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Thread: Thoughts on a full race motor setup

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Posts: 181-190 of 333
2008-12-17 06:39:36
#181
Originally Posted by mrslappy
Crankshaft balance is very important at high RPM. Thus why many manufacturers will use a 4cw crank for motors that are not reving as high. This is to lessen the load on the engine and thus it takes less power to keep it running so you get more power to the wheels. At higher RPM the 8CW crank is heavier but is more balanced for longevity. Ideally the Reciprocating weight of the Rod is balanced out by the counter weights. I always go for the lightest possible parts in the engine. In a 4 cylinder motor, I do not try to keep the amount I take off the crank the same as the rods and pistons. I will get the rods, pistons, piston pins, rings, bearings all done and balanced out. Then weigh the rotating and reciprocating weight. Then get the crankshaft as light as needed and knife edged. Having a heavier crankshaft with lighter rods and pistons will be more balanced and high RPM stability and longevity than a super light, wimpy crank with very strong, heavy, rods and pistons. Getting the crank knife edged and cut down for windage will be more beneficial than a super light crank with poor windage characteristics.

Of course most of this is out of most peoples pockets but if you do a few of the right things here and there, its definitely going to show on the dyno and in smoothness.

-Ted


That was pretty much what i had thought as it seemed logical to me to remove the same amount.
I completely agree what you say about lightening and windage.
The first motor i ever built was a mini motor and i spent a lot of time lightening the crank rods and new cast pistons (new cast pistons because i was only 17 and thats all i could afford) I then had the whole lot balanced to 10k including flywheel clutch etc and that motor was one of the smoothest fastest revving motors i've ever driven.
2008-12-17 06:42:48
#182
Originally Posted by nick
That was pretty much what i had thought as it seemed logical to me to remove the same amount.



Just to ask what was logical to you to remove the same amount from? Please explain.
2008-12-17 06:55:33
#183
Originally Posted by Doctor
My first built VE had the full 8cw crank, DET rods, 16VE pistons etc. I wasn't that impressed.

For this build I went back to the 4cw crank, knife edged, std ve rods and again, 16ve pistons. All balanced off course. I know a lot of guys are gonna say it's silly to go 4cw, but with everything being prefectly balanced, I just prefer the weight saving over the longevity issues. I honeslty don't see myself keeping this motor for longer than 40,000km. By then I'm sure I'd have rebuilt again to go 2.2l or even Vet.


mathmatically speaking it's impossible for a 4cw crank to be 'perfectly' balanced with a 4 cyl motor. I don't have the exact language in front of me but the 2nd or 3rd order harmonics are always going to be off because a 4cw crank doesn't balance each throw of each piston

I went with a gtir crank, cp pistons that are a few grams heavier than the 16ve pistons and carrillo rods that are lighter than stock (A beams) I had the whole rotating assembly balanced along with the flywheel
2008-12-17 07:08:24
#184
Originally Posted by nick
That was pretty much what i had thought as it seemed logical to me to remove the same amount.


1 pound = 453.59237 grams

.5 pound = 226.796185 grams

.25 pound = 113.3980925 grams

Lets say you knife edge an 8 CWC, taking off say 4 pounds off an 8 counter weight crank would equal 1/2 a pound per counterweight. There is no way you can take of 1 pound per rod and piston. So there is no way you are going to take off the same amount.

The most important part about knifedging and balancing a rotational assembly on a 4 cylinder motor is to make sure that all the pistons have the same weight, all the rods have the same weight while balancing the small end and large end of the rod. Also making sure the CWs are all balanced to each other.

Trust me when I tell you the counterweights from Nissan do not have the same weight as the rod and piston assembly.

Look at it like this.

A 4 CWC is about 33 pounds
A 8 CWC is about 39 pounds.

Thats a 6 pound diffrence. I can tell you this that the 20V (8 CWC ) and standard SR20VE (4 CWC ) have the same rods and the pistons are about the same weight. So having the pistons and rods matching the CW is not going to happen.

A 4 cylinder motor always has 2 piston up and 2 piston down. Each rod and piston being equal is what is counterbalancing the other rod and piston that is on the opposit side. So the pistons and rods balance eachother and the CW on the crank balance each other.
2008-12-17 07:08:29
#185
Originally Posted by Doctor
My first built VE had the full 8cw crank, DET rods, 16VE pistons etc. I wasn't that impressed.

For this build I went back to the 4cw crank, knife edged, std ve rods and again, 16ve pistons. All balanced off course. I know a lot of guys are gonna say it's silly to go 4cw, but with everything being prefectly balanced, I just prefer the weight saving over the longevity issues. I honeslty don't see myself keeping this motor for longer than 40,000km. By then I'm sure I'd have rebuilt again to go 2.2l or even Vet.


I agree on the 4cw for less weight for shorter life span motors such as all drag and if something happens its not a big deal. If the motor is built more to last forever and not planning on taking the bottom end apart again, i'd much rather have a knife edged 8cw crank. The ones i get done now make the 8cw crank the same weight as the 4 CW crank. I do have a very nice ( read very expensive) 4cw crank that is now a 90mm stroke and cut down it weighs about 23.8lbs I think. Its got a lot of custom crazy stuff done to it, nothing i'd consider for longevity though.

Originally Posted by nick
That was pretty much what i had thought as it seemed logical to me to remove the same amount.


Originally Posted by Andreas
Just to ask what was logical to you to remove the same amount from? Please explain.


I think he means about if you remove "x" amount from the rods and pistons then remove "x" amount from the crank.

I do not do it that way though. I do not focus on removing a specific amount of weight from the crank. This is because if you remove say 50 grams at the center point of the rod journal but you can not remove weight off the crank at that point lets say, but if you can 1/2 way down closer to the main journal, then you would have to remove 100grams of weight at that point.

I will only focus on removing weight from the rods/pistons as i can, and then maybe do some here or there on the crank as needed but most of that is more dependent on the longevity of the motor and its plans as being built. I never try to remove weight equally from Rod/piston to the crank.

Originally Posted by donttazmebro
mathmatically speaking it's impossible for a 4cw crank to be 'perfectly' balanced with a 4 cyl motor. I don't have the exact language in front of me but the 2nd or 3rd order harmonics are always going to be off because a 4cw crank doesn't balance each throw of each piston.

I went with a gtir crank, cp pistons that are a few grams heavier than the 16ve pistons and carrillo rods that are lighter than stock (A beams) I had the whole rotating assembly balanced along with the flywheel


That is correct about the 4cw crank. I believe its the 4th order harmonics though ( off the top of my head) but it could be 2nd or 3rd. If the crank is not counterbalanced correctly for higher RPMs it can actually create a harmonic and have slight "flex" to it. If I had to choose any crank for any motor build that was going to be for turbo/supercharged or road racing or anything not meant to be torn apart any time soon, I would go with a 8cw crank hands down. For really high reving, very responsive motors, the 4cw can have its advantages but you need to know what they are and how to use them.

-Ted
2008-12-17 08:16:08
#186
A 4 w crank will give a rocking couple that translates to a flex moment on the crankshaft, as the mainbearing inbetween one rod/piston and the counterweight acts as a see-saw pivot.

Mike
2008-12-17 08:28:05
#187
I hadn't thought of it from the fact of 4cw crank being so much lighter than the 8cw crank and still using the same piston/rod assy
I also realise the cranks counter weights aren't the same as the rod pistons weight,
my reasoning behind my statement was nissan would have put a lot of development into the design of the rotating assembly so it seemed logical to keep the same "ratio" if you will.

But i also now realise that it isn't as simple as just removing the same weight from the crank, I had kind of forgot the basic rotational inertia thing. Like Mr slappy was getting at, weight removed from further down the crank web will have a larger effect on rotational inertia than weight remved inboard.

I take back what I said, I agree with you miko after a bit further thought there isn't really any thing logical about that assumption!
2008-12-17 15:45:07
#188
Great discussion. Now would be a good time to clear up some of the misconception on undampened front pulleys and its's affect on sustained high rpm engines. Please, mrslappy your thoughts.
2008-12-17 16:19:37
#189
Originally Posted by SERacer
Great discussion. Now would be a good time to clear up some of the misconception on undampened front pulleys and its's affect on sustained high rpm engines. Please, mrslappy your thoughts.


On stock motors, using an aluminum crank pulley is a gamble. When you remove the dampening abilities then the harmonics get transfered across the crank and can actually loosen the rod bolts. You can sustain high rpm without a crank damper, but that is very dependent on how well the complete assembly is balanced as well as how well the motor is tuned. Detonation, pre ignition, are other factors that affect crank vibration. When building a motor for longevity it is always wise to keep a damper on the crank. If you have a very high HP motor, you may want a larger front crank damper because of the higher forces acting on the crank.
2008-12-17 18:01:18
#190
good work on the crankshaft discussion.. .. dosent ati make a dampener for the ve?
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