On Desktop Dyno 2000, are you able to and would it make a difference if I used my Bobweight Total, which is 2284.5 grams? I would think that would make a noticable difference in the power figures because my bobweight is way lighter than stock.
Does anyone know what the stock bobweight of a 400 is?
What does the lighter bobweight affect?
Any answers appreciated.
Thanks,
Tim

79 F150 4x4 410m Built Ford Tough!(9.1:1,KB pistons,custom Comp Cams X-Treme 210/218-.508/.524,roller rockers)/c6/3.5:1 gears,4.11:1 soon, Demon 4bbl, 285/75R16 BFG Mudders on alumn. mags with New Drk. Blue paint.
65 Ranchero 170-6, 3on the tree, solid mags, New Bright Yellow paint

 

Though your Bobweight is lower, how much did the new pistons add? My guess, going from the stock 8.3 to a 9.1, the addtional material in the piston would have added about 70 or 80 grams each. Also if you are going to take the time to figure in the bobweight then you should figure in the increase amount of force needed for the new valve springs compared to the old ones. I believe the news you have installed now have 50% more clasped pressure then the old ones. Also if you only installed roller tip rockers and not full roller rockers then you need to adjust the amount of power saved by 70% of the total power savings veruses ball rockers.

My point is this. Though these varibles are small they do add and/or subtract on the range of 1 to 3% of the total horsepower made by a given engine. But at the same time you can dyno test an engine one hour and it will show (for example) 354.5hp at 4800 rpms. Test that same engine again 10 minutes later and it will might pull 358.0 a difference of 1.02%. Test it again in an hour and you might get only 350hp or a -1.50% difference.

This is why an engine is based line by running 3 test then averaged. There are just to many uncontrollable varibles. Though programs like Mr Gasket's Desk Top Dyno are good for estimation of hp, they cannot give a 100% real world output reading. Thats one reason why they don't include a place for every varible and have the clause to the effect of "actual engine results will vary". Personally though I do use it to test out new combo and have found it to be close (within 3-7%) but always high for some reason.

 

>Though your Bobweight is lower, how much did the new pistons
>add? My guess, going from the stock 8.3 to a 9.1, the
>addtional material in the piston would have added about 70
>or 80 grams each.

Well, the entire rotating mass is still way lighter than a stock. I had to grind and drill the crap out of the stock crankshaft to get it to balance. This should make the engine rev quicker at the very least, right?

>Also if you are going to take the time to
>figure in the bobweight then you should figure in the
>increase amount of force needed for the new valve springs
>compared to the old ones. I believe the news you have
>installed now have 50% more clasped pressure then the old
>ones. Also if you only installed roller tip rockers and not
>full roller rockers then you need to adjust the amount of
>power saved by 70% of the total power savings veruses ball
>rockers.

But wouldn't the bobweight have more of an effect on the
engine than the extra pressure from the valve springs?
Also, These are full roller rockers, not just roller tips.
Thanks for the input.
79 F150 4x4 410m Built Ford Tough!(9.1:1,KB pistons,custom Comp Cams X-Treme 210/218-.508/.524,roller rockers)/c6/3.5:1 gears,4.11:1 soon, Demon 4bbl, 285/75R16 BFG Mudders on alumn. mags with New Drk. Blue paint.
65 Ranchero 170-6, 3on the tree, solid mags, New Bright Yellow paint

 

Whelp, um, okay let me put it this way. I guess I don't understand why you had to remove so much weight from the crank to make it balance. Did you static balance all the pistons, pins, and rods before you did the spin balance? Did you make sure that each assembly stayed together and no parts were mixed after you balanced it? Was the crank linearly static balanced with the piston assembly before it was spun balanced?

Which method of balancing the short block did you use? Did you use the weighted crank method (where weights that equal the total weight of each rod, pin, and piston assembly are used instead of the actual assembly) or the true method, where the short block is completely assembled then spun? The thing is, is that usually if you have to remove a lot of weight from the crank something wasn’t right or you have a bad crank. Factory setups can be anywhere from 30 to 50 grams out of weight and still run okay for a production engine. Also when a crank needs a lot of weight or needs to have a lot of it removed it’s a good sign that the crank could be twisted.

Manual trannies are hard on an engine. Remember that they have a direct connection to the engine where as an automatic has the torque converter that can take park of the shock load. This direct connection can, under hard use, twist the crank slightly as time goes on. I’ve seen cranks that were 2 and 3* off center from hard use with manuals. The only way to be sure is to have a known good crank and a suspected crank sitting side by side. Then using a protractor and compass take measurements of each journal and throw position and compare results.

The thing is though that unless your building a race only engine that makes goggles of power and torque (goggles=more then I can afford), and is going to run at 6500-7000 rpm’s and shift at 7500, this stuff isn’t needed. As far as which has more effect on making the engine rev faster, compare the bobweight of the 2284.5 grams, there is 24 grams in a ounce so that’s 95 oz’s or 5.9lbs, to the 100lbs+ added force of each new spring. This is one reason that super stock racers use low tension-high pressure springs and toss them out every 30-50 runs. Because the springs have fewer coils they go “flat” pretty quick, but they make a big difference in how fast the engine revs. Your new springs are going to a much larger effect then your bobweight ever would. The 6lbs of bobweight in the over all scheme of internal friction might make a .05 second difference.

 

>Well, um, okay let me put it this way. I guess I don't
>understand why you had to remove so much weight from the
>crank to make it balance.
Because the hypereutectic 351C pistons i used were intended for racing and therefore much lighter than a stock 351C or 400, to spin balance the crank withing plus or minus 2.5%, a lot of material had to be taken out of the counterweights. Here is a link that will tell you everything you need to know about them.
http://www.kb-silvolite.com/Claimer/Ford%20351C.html


>Did you static balance all the
>pistons, pins, and rods before you did the spin balance?
Yes, I did. Within a half of a gram.

>Did you make sure that each assembly stayed together and no
>parts were mixed after you balanced it?
Yes.

>Was the crank linearly static balanced with the piston assembly >before it was spun balanced?
Not sure what you mean by this, could you explain more?

>Which method of balancing the short block did you use? Did
>you use the weighted crank method (where weights that equal
>the total weight of each rod, pin, and piston assembly are
>used instead of the actual assembly).
Yes, the weighted crank method.

>The thing is though that unless your building a race only
>engine that makes goggles of power and torque (goggles=more
>then I can afford), and is going to run at 6500-7000 rpm’s
>and shift at 7500, this stuff isn’t needed.
True, but for the purpose of my mentorship at the machine shop, my mentor wanted me to experience everything I might encounter working in a machine shop. Which meant blueprinting and balancing.

Thanks for your responses.



79 F150 4x4 410m Built Ford Tough!(9.1:1,KB pistons,custom Comp Cams X-Treme 210/218-.508/.524,roller rockers)/c6/3.5:1 gears,4.11:1 soon, Demon 4bbl, 285/75R16 BFG Mudders on alumn. mags with New Drk. Blue paint.
65 Ranchero 170-6, 3on the tree, solid mags, New Bright Yellow paint

 

Ok this is a fun one because not to many are ever going to do this unless they want to run 8000 rpm's for a few hundred miles. Most people only think one dimensionally when it comes to balancing a crank and that is rotating mass. In other words they see a circle has flat. Now lets add 7 more circles and space those .5 inches apart. Now we have 8 circles in 3 dimensions (height, width and depth) on a linear plane (the crank shaft). Ok so you weighed all the pistons, pins and rods. They are now all within .5 grams of each other, so where are you going to put them on the crank? Who cares because they are all within .5 grams of each other right?

Well let’s say that you your assemblies weighed 400.0 thru 400.5 grams (just for the sake of making this easy on me). Let’s assume (I hate that word but hey) that your weights are as follows (assembly by weight) 1-400 2-400.3 3-400.3 4-400.2 5-400.4 6-400.5 7-400.1 and number 8-400.2. Has you can see that although you have only .5 grams of difference between each assembly you actually have 6 different weights. Ok so now where to install these on the crank.

You want to put the ones with the closest readings at 180* opposite of each other on the crank. This way you have almost equal opposing forces. But now you have mixed weights on the linear line of the crank. In other words you might have 1601.3 grams on the front four of the throws and 1600.2 on the back for. So now your linear or line balance (or crank center line balance) is off by 1.1 grams. Remember that 1-gram spinning at 2000 rpm’s is equal to 125 lbs of force (or something like that lol). Has the engine is spinning its changing the directional alignment of this force and pounding on the bearings.

Once again this is acceptable in a production engine. After all it’s going to take a lot more force then this to overcome the oil pressure and damage a main bearing or journal. But at 8000 rpm’s that force increases to over 2200lbs and that can overcome the oil surface clearances, forcing the cranking into the main bearings.


I have to get so i'll post more later.

 

Ok so now you have your crank on the stand. You measured it and found that its length is 34 inches. Doing a bit of basic math this places the center line at 17". When your going to do this don't forget to include the bolt and washer and key way for the harmonic balancer, but do not include the harmonic balancer. Now, with your piston assembly wieghts installed on the crank, you place a free standing .125" metal support at the exact center of the crank and go to town. I've been to this little garage in Chicago (called of all things "The Chicago Connection")where they use a laser and .001 scales to linear the crank assembly. These guys build mountian engines (from 540 to 760cid) that run at .05% total balance harmonics. You can balance one of there cranks (if memory serves the crank used on the 540 is a 4.5 inch Chevy forged unit weighing in at 101bls w/o assemblies) on a .100 metal rod. Now thats impressive.

 

>Ok this is a fun one because not to many are ever going to
>do this unless they want to run 8000 rpm's for a few hundred
>miles. Most people only think one dimensionally when it
>comes to balancing a crank and that is rotating mass. In
>other words they see a circle has flat. Now lets add 7 more
>circles and space those .5 inches apart. Now we have 8
>circles in 3 dimensions (height, width and depth) on a
>linear plane (the crank shaft). Ok so you weighed all the
>pistons, pins and rods. They are now all within .5 grams of
>each other, so where are you going to put them on the crank?
> Who cares because they are all within .5 grams of each
>other right?
>
>Well let’s say that you your assemblies weighed 400.0 thru
>400.5 grams (just for the sake of making this easy on me).
>Let’s assume (I hate that word but hey) that your weights
>are as follows (assembly by weight) 1-400 2-400.3 3-400.3
>4-400.2 5-400.4 6-400.5 7-400.1 and number 8-400.2. Has you
>can see that although you have only .5 grams of difference
>between each assembly you actually have 6 different weights.
>Ok so now where to install these on the crank.
>
>You want to put the ones with the closest readings at 180*
>opposite of each other on the crank. This way you have
>almost equal opposing forces. But now you have mixed weights
>on the linear line of the crank. In other words you might
>have 1601.3 grams on the front four of the throws and 1600.2
>on the back for. So now your linear or line balance (or
>crank center line balance) is off by 1.1 grams. Remember
>that 1-gram spinning at 2000 rpm’s is equal to 125 lbs of
>force (or something like that lol). Has the engine is
>spinning its changing the directional alignment of this
>force and pounding on the bearings.
>
>Once again this is acceptable in a production engine. After
>all it’s going to take a lot more force then this to
>overcome the oil pressure and damage a main bearing or
>journal. But at 8000 rpm’s that force increases to over
>2200lbs and that can overcome the oil surface clearances,
>forcing the cranking into the main bearings.

Wow, that is enlightning! If my crank at 8,000 rpm's with .015" main bearing clearance and 1 gram of linear unbalance, will run into the main bearings, then what keeps these stock rice rockets with factory specs from doing the same at 8000 rpm? These rice rockets must have way more linear unbalance than my engine, right?

Wanted: Winch for pulling out Chevys!

79 F150 4x4 410m built!(9.1:1,KB pistons,custom Comp Cams X-Treme 210/218-.508/.524,roller rockers)/c6/3.5:1 gears,4.11:1 soon, Demon 4bbl, 285/75R16 BFG Mudders on alumn. mags with New Drk. Blue paint.
65 Ranchero 170-6, 3on the tree, solid mags, New Bright Yellow paint

 

lol I was wondering when you were going to ask about the high rpm rice motors. You got to remeber two things. Nubmer one is that they have nearly 80% less rotating mass then your big *** V8. As such thats a lot less wieght being tossed around. Number two is that they have a main journal for every rod journal plus one more (same holds true for a I6). This means that the crank is held in almost perfect check by the oil pressure. A few years ago Offenhauser even made an Indy/Cart 4 that used no main bearings at all. Instead the crank "floated" on oil pressure. These engines would speed to 14,000 rpms so fast that cart made them run 32psi of boost as compare to 40 psi that other brands used.