I assume the 1.8 crank has less rotating mass than a 2.0 due to the 2:1 counter weight ratio. I figured using the 1.8 crank would drop compression slightly due to the less throw and would drop sprung mass and allow for quicker revolution of the crank.

Any ideas or suggestions?

Well, less displacement being compressed into the same combustion chamber will lower your compression ratio assuming you are using flat top pistons in both applications. U.S. 1.8l pistons have a slight dome as I recall and I don't think it will fit the e21 casting head. 2.0l pistons will work with the 1.8L crank but leave it "in the hole" too far for effective combusiton-not good for efficency.

The less rotating mass comes with a tradeoff too... With fewer counterweights it will flex more as the weight is concentrated in smaller areas rather than spread more evenly along the crank. This may or may not be an issue with longivity and probably depends on how high you plan to rev the engine and how well it's balanced by the machine shop.

Less rotating weight is like extra HP as documented by tests conducted by Herb Adams.

ken

I'm using customed JE dish 8:1 comp pistons. My only worry is that I'm going under a high amount of boost and I really do not want to see crank failure. I already knew the 1.8 had its advantage with the less rotation mass freeing up horse power, lowering compression with a smaller stroke. I just didnt want to be a know it all and assume to much about this little crank! Thanks Ken

I'm using customed JE dish 8:1 comp pistons. My only worry is that I'm going under a high amount of boost and I really do not want to see crank failure. I already knew the 1.8 had its advantage with the less rotation mass freeing up horse power, lowering compression with a smaller stroke. I just didnt want to be a know it all and assume to much about this little crank! Thanks Ken

Hi. You can cryo the crank, or nidrade (sp?) it. Cryoing a crank is extra protection to harden it. My 524td crank is forged but even forged cranks can be cryoed. The S50 one I have has been treated by Bmw already.
So I have been told that cryoing either cranks is a waste of money...
and I am planning on boosting as well.

If it was me, greg I would use a large crank as possible, and just use the lower compression pistons and boost it like there is not tommorow.
If the crank is forged - you should be fine.

You can ask JBOB about what crank he is using in his m10...he and attila are both running m10 turbos.

Extra displacement, is always better.
Thats why people choose the s50 crank over the 524td one, and then boost.

Well I'm just trying to edge out as much power as possible. Neither Atilla or JBob are going as extreme as me. I think GoodandTight is close but with how much I'm spending on the engine rebuild I feel I might as well free up what I can where I can as well.

Would an engine w/ less rotating mass improve vehicle dynamics, ie, quicker steering, and lighter engine braking keeps front end from plowing so much through turns ?

Would an engine w/ less rotating mass improve vehicle dynamics, ie, quicker steering, and lighter engine braking keeps front end from plowing so much through turns ?

No.

Less rotating mass allows for quicker revolutions of your motor. This free's up horse power, better throttle response and takes down on sprung mass(not much but in the racing game this matters.)

Would an engine w/ less rotating mass improve vehicle dynamics, ie, quicker steering, and lighter engine braking keeps front end from plowing so much through turns ?

The weight differences of the two cranks..is not that great if you where to weigh them.1.8 vs 2.0..

As Greg said:
Less rotating mass allows for quicker revolutions of your motor. This free's up horse power, better throttle response and takes down on sprung mass.
This is why people lighten their flywheels and knife edge cranks...


DHong..If we where to compare an M10 or S14 vs M20 Vs M30 in the front end of and e21 the engine weight does make a difference in what you are referring to as "vehicle dynamics."
People like engines that dont weigh that much in the front ends. Thats why boosted
engines are great.

I'll be the first to chime in with "There is no replacement for displacement".

I think the gain you'll have with the bump in displacement will be higher than what you'll see from slightly less rotating mass.

from what i've experienced, my current 2.0 (with lighter flywheel & crank pulley) revs up & down quite a bit faster than my old stock 1.8.

IMO, the torque gain from an extra 0.2 liters of displacement far outweighs the extra weight it comes with.

And Ken brought up a great point about the counterweights and how it can effect longitivity (thru excess/reduced flex & vibrations). On a related note: the hi revving & hi output m12 motor used a 1.5 crank with 8 counterweights

jbob
IMO, the torque gain from an extra 0.2 liters of displacement far outweighs the extra weight it comes with

jrcook320 I'll be the first to chime in with "There is no replacement for displacement".

Yes,
I agree. Thats why i said I would take either a s14 or 2.0 liter crank and use this as a basis for my m10 build if I was making one. They both can be croyed..

Bump the displacement than boost.


My first goal is to finish the buildup, and running of my 2.8 m20.
This should be done by spring. After which I am going to collect parts for a
2.9 m20 with the same bore, different pistons, and s50 rods. This will be a turbo engine, will boost. after that.

Cryogenic treatment can be used on ANY crank or any engine part for that matter-iron, steel or aluminum.

For a max effort engine, especially boosted, cryogenic treatment is worth the expense-and it's not really all that much. For a boosted m10 I would cryo just about everything including the block, crank, rods, pistons, head, valves and valve springs (keepers/retainers included) as well as those rocker arms.

If you are running custom forged pistons I would use the 2.0L crank (I like more displacement too) and have the pistons dished in the mirror image to the combustion chamber. Set the deck height (piston to head clearance) at .045" including the gasket thickness and ceramic coat the piston tops, combustion chambers and valve heads-throw in the exhaust ports too.

I have had the cylinders, cam, and lifters cryoed on my VW type 4 engine. The aftermarket cylinders have a reputation of distorting during hard use and the cam/lifters have wear problems-the cryo treatment should cure these easily.

If your new pistons are light enough your crank counterweights will have to be lightend some during balancing - that will help some. Also look into some aftermarket rods-some can be stronger while still being lighter.

A quality light flywheel/clutch assembly will have more effect on reving than a 1.8L crank...

ken