cars like HEAVIER flywheels; run the STOCK M20 flywheel.

Weight of flywheel provides load to engine and helps hot exhaust gas go to the turbocharger; the is the goal

Lower gear ratios also help turbochargers spool; if you compare a factory turbocharged BMW to its NA equal the data shows:

e92 335i 6mt (Turbo)

3,08 final drive
~27 lbs flywheel

e92 328i 6mt (non turbo)

3,73 final drive
~20 lbs flywheel

[JMAL]

would it be as noticable load difference if im only looking to hit 8-11psi boost? I have a holset hx35 12cm2 housing.

[Wanganstyle]

would it be as noticable load difference if im only looking to hit 8-11psi boost? I have a holset hx35 12cm2 housing.

Why would BMW do it if it was worthless?

How many PSi of boost does a Twin turbo 335i or single turbo 535i make?

[JMAL]

Why would BMW do it if it was worthless?

How many PSi of boost does a Twin turbo 335i or single turbo 535i make?

I think the tt is around 8-10psi, and the single is 6.5 - 7.5 from what i found.

I am not saying that it is for no reason, but those cars were built with all modern parts and for boost purposes. There are other load related parts as in drive shafts etc that might be lighter on the more modern cars making a different net effect.

This is just an exploration of options of maximizing manageable low boost street performance on a car that is old and not originally designed for boost.

That NA vs FI comparison is certainly food for thought.

[Wanganstyle]

I think the tt is around 8-10psi, and the single is 6.5 - 7.5 from what i found.

I am not saying that it is for no reason, but those cars were built with all modern parts and for boost purposes. There are other load related parts as in drive shafts etc that might be lighter on the more modern cars making a different net effect.

This is just an exploration of options of maximizing manageable low boost street performance on a car that is old and not originally designed for boost.

That NA vs FI comparison is certainly food for thought.

comparing the e92 vs e30 driveline and differential is 98% same weight as older cars; this idea is moot.
its easiest to just compare apples to apples; e92 328i NA vs e92 335i twin turbo

Driveshaft weight has not changed significantly since the B9 chassis.
Rotating mass of final drive differential/gear has also stayed pretty much same since 1980's


the DATA of: NA vs FI OEM BMW difference in Flywheel weight and Final drive ratio tells all;


If you don't want to believe in BMW OEM engineering ideas then.............

[The Dark Side of Will]

Turbocharged cars like HEAVIER flywheels; run the STOCK M20 flywheel.

Weight of flywheel provides load to engine and helps hot exhaust gas go to the turbocharger; the is the goal

Lower gear ratios also help turbochargers spool; if you compare a factory turbocharged BMW to its NA equal the data shows:

e92 335i 6mt (Turbo)

3,08 final drive
~27 lbs flywheel

e92 328i 6mt (non turbo)

3,73 final drive
~20 lbs flywheel

NOTHING likes a heavier flywheel.

The heavier your flywheel, the more of your engine's power goes into spinning up the flywheel and the less goes into accelerating the car. A turbo engine has a 2500# CAR bolted to it which provides plenty of load to spool the turbo... way more than a 25# flywheel.

BMW's flywheel weights are to smooth the engine's power pulses. The turbo engine has stronger power pulses than the non-turbo engine, so it gets a heavier flywheel to feel smooth.

The turbo engine's torque band and government fuel economy requirements dictate the different gearing.

The heavier flywheel ALSO makes first gear step off easier with the taller final gear.

It's not that turbo cars are better with taller gears... it's just that they don't lose as much acceleration with taller gears as N/A engines do.

[Wanganstyle]

NOTHING likes a heavier flywheel.

The heavier your flywheel, the more of your engine's power goes into spinning up the flywheel and the less goes into accelerating the car. A turbo engine has a 2500# CAR bolted to it which provides plenty of load to spool the turbo... way more than a 25# flywheel.

BMW's flywheel weights are to smooth the engine's power pulses. The turbo engine has stronger power pulses than the non-turbo engine, so it gets a heavier flywheel to feel smooth.

The turbo engine's torque band and government fuel economy requirements dictate the different gearing.

The heavier flywheel ALSO makes first gear step off easier with the taller final gear.

It's not that turbo cars are better with taller gears... it's just that they don't lose as much acceleration with taller gears as N/A engines do.

Try it for yourself:

2006 subaru sti
Stock flywheel gets up San Francisco hills happy

Light flywheel not so much.

I think cars were designed to be driven on public streets; flywheel weight reflects this.

The flywheel in my subaru wagon non turbo was about 10 lbs lighter than the sti turbo flywheel. Subaru ej22 non turbo and ej257 turbo are physically almost identical besides for the turbocharger setup

Both cars stock had no problems going up hills

San Francisco hills are a reality of life here.

[einstein57]

I run an s52 aluminum flywheel and a 3.23 LSD and love it.

[Wanganstyle]

I run an s52 aluminum flywheel and a 3.23 LSD and love it.

OK and what TURBO

[Northern]

OK and what TURBO

Some sort of Volvo T3 if I remember right.

[Wanganstyle]

Some sort of Volvo T3 if I remember right.

It's easier to go up hills with a heavy flywheel.