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Flywheels, to lighten or not to lighten, that is the question...


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Some L wheels are 29 lb and some are 21 lb. Also the L20B flywheel can have a 200/225 or even a 240mm clutch/pp bolt pattern. If shooting for a buck fifty likely you will want the flywheel from a car L20B as they are 200mm and will accept the very strong Roadster clutch. (truck L20Bs are 225mm) BTW the L20B will not accept the L16/18 200mm flywheel as they are 5 bolt to crank.


As stated above, dragging an extra 50% more hp from an L motor is a daunting task... for the wallet. Why not keep the L20B and find a Z22 or Z24 block and another L20B head for it? An LZ22/24 would automatically have 10/20 more hp than the L20B just from the size increase alone and should do it below 6K so there is a lot of money saved on parts needed to get the L20B to and alive at +6,500 RPMs. Visually it would pass as an L motor too.

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You'd have to talk you YOUR machine shop to figure out what THEY consider safe. A stock flywheel is better for driveability, and won't really hurt performance for a street car. Smaller motors need lighter flywheels since they are limited on torque, a bigger motor spins the flywheel easier... which is why with a small motor you gotta rev it up a bit more to get it off the line, with the L20b (or bigger), they move out easily enough without one...


But its up to you.

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A 150 horse L20B is going to have to rev pretty high so the building is critical to keep it together. A hybrid LZ22/24, as stated, will be starting 10 to 20 hp higher without any extra effort by the motor or expense on your part. Any mods done to a larger displacement motor will always produce more power than the exact same mods to the same motor with a smaller displacement.


ANY part that is turning or turned by the motor absorbs power as it is revved up. This includes the alternator, drive shaft, rear axles, brake drums, wheel rim and the rubber tire itself. I left out the crankshaft, transmission and differential as there's not really much you can lighten here without weakening it. Any reduction in weight frees up power that can be applied to moving the car. Yes the flywheel can be lightened but you could also install an aluminum drive shaft, aluminum drums, lighter alloy rims and lighter tires too.



Here is a guide for lightening the L flywheel that can be given to a machinist as a guide. Do NOT exceed these limits as an over lightened and weaker flywheel can explode with the force of a hand grenade. The outside edge of a 300mm diameter L flywheel spinning at 7,000 RPMs is traveling at a hair under 250 MPH! You do not want 10-15 lbs of metal this fast near your legs.




How much? I would guess 15 lb and down to 12 lb for a dd street car. The flywheel's job is to store kinetic energy used to get the vehicle moving from a stopped condition. Without a flywheel the engine would have to be revved much higher and the clutch slipped to get going. The flywheel also absorbs the pulsations from cylinder firings and engine vibrations and smooths the idle. There comes a point when it's too light and disadvantages outweigh any power advantages.

Edited by datzenmike
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So I’m guessing that for example if I were to have a 29lb flywheel and shaved it down to lets say 19lb it would have a goodly impact on HP and still maintain a reasonable amount of drivability. Is this a sound theory?


No. Not exactly. YOU WILL NOT INCREASE HP BY LIGHTENING YOU FLYWHEEL. You only reduce the amount of energy it takes to make it move. Yes, you will be able to get the RPM's up faster,BUT, not because you have created more HP by doing it.

Dat u racing runs a 15# flywheel in his circle track Datsun truck. I would think if you stayed around 20#'s for the street you should be OK.

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I had a bunch laying around. Found a stock one at 21 lb (225mm) and used on my Z24. I figured the larger Z24 crank wouldn't need any heavier. I do have a 240mm at 29 lb. from an '80 720 with the small L20B. It was a heavy 4X4 so I guess it would need the heavier f/w to get moving from a start. Most Z cars were 225mm and around 20 lb because they have a much heavier six crank shaft.

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No. Not exactly. YOU WILL NOT INCREASE HP BY LIGHTENING YOU FLYWHEEL. You only reduce the amount of energy it takes to make it move. Yes, you will be able to get the RPM's up faster,BUT, not because you have created more HP by doing it.

Dat u racing runs a 15# flywheel in his circle track Datsun truck. I would think if you stayed around 20#'s for the street you should be OK.


shaving the flywheel, you will loose some torque also

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I have a stock unit lightened by rebello to about 12 lbs and it is nice for a daily driver and I live in san francisco with lots of hills. no ill effects. if you want to go to 9 lbs i know there will not be a major issue but I would get a unit made for that weight versus milling a stock one down as taking off that much metal may make it scary and not as strong. I have used fidanza units in other cars and they work well.


you can make a motor with the z22 bottom with a good ported head and intake system and cam matchup and make good power. 150 whp is not impossible but I bet you can make at least 130 whp and your car would really move still. good luck

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The fidanza flywheel is what started this for me. Summits got 8lb & 9.5lb options, but I was not sure if a 9.5lb flywheel would be any good on a commuter. I know some weight reduction would have its benefits, but I got to be able to drive it every day.


Hainz, what

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Since this is kinda on the same subject and didn't think I needed to start a whole new thread on it, I thought I'd put my question here.


My question is to all the ka guys, Im doing a 96 ka dual cam swap into my 510 and need a whole clutch pack and flywheel anyway (since it was out of an automatic car). So is there any reason not to go with an 11lb or so light weight flywheel? (idle/stalling issues, etc...?) The car is mostly street, with some autocross, road racing and canyon runs. (I searched, but didn't come up with much on ka's)

Edited by Creepy Cruiser
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I personally have never seen a light flywheel cause idle issues. a 8 lb or 9.5 in my opinion and experience will be fine for daily driving. if you are concerned then go with the 9.5 lb wheel. I personally feel you will like the response from a light flywheel. the main thing you want to do is get clutch that is streetable. that is the item that a lot of people choose incorrectly and end up with something too aggressive for daily driving. if you are not drag racing a lot or making a lot of power you don't need anything too crazy. I have seen many use a oem style pressure plate with a disc with two different materials. one side may look a stock oem material and the other may be like ceramic or metallic so if anything gets worn it will be the pressure plate or some choose to use it on the face side of a flywheel if you have a wheel that has a removable/replaceable contact surface. hope this helps. good luck

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